Unit 1: Transport Across Cell Membranes From Achieve

Question 1

Name the structures within a cell membrane

Answer 1

Proteins, phospholipids

Question 2

Explain why the cell membrane is selectively permeable

Answer 2

This means that some substances freely cross the cell membrane in or out of the cell, whilst other substances cannot.

Question 3

Explain why different concentrations of substances exist between cells and their environments

Answer 3

Since the cell membrane is selectively permeable, this means some substances can pass across whilst others cannot. This can create differences in concentrations of substances on one side of the membrane compared to another. This creates a CONCENTRATION GRADIENT.

Substances can move from a HIGH concentration to a LOW concentration; this movement is DOWN a concentration gradient.

Substances can move from an area of LOW concentration to a HIGH concentration, this is AGAINST the concentration gradient.

If there is no difference between the substance on one side compared to the other, then a concentration gradient will NOT exist.

Question 4

Describe passive transport using diffusion as an example

Answer 4

Passive transport involves the movement of molecules from HIGH concentration to a LOW concentration DOWN a concentration gradient. This process does NOT require energy.

Diffusion is the movement of molecules from an area of high concentration to an area of low concentration down a concentration gradient. This process does not require energy, so diffusion is an example of passive transport.

Question 5

Explain the importance of the diffusion of oxygen, glucose, and carbon dioxide in terms of their concentration gradients

Answer 5

Glucose, oxygen and amino acids all move into the cell because the concentration outside the cell is higher than inside. By diffusion, these molecules enter the cell.

Carbon dioxide and waste all leave the cell, because the concentration is higher inside the cell than outside the cell. These leave the cell by diffusion.

Diffusion is important as it allows living cells to gain oxygen and glucose but also allows carbon dioxide and waste to leave the cell.

Question 6

Describe Osmosis as an example of passive diffusion involving water

Answer 6

Osmosis is the movement of water from an area of high water concentration to an area of low water concentration down a water concentration gradient across a selectively permeable membrane.

Question 7

Describe and explain the effects on animal cells when placed in solutions of different water concentrations

Answer 7

When a red blood cell is placed in a solution with a higher water concentration, it bursts.

When a red blood cell is placed in a solution with a lower water concentration, it shrinks.

When a red blood cell is placed in a solution with equal water concentration, it remains the same.

Question 8

Describe and explain the effects on plant cells when placed in solutions of different water concentrations

Answer 8

When a plant cell is placed in a solution with a lower water concentration, it becomes plasmolysed.

When a plant cell is placed in solution with a higher water concentration, it becomes turgid.

When a plant cell is placed in a solution with an equal water concentration, it stays the same.

Question 9

Describe active transport

Answer 9

Active transport is the movement of molecules from an area of LOW concentration to an area of HIGH concentration against the concentration gradient.

This process requires energy.

Example
Sodium/potassium pump.