Transport SystemsEdexcel IGCSE Biology: Revision notes

Simple unicellular organisms can rely on diffusion alone, because they have a large surface area to volume ratio and every part of the cell is close to the environment. Multicellular organisms are larger, with a smaller surface area to volume ratio, so many cells are too far from the exchange surface for diffusion alone to supply their needs quickly enough — they need a transport system to move substances between exchange surfaces and body cells.

Plants have two transport tissues:

• Xylem — transports water and mineral ions from the roots to other parts of the plant (one-way, upwards)
• Phloem — transports sucrose and amino acids between leaves (where they are made) and other parts of the plant (two-way, as needed)

Root hair cells have a large surface area (due to their long, thin extension) that increases the rate of water absorption from the soil by osmosis.

Transpiration is the evaporation of water from the surface of a plant (mainly through leaf stomata), which pulls more water up through the xylem. The rate of transpiration increases with higher temperature, higher wind speed, higher light intensity, and lower humidity.

Blood is composed of:

• Red blood cells — transport oxygen
• White blood cells — defend against disease
• Platelets — involved in clotting
• Plasma — the liquid component

Plasma transports carbon dioxide, digested food, urea, hormones and heat energy around the body.

Red blood cells are adapted for oxygen transport: a biconcave disc shape (increases surface area), no nucleus (more room for haemoglobin), and contain haemoglobin, which binds reversibly with oxygen.

White blood cells defend the body in two main ways:

• Phagocytes ingest and digest pathogens (phagocytosis)
• Lymphocytes release antibodies specific to a particular pathogen, which help destroy it

Vaccination introduces a small, safe amount of antigen, causing the body to produce memory cells. On future exposure to the same pathogen, memory cells enable a faster and greater antibody response, providing immunity.

Platelets help blood clot at a wound site, preventing blood loss and stopping micro-organisms entering the body.

The heart pumps blood around the body through the circulatory system, which includes vessels to and from the heart, lungs, liver and kidneys.

Heart rate increases during exercise and under the influence of adrenaline, to deliver more oxygen and glucose to respiring muscles. Risk factors for coronary heart disease include smoking, poor diet (high saturated fat), lack of exercise and high blood pressure.