IGCSE Biology Revision Guide: Cells, Nutrition, and Transport

Characteristics of Living Organisms and Cell Structure

• Organism: A living thing that possesses the seven characteristics of life.
• Seven Characteristics of Living Organisms (MRS GREN):
  • MOVEMENT: A change in position or place.
    • phonepe phototropism: Movement of plants towards sunlight.
    • geotropism: Growth of roots towards gravity.
  • Reproduction: The process of making more of the same kind of organism.
  • SENSITIVITY: The ability to detect or sense stimuli in the internal or external environment and make appropriate responses.
  • GROWTH: A permanent increase in size and dry mass.
  • RESPIRATION: A chemical process occurring in all cells to break down glucose and release energy for metabolism.
  • EXCRETION: The removal of toxic materials, waste products of metabolism, and substances in excess.
  • NUTRITION: Taking in materials and nutrients for energy, growth, and development.

Cell Organization and Types

• Cell Organization:
  • Cell division: $cell \div 2 = new\,cells$.
  • Unicellular: Organisms consisting of a single cell (e.g., bacteria, yeast).
  • Multicellular: Organisms consisting of many cells.
  • Hierarchy of Organization: $\text{cells (group)} \rightarrow \text{tissue (group)} \rightarrow \text{organ (group)} \rightarrow \text{organ system} \rightarrow \text{organism}$.
• Cell Types:
  • eukaryotic: Cells that have a nucleus (found in animal and plant cells).
  • prokaryotic: Cells that do not have a nucleus (e.g., bacteria).

Cell Organelles and Functions

• Cell Membrane: Found in all cells. It consists of thin protein-fat layers that are partially permeable. It controls the entry and exit of molecules.
• Cytoplasm: Found in all cells. A jelly-like fluid where metabolic reactions take place.
• Nucleus: Found in eukaryotes only. It stores genetic information in chromosomes and determines proteins, thereby controlling cell activities.
• Ribosomes: Found in all cells. They are the site of protein synthesis.
• Mitochondria: Found in eukaryotic cells. They are the site of aerobic respiration and produce $ATP$.
• Vesicles: Found in animal cells.
• Cell Wall (Cellulose): Found in plant cells only. It contains cellulose fibers and is fully permeable. It supports the cell and prevents it from bursting.
• Chloroplasts: Found in green plants. They contain chlorophyll and are the site of photosynthesis.
• Vacuole: Found in plant cells. Contains cell sap (sap vacuore) and sugar. It maintains turgor pressure.
• Bacterial Cell (Prokaryotic) Structures:
  • Circular DNA: The genetic material (no nucleus).
  • Plasmids: Extra small circles of DNA.
  • Cell Wall: Distinct from the plant cellulose cell wall.
  • Ribosomes: Present in bacterial cells (Friposomes).

Microscopy and Magnification

• Magnification Formula:
  • $\text{magnification} = \frac{\text{image size}}{\text{actual size}}$
  • Ensure units are the same during conversion. Add an $\times$ sign to the result.
• Unit Conversions:
  • $1\,cm = 10\,mm$
  • $1\,nm = \frac{1}{1000}\,\mu m \text{ (micrometers)}$

Specialized Cells

• Red Blood Cell (RBC):
  • Function: Transport oxygen.
  • Adaptations: Bi-concave disk shape for surface area, contains no nucleus to make room for haemoglobin.
• Root Hair Cell:
  • Function: Absorb water and mineral ions from the soil.
  • Adaptations: Long hair-like extension to provide a large surface area (SA). Located underground, so they contain no chloroplasts.
• Palisade Mesophyll Cell:
  • Function: Absorbs light for photosynthesis.
  • Adaptations: Packed with chloroplasts and has a large surface area; located on the surface of the leaf.
• White Blood Cell (WBC):
  • Function: Kills pathogens and neutralizes toxins by chemical means.
  • Adaptations: Can squeeze out of blood vessels and engulf microbes.
• Ciliated Epithelial Cells:
  • Function: Movement of mucus in the trachea and bronchi.
  • Adaptations: Possess many tiny hairs called cilia (e.g., nose hairs).
• Sperm and Egg Cells:
  • Function: Aid in reproduction.
  • Sperm Adaptations: Male gamete; long tail for swimming and many mitochondria for more energy.
  • Egg Adaptations: Female gamete; cytoplasm contains nutrients; membrane prevents entry of more than one sperm.
• Neurone:
  • Function: Conducts electrical impulses.
  • Adaptations: Has dendrites to attach to other cells.

Movement In and Out of Cells

• Diffusion:

  • Definition: Net movement of particles from a region of high concentration to a region of low concentration (down a concentration gradient).
  • Importance: Used to obtain raw materials for respiration, removing waste products, and photosynthesis ($CO_2$ in, $O_2$ out).
  • Passive Process: No energy is required.
  • Factors Affecting Rate:
    • $\uparrow$ SA:volume ratio: Faster diffusion due to more surface area for particles to move across.
    • $\downarrow$ diffusion distance: Faster diffusion due to a shorter length for particles to travel across a membrane.
    • $\uparrow$ temperature: Faster diffusion because increased kinetic energy ($KE$) leads to faster movement and more frequent collisions.
    • $\uparrow$ concentration gradient: Faster diffusion because a greater difference in concentration leads to more random collisions on the high-concentration side, pushing particles faster.

• Osmosis:

  • Definition: Net movement of water molecules from a region of high water potential (dilute) to a region of low water potential (concentrated) through a partially-permeable membrane.
  • Mechanism: Small molecules like water move down the gradient.

• Solutions Outside Cells:

  • Hypertonic: Higher solute concentration outside the cell. Water leaves the cell by osmosis, causing a decrease in mass.
  • Isotonic: Equilibrium. No change in mass or movement.
  • Hypotonic: Lower solute concentration outside the cell. Water enters the cell by osmosis, causing an increase in mass.

• Osmosis in Animal Cells:

  • In pure water (hypotonic solution): Water enters by osmosis, the cell membrane swells and bursts (lysis).
  • In concentrated solution (hypertonic solution): Water leaves, and the cytoplasm shrinks, making the cell smaller. There is no cell wall to support the cell from collapsing or bursting.

• Osmosis in Plant Cells:

  • Hypotonic solution: Water enters, and the vacuole swells, pushing the cytoplasm outward towards the cell wall. This creates turgor pressure (the pressure of water pushing outward against the cell wall). The cell becomes turgid (firm), and the cell wall prevents bursting.
  • Hypertonic solution: Water leaves, and the cytoplasm and vacuole shrink. The cell membrane pulls away from the cell wall. The cell becomes flaccid (soft/floppy). This process is known as plasmolysis and can kill a plant by damaging the cell membrane.

Active Transport (ACT)

• Definition: Movement of molecules or ions through a membrane from a region of low concentration to a region of high concentration (against a concentration gradient), using energy from respiration ($ATP$).
• Purpose: Used when cells need a substance present only in small amounts outside the cell. Without ACT, diffusion would move these substances out of the cell.
• Energy Source: Aerobic respiration produces $ATP$ (adenosine triphosphate), which powers carrier proteins.
• Mechanism: Carrier proteins in the cell membrane pick up specific molecules or ions and change shape to move them across the membrane into the cytoplasm (e.g., nitrate ion absorption).

Comparison of Transport Mechanisms

Biomolecules

• Organic Molecules: Large molecules made from smaller constituent molecules.
• Carbohydrates:
  • Composition: Carbon ($C$), Hydrogen ($H$), Oxygen ($O$).
  • Include: Sugars, starch, and cellulose.
  • Monosaccharides: Simplest form of sugar molecules (e.g., glucose $C_6H_{12}O_6$ used in respiration and dissolved in blood plasma).
  • Polysaccharides: Contain many sugar molecules (e.g., cellulose, glycogen, starch which serves as energy storage in plants).
• Lipids (Fats and Oils):
  • Composition: Carbon, Hydrogen, Oxygen ($1$ glycerol + $3$ fatty acid chains).
  • States: Fats (solid at room temperature); Oils (liquid at room temperature).
  • Functions: Vital for making cell membranes, energy storage (adipose tissue - prevents heat loss), and mechanical protection of organs.
• Proteins:
  • Composition: Carbon, Hydrogen, Oxygen, and Nitrogen ($N$).
  • Structure: Long chains of amino acids arranged in a particular sequence. There are $20$ different amino acids that can be arranged in any order, creating thousands of varieties of proteins.
  • Functions: Helps make enzymes, antibodies (pathogen protection), haemoglobins, cell membranes, keratin (hair/nails), and hormones (e.g., insulin).

Food Tests

• Iodine Test (for Starch): Add iodine solution (orange-brown) to the sample. Positive result: Blue/black. Negative result: Orange-brown.
• Benedict's Reagent (for Glucose): Add Benedict's (blue) to sample; heat in hot water bath. Positive result: Green $\rightarrow$ yellow $\rightarrow$ orange $\rightarrow$ red. Negative result: Blue.
• Ethanol Emulsion Test (for Fats): Add ethanol to food sample and shake (fat dissolves in ethanol). Pour mixture into cold water. Positive result: White/milky emulsion. Negative result: Stays clear.
• Biuret Test (for Proteins): Add drops of Biuret reagent to the sample and shake. Positive result: Violet/purple. Negative result: Stays blue.

Enzymes

• Definition: Biological catalysts. A catalyst increases the rate of reaction without being changed itself.

• Nature: Proteins that alter metabolic reactions. Without enzymes, metabolic reactions would be too slow to sustain life.

• Functions: Can break large molecules into small ones (digestion) or build large molecules from smaller ones (synthesis).

• Lock-and-Key Model:

  • Active Site: Specific region on an enzyme where the substrate binds.
  • Substrate: The substance on which the enzyme reacts. The shape of the active site and substrate are complementary.
  • Enzyme-Substrate Complex: A short-lived structure formed when the substrate binds to the enzyme.
  • Product: The molecules produced. When products are released, the enzyme is free to act again.

• Factors Affecting Enzyme Activity:

  • Temperature (ROR vs Temp):
    • Very Low Temp: Very slow or stops. Particles have very low $KE$, leading to few collisions.
    • Increasing Temp: Rate increases as particles gain $KE$, move faster, and have more frequent/successful collisions.
    • Optimum (Opt) Temp: Maximum rate where successful collisions are ideal. For most human enzymes, this is $37\,^{\circ}C$.
    • Above Optimum: Rate drops to zero. Enzyme is denatured; excessive vibration causes a permanent change in shape (irreversible). The active site is destroyed, and the substrate can no longer bind.
  • Effect of pH:
    • Most enzymes have an optimal pH where they work fastest (usually around pH $7$).
    • Exception: Pepsin in the stomach works best at pH $2$ (stomach acid).
    • Extreme pH levels break bonds in the protein, causing denaturation; the shape of the active site changes.
  • Buffer Solution: A liquid with a known, stable pH used to keep the pH constant during experiments.

Key Digestive Enzymes

Plant Nutrition and Photosynthesis

• Photosynthesis: The process by which plants synthesize carbohydrates from raw materials using energy from sunlight.

• Word Equation: $\text{carbon dioxide} + \text{water} \xrightarrow{\text{light energy + chlorophyll}} \text{glucose} + \text{oxygen}$

• Symbol Equation: $6CO_2 + 6H_2O \rightarrow C_6H_{12}O_6 + 6O_2$

• Mechanism: Chlorophyll (green pigment in chloroplasts) captures sunlight energy and transfers it to create water and $CO_2$ molecules to produce glucose. Oxygen is released into the atmosphere.

• Uses of Glucose in Plants:

  • Respiration: Energy release for cell activities (e.g., active transport, protein synthesis).
  • Starch: Converted to starch (long spiral chains) for storage as an energy source. Starch is insoluble and doesn't affect solution concentration (osmosis).
  • Sucrose: Glucose made in leaves is converted to sucrose for transport through phloem tubes to other parts of the plant.
  • Cellulose: Glucose molecules linked in long straight chains to build cell walls for structural support.
  • Nectar: Produced to attract pollinators (insects, birds, bats).
  • Protein Synthesis: Glucose + nitrate ions ($NO_3^-$) taken from soil via ACT make amino acids and proteins. Without nitrates, growth is limited.
  • Chlorophyll Production: Plants need Magnesium ($Mg^{2+}$) and Nitrate ions from soil. Without these, leaves turn yellow (chlorosis).

• Factors Affecting Photosynthesis Rate:

  • Supply of raw materials ($CO_2$ and $H_2O$).
  • Quantity of sunlight (Light intensity).
  • Temperature (affects enzymes controlling the process).

Leaf Structure

• External Features: Petiole, midrib, veins (vascular bundles), lamina (large surface area for sunlight/$CO_2$).
• Internal Layers:
  • Cuticle: Waxy, transparent, waterproof covering.
  • Upper Epidermis: Tightly packed cells to reduce water vapor escape; transparent to allow light through.
  • Palisade Mesophyll: Tall, narrow cells with many chloroplasts; main site of photosynthesis at the top of the leaf.
  • Spongy Mesophyll: Carries out photosynthesis and allows gas diffusion ($CO_2$, $O_2$, water vapor) through air spaces.
  • Lower Epidermis: Contains stomata; loses less water as it is not in direct sunlight.
  • Guard Cells: Pair of cells surrounding the stoma; change shape to open/close it.
  • Stomata: Openings allowing diffusion of gases in/out of the leaf. Transpiration also occurs here.
  • Vascular Bundle: Contains Xylem (brings water/minerals to the leaf) and Phloem (transports sugars/amino acids away).

Human Nutrition and Balanced Diet

• Components of a Balanced Diet:
  1. Carbohydrates: Needed for energy (Starch/Sugar). Sources: Potatoes, wheat, rice. Deficiency: Fatigue.
  2. Fats & Oils (Lipids): Secondary energy stores, insulation (adipose tissue), organ protection. Sources: Cheese, meat, butter. Excess: Obesity, heart risks.
  3. Protein: Building cells, growth/repair, enzymes, antibodies. Sources: Meat, fish, eggs, nuts. Deficiency: Poor growth.
  4. Vitamins:
     • C: Maintains skin/gums. Deficiency: Scurvy (bleeding gums).
     • D: For calcium absorption (bones/teeth). Sources: Oily fish, sunlight. Deficiency: Rickets (soft bones).
  5. Minerals:
     • Iron: Makes haemoglobin in RBC. Sources: Liver, red meat. Deficiency: Anaemia.
     • Calcium: Strengthens bones/teeth. Sources: Dairy.
  6. Fibre: Keeps alimentary canal working; stimulates peristalsis. Sources: Oats, plant foods (cellulose). Prevents constipation.
  7. Water: Solvent for metabolic reactions; transport in plasma; needed for digestion.

The Digestive System

• Processes:

  1. Ingestion: Intake of food into the body via mouth.
  2. Digestion: Mechanical (teeth grinding) and Chemical (enzymes breaking molecules).
  3. Absorption: Movement of nutrients/ions through intestine walls into the blood.
  4. Assimilation: Uptake and use of nutrients by cells (for energy or components).
  5. Egestion: Removal of undigested food as faeces.

• The Alimentary Canal Journey:

  • Mouth/Salivary Glands: Physical digestion. Saliva contains amylase (starch digestion) and mucus.
  • Oesophagus: Long tube to the stomach. Contains a lumen (hole for food).
  • Stomach: Strong muscular walls churn food with gastric juice. Goblet cells secrete mucus. Contains $HCl$ (pH $2$) to kill microorganisms. Proteases (pepsin) digest protein.
  • Small Intestine (SI):
    • Duodenum: Pancreatic juice with enzymes is added.
    • Ileum: Most absorption takes place here into the blood.
  • Large Intestine:
    • Colon: Absorbs remaining water.
    • Rectum: Stores faeces.
    • Anus: Egestion occurs here.

• Accessory Organs:

  • Pancreas: Secretes pancreatic juice.
  • Liver: Produces Bile.
  • Gallbladder: Stores bile. Bile is alkaline to neutralize stomach acid and emulsifies fats.

Transport in Plants

• Xylem: Stacked dead cells. Transports water and mineral ions UP from roots to stem. Helps support the plant.
• Phloem: Living cells. Transports sucrose and amino acids from leaves to roots/flowers.
• Vascular Bundle Locations:
  • In Roots: Xylem in center ($+$ shape), phloem outside.
  • In Stems: Xylem inside, phloem outside.
• Water Uptake: Root hair cells have long extensions for large SA. Water travels by osmosis through the epidermis and cortex to xylem vessels.
• Transpiration: Loss of water from leaves. Water evaporates from mesophyll into air spaces and out through stomata. This creates a transpiration stream pulling water up xylem vessels.
  • Factors Affecting Rate: Temperature ($\uparrow$ Temp = $\uparrow$ rate), Humidity ($\uparrow$ Humidity = $\downarrow$ rate), Wind ($\uparrow$ Wind = $\uparrow$ rate).
• Translocation: Movement of sucrose/amino acids in phloem from Sources (e.g., photosynthesizing leaves) to Sinks (e.g., roots for starch storage or flowers for fructose).

Gas Exchange and Respiration

• Gas Exchange: Diffusion of $CO_2$ and $O_2$ in and out of an organism.
• Respiratory Anatomy:
  • Trachea: Large tube below larynx; contains vocal cords.
  • Bronchi: Trachea divides into two bronchi (bronchus) which branch into bronchioles.
  • Alveoli: Tiny air sacs where gas exchange occurs. Surrounded by capillaries.
  • Adaptations of Alveoli: Large surface area, thin/permeable walls, good blood supply.
• Breathing Mechanism: Uses Intercostal muscles (between ribs) and the Diaphragm.
• Air Composition:
  • Oxygen: $21\%$ Inspired; $16\%$ Expired.
  • $CO_2$: $0.04\%$ Inspired; $4.0\%$ Expired.
  • Water Vapour: Variable.
• Exercise: Muscles need more energy and oxygen. Heart and breathing rates increase. If oxygen supply is insufficient, Anaerobic Respiration occurs.
  • Anaerobic Respiration: Breaking down nutrients without $O_2$, producing lactic acid.
  • Oxygen Debt: Extra $O_2$ required after exercise to break down lactic acid. Breathing and heart rates stay high until debt is paid.