Edexcel International GCSE (9-1) Science Double Award - Biology Comprehensive Study Guide

ORGANISMS AND LIFE PROCESSES

Essential Life Processes

• Most living things demonstrate eight specific life processes:     * Nutrition: Plants manufacture their own food (autotrophic), while animals consume other organisms (heterotrophic).     * Respiration: The release of energy from food substances.     * Excretion: The removal of waste products of metabolism.     * Response to Stimuli: Sensitivity to changes in the surroundings.     * Movement: Voluntary muscle action in animals and slow growth movements in plants.     * Control of Internal Conditions: Maintaining a steady state inside the body (Homeostasis).     * Reproduction: The production of offspring.     * Growth and Development: An increase in size and complexity using materials from food.

Cell Structure and Organelles

• Cells: The microscopic building blocks of life. Multi-celled (multicellular) organisms contain specialized cells with different structures for specific functions.
• Organelles: Detailed structures within cells visible under an electron microscope.     * Nucleus: The largest organelle. It contains $46$ chromosomes in human cells, carrying genetic material (DNA/genes). It controls cell activities by determining protein synthesis.     * Cytoplasm: A jelly-like complex material where chemical reactions occur.     * Cell Membrane: A thin "skin" that forms a boundary. It is partially permeable and selectively permeable, controlling the movement of substances.     * Mitochondria (Singular: Mitochondrion): Found in the cytoplasm of all living cells; they carry out respiration to release energy. Abundant in high-energy cells like muscle or nerve cells.     * Ribosomes: Tiny structures in the cytoplasm where proteins are assembled. Too small for light microscopes.     * Cell Wall: Found only in plants (made of cellulose), fungi (chitin), and bacteria (peptidoglycan). In plants, it is porous and freely permeable, providing structural support.     * Vacuole: Permanent in mature plant cells, filled with cell sap (watery liquid of sugars and mineral ions). Animal cells have small, temporary vacuoles.     * Chloroplasts: Found in green parts of plants (leaves). They contain chlorophyll to absorb light for photosynthesis.

Enzymes: Biological Catalysts

• Definition: Enzymes are proteins that act as biological catalysts, speeding up metabolic reactions without being consumed.
• Metabolism: The sum of all chemical reactions in a cell.
• Mechanism (Lock and Key Model):     * Substrate: The molecule the enzyme acts upon.     * Active Site: A specific area on the enzyme surface where the substrate fits.     * Action: The substrate enters the active site, the reaction occurs at a lower activation energy, products form and leave, and the enzyme is free to repeat the process.
• Factors Affecting Enzyme Activity:     * Temperature:         * Optimum Temperature: The temperature where the reaction is fastest ($37\,^{\circ}C$ for humans).         * Kinetic Energy: Increases with temperature, causing more frequent collisions between enzyme and substrate.         * Denaturing: Above approximately $40\,^{\circ}C$, the heat destroys the protein structure, permanently changing the shape of the active site so the substrate no longer fits.     * pH:         * Most enzymes work best at neutral $pH\,7$.         * Extremes of pH change the shape of the active site.         * Pepsin: An exception with an optimum $pH\,2$ for working in acidic stomach conditions.

Levels of Organisation

• Organelles: Structures within cells (e.g., nucleus).
• Cells: Units of life (e.g., muscle cells).
• Tissues: Groups of similar cells with a shared function (e.g., muscle tissue).
• Organs: Collections of several tissues carrying out a particular function (e.g., heart, stomach, leaf).
• Organ Systems: Groups of organs working together (e.g., digestive, circulatory, excretory, nervous, endocrine, reproductive, and gas exchange systems).

Energetics: Respiration and ATP

• Respiration: An oxidation reaction breaking down food molecules (glucose) to release chemical energy.
• ATP (Adenosine Triphosphate): The energy "currency" of the cell. It transfers energy from respiration to processes like muscle contraction, active transport, and cell division.     * Breakdown: $ATP + H_{2}O \rightarrow ADP + P + \text{Energy}$.     * Synthesis: Energy from glucose oxidation is used to add a phosphate to ADP ($ADP + P \rightarrow ATP$).
• Aerobic Respiration: Requires oxygen. Occurs in mitochondria.     * Equation: $C_{6}H_{12}O_{6} + 6O_{2} \rightarrow 6CO_{2} + 6H_{2}O + \text{Energy}$.
• Anaerobic Respiration: Occurs in the absence of oxygen. Less energy is released.     * In Yeast: $Glucose \rightarrow Ethanol + Carbon\,Dioxide + \text{Energy}$.     * In Muscles: $Glucose \rightarrow Lactate + \text{Energy}$.     * Oxygen Debt: The volume of oxygen needed to oxidize lactate built up during exercise.

Movement of Substances

• Diffusion: Net movement of particles from high to low concentration (down a concentration gradient). Factors: surface area to volume ratio, temperature, distance, and concentration gradient.
• Active Transport: Movement against a concentration gradient using energy from ATP (e.g., glucose absorption in the human gut, mineral uptake in plant roots).
• Osmosis: Net movement of water from a dilute solution (high water potential) to a concentrated solution (low water potential) across a partially permeable membrane.

THE VARIETY OF LIVING ORGANISMS

Eukaryotic Organisms

• Eukaryotic: Cells with a nucleus and membrane-bound organelles.     * Plants: Multicellular, contain chloroplasts for photosynthesis. Cell walls made of cellulose. Store carbs as starch or sucrose (e.g., maize, peas).     * Animals: Multicellular, no chloroplasts or cell walls. Gain nutrition by eating. Often have nervous coordination and movement. Store carbs as glycogen (e.g., mammals, insects).     * Fungi: Can be multicellular (mushrooms with hyphae forming a mycelium) or unicellular (yeast). Cell walls made of chitin. Saprotrophic Nutrition: Secrete extracellular enzymes to digest dead organic matter and absorb products (e.g., Mucor).     * Protoctists: Single-celled microscopic organisms. Some like animals (Amoeba), some like plants with chloroplasts (Chlorella). Pathogenic example: Plasmodium (malaria).

Prokaryotic Organisms

• Bacteria: Microscopic single-celled organisms, size $1$ to $5\,\mu m$. No nucleus; genetic material is a circular loop of DNA. Some have plasmids (rings of DNA) and flagella for movement. Cell wall made of peptidoglycan. Examples: Lactobacillus bulgaricus (yoghurt), Pneumococcus (pneumonia).

Viruses

• Small particles ($0.01$ to $0.1\,\mu m$), not considered living. Parasitic; reproduce only inside host cells. No cellular structure; just a protein coat and a core of DNA or RNA. Examples: Tobacco Mosaic Virus (discolors leaves), Influenza, HIV (causes AIDS).
• Pathogen: An organism that causes disease (includes fungi, bacteria, protoctists, or viruses).

ANIMAL PHYSIOLOGY: GAS EXCHANGE AND DIGESTION

Human Gas Exchange System (Thorax)

• Anatomy: Ribs, intercostal muscles (internal and external), diaphragm, trachea (windpipe), bronchi, bronchioles, and alveoli.
• Protection: Trachea and bronchi are lined with cilia and mucus-secreting cells to trap and sweep out dirt and bacteria.
• Ventilation Physics:     * Inhalation: External intercostals contract (ribs up/out), diaphragm contracts (flattens), volume increases, pressure decreases, air enters.     * Exhalation: Internal intercostals contract (ribs down/in), diaphragm relaxes (domes), volume decreases, pressure increases, air is forced out.
• Alveoli Adaptations:     * Huge surface area (approx. $60\,m^{2}$).     * Thin walls (one cell thick) for short diffusion distance.     * Moist lining for gases to dissolve.     * Extensive capillary network for steep concentration gradients.

Biological Consequences of Smoking

• Cilia Destruction: Chemicals destroy cilia; mucus builds up, leading to "smoker's cough" and bronchitis.
• Emphysema: Alveoli walls break down and fuse, reducing surface area for gas exchange.
• Lung Cancer: Tar contains carcinogens causing uncontrolled cell division (tumors).
• Carbon Monoxide: Binds to haemoglobin as carboxyhaemoglobin, reducing oxygen-carrying capacity. During pregnancy, this leads to lower birth mass.
• Coronary Heart Disease (CHD): Nicotine and carbon monoxide increase blood pressure and the risk of fatty build-ups (cholesterol) in coronary arteries, potentially leading to heart attacks.

Food and Digestion

• Balanced Diet: Requires Carbohydrates, Lipids (Fats/Oils), Proteins, Vitamins, Minerals, Dietary Fibre, and Water.     * Carbohydrates: Glucose (Single sugar), Starch/Glycogen (Polymers of glucose). For energy.     * Lipids: Glycerol and three fatty acids. For energy storage and insulation.     * Proteins: Polymers of $20$ different amino acids. For growth and repair.
• Nutrient Tests:     * Starch: Iodine solution (Yellow-brown to Blue-black).     * Glucose: Benedict's solution + Heat (Blue to Brick-red precipitate).     * Protein: Biuret test (Blue to Purple).     * Lipid: Ethanol emulsion test (Cloudy white layer).
• Digestive Enzymes:     * Amylase: Starch to Maltose (Salivary glands/Pancreas).     * Maltase: Maltose to Glucose (Small intestine wall).     * Proteases (Pepsin/Trypsin): Protein to Peptides (Stomach/Pancreas).     * Peptidases: Peptides to Amino Acids (Small intestine wall).     * Lipases: Lipids to Fatty acids and Glycerol (Pancreas).
• Villi Adaptations: In the ileum, villi and microvilli provide massive surface area. Lacteals absorb fats; capillaries absorb sugars and amino acids.
• Bile: Produced by the liver, stored in the gall bladder. Emulsifies lipids (large drops to tiny droplets to increase surface area) and neutralizes stomach acid.

ANIMAL PHYSIOLOGY: CIRCULATION AND COORDINATION

The Heart and Blood

• Heart Structure: Right side (atrium/ventricle) pumps to lungs; Left side (atrium/ventricle) has thicker walls to pump to the whole body via the aorta.
• Blood Components:     * Red Blood Cells: Biconcave, no nucleus, contains haemoglobin to transport oxygen.     * Lymphocytes: Produce antibodies specific to antigens on pathogens.     * Phagocytes: Ingest and digest pathogens (Phagocytosis).     * Platelets: Fragmented cells involved in blood clotting.     * Plasma: Liquid part transporting $CO_{2}$, urea, nutrients, hormones, and heat.

Coordination and Control

• Nervous System: Brain and spinal cord (CNS). Use rapid electrical impulses.     * Reflex Arc: Stimulus \rightarrow Receptor \rightarrow Sensory Neurone \rightarrow Relay Neurone (CNS) \rightarrow Motor Neurone \rightarrow Effector \rightarrow Response.     * Synapse: Gaps between neurones crossed by neurotransmitter chemicals.
• The Eye:     * Accommodation: Focusing on near (ciliary muscles contract, lens rounded) and distant (ciliary muscles relax, lens flat) objects.     * Iris Reflex: Pupils constrict in bright light (circular muscles contract).
• Endocrine System: Slower hormonal communication via blood.     * Adrenaline: Adrenal glands; "fight or flight" (increased heart/breathing rate).     * Insulin: Pancreas; lowers blood glucose by converting it to glycogen in the liver.

HOMEOTASIS AND REPRODUCTION

Homeostasis

• Temperature Regulation: Hypothalamus monitors blood temperature.     * Hot: Vasodilation (blood to skin surface), sweating (evaporative cooling).     * Cold: Vasoconstriction (blood away from surface), shivering (heat from respiration), hair erection.

Human Reproduction

• Sexual Reproduction: Fusion of haploid gametes (sperm/egg) to form a diploid zygote.
• Fetal Development: The placenta provides nutrition/gas exchange; amniotic fluid protects from bumps.
• Hormones:     * Testosterone: Male secondary sexual characteristics.     * Oestrogen: Female secondary sexual characteristics; repairs uterus lining.     * Progesterone: Maintains uterus lining during the menstrual cycle and pregnancy.

PLANT PHYSIOLOGY

Photosynthesis

• Process: Conversion of light energy to chemical energy (glucose).
• Equation: $6CO_{2} + 6H_{2}O \rightarrow C_{6}H_{12}O_{6} + 6O_{2}$.
• Leaf Structure: Palisade mesophyll (main photosynthesis), Spongy mesophyll (gas exchange), Stomata (pores), Guard cells (control stomata).
• Mineral Requirements:     * Nitrate: For amino acids/protein.     * Magnesium: For chlorophyll.

Transport in Plants

• Xylem: Dead, hollow lignified tubes transporting water and minerals from roots to leaves.
• Phloem: Living sieve tubes transporting sucrose and amino acids from leaves to rest of plant.