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, with the heads buried inside
• BA single layer of phospholipids with globular proteins coating both of its surfaces, giving a rigid sheet
• CA bilayer with hydrophilic heads facing the aqueous solutions and hydrophobic tails facing inward✓
• DPhospholipids form rigid covalent cross-links with their neighbours that prevent lateral movement in the bilayer

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 hydrophilic channel for the facilitated diffusion of ions and water
• BIt provides the receptor sites to which hormones such as insulin bind on the cell surface
• CIt catalyses the hydrolysis of membrane phospholipids to release fatty acids for respiration
• 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 globular protein (e.g. albumin)

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 the hydrolysis of ATP to change the shape of the carrier protein
• BMoves substances against their concentration gradient using energy released by respiration
• CUses carrier or channel proteins to move polar/charged substances down their gradient✓
• DOccurs only across the membranes of prokaryotic cells, which have no organelles

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 wall is already fully turgid and resists water uptake
• BOut of the cell, because the solutes in the external solution move in and displace an equal volume of the water
• CInto the cell, because water moves from a higher (less negative) to a lower (more negative) water potential✓
• DOut of the cell, because at −600 kPa 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 that open and close, but no input of metabolic energy
• CA favourable (downhill) concentration gradient alone
• DAquaporins and a high hydrostatic (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 protein in the luminal membrane itself
• BA sodium ion (Na⁺) concentration gradient maintained by the sodium-potassium pump✓
• COsmosis driven by the water-potential gradient across the epithelial cell-surface membrane
• DFacilitated diffusion of glucose down its own concentration gradient through a channel protein

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?

• AThe higher temperature lowered the kinetic energy of the pigment molecules in the vacuole
• BCholesterol between the phospholipids crystallised and sealed the tonoplast, forcing the pigment out of the cell
• CThe pigment was actively transported out of the vacuole by ATP-driven carrier proteins working faster at high temperature
• 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, so more solutes were pumped out of the cell
• BEthanol caused the cell to take in water and become turgid
• CEthanol provided an extra respiratory substrate, and the ATP released pumped the pigment out of the cell
• 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.

What does the fluid mosaic model describe?

• AA protein bilayer with phospholipids embedded throughout
• BA phospholipid monolayer with proteins embedded throughout
• CA cholesterol bilayer with phospholipids embedded throughout
• DA phospholipid bilayer with proteins embedded throughout✓

Answer: Phospholipids form a bilayer with hydrophilic heads outward and hydrophobic tails inward, and proteins of many kinds sit in and across it. The components drift laterally, which is the 'fluid' part; the scattered proteins are the 'mosaic'.

How does active transport differ from facilitated diffusion?

• AIt needs ATP and moves solutes down the gradient
• BIt needs ATP and moves solutes against the gradient✓
• CIt needs no ATP and moves solutes against the gradient
• DIt needs no ATP and moves solutes down the gradient

Answer: Moving a solute from low to high concentration is not spontaneous, so it must be driven by ATP hydrolysis at a carrier protein. Facilitated diffusion is passive and works only down the gradient — the last option describes it.

Why does a plant cell in pure water become turgid rather than bursting?

• AThe cell membrane resists further expansion
• BThe cellulose wall resists further expansion✓
• CThe vacuole pumps the excess water out again
• DThe cytoplasm stops being permeable to water

Answer: Water enters by osmosis until the inelastic cell wall pushes back hard enough to stop further net entry, generating turgor pressure. An animal cell has no wall, so under the same conditions it swells and lyses.

What is water potential?

• AThe tendency of water to leave a system by osmosis✓
• BThe tendency of solutes to leave a system by osmosis
• CThe total mass of water contained within a cell
• DThe total mass of solute dissolved within a cell

Answer: Water potential measures the free energy of water molecules and water always moves from a higher (less negative) to a lower (more negative) potential. Pure water is 0 kPa, and adding any solute makes it more negative.

What is the role of cholesterol in a cell membrane?

• AIncreasing phospholipid movement to regulate fluidity
• BForming the channels through which ions cross the membrane
• CForming the receptors to which hormones bind specifically
• DRestricting phospholipid movement to regulate fluidity✓

Answer: Cholesterol packs between phospholipid tails, limiting their movement when warm and preventing tight packing when cold, so the membrane stays workably fluid over a range of temperatures. Channels and receptors are proteins.

Why does glucose co-transport in the ileum depend on ATP indirectly?

• AA glucose gradient built by active transport drives it
• BThe co-transporter protein hydrolyses ATP directly
• CGlucose must be phosphorylated by ATP before entering
• DA sodium gradient built by active transport drives it✓

Answer: The sodium-potassium pump spends ATP to keep sodium low inside the epithelial cell. Sodium then diffuses back in through the co-transporter, dragging glucose against its own gradient — so the ATP is spent on the ion gradient, not on the glucose.

What happens to an animal cell placed in a solution of lower water potential?

• AWater leaves the cell and it shrinks✓
• BWater enters the cell and it swells
• CWater leaves the cell and it becomes turgid
• DWater enters the cell and it becomes plasmolysed

Answer: Water moves down the water potential gradient, so it leaves the cell, which becomes crenated. Turgor requires a cell wall and plasmolysis is the plant-cell equivalent, where the membrane pulls away from the wall.

Why are cells performing much active transport rich in mitochondria?

• AActive transport requires a steady supply of glucose
• BActive transport requires a steady supply of ATP✓
• CMitochondria carry the solutes across the membrane
• DMitochondria manufacture the membrane carrier proteins

Answer: Pumping against a gradient costs energy, so epithelial cells of the ileum and kidney tubules are packed with mitochondria to supply ATP. Mitochondria make ATP — they neither transport solutes nor build proteins.

Why can small non-polar molecules cross the membrane most readily?

• AThey dissolve in the hydrophobic phospholipid tails✓
• BThey dissolve in the hydrophilic phospholipid heads
• CThey are actively pumped through carrier proteins
• DThey pass through protein channels filled with water

Answer: The interior of the bilayer is hydrophobic, so lipid-soluble molecules such as oxygen and carbon dioxide diffuse straight through. Charged and polar particles are repelled by that core and need channel or carrier proteins.