Diffusion

Question 1

Define diffusion

Answer 1

Diffusion is the spreading out of particles resulting in a net movement from an area of of higher concentration to an area of lower concentration

net means overall

Question 2

State some of the substances that are transported in and out of cells by diffusion

Answer 2

Oxygen
Carbon dioxide
Urea

Question 3

Describe how oxygen moves into the body’s cells using diffusion

Answer 3

Cells need oxygen for respiration, which is carried out by mitochondria

Cells are surrounded by a high concentration of oxygen. This is because oxygen is transported in the bloodstream from the lungs.

Therefore the oxygen molecules move into the cell by diffusion from an area of higher concentration to an area of lower concentration

Question 4

The oxygen is used to generate energy in respiration and this produces the waste gas _________

Answer 4

The oxygen is used to generate energy in respiration and this produces the waste gas carbon dioxide

Question 5

Describe how carbon dioxide moves out of the body’s cells by diffusion

Answer 5

When the oxygen is used to generate energy in respiration, this produces the waste gas carbon dioxide.

This means that there is a higher concentration of carbon dioxide inside the cell than outside

Therefore the carbon dioxide molecules move out of the cell by diffusion from an area of higher concentration to an area of lower concentration

Question 6

Urea is another molecule that diffuses out of _____ as well as CO2

Answer 6

Urea is another molecule that diffuses out of cells as well as CO2

Question 7

Describe how urea moves out of the body’s cells by diffusion

Answer 7

Urea is a waste product produced inside cells.

This means that there is a higher concentration of urea inside the cell than outside

Therefore the urea move out of the cells by diffusion (from an area of higher concentration to an area of lower concentration)
into the blood plasma and is excreted by the kidneys

Question 8

What is urea

Answer 8

Urea is the waste product formed by the breakdown of excess amino acids in the liver

Question 9

State the factors which affect the rate of diffusion

Answer 9

Factors which affect the rate of diffusion are:

the difference in concentrations (concentration gradient)

• the temperature
• the surface area of the membrane.

Question 10

How to calculate the net movement of particles

Answer 10

net movement = particles moving in - particles moving out

Question 11

What is the concentration gradient

Answer 11

The concentration gradient is the difference in concentration

Question 12

Explain how the concentration gradient affects the rate of diffusion

Answer 12

The greater the concentration gradient, the faster diffusion takes place.

Question 13

Explain how temperature affects the rate of diffusion

Answer 13

The higher the temperature, the greater the rate of diffusion.

This is because the particles have more kinetic energy and are moving faster.
This speeds up diffusion as particles collide more often and harder and spread out faster

Question 14

Explain how the surface area of the cell membrane affects the rate of diffusion

Answer 14

The larger the surface area of the cell membrane, the greater the rate of diffusion.

Question 15

Explain why so many cells have folded membranes along at least one surface

Answer 15

Folded membranes provide increased surface area.
The greater the surface area, the more diffusion of dissolved
substances can take place across it.

Question 16

Use the idea of surface area to volume ratio to explain why multicellular organisms require exchange surfaces and a transport system

Answer 16

Multicellular organisms (organisms with more than one cell) have a low surface area to volume ratio.
Their surface area is not large enough for their volume

Cells on the surface can get enough oxygen simply by diffusion however not enough oxygen can diffuse into the cells in the centre of the organism; They are too far away from the surface.

This means that they need some sort of exchange surface and transport system for efficient diffusion

Question 17

What types of organisms have a huge surface area for their volume

Answer 17

Single-celled organisms have a huge surface area for their volume. ( a huge surface area to volume ratio)

Question 18

Explain the advantages that a single-celled organism has in terms of surface area to volume ratio

Answer 18

A single-celled organism has a relatively large surface area to volume
ratio. This allows sufficient transport of molecules into and out of the cell
to meet the needs of the organism.

Question 19

How to calculate the surface area to volume ratio of this organism

https://bam.files.bbci.co.uk/bam/live/content/zwf6y4j/large

Answer 19

Surface area = 6 x 1cm x 1cm = 6cm^2

Volume = 1cm x 1cm x 1cm = 1cm^3

Surface area to volume ratio = 6:1

Question 20

Calculate the surface area to volume ratio of this organism
Cube with sides of 2cm

Answer 20

https://bam.files.bbci.co.uk/bam/live/content/z9ytv9q/small

Surface area = 6 x 2cm x 2cm = 24cm^2

Volume = 2cm x 2cm x 2cm = 8cm^3

Surface area to volume ratio = 24: 8 = 3:1

Question 21

Relationship between the size of organisms and the surface area to volume ratio

Answer 21

As the organisms get larger, the surface area to volume ratio decreases

Question 22

Why is having a low surface area to volume ratio a problem for multicellular organisms

Answer 22

Multicellular organisms (organisms with more than one cell) have a low surface area to volume ratio.
Their surface area is not large enough for their volume

Cells on the surface can get enough oxygen simply by diffusion however not enough oxygen can diffuse into the cells in the centre of the organism; They are too far away from the surface.

Question 23

Organism B exchanges gases with the environment directly through its skin
.
Organism D exchanges gases with the environment using its respiratory system.

Explain why organism D requires a respiratory system, but organism B does not
require a respiratory system.

Answer 23

D has a smaller surface area to volume ratio (than B)

(so) diffusion distance is too large (to meet demands of cells/ organism)

Question 24

State how animals have solved the problem of low surface area to volume ratio which results in some cells not receiving enough oxygen

Use the idea of surface area to volume ratio to explain why multicellular organisms require exchange surfaces and a transport system

Answer 24

Multicellular organisms (organisms with more than one cell) have a low surface area to volume ratio.
Their surface area is not large enough for their volume

Cells on the surface can get enough oxygen simply by diffusion however not enough oxygen can diffuse into the cells in the centre of the organism; They are too far away from the surface.

Therefore these organisms have got special structures for gas exchange with a very high surface area e.g. lungs in mammals and have got a transport systems to carry gases around the body

Question 25

How can the effectiveness of an exchange surface be increased

Answer 25

The effectiveness of an exchange surface is increased by:

• having a large surface area
• a membrane that is thin, to provide a short diffusion path
• (in animals) having an efficient blood supply
• (in animals, for gaseous exchange) being ventilated.

Question 26

Explain why surfaces and organ systems are specialised for exchanging materials

Answer 26

In multicellular organisms, surfaces and organ systems are specialised
for exchanging materials. This is to allow sufficient molecules to
be transported into and out of cells for the organism’s needs.

Question 27

Explain how the small intestine is adapted for exchanging materials

Answer 27

The inside of the small intestine is covered in millions of tiny projections called villi
They increase the surface area so that the digested food is absorbed much more quickly into the blood

Villi have a single layer of surface cells(membrane is thine to provide a short diffusion pathway)
Villi have a very good blood supply to assist quick absorption

Question 28

Explain how the lungs are adapted for exchanging materials

Answer 28

In order to transfer oxygen to the blood and remove waste carbon dioxide from it, the lungs contain millions of little air sacs called alveoli where gas exchange takes place.

The alveoli are specialised to maximise the diffusion of oxygen and carbon dioxide. They have an enormous surface area for exchange of gases. Moist lining for dissolving gases. Very thin walls to provide a short diffusion pathway and a good blood supply.

Question 29

State the function of the lungs

Answer 29

The lungs transfer oxygen to the blood and remove waste carbon dioxide from the blood

Question 30

Explain how gills in fish are adapted for exchanging materials

Answer 30

The gill filaments give the gives the gills a big surface area for exchange of gases

The filaments have a thin membrane to provide a short diffusion pathway

The filaments have an efficient blood supply to take the oxygenated blood away. This ensure that the concentration gradient is always high

All these adaptations make diffusion as efficient as possible

Question 31

Explain how gases are exchanged in fish

Answer 31

The oxygen-rich water enters the fish through the mouth and passes out through the gills
As this happens, oxygen diffuses from the water into the blood in the gils and carbon dioxide diffuses from the blood into the wall

Question 32

What are gills made up of

Answer 32

Each gill is made up of lots of thin plates called gill filaments

Question 33

Explain how leaves in plants are adapted for exchanging materials

Answer 33

The underneath of the leaf is an exchange surface. It is covered in little holes called stomata which the carbon dioxide diffuses though
Oxygen (produced in photosynthesis) and water vapour also diffuse out through the stomata.

The size of the stomata is controlled by guard cells. These close the stomata if the plant is losing water faster than t is being replaced by the roots. Without these guard cells the plant would soon lint

The flattened shape of the leaf increases the area of this exchange surface so that it is more effective

The walls of the cells inside the leaf form another exchange surface. The air spaces inside the leaf increase the area of this surface so there’s more chance for carbon dioxide to get into the cells