3.2.3 Transport across cell membranes

State the three main examples of cell membranes. (3 marks)

- Cell surface membrane.

- Mitochondrial membrane.

- Chloroplast membrane.

Describe the fluid-mosaic model of membrane structure. (3 marks)

- Phospholipids and proteins can move laterally within the bilayer and hence fluid structure.

- Membranes contain multiple components, including phospholipids, proteins, glycoproteins, and glycolipids, hence mosaic structure.

- The basic structure of all cell membranes is the same.

Draw a labelled diagram of general structure of the cell membrane. (6 marks)

State the components of a typical cell membrane. (5 marks)

- Phospholipid bilayer

- Intrinsic proteins that span bilayer e.g. channel and carrier proteins

- Extrinsic proteins on the surface of membrane.

- Glycolipids and glycoproteins on exterior surface.

- Cholesterol that is sometimes present bound to phospholipid fatty acid tails.

Explain the arrangement of phospholipids in a cell membrane. (3 marks)

- Phospholipids form a bilayer with water on both sides.

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

- Hydrophilic phosphate heads face outwards towards water.

Describe the role of cholesterol in cell membranes. (2 marks)

- Restricts the movement of molecules in the membrane.

- Reduces fluidity and permeability while increasing rigidity.

Suggest two ways cell membranes are adapted for other functions. (2 marks)

- Fluid bilayer allows bending for vesicle formation and phagocytosis.

- Glycoproteins and glycolipids act as receptors or antigens in cell signalling and recognition.

Describe how movement across membranes occurs by simple diffusion. (4 marks)

- Lipid-soluble or very small molecules diffuse through the phospholipid bilayer.

- They move down a concentration gradient, from high to low concentration.

- Diffusion is passive and requires no energy from respiration.

- Only the kinetic energy of molecules is needed.

Explain why the phospholipid bilayer restricts some substances from passing through. (2 marks)

- Water-soluble, polar, and large molecules cannot pass through easily.

- This is due to the hydrophobic fatty acid tails in the bilayer's interior.

Describe facilitated diffusion across cell membranes. (4 marks)

- Polar or charged molecules, and slightly larger substances, move down their concentration gradient.

- Movement occurs through specific channel proteins or carrier proteins.

- Process is passive and does not require ATP.

- Only the kinetic energy of the molecules is used.

Explain the role of channel and carrier proteins in facilitated diffusion. (4 marks)

- The shape or charge of the protein determines which substances can pass.

- Channel proteins provide a hydrophilic pore for water-soluble substances, and may be gated.

- Carrier proteins bind specific molecules, then change shape to transport them across.

- Both types enable diffusion of substances that cannot pass through the bilayer directly.

Describe how osmosis moves water across cell membranes. (4 marks)

- Water moves from a high water potential to a low water potential.

- Movement is down a water potential gradient.

- It passes through a partially permeable membrane.

- Process is passive and requires no ATP.

Define isotonic solutions and state what has the highest water potential. (2 marks)

- Isotonic solutions are solutions that have the same water potential.

- Pure water has the highest Ψ at 0 kPa.

Describe what happens when you put a plant cell in a dilute solution. (3 marks)

- Water enters the cell via osmosis as the water potential inside the cell is less than the water potential in the surrounding solution.

- Swells but does not burst due to the cell wall of the plant.

- The cell becomes turgid (swollen).

Describe what happens when you put a animal cell in a dilute solution. (2 marks)

- Water enters the cell via osmosis as the water potential inside the cell is less than the water potential in the surrounding solution.

- This causes the animal cell to burst.

Describe how movement across membranes occurs by active transport. (2 marks)

- Substances move from an area of lower concentration to an area of higher concentration, against a concentration gradient.

- This process requires ATP hydrolysis and specific carrier proteins.

State what factors decrease the rate of respiration and hence, the rate of active transport. (3 marks)

- A decrease in temperature.

- A lack of oxygen.

- Metabolic and respiratory inhibitors such as cyanide.

Explain the role of carrier proteins and ATP in active transport. (4 marks)

- A complementary substance binds to a specific carrier protein.

- ATP is hydrolysed to ADP and Pi, releasing energy.

- The carrier protein changes shape, releasing the substance on the opposite side.

- Pi is released and the protein returns to its original shape.

Describe how co-transport works. (2 marks)

- Two different substances bind to a co-transporter protein and are moved at the same time.

- One substance moves against its gradient, while the other moves down its gradient.

Describe the absorption of sodium ions and glucose in the ileum as an example of co-transport. (3 marks)

- Sodium ions are actively transported from epithelial cells into the blood, creating a sodium gradient.

- Sodium enters the epithelial cell from the lumen with glucose via a co-transporter protein.

- Glucose moves into the blood down its concentration gradient by facilitated diffusion.

Explain how surface area, protein numbers, and gradients affect transport rates. (4 marks)

- Increasing membrane surface area increases movement rate.

- More channel or carrier proteins increase facilitated diffusion or active transport rates.

- Steeper concentration gradients increase rates of simple or facilitated diffusion.

- A steeper water potential gradient increases osmosis rates.

Describe adaptations of specialised cells for transport. (3 marks)

- Folded membranes, such as microvilli, increase surface area.

- Higher numbers of protein channels or carriers speed up facilitated diffusion or active transport.

- Many mitochondria provide ATP for active transport.