6.1 - Exchange between organisms and their environment

Question 1

What might affect the exchange rate of an organism

Answer 1

• the size of an organism
• its metabolic rate

Question 2

What certain materials must an animal maintain a certain supply of, and explain why

Answer 2

• oxygen + nutrients, e.g. glucose/amino acids/fatty acids
  —> needed for respiration and growth
  —> these are absorbed by diffusion and active transport (sometimes co-transport)

Question 3

Which has a higher SA:Vol, small organisms or large organisms

Answer 3

• smaller organisms

Question 4

Explain the importance of a high SA:Vol ratio

Answer 4

• cells need to release energy (via respiration) to survive and this requires the exchange of materials with the environment
• the rate of respiration of a cell is a function of its mass/volume (larger cells need more energy to sustain essential functions)
• the rate of material exchange is a function of its surface area (large membrane surface equates to more material movement)
• As a cell grows, volume increases faster than surface area, leading to a decreased SA:Vol ratio
• if metabolic rate exceeds the rate of exchange of vital materials and wastes (low SA:Vol ratio) the cell will eventually die
• Hence growing cells tend to divide and remain small in order to maintain a high SA:Vol ratio suitable for survival

Question 5

Describe diffusion in larger organisms

Answer 5

• larger, active organisms cannot rely upon their body surface
• even if the outer surface could supply enough of a substance, it would still take too long to reach the middle of an organism if diffusion alone was the method of transport.
• the SA:Vol ratio is insufficient for exchange by diffusion and distances are too great
• therefore a specialised exchange surface is required

Question 6

Describe exchange in a small organism

Answer 6

In small organisms, diffusion across their body surface is sufficient to meet needs:
- distances are small (less than 0.5 mm)
- surface area is relatively large
- shape tends to be flat and thin, e.g. flatworm, leaf

Question 7

What is tissue fluid, and why is it important for exchange surfaces

Answer 7

• “extracellular fluid which bathes the cells of most tissues arriving via blood capillaries and being removed via the lymphatic vessels”
• exchange of substances between the cells and the blood occurs via the tissue fluid

Question 8

Explain what plasma is

Answer 8

• plasma is a straw coloured liquid that constitutes around 55% of the blood
• Its mainly 95% water, and because water is a good solvent many substances can dissolve in it, allowing them to be transported around the body
• As blood passes through capillaries some plasma leaks out through gaps - resulting in forming tissue fluid

Question 9

What is the difference/similarity between plasma and tissue fluid

Answer 9

• both are about the same composition
• but tissue fluid contains far fewer proteins

Question 10

Which cells does tissue fluid bathe

Answer 10

Almost all the cells of the body outside the circulatory system

Question 11

Give an example of how an exchange of substances between cells and the blood occurs via the tissue fluid

Answer 11

• CO2 produced in aerobic respiration will leave a cell, dissolve into the tissue fluid surrounding it, and then diffuse into the capillary

Question 12

As the size of an organism increases, what else does and what are the consequences of this

Answer 12

• as size increases, volume increases disproportionately compared to SA
• Demands of cells for nutrients and waste removal also increased disproportionately
• The surface doesn’t increase sufficiently to accommodate these extra demands
• must be met by a specialist exchange surface with enhanced SA

Question 13

Describe the various features of a specialised exchange surface

Answer 13

• A large SA:Vol ratio 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, e.g. air to maintain a conc. gradient
• A transport system to ensure the movement of the internal medium, e.g. blood, to maintain a diffusion gradient
• moist (O2 and CO2 can dissolve —> aids passage through membrane)

Question 14

Microscopic organisms obtain their oxygen by diffusion in across their body surfaces. using the graph, explain how they are able to obtain sufficient oxygen for their needs

Answer 14

They are very small and so have a large SA:Vol ratio

Question 15

The blue whale is the largest organism on the planet. It spends much of its life in cold waters with temps between (0’C and 6’C. Use the graph to explain 1 way in which large size is an advantage to blue whales

Answer 15

The blue whale has a very small surface area to volume ratio and so loses less heat to the water than it would if it were small

Question 16

Because exchange surfaces are thin, what is the disadvantage of this?

Answer 16

• easily damaged
• easily dehydrated
  —> hence why they’re found internally to avoid this

Question 17

What is the equation to work out diffusion

Answer 17

Diffusion = (SA x difference in conc.) divided by length of diffusion path