Chapter 7



Chapter 6 A TOUR OF THE CELL

Summary of Chapter 6, BIOLOGY, 10TH ED Campbell, by J.B. Reece et al. 2014.

The Cell Theory

1. All organisms are made of cells.

2. Cells are the units of structure and function.

The cell is the simplest level of organized matter capable of life functions, capable of living.

All cells are related by their descent from earlier cells but they have been modified in various ways during their long evolutionary history.

There are single-cell organisms called unicellular organisms, and many-celled organisms called multicellular organisms.

CONCEPT 1. BIOLOGISTS USE MICROSCOPES AND THE TOOLS OF BIOCHEMISTRY TO STUDY CELLS

1. Microscopy

Microscopes are the most important tools of cytology, the study of cell structure.

Light or compound microscope.

• Visible light passes through a specimen and then through glass lenses.

• The lenses refract (bends) light and the image is enlarged.

• Good for magnifying objects up to 1000x; no more than 2 µm or 200 nm.

• Resolution or resolving power is the ability to distinguish fine detail.

-Minimum distance between two points that can be seen separately.

• Magnification is the ratio of an object’s image to its real size.

• Contrast is difference in brightness between the dark and light areas of an image.

Most cells range between 1 and 100 µm in diameter and can be seen only with the aid of a microscope. Most cell organelles are too small to be resolved by the light microscope.

The micrometer is the unit normally used to measure cells.

• 1 (m = 1 millionth of a meter (10-6) or 1 thousandths of a millimeter (0.001 ml).

Remember the equivalence of these units of length in the metric system. See Appendix C and D, Campbell text.

Starting with the meter, the mm, (m and nm are 0.001 of the previous unit.

• 1 meter (m) = 3.28 feet = 1.09 yards

• 1 millimeter (mm) = 10-3 m

• 1 micrometer (µm) = 10-3 mm = 10-6 m

• 1 nanometer (nm) = 10-3 µm = 10-9 m

Fluorescent microscopes are used to detect the location of certain molecules in the cell.

• Fluorescent stains (dyes and antibodies) absorb light of short wavelength and UV radiation, and release light of another wavelength.

• The emitted light allows observing the location of the molecules and the structures to which they are bound.

• Computing imaging methods have improved the resolution of structures labeled by fluorescent dyes.

Electron microscopes.

Most subcellular structures are too small to be seen with the compound microscope.

Energized electron beam is focused by electromagnets through the specimen or onto its surface.

Electron beams have wavelengths much shorter than visible light.

The term cell ultrastructure refers to the cell anatomy as seen with the electron microscope.

A. Transmission electron microscope (TEM).

• Magnification 200,000x or more.

• Used to study the internal structure of a cell.

• The electron beam passes through the specimen and is projected on a fluorescent screen or photographic plate.

• Electromagnets instead of prisms to focus and magnify the image onto a screen.

• It is used to study the internal structures of the cell.

B. Scanning electron microscopes (SEM).

• Magnification 3,000 to 100,000x.

• Used in the detailed study of the surface or topography of a specimen.

• The specimen is coated with metal.

• The electron beam strikes the metal and dislodges electrons from the metal coat. These electrons are then focused onto a screen.

• The intensity of these electrons varies with the contour of the surface and gives a three-dimensional figure of the surface. It is used to study the surface of the specimen.

Microscopes reveal the structure of the cell but not its function. Modern cell biology integrates cytology and biochemistry in order to understand the relationship between structures and functions of organelles.

See fig. 6.2, page 94, and fig. 6.3, page 95.

2. Cell Fractionation.

Organelles can be isolated.

Cell fractionation is a method of purifying organelles and separating them from other cell structures in order to study their individual functions. This is an example of reductionism in biology.

Centrifuges of different types are used to fractionate cells. The most powerful centrifuges are the ultracentrifuges that can spin at 130,000 revolutions per minute (rpm).

Cells are broken up, homogenization, and the mixture is centrifuged into the pellet and the supernatant.

In differential centrifugation the supernatant is spun at successively higher speeds in order to separate the components on the basis of their different sizes and densities.

• The pellet contains the heavier structures of the cell and become packed at the bottom of the test tube.

• The supernatant contains the liquids and the suspended lighter structures of the cell.

• The supernatant is decanted into another test tube and centrifuged again at a higher speed.

• This process is repeated several times at increasing higher speeds collecting pellets containing lighter and lighter structures.

Cell fractionation allows researchers to prepare specific cell organelles in bulk ad identify their functions.

See Fig. 6.4 on page 96.

CONCEPT 2. EUKARYOTIC CELLS HAVE INTERNAL MEMBRANES THAT COMPARTMENTALIZE THEIR FUNCTIONS - CELL STRUCTURES

1. Comparing prokaryotic and eukaryotic cells.

All organisms are made of one of two structurally different cells: prokaryotic and eukaryotic cells.

From the Greek

• Pro = before; karyon = kernel.

• Eu = true; karyon = kernel.

EUKARYOTIC CELLS PROKARYOTIC CELLS

Have a nuclear membrane: nucleus Lack a nuclear membrane: nucleoid.

Have two to hundreds of chromosomes. Have a single chromosome and plasmids.

DNA is double helix. DNA in a single circular strand.

Have membrane-bounded organelles. Lack membrane-bounded organelles.

Large ribosomes (80S). Small ribosomes (70S).

Sexual reproduction by fusion. Sexual reproduction unknown.

Asexual reproduction by mitosis. Asexual reproduction by fission.

Prokaryotic cells are simpler than eukaryotic cells.

Prokaryotic cells are considered to be more primitive than eukaryotic cell.

Prokaryotic cells have their DNA concentrated in a region called the nucleoid, but there is no membrane separating the nucleoid region from the rest of the cell.

Eukaryotic cells have highly organized membrane-bounded organelles.

The interior of each cell is called the cytoplasm. This is the region between the nucleus and plasma membrane. The nucleus is not part of the cytoplasm.

In eukaryotic cells, the DNA is in an organelle called the nucleus.

The nucleus is bound by a double membrane.

Nucleoplasm is the term used for the material inside the nuclear membrane.

Cytoplasm refers to the part outside the nucleus of the cell. This term is also used for the material inside prokaryotic cells.

Organelles are suspended on the cytosol of the cytoplasm. Organelles have specialized structures and functions.

As cells become larger, the volume increases at a greater rate than the surface area.

• The smaller the object, the greater its surface-to-volume ratio.

• See fig. 6.7 on page 98.

Once inside the cell, molecules must be transported to their place of utilization.

Above a critical size, the number of molecules needed by the cell could not be transported into the cell fast enough to sustain its needs.

Cells divide in order to maintain an optimal ratio of surface to volume.



Sizes and shape of cells are related to the functions they perform.

An approximate comparison of cell sizes:

• Micoplasmas: 0.1 - 1.0 µm in diameter. These are the smallest living cells known.

• Bacteria: mostly 1 - 10 µm in diameter.

• Eukaryotic cells: 10 -100 µm in diameter.

The plasma membrane separates the cell from the outside world and defines the cell as a distinct entity.

The plasma membrane helps maintain a life-supporting internal environment by regulating the passage of materials in and out of the cell.

Only so much of a particular substance can pass per second through the membrane.

Rates of chemical exchange with environment will be inadequate if the cell is very large (has a very large volume).

Cells must maintain an adequate surface area in order to allow enough substances in and out of the cell to support metabolism.

2. A Panoramic View of the Eukaryotic Cell.

Study pages 100 and 101 to learn the differences between animal and plant cells. Below you will see a summary of their differences in two columns.

General differences between plant and animal cells. Not every cell in a plant or animal has all these attributes:

Animal cells Plant cells

Have lysosomes seldom have lysosomes

Have centrioles lack centrioles

Have flagellated sperm cells lack of flagellated sperm cells in some groups

Lack chloroplasts have chloroplasts

Lack central vacuole and tonoplast have central vacuole and tonoplast

Lack cell wall have cell wall

Lack plasmodesmata have plasmodesmata

Animal cells divide by pinching the Plant cell division occurs by the formation

dividing cell into two daughter cells. of a cell plate during the telophase of mitosis.

Internal membranes

Divide the cells into compartments that allow cells to conduct specialized activities. They never have free ends and always contain an internal space.

This compartmentalization of the cell allows incompatible chemical process to occur simultaneously in the cell by separating them and providing the proper environment.

In general biological membranes consist of a layer of lipids and phospholipids with proteins embedded in this layer or attached to its surface.

Each type of membrane has its unique composition of lipids and proteins suited to its function, e.g. enzymes embedded in the membrane of mitochondria allow cellular respiration to occur here.

CONCEPT 3. THE EUKARYOTIC CELL’S GENETIC INSTRUCTIONS ARE HOUSED IN THE

NUCLEUS AND CARRIED OUT BY THE RIBOSOMES.

1. The Nucleus: Information Central.

• Controls the functions of the cell.

• Large structure surrounded by double membrane; contains nucleolus and chromosomes.

• It averages 5 (m in diameter.

• Nuclear envelope is made of a double membrane: two concentric membranes that separate its contents from the cytoplasm. Each is a lipid bilayer!

• The two layers are separated by a space of 20-40 nm.

• Nuclear pores about 100 nm in diameter allow the passage of material in and out of the nucleus.

• The inner and outer membranes are fused around the pore.

• Each pore is ringed by proteins, the pore complex that regulate the passage of macromolecules in and out of the nucleus.

• The nuclear side of the membrane is lined with a netlike structure of proteins called the nuclear lamina that maintains the shape of the nucleus.

• The nuclear matrix is made by framework of fibers extending throughout the nuclear interior. Its function is not completely understood.

• See fig. 6.9.

• Check this site:

2. Ribosomes: Protein Factories

• Granules composed of RNA and protein.

• They do not have a surrounding membrane.

• Some attached to inner membranes of the endoplasmic reticulum or nuclear membrane and others are free in cytosol.

• Synthesis of polypeptides.

• Bound ribosomes make proteins mainly destined to make membranes.

• Free ribosomes make proteins that are usually dissolved in the cytosol.

• Ribosomes may alternate their roles in protein making.

• Ribosomes are made of two subunits.

• The subunits are identified by the letter “S”, which means: “The unit S means Svedberg units, a measure of the rate of sedimentation of a particle in a centrifuge, where the sedimentation rate is associated with the size of the particle. Svedberg units are not additive - two subunits together can have Svedberg values that do not add up to that of the entire ribosome.”



“The svedberg is actually a measure of time; it is defined as exactly 10−13 seconds(100 fs).

The Svedberg unit (S) offers a measure of particle size based on its rate of travel in a tube subjected to high.”

3. Chromosomes.

• DNA and its associated proteins form thin fibers called chromatin.

• Chromatin condenses as it coils up before cell division and form the chromosomes.

• Chromosomes become visible as rod-like structures that can be stained dark.

• Each eukaryotic species has its characteristic number of chromosomes, e. g. 46 in human.

• Contain genes (units of hereditary information that govern structures and activity of cell).

• Information in DNA is transcribed in RNA synthesis. Messenger RNA, mRNA, is made.

4. Nucleolus

• Granular body within nucleus; consists of RNA and protein.

• There may be one, two or more nucleoli in a cell.

• The nucleolus is not surrounded by a membrane.

• Site of ribosomal RNA synthesis and ribosome submit assembly.

• It adjoins part of the chromatin where the instructions for making rRNA are found.

• Ribosome subunits leave the nucleus through the nuclear pores to the cytoplasm where they assemble to form the ribosomes.

• Sometimes there are two or more nucleoli. The number depends on the species and stage of cell's reproductive cycle.

CONCEPT 4. THE ENDOMEMBRANE SYSTEM REGULATES PROTEIN TRAFFIC AND

PERFORMS METABOLIC FUNCTION IN THE CELL.

Membranes of the eukaryotic system pass material to each other either in direct physical contact or by means of vesicles, tiny sacs made of membrane.

The various membranes that make the system vary in structure and function.

The thickness, molecular composition and metabolic functions may several times during the membrane life.

The endomembrane system consists of the following membranes:

1. Nuclear envelope.

2. Endoplasmic reticulum.

3. Golgi apparatus,

4. Lysosomes.

5. Vacuoles.

6. Plasma membrane: this membrane is not internal but it is connected to the endoplasmic reticulum.)

1. Endoplasmic reticulum (ER)

The endoplasmic reticulum, ER, manufactures membranes and performs many biosynthetic functions.

The ER consists of a network of membranous tubules and sacs called cisternae. The ER separates the internal space of the ER called the cisternal space, from the cytosol.

The ER is continuous with the nuclear envelope. The space between the nuclear envelope membranes is continuous with the cisternal space.

The ER has two regions: the rough ER has ribosomes attached to the cytoplasmic surface of the membrane; the smooth ER lack ribosomes.

Ribosomes are also attached to the cytoplasmic side of the nuclear envelope, which is continuous with the rough ER.

A. Functions of the Smooth ER

Smooth ER: In various cell types, the smooth ER functions in various processes:

• Synthesis of lipids: steroids (sex and adrenal hormones), phospholipids and oils, including the lipid components of lipoproteins.

• Metabolism of carbohydrates: liver cells hydrolyze glycogen and release glucose into the blood

• Detoxification of drugs and poisons: by adding hydroxyl groups to foreign chemicals, they become more soluble and easier to flush out of the body by the kidneys.

• Ca2+ in the cisternal space is released into the cytosol of muscle cells and triggers the contraction of the muscle cell.

• Smooth ER is found in kidney and liver cells, cardiac and skeletal muscle cell, intestinal cells, and testosterone-synthesizing cells. Other cells have little or no smooth ER.

• Barbiturates, alcohol and many drugs induce the proliferation of the smooth ER thus increasing the rate of detoxification.

B. Functions of the Rough ER

• Many specialized cells secrete proteins synthesized by ribosomes attached to the ER.

• Polypeptides are made by the ribosomes attached to the ER membrane. These polypeptides enter the cisternal space through a pore formed by a protein in the ER membrane. As it enters, the polypeptide acquires its final conformation.

• Many secretory proteins are glycoproteins, proteins covalently bonded to a carbohydrate. Specialized molecules built into the ER membrane catalyzed the bond between the polypeptide and carbohydrate. These carbohydrates are called oligosaccharides, short-chain saccharides.

• Secretory proteins are kept separate in the ER from cytosol proteins. These proteins leave the ER wrapped by membranes forming a vesicle, small sac.

• The rough ER makes the integral proteins, phospholipids, and cholesterol to form part of all cellular membranes.

• As the ER membrane expands some of it is used to make transport vesicles.

2. Golgi apparatus: Shipping and Receiving Center

• Stacks of flattened membrane sacs. Its internal space is called cisternae. The number may vary from several to hundreds.

• Receives proteins made in the ER and transport them by vesicles from the ER.

• Modifies proteins made in the ER and packages them into vesicles to be sent to other parts of the cell.

• The Golgi apparatus also manufactures certain macromolecules, e. g. many polysaccharides like pectin.

• The cis face of the Golgi stack faces the ER and receives the vesicles from the ER.

• The trans face gives rise to transport vesicles which pinch off and transport materials to other parts of the cells.

• Products of the ER are modified during their transit through the Golgi apparatus. E.g. modifies the carbohydrates of glycoproteins.

• These modified molecules are sorted out and targeted for various parts of the cell.

• The cisternae move forward from the cis to the trans face carrying and modifying their protein cargo as they move. This is called the cisternal maturation model.

• Finished products may be excreted, incorporated into the plasma membrane or in lysosomes, etc.

• See fig. 6.12, page 106.

•

3. Lysosomes

• Membranous sacs found in most eukaryotic cells. Their presence in plant cells is debatable.

• Digestive (hydrolytic) enzymes are stored here and isolated from the rest of the cell.

• Hydrolytic enzymes work best at a pH 5. In a neutral pH they are not very active.

• Some lysosomes arise from the trans face of the Golgi apparatus.

• Fuse with food vacuoles for the digestion of engulfed particles (phagocytosis).

• Digest infecting bacteria and damaged organelles.

• Lysosomes engulf small portions of the cytoplasm and organelles, a process called autophagy, break them down and the products are sent back to the cytosol for reuse.

• Apoptosis: programmed cell death.

•

4. Vacuoles

• Vacuoles have a variety of sizes and functions.

• Food vacuoles are formed by phagocytosis, the engulfing of food particles.

• Contractile vacuoles regulate the amount of water in the cell.

• The central vacuole of plants is a large membranous sac occupying about 80% of the cell.

• It is formed by the fusion of many small vacuoles derived from the ER and Golgi apparatus.

• Single membrane called tonoplast, is part of the endomembrane system.

• It stores ions like K+ and Cl-, proteins, food, pigments, poisons, salts and wastes.

• Involved in regulating plant cell growth by absorbing water and expanding.

CONCEPT 5. MITOCHONDRIA AND CHLOROPLASTS CHANGE ENERGY FROM ONE

FORM TO ANOTHER.

2. ENDOSYMBIONT THEORY: Organelles derived from certain prokaryotes entering an early

eukaryotic-like organism and becoming symbionts within that organism. See fig. 6.16, p. 109.

• An early ancestor of eukaryotic cell engulfed an oxygen- using non-photosynthetic prokaryotic cell.

• Eventually, the engulfed cell formed a relationship with the host cell in which it was enclosed, becoming and endosymbiont, a cell living within another cell.

• In millions of years, the endosymbiont and the host cell merged into a single organism, the mitochondrion (former endosymbiont) and the eukaryotic cell.

Structural features of the mitochondria and chloroplasts support this theory.

• Bounded by a double membrane rather than a single membrane like the endomembrane system.

• Chloroplasts have an internal system of membranous sacs.

• Ribosomes present

• Circular DNA associated with their inner membrane that function in the programming the synthesis of certain proteins.

• They grow and reproduce within the cell.

2. Mitochondrion (plural mitochondria): Chemical Energy Conversion

• Mitochondria are the sites of cellular respiration.

• Mitochondria are found in almost all eukaryotic cells.

• Some cells have a single large mitochondrion; others may have hundreds or thousands. The number is correlated to the metabolic level of cell activity.

• 1 - 10 µm long.

• Sacs consisting of two membranes, each a phospholipid bilayer; the intermembrane space is found between the two membranes.

• The inner membrane is folded to form cristae and encloses the matrix.

• The matrix contains many enzymes, RNA, DNA and ribosomes.

• Mitochondrial DNA codes for 13 of the enzymes used in cellular respiration; the nuclear DNA encodes the others.

• Enzymes found in the matrix catalyze some steps of cellular respiration.

• The enzyme that makes ATP is part of the inner membrane.

• Mitochondria are capable of self-replication by fission and then growth.

3. Chloroplasts: Capture light energy

• The chloroplast is a specialized plastid that convert light (radiant energy) to chemical energy during photosynthesis.

• Chloroplasts contain chlorophyll and enzymes that function in production of sugars

• Chlorophyll captures light energy.

• ATP and other energy-rich compounds are formed and then used to convert CO2 to glucose.

• Double membrane structure separated by a narrow intermembrane space and enclosing internal thylakoid membranes.

• Thylakoids are flattened membranous sacs, arranged into stacks called grana (singular granum). Inside the thylakoid is the thylakoid space.

• Fluid outside the thylakoids is called the stroma, which contains DNA and enzymes.

• Chloroplasts can divide by pinching into two chloroplasts, and can move around the cell.

Peroxisomes and glyoxysomes are not made by the endoplasmic reticulum. Their membranes grow by incorporating proteins and lipids from the cytosol. When they reach certain size they split.

4. Peroxisome: Oxidation

• Roughly spherical with a crystalline core.

• Single membrane sacs involved in metabolic reactions where hydrogen is transferred to oxygen to make H2O2.

• Use molecular oxygen (O2) to detoxify harmful substances like alcohol and formaldehyde.

• Render free radicals harmless; then convert O2- (superoxide ion) and hydroxyl radicals to H2O2, then catalase reduces the H2O2 to water.

• Break down fatty acids into smaller molecules that can be transported to mitochondria for fuel in cellular respiration.

• Abundant in the liver, they detoxify alcohol by transferring hydrogen from the alcohol to oxygen.

• Peroxisomes are involved in the formation of bile acids in the liver that aid in the digestion of lipids. Bile acids are hydrophilic derivatives of cholesterol which are synthesized in the liver. They are then exported to the small intestine via the gall bladder where they aid in lipid digestion.

• Enzymes involved in cholesterol synthesis are found in peroxisomes.

• Also abundant in the kidneys.





5. Glyoxysomes.

• A type of peroxisomes found in fat-storing tissues of plant seeds.

• Convert fatty acids into sugars to provide energy to the emerging seedling.

6. Proplastids.

• All plastids develop from proplastids, which are small, pale green or colorless organelles of the size of a mitochondrion.

• Proplastids are formed from the division of proplastids or mature plastids.

7. Chromoplasts.

• A type of plastid.

• Found in plant cells.

• Contain pigments that give flowers and fruits their characteristic color.

8. Leucoplasts.

• A type of plastid.

• Amyloplasts synthesize starch, especially in roots and tubers.

• Elaioplasts synthesize plant oils.

CONCEPT 6. THE CYTOSKELETON

The cytoskeleton is a network of fibers extending throughout the cytoplasm. It plays a major role in organizing the structures and activities of the cell.

The cytoskeleton provides structural support to the cell by maintaining its shape helps in cell motility separates the chromosomes during cell division anchorage and movement of organelles within the cell

Cell motility includes displacement of cells from one location to another as well as movements of part of the cell.

Examples of motility: cilia and flagella movements, shape changes, muscle cell contraction, phagocytosis and vacuole formation, travel of vesicles within the cell, etc.

The cytoskeleton is made of three types of fibers: microtubules, microfilaments and intermediate filaments.

The cytoskeleton interacts with motor proteins in cell motility. See fig. 6.21, page 113.

Look at a summary of the cytoskeleton structures on page 113, table 6.1.

Components of the cytoskeleton:

1. Microtubules:

• Hollow tubes made of subunits of the globular protein tubulin. Reversible assembling.

• 25 nm in diameter and 200 nm to 25 µm in length.

• Tubulin is made of dimers of α and β tubulin, two slightly different polypeptide subunits.

• Provide structural support to the cell and gives its shape.

• Provide anchorage to organelles and allow organelles to move within the cytoplasm.

• Guide the movement of chromosomes when the cell divides.

• Important in the movement of cilia and flagella.

• Components of centrioles and basal bodies.

• Constantly growing, disassembling and reassembling.

A. Centrosomes

• Clouds of cytoplasmic material that in animal cells contains the centrioles.

• Also known as the microtubule-organizing center (MTOC). Microtubules are produced and grow out of this region of the cytoplasm.

• Centrosomes present in both animal and plant cells.

B. Centrioles

• Found in the centrosome of animal cells; at right angles to each other.

• Nine sets of three microtubules forming a hollow tube; 9 triplets or 9 x 3 structures.

• Centrioles may help organize microtubule assembly and the mitotic spindle in cell division.

• They are the main components of cilia and flagella.

• They are essential to eukaryotic cell functioning.

• Centrosomes of plant and other cells lack centrioles.

• See fig. 6.22, page 114.

C. Cilia (singular cilium)

• Relatively short projections extending from the surface of the cell.

• Usually many present on the cell surface.

• 2 - 20 µm in length and 0.25 µm in diameter.

• Covered by plasma membrane.

• Made of two central and nine peripheral microtubules (9 + 2 structures). This arrangement is common to almost all eukaryotic cells but different from motile prokaryotes.

• Anchored inside the cell by a basal body similar in structure to centrioles.

• Movement of some single-celled organisms.

• Use to move materials on the surface of some tissues, e.g. in animals, lining of internal ducts of the body.

• How the activity of cilia is coordinated is not understood.

• Cilia may act as signal-receiving antenna for the cell. This is called a “primary cilium”.

• It seems that almost all cells have this primary cilium.

• Cilia and flagella have the same internal structure.

D. Flagella (singular flagellum)

• Long projections, usually one or few.

• 10 - 20 µm in length.

• Covered by plasma membrane.

• Anchored inside the cell by a basal body similar in structure to centrioles.

• Made of two central and nine peripheral microtubules (9 + 2 structures).

• Movement of some single-celled organisms and gametes.

• In many animals, the basal body of the fertilizing sperm enters the egg and becomes a centriole.

• Dynein is a motor protein responsible for the movement of the flagellum.

See fig. 6.24, page 116.

For microphotographs of the internal structure of cilia and flagella visit the following sites:





2. Microfilaments.

• They are about 6-7 nm in diameter.

• Solid, helical, twisted two subunits of the protein actin. Reversible assembling.

• A network of microfilaments (cortical microfilaments) just inside the plasma membrane helps support the cell’s shape.

• Attached to membrane proteins and fix their position in the membrane.

• This network gives the outer layer of the cytoplasm called the cortex, the consistency of a gel.

• Pinching in of mother cell during animal cell mitosis.

• Involved in the changing of shape of cells and their movement, e.g. amoeboid movements, endocytosis.

• Interact with filaments of myosin to make cells contract, e.g. muscle cells, pseudopodia of amoebas.

• Involved in cytoplasmic streaming.

• Extend or reduce microvilli on the surface of the cell, e.g. intestines.

• Each cell appears to have its own arrangement of microfilaments, which are many and cross-linked forming a network.

• They are constantly breaking down and reassembling wherever they are needed in the cell.

• Microfilaments appear to be present in all eukaryotic cells.

• See fig. 6.26 on page 117.

3. Intermediate filaments.

• About 8-12 nm in diameter, intermediate between microtubules and microfilaments.

• Play an important role in maintaining the original shape of the cell.

• They are diverse made from a family of proteins called keratins.

• Abundant in parts of the cell subject to mechanical stress; for strengthening the cell.

• Form the nuclear lamina inside the nuclear envelope; involved in disassembling and reassembling of the nuclear membrane during mitosis.

• Fixe the nucleus in a more or less permanent position.

• Involved in the formation of desmosomes.

• Probably irreversible assembling of polypeptides.

CONCEPT 7. EXTRACELLULAR COMPONENTS AND CONNECTIONS BETWEEN CELLS

HELP COORDINATE CELLULAR ACTIVITIES.

The plasma membrane is considered to be the boundary of the cell.

Many cells secrete coats of different kinds that are outside the plasma membrane.

Plants, fungi, some protists and some prokaryotes produce cell walls. These cell walls will be considered in the chapters dealing with these organisms.

1. Cell wall of plants

Absence or presence of cell wall is one of the distinctions between animal and plant cells.

Function:

1. Protects the cell.

2. Maintains its shape.

3. Prevents excessive intake of water.

4. Collectively, it allows the plant to stand upright against gravity.

Cell walls range from 1 µm to several micrometers.

The composition of the cell wall varies from species to species and from one cell type to another. The basic structure of the cell wall is consistent, however.

• Microfibrils of cellulose are embedded in a matrix of polysaccharides and proteins.

Young plant cells secrete a flexible primary cell wall that is hardened when the cell stops growing by the addition of certain substances into the primary cell wall.

The middle lamella is a layer of polysaccharides called pectins located in between the cell walls of adjacent cells. It serves to glue cells together.

When the plant cell stops growing, it secretes hardening substances to the primary wall.

Other plant cells add several layers of strong secondary cell wall between the plasma membrane and the primary cell wall

Wood consists mostly of secondary cell wall.

2. Extracellular matrix (ECM) of animal cells.

Most eukaryotic cells are surrounded by a glycocalyx, or cell coat, formed by polysaccharide side chains of proteins (glycoproteins) and lipids (glycolipids) that are part of the plasma membrane.

These substances surround the cell and form the extracellular matrix, ECM.

Different cell types have different glycocalyx, which provides a highly specific biological marker by which cells recognize each other.

Molecules of the ECM allow cells to recognize one another, make contact and form associations. Other molecules contribute to the mechanical strength of tissues.

Animal cells have a glycocalyx gel made of carbohydrates and fibrous proteins.

Collagen, a glycoprotein, is the main structural protein of the ECM.

Collagen fibers are embedded in a network of fibers of proteoglycans.

Proteoglycans are rich in carbohydrates - up to 95%, the rest is protein. These carbohydrates are covalently bound to the proteins.

Fibronectin proteins (glycoproteins) are bound to collagen and to receptor proteins called integrins that are part of the plasma membrane.

Integrins cross the plasma membrane and bind to the cell cytoskeleton, making integrins capable of transmitting changes in the ECM to the cytoskeleton and help regulate cell functions.

Changes in the ECM may influence activity of genes in the nucleus by triggering a combination of mechanical and chemical pathways.

See fig. 6.28 on page 119.

3. Cell junctions

Intercellular junctions help integrate cells into higher levels of structure and function.

In plant cells:

A. Plasmodesmata (singular plasmodesma)

• They are channels between adjacent plant cells.

• Openings in the cell wall and plasma membrane allow the cytoplasm to be continuous between the cells. The plasma membrane lines the channel and is continuous from cell to cell. See fig. 6.29, page 119.

• Molecules and ions can pass from cell to cell directly.

• The macromolecules that are going to pass from cell to cell seem to reach the channel by moving along fiber of the cytoskeleton.

• Desmotubules pass through the plasmodesmata and connect the ERs of adjacent cells.

and are derived from rough endoplasmic reticulum that has been stretched to form a rod like structure. Molecules pass along the channel that surrounds the desmotubule.

• The cytoplasm of all cells in a plant is connected via the plasmodesmata. Plant viruses spread via plasmodesmata

In animal cells:

B. Tight junctions (See fig. 6.30)

• These are areas of contact between two adjacent cells held together by proteins linking the two cells. The plasma membranes of adjacent cells are fused together.

• No space remains between the cells and the intercellular space is obliterated.

• Prevents leakage. No substance can pass through the junction of the epithelial cells.

• Commonly found in intestine, glands and bladder.

C. Desmosomes. (See fig. 6.30)

• Points of attachment between two adjacent animal cells. They act like rivets.

• Button-like discs associated with the plasma membranes of adjacent cells.

• Protein filaments made of keratin cross the narrow intercellular space between the cells.

• Protein filaments are connected to systems of intermediate filaments inside each cell.

• Filaments are anchored to a dense mixture of attachment proteins called a plaque, making them button-like

• Cells are fastened into strong sheets.

• Found in tissues that are subject to mechanical stress: skin, heart muscle, and uterus.

D. Gap junction (See fig. 6.30)

• Gap junctions are protein channels between adjacent cells. These are communication junctions.

• They allow the passage of small molecules (ions, sugars, amino acids) between adjacent cells, similar to the plasmodesmata of plants.

• The gap junction can open or close the pores.

• Common in cardiac and smooth muscles.

• For interesting pictures of gap junctions visit

Interesting site with animations:







The cell is a living unit. None of the organelles works alone.

Summary:

See page 122 in your textbook.

You must learn the structure and function of all organelles and cell parts.

1. Microscopy

• Resolution and magnification

• Measuring units

2. Microscopes

• Light or compound microscopes

• Fluorescent microscopes – how are they different from the standard compound microscope?

• Electron microscopes: TEM and SEM. Difference between the two EM.

3. Methods

• Cell fractionation

• Centrifuges and ultracentrifuges

• Differential centrifugation

4. Cell types

• Differences between prokaryotic and eukaryotic cells - terminology

• Differences between animal and plant cells - terminology

5. Cell structures

• Nuclear structures

o Nucleus

o Nucleolus

o Chromosomes

o Ribosomes

6. Endomembrane system

• Endoplasmic reticulum

• Golgi apparatus

• Lysosomes

• Vacuoles

7. Other membranous organelles

• Mitochondrion

• Proplastids

• Chromoplasts

• Leucoplasts

• Chloroplasts

• Peroxisomes

• Glyoxisomes

8. Cytoskeleton

• Microtubules: tubulin

o Centrosomes

o Centrioles

o Cilia

o Flagella

• Microfilaments: actin, myosin

• Intermediate filaments: keratin

9. Cell surfaces and junctions

• Cell wall: primary, secondary, middle lamella

• Extracellular matrix: glycoproteins, glycolipids

• Plant cells

o Plasmodesmata

• Animal cells

o Tight junctions

o Desmosomes

o Gap junctions

Many good TEM pictures here:



August 16, 2017

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