Organisation

1. From cells to organ systems

A cell is the smallest living unit. A tissue contains cells with similar structures working together. An organ contains different tissues working together. An organ system contains organs that carry out a major body function.

For example: a muscle cell → muscle tissue → stomach → digestive system → human organism. The stomach also contains glandular tissue that makes digestive substances and epithelial tissue that covers surfaces. One organ needs several tissues because it has several jobs.

2. Digestion and absorption

Large food molecules cannot pass easily through the gut wall. Digestion breaks them into small, soluble molecules. Absorption is their movement through the small-intestine wall into the body. These are different processes.

• Mouth: teeth break food into smaller pieces; saliva adds amylase.
• Oesophagus: muscular contractions push food to the stomach.
• Stomach: mixes food, releases protease and hydrochloric acid. Acid helps kill swallowed microbes and provides the low pH needed by stomach protease.
• Pancreas: releases digestive enzymes into the small intestine.
• Liver and gall bladder: make and store bile respectively; bile enters the small intestine.
• Small intestine: digestion is completed and soluble products are absorbed. Villi provide a large surface area, a thin surface and a good blood supply.
• Large intestine: absorbs water. Undigested material leaves through the rectum and anus.

Absorbed sugars, amino acids and lipid products provide raw materials for growth and repair. Cells use them to build new carbohydrates, proteins and lipids. Glucose also releases energy through respiration. These chemical reactions form part of metabolism.

3. Enzymes: shape makes them specific

An enzyme is a protein that acts as a biological catalyst. It speeds up a reaction without being used up. A substrate is the molecule an enzyme acts on. The substrate binds at a specially shaped region called the active site.

In the simplified lock-and-key model, only a substrate with a matching shape fits. An enzyme therefore catalyses a specific reaction. Products leave the active site, allowing the enzyme to be used again.

In cooler conditions, particles move more slowly and collide less often, so the reaction is slower. Warming towards the optimum temperature increases successful collisions. Above the optimum, bonds maintaining the enzyme's shape are disrupted. Its active site changes shape, so the substrate no longer fits well. This is denaturation.

Each enzyme also has an optimum pH. Moving far from it can change the active-site shape. A stomach protease works in acidic conditions; many small-intestine enzymes work best in alkaline conditions.

4. Which enzymes digest which foods?

Carbohydrases as a group break carbohydrates down into simple sugars. At this level, use the word equations above; digestive chemical symbol equations are not required.

Bile is not an enzyme. It is made by the liver, stored in the gall bladder and released into the small intestine. It is alkaline, so it neutralises acid arriving from the stomach. It also emulsifies fat: large fat globules become many small droplets. This provides more surface area for lipase to act on. Emulsification does not itself digest fat molecules.

5. Required practical 3: testing foods

These are qualitative tests: they identify whether a substance is present. Prepare food extracts by crushing solid food with distilled water and filtering if needed. Use separate clean samples for each test. Wear eye protection, avoid skin contact with reagents and never taste food in a laboratory.

An alternative lipid test is the ethanol emulsion test: shake the sample with ethanol, then add the extract to water. A cloudy white emulsion indicates lipid. Ethanol is flammable: use no naked flame. Keep hot water baths away from ethanol.

Use a known positive sample to check the reagent works and water as a negative control. Compare samples using the same volumes and conditions. A negative result means the substance was not detected; very small amounts may be below the test's detection limit.

6. Required practical 4: pH and amylase

Question: how does pH affect the time amylase takes to digest starch? Use solutions prepared at different known pH values.

1. Put drops of iodine in the wells of a spotting tile.
2. Measure equal volumes of starch and amylase solutions. Place the prepared solutions in a water bath so they reach the chosen temperature.
3. Mix the starch and amylase solutions at the chosen pH; start timing immediately.
4. Every 30 seconds, remove a drop and add it to a fresh iodine well. Blue-black means starch remains.
5. Record the first sampling time at which iodine stays orange-brown. Starch is no longer detected.
6. Repeat at each pH, keeping temperature, enzyme and starch concentrations, volumes and sampling intervals the same.
7. Repeat measurements and calculate mean times, checking unexpected results rather than silently removing them.

The independent variable is pH; the measured dependent variable is time until starch is no longer detected. The water bath controls temperature. Use fresh pipettes or wash them to prevent contamination. Take care with hot water and iodine.

For equal amounts of starch, an estimated rate is 1 ÷ time. If time is 60 s, rate = 1/60 = 0.0167 s⁻¹. A shorter time means a faster reaction.

Plot pH on the horizontal axis and rate on the vertical axis. Do not assume every enzyme has an optimum of pH 7: use the evidence supplied.

7. The heart and double circulation

The heart is a muscular pump. In a double circulatory system, blood passes through the heart twice during one complete journey around the lungs and body.

Body → vena cava → right atrium → right ventricle → pulmonary artery → lungs → pulmonary vein → left atrium → left ventricle → aorta → body.

The right ventricle pumps blood to the nearby lungs. The left ventricle has a thicker muscular wall because it must produce higher pressure to supply the whole body. Valves prevent backflow. The septum separates the left and right sides so oxygenated and deoxygenated blood do not mix.

The coronary arteries supply the heart muscle itself with oxygen and glucose for respiration. Blood inside the chambers does not replace this supply.

8. Heart rate and blood-flow calculations

A group of cells in the right atrium acts as the heart's natural pacemaker. Electrical signals coordinate the heartbeat. An artificial pacemaker supplies electrical impulses to help control an abnormal heart rhythm.

For flow calculations, use volume per unit time. If a heart pumps 70 cm³ per beat at 72 beats per minute, blood flow = 70 × 72 = 5,040 cm³/min, or 5.04 dm³/min. Remember: 1,000 cm³ = 1 dm³.

9. Lungs and gas exchange

Air travels down the trachea, into the two bronchi and through smaller branches to the alveoli, the tiny air sacs. A capillary network surrounds each alveolus.

Oxygen moves by diffusion from alveolar air into blood. Carbon dioxide diffuses in the opposite direction. The many alveoli provide a large surface area. Alveolar and capillary walls are thin, giving a short diffusion distance. Breathing refreshes alveolar air and blood flow carries oxygen away, maintaining steep concentration gradients.

Gas exchange and respiration are different. Gas exchange moves gases between air and blood. Respiration is a set of chemical reactions in cells that release energy.

10. Blood vessels: structure fits function

A lumen is the space inside a vessel. Arteries do not all carry oxygenated blood. Their name describes their direction relative to the heart.

11. Blood is a tissue

Blood contains a liquid called plasma with cells and cell fragments suspended in it.

• Red blood cells: carry oxygen using haemoglobin. Their biconcave shape gives a large surface area and short diffusion distance. Mature human red cells have no nucleus, leaving more space for haemoglobin.
• White blood cells: defend against pathogens. Some engulf pathogens; others produce antibodies or antitoxins. They have nuclei and are usually larger and fewer than red cells.
• Platelets: small cell fragments involved in clotting. Clots reduce blood loss and help prevent pathogens entering a wound.
• Plasma: transports cells and dissolved substances, including glucose, amino acids, carbon dioxide, urea and hormones. It also distributes thermal energy.

In an image, look for numerous disc-shaped red cells, larger nucleated white cells and very small platelet fragments. Blood products can be life-saving, but compatibility and screening matter because immune reactions and infection transmission are possible risks.

12. Coronary heart disease and treatments

In coronary heart disease, fatty deposits narrow coronary arteries. Less blood reaches heart muscle, so it receives less oxygen. Aerobic respiration falls; severe blockage can damage or kill heart muscle.

Choose a treatment by weighing the problem, likely benefit and risks for the patient. A pacemaker corrects rhythm; a stent opens an artery. They solve different problems.

13. Health and interacting diseases

Health includes physical and mental well-being. A person can have no diagnosed infection but still have poor health because of stress, diet or difficult living circumstances.

Communicable diseases can spread between organisms, usually through pathogens. Non-communicable diseases do not spread by infection, although infection can contribute to some of them.

• A damaged immune system makes infections more likely.
• Some viruses cause changes in cells that increase cancer risk.
• An immune response initially triggered by infection can contribute to allergies, such as some rashes or asthma.
• Severe physical illness can contribute to depression and other mental illness.

Illness can affect family life, employment and independence. Communities and nations bear costs through healthcare, caring responsibilities and lost productivity. For example, a person may lose income or independence, relatives may provide unpaid care, local services may face greater demand, and national health systems may pay for long-term treatment. Comparing local, national and global patterns also requires considering differences in age, access to healthcare and data collection. These impacts are not just financial.

14. Lifestyle, risk and evidence

A risk factor is associated with a greater chance of disease. It does not guarantee that someone will develop it. Multiple factors can interact, including inherited factors, age, environment and lifestyle.

• Diet, smoking and physical activity influence cardiovascular-disease risk.
• Obesity increases the risk of Type 2 diabetes.
• Alcohol can damage the liver and affect brain function.
• Smoking damages lungs and increases the risk of lung cancer.
• Smoking during pregnancy can reduce oxygen delivery to the developing baby. Alcohol can harm development of the baby's brain and other organs.
• Carcinogens are cancer-causing agents. Tobacco-smoke chemicals and ionising radiation are examples.

A diet high in saturated fat can raise blood cholesterol and contribute to fatty artery deposits. Regular physical activity helps maintain cardiovascular fitness and a healthy body mass. If energy intake regularly exceeds energy use, body fat can increase. Smoking damages blood vessels as well as lungs. These mechanisms help explain some associations; they do not make disease inevitable.

A correlation means two variables are associated. It does not by itself establish a cause. An observed link may involve a third variable, or confounding factor. Researchers look for repeatable patterns and plausible mechanisms, and compare groups while considering other differences.

Use large, representative samples. A survey of one sports club may not represent a whole town. Compare rates per equal population sizes, not just case totals.

15. Presenting and interpreting health data

Use a bar chart for distinct categories, such as smoker/non-smoker. Use a histogram for continuous data grouped into intervals, such as age. Use a scatter graph to investigate an association between two numerical variables. Give both axes labels and units where appropriate.

In a scatter graph, points rising from left to right suggest positive correlation. Falling points suggest negative correlation. A scattered pattern may show no clear correlation. Describe the pattern before explaining it.

Frequency tables organise counts. Include every group, label intervals without overlap and check that frequencies add to the sample size. When drawing histograms here, use equal-width intervals so frequencies can be compared directly.

16. Cancer: uncontrolled cell division

Cancer develops when changes in cells lead to uncontrolled growth and division. A mass of abnormal cells is a tumour.

A benign tumour remains in one area and does not invade other tissues. It may still cause harm by pressing on structures. A malignant tumour is cancerous: cells invade neighbouring tissue and can travel in the blood to establish secondary tumours elsewhere.

Risk factors include inherited genetic variants and lifestyle or environmental exposure to carcinogens. A risk factor increases probability; it is not proof of why one individual developed cancer.

17. A leaf is an organ

A leaf contains several tissues that work together to carry out photosynthesis and exchange gases.

• Epidermis: covers and protects the leaf. The upper epidermis allows light to reach photosynthesising cells. A waxy cuticle helps reduce water loss.
• Palisade mesophyll: closely packed cells near the top of the leaf with many chloroplasts to absorb light.
• Spongy mesophyll: loosely arranged cells with air spaces, allowing gases to diffuse through the leaf.
• Xylem: carries water and mineral ions into the leaf.
• Phloem: carries dissolved sugars to other parts of the plant.
• Guard cells: surround each stoma and change the pore's opening, balancing gas exchange and water loss.

Meristems at root and shoot tips contain cells that divide and can differentiate into new plant tissues. Meristems enable growth; they are not a mature leaf layer.

18. Transport through a plant

Roots, stems and leaves work as an organ system. Root hair cells have a long extension giving a large surface area. Water enters by osmosis; mineral ions can enter by active transport, using energy from respiration when ions move against a concentration gradient.

Xylem vessels are continuous hollow tubes formed from dead cells. Thickened lignified walls strengthen the tubes and help keep them open. They carry water and dissolved mineral ions upwards from roots to stems and leaves.

Phloem contains elongated living cells joined end to end. Pores in the end walls let cell sap pass between cells. Movement of dissolved sugars through phloem is translocation. Sugars move from sources, such as photosynthesising leaves, to places where they are used or stored. Movement can be upwards or downwards in different tubes.

19. Transpiration and environmental conditions

Transpiration is the loss of water vapour from the aerial parts of a plant, mainly leaves. Water evaporates from moist mesophyll cell surfaces into leaf air spaces. Water vapour then diffuses out through open stomata. This loss helps draw a continuous stream of water up xylem.

These are usual trends when other factors are controlled. Severe water shortage can cause stomata to close, reducing water loss but also limiting carbon-dioxide entry.

20. Investigating water uptake and stomata

A potometer estimates water uptake by following an air bubble along a narrow capillary tube connected to a leafy shoot. It estimates transpiration because most absorbed water replaces evaporated water, but some is used in growth and photosynthesis.

1. Cut and fit the shoot under water to prevent unwanted air entering xylem. Seal joints so the apparatus is airtight and watertight.
2. Allow the shoot to adjust to the conditions. Introduce one measurable bubble into the capillary.
3. Measure bubble distance over a fixed time. Use a reservoir to reset it if the apparatus has one.
4. Change only one environmental factor; control temperature, light, humidity and airflow as appropriate. Repeat and calculate means.

Distance rate = distance ÷ time. To find volume uptake, use volume = capillary cross-sectional area × distance. For a circular tube, area = πr². If r = 0.5 mm and distance = 40 mm, volume = π × 0.5² × 40 = 31.4 mm³. In 5 min, rate = 6.28 mm³/min.

For stomata, examine epidermal impressions with a microscope. Count stomata in several randomly selected fields of known area. Calculate stomatal density = number ÷ area, then a mean. Sample both surfaces and several leaves rather than choosing the most crowded field.

For a continuous variable such as light intensity, plot it on the horizontal axis and water-uptake rate on the vertical axis. Select a scale that uses the graph area effectively and include units.

Use iodine to follow starch digestion

A student mixes starch and amylase at a constant temperature. Every 30 seconds, a drop of the mixture is added to fresh iodine on a spotting tile.

At 30 and 60 seconds, the iodine turns blue-black. At 90 seconds, it stays orange-brown.

Build a frequency table and histogram

A sample has these ages in years: 12, 14, 17, 21, 23, 28, 29, 31, 35, 38, 41, 43. On paper, group them into 10–19, 20–29, 30–39 and 40–49. Draw a histogram using equal-width intervals and touching bars.

Read a transpiration pattern

On paper, plot humidity horizontally and uptake vertically. Describe the pattern, then explain it using a concentration gradient.

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One final thought

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