Exam Questions

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.