Animal tissues, organs and organ systems

The digestive systemSpec 4.2.2.1

The digestive system is an example of an organ system in which several organs work together to digest and absorb food.

Food contains large molecules of carbohydrate, protein and lipid. These are too big to pass into the blood. Digestive enzymes convert food into small soluble molecules that can be absorbed into the bloodstream.

Tap to enlarge

EnzymesSpec 4.2.2.1

Enzymes catalyse specific reactions in living organisms. A catalyst speeds up a reaction and is not used up. Enzymes are large protein molecules, and each one has a special shape.

Each enzyme has an active site. The shape of the active site is what makes the enzyme specific: it only works on one type of molecule, called the substrate.

The lock and key theory

The ‘lock and key theory’ is a simplified model of how enzymes work.

1. The substrate collides with the enzyme and fits into the active site, like a key into a lock. The shapes are complementary.
2. The enzyme catalyses the reaction while the substrate is in the active site.
3. The products leave the active site. The enzyme is unchanged and can be used again.

Scientists also use other models to explain enzyme action. If a question describes a different model, use the information given in the question.

Temperature and pH

As temperature rises, the reaction gets faster because the particles move faster and collide more often. Each enzyme has an optimum temperature where its rate is highest.

If the temperature is too high, or the pH is too acidic or too alkaline, the shape of the active site changes. The substrate no longer fits. The enzyme is denatured, and it cannot be changed back.

Tap to enlarge

Rate calculations

When the end point is a colour change, such as the loss of starch, you can use rate = 1 ÷ time taken. A shorter time means a faster rate.

Amylase, proteases and lipasesSpec 4.2.2.1

Three types of digestive enzyme break the three main food molecules into small, soluble molecules. You need to recall where each is made and what it does.

Carbohydrases break down carbohydrates to simple sugars. Amylase is a carbohydrase which breaks down starch.

You only need simple word equations, not chemical symbol equations.

The products of digestion are used to build new carbohydrates, lipids and proteins. Some glucose is used in respiration.

BileSpec 4.2.2.1

Bile is made in the liver and stored in the gall bladder. It is released into the small intestine.

Bile does two jobs:

• It is alkaline, so it neutralises hydrochloric acid that comes from the stomach. This gives the enzymes in the small intestine the alkaline conditions they work best in.
• It emulsifies fat, breaking it into small droplets. This increases the surface area of the fat.

The alkaline conditions and the large surface area increase the rate of fat breakdown by lipase.

Required practical: food testsSpec 4.2.2.1

You can use qualitative reagents to test for a range of carbohydrates, lipids and proteins. Qualitative means the test tells you whether a food molecule is present, not how much.

1. Prepare a food sample: grind a small amount of food with water and filter it or let it settle. Test the liquid.
2. Add the test reagent to a sample of the food solution.
3. Look for the colour change and compare with the table below.

Some schools use the ethanol emulsion test for lipids instead: shake the sample with ethanol, then pour it into water. A cloudy white emulsion shows that lipid is present.

The more the Benedict's colour moves towards brick-red, the more sugar there is. If the colour stays blue, no sugar is present.

Required practical: the effect of pH on amylaseSpec 4.2.2.1

This practical investigates how pH affects the rate of reaction of the enzyme amylase. You use a continuous sampling technique to find the time taken to completely digest a starch solution at a range of pH values.

1. Put a drop of iodine solution into each well of a spotting tile.
2. Add starch solution and a buffer solution of a known pH to a test tube. Add amylase solution.
3. Place the tube in a water bath (or use an electric heater) so the temperature stays constant. Start a stopwatch.
4. Every 30 seconds, take a sample from the tube and add it to a fresh drop of iodine.
5. Stop when the iodine stays orange-brown. All the starch has been digested. Record the time taken.
6. Repeat at a range of pH values, using a different buffer each time.

The shorter the time, the faster the rate. The rate is highest at the optimum pH. Away from the optimum, amylase is denatured and the starch takes much longer to digest, or is not digested at all.

Tap to enlarge

The heart and the double circulatory systemSpec 4.2.2.2

The heart is an organ that pumps blood around the body in a double circulatory system. There are two separate loops, and blood passes through the heart twice on each complete circuit.

• The right ventricle pumps blood to the lungs, where gas exchange takes place.
• The left ventricle pumps blood around the rest of the body.

Blood returns from the body to the right atrium and from the lungs to the left atrium. The ventricles have thick muscular walls. The left ventricle has a thicker wall than the right, because it has to pump blood all the way around the body at a higher pressure. Valves in the heart stop blood flowing backwards. You do not need to know the names of the valves.

1. Deoxygenated blood from the body enters the right atrium through the vena cava.
2. It passes into the right ventricle, which pumps it to the lungs through the pulmonary artery.
3. In the lungs the blood picks up oxygen and loses carbon dioxide.
4. Oxygenated blood returns through the pulmonary vein to the left atrium.
5. It passes into the left ventricle, which pumps it around the body through the aorta.

Tap to enlarge

Controlling the heart rateSpec 4.2.2.2

The natural resting heart rate is controlled by a group of cells located in the right atrium that act as a pacemaker.

Sometimes the heart rate is irregular. Artificial pacemakers are electrical devices used to correct irregularities in the heart rate.

The lungs and gas exchangeSpec 4.2.2.2

You need to know the structure of the lungs and how they are adapted for gaseous exchange. Knowledge of the lungs is restricted to the trachea, bronchi, alveoli and the capillary network surrounding the alveoli.

1. Air passes down the trachea.
2. The trachea divides into two bronchi, one to each lung.
3. The bronchi branch into smaller tubes that end in alveoli (air sacs).
4. At the alveoli, oxygen diffuses into the blood in the surrounding capillaries and carbon dioxide diffuses out of the blood into the air.

Blood arriving at the lungs has a low concentration of oxygen and a high concentration of carbon dioxide. Oxygen therefore diffuses from the air in the alveoli into the blood, and carbon dioxide diffuses the other way.

Tap to enlarge

Blood vesselsSpec 4.2.2.2

The body contains three different types of blood vessel: arteries, veins and capillaries. Their structures are related to their functions.

Arteries have thick, strong walls because the blood is at high pressure. The elastic fibres let the wall stretch and spring back. The muscle strengthens the wall.

Veins have a wide lumen and valves. The blood is at low pressure, so the valves stop it flowing backwards.

Capillaries are tiny and have very thin walls. This gives a short diffusion distance, so oxygen and nutrients can pass to the cells and carbon dioxide and waste can pass back into the blood. Capillary walls are permeable.

Blood flow rate

Tap to enlarge

BloodSpec 4.2.2.3

Blood is a tissue consisting of plasma, in which the red blood cells, white blood cells and platelets are suspended.

In the lungs, haemoglobin combines with oxygen to form oxyhaemoglobin. In the body tissues the oxyhaemoglobin releases the oxygen.

Recognising blood cells

You may need to identify cells in a photograph or diagram.

• Red blood cells are the most numerous. They are small, round and have no nucleus, and look paler in the middle.
• White blood cells are larger and have a clear nucleus, often stained dark purple.
• Platelets are tiny fragments, much smaller than the other cells.

Blood products, such as donated blood used in a transfusion, can save lives. There are risks, for example infection can be passed on from the donor, so donated blood has to be checked and matched to the patient.

Tap to enlarge

Coronary heart diseaseSpec 4.2.2.4

Coronary heart disease (CHD) is a non-communicable disease: it is not caused by a pathogen and cannot be passed from person to person.

In CHD, layers of fatty material build up inside the coronary arteries, narrowing them. This reduces the flow of blood through the coronary arteries, resulting in a lack of oxygen for the heart muscle.

• Stents are used to keep the coronary arteries open.
• Statins are widely used to reduce blood cholesterol levels, which slows down the rate of fatty material deposit.

1. Fatty material builds up inside a coronary artery.
2. The artery narrows, so less blood flows through it.
3. The heart muscle receives less oxygen.

Tap to enlarge

Treating cardiovascular diseaseSpec 4.2.2.4

Faulty heart valves

In some people heart valves may become faulty. A valve may be prevented from opening fully, or it might develop a leak.

• If a valve does not open fully, less blood can flow through, so the heart has to work harder and the body may receive less oxygen.
• If a valve leaks, blood flows backwards, so the heart pumps less blood forward with each beat.

Faulty heart valves can be replaced using biological or mechanical valves.

Heart failure

In the case of heart failure, a donor heart, or heart and lungs, can be transplanted. Artificial hearts are occasionally used to keep patients alive whilst waiting for a heart transplant, or to allow the heart to rest as an aid to recovery.

Evaluating treatments

You need to evaluate the advantages and disadvantages of treating cardiovascular diseases by drugs, mechanical devices or transplant.

Mechanical valves last a long time, but blood clots are more likely to form on them, so the patient must take drugs to stop clotting for the rest of their life. Biological valves are less likely to cause clots, but they wear out sooner and may need replacing.

Health and diseaseSpec 4.2.2.5

Health is the state of physical and mental well-being.

Diseases, both communicable and non-communicable, are major causes of ill health. A communicable disease is caused by a pathogen and can spread from person to person. A non-communicable disease cannot. Other factors including diet, stress and life situations may have a profound effect on both physical and mental health.

How different types of disease interact

Handling data about diseaseSpec 4.2.2.5

You need to be able to translate disease incidence information between graphical and numerical forms. Incidence is how often a disease occurs in a population.

• Construct and interpret frequency tables and diagrams.
• Use bar charts for data in separate categories, and histograms for continuous data grouped into ranges.
• Use a scatter diagram to identify a correlation between two variables. A positive correlation means one variable increases as the other increases. A negative correlation means one decreases as the other increases. No pattern means no correlation.

Sampling

It is not possible to test everyone, so scientists collect data from a sample. In epidemiological studies, a larger sample that is representative of the population gives more reliable data.

Tap to enlarge

Risk factors and causesSpec 4.2.2.6

Risk factors are linked to an increased rate of a disease. They can be:

• aspects of a person's lifestyle
• substances in the person's body or environment.

A causal mechanism has been proven for some risk factors, but not for others. A correlation between a risk factor and a disease does not prove that the risk factor causes the disease. A causal mechanism explains how the risk factor leads to the disease.

Many diseases are caused by the interaction of a number of factors. For example, CHD is affected by diet, smoking and how much exercise a person takes, as well as other factors.

The effect of lifestyle on diseaseSpec 4.2.2.6

You need to be able to explain the effect of lifestyle factors including diet, alcohol and smoking on the incidence of non-communicable diseases.

Because lifestyles differ, the incidence of these diseases is different in different places. Diseases linked to diet and obesity, for example, are more common in places where people eat more high-energy food and take less exercise. This is true at local, national and global levels.

The cost of non-communicable diseasesSpec 4.2.2.6

You need to be able to discuss the human and financial cost of these diseases to an individual, a local community, a nation or globally.

Prevention through diet and lifestyle can reduce these costs, because many interventions would not be necessary if individuals reduced their risks.

Using data about risk factorsSpec 4.2.2.6

You need to interpret data about risk factors for specified diseases. The same skills apply as for incidence data.

• Translate information between graphical and numerical forms.
• Extract and interpret information from charts, graphs and tables.
• Use a scatter diagram to identify a correlation between a risk factor and a disease.
• Understand sampling: the sample should be large and representative for the conclusion to be reliable.

Describe what the data show first, then say whether there is a correlation. Then say whether the data prove a cause. A correlation alone does not. You also need a causal mechanism, or evidence that other factors were controlled.

CancerSpec 4.2.2.7

You need to be able to describe cancer as the result of changes in cells that lead to uncontrolled growth and division. These cells can form a lump called a tumour.

Scientists have identified lifestyle risk factors for various types of cancer, such as smoking, obesity and drinking alcohol. There are also genetic risk factors for some cancers. Viruses living in cells can also trigger some cancers.