Organisation, for the exam.

Part 1From cells to organ systems

Living things are organised in levels, each built from the one below. A cell is the basic building block. A group of similar cells working together is a tissue. Several tissues working together form an organ. A group of organs working together is an organ system, and organ systems together make the whole organism.

For example, the stomach is an organ: it contains muscular tissue to churn food, glandular tissue to make digestive juices, and epithelial tissue lining the inside. The stomach is one organ in the digestive system.

Part 2What enzymes do

An enzyme is a biological catalyst — it speeds up a reaction without being used up. Enzymes are large protein molecules made by living cells, and each has a special shape with a gap called the active site.

The molecule an enzyme works on is its substrate. Only a substrate with the matching shape fits the active site — this is the lock-and-key idea. Because the fit is so specific, each enzyme only catalyses one type of reaction.

Enzymes work best at a particular temperature and pH — their optimum. Raise the temperature and reactions speed up at first, because molecules collide more often. But too hot and the enzyme is ruined: the active site changes shape so the substrate no longer fits. This is called denaturation. The wrong pH denatures an enzyme too.

Part 3Investigating enzyme rate

You can measure how a condition such as pH changes how fast amylase breaks down starch. The trick is to use iodine solution: it goes blue-black with starch, but stays orange-brown once all the starch is gone — so the colour tells you when the reaction has finished.

Part 1Breaking big molecules down

The food you eat is made of large, insoluble molecules. Digestive enzymes break them down into small, soluble molecules that can be absorbed into the blood. There are three groups, each with its own substrate and products.

Each enzyme is made in particular places. Amylase is made in the salivary glands, the pancreas and the small intestine. Protease is made in the stomach (where it is called pepsin), the pancreas and the small intestine. Lipase is made in the pancreas and the small intestine.

Part 2The job of bile

Bile is made in the liver and stored in the gall bladder. It is released into the small intestine, where it does two things. First, it is alkaline, so it neutralises the acid that arrives from the stomach — this gives the right pH for the enzymes in the small intestine to work.

Second, bile emulsifies fat: it breaks large fat droplets into many tiny droplets. This gives a much larger surface area for lipase to work on, so fat is digested faster. Note that bile is not an enzyme — it doesn't break any chemical bonds itself.

Part 1The organs of the digestive system

The digestive system is an organ system in which several organs work together to break down and absorb food. Food travels through a long tube and is digested along the way.

The lining of the small intestine is folded into millions of tiny finger-like villi. They give a huge surface area, have a thin wall (one cell thick) and a rich blood supply — all of which speed up the absorption of digested food.

Part 2Testing food for its molecules

You can test a food sample to find out which molecules it contains. Each test gives a colour change. Learn the reagent, the colour change and which molecule it shows.

Part 1The heart & double circulation

The heart is an organ made mostly of muscle that pumps blood around the body. Humans have a double circulation: the blood passes through the heart twice for one trip around the body.

One loop carries blood from the heart to the lungs to pick up oxygen and return. The other loop carries oxygenated blood from the heart to the rest of the body and back. A double circulation keeps the pressure high so blood reaches the body quickly and efficiently.

The heart's natural resting rhythm is set by a group of cells in the right atrium called the pacemaker. If it fails, doctors can fit an artificial pacemaker — a small electrical device that keeps the heartbeat regular.

Part 2The three blood vessels

There are three types of blood vessel, each adapted to its job.

Part 1The four components of blood

Blood is a tissue. It is a straw-coloured liquid called plasma with three kinds of cell suspended in it: red blood cells, white blood cells and platelets.

Red blood cells are beautifully adapted to carry oxygen. They have no nucleus, leaving more room for haemoglobin; their biconcave disc shape gives a large surface area for oxygen to diffuse across; and they are packed with haemoglobin, which binds oxygen in the lungs and releases it in the tissues.

Part 1The breathing system

The lungs are the organs of gas exchange. Air travels in through the trachea, which splits into two bronchi, then into smaller bronchioles, ending in millions of tiny air sacs called alveoli.

At the alveoli, oxygen diffuses from the air into the blood, and carbon dioxide diffuses from the blood into the air to be breathed out. The job is to make this exchange happen as quickly as possible.

Part 2How alveoli are adapted

The alveoli are superbly adapted to maximise the rate of diffusion. Learn these four adaptations and what each one does:

Part 1Health and risk factors

Health is the state of physical and mental wellbeing. Diseases are a major cause of ill health. A communicable disease can be spread between people (caused by pathogens); a non-communicable disease cannot be spread — it tends to last a long time and worsen slowly.

A risk factor is anything that increases the chance of getting a disease. Some are aspects of lifestyle (such as diet, smoking and exercise); others are substances in the body or environment. Risk factors are correlated with disease, but a correlation does not always prove that one thing causes the other.

Part 2Coronary heart disease & cancer

In coronary heart disease (CHD), layers of fatty material build up inside the coronary arteries — the vessels that supply the heart muscle itself. This narrows them, so less blood (and less oxygen) reaches the heart muscle, which can cause chest pain or a heart attack.

One treatment is a stent — a tube inserted to hold a narrowed artery open so blood flows freely. Another is statins — drugs that reduce blood cholesterol, slowing the build-up of fatty deposits.

A tumour forms when cells divide uncontrollably. There are two kinds. A benign tumour stays in one place inside a membrane and does not spread — it is usually not dangerous. A malignant tumour is a cancer: its cells invade neighbouring tissues and can spread in the blood to other parts of the body, forming secondary tumours.

Part 1Leaf structure & plant tissues

A leaf is a plant organ adapted for photosynthesis. It contains several tissues. The upper epidermis is transparent to let light through. Below it, the palisade mesophyll is packed with chloroplasts to absorb light. The spongy mesophyll has air spaces for gases to diffuse, and guard cells control tiny pores called stomata on the underside.

Running through the plant are two transport tissues. Xylem carries water and dissolved mineral ions up from the roots to the leaves. Phloem carries dissolved sugars (food made in the leaves) to the rest of the plant.

Part 2Transpiration & translocation

Transpiration is the loss of water vapour from a plant, mostly through the stomata in the leaves. As water evaporates from the leaves, more water is pulled up through the xylem from the roots — this is the transpiration stream. It supplies water for photosynthesis and keeps the plant cool and supported.

The rate of transpiration increases with higher temperature, more wind (air movement), lower humidity and brighter light. The guard cells control water loss: in the light they swell and open the stomata to let carbon dioxide in; in the dark, or when water is short, they close the stomata to reduce water loss.

Translocation is the transport of dissolved sugars (and other foods) through the phloem, from the leaves where they are made to other parts of the plant — for use straight away, or for storage.