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Explain why many multicellular organisms need a transport system (4)
The cells of all organisms need a supply of oxygen & glucose, & removal of CO2 and urea
Organisms with a high SA : V can meet these requirements by diffusion alone.
Organisms with a low SA:V have an inadequate rate of diffusion.
Respiration can’t occur, & cells stop functioning (lack of ATP)
Explain how circulatory systems improve the rate of supply & removal of substances within organisms (6)
Transport medium acts as a solvent for biologically important molecules & can transport them.
The heart contracts to put pressure on transport medium, transport medium moves from high pressure to low pressure.
Dissolved substances get transported by mass flow.
Blood vessels maintain high pressure on transport medium, so it can move greater distances.
This maintains high conc gradients near cells.
So high rate of diffusion is maintained for all substances & all cells.
Describe how an open circulatory system works
The heart contracts increasing pressure on the haemolymph
The haemolymph moved through the vessels toward the haemocoel
This maintains the conc gradient near cells
Substances are exchanged across the cells by diffusion.
The haemolymph returns to the heart (when heart relaxes)
Because the vessels are open-ended the haemolymph is not under high pressure.
Small organisms e.g. insects have an open circulatory system.
Describe the key features of closed circulatory systems & how they work
They have the transport medium enclosed in blood vessels at all times.
The heart contracts to put pressure on the transport medium.
The transport medium can be put under higher pressure.
So it can be moved at greater distances.
The transport medium doesn’t make direct contact with cells & tissues.
Responsible for transporting gases.
Describe how a double circulatory system works.
Blood moves through the heart twice in each circuit of the body.
The heart contracts to put pressure on blood
The blood moves to the gas exchange organs & becomes oxygenated (pulmonary circuit).
It then returns to the heart, and is pumped again to the rest of the body (systemic circuit)
While moving through the systemic circuit, the blood is deoxygenated.
Describe the advantages of double circulatory systems
The pulmonary & systemic circuits are separate.
Deoxygenated blood can be pumped at a lower pressure to the gas exchange organs, so they aren’t damaged.
Oxygenated blood can be pumped at a higher pressure in the systemic circuit.
So that the gas exchange is more efficient
Organism can be larger & more metabolically active.
Explain why water is a good transport medium.
Water is composed of 2 hydrogen atoms covalent bonded to oxygen
Each O-H bond has a dipole, which makes the water molecule polar overall with a partially negative oxygen & partially positive hydrogens
Water molecules have hydrogen bonds between them so;
They are cohesive, & result in water being a liquid that can transport substances according to pressure gradients.
They are excellent solvents; interacts with opposite charges on other molecules to dissolve them.
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