Membrane Structure and Transport – Key Concepts

A set of practice flashcards covering membrane structure, diffusion, osmosis, osmolarity/osmolality, tonicity, and membrane transport mechanisms based on the lecture notes.

What is the primary function of the plasma (cell) membrane?

To protect the cell and regulate movement of substances in and out; it acts as a semi-permeable barrier.

What is the composition of a phospholipid bilayer?

Two layers of phospholipids with hydrophilic heads facing outward and hydrophobic tails inward; cholesterol and membrane proteins are also present.

What role does cholesterol play in the cell membrane?

Maintains membrane fluidity and stability, influencing membrane movability.

What are integral and peripheral proteins in the membrane?

Integral proteins span the membrane and often function as channels or carriers; peripheral proteins attach to the membrane surface.

What is a membrane channel protein?

A protein that forms a pore allowing specific substances to diffuse across the membrane (facilitated diffusion).

What is a carrier protein?

A membrane protein that binds a molecule and changes shape to shuttle it across the membrane (facilitated diffusion or active transport when energy is involved).

Define diffusion.

Movement of solute particles from high concentration to low concentration; no energy required; can be simple or facilitated diffusion.

Define facilitated diffusion.

Diffusion that requires a membrane protein (channel or carrier) but still does not use energy.

Define osmosis.

Diffusion of water across a semipermeable membrane from higher water concentration to lower water concentration (or from low solute to high solute concentration).

What is a semipermeable membrane?

A membrane that allows some substances to pass while restricting others; selective permeability.

Define osmolarity.

The total number of solute particles per liter of solution (or per kilogram for osmolality); measures solute concentration.

Define osmotic pressure.

The pressure required to stop osmosis; relates to solute concentration driving water movement.

What is tonicity and its three types?

The effect of a solution on cell volume: hypertonic (cell shrinks), isotonic (no net change), hypotonic (cell swells).

Where are extracellular and intracellular fluids located?

Intracellular fluid is inside cells; extracellular fluid (including plasma) is outside cells.

Which solutes are higher outside vs inside the cell?

Sodium (Na+) is higher outside; potassium (K+) is higher inside.

What factors affect the rate of diffusion across the membrane?

Concentration gradient, temperature, particle size, surface area of the membrane, and membrane thickness.

What is active transport and what does it require?

Movement against the concentration gradient (low to high); requires energy, usually ATP, and uses pumps (e.g., Na+/K+ ATPase).

What are endocytosis and exocytosis?

Endocytosis is uptake of material into the cell; exocytosis is release of material from the cell; both require energy.

Why are lipid-soluble (lipophilic) molecules able to cross membranes easily?

Because they dissolve in the lipid bilayer, facilitating diffusion across the membrane.

What happens to red blood cells in a hypotonic (pure water) solution?

Water enters the cells, causing swelling and potential hemolysis (rupture) of the cells.

What is the difference between osmolarity and osmolality?

Osmolarity is particles per liter of solution; osmolality is particles per kilogram of solvent; both describe solute concentration and are often used interchangeably in biology.

What is the difference between a channel and a pump in membrane transport?

Channels allow passive diffusion through a gate; pumps actively transport substances using energy (ATP) against the gradient.