Organisation

Cells are the basic building blocks of living organisms. Similar cells working together form tissues, tissues combine to form organs, and several organs working together form an organ system. Understanding this hierarchy is central to biology of tissues and organization, cell structure and function in biology, anatomy and physiology basics, and understanding organs and their definitions.

A tissue is a group of cells with a similar structure that work together to perform a particular function. An organ is made from different tissues working together, while an organ system contains several organs cooperating to carry out major body functions. Examples include the human digestive system, circulatory system and respiratory system.

The human digestive system is an organ system in which organs work together to digest food and absorb soluble biological molecules. Digestive enzymes break large insoluble molecules into smaller soluble molecules that can be absorbed into the bloodstream and used by cells.

Enzymes are biological catalysts that increase the rate of chemical reactions without being permanently changed. Enzymes and their functions depend on the specific shape of their active sites. The enzyme lock key model explains that only a substrate with a complementary shape can fit into a particular enzyme's active site. Enzyme activity is affected by factors including temperature and pH.

Carbohydrases break carbohydrates into simple sugars; amylase is a carbohydrase that breaks starch down into sugars. Proteases break proteins into amino acids, while lipases break lipids into glycerol and fatty acids. Amylase is produced in the salivary glands, pancreas and small intestine; proteases are produced in the stomach, pancreas and small intestine; and lipases are produced mainly in the pancreas and small intestine.

Bile is produced by the liver and stored in the gall bladder. It is alkaline, so it neutralises hydrochloric acid entering the small intestine from the stomach. Bile also emulsifies fats into small droplets, increasing their surface area so lipase can digest them more rapidly.

Food tests can identify important biological molecules. Benedict's reagent is used to test for reducing sugars, iodine solution is used to test for starch, and Biuret reagent is used to test for protein. GCSE practical work also includes investigating how pH affects the rate at which amylase breaks down starch while controlling variables such as temperature.

The heart is a muscular organ located in the chest and forms the centre of the human circulatory system. A labelled heart diagram shows four chambers: the right atrium, right ventricle, left atrium and left ventricle. The right side pumps deoxygenated blood to the lungs, while the left side pumps oxygenated blood around the body, producing a double circulatory system.

Important blood vessels associated with the heart include the aorta, vena cava, pulmonary artery, pulmonary vein and coronary arteries. Arteries, veins and capillaries have structures adapted to their functions: arteries have thick muscular elastic walls, veins have a larger lumen and valves, and capillaries have walls only one cell thick to allow efficient exchange of substances.

The natural resting heart rhythm is controlled by a group of cells in the right atrium that act as a pacemaker. Artificial pacemakers can correct irregular heart rhythms. The contraction of the heart produces systolic pressure and relaxation produces diastolic pressure, which are the two values recorded when blood pressure is measured.

Blood is a tissue containing plasma, red blood cells, white blood cells and platelets. Plasma transports dissolved substances, red blood cells carry oxygen using haemoglobin, white blood cells defend against pathogens, and platelets help blood clot. These adaptations allow blood to transport substances and protect the body efficiently.

Coronary heart disease occurs when fatty material builds up inside coronary arteries, narrowing them and reducing blood flow to heart muscle. Treatments include stents to keep arteries open, statins to reduce blood cholesterol, replacement heart valves, heart transplants and in some circumstances artificial hearts. Each treatment has benefits, limitations and possible risks.

Health is a state of physical and mental well-being. Both communicable and non-communicable diseases can affect health, and different diseases can interact. Lifestyle and environmental risk factors include diet, obesity, smoking, alcohol consumption and exposure to carcinogens. Cancer develops when changes in cells cause uncontrolled growth and division; benign tumours remain contained whereas malignant tumours invade tissues and may spread around the body.

Plant tissues include epidermal tissue, palisade mesophyll, spongy mesophyll, xylem, phloem and meristem tissue. The leaf is a plant organ containing specialised tissues. Palisade cells contain many chloroplasts for photosynthesis, the epidermis protects the leaf, and stomata with guard cells control gas exchange and water loss.

A plant's transport system includes root hair cells, xylem and phloem. Root hair cells absorb water by osmosis and mineral ions by active transport. Xylem carries water and mineral ions from the roots to the stems and leaves as part of the transpiration stream, while phloem carries dissolved sugars between different parts of the plant by translocation. The rate of transpiration is affected by temperature, humidity, air movement and light intensity.