Biology 102 Fall 2001
R. Brundage
Lecture 6
VII.Components of the Cytoskeleton
A.The cytoskeleton gives cells their internal organization, shape,
and capacity to move.
1.It forms an interconnected system of bundled fibers, slender threads,
and lattices that extends from the nucleus to the plasma membrane.
2.The main components are microtubules, microfilaments, and intermediate
filaments–all assembled from protein subunits.
3.Some portions are transient, such as the "spindle" microtubules used in
chromosome movement during cell division; others are permanent,
such as filaments operational in muscle contraction.
B.Microtubules
1.Microtubules, the largest structural elements in the cytoskeleton,
are composed of tubulin subunits which compose a cylinder.
2.Microtubule organizing centers (MTOCs) are small masses of proteins
in the cytoplasm that give rise to microtubules.
3.Microtubules govern the division of cells and some aspects of their shape
as well as many cell movements.
C.Microfilaments
1.Microfilaments, the thinnest elements, consist of two helically twisted
polypeptide chains assembled from actin monomers.
2.Microfilaments are particularly important in movements that take place at
the cell surface; they also contribute to the shapes of animal cells.
D.Myosin and Other Accessory Proteins
1.Extending from the microfilaments of muscle cells, myosin plays a vital
role in contraction.
2.Other proteins attach microfilaments to the inner surface of the plasma
membrane (spectrin) or span the plasma membrane to connect
microfilaments to outside proteins (integrins).
E.Intermediate Filaments
1.Intermediate filaments, the most stable of the cytoskeleton elements,
occur only in animal cells of specific tissues.
2.Examples include desmins and vimentins (support machinery by which
muscle cells contract) and lamins (form a scaffold that reinforces
the nucleus).
VIII.The Structural Basis of Cell Motility
A.Mechanisms of Cell Movements
1.Through controlled assembly and disassembly of their subunits,
microtubules, and microfilaments grow or diminish in length, thereby
the structures attached to them are thereby pushed or dragged through the
cytoplasm (example: pseudopod movement in Amoeba.
2.Parallel arrays of microfilaments or microtubules actively slide past one
another to bring about contraction, as in muscle.
3.Microtubules or microfilaments shunt organelles from one location to
another as in cytoplasmic streaming.
B.Case Study: Flagella and Cilia
1.Microtubular extensions of the plasma membrane have a 9 + 2
cross-sectional array that arises from a centriole (a type of MTOC)
and are useful in propulsion.
2.Flagella are quite long, not usually numerous, and found on one-celled
protistans and animal sperm cells.
3.Cilia are shorter and more numerous and can provide locomotion for
free-living cells or may move surrounding water and particles if the
ciliated cell is anchored.
IX.Cell Surface Specializations
A.Eukaryotic Cell Walls
1.Cell walls are carbohydrate frameworks for mechanical support in
bacteria, protistans, fungi, and plants; cell walls are not found in animals.
2.In growing plant parts, bundles of cellulose strands form a primary cell
wall that is pliable enough to allow enlargement under pressure.
3.Later, more layers are deposited on the inside of the primary wall to form
the secondary wall.
4.Lignin composes up to 25 percent of the secondary wall in woody plants;
it makes plant parts stronger, more waterproof, and less inviting
to insects.
B.Matrixes Between Animal Cells
1.The matrix between animal cells includes cell secretions and materials
drawn from the surroundings between cells.
2.For example, cartilage consists of scattered cells and collagen embedded
in a "ground substance" of modified polysaccharides; bone is similarly
constructed.
C.Cell-to-Cell Junctions
1.In plants tiny channels called plasmodesmata cross the adjacent primary
walls and connect the cytoplasm
.Animal cells display three types of junctions:
a.Tight junctions occur between cells of epithelial tissues in which
cytoskeletal strands of one cell fuse with strands of neighboring
cells causing an effective seal.
b.Adhering junctions are like spot welds at the plasma membranes
of two adjacent cells that need to be held together during
stretching as in the skin and heart.
c.Gap junctions are small, open channels that directly link the
cytoplasm of adjacent cells.
X.Prokaryotic Cells–The Bacteria
A.The term prokaryotic ("before the nucleus") indicates existence of bacteria before
evolution of cells with a nucleus; bacterial DNA is clustered in a distinct region of
the cytoplasm (nucleoid).
B.Bacteria are some of the smallest and simplest cells.
1.Bacterial flagella project from the membrane and permit rapid movement.
2.A somewhat rigid cell wall supports the cell and surrounds the plasma
membrane, which regulates transport into and out of the cell.
3.Ribosomes, protein assembly sites, are dispersed throughout the
cytoplasm.
A Closer Look at Cell Membranes
A.Because the concentration of ions and other substances outside a cell may rapidly
become too high or low, a mechanism is needed to selectively permit substances
to enter or leave the cell.
B.The plasma membrane–a surface of lipids, proteins, and some carbohydrate
groups–regulates exchange of materials between cytoplasm and surroundings.
C.Within the cytoplasm, exchanges are made across internal membranes of the organelles.
I.Membrane Structure and Function
A.The Lipid Bilayer of Cell Membranes
1.The "fluid" portion of the cell membrane is made of phospholipids.
a.A phospholipid molecule is composed of a hydrophilic head and two
hydrophobic tails
b.If phospholipid molecules are surrounded by water, their hydrophobic
fatty acid tails cluster and a bilayer results; hydrophilic heads are at the outer faces of a two-layer sheet.
2.Bilayers of phospholipids are the structural foundation for all cell membranes.
B.Fluid Mosaic Model of Membrane Structure
1.Cell membranes are of mixed composition including the following:
a.Phospholipids differ in their hydrophilic heads and the length and
saturation of their fatty acid tails.
b.Glycolipids have sugar monomers attached at the head end.
c.Cholesterol is abundant in animal membranes;
phytosterols occur in plants.
2.Within a bilayer, phospholipids show quite a bit of movement; they diffuse
sideways, spin, flex their tails to prevent close packing and promote fluidity, which also results from short-tailed lipids and unsaturated tails (kink at double bonds).
3.The arrangement of molecules on one side of the membrane differs from that on
the other side (asymmetrical).
C.Overview of Membrane Proteins
1.Transport proteins allow water-soluble substances to move through their interior,
which opens on both sides of the bilayer.
2.Receptor proteins have binding sites for hormones (and like substances) that can
trigger changes in cell action, as in growth processes.
3.Recognition proteins identify the cell as a certain type, help guide cells into
becoming issues, and function in cell-to-cell recognition and coordination.
4.Adhesion proteins are glycoproteins that help cells stay connected to one another
in a tissue.
II.How Substances Cross Cell Membranes
A.All cell membranes show selective permeability, that is, some substances can cross,
others cannot.
1.Gases and small electrically-neutral molecules can readily cross the lipid bilayer.
2.Glucose and other large, polar molecules cannot pass through the bilayer directly
but must rely on passage through the interior of transport proteins.
B.Concentration Gradients and Diffusion
1.Concentration refers to the number of molecules (or ions) of a substance
in a given volume of fluid.
2.Molecules constantly collide and tend to move down a concentration gradient
(high to low).
3.The net movement of like molecules down a concentration gradient is called
diffusion; each substance diffuses independently of other substances present as illustrated by dye molecules in water
C.Factors Influencing the Rate and Direction of Diffusion
1.The rate of diffusion depends on concentration differences, temperature
(higher = faster), molecular size (smaller = faster), electric gradients
(a difference in charge), and pressure gradients .
2.When gradients no longer exist, there is no net movement
(dynamic equilibrium).
D.Mechanisms By Which Solutes Cross Cell Membranes
1.In passive transport, material passes through the interior of transport
proteins without an energy boost; this is also known as "facilitated"
diffusion.
2.In active transport, proteins become activated to move a solute against its
concentration gradient.
3.Substances move in bulk across the cell membrane by exocytosis and
endocytosis.
III.The Directional Movement of Water Across Membranes
A.Water Movement by Osmosis
1.Bulk flow is the tendency of different substances in a fluid to move
together in the same direction due to a pressure gradient
(as in animal circulatory systems).
2.Osmosis is the passive movement of water across a differentially permeable
membrane in response to solute concentration gradients, pressure gradients, or both.
3.For example, if a bag containing a sugar solution is placed in pure water, the
water will diffuse inward (higher to lower).
B.Effects of Tonicity
1.Tonicity denotes the relative concentration of solutes in two fluids–extracellular
fluid and cytoplasmic fluid, for example.
2.Three conditions are possible:
a.An isotonic fluid has the same concentration of solutes as the fluid in the
cell; immersion in it causes no net movement of water.
b.A hypotonic fluid has a lower concentration of solutes than the fluid in
the cell; cells immersed in it may swell.
c.A hypertonic fluid has a greater concentration of solutes than the fluid in
the cell; cells in it may shrivel.
3.Cells either are dependent on relatively constant (isotonic) environments or are
adapted to hypotonic and hypertonic ones.
C.Effects of Fluid Pressure
1.Hydrostatic pressure is a force directed against a membrane by a fluid; the greater
the solute concentration, the greater will be the hydrostatic pressure it exerts.
2.This force is countered by osmotic pressure, which prevents any further increase
in the volume of the solution.
3.When plants lose water, there is a shrinkage of the cytoplasm called plasmolysis.
IV.Protein-Mediated Transport
A.When water-soluble molecules bind to transport proteins, they trigger changes in shape
that "ease" the solute through the protein and hence through the membrane.
B.Passive Transport
1.A carrier protein that functions in passive transport (also called "facilitated
diffusion") tends to move molecules to the side of the membrane where they are less concentrated.
2.Passive transport will continue until solute concentrations are equal on both sides
of the membrane or other factors intervene.
C.Active Transport
1.To move ions and large molecules across a membrane against a concentration
gradient, special proteins are induced to change shape (in a series), but only with an energy boost from ATP.
2.An example of active transport is the sodium-potassium pump of the neuron
membrane, and the calcium pump of most cells.
V.Exocytosis and Endocytosis
A.Transport To The Plasma Membrane
1.In exocytosis, a cytoplasmic vesicle moves substances from cytoplasm to plasma
membrane where the membranes of the vesicle and cell fuse.
2.The vesicle contents are released to the surroundings
(commonly called secretion).
B.Transport From The Plasma Membrane
1.Endocytosis encloses particles in small portions of plasma membrane to form
vesicles that then move into the cytoplasm.
2.Three pathways of endocytosis have been recognized:
a.In receptor-mediated endocytosis, specific molecules are brought into
the cell by specialized regions of the plasma membranes that form coated pits which sink into the cytoplasm.
b.In bulk-phase endocytosis, a vesicle forms around a small volume of
extracellular fluid without regard to what substances might be dissolved in it.
c.Phagocytosis, is an active form of endocytosis by which a cell engulfs
microorganisms, particles, or other debris; this is seen in protistans and white blood cells.
3.Membrane Cycling
1.Even as exocytosis and endocytosis disrupt the plasma membrane, the
rates are such that the plasma membrane is continually replaced.
2.For example in neurotransmitter release, an episode of exocytosis was
immediately followed by counterbalancing endocytosis.