Cell Membranes & Transport

According to the fluid-mosaic model of the cell-surface membrane, which statement best describes the arrangement of phospholipids?

• AHydrophilic tails face outward into the aqueous solutions on both sides
• BA single layer with proteins coating both surfaces
• CA bilayer with hydrophilic heads facing the aqueous solutions and hydrophobic tails facing inward✓
• DPhospholipids form rigid covalent cross-links that prevent lateral movement

Answer: Phospholipids form a bilayer: the hydrophilic (phosphate) heads face the watery environments inside and outside the cell, while the hydrophobic fatty-acid tails point inward, away from water. Molecules can move laterally, giving fluidity.

Cholesterol is a component of many animal cell-surface membranes. What is its main role?

• AIt acts as a channel for facilitated diffusion of ions
• BIt provides receptor sites for hormones such as insulin
• CIt catalyses the hydrolysis of membrane phospholipids
• DIt regulates membrane fluidity, restricting movement of phospholipids and adding mechanical stability✓

Answer: Cholesterol fits between phospholipid molecules and binds to their fatty-acid tails, restricting their movement. This stabilises the membrane and reduces fluidity and permeability, especially to ions and polar molecules.

Which one of the following molecules would diffuse most readily by SIMPLE diffusion directly through the phospholipid bilayer?

• AOxygen (a small, non-polar molecule)✓
• BSodium ions (Na⁺)
• CGlucose
• DA large globular protein

Answer: Small, non-polar (lipid-soluble) molecules such as O₂ and CO₂ dissolve in and pass directly through the hydrophobic core of the bilayer. Charged ions (Na⁺) and large polar molecules (glucose) cannot, and require transport proteins.

Facilitated diffusion differs from simple diffusion because facilitated diffusion:

• ARequires ATP hydrolysis to move molecules
• BMoves substances against their concentration gradient
• CUses carrier or channel proteins to move polar/charged substances down their gradient✓
• DOnly occurs across the membranes of prokaryotic cells

Answer: Facilitated diffusion is passive (no ATP) and moves substances DOWN their concentration gradient, but unlike simple diffusion it uses intrinsic channel or carrier proteins to transport polar molecules and ions that cannot cross the hydrophobic bilayer.

A plant cell with a water potential of −600 kPa is placed in a solution with a water potential of −300 kPa. In which direction will there be a net movement of water?

• ANo net movement, because the cell is already turgid
• BOut of the cell, because solutes always move into the cell
• CInto the cell, because water moves from a higher (less negative) to a lower (more negative) water potential✓
• DOut of the cell, because the cell has the higher water potential

Answer: Water moves by osmosis from a region of higher (less negative) water potential to lower (more negative). The solution (−300 kPa) is higher than the cell (−600 kPa), so water moves INTO the cell.

Pure water has the highest possible water potential. What is the water potential (ψ) of pure water at standard temperature and pressure?

• A+100 kPa
• BDependent on the volume of water present
• C−100 kPa
• D0 kPa✓

Answer: By definition, pure water at standard temperature and atmospheric pressure has a water potential of 0 kPa. Adding solute lowers ψ, making it negative; solutions therefore always have a water potential below zero.

Active transport of ions across a cell membrane requires which of the following?

• ACarrier proteins and metabolic energy from the hydrolysis of ATP✓
• BChannel proteins but no energy input
• CA favourable (downhill) concentration gradient only
• DAquaporins and a high turgor pressure

Answer: Active transport moves substances against their concentration gradient using carrier proteins that change shape, driven by energy released from ATP hydrolysis. It is selective and stops if respiration (ATP supply) is inhibited.

In the absorption of glucose from the lumen of the ileum into epithelial cells, glucose enters against its concentration gradient by co-transport. What directly powers this entry?

• ADirect hydrolysis of ATP by the glucose carrier itself
• BA sodium ion (Na⁺) concentration gradient maintained by the sodium-potassium pump✓
• COsmosis driven by a water-potential gradient
• DFacilitated diffusion of glucose down its own gradient

Answer: The Na⁺/K⁺ pump actively pumps Na⁺ out of the epithelial cell (using ATP), creating a low cytoplasmic Na⁺ concentration. Na⁺ then diffuses back in through a co-transporter protein, dragging glucose in with it against the glucose gradient (indirect/secondary active transport).

Beetroot pieces were placed in water at a range of temperatures, and the leakage of red pigment was measured. Pigment leakage increased sharply above about 45 °C. Which explanation best accounts for this?

• AIncreased temperature lowered the kinetic energy of the pigment molecules
• BCholesterol crystallised, sealing the membrane
• CThe pigment was actively transported out by ATP-driven pumps
• DHigh temperature denatured membrane proteins and disrupted the phospholipid bilayer, increasing permeability✓

Answer: Above roughly 45 °C the phospholipid bilayer becomes more fluid and membrane (carrier/channel) proteins denature, leaving gaps. This sharply increases permeability, so more pigment leaks out of the vacuole and cell.

An investigation found that increasing the concentration of ethanol in the surrounding solution increased the permeability of a cell-surface membrane. What is the most likely reason?

• AEthanol increased the activity of the sodium-potassium pump
• BEthanol caused the cell to become turgid
• CEthanol provided extra glucose for respiration
• DEthanol is a non-polar solvent that dissolves phospholipids, disrupting the bilayer structure✓

Answer: Organic solvents such as ethanol dissolve membrane lipids and disrupt the orderly phospholipid bilayer, creating gaps. This increases membrane permeability so substances leak across more readily.