Living things are organised on increasing scales: specialised cells group into tissues, tissues into organs, and organs into organ systems that work together to keep an organism alive. This topic follows that idea through real systems. You will study how digestive enzymes break large food molecules into small soluble ones, how the heart and blood vessels transport materials, what raises the risk of non-communicable diseases such as coronary heart disease and cancer, and how plants move water, mineral ions and sugars. Throughout, look for the link between a structure and the job it does — AQA rewards answers that connect form to function.
Cells are the basic building blocks of all living organisms. A tissue is a group of similar cells that work together to carry out a particular function, for example muscular tissue that contracts. An organ is a group of different tissues working together to perform a specific function; the stomach contains muscular tissue to churn food, glandular tissue to make digestive juices, and epithelial tissue to cover its surfaces. An organ system is a group of organs that work together to perform a job — the digestive system is an example. Several organ systems together make up the whole organism. This pattern (cell → tissue → organ → organ system → organism) underpins the rest of the topic.
The digestive system breaks down food so that small soluble molecules can be absorbed into the blood. Large food molecules are insoluble, so digestion converts them into smaller soluble products. This is done by enzymes, which are biological catalysts: protein molecules that speed up reactions without being used up.
Each enzyme has a specific shape with an active site that fits only one substrate — the lock and key model. If the temperature or pH is wrong, the active site changes shape and the enzyme is denatured, so the substrate no longer fits. The three main digestive enzymes are: carbohydrases (e.g. amylase) which break starch into sugars; proteases which break proteins into amino acids; and lipases which break lipids into fatty acids and glycerol. Amylase is made in the salivary glands, pancreas and small intestine; protease is made in the stomach (as pepsin), pancreas and small intestine; lipase is made in the pancreas and small intestine.
Bile, made in the liver and stored in the gall bladder, is released into the small intestine. It is alkaline, so it neutralises the acid that leaves the stomach, and it emulsifies fats — breaking large drops into small droplets to increase surface area for lipase to work faster.
Four qualitative food tests are required practical knowledge. For starch, add iodine solution: orange-brown turns blue-black if starch is present. For sugars (reducing sugars such as glucose), add Benedict's solution and heat in a water bath: blue changes through green, yellow and orange to brick-red. For protein, add Biuret reagent: blue turns purple/lilac. For lipids, add ethanol then water (the emulsion test): a cloudy white emulsion forms.
Enzyme activity depends on temperature and pH. As temperature rises, rate increases until the optimum, then falls sharply as the enzyme denatures. You can measure the effect of pH on amylase by timing how long starch takes to disappear (testing samples with iodine) and calculating rate = 1000 / time. Plotting rate against pH shows a peak at the optimum pH.
The heart is a double pump made mostly of muscle. The right side pumps deoxygenated blood to the lungs (the pulmonary circulation); the left side pumps oxygenated blood around the body (the systemic circulation). This is a double circulatory system. Blood enters the atria, which pump it into the ventricles, which pump it out of the heart. The left ventricle has a thicker muscular wall because it must generate higher pressure to reach the whole body. Valves prevent blood flowing backwards.
The heart's natural resting heart rate is controlled by a group of cells in the right atrium acting as a pacemaker. An artificial pacemaker is an electrical device used to correct an irregular heartbeat. The lungs contain millions of tiny air sacs called alveoli, which provide a huge surface area for gas exchange: oxygen diffuses into the blood and carbon dioxide diffuses out.
There are three types of blood vessel, each suited to its job. Arteries carry blood away from the heart at high pressure, so they have thick, elastic, muscular walls and a narrow lumen. Veins carry blood back to the heart at low pressure, so they have thinner walls, a wider lumen and valves to stop backflow. Capillaries are one cell thick, so substances such as oxygen, glucose and carbon dioxide can diffuse quickly between the blood and the tissues.
Blood is a tissue made of plasma and three types of cell. Plasma is a yellow liquid that transports dissolved substances including carbon dioxide, glucose, urea, hormones and antibodies. Red blood cells carry oxygen; they have no nucleus (more room for haemoglobin) and a biconcave disc shape (large surface area). Haemoglobin binds oxygen in the lungs to form oxyhaemoglobin and releases it in the tissues. White blood cells defend against pathogens, and platelets are fragments that help blood to clot.
The heart muscle is supplied with oxygenated blood by the coronary arteries. In coronary heart disease (CHD), layers of fatty material build up inside these arteries, narrowing them. This reduces blood flow, so the heart muscle receives less oxygen, which can cause a heart attack.
CHD can be treated in several ways. Stents are wire mesh tubes inserted to keep a narrowed artery open and maintain blood flow; they are effective but carry a risk of complications. Statins are drugs taken to reduce blood cholesterol, slowing the rate of fatty deposit formation; they must be taken long term and may have side effects. Faulty heart valves can be replaced with mechanical or biological valves. In heart failure, a donor heart (and sometimes lungs) may be transplanted, or an artificial heart used — for example to keep a patient alive while waiting for a transplant or to allow the heart to recover. Each treatment has benefits and risks that should be weighed up.
Health is the state of physical and mental wellbeing. Diseases are a major cause of ill health and can interact: a defective immune system makes a person more likely to suffer from infectious disease; some viruses can trigger cancers; immune reactions caused by a pathogen can trigger allergies; and physical ill health can lead to mental illness such as depression.
Diseases are either communicable (caused by pathogens and able to spread) or non-communicable (not transmitted, often lasting a long time). Risk factors are aspects of lifestyle or substances in the body and environment that increase the chance of developing a disease. Examples include a poor diet, lack of exercise, smoking and alcohol. A risk factor may have a causal mechanism for some diseases but only a correlation for others — AQA expects you to distinguish correlation from cause when interpreting data. Non-communicable diseases have a large human and financial cost, locally, nationally and globally.
Cancer is caused by changes in cells that lead to uncontrolled cell division and growth. This produces a mass of cells called a tumour. Benign tumours grow within a membrane and do not spread to other parts of the body, so they are usually not dangerous. Malignant tumours are cancers: their cells invade neighbouring tissues and can spread to different parts of the body in the blood, forming secondary tumours. Lifestyle risk factors (such as smoking, obesity, UV exposure and some carcinogens) and genetic risk factors both contribute to cancer.
Plant organs include roots, stems and leaves, each built from tissues with specific jobs. The leaf is the main organ for photosynthesis. Its epidermal tissues cover the leaf; the waxy upper epidermis is transparent to let light through. The palisade mesophyll just below is packed with chloroplasts for photosynthesis, while the spongy mesophyll has air spaces for gas exchange. Guard cells control the opening and closing of stomata (pores on the lower surface) to allow gas exchange while limiting water loss. Xylem and phloem tissues transport materials around the plant.
Xylem tissue is made of dead, hollow cells strengthened with lignin; it carries water and dissolved mineral ions upwards from the roots to the stems and leaves in the transpiration stream. Phloem tissue is made of living columns of cells; it transports dissolved sugars (made in the leaves) to where they are needed for use or storage, a process called translocation, which moves substances in both directions.
Transpiration is the loss of water vapour from the leaves, mainly through the stomata, which pulls water up the xylem from the roots. The rate of transpiration is increased by higher temperature (faster evaporation and diffusion), increased air movement (wind carries water vapour away), increased light intensity (stomata open wider) and lower humidity (steeper concentration gradient for water vapour). Guard cells balance the need for carbon dioxide against water loss: in bright light they swell and open the stomata; when water is short they become flaccid, closing the stomata to conserve water. Plants in dry conditions often have fewer stomata or sunken stomata to reduce water loss. Transpiration rate can be measured using a potometer, which tracks the uptake of water.
Practise exam-style questions on this topic.