AQA GCSE Biology Past Paper - Cell Biology PDF

## AQA GCSE Biology Topic 1: Cell Biology ## ### Required practical activity 1 ### **Describe how to prepare a microscope slide** * **Plant** example: Peel off a thin layer of onion tissue using forceps, and then place on a water drop on a slide. * **Animal** example: Swab the inside of your c...

## AQA GCSE Biology Topic 1: Cell Biology ## ### Required practical activity 1 ### **Describe how to prepare a microscope slide** * **Plant** example: Peel off a thin layer of onion tissue using forceps, and then place on a water drop on a slide. * **Animal** example: Swab the inside of your check with a cotton bud, and then smear on the center of the slide. **Add a drop of stain with a pipette** **Lower a coverslip at an angle without trapping air bubbles** **Describe how to use a microscope to view cells** 1. **Clip** the slide onto the stage, and turn on the light. 2. **Select** the lowest power objective lens (usually x4). * Use the **coarse focusing dial** to move the stage close to the lens * Turn the **coarse focusing dial** to move the stage away from the lens until the image comes into focus. 3. **Adjust** the fine focusing dial to get a clear image 4. **Swap** to a higher power objective lens, and then refocus. **How is the total magnification of a microscope calculated?** Magnification of eyepiece lens x magnification of objective lens. For example, 10 x 4 = x 40 **How can the sizes of cells be estimated using a microscope?** 1. **Measure** the diameter of the field of view (using the microscope to observe a transparent ruler). 2. **Divide** this by the number of cells that span the area of the field of view. **What are the rules of scientific drawing?** * Use **clear, continuous lines** with a sharp pencil, and **no sketching or shading**. * **Include** a magnification scale (e.g., x 400). * **Label** important features (e.g., nucleus). **Example application questions** * **Why use a stain?** To make sub-cellular structures visible. * **Why is a thin layer of onion tissue used?** To let light pass through. * **Why is the cover slip lowered at an angle?** To avoid trapping air bubbles. * **Why is it easier to view cells using the low power objective lens first?** The image is easier to focus. * **Why do you need to wear gloves?** To prevent the spread of possible infection. * **How could a student observe structures within a cell in greater detail?** Use a higher power objective lens. ### 1.1.3 Cell specialisation ### **Describe 3 examples of specialised cells in animals** | **Cell** | **Function** | **How is its structure related to its function?** | |---|---|---| | Sperm cell | Fertilise an egg | Long tail/flagellum: Allows sperm to swim/move towards egg. Many mitochondria: ↑ Rate of respiration to release energy for swimming. | | Nerve cell | Carry electrical impulses | Long: Carry electrical impulses/signals over long distances. Many branches: Connect to many other cells forming a network. Insulation: Speeds up transmission of impulses.| | Muscle cell | Contract | Many mitochondria: ↑ Rate of respiration to release energy for contraction. | **Describe 3 examples of specialised cells in plants** | **Cell** | **Function** | **How is its structure related to its function?** | |---|---|---| | Root hair cell | Absorb water & mineral ions from soil | Long projection: ↑ Surface area for the absorption of water by osmosis. Many mitochondria: ↑ Rate of respiration to release energy for active transport of mineral ions.| | Xylem cell | Transport water & mineral ions from roots to leaves | Lignin in cell wall: Strength to withstand the pressure of water moving. Hollow tubes/no cytoplasm: Water/mineral ions move easily. End walls between cells broken down: Cells form a long tube so water/mineral ions can flow easily.| | Phloem cell | Transport dissolved sugars up & down the plant | No nucleus: Maximise space for the movement of dissolved sugars. Pores in end walls: Dissolved sugars can move from cell to cell | **Example application question** * **Root hair cells do not contain chloroplasts. Suggest why?** Root hair cells are not exposed to light, so do not photosynthesise. * **Suggest how a cell in the salivary glands would be adapted for producing amylase (enzyme)?** Many ribosomes would be needed to produce amylase protein. Many mitochondria would be needed to release energy to make protein. ### 1.1.4: Cell differentiation ### **What happens as a cell differentiates?** * It acquires different sub-cellular structures * It is able to carry out a specific function * It becomes a specialised cell. **When do animal cells differentiate?** * Most types of animals differentiate at an early stage, in embryo development. * In mature animals, cell division is mainly restricted to repair and replacement (e.g., new blood/skin cells). **When do plant cells differentiate?** * Plant cells retain the ability to differentiate throughout their life. **Why is cell differentiation important?** * As an organism develops, cells differentiate to form different types of cells. * Specialised cells make different parts form the body, tissues, or organs. **Exam insight: common misconception** * **"All cells are differentiated in adults."** This is incorrect, STEM cells remain in certain parts of the body (e.g., bone marrow). These are undifferentiated cells that can differentiate into other cell types (e.g., blood cells). ### 1.1.5 Microscopy### **What is the difference between magnification and resolution?** * **Magnification:** The number of times larger an image is compared to a real object. * **Resolution:** The shortest distance between 2 points that can be seen as separate points (level of detail). **What is the difference between light and electron microscopes?** * **Light Microscopes** use light to form the image, whereas **electron microscopes** use electrons to form the image. * **Electron microscopes** have higher magnification and resolution, so can study cells in much finer detail (e.g., see more sub-cellular structures, such as ribosomes). **List the steps in calculations involving magnification, real size, and image size** 1. **Note** the formula, and then rearrange it, if necessary (I = AM). 2. **Convert** units if necessary, making sure that the image and real sizes are in the **same unit**. 3. **Calculate** the answer, and then **check** for standard form, if required. **How can you convert between different units?** See the table on page 9 of the document. **Exam insight: common misconception** * **"An electron microscope is a microscope that uses electricity."** This is incorrect. An electron microscope uses electrons to form an image. * **"An electron microscope focuses better, or is clearer."** While electron microscopes have higher resolution, magnification is a separate matter. **Example application questions** * **Why can't you see ribosomes with a light microscope?** They are too small. * **One muscle cell has a diameter of 0.15 mm. When viewed with a microscope the muscle cell in the image had a diameter of 0.6 cm. Calculate the magnification used.** Magnification= 6 mm / 0.15 mm = x 40 * **The actual length of a cell structure is 30 µm. It is magnified 40 times. Calculate the length of the magnified cell structure in mm.** The length of the magnified cell structure is 1200 micrometers or 1.2 mm. * **The figure shows a root hair viewed using a microscope at a magnification of x 50. The image length of the root hair X-Y is 43 mm. Calculate the real length of the root hair in millimeters.** 43 mm / 50 = 0.86 millimeters. ### 1.2 Cell division ### **Fundamental biological concept/principle 1** "The structure and functioning of cells and how they divide by mitosis and meiosis from sections Cell biology and Meiosis." **Describe the stages of the cell cycle, including mitosis** The cell cycle is a series of stages in which a cell divides (DNA is doubled and then divided into two identical cells). * **Stage 1:** DNA replicates to form two copies of each chromosome. The cell grows and the number of sub-cellular structures increases (e.g., ribosomes and mitochondria). * **Stage 2 (Mitosis):** One set of chromosomes is pulled to each end of the cells, and the nucleus divides to form two nuclei. * **Stage 3:** The cytoplasm and cell membrane divide, forming two genetically identical cells. **Exam insight: common misconception** * **"DNA divides before mitosis"** This is incorrect. DNA has to replicate/double before mitosis, so that the new cells formed each have the same number of chromosomes as the original cell. * **"Mitosis repairs cells"** Mitosis does not **repair** cells. Rather, mitosis creates new cells to **replace** damaged cells, helping to repair tissue. **Example application question** * **What is happening in the photograph of this animal cell during a phase of the cell cycle?** The cytoplasm and cell membrane are dividing, forming two identical daughter cells. ### 1.2.3: Stem cells ### **What is a stem cell?** A stem cell is an undifferentiated cell that can: * Give rise to many more cells of the same type * Differentiate into certain other cells **Describe the two types of animal stem cells** * **Embryonic stem cells:** Found in early embryos, they can differentiate into any type of body cell. * **Adult stem cells:** Found in bone marrow, they can differentiate into many (not all) types of cells (e.g., blood cells). **What are the potential uses of stem cells in medicine?** * **Embryonic stem cells:** Can be cloned and made to differentiate into most cell types. They can then be transplanted into a patient to replace faulty cells. * **Adult stem cells:** Could treat Type 1 diabetes (replace pancreas cells) and paralysis (replace nerve cells). They can only differentiate into some cell types, so can only treat a smaller variety of diseases (e.g., bone marrow transplant for blood cancer). **What is therapeutic cloning?** When a donor egg cell is used to produce an embryo with the same genes as a patient. Stem cells from the embryo are then stimulated to divide to form any cells the patient needs for medical treatment. **What is the key advantage of therapeutic cloning?** Stem cells are less likely to be rejected by the patient's immune system. **What are some issues associated with the use of stem cells in medicine?** * **Transfer of viral infection** * **Risk of rejection by the immune system** requires immunosuppressant drugs. * **Ethical and religious objections** (e.g., potential life destroyed/embryo cannot give consent). **Describe stem cells in plants:** * Found in meristem regions (growing regions) in roots and shoot tips * They can differentiate into any type of plant cell, throughout a plant's lifetime (e.g., xylem/phloem). **How can stem cells from meristems in plants be used?** To produce clones of plants more quickly and economically, for example: * Rare species to protect from extinction * Crop plants with special features (e.g., disease resistance). This results in a large number of identical plants for farmers. **Exam insight: common misconception** * **"People object to using stem cells from embryos as it is playing god, cruel and not natural."** This misconception is broad. Instead of stating this idea directly, focus on the ethical and religious objections, along with the potential destruction of a potential life. * **"Stem cells are found in the stem of plants."** Stem cells are found in the root and shoot tips, which are actively dividing. **Example application questions** * **Explain how a transplant of bone marrow stem cells can treat medical conditions.** Bone marrow stem cells can be cloned and made to differentiate into new cells, which can replace damaged cells in the body. This could be used to treat conditions such as leukemia or other disorders where healthy blood cells are not produced. * **Evaluate the use of stem cells from a patient’s own bone marrow instead of stem cells from an embryo. Give a conclusion to your answer.** Stem cells from a patient's own bone marrow are a safe and reliable option because there are no ethical issues, the risk of rejection is low, and they can be readily obtained. However, they can only differentiate into a limited range of cell types while embryonic stem cells have much greater potential for treating a wider range of diseases. * **Give two advantages of using stem cells made from a person’s own cells, over embryonic stem cells.** (1) The risk of rejection is lower. (2) It avoids the ethical issues with embryonic stem cells. * **Suggest two disadvantages of therapeutic cloning.** (1) The procedure is expensive and complex. (2) The success rate is lower than with more traditional methods. ### 1.2.4: Coronary heart disease ### **Explain the cause of coronary heart disease (CHD).** 1. Fatty material (e.g., cholesterol) builds up inside the coronary arteries, which supply blood to the heart muscle. 2. This narrows the coronary arteries, reducing blood flow. 3. This results in a lack of oxygen and glucose for the heart muscle. **Explain how CHD can cause a heart attack.** 1. Restricted blood flow through the coronary arteries means that not enough oxygen reaches the heart muscle. 2. This results in insufficient aerobic respiration and energy release. **Describe how coronary heart disease can be treated.** * **Stent:** A metal mesh tube is inserted into the blocked coronary artery, keeping it open so that blood continues to flow. This has a higher risk of potential surgery, infections, and blood clots. * **Statins:** These are drugs that reduce blood cholesterol, slowing the build-up of fatty material inside the coronary arteries. While generally safe, statins can have side effects that take time to manifest. **Explain the consequences of faulty heart valves.** * **Faulty heart valves** can prevent blood from flowing correctly through the heart. This can cause the heart to work harder to pump blood around the body. * **A valve may not open fully** which can restrict blood flow. * **A valve may develop a leak**, causing a backflow of blood. * **Less oxygenated blood** is pumped around the body. * **Less aerobic respiration** leads to **less* energy release. * **More anaerobic respiration** takes place, producing **lactic acid**. * **Muscle fatigue and increased breathing rate** occur. **Describe how faulty heart valves can be treated.** * **Biological Valve Replacement:** Using valves from animals or human donors. These can wear out or stiffen over time and may need to be replaced, and present ethical issues. * **Mechanical Valve Replacement:** Using an artificial valve. These are generally more durable, but may cause blood clots, requiring anti-clotting drugs to be taken. **Describe how heart failure (failure to pump enough blood) can be treated.** * **Heart Transplant:** Receiving a donated heart or heart and lungs. This requires a donor, which often requires a long wait. There is also the risk of rejection, so patients must take immunosuppressant drugs. * **Artificial heart:** Used to keep patients alive while waiting for a transplant, or to allow the heart to rest as an aid to recovery. Artificial hearts are expensive, do not last long, and can cause blood clots, so anti-clotting drugs are required. ### 1.2.7: Cancer ### **Describe how tumours (and cancers) form.** Tumors form when abnormal cells grow and divide uncontrollably, due to changes in the cells. **Describe the differences between benign and malignant tumours.** | | **Benign Tumour** | **Malignant Tumour** | |---|---|---| | | Not cancerous | Cancerous | | | Do not invade neighboring tissue. | Invade neighboring tissue | | | Do not spread to other parts of the body. | Spread to other parts of the body. | | | Usually grow more slowly. | Usually grow faster. | | | Stay in one place, often contained within a membrane. | Spread to other parts of the body. | **Describe how a tumour can spread to different parts of the body.** Cells break off from the tumor and travel in the blood, forming secondary tumors in other parts of the body. **What are the two types of risk factors for cancer?** * **Lifestyle risk factors:** Smoking, carcinogens, ionising radiation, and some viruses. * **Genetic risk factors:** A predisposition for certain diseases. **Exam insight: common misconception** * **"All tumours are cancerous."** Only **malignant** tumours are cancerous, so can spread. * **"Cancer is caused by cells growing too fast."** While cancer cells do grow and divide faster, the key point is that their **growth and division is uncontrolled**. **Example application questions** * **Suggest two reasons why the survival rates for all cancers have increased.** (1) Improved treatment/drugs (e.g., chemotherapy, radiotherapy, and immunotherapy). (2) Earlier diagnosis as a result of increased awareness, more effective screening, and improved access to healthcare. **Example application questions - relating to coronary heart disease:** * **The number of people who are obese in the UK is increasing. Explain the financial impact on the UK economy of an increasing number of people who are obese.** An increasing number of obese people leads to higher healthcare costs. This is because: (1) **Increased demand for services:** obese people are more likely to need treatment for conditions such as heart disease, diabetes, and some types of cancer. (2) **Higher costs of care:** Obese people often require longer hospital stays and more complex treatments. This increases the cost of providing healthcare. * **Other than stopping smoking, suggest two lifestyle changes a person can make to reduce the risk of CHD.** (1) Taking part in regular physical activities, like brisk walking or jogging, can help reduce the risk of cardiovascular disease. (2) Following a balanced diet that is low in saturated fats, trans fats, cholesterol, and sugar can help lower the risk of developing heart disease. * **In a scientific investigation over 44,000 men were monitored for 12 years. The men were divided into four groups based on their number of hours of physical activity they did per week. The results are shown in the table.** | Group | Mean number of hours of physical activity per week | Percentage who are smokers | Percentage with high cholesterol | Percentage with family history of heart disease | Mean percentage of fat in diet | Number of cases of heart disease | |---|---|---|---|---|---|---| | W | 1 | 14 | 11 | 12 | 34 | 423 | | X | 5 | 11 | 10 | 12 | 33 | 370 | | Y | 12 | 9 | 11 | 12 | 32 | 336 | | Z | 24 | 7 | 11 | 12 | 30 | 294 | * **Evaluate whether the data shows that increased hours of physical activity reduces the risk of heart disease.** The data suggests that there is a correlation between increased hours of physical activity and a lower risk of heart disease. The data shows a decrease in the number of cases of heart disease as the amount of physical activity increases. **Supporting the findings:** * The largest decrease in the number of cases occurs when physical activity increases from low levels to slightly higher levels (groups W-X). * The decrease in the number of cases is less pronounced when physical activity increases from moderate levels to high levels (groups Y-Z). * The data does not show a clear correlation between the risk of heart disease and other factors (e.g., smoking, cholesterol, or family history). This could be due to the limited scope of the study. **Limitations:** * However, the study only looked at men, not women. The results might be different for women. * The study did not show how many people were in each category of physical activity, and it is not possible to determine what percentage of each group had heart disease. Further research is needed to confirm these findings and to explore other factors that could influence the risk of heart disease. ### 1.3 Transport in Plants ### **Example application questions** * **Adult horney moths lay eggs that hatch into larvae. These live inside roots of trees, using tree roots as a source of food and causing damage to the tree roots.** This is a classic example of a **parasite / pest** relationship. The horney moth larvae are parasites that live in and harm the tree roots. **Explain why a tree might die if the roots of the tree are damaged.** (1) The roots absorb water and mineral ions from the soil. (2) The roots anchor the tree in the ground. (3) Damaged roots cannot absorb water and mineral ions efficiently. (4) Water moves from the roots to the leaves by **transpiration**. (5) Damaged roots cannot carry out photosynthesis. (6) If photosynthesis is not happening, the tree cannot produce enough energy to grow, and the tree will eventually die. * **Describe two differences between the transpiration stream and translocation.** **Transpiration stream:** The movement of water through the xylem from the roots to the leaves. It is driven by the forces of transpiration pull, adhesion, and cohesion. **Translocation:** The movement of dissolved sugars (produced by photosynthesis) through the phloem from the leaves to other parts of the plant. It is driven by active transport and **pressure flow**. * **A teacher used celery to show water uptake in plants. Celery was left in water with red dye for 3 hours. After, the leaves of the celery were dark red. Explain how.** The celery absorbed the red dye through its xylem. The xylem is found in the roots, stem, and leaves. The red dye moved up the celery in the water, from the roots to the leaves. * **Having a large number of stomata per mm2 of leaf surface can be a disadvantage to a plant. Explain how.** A higher number of stomata means an increased surface area. This leads to more loss of water through transpiration, which can be damaging if a plant does not have enough access to water. * **The rate of water loss from leaves at midnight is much lower than at midday. Explain why.** (1) **Light intensity**: Stomata open during the day, in the presence of light, to allow for gaseous exchange (CO2 uptake for photosynthesis and oxygen release). Stomata close at night, to reduce water loss. * **Most plants have more stomata on the lower surface of a leaf than on the upper surface. Explain why.** This is because: (1) **Air currents**: Stomata on the upper surface are exposed to more vigorous air currents, which cause more water loss. (2) **Temperature**: The lower surface faces the ground, and so is cooler. This helps to reduce transpiration rates. * **A student used a potometer to investigate the rate of water uptake in a plant shoot. Explain why the air bubble would not move if the investigations were done in the dark.** (1) In the dark, **photosynthesis** does not occur. (2) **Photosynthesis** is the process that produces the sugars that provide the energy to drive **transpiration**, the movement of water through the plant. (3) Without photosynthesis, the plant does not need to take up water, so the air bubble in the potometer will not move. **Less absorption of water:** * **Lower rate of photosynthesis:** So less glucose produced * **So less respiration:** So less cellulose made for cell walls/amino acids for making new proteins. * **Less absorption of ions:** * **Fewer nitrates:** So fewer proteins made for growth * **Fewer magnesium ions:** So less chlorophyll produced, so a lower rate of photosynthesis. **Damage to phloem** * **Less transport of sugars to root cells:** For respiration. **Damage to xylem:** * **Less water transported** * **Fewer nitrates:** Reaches cells so fewer proteins made. * **Less anchorage** **Other ways the tree might die due to damaged roots.** * **Less support:** The damaged roots may not be able to support the weight of the tree. * **Wind damage:** The tree may be more vulnerable to being blown over by strong winds. * **Disease:** Damaged roots can leave the tree more susceptible to diseases. **Exam insight: common misconception:** * **"Water can move by active transport."** **Water only moves by osmosis.** Active transport is needed to move substances against a concentration gradient. **Example application question:** * **Explain why a plant growing in soil flooded with water cannot absorb enough nitrate ions.** (1) Plants absorb ions by active transport. This requires energy from respiration. (2) Respiration requires oxygen. (3) Waterlogged soil has little oxygen. Therefore, plants growing in waterlogged soil cannot absorb enough nitrate ions, because they do not have enough energy for active transport. ### 2.1 Principles of organisation ### **Describe the levels of organization in organisms, using examples:** 1. **Cells:** The smallest structural and functional unit of an organism. * Example: muscle cell 2. **Tissues:** A group of cells with a similar structure and function. * Example: muscle tissue 3. **Organs:** A group of tissues performing specific functions. * Example: Stomach 4. **Organ Systems:** A group of organs performing specific functions. * Example: digestive system **Name and label the organs in the digestive system.** * **Mouth:** The first stage of the digestive system, food is broken down physically and chemically. * **Oesophagus:** Transports food from the mouth to the stomach. * **Stomach:** A muscular bag where food is churned and further broken down by enzymes. * **Small intestine:** The main site of absorption of nutrients into the bloodstream. * **Large intestine:** Absorbs water and forms and stores waste products. **Additional digestive system organs:** * **Salivary glands:** Produce saliva, which contains enzymes that begin the breakdown of carbohydrates. * **Liver:** Produces bile and stores glucose. * **Gall bladder:** Stores and releases bile. * **Pancreas:** Produces digestive enzymes, such as amylase, protease, and lipase. **Exam insight: common misconception:** * **"The nucleus of a cell is an organ."** The nucleus is a sub-cellular structure (an organelle), not an organ. * **"Food passes through the liver and pancreas in the digestive system."** While essential to digestion, food does not directly pass through the liver and pancreas. These organs produce substances (e.g., bile and enzymes) that are released into the small intestine. **Example application questions:** * **Rewrite these in the correct order, from smallest to largest: brain, neurone, dog, nervous system, nerve.** Neurone (cell) → Nerve (tissue) → Brain (organ) → Nervous system (organ system) → Dog (organism) ### 2.2 Animal tissues, organs and systems ### **2.2.1: The Human digestive system:** **What is digestion?** Digestion is the breakdown of large, insoluble food molecules into smaller, soluble molecules that can be absorbed into the blood. **What is an enzyme?** An enzyme is a biological catalyst. They are typically proteins and speed up reactions in organisms without being used up or changed. **How do enzymes work?** The ‘lock and key theory’ provides a simplified model of enzyme action: 1. The **substrate** (the molecule that the enzyme acts upon) fits into the enzyme's **active site**. This is possible because the shape of the active site and the substrate are complementary. 2. This causes a **chemical reaction**. (e.g., bonds are broken down to produce smaller molecules). 3. The unchanged enzyme releases the **products** of the reaction. **Why can each enzyme only catalyse a specific reaction?** Each enzyme has a specific three-dimensional shape, meaning that only a substrate with a complementary shape can bind and fit into the active site. **How are enzymes related to metabolism?** * **Metabolism** is the sum of all the reactions inside a cell or the body of an organism * **Different enzymes** catalyse specific metabolic reactions (e.g., respiration). * **Reactions:** may join smaller molecules to form larger molecules or break down molecules. **Describe the role of lipase, carbohydrase, and protease in digestion** | **Enzyme** | **What does it break down?** | **Where is it produced?** | **Where does it act?** | |---|---|---|---| | **Lipase** | Lipids (fats) into fatty acids and glycerol | Pancreas | Small intestine | | **Carbohydrase** (e.g., amylase) | Carbohydrates into simple sugars (e.g., starch into glucose). | Salivary glands, pancreas, and small intestine. | Mouth and small intestine | | **Protease** | Proteins into amino acids. | Stomach, pancreas, and small intestine | Stomach and small intestine | **How are the products of digestion used in the body once absorbed?** * **Fatty acids and glycerol:** Used to build new lipids (fats) and carbohydrates. * **Simple sugars (e.g., glucose):** Used to build new carbohydrates. Glucose is used for respiration to release energy. * **Amino acids:** Used to build new proteins. **Where is bile produced, stored, and released into?** * **Produced:** Liver * **Stored:** Gall bladder * **Released into:** Small intestine **What is the role of bile in digestion?** * **Neutralises hydrochloric acid:** Bile is alkaline, neutralising the hydrochloric acid produced in the stomach, which creates the optimum conditions for the enzymes in the small intestine. * **Emulsifies lipids:** Bile breaks down large fat droplets into smaller droplets to increase surface area for the lipase enzyme to work on them. This speeds up lipase breakdown of fat. **What does it mean when an enzyme denatures?** If the enzyme's active site changes shape due to high temperatures or changes in pH, the substrate will no longer fit and bind to the active site, preventing the reaction from occurring. **Describe and explain how temperature affects enzyme activity.** * **Describe:** Enzyme activity increases as temperature increases up to an optimum temperature, but then decreasing rapidly as the temperature increases beyond the optimum temperature. * **Explain:** (1) Increasing temperatures mean the molecules have more kinetic energy, increasing the rate of enzyme-substrate collisions. (2) When the temperature increases above the optimum, the enzyme denatures, causing the active site to change shape, and the substrate can no longer bind to the active site. **Describe and explain how pH affects enzyme activity.** * **As pH increases** from its optimum, enzyme activity decreases. * **As pH decreases** from its optimum, enzyme activity decreases. * **Explain:** Each enzyme has an optimum pH range. Changes in pH above or below the optimum pH cause the enzyme to denature, so the substrate can no longer bind to the active site. **Exam insight: common misconception:** * **"Enzymes are cells."** Enzymes are protein molecules, not cells. * **"All enzymes break molecules down."** Some enzymes also join molecules to form larger molecules. * **"Bile is an enzyme."** Bile is not an enzyme. It is a substance produced by the liver that helps the enzyme lipase by emulsifying fats and neutralising acid. * **"Enzymes die at high temperatures."** Enzymes are not living, so they cannot die. At high temperatures they can denature. **Example application question:** * **The enzyme amylase can only digest starch. Explain why amylase cannot digest other substances.** Amylase has a specific, three-dimensional shape to its active site. This shape is complementary to the shape of the starch molecule. The active site of amylase is designed to only bind to starch, preventing it from binding to other molecules. ### 2.2.3 Blood### **Why is blood a tissue?** It is a group of cells with a similar structure and function. **List the components of the blood & their functions:** * **Plasma:** (liquid). Transports dissolved substances such as glucose, urea, carbon dioxide, hormones, and suspends blood cells. * **Red blood cells:** Transports oxygen from the lungs to the body cells. * **White blood cells:** Part of the immune system. Destroys pathogens that enter the body. * **Platelets:** Small fragments of cells that help clot blood at wounds. **How are red blood cells adapted to their function?** * **Contain hemoglobin:** Hemoglobin is a protein that binds to oxygen. * **Biconcave discs:** This shape provides a large surface area for rapid diffusion of oxygen. * **No nucleus:** This leaves more space for hemoglobin. **How are white blood cells adapted to their function?** * **Produce antibodies:** Bind to pathogens and mark them for destruction. * **Engulf pathogens:** (phagocytosis). Engulf and digest invading pathogens. **Exam insight: common misconception:** * **"White blood cells fight disease."** This is too vague. White blood cells help to destroy the microorganisms that cause disease. **Example application question:** * **Some types of cancer can cause the numbers of blood components in a person’s body to fall to a dangerously low level. A person with one of these types of cancer may experience symptoms such as tiredness, frequent infections, and bleeding that will not stop after the skin is cut.** **Explain how a very low number of blood components in the body can cause these symptoms.** * **Tiredness:** Fewer red blood cells mean less hemoglobin. This means that less oxygen is transported around the body, resulting in less aerobic respiration and more anaerobic respiration. Anaerobic respiration releases less energy than aerobic respiration, resulting in tiredness. * **Frequent infections:** Fewer white blood cells mean that the body's immune system is less effective at fighting off pathogens. This leads to more frequent infections. * **Bleeding:** Fewer platelets mean that the blood clots less easily, making it difficult to stop bleeding after cuts or injuries. ### 2.2.4 Coronary heart disease ### **Explain the cause of coronary heart disease (CHD)** 1. Fatty material (e.g., cholesterol) builds up inside the coronary arteries, which are the blood vessels that supply blood to the heart muscle. 2. This buildup narrows the coronary arteries, reducing the flow of blood to the heart muscle. 3. This can lead to a lack of oxygen and glucose for the heart muscle, which can cause pain, damage, or even death. **Explain how CHD can cause a heart attack** 1. Blockage of one or more coronary arteries, resulting in a restricted blood flow limits the amount of oxygen that reaches the heart muscle. 2. Without sufficient oxygen, the heart muscle can't perform aerobic respiration efficiently. This can lead to a lack of energy, resulting in chest pain. 3. If the blockage persists, the heart muscle cells can die due to lack of oxygen, resulting in a heart attack. **Describe how coronary heart disease can be treated:** * **Stent:** A small, metal mesh tube (stent) is inserted into the blocked coronary artery. This helps to keep the artery open and improve blood flow to the heart. * **Statins:** These are medications that help lower the level of cholesterol in the blood. Cholesterol is a type of fat that can build up in the coronary arteries and contribute to CHD. **Explain the consequences of faulty heart valves** * **Heart valves:** Control the flow of blood through the heart, ensuring that blood flows in the correct direction. Healthy heart valves open and close smoothly, preventing backflow. * **Faulty heart valves:** Faulty heart valves may not function properly, either not opening fully or leaking. * **Consequences:** * **Restricted blood flow:** This can stress the heart, as it needs to work harder to pump enough blood to the body. * **Backflow of blood:** Can cause a buildup of blood pressure in the heart, putting more stress on the heart muscle and increasing the risk of heart failure. * **Symptoms:** * Shortness of breath * Fatigue * Chest pain * Swollen ankles and feet, due to fluid buildup * Irregular heartbeat **Describe how faulty heart valves can be treated:** * **Valve Replacement:** (1) **Biological valve replacement** involves using valves from animals or human donors. These valves are less likely to cause the body