Cell Membrane

Question 1

What is the function of the cell-surface (plasma) membrane, and how does it control movement of substances?

Answer 1

Cell-surface membranes surround cells and form a barrier between the cell and its environment. They are partially permeable – control what enters/exits via simple diffusion, facilitated diffusion, osmosis, and active transport.

Question 2

What are the roles of membranes within cells, and give two examples?

Answer 2

Membranes within cells surround organelles and compartmentalise functions. They are also partially permeable.
Example: Mitochondria – keep respiration enzymes together.
Example: Nucleus – RNA exits via nuclear envelope; DNA too large to pass.

Question 3

Describe the structure of membranes according to the fluid mosaic model.

Answer 3

The membrane contains phospholipids (form bilayer), proteins (channel, carrier, receptor), carbohydrates (glycoproteins, glycolipids), and cholesterol. It is fluid as phospholipids move laterally and mosaic as proteins are scattered throughout.

Question 4

What is the function of glycoproteins, glycolipids, and receptor proteins in membranes?

Answer 4

Glycoproteins and glycolipids are for cell signalling and recognition. Receptor proteins bind signalling molecules (e.g. insulin).

Question 5

Describe how phospholipids are arranged and their role in membrane function.

Answer 5

Phospholipids have hydrophilic heads facing outwards and hydrophobic tails inwards. They form a bilayer that acts as a barrier to water-soluble substances. They allow small, non-polar molecules (e.g. CO₂, O₂) and water to diffuse through.

Question 6

What is the role of cholesterol in the membrane?

Answer 6

Cholesterol is found in all membranes (not bacteria). It binds to phospholipid tails, increasing rigidity and reducing fluidity/permeability. It helps maintain cell shape (especially in animal cells like RBCs). Its hydrophobic regions further restrict polar substances.

Question 7

Describe how different temperature ranges affect membrane permeability.

Answer 7

How temperature affects membrane permeability:
• Below 0°C:
• Phospholipids have little kinetic energy and are packed closely, making the membrane rigid.
• However, channel and carrier proteins may denature, disrupting transport.
• Ice crystals can form, piercing the membrane, causing physical damage and increasing permeability when the cell thaws.
• 0–45°C:
• Phospholipids gain kinetic energy, increasing their movement and making the membrane more fluid.
• As temperature increases, permeability increases gradually due to the loosening of the membrane structure.
• Proteins remain functional in this range.
• Above 45°C:
• The phospholipid bilayer becomes increasingly fluid and may begin to melt (disintegrate).
• Transport and structural proteins denature, leading to loss of membrane function.
• This results in very high permeability, as the membrane becomes disordered.
• Internal pressure may increase due to water and solute influx.

Question 8

Describe how the beetroot experiment is used to investigate membrane permeability.

Answer 8

Cut equal beetroot cubes → rinse pigment. Incubate in water baths at different temperatures. Measure pigment leakage via colorimeter (absorbance at 470 nm). Higher absorbance = higher permeability.

Question 9

How does solvent concentration affect membrane permeability?

Answer 9

Solvent concentration affects membrane permeability because solvents like ethanol and acetone dissolve lipids in the phospholipid bilayer. As the concentration of the solvent increases, there is more disruption to the membrane structure, making it more permeable

Question 10

Define simple diffusion and state three factors affecting its rate.

Answer 10

Simple diffusion is the passive movement of particles from a region of high concentration to a region of low concentration, down their concentration gradient, without the use of energy (ATP).

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Factors affecting rate of simple diffusion:
1. Concentration gradient – A steeper gradient increases the rate of diffusion, as there’s a greater difference in particle concentration.
2. Thickness of the exchange surface – A thinner surface (shorter diffusion distance) increases the rate of diffusion.
3. Surface area – A larger surface area allows more particles to diffuse at once, increasing the rate (e.g. microvilli increase surface area in epithelial cells of the small intestine).

Question 11

What type of particles require facilitated diffusion, and what proteins are involved?

Answer 11

Used by large or charged/polar particles (e.g. glucose, ions). Uses channel proteins (pores for ions) and carrier proteins (change shape for specific molecules).

Question 12

State two factors that affect the rate of facilitated diffusion.

Answer 12

1. Concentration gradient
   
   2. Number of channel/carrier proteins (rate plateaus when saturated)

Question 13

Give two examples of facilitated diffusion in cells.

Answer 13

• GLUT1 proteins for glucose in RBCs
• Aquaporins: channel proteins for water (found in kidney cells)

Question 14

How do you calculate the rate of diffusion from different types of graphs?

Answer 14

For straight-line graphs: calculate gradient (Δy / Δx). For curved graphs: draw a tangent at the point, then calculate gradient.

Question 15

Define osmosis and explain water potential.

Answer 15

Osmosis is the diffusion of water across a partially permeable membrane from a region of higher water potential to a region of lower water potential.

Water potential (Ψ) is the tendency of water to move and is measured in kilopascals (kPa). Pure water has the highest possible water potential, which is 0. Adding solutes lowers the water potential, making it more negative. During osmosis, the net movement is of water, not solutes.

Question 16

Define isotonic, hypotonic, and hypertonic.

Answer 16

• Isotonic: equal Ψ → no net water movement
• Hypotonic: external Ψ higher → water enters → cell swells
• Hypertonic: external Ψ lower → water leaves → cell shrinks

Question 17

What are the three main factors affecting the rate of osmosis?

Answer 17

1. Water potential gradient
   
   2. Exchange surface thickness
   3. Surface area

Question 18

How do you make a serial dilution from a 2 M sucrose solution?

Answer 18

1. Label five test tubes.
   
   2. Add 10 cm³ of 2 M sucrose to the first tube.
   3. Add 5 cm³ of distilled water to each of the remaining four tubes.
   4. Transfer 5 cm³ from the first tube to the second; mix thoroughly → 1 M.
   5. Repeat to get:
      • 3rd tube: 0.5 M
      • 4th tube: 0.25 M
      • 5th tube: 0.125 M

Question 19

How do you make 15 cm³ of 0.4 M sucrose from 1 M?

Answer 19

Scale factor = 1 ÷ 0.4 = 2.5
Use 15 ÷ 2.5 = 6 cm³ of 1 M
Add 9 cm³ distilled water (15 - 6)

Question 20

Describe how to measure the change in mass of plant tissue in osmosis.

Answer 20

1. Use a cork borer to cut equal-sized potato cylinders (~1 cm diameter).
   
   2. Weigh 3 chips per group using a mass balance.
   3. Place each group in different sucrose concentrations for at least 20 minutes.
   4. Remove, pat dry, and reweigh.
   5. Calculate % change in mass for each concentration:

Percentage change = ((Final mass − Initial mass) ÷ Initial mass) × 100

Question 21

How do you interpret the change in mass in this experiment?

Answer 21

• Gain mass → water entered → solution has higher Ψ than cells
• Lose mass → water left cells → solution has lower Ψ than cells
• No change → solution Ψ = cell Ψ

Question 22

How is a calibration curve used to find the water potential of plant tissue?

Answer 22

1. Plot % change in mass (y-axis) vs. sucrose concentration (M) (x-axis).
   
   2. Curve should cross x-axis where % change = 0.
   3. The sucrose concentration at this point = Ψ of the potato tissue.
   4. Use a reference table or textbook to find the actual water potential (Ψ) of that sucrose concentration.

Question 23

What is active transport and what does it require?

Answer 23

Active transport uses ATP to move substances against their concentration gradient. Involves carrier proteins that change shape when ATP binds to the carrier proteins and hydrolyses (ATP → ADP + Pi) to provide energy.

Question 24

What is co-transport and how does it function?

Answer 24

Co-transporters are carrier proteins that bind two molecules simultaneously. The movement of one down its gradient drives the other against its gradient.

Question 25

What is co-transport and how does it function?

Answer 25

Co-transporters are carrier proteins that bind two molecules simultaneously. The movement of one down its gradient drives the other against its gradient.

Question 26

Explain glucose absorption in the ileum via co-transport.

Answer 26

In the ileum, sodium ions (Na⁺) are actively transported out of epithelial cells into the blood using the sodium-potassium pump. This creates a concentration gradient, with a lower concentration of Na⁺ inside the cell than in the lumen. Sodium ions then diffuse from the lumen of the ileum into the epithelial cell via a sodium-glucose co-transporter protein. As sodium moves in, glucose is co-transported into the cell against its concentration gradient. Finally, glucose moves into the blood from the epithelial cell by facilitated diffusion through a channel protein. 1. Na⁺ actively transported out of epithelial cells into blood (via Na⁺/K⁺ pump) → creates a Na⁺ gradient. 2. Na⁺ diffuses from lumen into epithelial cell via Na⁺-glucose co-transporter. 3. Glucose is dragged in with Na⁺ (against its gradient). 4. Glucose diffuses into blood via facilitated diffusion through protein channels.

Question 27

What factors affect the rate of active transport?

Answer 27

• Speed of carrier proteins • Number of carrier proteins • Rate of respiration (availability of ATP) If respiration is inhibited, active transport cannot occur.

Question 28

Complete the transport comparison table: Diffusion Facilitated Diffusion Osmosis Active Transport Co-Transport

Answer 28

Diffusion-Net movement from high to low concentration. No energy required Facilitated Diffusion-Uses carrier/channel proteins for large/polar molecules. No energy required but energy required for proteins Osmosis-Water from high to low water potential across a partially permeable membrane. No energy required, energy required for proteins Active Transport-Movement from low to high concentration using ATP and carrier proteins. Energy is required and for proteins aswell Co-Transport-One molecule moves down gradient to pull another against its gradient. Energy is required for proteins aswell

Question 29

What calculations and skills should you master for transport questions in exams?

Answer 29

• Know how to calculate percentage change in mass. • Understand that active transport is the only transport needing energy. • Be able to explain glucose absorption using co-transport. • Know how to construct and interpret a calibration curve to determine Ψ. • Learn and apply the water potential of sucrose values from a data source (e.g. textbook table).

Question 30

How do channel and carrier proteins differ in facilitated diffusion?

Answer 30

• Carrier proteins: bind specific large molecules (e.g. glucose), change shape to transport. • Channel proteins: form pores for charged particles (e.g. ions).