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Notes:

 

1- We ask you kindly to go through the below listed topics of biology and to concentrate on the main concepts

and outlines i.e. you don’t need to learn by heart all texts, figures and tables, it is just to give you a

sufficient explanation for the  different topics and to deepen your understanding to the material.

 2- Please notify, for the interview you need to study all the below listed topics and concentrate on at least ten

topics.

TOPICS ON BIOLOGY FOR ADMISSION TEST

1. Basic structure and characteristics of the eukaryotic cell (cellular organelles, structure, function).

2. Basic metabolic pathways: glycolysis, biological oxidation, photosynthesis (the biological role and basic characteristics of enzymes).

3. The DNA and its role in heredity: the structure of DNA, the genetic code, the replication of the genetic material.

4. Cell division I.: Chromatin, chromosomes. The behaviour of chromosomes during mitosis.

5. Cell division II.: Meiosis. The role of meiosis in sexually reproducing organisms.

6. Genetics I.: Genotype, phenotype, genes, alleles. Monohybrid cross, dominant-recessive type of inheritance, co-dominance. The first Mendelian law of inheritance.

7. Genetics II.: X-linked inheritance. Dihybrid cross: the second Mendelian law of inheritance. Genetic linkage, crossing-over.

8. From DNA to protein: Translation, mRNA, rRNA, tRNA, ribosomes.

9. Basic anatomy and physiology of the human respiratory system.

10. Basic anatomy and physiology of the human circulatory system.

11. Basic anatomy and physiology of the human digestive system.

12. Basic anatomy and physiology of the human excretory(Urinary) system., importamt

13. Homeostasis: the basic structure and function of the human nervous system.

14. Homeostasis: hormones, the human endocrine system.

15. Basic structure and function of skeletal muscle cells, locomotion in humans.

16. The basic defense systems against infections: the humoral and cellular immune response in humans.

Textbook: Sadava, Heller, Orians, Purves, Hillis (ed.): Life. The Science of Biology, 8th. Edition. Sinauer Associates, INC., VHPS/W.H. Freeman and Co. Gordonsville, VA, U.S.A.

Almost the answer of all of the above questions can be found on the below website:

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The answer of topics no.15: Basic structure and function of skeletal muscle cells, ,

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In addition you can find some answers below:

Answers

Macromolecules: Giant Polymers

Macromolecules are polymers constructed by the formation of covalent bonds between smaller molecules called monomers. Macromolecules in living organisms include polysaccharides, proteins, nucleic acids and lipids.

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Proteins: Polymers of Amino Acids

The functions of proteins include support, protection, catalysis, transport, defense, regulation, and movement. Protein function sometimes requires an attached prosthetic group.

There are 20 amino acids found in proteins. Each amino acid consists of an amino group, a carboxyl group, a hydrogen, and a side chain bonded to the α carbon atom.

The side chains, or R groups, of amino acids may be charged, polar, or hydrophobic; there are also special cases, such as the —SH groups of cysteine, which can form disulfide bridges. The side chains give different properties to each of the amino acids.

Basic structure of amino acid:

COOH

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H2N-C-H

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R

Amino acids are covalently bonded together into polypeptide chains by peptide linkages, which form by condensation reactions between the carboxyl and amino groups.

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Polypeptide chains are folded into specific three-dimensional shapes to form functional proteins. Four levels of protein structure are possible: primary, secondary, tertiary, and quaternary.

The primary structure of a protein is the sequence of amino acids bonded by peptide linkages. This primary structure determines both the higher levels of structure and protein function.

The two types of secondary structure-α helices and β pleated sheets-are maintained by hydrogen bonds between atoms of the amino acid residues.

The tertiary structure of a protein is generated by bending and folding of the polypeptide chain.

The quaternary structure of a protein is the arrangement of two or more polypeptides into a single functional protein consisting of two or more polypeptide subunits.

Carbohydrates: Sugars and Sugar Polymers

All carbohydrates contain carbon bonded to hydrogen atoms and hydroxyl groups.

Hexoses are monosaccharides that contain six carbon atoms. Examples of hexoses include glucose, galactose, and fructose, which can exist as chains or rings.

The pentoses are five-carbon monosaccharides. Two pentoses, ribose and deoxyribose, are components of the nucleic acids RNA and DNA, respectively.

Glycosidic linkages may have either α or β orientation in space. They covalently link monosaccharides into larger units such as disaccharides, oligosaccharides, and polysaccharides.

Cellulose, a very stable glucose polymer, is the principal component of the cell walls of plants. It is formed by glucose units linked together by β-glycosidic linkages

Lipids: Water-Insoluble Molecules

Fats and oils are triglycerides, composed of three fatty acids covalently bonded to a glycerol molecule by ester linkages.

Saturated fatty acids have a hydrocarbon chain with no double bonds.

Phospholipids have a hydrophobic hydrocarbon "tail" and a hydrophilic phosphate "head."

In water, the interactions of the hydrophobic tails and hydrophilic heads of phospholipids generate a phospholipid bilayer that is two molecules thick. The head groups are directed outward, where they interact with the surrounding water. The tails are packed together in the interior of the bilayer.

Nucleic Acids: Informational Macromolecules

DNA is the hereditary material. Both DNA and RNA play roles in the formation of proteins. Information flows from DNA to RNA to protein.

Nucleic acids are polymers made up of nucleotides. A nucleotide consists of a phosphate group, a sugar (ribose in RNA and deoxyribose in DNA), and a nitrogen-containing base. In DNA the bases are adenine, guanine, cytosine, and thymine, but in RNA uracil substitutes for thymine.

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Not to be memorized

In the nucleic acids, the bases extend from a sugar-phosphate backbone. The information content of DNA and RNA resides in their base sequences. RNA is single-stranded. DNA is a double-stranded helix in which there is complementary, hydrogen-bonded base pairing between adenine and thymine (A-T) and guanine and cytosine (G-C). The two strands of the DNA double helix run in opposite directions

The Cell: The Basic Unit of Life

• All cells come from preexisting cells and have certain processes, types of molecules, and structures in common.

• To maintain adequate exchanges with its environment, a cell's surface area must be large compared with its volume.

• Cells can be visualized by various methods using microscopes.

• All cells are surrounded by a plasma membrane

Prokaryotic Cells

• All prokaryotic cells have a plasma membrane, a nucleoid region with DNA (no membrane-enclosed organelles (nucleus, no Mitochondria) and a cytoplasm that contains ribosomes, water, and dissolved proteins and small molecules.

• Prokaryotes doesn’t contain membrane-enclosed organelles (Nucleus, Mitochondria, golgi apparatus…etc

• Some prokaryotes have additional protective structures: cell wall, outer membrane, and capsule.

Eukaryotic Cells

Like prokaryotic cells, eukaryotic cells have a plasma membrane, cytoplasm, and ribosomes. However, eukaryotic cells are larger and contain many membrane-enclosed organelles. The membranes that envelop organelles in the eukaryotic cell are partial barriers, ensuring that the chemical composition of the interior of the organelle differs from that of the surrounding cytoplasm.

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Cell organelles (cell ultra structures)

1- The Nucleus

2- Endoplasmic reticulum

3- Golgi apparatus

4- Mitochondria and Chloproplast

5- Lysosomes

Organelles that Process Information (The nucleus)

The nucleus is usually the largest organelle in a cell. It is surrounded by a double membrane (the nuclear envelope), which disassembles during cell division. Within the nucleus, the nucleolus is the source of the ribosomes which is produce their and export to the cytoplasm.

The cell nucleus is a remarkable organelle because it forms the package for our genes and their controlling factors. It functions to:

[pic]Store genes on chromosomes

[pic]Organize genes into chromosomes to allow cell division.

[pic]Transport regulatory factors & gene products via nuclear pores

[pic]Produce messages ( messenger Ribonucleic acid or mRNA) that code for protein synthesis.

[pic]Produce ribosomes in the nucleolus

[pic]

The nucleus contains most of the cell's DNA, which associates with protein to form chromatin. Chromatin is diffuse throughout the nucleus until just before cell division, when it condenses to form chromosomes.

The Endomembrane System

The endoplasmic reticulum system is made up of a series of interrelated compartment enclosed by membranes: endoplasmic reticulum (rough and smooth), the Golgi apparatus and lysosomes

• The rough endoplasmic reticulum has attached ribosomes that synthesize proteins. The smooth endoplasmic reticulum lacks ribosomes and is associated with the synthesis of lipids and detoxification of poisons and drugs.

Endoplasmic reticulum is a network of tubules, vesicles and sacs that are interconnected. They may serve specialized functions in the cell including

- protein synthesis,

- Sequestration of calcium

- production of steroids

- Storage and production of glycogen

-Rough endoplasmic reticulum bears the ribosomes during protein synthesis. The newly synthesized proteins are sequestered in sacs, called cisternae . The system then sends the proteins via small vesicles to the Golgi Complex , or, in the case of membrane proteins, it inserts them into the membrane. The Ribosomes sit on the outer surfaces of the sacs (or cisternae). They resemble small beads sitting in rosettes or in a linear pattern

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The Golgi Apparatus

• The Golgi apparatus consists of a stack of membrane-bounded cisternae located between the endoplasmic reticulum and the cell surface. A myriad of enzymes (proteins) are present in the Golgi to perform its various synthetic activities.

• The Golgi apparatus receives materials (proteins) from the rough ER by means of vesicles that fuse its cis region:

• Some of these (proteins) will eventually end up as integral membrane proteins embedded in the plasma membrane.

• Other proteins moving through the Golgi will end up in lysosomes

• or be secreted by exocytosis (e.g., digestive enzymes).

• The major processing activity is glycosylation: the adding of sugar molecules to form glycoproteins.

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• Lysosomes contain many digestive enzymes. Lysosomes fuse with the phagosomes produced by phagocytosis to form secondary lysosomes, in which engulfed materials are digested. Undigested materials are secreted from the cell when the secondary lysosome fuses with the plasma membrane.

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Organelles that Process Energy (Mitochondria and chloroplasts)

• Mitochondria are enclosed by an outer membrane and an inner membrane that folds inward to form cristae. Mitochondria contain the proteins needed for cellular respiration (electrons transport chain, proton pumping and ATP synthesis), lipid synthesis and the Kreb’s cycle (Citric acid cycle) Kreb’s cycle takes place in the matrix.

Mitochondria contain their own DNA and ribosomes and are capable of making some of their own proteins

Mitochondrial Substructure

|[pic] |

|Mitochondria contain two membranes, separated by a space.  Both are the typical "unit membrane" (railroad track)|

|in structure. The space which is enclosed by the inner membrane is the matrix.  This appears moderately dense |

|and one may find strands of DNA, ribosomes, or small granules in the matrix. The mitochondria are able to code |

|for part of their proteins with these molecular tools. The above cartoon shows the diagram of the mitochondrial |

|membranes and the enclosed compartments. |

How are mitochondria organized to be powerhouses?

The food we eat is oxidized to produce high-energy electrons that are converted to stored energy. This energy is stored in high energy phosphate bonds in a molecule called adenosine triphosphate, or ATP. ATP is converted from adenosine diphosphate by adding the phosphate group with the high-energy bond. Various reactions in the cell can either use energy (whereby the ATP is converted back to ADP, releasing the high energy bond) or produce it (whereby the ATP is produced from ADP).

Steps from glycolysis to the electron transport chain. Why are mitochondria important?

Lets break down each of the steps so you can see how food turns into ATP energy packets and water. The food we eat must first be converted to basic chemicals that the cell can use. Some of the best energy supplying foods contain sugars or carbohydrates ...bread, for example. Using this as an example, the sugars are broken down by enzymes that split them into the simplest form of sugar which is called glucose. Then, glucose enters the cell by special molecules in the membrane called “glucose transporters”.

Once inside the cell, glucose is broken down to make ATP in two pathways. The first pathway requires no oxygen and is called anaerobic metabolism. This pathway is called glycolysis and it occurs in the cytoplasm outside the mitochondria. During glycolysis, glucose is broken down into pyruvate. Other foods like fats can also be broken down for use as fuel (see following cartoon). Each reaction is designed to produce some hydrogen ions (electrons) that can be used to make energy packets (ATP). However, only 4 ATP molecules can be made by one molecule of glucose run through this pathway. That is why mitochondria and oxygen are so important. We need to continue the breakdown process with the Kreb’s cycle inside the mitochondria (matrix) in order to get enough ATP (~ 28 ATP/one glucose sugar) to run all the cell functions

The events that occur inside and outside mitochondria are diagrammed in the above cartoon. Pyruvate is carried into the mitochondria and there it is converted into Acetyl Co-A which enters the Kreb's cycle.  This first reaction produces carbon dioxide because it involves the removal of one carbon from the pyruvate.

How does the Kreb's cycle work?

The whole idea behind respiration in the mitochondria is to use the Kreb’s cycle (also called the citric acid cycle) to get as many electrons out of the food we eat as possible. These electrons (in the form of hydrogen ions) are then used to drive pumps that produce ATP.  The energy carried by ATP is then used for all kinds of cellular functions like movement, transport, entry and exit of products, division, etc.  To produce ATP from pyruvate it is a complicated process in which many factors (like Acetyl CoA , NAD, FAD, enzymes (electron transport chain which is located int he inner membrane of mitochondria, ATP synthase…), proton gradient and Oxygen are need. ATP is produced in matrix, also O2 couple to H+ (proton or Hydrogen ion) and electrons to produce water also this take place in matrix.

why do we need mitochondria?

The whole idea behind this process is to get as much ATP out of glucose (or other food products) as possible. If we have no oxygen, we get only 4 molecules of ATP energy packets for each glucose molecule (in glycolysis). However, if we have oxygen, then we get to run the Kreb’s cycle to produce many more hydrogen ions that can run those ATP pumps. From the Kreb’s cycle we get 24-28 ATP molecules out of one molecule of glucose converted to pyruvate (plus the 4 molecules we got out of glycolysis). So, you can see how much more energy we can get out of a molecule of glucose if our mitochondria are working and if we have oxygen.

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Plasma Membrane Composition and Structure

Biological membranes consist of lipids, proteins, and carbohydrates. The fluid mosaic model of membrane structure describes a phospholipid bilayer in which proteins can move about laterally within the membrane.

Integral membrane proteins are at least partially inserted into the phospholipid bilayer. Peripheral membrane proteins are attached to the surface of the bilayer by ionic bonds. The two surfaces of a membrane may have different properties because of their different phospholipid composition, exposed domains of integral membrane proteins, and peripheral membrane proteins.

Carbohydrates attached to proteins or phospholipids project from the external surface of the plasma membrane and function as recognition signals for interactions between cells.

Membrane components may:

[pic]be protective

[pic]regulate transport in and out of cell or subcellular domain

[pic]allow selective receptivity and signal transduction by providing transmembrane receptors that bind signaling molecules

[pic]allow cell recognition

[pic]provide anchoring sites for cytoskeletal filaments or components of the extracellular matrix. This allows the cell to maintain its shape and perhaps move to distant sites.

[pic]provide a stable site for the binding and catalysis of enzymes.

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Membrane transport

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Passive Membrane Transport

Substances can diffuse passively across a membrane by three processes: simple diffusion through the phospholipids bilayer, facilitated diffusion through protein channels, or facilitated diffusion by means of a carrier protein.

A solute diffuses across a membrane from a region with a greater concentration of that solute to a region with a lesser concentration of that solute. Equilibrium is reached when the concentrations of the solute are identical on both sides of the membrane.

The rate of simple diffusion of a solute across a membrane is directly proportional to its concentration gradient across the membrane. An important factor in simple diffusion across a membrane is the lipid solubility of the solute.

In osmosis, water diffuses from regions of higher water concentration to regions of lower water concentration.

Channel proteins and carrier proteins function in facilitated diffusion.

The rate of carrier-mediated facilitated diffusion reaches a maximum when a solute concentration is reached that saturates the carrier proteins so that no increase in rate is observed with further increases in solute concentration. [pic]

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Active Transport

Active transport requires the use of chemical energy to move substances across a membrane against a concentration gradient.

Active transport proteins may be uniports, symports, or antiports.

In primary active transport, energy from the hydrolysis of ATP is used to move ions into or out of cells against their concentration gradients.

Secondary active transport couples the passive movement of one solute with its concentration gradient to the movement of another solute against its concentration gradient. Energy from ATP is used indirectly to establish the concentration gradient that results in the movement of the first solute

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Endocytosis and Exocytosis

Endocytosis transports macromolecules, large particles, and small cells into eukaryotic cells by means of engulfment by and vesicle formation from the plasma membrane. Phagocytosis and pinocytosis are both nonspecific types of endocytosis.

In receptor-mediated endocytosis, a specific membrane receptor protein binds to a particular macromolecule.

In exocytosis, materials in vesicles are secreted from the cell when the vesicles fuse with the plasma membrane.

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The Human Respiratory System

The Pathway

• Air enters the nostrils

• passes through the nasopharynx,

• the oral pharynx

• through the glottis

• into the trachea

• into the right and left bronchi, which branches and rebranches into

• bronchioles, each of which terminates in a cluster of

• alveoli

[pic]

Only in the alveoli does actual gas exchange takes place. There are some 300 million alveoli in two adult lungs. These provide a surface area of some 160 m2 (almost equal to the singles area of a tennis court and 80 times the area of our skin!).

Breathing

In mammals, the diaphragm divides the body cavity into the

• abdominal cavity, which contains the viscera (e.g., stomach and intestines) and the

• thoracic cavity, which contains the heart and lungs.

• During inspiration (inhaling),

o The external intercostal muscles contract, lifting the ribs up and out.

o The diaphragm contracts, drawing it down .

• During expiration (exhaling), these processes are reversed and the natural elasticity of the lungs returns them to their normal volume. At rest, we breath 15-18 times a minute exchanging about 500 ml of air.

• In more vigorous expiration,

o The internal intercostal muscles draw the ribs down and inward

o The wall of the abdomen contracts pushing the stomach and liver upward.

Under these conditions, an average adult male can flush his lungs with about 4 liters of air at each breath. This is called the vital capacity. Even with maximum expiration, about 1200 ml of residual air remain.

The table shows what happens to the composition of air when it reaches the alveoli. Some of the oxygen dissolves in the film of moisture covering the epithelium of the alveoli. From here it diffuses into the blood in a nearby capillary then it enters a red blood cell and combines with the hemoglobin.

Hemoglobin consist of four globulin (proteins) and heam group which contains Iron ion (Fe+2 ) to which Oxygen used to bind.(Iron gives the red colour to the blood)

Myoglobin has a high affinity for O2 and serves as an O2 reserve in muscle.

At the same time, some of the carbon dioxide in the blood diffuses into the alveoli from which it can be exhaled.

|Composition of atmospheric air and expired air in a typical subject. |

|Note that only a fraction of the oxygen inhaled is taken up by the lungs. |

|Component |Atmospheric Air (%) |Expired Air (%) |

|N2 (plus inert gases) |78.62 |74.9 |

|O2 |20.85 |15.3 |

|CO2 |0.03 |3.6 |

|H2O |0.5 |6.2 |

|  |100.0% |100.0% |

Gas Exchange in Human Lungs

In mammalian lungs, the gas exchange surface area provided by the millions of alveoli is enormous, and the diffusion path length between the air and perfusing blood is very short.

Surface tension in the alveoli would make inflation of the lungs difficult if the alveoli did not produce surfactant.

Transport of Respiratory Gases

Oxygen is reversibly bound to hemoglobin in red blood cells. Each molecule of hemoglobin can carry a maximum of four molecules of O2. Because of positive cooperativity, the affinity of hemoglobin for O2 depends on the PO2 (concentration of Oxygen) to which the hemoglobin is exposed. Therefore, hemoglobin picks up O2 as it flows through respiratory exchange structures and gives up O2 in metabolically active tissues.

Carbon dioxide (Co2) is transported in the blood principally as bicarbonate ions (dissolved in blood).

Human circulatory system

The circulatory system is a complex arrangement of tubes that transport blood as well as waste products throughout the entire body.

The heart is the main pump. The heart is divided into four chambers. The top two chambers are the atriums and the bottom two chambers are the ventricles. The atriums both contract at the same time as do the two ventricles. Blood enters the heart via the superior and the inferior vena cava. These are the two largest veins in the body. The right atrium receives the blood first. The right atrium contracts and forces the blood into the right ventricle.

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When the right ventricle contracts the blood is pumped into both lungs via the pulmonary artery. This portion of the circulatory system is sometimes referred to as pulmonary circulation or lesser circulation. The pulmonary artery is the only artery in the body that carries deoxygenated blood. Blood is returned from the lungs via the pulmonary veins. These are the only veins in the body that carry oxygenated blood. The oxygenated blood is returned to the left atrium. When the atrium contracts the blood is forced into the left ventricle. The left ventricle is the strongest and most muscular portion of a healthy heart.

This is due to the fact that the left ventricle works the hardest. It must force blood throughout the body. When the left ventricle contracts, blood is forced into the aorta. The aorta is the main artery leaving the heart.

The oxygenated blood is now forced throughout the body through a series of arteries that gradually become smaller and smaller. Blood flows from arteries into arterioles. From arterioles into capillaries. At this point the blood is able to make close contact with individual cells. Here is were waste products are picked up and oxygen is delivered.

The blood now starts its return trip to the heart. From the capillaries the blood flows into venules. These are very small veins. (About the same size as capillaries) From the venules the blood is forced into veins These veins all return blood either into the superior or the inferior vena cava. As stated earlier, the inferior and the superior vena cava return the deoxygenated blood to the right atrium of the heart.

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The Human Heart: Two Pumps in One

The human heart has four chambers. Valves in the heart prevent the backflow of blood.

The cardiac cycle has two phases: systole, in which the ventricles contract; and diastole, in which the ventricles relax. The sequential heart sounds ("lub-dub") are made by the closing of the heart valves.

Blood pressure can be measured using a sphygmomanometer and a stethoscope.

The autonomic nervous system controls heart rate: Sympathetic activity increases heart rate, and parasympathetic activity decreases it.

Action Potential

 

Resting Membrane Potential

When a neuron is not sending a signal, it is "at rest." When a neuron is at rest, the inside of the neuron is negative relative to the outside. Although the concentrations of the different ions attempt to balance out on both sides of the membrane, they cannot because the cell membrane allows only some ions to pass through channels (ion channels). At rest, potassium ions (K+) can cross through the membrane easily. Also at rest, chloride ions (Cl-)and sodium ions (Na+) have a more difficult time crossing. The negatively charged protein molecules (A-) inside the neuron cannot cross the membrane. In addition to these selective ion channels, there is a pump that uses energy to move three sodium ions out of the neuron for every two potassium ions it puts in. Finally, when all these forces balance out, and the difference in the voltage between the inside and outside of the neuron is measured, you have the resting potential. The resting membrane potential of a neuron is about -70 mV (mV=millivolt) - this means that the inside of the neuron is 70 mV less than the outside. At rest, there are relatively more sodium ions outside the neuron and more potassium ions inside that neuron.

An action potential is caused by positive ions moving in and then out of the neuron at a certain spot on the neuron membrane.

An action potential is initiated by a stimulus above a certain intensity or threshold. Not all stimuli initiate an action potential. The stimulus could be a pin prick, light, heat, sound or an electrical disturbance in another part of the neuron. An action potential occurs when a neuron sends information down an axon, away from the cell body. Neuroscientists use other words, such as a "spike" or an "impulse" for the action potential. The action potential is an explosion of electrical activity that is created by a depolarizing current. This means that some event (a stimulus) causes the resting potential to move toward 0 mV. When the depolarization reaches about -55 mV a neuron will fire an action potential. This is the threshold

action potential

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Depolarization

A stimulus causes a gate in the Na+ Channel to open. Since there is a high concentration of Na+ outside, Na+ diffuses into the neuron. The electrical potential changes to ~ +40 mV.

Repolarization

Depolarization causes the K+ Channel gate to immediately open. K+ diffuses out of the neuron. This reestablishes the initial electrical potential of ~ -70 mV.

Refractory Period

During this time (~ 1 msec), the Na+ and K+ Channels cannot be opened by a stimulus.

The Na+/K+ Pump actively pumps Na+ out of the neuron and K+ into the neuron. This reestablishes the initial ion distribution of the resting neuron.

This single action potential acts as a stimulus to neighbouring proteins and initiates an action potential in another part of the neuron. Ultimately a wave of action potentials travel from the dendrites all the way to the axon terminals. At the axon terminal, the electrical impulse is converted to a chemical signal

Blood component: A Fluid Tissue

Blood can be divided into:

1- A plasma portion (water, salts, gases, ions, nutrient molecules, and proteins) Plasma is the

liquid portion of the blood - protein-salt solution in which red and white blood cells and platelets

are suspended. Plasma, which is 90 percent water, constitutes about 55 percent of blood volume.

Plasma contains albumin (the chief protein constituent), fibrinogen (responsible, in part, for the

clotting of blood), globulins (including antibodies), and other clotting proteins. Plasma serves a

variety of functions:

- Maintaining a satisfactory blood pressure and volume to supplying critical proteins for blood

clotting and immunity.

- It also serves as the medium of exchange for vital minerals such as sodium and potassium, thus

helping maintain a proper balance in the body, which is critical to cell function.

2- A cellular portion (red blood cells, white blood cells, and platelets). All of the cellular

components are produced from stem cells in the bone marrow:

Red blood cells transport respiratory gases. Red blood cells contain hemoglobin, a complex iron-containing protein that carries oxygen throughout the body and gives blood its red color. The percentage of blood volume composed of red blood cells is called the “hematocrit.” The average hematocrit in an adult male is 47 percent. Their production in the bone marrow is stimulated by erythropoietin, which is produced in response to hypoxia in the tissues. They live for approximately 120 days in the circulatory system and are eventually removed by the spleen

White blood cells are responsible for protecting the body from invasion by foreign substances such as bacteria, fungi, and viruses. The majority of white blood cells are produced in the bone marrow, where they outnumber red blood cells by two to one. However, in the blood stream, there are about 600 red blood cells for every white blood cell. There are several types of white blood cells; Granulocytes and macrophages protect against infection by surrounding and destroying invading bacteria and viruses, and lymphocytes aid in immune defense.

Platelets, along with circulating proteins, are involved in blood clotting. Platelets are made in the bone marrow and survive in the circulatory system for an average of 9–10 days before being removed from the body by the spleen.

ABO Blood Types

The most well known and medically important blood types are in the ABO group.  They were discovered in 1900 and 1901 at the University of Vienna by Karl Landsteiner in the process of trying to learn why blood transfusions sometimes cause death and at other times save a patient.  In 1930, he belatedly received the Nobel Prize for this discovery.

All humans and many other primates can be typed for the ABO blood group.  There are four types: A, B, AB, and O.  There are two antigens and two antibodies that are mostly responsible for the ABO types.  The specific combination of these four components determines an individual's type in most cases.  The table below shows the possible permutations of antigens and antibodies with the corresponding ABO type ("yes" indicates the presence of a component and "no" indicates its absence in the blood of an individual).

|ABO | |

|Blood Type | |

| | |  Antigen    |  Antigen  | |  Antibody  |  Antibody  |

| | |A |B | |anti-A |Anti-B |

|[pic] | |[pic] | |[pic] |

|A | |yes |No | |no |yes |

|B | |no |Yes | |yes |no |

|O | |no |No | |yes |yes |

|AB | |yes |Yes | |no |no |

|[pic] | |[pic] | |[pic] |

For example, people with type A blood will have the A antigen on the surface of their red cells (as shown in the table below).  As a result, anti-A antibodies will not be produced by them because they would cause the destruction of their own blood.  However, if B type blood is injected into their systems, anti-B antibodies in their plasma will recognize it as alien and burst or agglutinate the introduced red cells in order to cleanse the blood of alien protein.

Please learn the following two expressions:

Phenotype: Appearance of an organism, resulting from the interaction of its genotype and its environment

Genotype: Genetic constitution of an individual

|Blood Group |Antigens on |Antibodies in Serum |Genotypes |

| |RBCs | | |

|A |A |Anti-B |AA or AO |

|B |B |Anti-A |BB or BO |

|AB |A and B |Neither |AB |

|O |Neither |Anti-A and anti-B |OO |

Blood with phenotype A has AA or AO genetypes

Blood with phenotype B has BB or BO genotypes

Blood with pheotype O has OO genotype

Blood with phenotype AB has AB genotype

Individuals with type O blood do not produce ABO antigens.  Therefore, their blood normally will not be rejected when it is given to others with different ABO types.  As a result, type O people are universal donors for transfusions, but they can receive only type O blood themselves.  Those who have type AB blood do not make any ABO antibodies.  Their blood does not discriminate against any other ABO type.  Consequently, they are universal receivers for transfusions, but there blood will be agglutinated when given to people with every other type because they produce both kinds of antigens.

Enzymes: Biological Catalysts

Enzymes are biological catalysts, proteins that are highly specific for their substrates. Substrates bind to the active site, where catalysis takes place, forming an enzyme-substrate complex.

At the active site, a substrate can be oriented correctly, chemically modified, or strained. As a result, the substrate readily forms its transition state, and the reaction proceeds.

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The active site where substrate binds determines the specificity of an enzyme. Upon binding to substrate, some enzymes change shape, facilitating catalysis

Some enzymes require cofactors to carry out catalysis. Prosthetic groups are permanently bound to the enzyme. Coenzymes are not usually bound to the enzyme. They can be considered substrates, as they are changed by the reaction and then released from the enzyme.

Metabolism is organized into pathways in which the product of one reaction is a reactant for the next reaction. Each reaction in the pathway is catalyzed by an enzyme.

Enzymes are sensitive to their environment.Both pH and temperature affect enzyme activity.

Human endocrine system

Endocrine cells secrete chemical messages called hormones, which bind to receptors on or in target cells.

Most hormones are peptides, proteins, steroids, or amines. Peptide and protein hormones and some amines are water-soluble; steroids and some amines are lipid-soluble.

The receptors for water-soluble hormones are on the cell surface. The receptors for lipid-soluble hormones are inside the cell.

Some hormones diffuse to targets near the site of secretion. Autocrine hormones influence the cell that secretes them; paracrine hormones influence nearby cells.

Most hormones are distributed throughout the body by the circulatory system.

Hormones cause different responses in different target cells.

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Vertebrate Endocrine Systems

Humans have eight major endocrine glands that secrete many hormones

1- The pituitary gland is divided into two parts. The anterior pituitary and the posterior pituitary

The posterior pituitary secretes two neurohormones, antidiuretic hormone and oxytocin.

The anterior pituitary secretes tropic hormones (thyrotropin, adrenocorticotropin, luteinizing

hormone, and follicle- stimulating hormone), as well as growth hormone, prolactin, melanocyte-

stimulating hormone.

2- The anterior pituitary is controlled by neurohormones produced by cells in the hypothalamus and

transported through portal blood vessels to the anterior pituitary.

3, 4- The thyroid gland is controlled by thyrotropin and secretes thyroxine, which controls cell

metabolism. The level of calcium in the blood is regulated by three hormones. Calcitonin

(produced by Parathyroid gland) lowers blood calcium by promoting bone deposition. Parathyroid

hormone (produced by Parathyroid gland) raises blood calcium by promoting bone turnover and

decreased calcium excretion.

5- The pancreas secretes three hormones. Insulin stimulates glucose uptake by cells and lowers

blood glucose, glucagon raises blood glucose, and somatostatin slows the rate of nutrient

absorption from the gut.

6- The adrenal gland has two portions, one within the other. The hormones of the adrenal medulla,

epinephrine and norepinephrine, stimulate the liver to supply glucose to the blood, as well as other

fight-or-flight reactions. The adrenal cortex produce three classes of corticosteroids:

glucocorticoids , mineralocorticoids, and small amounts of sex steroids. Aldosterone is a

mineralocorticoid that stimulates the kidney to conserve sodium and to excrete potassium.

Cortisol is a glucocorticoid that decreases glucose utilization by most cells.

7- Sex hormones (androgens in males, estrogens and progesterone in females) are produced by the

gonads in response to tropic hormones. Sex hormones control sexual development, secondary

sexual characteristics, and reproductive functions.

8- The pineal hormone produce melatonin is involved in controlling biological rhythms and photoperiodism.

Human Excretory System

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|Front view of urinary tract |

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Our kidneys are located on either side of the spine, just up under the bottom ribs. They are well supplied with blood via the renal artery and renal vein. Urine made in the kidney collects in the renal pelvis within the kidney, then flows down the ureter to the bladder where it is stored until voided. From the bladder, the urine flows to the outside via the urethra, (which in the male also serves as part of the reproductory tract).

The kidney is composed of an outer layer, the cortex, and an inner core, the medulla. The kidney consists of repeating units (tubules) called nephrons. The “tops” of the nephrons make up or are in the cortex, while their long tubule portions make up the medulla. To the right is a diagram of an individual nephron. Each nephron has a closely associated blood supply. Blood comes in at the glomerulus and transfers water and solutes to the nephron at Bowman’s capsule. In the proximal tubule, water and some “good” molecules are absorbed back into the body, while a few other, unwanted molecules/ions are added to the urine. Then, the filtrate goes down the loop of Henle (in the medulla) where more water is removed (back into the bloodstream) on the way “down”, but the “up” side is impervious to water. In the distal tubule, more water and some “good” solutes are removed from the urine, while some more unwanted molecules are put in. From there, the urine flows down a collecting duct which gathers urine from several nephrons. As the collecting duct goes back through the medulla, more water is removed from the urine. The collecting ducts eventually end up at the renal pelvis which collects the urine from all of them. The area where the collecting ducts enter the renal pelvis is a common area for formation of kidney stones, often giving them a “staghorn” shape.

As you can see above: The glomeruli and the proximal and distal convoluted tubules are located in the cortex of the kidney. Certain molecules are actively resorbed from the glomerular filtrate by the tubule cells, and other molecules are actively secreted. Straight sections of

renal tubules called loops of Henle and collecting ducts are arranged in parallel in the medulla of the kidney.

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Regulation of Kidney Functions

Kidney function in mammals is controlled by autoregulatory mechanisms that maintain a constant high glomerular filtration rate even if blood pressure varies.

An important autoregulatory mechanism is the release of renin by the kidney when blood pressure falls.

Chromosomes, the Cell Cycle, and Cell Division

Cell division must be initiated by a reproductive signal. Cell division consists of three steps: replication of the genetic material (DNA), segregation of the two DNA molecules to separate portions of the cell, and cytokinesis, or division of the cytoplasm.

In prokaryotes, cellular DNA is a single molecule, or chromosome. Prokaryotes reproduce by cell fission.

In eukaryotes, cells divide by either mitosis or meiosis.

Interphase and the Control of Cell Division

The mitotic cell cycle has two main phases: interphase (during which cells are not dividing) and mitosis (when cells are dividing).

During most of the cell cycle, the cell is in interphase, which is divided into three subphases: S, G1, and G2. DNA is replicated during the S phase.

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Eukaryotic Chromosomes

A eukaryotic chromosome contains a DNA molecule bound to proteins in a complex called chromatin. At mitosis, the replicated chromatids are held together at the centromere. Each chromatid consists of one double-stranded DNA molecule.

During interphase, the DNA in chromatin is wound around cores of histones (protein) to form nucleosomes. DNA folds over and over again, packing itself within the nucleus. During mitosis or meiosis, it folds even more.

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Mitosis: Distributing Exact Copies of Genetic Information

After DNA is replicated during the S phase, the first sign of mitosis is the separation of the replicated centrosomes, which initiate microtubule formation for the spindle.

Mitosis can be divided into several phases, called prophase, prometaphase, metaphase, anaphase, and telophase.

During prophase, the chromosomes condense and appear as paired chromatids, and the spindle forms.

During prometaphase, the chromosomes move toward the middle of the spindle. In metaphase, they gather at the middle of the cell with their centromeres on the equatorial plate. At the end of metaphase, the centromeres holding the sister chromatids together separate, and during anaphase, each chromatid, now called the daughter chromosome, migrates to its pole along the microtubule track.

During anaphase sister chromatids are separate.

During telophase, the chromosomes become less condensed. The nuclear envelopes and nucleoli re-form, thus producing two nuclei whose chromosomes are identical to each other and to those of the cell that began the cycle.

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Cytokinesis: The Division of the Cytoplasm

Nuclear division is usually followed by cytokinesis. Animal cell cytoplasm usually divides by a furrowing of the plasma membrane, caused by the contraction of cytoplasmic microfilaments

Reproduction: Asexual and Sexual

The cell cycle can repeat itself many times, forming a clone of genetically identical cells.

Asexual reproduction produces a new organism that is genetically identical to the parent. Any genetic variety is the result of mutations.

In sexual reproduction, two haploid gametes—one from each parent—unite in fertilization to form a genetically unique, diploid zygote.

In sexually reproducing organisms, certain cells in the adult (testes and ovary) undergo meiosis, a process by which a diploid cell (cell with 46 chromosomes) produces haploid gametes (23 chromosomes).

Each gamete contains a random selection of one of each pair of homologous chromosomes(one from mother and one from father) as it comes to gether it forms diploid cell (46 chromosomes) the zygote which grows to form embryo.

Meiosis: A Pair of Nuclear Divisions

Meiosis reduces the chromosome number from diploid to haploid, ensures that each haploid cell contains one member of each chromosome pair, and results in genetically diverse products. It consists of two nuclear divisions.

During prophase I of the first meiotic division, homologous chromosomes pair up with each other, and material may be exchanged between the two homologs by crossing over. In metaphase I, the paired homologs line up at the equatorial plate.

In anaphase I, entire chromosomes, each with two chromatids, migrate to the poles. By the end of meiosis I, there are two nuclei, each with the haploid number of chromosomes.

In meiosis II, the sister chromatids separate. No DNA replication precedes this division, which in other aspects is similar to mitosis. The result of meiosis is four cells, each with a haploid chromosome content.

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The Genetic Code

The genetic code consists of triplets of nucleotide bases (codons). There are four bases, so there are 64 possible codons.

One mRNA codon indicates the starting point of translation and codes for methionine. Three stop codons indicate the end of translation. The other 60 codons code only for particular amino acids.

Because there are only 20 different amino acids, the genetic code is redundant; that is, there is more than one codon for certain amino acids. But the code is not ambiguous: A single codon does not encode more than one amino acid.

6.3 What are enzymes?

The rate of a chemical reaction is independent of ΔG, but is determined by the energy barrier. Enzymes are protein catalysts that affect the rates of biological reactions by lowering the energy barrier, supplying the activation energy needed to initiate a reaction.

Substrates bind to the enzyme's active site—the site of catalysis—forming an enzyme–substrate complex. Enzymes are highly specific for their substrates.

6.4 How do enzymes work?

Binding substrate causes many enzymes to change shape, exposing their active site(s) and allowing catalysis. The change in enzyme shape caused by substrate binding is known as induced fit. Some enzymes require other substances, known as cofactors, to carry out catalysis. Prosthetic groups are permanently bound to the enzyme; coenzymes are not. Coenzymes can be considered substrates, as they are changed by the reaction and then released from the enzyme.

44.1 What cells are unique to the nervous systems?

Nervous systems include neurons and glial cells. Neurons are organized in circuits with sensory inputs, integration, and outputs to effectors. Glial cells serve support functions.

In vertebrates, the brain and spinal cord form the central nervous system, which communicates with the rest of the body via the peripheral nervous system. The CNS increases in complexity from invertebrates to vertebrates and from fish to mammals.

Neurons generally receive information via their dendrites, of which there can be many, and transmit information via their single axons, which end in axon terminals. Review Figure 44.3

Where neurons and their target cells meet, information is transmitted across specialized junctions called synapses.

44.2 How do neurons generate and conduct signals?

See Web/CD Tutorial 44.1

Neurons have an electric charge difference across their plasma membranes, the membrane potential. The membrane potential is created by ion pumps and ion channels. When a neuron is not active, its membrane potential is a resting potential.

The sodium-potassium pump concentrates K+ on the inside of a neuron and Na+ on the outside. Potassium channels allow K+ to diffuse out of the neuron, leaving behind unbalanced negative charges.

The resting potential is perturbed when ion channels open or close, changing the permeability of the plasma membrane to charged ions. Through this mechanism, the plasma membrane can become depolarized or hyperpolarized. Review Figure 44.9

An action potential is a rapid reversal in charge across a portion of the plasma membrane resulting from the sequential opening and closing of voltage-gated Na+ and K+ channels. These changes in voltage-gated channels occur when the plasma membrane depolarizes to a threshold level.

Action potentials are all-or-none, self-regenerating events. They are conducted down axons because local current flow depolarizes adjacent regions of membrane and brings them to threshold.

In myelinated axons, action potentials appear to jump between nodes of Ranvier,

44.3 How do neurons communicate with other cells?

Neurons communicate with each other and with other cells by transmitting information over chemical synapses (with neurotransmitters) or electrical synapses.

The neuromuscular junction is a well-studied chemical synapse between a motor neuron and a skeletal muscle cell. Its neurotransmitter is ACh, which causes a depolarization of the postsynaptic membrane when it binds to its receptor.

When an action potential reaches an axon terminal, it causes the release of neurotransmitters, which diffuse across the synaptic cleft and bind to receptors on the postsynaptic membrane.

Ionotropic receptors are ion channels or directly influence ion channels. Metabotropic receptors are G protein-linked receptors that influence the postsynaptic cell through various signal transduction pathways and result in the opening or closing of ion channels. The actions of ionotropic synapses are generally faster than those of metabotropic synapses

50.1 What do animals require from food?

Animals are heterotrophs that derive their energy and molecular building blocks, directly or indirectly, from autotrophs.

Carbohydrates, fats, and proteins in food supply animals with metabolic energy. A measure of the energy content of food is the kilocalorie. Excess caloric intake is

Humans require eight essential amino acids in the diet. Different animals need mineral elements in different amounts. Macronutrients are needed in large quantities. Micronutrients are needed in small amounts. Review Figure 50.5 and

Vitamins are organic molecules that must be obtained in food. Review Table 50.2 (Part 1), 50.2 (Part 2),

Fat Soluble Vitamins

Fat-soluble vitamins are absorbed, together with fat from the intestine, into the circulation. Once absorbed into the circulation these vitamins are carried to the liver where they are stored.

Vitamins A, D, E and K make up the fat soluble vitamins. Vitamins A, D and K are stored in the liver and vitamin E is distributed throughout the body's fatty tissues.

Water Soluble Vitamins

Water-soluble vitamins, such as Vitamin C and the B vitamins are stored in the body for only a brief period of time and are then excreted by the kidneys. The one exception to this is vitamin B12, which is stored in the liver. Water-soluble vitamins need to be taken daily.

Vitamin sources, uses and deficiency problems

Vitamin A (fat-soluble)

• Sources: Dairy products, eggs, liver. Can be converted by the body from the beta-carotene found in green vegetables, carrots and liver.

• Uses: Maintains the health of the epithelium and acts on the retina's dark adaptation mechanism.

• Deficiency leads to night blindness

Vitamin B1 (thiamine) (water-soluble)

• Sources: Yeast, egg yolk, liver, wheatgerm, nuts, red meat and cereals

• Uses: Carbohydrate metabolism

• Deficiency leads to: Fatigue, irritability, loss of appetite; severe deficiency can lead to beri-beri

Vitamin B2 (riboflavin) (water-soluble)

• Sources: Dairy products, liver, vegetables, eggs, cereals, fruit, yeast

• Uses: Intracellular metabolism

• Deficiency leads to: Painful tongue and fissures

Vitamin B12 (water-soluble)

• Sources: Liver, red meat, dairy products and fish

• Uses: Essential for manufacturing of genetic material in cells. Involved in the production of erythrocytes

• Deficiency leads to: pernicious anaemia

Vitamin C (ascorbic acid) (water-soluble)

• Sources: Green vegetables and fruit

• Uses: Essential for the maintenance of bones, teeth and gums, ligaments and blood vessels. It is also necessary for ensuring a normal immune response to infection

• Deficiency leads to: Scurvy

Vitamin D (fat-soluble)

• Sources: Fish liver oils, dairy produce. Vitamin D is formed in the skin when it is exposed to sunlight

• Uses: Has a role in the absorption of calcium, which is essential for the maintenance of healthy bones

• Deficiency leads to: Rickets

Vitamin E (fat-soluble)

• Sources: Pure vegetable oils; wheatgerm, wholemeal bread and cereals, egg yoke, nuts sunflower seeds

• Uses: Protects tissues against damage; promotes normal growth and development; helps in normal red blood cell formation

• Deficiency leads to: May cause muscular dystrophy

Vitamin K (fat-soluble)

• Sources: Green vegetables

• Uses: Used by the liver for the formation of prothrombin

• Deficiency leads to: Bleeding due to delayed clotting times caused by lack of clotting factors.

5.2 How do animals ingest and digest food?

Digestion involves the breakdown of complex food molecules into monomers that can be absorbed and utilized by cells. In most animals, digestion takes place in a tubular gut. Review Figure 50.8

Absorptive areas of the gut are characterized by a large surface area produced by extensive folding and numerous villi and microvilli. Review Figure 50.9

Hydrolytic enzymes break down proteins, carbohydrates, and fats into their monomeric units. To prevent the organism itself from being digested, many of these enzymes are released as inactive zymogens, which become activated when secreted into the gut.

50.3 How does the vertebrate gastrointestinal system function?

The vertebrate gut can be divided into several compartments with different functions. Review Figure 50.10, Web/CD Activity 50.4

The cells and tissues of the vertebrate gut are organized in the same way throughout its length. The innermost tissue layer, the mucosa, is the secretory and absorptive surface. The submucosa contains blood and lymph vessels, and a nerve plexus. External to the submucosa are two smooth muscle layers. Between the two muscle layers is another nerve plexus that controls the movements of the gut. Review Figure 50.11

Swallowing is a reflex that pushes the bolus of food into the esophagus. Peristalsis and other movements of the gut move the bolus down the esophagus and through the entire length of the gut. Sphincters block the gut at certain locations, but they relax as a wave of peristalsis approaches. Review Figure 50.12

Digestion begins in the mouth, where amylase is secreted with the saliva. Digestion of protein begins in the stomach, where parietal cells secrete HCl and chief cells secrete pepsinogen, a zymogen that becomes pepsin when activated by low pH and autocatalysis. The mucosa also secretes mucus, which protects the tissues of the gut. Review Figure 50.13

In the duodenum, pancreatic enzymes carry out most of the digestion of food. Bile from the liver and gallbladder emulsify fats into micelles. Bicarbonate ions from the pancreas neutralize the pH of the chyme entering from the stomach to produce an environment conducive to the actions of pancreatic enzymes such as trypsin. Review Figure 50.15 and Table 50.3 (Part 1), 50.3 (Part 2)

Final enzymatic cleavage of polypeptides and disaccharides occurs among the microvilli of the intestinal mucosa. Amino acids, monosaccharides, and inorganic ions are absorbed by the microvilli. Specific transporter proteins are sometimes involved. Sodium co-transport often powers the active transport of nutrients.

Fats broken down by lipases are absorbed mostly as monoglycerides and fatty acids and are resynthesized into triglycerides within cells. The triglycerides are combined with cholesterol and phospholipids and coated with protein to form chylomicrons, which pass out of the mucosal cells and into lymphatic vessels in the submucosa. Review Figure 50.16, Web/CD Tutorial 50.1

Water and ions are absorbed in the large intestine as waste matter is consolidated into feces, which is periodically eliminated.

50.4 How is the flow of nutrients controlled and regulated?

Autonomic reflexes coordinate activity of the digestive tract, which has an intrinsic nervous system that can act independently of the CNS.

The actions of the stomach and small intestine are largely controlled by the hormones gastrin, secretin, and cholecystokinin. Review Figure 50.18

The liver plays a central role in directing the traffic of fuel molecules. During the absorptive period, the liver takes up and stores fats and carbohydrates, converting monosaccharides to glycogen or fats. The liver also takes up amino acids and uses them to produce blood plasma proteins, and can engage in gluconeogenesis.

Fat and cholesterol are shipped out of the liver as low-density lipoproteins. High-density lipoproteins act as acceptors of cholesterol and are believed to bring fat and cholesterol back to the liver.

Insulin largely controls fuel metabolism during the absorptive period and promotes glucose uptake as well as glycogen and fat synthesis. During the postabsorptive period, lack of insulin blocks the uptake and utilization of glucose by most cells of the body except neurons. If blood glucose levels fall, glucagon secretion increases, stimulating the liver to break down glycogen and release glucose to the blood. Review Figure 50.19, Web/CD Tutorial 50.2

Food intake is governed by sensations of hunger and satiety, which are determined by brain mechanisms. Review Figure 50.20

18.1 What are the major defense systems of animals?

Animal defenses against pathogens are based on the body's ability to distinguish between self and nonself.

Nonspecific (innate) defenses are inherited mechanisms that protect the body from many kinds of pathogens and typically act rapidly.

Specific defenses are adaptive mechanisms that respond to a specific pathogen. They develop slowly but are long-lasting.

Many defenses are implemented by cells and proteins carried in the blood plasma and lymph. Review Figure 18.1, Web/CD Activity 18.1

White blood cells fall into two broad groups. Phagocytes include macrophages that engulf pathogens by phagocytosis. Lymphocytes, which include B cells and T cells, participate in specific responses. Review Figure 18.2, Web/CD Tutorial 18.1

18.2 What are the characteristics of the nonspecific defenses?

Humoral and cellular immune response in humans.

An animal's nonspecific defenses include physical barriers such as the skin and competing resident microorganisms known as normal flora. Review Table 18.1 Part 1, 18.1 Part 2

Circulating defensive cells, such as phagocytes and natural killer cells, eliminate invaders.

The inflammation response calls on several cells and proteins that act against invading pathogens. Mast cells release histamines, which cause blood vessels to dilate and become "leaky." Review Figure 18.4, Web/CD Activity 18.2

A cell signaling pathway involving the toll receptor stimulates the body's defenses.

18.3 How does specific immunity develop?

See Web/CD Tutorial 18.2

The specific immune response is characterized by recognition of specific antigens, mechanisms for developing a response to an enormous diversity of antigenic determinants, the ability to distinguish self from nonself, and immunological memory of the antigens it has encountered.

Each antibody and each T cell is specific for a single antigenic determinant. T cell receptors bind to antigens on the surface of virus-infected cells.

The humoral immune response is directed against pathogens in the blood, lymph, and tissue fluids. The cellular immune response is directed against an antigen established within a host cell. Both responses are mediated by antigenic fragments being presented on a cell surface along the proteins of the major histocompatibility complex.

Clonal selection accounts for the specificity and diversity of the immune response as well as immunological memory and tolerance to self. Review Figure 18.6

An activated lymphocyte produces effector cells that carry out an attack on the antigen and memory cells that retain the ability to divide to produce more effector and memory cells. Effector B cells are called plasma cells.

18.4 What is the humoral immune response?

See Web/CD Tutorial 18.3

B cells are the basis of the humoral immune response. Activated B cells, stimulated by signals from helper T cells with the same specificity, form plasma cells, which synthesize and secrete specific antibodies.

The basic unit of an antibody, or immunoglobulin, is a tetramer of four polypeptides: two identical light chains and two identical heavy chains, each consisting of a constant region and a variable region. Review Figure 18.9, Web/CD Activity 18.3

18.5 What is the cellular immune response?

See Web/CD Tutorial 18.4

T cells are the effectors of the cellular immune response. T cell receptors are similar in structure to the immunologlobulins, having variable and constant regions. Review Figure 18.12

There are two types of T cells. Cytotoxic T cells recognize and kill virus-infected cells or mutated cells. Helper T cells direct both the cellular and humoral immune responses.

The genes of the major histocompatibility complex (MHC) encode membrane proteins that bind antigenic fragments and present them to T cells. Review Figure 18.13

18.6 How do animals make so many different antibodies?

See Web/CD Tutorial 18.5

B cell genomes undergo changes as the cell develops so that each cell can produce a specific antibody protein. The immunoglobulin chains derive from "supergenes" that are constructed from one each of numerous V, D, J, and C genes. This DNA rearrangement and rejoining yields a unique immunoglobulin chain. Review Figures 18.16 and 18.17

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