Chapter 5 Membrane Structure and Function

I. Water, Water Everywhere
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.

II. Fluid Membranes in a Largely Fluid World
A. The Lipid Bilayer
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. Three types of lipids are common in cell membranes:
a. Phospholipids all have a hydrophilic head and hydrophobic tails but both of these regions show considerable variation.
b. Glycolipids have sugar monomers attached at the head end.
c. Cholesterol is abundant in animal—but not in plant—membranes.
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. A variety of proteins are associated with the lipid bilayer to produce a mosaic effect.
a. Glycoproteins have oligosaccharides attached to the extracellular side of the bilayer.
b. Proteins help maintain the fluid nature of the membrane.
4. The arrangement of molecules on one side of the membrane differs from that on the other side (asymmetrical).
C. Functions of Membrane Proteins
1. Two classes of proteins have transport functions:
a. A channel protein, whether it be perpetually open or gated, serves as a pore through which ions, water, and soluble substances can move.
b. A carrier protein binds specific substances and changes shape to shunt the materials across; some work passively, others require energy for “pumping.”
2. A recognition protein identifies the cell as a certain type, helps guide cells into becoming issues, and functions in cell-to-cell recognition and coordination.
3. A receptor protein has binding sites for hormones (and like substances) that can trigger changes in cell action, as in growth processes.

III. Diffusion
A. Gradients Defined
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. Gradients in pressure, temperature, and electric charge can also influence movements.
B. Simple Diffusion
1. Simple diffusion is the movement of like molecules or ions down a concentration gradient.
2. Each substance diffuses independently of other substances present as illustrated by dye molecules in water.
3. When gradients no longer exist, there is no net movement (dynamic equilibrium).
4. The rate of diffusion depends on concentration differences, temperature (higher = faster), and molecular size (smaller = faster).
C. Bulk Flow
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. Bulk flow has the effect of shrinking the distance that molecules must travel; this tends to enhance diffusion rates.

IV. Osmosis
A. Osmosis Defined
1. Osmosis is the passive movement of water across a differentially permeable membrane in response to solute concentration gradients, pressure gradients, or both.
2. For example, if a bag containing a sugar solution is placed in pure water, the water will diffuse inward (higher to lower).
B. 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. Water Potential
1. Because soil is hypotonic, water tends to move into a plant where it accumulates causing turgor pressure.
2. Water potential is the sum of the opposing forces of osmosis and turgor pressure.
3. Wilting occurs when water no longer enters the plant in sufficient quantities.

V. Movement of Water and Solutes Across Cell Membranes
A. The Available Routes
1. Small, electrically neutral molecules (for example, oxygen, carbon dioxide, and water) cross the lipid bilayer by simple diffusion.
2. Larger molecules (such as glucose) and charged ions (such as Na+, Ca+, HCO o(3,–) ) must be moved by membrane transport proteins.
a. In active transport, proteins become activated to move a solute against its concentration gradient.
b. In passive transport, material passes through proteins without an energy boost.
B. Facilitated Diffusion
1. The membrane protein only helps a solute move according to the dictates of simple diffusion.
2. When water-soluble molecules bind to the proteins’ hydrophilic groups, it triggers a change in shape that “eases” the solute through the protein and hence through the membrane.
C. Active Transport
1. To move ions and large molecules across a membrane, 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.
D. Exocytosis and Endocytosis
1. Vesicles, small sacs made of membranes, can transport and store substances within the cytoplasm.
2. Exocytosis moves substances from cytoplasm to plasma membrane during secretion.
3. Endocytosis encloses particles in small portions of plasma membrane to form vesicles that then move into the cytoplasm.
a. Amoebas are phagocytic (“cell eater”), as are white blood cells; lysosomes fuse with the endocytic vesicles to digest the contents.
b. Droplets of liquid are also taken in (endocytosis).
c. 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.