Transport across cell membranes Flashcards – AQA A-Level Biology - (7402)

What basic structure do cell-surface membranes and eukaryotic organelle membranes share?

A phospholipid bilayer containing proteins.

How are phospholipid heads and tails oriented in a membrane bilayer?

The hydrophilic heads face the aqueous environments on either side; the hydrophobic tails face inward.

What makes a membrane fluid in the fluid-mosaic model?

Phospholipids and some proteins can move laterally within the layer.

What gives a membrane its mosaic appearance?

The mixture of proteins within the bilayer.

What are the two ways proteins can be positioned in a membrane?

They can extend across the bilayer or be associated with one surface.

What is the difference between a glycoprotein and a glycolipid?

A glycoprotein is a protein with a carbohydrate chain; a glycolipid is a lipid with a carbohydrate chain.

Where do the carbohydrate portions of membrane glycoproteins and glycolipids project?

From the membrane surface.

How does cholesterol affect membrane fluidity?

It fits between phospholipids and restricts the movement of other membrane molecules.

What is simple diffusion across a membrane?

The net movement of particles down their concentration gradient, directly through the membrane without a transport protein or ATP input.

How does random particle movement relate to net movement during simple diffusion?

Particles move randomly in both directions, but net movement is down the concentration gradient until concentrations are equal.

Which substances cross the hydrophobic core of a phospholipid bilayer readily, and which do not?

Small non-polar or lipid-soluble molecules cross relatively readily; charged particles and large polar molecules do not.

What is facilitated diffusion?

The passive net movement of substances down their concentration gradient through membrane proteins, without ATP.

What does facilitated diffusion enable substances to cross that they cannot readily cross on their own?

The hydrophobic phospholipid bilayer of a membrane.

How do channel proteins enable particles to cross a membrane?

They provide hydrophilic pathways through which ions or other particles can pass.

How do carrier proteins move substances across a membrane?

They bind substances and change shape, releasing them on the other side.

What do channel and carrier proteins have in common in terms of which substances they transport?

Both are selective for particular substances.

What is osmosis?

The net movement of water molecules across a partially permeable membrane from higher to lower water potential.

During osmosis, how does water movement in both directions relate to net movement?

Water moves in both directions, but net movement continues until equilibrium is reached.

What does water potential describe?

The tendency of water to move from one region to another.

Which two factors affect water potential?

Solute concentration and pressure.

How does adding solute affect water potential?

It lowers water potential.

How does the water potential of pure water at atmospheric pressure compare with that of solutions?

Pure water has a water potential of zero; solutions have lower water potentials.

What is active transport?

The movement of substances across a membrane against their concentration gradient, from lower to higher concentration.

How do carrier proteins move substances across a membrane during active transport?

They bind specific substances and change shape to transfer them across the membrane.

What supplies energy for carrier-mediated active transport?

ATP hydrolysis.

What is co-transport?

The coupled movement of two substances through the same carrier protein.

How do sodium ions and glucose enter cells lining the mammalian ileum from the intestinal lumen?

They enter together through a sodium-glucose co-transporter.

What drives glucose uptake by the sodium-glucose co-transporter in ileum cells?

Sodium moves down its electrochemical gradient, driving glucose entry against its concentration gradient.

How is the sodium gradient maintained in cells lining the ileum?

Sodium-potassium pumps in the membrane facing the blood use energy from ATP hydrolysis to move sodium ions out of the cell.

How does glucose leave an ileum cell towards the blood?

By facilitated diffusion through carrier proteins.

Why is ATP used indirectly in glucose uptake by ileum cells?

ATP powers the sodium-potassium pumps that maintain the sodium gradient driving glucose co-transport.

Why can a larger membrane surface area increase the rate of transport?

It provides more membrane through which transport can occur.

How can specialised cells increase the membrane surface area available for transport?

By having folded external or internal membranes.

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

A steeper gradient increases the net rate; as the gradient decreases, net diffusion slows.

How does the water potential gradient affect the net rate of osmosis?

A steeper gradient increases the net rate; as the gradient decreases, net osmosis slows.

How can increasing the number of suitable channels or carriers affect protein-mediated transport?

It can increase the transport rate, provided other factors are not limiting.

What limits the rate of carrier-mediated transport?

The number and turnover of available carriers.

How is a dilution series prepared for investigating plant tissue water potential?

Mix measured volumes of a stock solution and water to produce solutions of known solute concentrations.

How are mass measurements taken from plant tissue in a dilution series?

Measure each piece's initial and final mass, blotting consistently before weighing.

How is the percentage change in mass of a plant tissue piece calculated?

$\frac{\text{final mass} - \text{initial mass}}{\text{initial mass}} \times 100\%$

How are repeats used to improve the reliability of mass-change measurements?

Use repeated measurements to calculate a mean percentage change in mass for each concentration.

What is plotted to produce a calibration curve for plant tissue water potential?

Mean percentage change in mass against solute concentration.

What does the zero mass-change crossing of a plant tissue calibration curve indicate?

The concentration of a solution in which there is no net water movement into or out of the tissue.

How is plant tissue water potential estimated from the zero mass-change concentration?

Find the solution's water potential from its concentration under the relevant conditions; this estimates the tissue's water potential.

Which four variables should be controlled when measuring plant tissue mass changes in solutions?

Tissue size, solution volume, temperature and immersion time.

How can beetroot tissue be used to investigate the effect of temperature on membrane permeability?

Place comparable, rinsed pieces in water at different controlled temperatures for the same time, then measure the pigment released.

Why are beetroot pieces rinsed after cutting in a membrane permeability investigation?

To remove pigment released by cutting.

In a beetroot temperature investigation, what are the independent variable and the dependent measurement?

Independent variable: temperature.
Dependent measurement: pigment release.

Which three conditions should be kept consistent in a beetroot temperature investigation?

Tissue size, solution volume and treatment time. Other conditions should also be kept consistent.

How should measurements be repeated in a beetroot temperature investigation?

Take repeat measurements at each temperature.

What does greater colorimeter absorbance indicate in a beetroot pigment solution?

More pigment in the solution and therefore greater leakage from the tissue.

What does increased pigment leakage from beetroot tissue indicate about membrane permeability?

Membrane permeability has increased.

What is a limitation of using beetroot pigment leakage to assess the permeability of one membrane?

The measurement reflects pigment passing through the relevant cell membranes, not direct observation of one membrane alone.