3A - Size and surface area

Question 1

What is the environment around the cells of multicellular organisms made of?

Answer 1

Tissue fluid

Question 2

What 2 environments do organisms have to transfer materials between?

Answer 2

The external and internal environments found within an organism and its cells.

Question 3

What is it hard for cells to do if they are too far away from exchange surfaces?

Answer 3

Too far away for diffusion alone to supply or remove their tissue fluid with various materials needed to keep its composition relatively constant.

Question 4

Because cells are too far away from exchange surfaces for diffusion alone to occur, what happens after materials are absorbed?

Answer 4

They are rapidly distributed to the tissue fluid and the waste products returned to the exchange surface for removal.

Question 5

What does the exchange of materials in cells involve the use of?

Answer 5

A mass transport system.

Question 6

What does a mass transport system do?

Answer 6

Maintains the diffusion gradients that bring materials to and from the cell-surface membranes.

Question 7

What will affect the amount of each material that is exchanged in an organism?

Answer 7

The size and metabolic rate of an organism.

Question 8

What will the size and metabolic rate of an organism affect?

Answer 8

The amount of each material exchanged in the organism.

Question 9

What is the link between the metabolic rate of an organism and how much material is exchanged?

Answer 9

High metabolic rate = more materials exchanged.

Question 10

What do organisms with a high metabolic rate and that exchange more materials need?

Answer 10

A large surface area to volume ratio.

Question 11

What things need to be exchanged between an organism and its environment?

Answer 11

Respiratory gases (oxygen and carbon dioxide).
Nutrients (glucose, fatty acids, amino acids, vitamins and minerals).
Excretory products (urea and carbon dioxide).
Heat

Question 12

In what 2 ways does exchange happen (apart from the exchange of heat)?

Answer 12

Passively (no metabolite is required, by diffusion and osmosis).

Actively (metabolic energy is required, by active transport).

Question 13

What does it mean if exchange happens passively?

Answer 13

No metabolic energy is required - by diffusion and osmosis.

Question 14

Through what processes does exchange happen passively?

Answer 14

Diffusion and osmosis.

Question 15

What does it mean if exchange happens actively?

Answer 15

Metabolic energy is required - by active transport.

Question 16

Through what process does exchange happen actively?

Answer 16

Active transport.

Question 17

What happens to simple diffusion as organisms become larger?

Answer 17

Simple diffusion of substances across the outer surface can only meet the needs of relatively inactive organisms.

Question 18

How do you work out ratio of surface area to volume?

Answer 18

Area/Volume.

Question 19

In a cell, where is the lowest concentration of oxygen?

Answer 19

Mitochondria because it is used up in respiration.

Question 20

In a cell, where is the highest concentration of carbon dioxide?

Answer 20

Mitochondria as it is produced in respiration.

Question 21

What does the fact that mitochondria have the lowest concentration of oxygen but the highest concentration of carbon dioxide mean?

Answer 21

The diffusion gradient for these gases is maintained in and out of the cell.

Question 22

What features have organisms evolved?

Answer 22

E.g. flattened shape so that no cell is ever far from the surface. Specialised exchange surfaces with large areas to increase the SA:V ratio.

Question 23

What are the features of specialised exchange surfaces?

Answer 23

• Large SA relative to the V of the organism which increases the rate of exchange.
• Very thin so that the diffusion distance is short and therefore materials cross the exchange surface rapidly.
• Selectively permeable to allow selected materials to cross.
• Movement of the environmental medium, for example, air, to maintain a diffusion gradient.
• A transport system to ensure the movement of the internal medium, for example blood, in order to maintain a diffusion gradient.

Question 24

What does a large SA:V ratio help do?

Answer 24

Increases the rate of exchange.

Question 25

How does the fact that the exchange surface is thin help?

Answer 25

Short diffusion pathway so materials cross the exchange rapidly.

Question 26

How does selectively permeable surfaces affect the rate of exchange?

Answer 26

Allows selected materials to cross.

Question 27

How does movement of the environmental medium affect exchange?

Answer 27

Maintains diffusion gradient.

Question 28

How does a transport system affect exchange?

Answer 28

Ensures movement of the internal medium in order to maintain a diffusion gradient.

Question 29

What things do organisms exchange with their environment?

Answer 29

oxygen and carbon dioxide
waste products
heat
nutrients

Question 30

How easy the exchange of a substance is depends on what?

Answer 30

The organism’s surface area to volume ratio.

Question 31

Do smaller animals have a higher or lower surface area to volume ratio?

Answer 31

Higher

Question 32

What things do multicellular organisms need to help exchange?

Answer 32

Exchange organs and mass transport systems.

Question 33

How does exchange occur in single-celled organisms?

Answer 33

Diffusion across the cell membrane.

Question 34

What is the diffusion rate like in single-celled organisms?

Answer 34

Quick as there is a short diffusion pathway.

Question 35

Why is diffusion across the outer membrane too slow in multicellular organisms?

Answer 35

Some cells are deep within the body so there is a long diffusion pathway.

Larger animals have a low surface area to volume ratio so it is difficult to exchange enough substances to supply a large volume of animal through a relatively small outer surface.

Question 36

What are examples of mass transport systems?

Answer 36

Circulatory system - uses blood to carry oxygen and glucose around the body as well as hormones, antibodies and waste like carbon dioxide.

In plants, the xylem and phloem transport water and solutes.

Question 37

What 2 factors affect heat exchange?

Answer 37

Body size and shape.

Question 38

How does size affect heat exchange?

Answer 38

Small surface area = harder to lose heat from body.

Question 39

What do large animals have to help them generate enough heat for them to stay warm?

Answer 39

A relatively high metabolic rate.

Question 40

How does shape affect heat exchange?

Answer 40

Animals with a compact shape have a small SA:V - minimising heat loss.

Question 41

Whether an animal is compact or not depends on what?

Answer 41

The temperature of its environment.

Question 42

What type of adaptations do organisms have to aid exchange?

Answer 42

Behavioural and physiological.

Question 43

What behavioural and physiological adaptations do organisms have to aid exchange?

Answer 43

Some small desert mammals have kidney structure adaptations so that they produce less urine to compensate for the increased loss of water from their large SA:V.

To support their high metabolic, small mammals living in cold regions need to eat large amounts of high energy foods such as seeds and nuts.

Smaller mammals may have thick layers of fur or hibernate when the weather gets really cold.

Larger organisms in hot regions find it hard to keep cool as their heat loss is relatively slow so elephants have developed large flat ears to increase SA allowing them to lose more heat. Hippos spend most of the day in water - a behavioural adaptation to help them lose heat.