Chapter 7: Membrane Structure and Function

Vocabulary terms and definitions covering the structure, fluidity, transport mechanisms, and signaling roles of the cellular plasma membrane.

Selective permeability

The property of the plasma membrane that allows some substances to cross it more easily than others.

Phospholipids

The most abundant lipid in the plasma membrane; they are amphipathic molecules containing both hydrophobic and hydrophilic regions.

Fluid mosaic model

A model of membrane structure stating that the membrane is a fluid structure with a “mosaic” of various proteins embedded in it in an organized fashion.

Sandwich model

A membrane model proposed in 1935 by Hugh Davson and James Danielli where the phospholipid bilayer lies between two layers of globular proteins.

Singer and Nicolson model (1972)

The proposal that the membrane is a mosaic of proteins dispersed within the bilayer, with only the hydrophilic regions exposed to water.

Freeze-fracture

A specialized preparation technique that splits a membrane along the middle of the phospholipid bilayer, supporting the fluid mosaic model.

Peripheral proteins

Proteins that are bound to the surface of the membrane.

Integral proteins

Proteins that penetrate the hydrophobic core of the membrane; those that span the membrane are called transmembrane proteins.

Membrane Fluidity (Temperature Effects)

Membranes rich in unsaturated fatty acids are more fluid than those rich in saturated fatty acids; they usually have the fluidity of salad oil to work properly.

Cholesterol

A steroid that reduces membrane fluidity at moderate temperatures (such as $37^{\circ}\text{C}$) but hinders solidification at cool temperatures by preventing tight packing.

Six major functions of membrane proteins

(a) Transport, (b) enzymatic activity, (c) signal transduction, (d) cell-cell recognition, (e) intercellular joining, and (f) attachment to the cytoskeleton and extracellular matrix (ECM).

Glycolipids

Membrane carbohydrates that are covalently bonded to lipids.

Glycoproteins

Membrane carbohydrates that are covalently bonded to proteins.

Cell-Cell Recognition (HIV)

The process where HIV must bind to the immune cell surface protein $CD4$ and a “co-receptor” $CCR5$ to infect a cell.

Sidedness of membranes

The asymmetrical distribution of proteins, lipids, and carbohydrates determined when the membrane is built by the ER and Golgi apparatus.

Aquaporins

Specific channel proteins that facilitate the passage of water through the membrane.

Carrier proteins

Transport proteins that bind to molecules and change shape to shuttle them across the membrane.

Diffusion

The tendency for molecules to spread out evenly into available space, moving down their concentration gradient.

Passive transport

The diffusion of a substance across a biological membrane with no energy expenditure by the cell.

Osmosis

The diffusion of water across a selectively permeable membrane from a region of lower solute concentration to a region of higher solute concentration.

Tonicity

The ability of a surrounding solution to cause a cell to gain or lose water.

Isotonic solution

A solution where solute concentration is the same as inside the cell, resulting in no net water movement.

Hypertonic solution

A solution where solute concentration is greater than inside the cell, causing the cell to lose water.

Hypotonic solution

A solution where solute concentration is less than inside the cell, causing the cell to gain water.

Osmoregulation

The control of solute concentrations and water balance, an adaptation used by organisms like Paramecium via a contractile vacuole.

Turgid

A firm state for a plant cell, occurring when it is in a hypotonic solution and the cell wall opposes further water uptake.

Plasmolysis

A lethal effect in hypertonic environments where the plant cell membrane pulls away from the cell wall as the cell loses water.

Facilitated diffusion

The passive movement of molecules across the plasma membrane aided by transport proteins like channel or carrier proteins.

Active transport

The use of energy (usually $ATP$) to move solutes against their concentration gradients through specific membrane proteins.

Sodium-potassium pump

A type of active transport system that functions as the major electrogenic pump of animal cells.

Membrane potential

The voltage difference across a membrane created by the distribution of positive and negative ions.

Electrochemical gradient

The combination of a chemical force (ion concentration gradient) and an electrical force (effect of membrane potential) driving ion diffusion.

Electrogenic pump

A transport protein that generates voltage across a membrane, such as the proton pump used by plants, fungi, and bacteria.

Cotransport

Occurs when the active transport of a solute indirectly drives the transport of other solutes, such as using a $H^+$ gradient to drive nutrient transport.

Exocytosis

The process where transport vesicles migrate to the membrane, fuse with it, and release their contents outside the cell.

Endocytosis

The process of taking in macromolecules by forming vesicles from the plasma membrane; types include phagocytosis, pinocytosis, and receptor-mediated endocytosis.

Phagocytosis

A type of endocytosis often called “cellular eating” where a cell engulfs a particle in a vacuole.

Pinocytosis

A type of endocytosis often called “cellular drinking” where molecules dissolved in droplets are taken up into tiny vesicles.

Ligand

Any molecule that binds specifically to a receptor site of another molecule, triggering vesicle formation in receptor-mediated endocytosis.