A1. Size and Surface Area

Question 1

Exchange of substances with the environment
Every organism, whatever its size, needs to exchange things with its ______________. Cells need to take in ________(for __________ _____________) and ___________. They also need to excrete ________products like __________ ___________and _____. Most organisms need to stay at roughly the same _______________, so heat needs to be exchanged too.

Answer 1

Every organism, whatever its size, needs to exchange things with its environment. Cells need to take in oxygen (for aerobic respiration) and nutrients. They also need to excrete waste products like carbon dioxide and urea. Most organisms need to stay at roughly the same temperature, so heat needs to be exchanged too.

Question 2

Surface area: volume ratio
An organism’s surface area: volume ratio affects how quickly substances are exchanged. But before going into the effects of surface area: volume ratios, you need to understand a bit more about them. Smaller organisms have __________surface area: volume ratios than larger organisms,

Answer 2

An organism’s surface area: volume ratio affects how quickly substances are exchanged. But before going into the effects of surface area: volume ratios, you need to understand a bit more about them. Smaller organisms have higher surface area: volume ratios than larger organisms,

Question 3

Tip: The formula for calculating the surface area of a sphere is …

Answer 3

Tip: The formula for calculating the surface area of a sphere is 4πr²

Question 4

Exchange organs and mass transport systems - Single-celled organisms
In single-celled organisms, these substances can ________ _________into (or out of the cell across the ____-___________ ____________. The diffusion rate is quick because of the ______ _________the substances have to travel

Answer 4

Exchange organs and mass transport systems - Single-celled organisms
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

Question 5

Exchange organs and mass transport systems - Multicellular organisms

In multicellular organisms, diffusion across the outer membrane is too slow,
for two reasons: (2 reasons)

They also need an efficient system to carry substances to and from their individual cells-this is mass transport. In mammals, ‘mass transport’ normally refers to the _____________ ___________, which uses _______to carry glucose and oxygen around the body. It also carries _________, __________and waste like ____. Mass transport in plants involves the transport of __________and _________in the _______and _________.

Answer 5

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. So rather than using straightforward diffusion to absorb and excrete substances, multicellular organisms need specialised exchange organs(like lungs).

They also need an efficient system to carry substances to and from their individual cells-this is mass transport. In mammals, ‘mass transport’ normally refers to the circulatory system, which uses blood to carry glucose and oxygen around the body. It also carries hormones, antibodies and waste like CO2. Mass transport in plants involves the transport of water and solutes in the xylem and phloem.

Question 6

Heat exchange
As well as creating waste products that need to be transported away, the metabolic activity inside cells creates heat. Staying at the right temperature is difficult, and it’s pretty heavily influenced by your … (3 things)

Answer 6

Body Size
Body Shape
Adaptations for heat exchange

Question 7

Heat exchange - Body size

What does the rate of heat loss depend on?
Example of an organism with large volume and its effect?
Example of an organism with small volume and its effect?
What does this mean for small organisms?

Answer 7

The rate of heat loss from an organism depends on its surface area. If an organism has a large volume, eg, a hippo, its surface area is relatively small. This makes it harder for it to lose heat from its body. If an organism is small, eg, a mouse, its relative surface area is large, so heat is lost more easily. This means smaller organisms need a relatively high metabolic rate, in order to generate enough heat to stay warm.

Question 8

Heat exchange - Body shape

What shape do animals with a small surface area have and what is its effect?
What shape do animals with a large surface area have and what is its effect?

Answer 8

Animals (of any size) 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 or have sticky outy bits) have a larger surface area relative to their volume- this increases heat loss from their surface.

Question 9

Heat exchange - Adaptations for heat exchange
(1 point and examples?)

Answer 9

Whether an animal is compact or not depends on the temperature of its environment—the animal’s body shape is adapted to suit its environment.

Question 10

Behavioural and physiological adaptations to aid exchange
Not all organisms have a body size or shape to suit their climate — some have other adaptations to aid exchange instead…(4 adaptations)

Answer 10

• Animals with a high surface area: volume ratio tend to lose more water as it evaporates from their surface. This is a problem particularly for animals living in hot regions where water evaporates quickly. Some small desert mammals have kidney structure adaptations so that they produce less urine to compensate.
• 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.
• Larger organisms living in hot regions, such as elephants and hippos, find it hard to keep cool as their heat loss is relatively slow. Elephants have developed large flat ears which 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.