Biology Cells Lecture B



Biology Cells Lecture B. Rife SOHI 2001

Cell is the smallest unit of living matter. All organisms are made up of cells.

Cell Theory - Cells are capable of self-reproduction. Cells come only from preexisting cells.

Plasma membrane - is a thin layer of protoplasm, consisting mainly of phospholipids and proteins (cholesterol), which is present on the surface of all cells.

The plasma membrane separates the cell interior from the outside environment and regulates the passage of molecules into and out of the cell. (Fluid Mosaic Model)

Organelles - are small membranous bodies whose structure suits their function.

Cytoplasm - refers to the contents of a cell that lies between the nucleus (or nucleoid) and the plasma membrane.

Microscopes Provide Windows To The World Of The Cell

Images formed by microscopes represent the object “under” the microscope.

Two aspects of microscope images are important.

1. Magnification: the number of times larger the image appears than the object actually is.

M total = M ocular x M objective

2. Resolution: clarity of the image

Light Microscope (LM) bend the light coming through an object. The bent light rays form larger images in the viewer’s eyes. Well-resolved LM images are limited to 1000-2000 times larger than life size.

Always start viewing with low power

The LM is particularly good for looking at living cells and tissues.

Scanning electron microscope (SEM) composes images on a TV screen, from electrons that bounce off the surfaces of the object. SEM images are usually about 10,000-20,000 times larger than life size. The SEM is particularly good for showing organismal and cellular surfaces under high magnification.

Transmission electron microscopes (TEM) compose images on camera film, from electrons that have traveled through very thin slices of the object and have been bent by magnetic lenses. TEM images are usually about 100,000-200,000 times larger than life size. The TEM is particularly useful for showing the internal structures of cells.

Chemical Basis of Life

Element - A substance that cannot be broken down into two or more different substances.

(C, H, O, N = 96% of the elements in the human body)

Compounds - A substance composed of two or more elements chemically combined.

(Lipids, Carbohydrates, Proteins, Nucleic Acids)

Cell Sizes Vary With Their Functions

Nerve cells are very long, to communicate between different parts of an animal’s body.

Bird eggs are very large, mostly composed of food reserves.

Blood cells are very small, to allow them to flow through blood vessels, and to provide a large surface area (from many cells) for efficient gas exchange.

Natural Laws Limit Cell Size

Prokaryotic cells are between 1 µm and 10 µm

Eukaryotic cell are between 10 µm and 100 µm

1 µm = 1 x 10 -6 m (REVIEW ALL SI UNITS)

Nutrients enter a cell and wastes exit a cell at the plasma membrane.

A cell needs a surface area (plasma membrane) that can adequately exchange materials with the environment. Surface area to volume relationships require that cells stay small.

Surface Area Volume A:V Ratio

1 x 1 x 1 mm cell 6 mm2 1 mm3 6:1

2 x 2 x 2 mm cell 24 mm2 8 mm3 3:1

4 x4 x 4 mm cell 96 mm2 64 mm3 1.5:1

Prokaryotic Cells Are Small And Structurally Simple

Prokaryotic cells - lack a true nucleus. Their DNA is in a single chromosome located within a region called the nucleoid. There are few organelles.

Prokaryotic cells are bacteria and cyanobacteria (blue-green algae

Eukaryotic Cells Are Partitioned Into Functional Compartments

Eukaryotic cells - have a true nucleus, a membrane-bound compartment that houses DNA within chromosomes.

All cells, aside from bacteria, are eukaryotic. Eukaryotic organisms include the algae, protozoa, and fungi, in addition to plants and animals.

Eukaryotic organisms are internally complex, with organelles of two types: membranous and nonmembranous.

Membranous organelles found in eukaryotic cells include the nucleus, endoplasmic reticulum, Golgi apparatus, mitochondria, lysosomes, and microbodies.

Nonmembranous organelles found in eukaryotic cells include ribosomes, microtubules, centrioles, flagella, and the cytoskeleton.

Animal cells are bounded by the plasma membrane alone, often have flagella, and lack a cell wall.

Plant cells are bounded by both a plasma membrane and a rigid cell wall. In addition, plant cells usually have a central vacuole and chloroplasts, lack centrioles, and usually lack flagella.

The Nucleus Is The Cell’s Genetic Control Center

Nucleus - is a small mass of nucleoproteins surrounded by a double membrane. It contains chromosomes (DNA), and functions in metabolism, growth, and reproduction.

DNA always remains within the nucleus, but messenger RNA (mRNA), carries the instructions from DNA out into the cytoplasm where protein synthesis occurs (ribosomes)

Nucleoli - are small spherical bodies composed principally of ribosomal RNA (rRNA) located in the nucleus.

Nuclear membrane - The envelope surrounding the cell nucleus, separating the nucleoplasma from the cytoplasm; composed of two membranes and contains numerous pores.

Many Cell Organelles Are Related Through The Endomembrane System

An extensive system of membranous organelles works together in the synthesis, storage, and export of molecules.

A single membrane bound each of these organelles. Some are in the form of flattened sacs, some are rounded sacs, and some are tube-shaped.

Rough Endoplasmic Reticulum Makes Membrane And Proteins

Ribosomes - are small organelles composed of proteins and RNA (ribonucleic acid) which synthesize proteins from amino acids within the living cell.

Some ribosomes may lie free within the cytoplasm but most are attached to the endoplasmic reticulum (rough ER)Rough ER specializes in protein synthesis.

The endoplasmic reticulum is a transport system. Protein molecules move from the lumen of rough ER to that of smooth ER, which sends them enclosed within vesicles usually to the Golgi complex.

Vesicles move to the plasma membrane where they discharge their contents and add new membrane as the vesicle fuses with the plasma membrane.

Smooth Endoplasmic Reticulum Has Many Functions

Smooth ER has a function suited to the particular type cell; sometimes it specializes in lipid synthesis.

In other forms of smooth ER, enzymes help process materials as they are transported from one place to another. An example of this function is the detoxification of drugs (alcohol) by smooth ER in liver cells.

The Golgi Apparatus Finishes, Stores, And Ships Cell Products

Golgi Complex - A cellular organelle that is part of the cytoplasmic membrane system; it is composed of regions of stacked cisternae and it functions in manufacture, storage and secretory processes. (via vesicles)

The hormone insulin is received by the Golgi in an inactive form, but after certain amino acids are removed, the hormone is active and ready to be secreted.

Most common processing by the Golgi is the synthesis of glycoproteins

Golgi complexes produce lysosomes.

ILysosomes Digest The Cell’s Food And Wastes

Lysosomes - are membrane-bounded vesicles that contain hydrolytic digestive enzymes.

The lysosomal enzymes digest macromolecules into simpler subunits. They also digest bacteria and even parts of the cell. (autodigestion - suicide packets)

Microbodies - are similar to lysosomes because they are vesicles bounded by a single membrane and contain specific enzymes.

They are believed to help detoxify alcohol.

Vacuoles Function In General Cell Maintenance

Vacuole - A membrane-bound cavity within a cell; may function in digestion, storage, secretion, or excretion.

Typically, plant cells have one or two large vacuoles so filled with a watery fluid that it gives added support to the cell.

Plant vacuoles contain not only water, sugars, and salts, but also pigments and toxic substances.

Contractile vacuoles in cells of freshwater protists (both protozoans and algae) function in water balance.

Mitochondria Harvest Chemical Energy From Food

Cellular respiration, which occurs in mitochondria, is the process by which the chemical energy of carbohydrates is converted to that of ATP (adenosine triphosphate). ATP energy is use for synthetic reactions, active transport, and all energy-requiring processes in cells

C6H12O6 + 6O2 ---> 6CO2 + 6H2O + ATP ENERGY

Mitochondria - is a double membrane-bound organelle of the cell known as the powerhouse because it transforms the energy of carbohydrates and fats to ATP energy.

The inner membrane is folded to form little shelves, called cristae, which project into the matrix, an inner space filled with enzymes.

Like chloroplasts, mitochondria also contain their own DNA and ribosomes, and can reproduce themselves by division.

Origin The endosymbiotic theory states that chloroplasts and mitochondria were originally prokaryotes that came to reside inside a eukaryotic cell, establishing a symbiotic relationship known as mutualism.

Cells Have An Internal Skeleton

Eukaryotic cell shape and motility are dependent upon the cytoskeleton, an internal framework that contains microtubules, intermediate filaments, and microfilaments that differ as to composition and size.

Microfilaments - are long, solid and extremely thin fibers, composed of globular proteins, that usually occur in bundles or networks.

Actin microfilaments are well known for their role in muscle contraction where they interact with other protein microfilaments composed of myosin.

Intermediate Filaments - are intermediate in size between microtubules and microfilaments and are composed of fibrous proteins.

In the skin, where the fibers are made of keratin, the the presence of intermediate filaments gives great mechanical strength to cells.

Microtubules - are hollow tube-like filaments composed of globular proteins and are found free within the cytoplasm or in definite patterns within cilia and flagella and are composed of repeating subunits of the protein tubulin.

Cilia And Flagella Move When Microtubules Bend

Cilia - are relatively short, numerous and centriole-based, hairlike processes on certain motile organisms.

Flagella - are relatively long, whiplike, centriole-based locomotor organelles on some motile cells.

In both cases, these nonmembranous organelles are minute, tubular extensions of the plasma membrane that surround a complex arrangement of microtubules.

The underlying structure consists of nine microtubule doublets arranged in a cylinder around a central pair of microtubules. At the base within the cell body (basal body), the structure is slightly different.

Cilia and flagella function to move whole cells or to move materials across or into cells.

Eukaryotic Organelles Comprise Four Functional Categories

Manufacture: Nucleus, Ribosomes, Rough ER, Smooth ER, Golgi apparatus - synthesis of macromolecules and transport within the cell

Breakdown: Lysosomes, Microbodies, Vacuoles

- elimination and recycling of cellular materials

Energy processing: Chloroplasts, Mitochondria

- conversion of energy from one form to another

Support, movement, and communication: Cytoskeleton, Cell walls, Cell surfaces, Cell junctions

- relationships with extracellular environments

Within each of the four categories there are structural similarities that underlie their functions.

All four categories work together as an integrated team, producing the emergent properties at the cellular level.

Membranes Organize The Chemical Activities Of Cells (Function)

Membranes separate cells from their outside environments, including, in multicellular organisms, that environment in other cells that performs different functions..

They control the passage of molecules form one side of the membrane to the other.

In eukaryotes, they partition function into organelles.

The provide reaction surfaces, and organize enzymes and their substrates.

The plasma membrane regulates the passage of molecules into and out of the cell.

The plasma membrane is selectively permeable - it has special mechanisms to regulate the passage of most molecules into and out of the cell.

Membrane Phospholipids Form A Bilayer

Phospholipids are like fats, with two nonpolar (hydrophobic) fatty acid “tails” and one polar (hydrophilic) phosphate “head” attached to the glycerol.

In water, thousands of individual molecules form a stable bilayer, aiming their heads (hydrophilic) out and their tails (hydrophobic) in.

The hydrophobic interior of this bilayer offers an effective barrier (selectively permeable) to the flow of most hydrophilic molecules.

The Membrane Is A Fluid Mosaic Of Phospholipids And Proteins

The fluid-mosaic model of membrane structure is widely accepted at this time.

It is fluid because the individual molecules are more-or-less free to move about laterally.

Cholesterol is a common constituent of animal cell membranes and helps stabilize the fluidity at different temperatures.

Some proteins extend through both sides of the bilayer.

Proteins Make The Membrane A Mosaic Of Function

The movement of molecules through the plasma membrane is dependent upon its protein component.

Functions for plasma membrane proteins:

1. cell to cell recognition (Identification tags - particularly glycoproteins)

2. receptors for chemical messengers (trigger cell activity when a messenger molecule attaches.

3. enzymes catalyzing intracellular and extracellular reactions

4. passage of (hydrophilic) molecules across the membrane

5. Cell junctions - either attachments to other cells or the internal cytoskeleton.

Permeable allows all molecules to pass through

Impermeable allows no molecules to pass through

Cell membranes are selectively permeable. They permit the free passage of some materials and restrict the passage of others.

Passive Transport Is Diffusion Across A Membrane

Diffusion is the tendency for particles of any kind to spread out spontaneously from an area of high concentration to an area of low concentration.

Only a few types of small molecules freely diffuse through the plasma membrane.

Passive transport across membranes occurs when a molecule diffuses down a concentration gradient. At equilibrium, molecules continue to diffuse back and forth, but there is no net change in concentration anywhere.

Different molecules diffuse independently of one another.

Passive transport is an extremely important way for small molecules to get into and out of cells. For example, O2 moves into red blood cells and CO2 moves out of these cells by this process in the lungs.

Osmosis Is The Passive Transport Of Water

Osmosis is the diffusion of water across a selectively permeable membrane. The presence of osmotic pressure is evident when there is an increased amount of water on t he side of the membrane that has the higher solute concentration.

When a cell is placed in an isotonic solution (same solute concentration), it neither gains nor loses water.

When a cell is placed in a hypotonic solution (less solute conc. - more water conc.) the cell gains water.

When a cell is placed in a hypertonic solution (greater solute conc. - less water conc.) the cells loses water and the cytoplasm shrinks.

Water Balance Between Cells And Their Surroundings Is Crucial To Organisms

If a plant or animal cell is isotonic with its surroundings, no osmosis occurs, and the cells do not change. however plant cells in such environments are flaccid or wilted, lacking the turgor that helps support some plant tissues.

An animal cell in a hypotonic solution will gain water and lyse (burst)

A plant cell in a hypotonic solution, the cytoplasm and central vacuoles gain water, and plasma membrane pushes against the rigid cell wall. The resulting pressure, called turgor pressure, helps give internal support to the cell.

An animal cell in a hypertonic solution will lose water and shrivel. A plant cell in a hypertonic solution will pull the plasma membrane away from the cell wall and lose turgor.

Specific Proteins Facilitate Diffusion Across Membranes

Facilitated diffusion occurs when a pored protein, spanning the membrane bilayer, allows a solute to diffuse down a concentration gradient. No energy expenditure is required in this case. The rate of facilitated diffusion depends on the number of such transport proteins, in addition to the strength of the concentration gradient. Hormones often enter cells this way.

Filtration: movement of water and small solute particles, but not larger particles, through a filtration membrane; movement occurs from area of high pressure to area of low pressure (Down a pressure gradient). Materials are removed from a solution on the basis of size. Urine formation involves filtration in the nephron.

Cells Expend Energy For Active Transport

Active transport involves the aid of a transport protein in moving a solute up a concentration gradient.

Energy expenditure in the form of ATP-mediated phosphorylation is required to help the protein change its structure and thus move the solute molecule.

Active transport proteins often couple the passage of two solutes in opposite directions across membranes. A very important example of a coupled active transport system is the Na+ - K+ pump (ion pump), which functions in nerve impulse transmission

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