Cells are organised into tissues, organs and systems. This topic follows that organisation through the digestive, circulatory and respiratory systems, and through the diseases that affect them.
Similar cells working together form a tissue; several tissues working together form an organ; several organs working together form an organ system; organ systems together form the organism.
A single-celled organism can rely on diffusion, because it has a large surface area compared with its volume and nothing is far from the surface.
As an organism gets larger its surface area to volume ratio falls, and the distances inside it grow. Diffusion alone becomes far too slow, so multicellular organisms need specialised exchange surfaces such as lungs, gills and roots, and a transport system to carry substances between them and the cells.
Enzymes are biological catalysts: protein molecules that speed up reactions without being used up.
The lock and key model explains their specificity. The substrate fits the enzyme's active site, which has a shape complementary to it. The shape and chemistry of the active site make each enzyme highly specific to particular substrates and reactions.
Raising the temperature speeds the reaction up until the optimum, after which the active site changes shape and the enzyme is denatured. A pH far from the optimum can alter the active site and denature the enzyme. Denaturing is not the same as killing, because enzymes are not alive.
Increasing substrate concentration raises the rate until every active site is occupied, after which adding more makes no difference.
Large insoluble food molecules are broken into small soluble ones that can be absorbed into the bloodstream.
Carbohydrases such as amylase break starch into simple sugars; proteases break protein into amino acids; lipases break lipids into fatty acids and glycerol. Amylase is made in the salivary glands, pancreas and small intestine; protease in the stomach, pancreas and small intestine; lipase in the pancreas and small intestine.
Bile is made in the liver and stored in the gall bladder. It does two jobs: it neutralises the stomach acid, giving the alkaline conditions the small-intestine enzymes need, and it emulsifies fat into small droplets, increasing the surface area for lipase to work on.
The energy in a food sample can be estimated by burning it under a boiling tube of water and measuring the temperature rise.
The measured value is usually an underestimate, because much of the energy escapes to the surroundings rather than heating the water, and the food may not burn completely.
The heart is a double pump. The right side sends deoxygenated blood to the lungs; the left side sends oxygenated blood to the rest of the body, and has a thicker muscular wall because it must generate a much higher pressure to pump blood around the whole body. Valves stop blood flowing backwards, and the resting rate is set by a group of cells in the right atrium acting as a natural pacemaker.
Arteries carry blood away from the heart at high pressure, with thick muscular elastic walls and a narrow lumen. Veins return blood at low pressure, with thinner walls, a wider lumen and valves. Capillaries are one cell thick, so substances can diffuse in and out.
Blood is a tissue with four components:
Cardiac output is the volume of blood the heart pumps each minute: cardiac output = heart rate × stroke volume, where stroke volume is the volume pumped by each beat.
A heart beating 70 times a minute with a stroke volume of 70 cm³ has an output of 4900 cm³ per minute.
In coronary heart disease, layers of fatty material build up inside the coronary arteries, narrowing them so less blood and therefore less oxygen reaches the heart muscle.
Stents hold the artery open, restoring blood flow, but do not treat the cause. Statins reduce blood cholesterol, which slows the build-up, but must be taken long-term and can have side effects. Faulty valves can be replaced with biological or mechanical valves, and a heart transplant or an artificial heart may be used. Artificial hearts avoid rejection but carry a risk of clotting.
Health is a state of physical and mental wellbeing, and disease is a major cause of ill health, but diet, stress and life situation all affect it too.
Diseases can interact in several ways. A weakened immune system increases susceptibility to infectious disease. Some viral infections can contribute to cancer. Inappropriate immune responses can cause allergies such as skin rashes and asthma. Severe physical illness can affect mental health.
Risk factors are things linked to an increased chance of disease. Smoking is linked to lung disease and lung cancer; obesity to type 2 diabetes; alcohol to liver and brain damage; smoking and alcohol in pregnancy to harm to an unborn baby; and carcinogens, including ionising radiation, to cancer.
A correlation between a factor and a disease does not by itself prove that the factor causes it. Other evidence is needed to rule out alternative explanations and support a causal relationship.
Cancer is the result of uncontrolled cell division. A benign tumour remains localised and does not invade surrounding tissues or spread to other parts of the body. A malignant tumour invades neighbouring tissue, and its cells can spread through the blood or lymph to form secondary tumours elsewhere.
Air passes down the trachea, into two bronchi, then into bronchioles ending in the alveoli, which are surrounded by a network of capillaries.
The alveoli are adapted for exchange: a very large total surface area, walls one cell thick, a moist lining, and a rich blood supply that maintains steep concentration gradients. Oxygen diffuses into the blood and carbon dioxide diffuses out.
A leaf is an organ made of several tissues: the waxy cuticle reduces water loss; the upper epidermis is transparent to let light through; the palisade mesophyll is packed with chloroplasts and does most of the photosynthesis; the spongy mesophyll has air spaces for gases to diffuse; the lower epidermis contains the stomata and guard cells; and xylem and phloem run through in veins.
Xylem carries water and mineral ions from the roots upwards. Its cells are dead, hollow and strengthened with lignin, forming a continuous tube. The flow is called the transpiration stream, driven by water evaporating from the leaves.
Phloem carries dissolved sugars from the leaves to the rest of the plant, in either direction. This is translocation, and it happens in living cells with perforated end plates.
Transpiration is faster in higher temperature, higher light intensity, lower humidity and moving air. Guard cells open the stomata to let carbon dioxide in and close them to limit water loss.
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