Cell Structure And Membranes

Question 1

What molecules can pass through the plasma membrane?

Answer 1

Lipid soluble substances (e.g. some hormones) and very small molecules eg O and CO

Question 2

Molecules that cannot pass through the membrane

Answer 2

• Water soluble (polar) substances (eg sodium ions) and large molecules (glucose)

Question 3

Simple diffusion

Answer 3

net movement of molecules from an area of high concentration to an area of low concentration until equilibrium is reached. It is a passive process as it requires no additional energy.

Where lipid soluble and uncharged molecules can pass straight through the phospholipid bilayer. These molecules are small and non polar eg O

Question 4

Facilitated diffusion -

Answer 4

This is a passive process but it differs from simple diffusion as proteins are used to transport molecules.

The movement of ions and polar molecules, which cannot simply diffuse, can be transported across membranes by facilitated diffusion using protein channels and carrier proteins.

Question 5

Channel protiens role in facilliated diffusion

Answer 5

Protein channels form tubes filled with water and this enables water-soluble ions to pass through the membrane. This is still selective, as the channel proteins only open in the presences of certain ions when they bind to the protein.

Question 6

Carrier protiens role in faciliated diffusion

Answer 6

Carrier proteins will bind with a molecule, such as glucose, which causes a change in the shape of the protein. This shape change enables the molecule to be released to the other side of the membrane.

Question 7

What is osmosis

Answer 7

Osmosis is the movement of water from an area of higher water potentials to an area of lower water potential across a partially permeable membrane.

Question 8

What is water potential

Answer 8

Water potential is the pressure created by water molecules and is measured in kPa and represented with the symbol . Pure water has a water potential of zero, so when solutes are dissolved in water the water potential will become negative. The more negative the water potential, the more solute must be dissolved in it.

Question 9

What is an isotonic solution

Answer 9

An isotonic solution is when the water potential is the same in the solution and the cell within the solution.

Question 10

What is a hypotonic solution

Answer 10

Hypotonic is when the water potential of a solution is more positive (closer to zero) than the cell.

Question 11

What is a hypertonic solution

Answer 11

Hypertonic is when the water potential of a solution is more negative than the cell.

Question 12

Animal and plant cells in hypotonic and hypertonic

Answer 12

In animal cells, if they are placed in a hypotonic solution such as pure water, a lot of water will move into the cell by osmosis.

Both animal and plant cells will shrink and become shrivelled if they are placed in hypertonic solutions, due to large volumes of water leaving the cell by osmosis.

Question 13

What is active transport

Answer 13

The movement of molecules and ions from an area of lower concentration to an area of higher concentration (against the concentration gradient) using ATP and carrier proteins.

Question 14

What do the carrier protiens do in active transport

Answer 14

The carrier proteins act as pumps to move substances across the membrane. This is very selective, as only certain molecules can bind to the carrier proteins to be pumped.

Question 15

How does active transport differ from facilliated diffusion

Answer 15

Although facilitated diffusion also uses carrier protein, active transport is different as it requires energy

Question 16

Energy in active transport

Answer 16

The energy is required to make the carrier protein change shape, allowing it to transfer the molecules or ions across the cell membrane

The energy required is provided by ATP (adenosine triphosphate) produced during respiration. The ATP is hydrolysed to release energy

Question 17

What is active transport important in

Answer 17

1. Reabsorption of useful molecules and ions into the blood after filtration into the kidney tubules
2. Absorption of some products of digestion from the digestive tract
3. Loading sugar from the photosynthesising cells of leaves into the phloem tissue for transport around the plant
4. Loading inorganic ions from the soil into root hairs

Question 18

What is co-transport

Answer 18

Co-transport is the coupled movement of substances across a cell membrane via a carrier protein

This means that two types of molecule are moved across the membrane at the same time; the movement of one is dependent on the movement of the other

Question 19

What is the specific example of co transport

Answer 19

This specific co-transport protein is involved in the absorption of glucose and sodium ions as follows:

The active transport of sodium ions from the epithelial cell into the blood lowers the sodium ion concentration inside the cell and generates a sodium ion concentration gradient between the ileum and the epithelial cell

Question 20

Facilliated diffusion in active transport

Answer 20

Sodium ions move into the cell from the ileum by facilitated diffusion, carrying glucose molecules along with them via a cotransport protein

The glucose concentration inside the epithelial cell increases, and glucose molecules enter the blood via facilitated diffusion

Question 21

What is plasmolysis

Answer 21

f a plant cell is placed in a solution with a lower water potential than the plant cell (such as a concentrated sucrose solution), water will leave the plant cell through its partially permeable cell surface membrane by osmosis

As water leaves the vacuole of the plant cell, the volume of the plant cell decreases
The protoplast gradually shrinks and no longer exerts pressure on the cell wall

As the protoplast continues to shrink, it begins to pull away from the cell wall
This process is known as plasmolysis – the plant cell is plasmolysed

Question 22

What happens when a plant cell is placed in pure water or a dilute solution,

Answer 22

as the pure water or dilute solution has a higher water potential than the plant cell

the volume of the plant cell increases

The expanding protoplast (living part of the cell inside the cell wall) pushes against the cell wall and pressure builds up inside the cell – the inelastic cell wall prevents the cell from bursting

The pressure created by the cell wall also stops too much water entering a

a plant cell has become rigid and firm - turgid

If plants do not receive enough water the cells cannot remain rigid and firm (turgid) and the plant wilts