Comprehensive Study Guide: Edexcel International GCSE (9-1) Science Double Award - Biology Units 1-3

Characteristics of Living Organisms and Life Processes

  • All living organisms, regardless of their complexity, are composed of microscopic units known as cells.

  • There are eight fundamental life processes that take place in most living things:   - Nutrition: Plants make their own food via photosynthesis; animals eat other organisms.   - Respiration: The release of energy from food.   - Excretion: The removal of metabolic waste products from the body.   - Sensitivity: Responding to stimuli or changes in their surroundings.   - Movement: Animals use muscle action; plants show slow growth movements.   - Control: Maintaining steady internal conditions (homeostasis).   - Reproduction: The production of offspring.   - Growth and Development: Increasing in size and complexity throughout life.

Cell Structure and Organelles

  • Cells of "higher" organisms (animals, plants, fungi) share common features, though specific cells like red blood cells (no nucleus) are specialized.

  • Cytoplasm: A complex material with a sloppy-jelly texture where most chemical reactions occur.

  • Organelles: Small structures within cells visible under an electron microscope (10001000-fold higher magnification than light microscopes).

  • The Nucleus: The largest organelle; controls cell activities and contains chromosomes (4646 in humans). Chromosomes carry genes (DNA) that determine protein production.

  • Ribosomes: Tiny structures in the cytoplasm where proteins are assembled (25 nm25\,nm in diameter).

  • Enzymes: A group of proteins that act as biological catalysts to control chemical reactions.

  • Cell Membrane: A thin layer sometimes called the cell surface membrane. It is selectively permeable (controls what enters/exits) or partially permeable.

  • Mitochondria (Singular: Mitochondrion): Found in all living cells; the site of aerobic respiration where energy is released.

  • Plant-Specific Structures:   - Cell Wall: A layer of non-living material made of cellulose. It is porous and freely permeable, helping the cell keep its shape and resist internal water pressure.   - Vacuole: A large central space in mature plants filled with cell sap (watery liquid with sugars and mineral ions).   - Chloroplasts: Found in green plant parts; contain chlorophyll to absorb light for photosynthesis.

Enzymes and Metabolism

  • Metabolism: The sum of all chemical reactions taking place in a cell.

  • Catalysts: Chemicals that speed up reactions without being used up. Enzymes are biological catalysts made of protein.

  • Lock and Key Model: The molecule an enzyme acts on is the substrate, which fits into the active site. This specificity ensures one enzyme catalyses only one reaction.

  • Factors Affecting Enzymes:   - Temperature: Increasing temperature increases kinetic energy and collision frequency. However, above the optimum temperature (usually 37 ∘C37\,^{\circ}C in humans), heat destroys the enzyme shape; it is then denatured.   - pH: Most enzymes work best at neutral pH (77), but some (like pepsin in the stomach) have an optimum as low as 22.

  • Intracellular vs. Extracellular: Intracellular enzymes work inside cells; extracellular enzymes (like digestive enzymes) are secreted to work outside.

Respiration and ATP

  • Respiration: An oxidation reaction that breaks down food molecules (typically glucose) to release stored chemical energy.

  • Aerobic Respiration: Requires oxygen. Complete breakdown of glucose into carbon dioxide and water.   - Equation: C6H12O6+6O2→6CO2+6H2O+energyC_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{energy}.

  • ATP (Adenosine Triphosphate): The energy 'currency' of the cell. Respiration makes ATP from ADP and phosphate; breaking ATP back to ADP releases energy for work (movement, active transport, etc.).

  • Anaerobic Respiration: Respiration without oxygen; glucose is partially broken down, releasing less energy.   - In Yeast: glucose→ethanol+carbon dioxide+some energyglucose \rightarrow ethanol + carbon\ dioxide + \text{some energy}.   - In Muscle Cells: glucose→lactate+some energyglucose \rightarrow lactate + \text{some energy}. Lactate buildup causes an oxygen debt.

Movement of Materials across Membranes

  • Diffusion: The net movement of particles from high to low concentration down a concentration gradient. Influenced by surface area to volume ratio, distance, temperature, and concentration gradient.

  • Active Transport: Movement of substances against a concentration gradient using energy from ATP. Utilizes protein 'pumps' in the membrane (e.g., glucose absorption in the gut).

  • Osmosis: The net movement of water from a dilute solution (high water potential) to a more concentrated solution (low water potential) across a partially permeable membrane.

  • Specialized Exchange Surfaces: Organs like the alveoli in lungs and villi in the small intestine are adapted with large surface areas and thin walls to maximize diffusion rate.

Levels of Organization and Classification

  • Organization: Organelles $\rightarrow$ Cells $\rightarrow$ Tissues (similar cells) $\rightarrow$ Organs (different tissues) $\rightarrow$ Organ Systems (different organs).

  • Five Kingdoms:   - Plants: Multicellular, photosynthesize, cellulose cell walls, store carbohydrate as starch or sucrose.   - Animals: Multicellular, no cell walls, move using nervous coordination, store carbohydrate as glycogen.   - Fungi: Bodies made of mycelium (network of thread-like hyphae). Saprotrophic nutrition (extracellular digestion of dead matter). Chitin cell walls. Examples: Mucor, Yeast.   - Protoctists: "Dustbin kingdom." Microscopic single-celled organisms. Example: Amoeba (animal-like), Chlorella (plant-like), Plasmodium (pathogen causing malaria).   - Bacteria: Prokaryotic (no nucleus). Smaller (1−5 μm1-5\,\mu m). Have circular DNA, cell walls (peptidoglycan), and sometimes plasmids. Examples: Lactobacillus bulgaricus, Pneumococcus.

  • Viruses: Non-living parasites (0.01−0.1 μm0.01-0.1\,\mu m). Composed of DNA or RNA inside a protein coat. Examples: Influenza, HIV (causes AIDS), Tobacco Mosaic Virus.

  • Pathogen: Any organism that causes disease (can be fungi, bacteria, protoctists, or viruses).

Human Physiology: Gas Exchange and Smoking

  • Thorax Structure: Includes ribs, intercostal muscles, diaphragm, trachea (with C-shaped cartilage), bronchi, bronchioles, and alveoli.

  • Ventilation:   - Inhalation: External intercostals contract (ribs up/out), diaphragm contracts (flattens), volume increases, pressure drops, air enters.   - Exhalation: Internal intercostals contract (ribs down/in), diaphragm relaxes (domed), volume decreases, pressure rises, air leaves.

  • Alveoli Adaptations: Enormous surface area (60 m260\,m^2), thin walls (two cell layers thick), moist lining, and extensive capillary network.

  • Smoking Hazards:   - Cilia Destruction: Mucus builds up, causing "smoker's cough" and bronchitis.   - Emphysema: Alveolar walls break down, reducing surface area for gas exchange.   - Lung Cancer: Tar contains carcinogens (over 6060 known) causing cell mutation.   - Carbon Monoxide: Binds to haemoglobin forming carboxyhaemoglobin, reducing oxygen carrying capacity. Causes heart disease and low birth weight in babies.

Human Physiology: Digestion

  • Balanced Diet: Requires Carbohydrates (energy), Lipids (insulation/storage), Proteins (growth/repair), Vitamins (A for vision, C for tissues, D for bones), Minerals (Calcium for bones, Iron for haemoglobin), Water, and Fibre (peristalsis).

  • Alimentary Canal:   - Mouth: Mechanical breakdown (teeth) and chemical (amylase digests starch to maltose).   - Stomach: Secretes hydrochloric acid (pH 22, kills bacteria) and pepsin (digests protein).   - Duodenum: Receives bile and pancreatic juices (amylase, protease, lipase).   - Bile: Produced in liver, stored in gall bladder; neutralises stomach acid and emulsifies lipids to increase surface area for lipase.   - Ileum: Adapted for absorption via villi and microvilli. Villi have capillaries for sugars/amino acids and lacteals for lipids.

Human Physiology: Circulation and Control

  • Circulatory System: Humans have a double circulatory system (pulmonary and systemic).

  • The Heart: Right side pumps to lungs (deoxygenated); left side pumps to body (oxygenated) with a thicker muscular wall. Cardiac cycle involves atrial and ventricular contraction.

  • Blood Vessels: Arteries (thick, elastic, high pressure), Veins (thin, valves, low pressure), Capillaries (one cell thick, exchange site).

  • Blood Composition: Plasma (liquid), Red blood cells (no nucleus, biconcave, haemoglobin), White blood cells (Phagocytes engulfing; Lymphocytes producing antibodies), Platelets (clotting).

  • Endocrine System: Coordination via hormones secreted by ductless glands into the blood.   - Adrenaline: Prepares body for "fight or flight" (increased heart/breathing rate, glycogen conversion).   - Insulin: Made in pancreas; stimulates liver to convert blood glucose into glycogen. Deficiency leads to Diabetes.

Human Physiology: Homeostasis and Reproduction

  • Thermoregulation: Controlled by the hypothalamus. Skin responses include sweating, vasodilation (widening of surface arterioles to lose heat), and vasoconstriction.

  • Excretion: Lungs (CO2CO_2), Kidneys (Urea, excess water/salts), and Skin (salts/urea in sweat).

  • Sexual Reproduction: Fusion of haploid gametes (23 chromosomes) to form a diploid zygote (46 chromosomes).

  • Menstrual Cycle: Day 1 (menstruation), Day 14 (ovulation). Oestrogen repairs lining; Progesterone maintains it.

  • Pregnancy: Placenta allows nutrient/waste exchange; amniotic fluid protects fetus from physical shock.