Size and surface area

Question 1

1) Give two substances that living organisms need to take into their cells from the environment.

Answer 1

E.g. oxygen, glucose, amino acids.

Question 2

2) Give two waste products that living organisms need to excrete from their cells.

Answer 2

2) E.g. carbon dioxide and urea.

Question 3

2) Give two waste products that living organisms need to excrete from their cells.

Answer 3

2) E.g. carbon dioxide and urea.

Question 4

3) True or false? Larger animals usually have larger surface area to volume ratios than smaller animals.

Answer 4

3) False - smaller animals usually have a bigger surface area compared to their volume.

Question 5

4) Explain why single-celled organisms don’t need specialised exchange organs in order to exchange substances with their environment, but multicellular organisms do.

Answer 5

4) In single-celled organisms, substances that need to be exchanged can diffuse directly in or out of the cell across the cell-surface membrane. However, in multicellular organisms some cells are far from the outer surface, and there are lots of cells in relation to the area of the outer surface. This means diffusion of substances across the outer surface would be too slow to meet the needs of all cells, so they need specialised exchange surfaces.

Question 6

Explain why a mouse has a larger surface area to volume ratio than an elephant.

Use this example:

The hippo could be represented by a block measuring
2 cm x 4 cm x 4 cm.
Compare this to a cube mouse measuring 1 cm x 1 cm x 1 cm.

Answer 6

The hippo could be represented by a block measuring
2 cm x 4 cm x 4 cm.

Its volume is 2 x 4 x 4 = 32 cm³
Its surface area is 2 x 4 x 4 = 32 cm² (top and bottom surfaces of cube) + 4 x 2 x 4 = 32 cm² (four sides of the cube)

Total surface area = 64 cm²
So the hippo has a surface area: volume ratio of 64:32 or 2:1.

Compare this to a cube mouse measuring 1 cm x 1 cm x 1 cm.

Its volume is 1 x 1 x 1 = 1 cm³
Its surface area is 6 x 1 x 1 = 6 cm²
So the mouse has a surface area: volume ratio of 6:1.

The cube mouse’s surface areais six times its volume, but the cube hippo’s surface area isonly twice its volume. Smaller animals have a bigger surface area compared to their volume.

Question 7

Why do multicellular organisms need exchange organs and mass transport?

Answer 7

An organism needs to supply every one of its cells with substances like glucose and oxygen (for respiration). It also needs to remove waste products from every cell to avoid damaging itself.

In single-celled organisms, these substances can diffuse directly into (or out of) the cell across the cell-surface membrane. The diffusion rate is quick because of the small distances the substances have to travel.

In multicellular animals, diffusion across the outer membrane is too slow.

Question 8

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

Answer 8

1. Some cells are deep within the body - there’s a big distance between them and the outside environment.
2. Larger animals have a low surface area to volume ratio it’s difficult to exchange enough substances to supply a large volume of animal through a relatively small outer surface.

Question 9

What is mass transport?

Answer 9

efficient exchange systems used to carry substances to and from individual cells.

Question 10

What are exchange organs?

Answer 10

Specialised organs used to absorb and excrete substances as in large multicellular organisms as this process can’t happen through diffusion alone.

Question 11

What is the main mass transport system in mamels? What does it transport?

Answer 11

the circulatory system uses blood to carry glucose and oxygen around the body. It also carries hormones, antibodies and waste like CO₂.

Question 12

What are the main transport stems in plants?

Answer 12

Mass transport in plants involves the transport of water and solutes in the xylem and phloem.

Question 13

How does body size affect heat exchange in mammels?

Answer 13

The rate of heat loss from an organism depends on its surface area. If an animal has a large volume, its S.A. is relatively small. This makes it harder for it to lose heat from its body.
If an organism is small, its relative surface are is large, so heat is lost more easily.

Question 14

Why do small mammels need a relatively high metabolic rate to stay warm?

Answer 14

If an organism is small, its relative surface are is large, so heat is lost more easily. This means smaller organisms need a relativey high metabolic rate, in order to generate enough heat to stay warm.

Question 15

How does body shape affect heat exchange in mammels?

Answer 15

Animals with a compact shape have a small surface area relative to their volume - minimising heat loss from their surface.

Animals with a less compact shape (those that are a bit gangly) have a larger surface area relative to their volume - this increases heat loss from their surface.

Question 16

Norwegian lemmings are rodents that have very short tails.

Suggest how this adaptation helps them to survive in the cold environments in which they live.

Answer 16

Having short tails will mean the Norwegian lemmings have a lower surface area to volume ratio than if they had long tails. This will make it easier for them to keep warm in their cold environment, as it will reduce the rate at which heat is lost from their bodies.

Question 17

Are Norwegian lemmings likely to have a higher or lower metabolic rate
than larger mammals that live in the same habitat? - a cold environment
Explain your answer.

Answer 17

A higher metabolic rate. Smaller animals tend to have larger surface area to volume ratios than larger animals. This means that they lose heat more easily from their body, so they need a higher metabolic rate in order to generate the heat needed to stay warm.

Question 18

When water evaporates from the surface of the body, some heat is lost to the environment. Using this information, explain which animal from the table (A-C) is most likely to lick its forearms.

Answer 18

C-licking its forearms will help to cool an animal down because heat will be lost from the body when the saliva evaporates, so the animali s more likely to live in a hot habitat. The adaptation is more likely to be seen in an animal with a lower surface area to volume ratio because these animals find it more difficult to lose heat.

Question 19

Why do some animals have behavioural or physiological adaptions to eaid exchange?

Answer 19

They may not have a body size or shape to suit their climate.

Question 20

Give some examples of the adaptions some desert animals need to aid exchange when they don’t have a body size or shape to suit their climate.

Answer 20

1) Animals with a high SA: volume ratio tend to lose more water as it evaporates from their surface. Some small desert mammals have kidney structure adaptations so that they produce less urine to compensate.

Question 21

Give some examples of adaptions small animals living in cold environments need to aid exchange.

Answer 21

To support their high metabolic rates, 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.

Question 22

Give an example of an adaption large animals living in hot environments need to aid exchange.

Answer 22

4) Larger organisms living in hot regions, e.g. elephants and hippos, find it hard to keep cool as their heat loss is relatively slow. Elephants have developed large flat ears to increase their surface area, allowing them to lose more heat. Hippos spend much of the day in the water - a behavioural adaptation to help them lose heat.