5.1 Plasma Membrane Structure and Function
A. Components of the Plasma Membrane
The plasma membrane is a phospholipid bilayer with embedded proteins and cholesterol.
Phospholipids are amphipathic molecules, having both hydrophilic and hydrophobic regions; nonpolar tails (hydrophobic) are directed inward, polar heads (hydrophilic) are directed outward to face both extracellular and intracellular fluid.
Proteins in the plasma membrane can be peripheral (on one side of the membrane), or integral (embedded in the plasma membrane).
Integral proteins are usually found in the membrane and are held in place by the cytoskeleton and the extracellular matrix (ECM).
The ECM is only found in animals and its function includes supporting the plasma membrane and communicating between cells.
B. Fluid-Mosaic Model
1. The fluid-mosaic model describes the plasma membrane.
2. The fluid component refers to the phospholipid bilayer of the plasma membrane.
3. Fluidity of the plasma membrane allows cells to be pliable.
4. Fluidity is affected by cholesterol molecules in the plasma membrane.
5. The mosaic component refers to the protein content in the plasma membrane.
6. Proteins bond to the ECM and/or cytoskeleton to prevent movement in the fluid phospholipid bilayer.
C. Carbohydrate Chains
1. Glycolipids have a structure similar to phospholipids except the hydrophilic head is a variety of sugar; they are protective and assist in various functions.
2. Glycoproteins have an attached carbohydrate chain of sugar that projects externally.
3. In animal cells, the glycocalyx is a “sugar coat” of carbohydrate chains and has several functions.
4. Cells are unique in that they have highly varied carbohydrate chains (a “fingerprint”).
5. The immune system recognizes foreign tissues that have inappropriate carbohydrate chains.
6. Carbohydrate chains are the basis for A, B, and O blood groups in humans.
D. The Functions of the Proteins
1. Channel proteins allow a particular molecule to cross a membrane freely (e.g., Cl- channels).
2. Carrier proteins selectively interact with a specific molecule so it can cross the plasma membrane (e.g., Na+K+ pump).
Cell recognition proteins are glycoproteins that allow the body’s immune system to distinguish between foreign invaders and body cells.
4. Receptor proteins are shaped so a specific molecule (e.g., hormone) can bind to it.
5. Enzymatic proteins carry out specific metabolic reactions.
6. Junction proteins join animal cells so tissues can function.
E. Permeability of the Plasma Membrane
1. The plasma membrane is selectively permeable; only certain molecules can pass through.
a. Small non-polar molecules (carbon dioxide, oxygen, glycerol and alcohol) pass through the membrane freely.
b. Small molecules (carbon dioxide, oxygen) move “down” a concentration gradient, i.e., from high to low concentration.
c. Cells have channel proteins called aquaporins that allow water to cross a plasma membrane more quickly than expected.
d. Ions and charged molecules cannot readily pass through the hydrophobic component of the bilayer and usually combine with carrier proteins.
e. Large particles enter or exit a cell through bulk transport.
F. How Do Cells Talk to One Another? (Biological Systems reading)
1. Cell Signaling
a. Signaling molecules, or chemical messengers, “talk” to other cells and may change cells, tissues, or organs.
b. These cells do not respond to all molecules. They require binding to a receptor protein.
c. Once the signaling molecule is bound to a receptor, the signal follows through a transduction pathway.
d. The cell’s response to the transduction pathway can change the shape or movement of the cell, alter the metabolism or function of the cell, or alter the gene expression and amount of a cell protein.
5.2 Passive Transport Across a Membrane
1. Diffusion is the movement of molecules from higher to lower concentration (i.e., “down” the concentration gradient).
a. A solution contains a solute, usually a solid, and a solvent, usually a liquid.
b. In the case of a dye diffusing in water, the dye is a solute and water is the solvent.
c. Once a solute is evenly distributed, random movement continues but with no net change.
d. The chemical and physical properties of a membrane allow only a few types of molecules to cross by diffusion.
e. Gases readily diffuse through the lipid bilayer; e.g., the movement of oxygen from air sacs (alveoli) to the blood in lung capillaries depends on the concentration of oxygen in alveoli.
f. Temperature, pressure, electrical currents, and molecular size influence the rate of diffusion.
1. Osmosis is the diffusion of water across a differentially (selectively) permeable membrane.
a. Osmosis is illustrated by the thistle tube example:
i. A differentially permeable membrane separates two solutions.
ii. The beaker has more water (lower percentage of solute) and the thistle tube has less water (higher percentage of solute).
iii. The membrane does not permit passage of the solute; water enters but the solute does not exit the membrane.
iv. The membrane permits passage of water with a net movement of water from the beaker to the inside of the thistle tube.
b. Osmotic pressure is the pressure that develops in a system due to osmosis.
c. Osmotic pressure results in water being absorbed by the kidneys and water being taken up from tissue fluid.
d. Isotonic solutions are solutions where the solute concentration and the water concentrations inside and outside the cell are equal.
e. Tonicity refers to the strength of the solution
f. A 0.9% salt solution is used in injections because it is isotonic to red blood cells (RBCs).
g. A hypotonic solution has a solute concentration that is less than another solution; when a cell is placed in a hypotonic solution, water enters the cell and it may undergo cytolysis (“cell bursting”).
h. Swelling of a plant cell in a hypotonic solution creates turgor pressure; this is how plants maintain an erect position.
i. A hypertonic solution has a solute concentration that is higher than another solution; when a cell is placed in a hypertonic solution, water leaves the cell and it shrivels (a condition called crenation).
Plasmolysis is shrinking of the cytoplasm due to osmosis in a hypertonic solution; as the central vacuole loses water, the plasma membrane pulls away from the cell wall.
B. Facilitated Transport
1. Facilitated transport is the transport of a specific solute “down” or “with” its concentration gradient (from high to low), facilitated by a carrier protein; glucose and amino acids move across the membrane in this way.
5.3 Active Transport Across a Membrane
1. Active transport is transport of a specific solute across plasma membranes “up” or “against” (from low to high) its concentration gradient through use of cellular energy (ATP).
a. Iodine is concentrated in cells of the thyroid gland, glucose is completely absorbed into the lining of the digestive tract, and sodium is mostly reabsorbed by the kidney tubule lining.
b. Active transport requires both carrier proteins and ATP; therefore cells must have a high number of mitochondria near membranes where active transport occurs.
c. Proteins involved in active transport are often called “pumps”; the sodiumpotassium pump is an important carrier protein in nerve and muscle cells.
d. Salt (NaCl) crosses a plasma membrane because Na+ are pumped across, and the Cl- is attracted to Na+ and simply diffuses across specific channels in the membrane.
e. Cystic fibrosis results from a faulty chloride channel protein, causing thick mucus in bronchial tubes and pancreatic ducts.
A. Bulk Transport
1. In exocytosis, a vesicle formed by the Golgi body fuses with the plasma membrane as secretion occurs; insulin leaves insulin-secreting cells by this method.
2. During endocytosis, cells take in substances by vesicle formation as plasma membrane pinches off by phagocytosis, pinocytosis, or receptor-mediated endocytosis.
In phagocytosis, cells engulf large particles (e.g., bacteria, food particles), forming an endocytic vesicle.
a. Phagocytosis is commonly performed by amoeboidtype cells (e.g., amoebas and macrophages).
b. When the endocytic vesicle fuses with a lysosome, digestion of the internalized substance occurs.
4. Pinocytosis occurs when vesicles form around a liquid or very small particles; this is only visible with electron microscopy.
a. Cells that use pinocytosis to ingest substances are: blood cells, cells that line the kidney tubules or the intestinal wall, and plant root cells.
5. Receptor-mediated endocytosis, a form of pinocytosis, occurs when specific macromolecules bind to plasma membrane receptors.
a. The receptor proteins are shaped to fit with specific substances (vitamin, hormone, lipoprotein molecule, etc.), and are found at one location in the plasma membrane.
b. This location is a coated pit with a layer of protein on the cytoplasmic side.
c. Once formed, the vesicle is uncoated and may fuse with a lysosome.
d. Pits are associated with exchange of substances between cells (e.g., maternal and fetal blood).
e. This system is selective and more efficient than pinocytosis; it is important in moving substances from maternal to fetal blood.
f. Cholesterol (transported in a molecule called a low-density lipoprotein, LDL) enters a cell from the bloodstream via receptors in coated pits; in familial hypocholesterolemia, the LDL receptor cannot bind to the coated pit and the excess cholesterol accumulates in the circulatory system.
5.4 Modification of Cell Surfaces
A. Cell Surfaces in Animals
1. The extracellular matrix is a meshwork of polysaccharides and proteins in animal cells.
Collagen gives the matrix strength and elastin gives it resilience.
Fibronectins and integrins bind to membrane receptors and permit communication between matrix and cytoplasm; these proteins also form “highways” that direct the migration of cells during development.
Proteoglycans are glycoproteins that provide a packing gel that joins the various proteins in matrix and most likely regulate signaling proteins that bind to receptors in the plasma protein.
Junctions Between Cells are points of contact between cells that allow them to behave in a coordinated manner.
Adhesion junctions mechanically attach adjacent cells.
In desmosomes, internal cytoplasmic plaques, firmly attached to cytoskeleton within each cell, are joined by integral membrane proteins; they hold cells together where tissues stretch (e.g., in heart, stomach, bladder).
In tight junctions, plasma membrane proteins attach in zipper-like fastenings; they hold cells together so tightly that the tissues are barriers (e.g., epithelial lining of stomach, kidney tubules, blood-brain barrier).
A gap junction allows cells to communicate; formed when two identical plasma membrane channels join.
i. They provide strength to the cells involved and allow the movement of small molecules and ions from the cytoplasm of one cell to the cytoplasm of the other cell.
ii. Gap junctions permit flow of ions for heart muscle and smooth muscle cells to contract.
B. Plant Cell Walls
Plant cells are surrounded by a porous cell wall; it varies in thickness, depending on the function of the cell.
Plant cells have a primary cell wall composed of cellulose polymers united into threadlike microfibrils that form fibrils.
Cellulose fibrils form a framework whose spaces are filled by non-cellulose molecules.
Pectins allow the cell wall to stretch and are abundant in the middle lamella that holds cells together.
Non-cellulose polysaccharides harden the wall of mature cells.
Lignin adds strength and is a common ingredient of secondary cell walls in woody plants.
Plasmodesmata are narrow membrane-lined channels that pass through cell walls of neighboring cells and connect their cytoplasms, allowing direct exchange of some molecules and ions between neighboring plant cells.