2.3 Transport across cell membranes

What is the name of the model describing cell membrane structure?

• The fluid-mosaic model.

What does the 'fluid' part of the fluid-mosaic model refer to?

• The phospholipid and protein molecules are free to move laterally within the bilayer.

What does the 'mosaic' part of the model refer to?

• The membrane is composed of a variety / mosaic of different components.

• E.g., phospholipids, proteins, glycoproteins, glycolipids, cholesterol.

Where is this basic membrane structure found?

• In all cell membranes: both the cell-surface (plasma) membrane and the membranes around eukaryotic organelles.

How are phospholipids arranged in the bilayer?

• They form a bilayer.

• Hydrophobic fatty acid tails face inwards, away from water.

• Hydrophilic phosphate heads face outwards, towards the aqueous environments.

What are intrinsic/integral proteins, and give two examples?

• Proteins that span the entire bilayer.

• Examples: Channel proteins and carrier proteins (for transport).

What are extrinsic/peripheral proteins?

• Proteins that are present on the surface of the membrane (inner or outer), not spanning it.

Where are glycolipids and glycoproteins found, and what are they?

• Found on the exterior surface of the membrane.

• Glycolipids: Lipids with attached polysaccharide chains.

• Glycoproteins: Proteins with attached polysaccharide chains.

Where is cholesterol found in the membrane?

• It is located within the phospholipid bilayer.

• It binds to the hydrophobic fatty acid tails of the phospholipids.

What is the main role of cholesterol in the membrane?

• It restricts the lateral movement of phospholipids and other molecules.

• This decreases membrane fluidity and permeability, and increases rigidity / stability.

How does membrane fluidity allow for processes like phagocytosis?

• The fluid phospholipid bilayer allows the membrane to change shape, bend, and fuse.

• This is essential for vesicle formation, phagocytosis, and exocytosis.

How are glycoproteins and glycolipids adapted for cell communication?

• They act as receptors for hormones/neurotransmitters (cell signalling).

• They act as antigens for cell recognition (e.g., by the immune system).

What types of substances can cross a membrane by simple diffusion?

• Lipid-soluble (non-polar) molecules (e.g., steroid hormones, O₂).

• Very small molecules (e.g., CO₂).

How do these substances move, and what route do they take?

• They move from an area of higher concentration to lower concentration (down a concentration gradient).

• They move directly across the phospholipid bilayer.

Is simple diffusion active or passive? What provides the energy?

• It is passive.

• It uses only the kinetic energy of the molecules themselves.

Why can't water-soluble (polar) or larger substances cross by simple diffusion?

• The interior of the bilayer has hydrophobic fatty acid tails.

• This creates a hydrophobic barrier that repels polar/charged molecules and blocks large ones.

What types of substances require facilitated diffusion?

• Water-soluble (polar) molecules.

• Charged ions (e.g., Na⁺).

• Slightly larger molecules (e.g., glucose, amino acids).

How do these substances move, and what is their route?

• They move down a concentration gradient.

• They move through specific channel or carrier proteins embedded in the membrane.

Is facilitated diffusion active or passive?

• It is passive (does not require ATP).

What determines which substance a channel or carrier protein transports?

• The specific shape and charge of the protein's channel or binding site.

How does a channel protein work?

• It forms a hydrophilic, water-filled pore.

• This allows specific ions/charged particles to pass through.

• Some are gated (can open or close in response to a signal).

How does a carrier protein work in facilitated diffusion?

• The specific molecule binds to a complementary binding site on the protein.

• This causes the protein to change shape.

• The shape change releases the molecule on the other side of the membrane.

What is osmosis?

• The net movement / diffusion of water molecules.

What is the direction of water movement in osmosis?

• From an area of high water potential (ψ) to an area of low water potential.

• (Down a water potential gradient).

Through what does osmosis occur, and is it active or passive?

• It occurs through a partially permeable membrane (the phospholipid bilayer).

• It is a passive process (no ATP required).

What is water potential (ψ)?

• A measure of the tendency of water molecules to move out of a solution.

• Pure water has the highest possible ψ = 0 kPa.

• Adding solute decreases (makes more negative) the water potential.

How does the direction of movement in active transport differ from passive processes?

• Substances move from an area of lower concentration to higher concentration (against the concentration gradient).

• It requires energy from ATP hydrolysis.

What membrane structure facilitates active transport?

• Specific carrier proteins.

What is the first step in the mechanism of active transport?

• The specific substance binds to a complementary site on the carrier protein.

What provides the energy for the shape change?

• ATP binds to the protein and is hydrolysed into ADP + Pi.

• This releases energy.

How is the substance transported across the membrane?

• The released energy causes the carrier protein to change shape.

• This shape change releases the substance on the side of higher concentration.

How does the carrier protein reset to its original state?

• The inorganic phosphate (Pi) is released from the protein.

• This causes the protein to return to its original shape.

What is co-transport?

• The simultaneous transport of two different substances.

• It occurs via a co-transporter protein (a type of carrier protein).

How are the movements of the two substances typically linked?

• The movement of one substance against its gradient is coupled to / powered by the movement of another substance down its gradient.

What is a key example of co-transport in mammals?

• The absorption of sodium ions (Na⁺) and glucose by epithelial cells lining the ileum (small intestine).

How is the essential sodium ion gradient created?

• Na⁺ is actively transported out of the epithelial cell into the blood by the Na⁺/K⁺ pump.

• This creates a low Na⁺ concentration inside the epithelial cell compared to the gut lumen.

What happens at the luminal membrane of the epithelial cell?

• Na⁺ moves into the cell down its concentration gradient.

• It does so via a co-transporter protein, which simultaneously brings glucose into the cell against its concentration gradient.

How does the absorbed glucose then enter the blood?

• Glucose moves down its concentration gradient from the epithelial cell into the blood.

• This occurs via facilitated diffusion through a different carrier protein.

Why is this co-transport example sometimes called indirect or secondary active transport?

• Because the movement of glucose against its gradient is indirectly powered by ATP.

• The ATP was used to create the Na⁺ gradient (via the Na⁺/K⁺ pump), which then drives the co-transport.

How does increasing the surface area of a membrane affect transport rate?

• It increases the rate of movement for all forms of transport (diffusion, osmosis, active transport).

How does increasing the number of channel/carrier proteins affect transport?

• It increases the rate of facilitated diffusion and active transport.

How does the concentration gradient affect the rate of simple diffusion?

• Increasing the concentration gradient increases the rate of simple diffusion.

How does the concentration gradient initially affect the rate of facilitated diffusion?

• Increasing the concentration gradient increases the rate of facilitated diffusion.

Why does the rate of facilitated diffusion eventually plateau even if the gradient increases?

• When all the available channel/carrier proteins are in use / saturated.

• At this point, protein number becomes the limiting factor.

How does the water potential gradient affect osmosis?
Answer:

• Increasing the water potential gradient increases the rate of osmosis.

What is one structural adaptation to increase transport rate, and give an example?

• Folding the cell membrane to increase surface area.

• Example: Microvilli on epithelial cells in the ileum.

How can a cell adapt to increase the rate of facilitated diffusion or active transport?

• By having a higher density / more channel proteins and carrier proteins in its membrane.

Why do cells specialised for active transport have many mitochondria?

• Mitochondria produce ATP via aerobic respiration.

• A large number of mitochondria ensures a high rate of ATP production to supply energy for active transport.