3.4.1Exchange and Transport
Mass Transport — AQA A Level Biology 7402 Revision Notes & Topical Questions
5 min read 9 parts Lesson 27 of 48 1 quiz
3.4.1 Mass Transport — AQA A Level Biology 7402 notes: Explain why large animals need a mass transport system and describe the main features of the mammalian transport system.Single-celled organisms and very small animals exchange substances directly with their environment becaus…
- specialised exchange surfaces
- closed, double circulatory system
- pulmonary circulation
- systemic circulation
- Exam-style question 1
- Exam-style question 2
Section 1 of 9Why transport systems are needed
Single-celled organisms and very small animals exchange substances directly with their environment because their surface area to volume ratio is large and the diffusion distance is tiny.
- so the surface area is too small to supply the volume of cells,
- and the diffusion distance from the surface to the middle of the body becomes too long.
- so the demand for oxygen and glucose is large and carbon dioxide and urea must be removed quickly.
Therefore larger animals have specialised exchange surfaces (lungs, gut) and a mass transport system: a circulatory system that moves substances in bulk, by mass flow, between the exchange surfaces and the cells.
Section 2 of 9Features of an efficient circulatory system
Mammals have a closed, double circulatory system: blood remains within vessels and passes through the heart twice in each complete circuit. The pulmonary circulation carries deoxygenated blood from the right side of the heart to the lungs and back to the left side; the systemic circulation carries oxygenated blood from the left side to the body and back to the right side.
Section 3 of 9Single, double and open systems compared
| Feature | Fish (single, closed) | Mammal (double, closed) | Insect (open) |
|---|---|---|---|
| Passes through heart per circuit | Once | Twice | Fluid is pumped into body cavity |
| Blood pressure to body | Low, because it falls after the gills | High, because it is boosted after the lungs | Low |
| Speed of delivery | Slower | Fast | Slow |
| Role in oxygen transport | Yes, by haemoglobin | Yes, by haemoglobin | Little; tracheae supply oxygen directly |
Section 4 of 9Advantages of the double circulation
Blood passing through the narrow capillaries of the lungs loses a lot of pressure.
- so that it reaches the tissues at high pressure and flows quickly.
- which protects the delicate capillaries of the lungs from damage and prevents fluid from leaking into the alveoli.
Section 5 of 9What the blood carries
Blood carries oxygen (mostly bound to haemoglobin in red blood cells), carbon dioxide (mostly as hydrogencarbonate ions in the plasma), nutrients such as glucose, amino acids and fatty acids, urea to the kidneys, hormones from endocrine glands to target cells, and heat from the active organs.
Section 6 of 9Blood, tissue fluid and lymph
The circulatory system is not the only transport route.
Plasma is about 90% water and contains dissolved proteins (albumin, fibrinogen, antibodies), nutrients, hormones, urea and ions. Red blood cells have no nucleus, giving more space for haemoglobin, and a biconcave disc shape that increases surface area for diffusion of oxygen. White blood cells defend against infection, and platelets are fragments of cells that help clotting.
An exam question may give the heart rate and stroke volume and ask for the cardiac output (heart rate × stroke volume). For example, 72 beats per minute × 70 cm3 = 5040 cm3 min−1, about 5 dm3 min−1, which is roughly the whole blood volume each minute at rest.
Section 7 of 9Exam-style questions
Explain why large, active animals need a circulatory system but a single-celled organism does not. [3]3 marks
Small organisms have a large surface area to volume ratio and a short diffusion distance, so diffusion alone supplies their needs (1). Large animals have a small surface area to volume ratio and long diffusion distances (1). They also have a high metabolic rate, so substances must be delivered quickly by mass flow (1).
Explain the advantage of a double circulatory system for a mammal. [3]3 marks
Blood loses pressure when it passes through the lung capillaries (1). It returns to the heart to be pumped again to the body at high pressure (1). Therefore oxygen and nutrients are delivered faster, supporting a high metabolic rate (1).
A mouse has a mass of 25 g and a surface area of 50 cm2 and a rat has a mass of 400 g and a surface area of 300 cm2. Taking mass as proportional to volume, compare the surface area to mass ratios. [3]3 marks
Mouse: 50 ÷ 25 = 2.0 cm2 g−1 (1). Rat: 300 ÷ 400 = 0.75 cm2 g−1 (1). The smaller animal has the larger ratio, so loses heat faster and needs a higher metabolic rate and faster transport (1).
Section 8 of 9Key terms
| Term | Meaning |
|---|---|
| Mass transport | Bulk movement of substances in a fluid by pressure differences. |
| Closed circulation | System where blood stays inside vessels. |
| Double circulation | System in which blood passes through the heart twice in each circuit. |
| Pulmonary circulation | Blood flow between heart and lungs. |
| Systemic circulation | Blood flow between heart and the rest of the body. |
| Surface area to volume ratio | Surface area divided by volume; decreases as size increases. |
Section 9 of 9Exam technique
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