AQA Biology Paper 1 Comprehensive Study Notes

Cell Biology

• B1.1 Prokaryotic vs Eukaryotic Cell Ultrastructure: Eukaryotic cells, such as animal and plant cells, contain their genetic material (DNA) enclosed within a nucleus. They also possess membrane-bound organelles like mitochondria and chloroplasts. Prokaryotic cells, such as bacteria, are much smaller ($0.1$ to $5.0\,\mu m$). They do not have a nucleus; their DNA is found as a single circular loop in the cytoplasm. They may also contain small rings of DNA called plasmids. Prokaryotes lack membrane-bound organelles but have a cell wall (composed of peptidoglycan), cytoplasm, and a cell membrane.

• B1.2 Sub-cellular Structures and Function:

  • Nucleus: Contains genetic material and controls cell activities.

  • Cytoplasm: A jelly-like substance where chemical reactions occur, containing enzymes.

  • Cell Membrane: Controls the passage of substances into and out of the cell.

  • Mitochondria: The site of aerobic respiration, providing energy for the cell.

  • Ribosomes: The site of protein synthesis.

  • Chloroplasts (Plant only): Contain chlorophyll; the site of photosynthesis.

  • Permanent Vacuole (Plant only): Filled with cell sap to keep the cell turgid.

  • Cell Wall (Plant only): Made of cellulose to strengthen the cell.

• B1.3 Cell Specialisation: Cells undergo differentiation to perform specific functions.

  • Sperm Cells: Feature a tail (flagellum) for swimming, many mitochondria for energy, and an acrosome containing enzymes to penetrate the egg.

  • Nerve Cells: Long axons to carry impulses over distances, with dendrites to connect to other nerve cells.

  • Root Hair Cells: Large surface area for absorbing water and mineral ions from the soil.

• B1.4 Cell Differentiation: This is the process by which a cell changes to become specialized. In animals, most cells differentiate at an early stage. In plants, many cells retain the ability to differentiate throughout their entire life.

• B1.5 Light vs Electron Microscopy: Light microscopes use light and lenses to form an image, with limited magnification and resolution. Electron microscopes use a beam of electrons, providing much higher magnification and higher resolution, allowing scientists to see sub-cellular structures like ribosomes.

• B1.6 Magnification Equation: The formula to calculate magnification is: $I = A \times M$ Where:

• $I$ is the size of the Image.

• $A$ is the Actual size of the specimen.

• $M$ is the Magnification. Standard units like millimeters ($mm$), micrometers ($\mu m$), and nanometers ($nm$) must be used consistently ($1\,mm = 1000\,\mu m$).

• B1.7 Chromosomes, Genes, and DNA Configuration: The nucleus contains chromosomes made of DNA molecules. Each chromosome carries a large number of genes. In body cells, chromosomes are normally found in pairs (e.g., $23$ pairs in humans).

• B1.8 Cell Cycle and Mitosis: The cell cycle involves growth and division. During the cell cycle:

  • Stage 1 (Interphase): The cell grows and increases the number of sub-cellular structures (ribosomes/mitochondria). The DNA replicates to form two copies of each chromosome.

  • Stage 2 (Mitosis): One set of chromosomes is pulled to each end of the cell and the nucleus divides.

  • Stage 3 (Cytokinesis): The cytoplasm and cell membranes divide to form two identical daughter cells.

• B1.9 Stem Cells: A stem cell is an undifferentiated cell capable of giving rise to many more cells of the same type.

  • Embryonic Stem Cells: Can differentiate into most types of human cells.

  • Adult Stem Cells (Bone Marrow): Can form many types of cells, including blood cells.

  • Meristem (Plants): Can differentiate into any type of plant cell throughout the plant's life.

• B1.10 Therapeutic Cloning and Bioethics: In therapeutic cloning, an embryo is produced with the same genes as the patient. Stem cells from the embryo are not rejected by the patient's body and can be used for medical treatment. Ethical concerns include the potential destruction of human embryos and the risks of viral transfer.

• B1.11 Diffusion: The spreading out of particles from an area of higher concentration to an area of lower concentration. Rates are affected by the concentration gradient, temperature, and surface area. In multicellular organisms, the Surface Area to Volume Ratio ($SA:V$) is critical; as organisms get larger, their $SA:V$ ratio decreases, requiring specialized exchange surfaces.

• B1.12 Osmosis: The diffusion of water from a dilute solution (high water potential) to a concentrated solution (low water potential) through a partially permeable membrane. This affects plant turgidity and can cause animal cells to burst (lysis) or shrivel (crenation).

• B1.13 Active Transport: Moves substances from a more dilute solution to a more concentrated solution (against a concentration gradient). This requires energy from respiration (ATP) and involves carrier proteins in the cell membrane.

Organisation

• B2.1 Hierarchical Scale: The body is organized into a hierarchy: Cells $→$ Tissues (groups of similar cells) $→$ Organs (groups of tissues performing a function) $→$ Organ Systems (groups of organs working together).

• B2.2 Human Digestive System: Organs work together to digest and absorb food. The mouth breaks food mechanically, the stomach provides acidic conditions for proteases, and the small intestine is the site of nutrient absorption into the blood.

• B2.3 - B2.5 Enzymes and Digestion: Enzymes are biological catalysts with a specific active site that fits a substrate (Lock-and-Key Model).

  • Amylase: Produced in salivary glands and pancreas; breaks starch into sugars.

  • Protease: Produced in stomach, pancreas, and small intestine; breaks proteins into amino acids.

  • Lipase: Produced in pancreas and small intestine; breaks lipids into glycerol and fatty acids.

• B2.6 Bile: Produced in the liver and stored in the gallbladder. It is alkaline to neutralize hydrochloric acid from the stomach and emulsifies fat to form small droplets, increasing the surface area for lipase action.

• B2.7 - B2.9 Heart and Blood Vessels: The heart is a double pump. The right ventricle pumps blood to the lungs; the left ventricle (thicker walls) pumps blood to the body.

  • Arteries: Carry blood under high pressure; thick muscular/elastic walls.

  • Veins: Carry blood under low pressure; thinner walls and valves to prevent backflow.

  • Capillaries: One-cell thick walls for efficient exchange of substances.

• B2.10 Blood Components:

  • Plasma: Transports CO2, urea, and hormones.

  • Erythrocytes (Red Blood Cells): Transport oxygen via hemoglobin.

  • Leucocytes (White Blood Cells): Part of the immune system.

  • Platelets: Cell fragments for blood clotting.

• B2.11 Coronary Heart Disease (CHD): Involves the buildup of fatty material in coronary arteries. Treatments include stents (to keep arteries open) and statins (to reduce cholesterol). Damaged heart valves can be replaced with biological or mechanical valves.

• B2.14 Cancer: Caused by uncontrolled cell division (mitosis).

  • Benign Tumours: Growth of abnormal cells contained in one area (usually within a membrane).

  • Malignant Tumours: Cancers that invade neighboring tissues and spread to different parts of the body (metastasis).

• B2.15 - B2.18 Plant Transport:

  • Xylem: Transports water/minerals from roots to leaves (Transpiration Stream).

  • Phloem: Transports dissolved sugars from leaves to the rest of the plant (Translocation).

  • Stomata/Guard Cells: Regulate gas exchange and water loss via turgor pressure.

Infection and Response

• B3.1 - B3.5 Pathogens:

  • Viruses: Live and replicate inside cells, causing cell damage. Examples: Measles, HIV, Tobacco Mosaic Virus (TMV).

  • Bacteria: Produce toxins that damage tissues. Examples: Salmonella (food poisoning), Gonorrhoea (STD).

  • Fungi: Rose black spot causes purple/black spots on leaves and chlorosis (yellowing).

  • Protists: Malaria is caused by a protist spread by mosquitoes (vectors).

• B3.6 Non-Specific Defenses: Includes skin (barrier), nose (hairs/mucus), trachea/bronchi (cilia/mucus), and stomach (hydrochloric acid).

• B3.7 Immune System: White blood cells defend against pathogens via Phagocytosis (engulfing), Antibody production (targeting specific antigens), and Antitoxin production (neutralizing bacterial toxins).

• B3.8 Vaccination: Involves introducing small quantities of dead or inactive pathogens into the body to stimulate white blood cells to produce antibodies. If the same pathogen re-enters, the body responds rapidly (secondary response). Herd immunity occurs when a large proportion of the population is immune.

• B3.11 Drug Trials: New drugs are tested for toxicity, efficacy, and dose.

  • Preclinical: Done in labs using cells, tissues, and live animals.

  • Clinical Stage 1: Healthy volunteers receive low doses.

  • Clinical Stage 2/3: Tested on patients to find the optimum dose and efficacy.

• B3.12 Blind and Double-Blind Tests: Patients are divided into two groups. One receives the drug and the other a placebo. In a double-blind trial, neither the doctor nor the patient knows who has the drug until the end.

• B3.13 - B3.14 Monoclonal Antibodies (HT): Produced from a single clone of cells. A mouse B-lymphocyte is fused with a tumour cell to create a hybridoma, which clones itself and produces specific antibodies. Uses include pregnancy tests and targeting cancer cells.

• B3.16 Ion Deficiencies (HT):

  • Nitrate: Needed for protein synthesis; deficiency leads to stunted growth.

  • Magnesium: Needed to make chlorophyll; deficiency leads to chlorosis.

Bioenergetics

• B4.1 Photosynthesis Equation: An endothermic reaction occurring in chloroplasts. $6CO_2 + 6H_2O \rightarrow C_6H_{12}O_6 + 6O_2$

• B4.3 Limiting Factors: Temperature, light intensity, carbon dioxide concentration, and the amount of chlorophyll can limit the rate of photosynthesis.

• B4.4 Inverse Square Law (HT): Light intensity is inversely proportional to the square of the distance from the source: $\text{Light Intensity} \propto \frac{1}{d^2}$

• B4.5 - B4.7 Respiration: An exothermic reaction continually occurring in living cells.

  • Aerobic: $\text{Glucose} + \text{Oxygen} \rightarrow \text{Carbon Dioxide} + \text{Water}$.

  • Anaerobic (Animals): $\text{Glucose} \rightarrow \text{Lactic Acid}$.

  • Anaerobic (Plants/Yeast): $\text{Glucose} \rightarrow \text{Ethanol} + \text{Carbon Dioxide}$ (Fermentation).

• B4.9 Oxygen Debt (HT): The amount of extra oxygen the body needs after exercise to react with the accumulated lactic acid and remove it from the cells via the liver.

Required Practicals (RP)

• RP1 Microscopy: Prepare a slide (e.g., onion skin), use various lens powers, and calculate magnification of viewed structures.

• RP3 Osmosis: Measure the percentage change in mass of plant tissue (e.g., potato) placed in varying concentrations of sugar solutions to determine the internal concentration.

• RP4 Food Tests: Use Iodine (Starch/Blue-black), Benedict’s (Sugars/Red-Orange), Biuret (Proteins/Purple), and Sudan III (Lipids/Red layer).

• RP6 Photosynthesis: Counting oxygen bubbles from pondweed at varying distances from a light source to measure the effect of light intensity on the rate of photosynthesis.