Chapter 3: Movement into and out of cells

Question 1

Define diffusion.

Answer 1

Diffusion is the net (overall) movement of a substance from high concentration to low concentration (down the concentration gradient)

Question 2

How do dissolved substances enter the cells?

Answer 2

Dissolved substances such as gases and solutes must pass through the cell’s partially permeable membrane in order to enter or leave the cell?

Question 3

Where does the energy for diffusion come from?

Answer 3

It relies on the kinetic energy possessed by the movement of particles.

Diffusion requires no additional energy to occur

Question 4

Why is the diffusion of gases important?

Answer 4

Most living things require a reliable source of oxygen for respiration.

This moves into the organism by diffusion down a concentration gradient.

Carbon dioxide, produced during aerobic respiration is potentially toxic if it builds up.
This is then removed using diffusion

Photosynthetic plants need CO2 for making their food, which diffuses into the stomata and leaves into the air spaces in the mesophyll and reach the palisade cells.

Oxygen produced as well as water vapour diffuses out of the leaf in the same way.

Question 5

How do small animals obtain oxygen?

Answer 5

Small animals with a large surface area to volume ratio may obtain oxygen through their body surface.

Question 6

How do larger animal obtain oxygen?

Answer 6

They rely on gas exchange organs (e.g lungs or gills) which provide a large surface area for gas exchange and a circulatory system to transport the oxygen to all their cells.

Question 7

Why is the diffusion of solutes important?

Answer 7

Mineral ions in solution (e.g. nitrates and magnesium) are thought to diffuse across the tissues of plant roots.

However, most are absorbed by active transport.

Water soluble vitamins (e.g. Vitamin B & C ) are absorbed into the blood from the ileum by diffusion.

In the kidneys, some solutes in the renal capsule (e.g. urea and salts) pass into the bloodstream by diffusion.

Dialysis machines use diffusion to remove small solutes from the blood.

Question 8

List the factor affecting diffusion.

Answer 8

Temperature
Concentration gradient
Distance
Surface area

Question 9

How does temperature affect diffusion?

Answer 9

The higher the temperature, the increase in the amount of kinetic energy the particles possess

Question 10

How does concentration gradient affect diffusion?

Answer 10

A steeper gradient means a faster diffusion rate

Question 11

How does distance affect the rate of diffusion?

Answer 11

A shorter distance results in a faster diffusion rate

Question 12

How does surface area affect diffusion?

Answer 12

A greater surface area to volume ratio results in a faster diffusion rate

Question 13

Why is water important?

Answer 13

Dissolved substances are easily transported around organisms (e.g. dissolved food molecules in the blood)

Digested food molecules in the alimentary canal cannot be moved to all cells in the body without water.

Toxic substances and substances in excess of requirements dissolve in water so that they can be easily removed from the body in urine.

Water is an important part of the cytoplasm

Water ensures metabolic reactions can happen.

Question 14

Define osmosis.

Answer 14

The net movement of water molecules form a region of higher water potential to a region of lower water potential through a partially permeable membrane.

Question 15

How does water move into and out of cells?

Answer 15

By osmosis through the cell membrane

Question 16

How do you investigate osmosis using dialysis tubing?

Answer 16

A dialysis tubing containing sucrose solution is partly submerged in water and the liquid rises

When the dialysis tubing is half-full of water, the liquid containing sucrose solution falls.

When the dialysis tubing is full of water, there is no change in the liquid level (as both the tube and its surroundings are water)

Question 17

When is a plant cell said to be turgid?

Answer 17

A plant cell with the vacuole pushing out on the cell wall is said to be turgid as it is exerting turgor pressure on the inelastic cell wall

Question 18

When do plant cells become flaccid?

Answer 18

When the vacuoles lose water for any reason, the cells will lose their turgor and become flaccid

Question 19

When does plasmolysis occur?

Answer 19

When there is too little water in cells.

Question 20

What happens when too much fertiliser is used?

Answer 20

Too much fertiliser can lower the osmotic potential of the soil water.

This causes water to be drawn out of the plant root hair cells by osmosis, leading to wilting and crop death.

Question 21

What happens if their is too much build up of salt in the soil?

Answer 21

The slats causes the soil water to have a lower water potential than the plant root hair cells.

Crops can no longer be grown on the land as they wilt and die due to water loss by osmosis.

Question 22

What happens when too much water enters animal cells?

Answer 22

Lysis

The cell will burst as it does not have a cell wall.

Question 23

What happens when the concentration of water on the outside is the same as that of inside?

Answer 23

The cell remains the same.

Question 24

What happens if the water concentration is lower on the outside of the animal cells?

Answer 24

The become shrivelled.

This is because water moves out of the cell by osmosis.

Question 25

What is diarrhoea?

Answer 25

The loss of watery faeces.

Question 26

How is diarrhoea caused?

Answer 26

When water cannot be absorbed form the contents of the large intestine or when extra water is secreted into the large intestine due to a viral or bacterial infection.

Question 27

How is diarrhoea treated?

Answer 27

Oral rehydration therapy

Question 28

Define active transport.

Answer 28

The movement of water through a cell membrane from a region of lower concentration to a region of higher concentration (against the concentration gradient)

Question 29

Where does the energy for active transport come from?

Answer 29

ATP generated through cellular respiration.

Question 30

State the role of protein carriers during active transport.

Answer 30

They move molecules or ions across a membrane