Cells, tissues, organs and organ systems; enzymes and the digestive system; the heart, blood vessels and blood; coronary heart disease and health; and transport in plants by the xylem and phloem with transpiration.

What this topic is asking

AQA wants you to describe how cells are organised into tissues, organs and systems, explain how enzymes work and how food is digested, describe the structure and function of the heart, blood vessels and blood, explain risk factors for coronary heart disease, and describe transport in plants.

Levels of organisation, enzymes and digestion

Cells are organised in a hierarchy: similar cells form tissues (such as muscle tissue), different tissues form organs (such as the stomach, which contains muscular, glandular and epithelial tissue), and organs work together in organ systems (such as the digestive system) to carry out a function for the whole organism.

Digestive enzymes break large, insoluble food molecules into small, soluble ones that can be absorbed: amylase (a carbohydrase, made in the salivary glands and pancreas) breaks starch into sugars; proteases (made in the stomach, pancreas and small intestine) break proteins into amino acids; and lipases (from the pancreas and small intestine) break lipids into fatty acids and glycerol. Bile, made in the liver and stored in the gall bladder, is alkaline (it neutralises stomach acid so enzymes work) and emulsifies fats into small droplets, increasing the surface area for lipase to act on.

The heart, blood and blood vessels

The heart is a double pump: the right side pumps deoxygenated blood to the lungs (the pulmonary circuit), and the left side pumps oxygenated blood around the rest of the body (the systemic circuit). The left ventricle has a thicker, more muscular wall because it must generate enough pressure to push blood around the whole body. The natural resting heart rate is controlled by a group of cells in the right atrium acting as a pacemaker; an artificial pacemaker can correct an irregular rhythm. Blood from the lungs is reoxygenated at the alveoli, which are adapted with a large surface area, thin walls and a rich blood supply for efficient gas exchange.

Coronary heart disease (CHD) is a non-communicable disease in which layers of fatty material build up in the coronary arteries, narrowing them and reducing blood flow to the heart muscle. Treatments include stents (to hold arteries open) and statins (drugs that reduce blood cholesterol and slow the build-up). Lifestyle risk factors for non-communicable diseases such as cardiovascular disease, type 2 diabetes and some cancers include poor diet, smoking, lack of exercise and excess alcohol; these diseases place a large human and financial cost on society.

Transport in plants

Plant organs (roots, stem, leaves) contain two transport tissues:

• Xylem: carries water and dissolved mineral ions in one direction, up from the roots to the leaves. It is made of dead, hollow cells strengthened with lignin.
• Phloem: carries dissolved sugars (mainly sucrose) made in the leaves to the rest of the plant, in both directions, a process called translocation.

Transpiration is the loss of water vapour from the leaves, mainly through tiny pores called stomata whose opening is controlled by guard cells. As water evaporates from the leaves it pulls a continuous column of water up the xylem from the roots, the transpiration stream. The rate of transpiration increases with higher temperature, more air movement (wind), lower humidity and brighter light (which opens the stomata), so plants must balance gas exchange for photosynthesis against water loss.