B2

• Diffusion


  1. Spreading out of particles resulting in a net movement from an area of higher concentration to an area of lower concentration

  2. Passive process as no energy is required

  3. Molecules like oxygen, glucose, amino acids, and water can move across, but larger molecules like starch and proteins cannot
• Diffusion in living organisms

  • Single-celled organisms use diffusion due to their large surface area to volume ratio
  • Multicellular organisms cannot rely on diffusion alone and have adaptations for transportation, e.g., alveoli in the lungs, villi in the small intestines, and root hair cells in plants
• Factors affecting the rate of diffusion

  • Concentration gradient
  • Temperature
  • Surface area of the membrane
• Osmosis

  1. Movement of water from a less concentrated solution to a more concentrated one through a partially permeable membrane

  2. Dilute solution of sugar has high water potential, concentrated solution has low water potential

  3. Passive process as it does not use energy
• Types of solutions in osmosis

  • Isotonic
  • Hypertonic
  • Hypotonic
• Osmosis in living organisms

  • Osmosis in animals can cause cells to burst or become shrivelled
  • Osmosis in plants can lead to turgor pressure essential for rigidity in leaves and stems
• Effect of external solution on cells

  1. If more dilute, water moves into the cell and vacuole causing swelling and pressure called turgor

  2. If less dilute, water moves out of the cell causing softening, plasmolysis, and cell death
• Active transport

  Movement of particles from an area of lower concentration to an area of higher concentration, against the concentration gradient
• Active transport requires

  Energy (ATP) from respiration as it works against the gradient
• Examples of active transport in living organisms

  • In root hair cells: Take up water and mineral ions from the soil, In the gut: Move substances like glucose and amino acids from the gut into the bloodstream
• Mitosis is a type of cell division where one cell divides to form two identical daughter cells
• The cell cycle is a series of steps that the cell undergoes in order to undergo mitosis
• Stages of Mitosis

  Interphase: Cell growth, organelles increase, protein synthesis occurs, chromosomes are replicated, energy stores increase, Mitosis: Chromosomes line up at the equator and spindle fibres pull them to either side of the cell, Cytokinesis: Cytoplasm and cell membranes divide to form two identical daughter cells
• Mitosis in multicellular organisms is important for


  Growth, development, and replacing damaged cells
• Mitosis is vital for asexual reproduction because


  It allows the organism to replicate its own cells to produce offspring
• Differentiation


  Process where cells gain new sub-cellular structures to be suited to their role
• Examples of specialised cells in animals

  • Sperm cells: Specialised to carry male DNA to egg cell, Nerve cells: Specialised to transmit electrical signals quickly
• Sperm cells

  • Streamlined head and long tail for swimming, Many mitochondria for energy, Acrosome with digestive enzymes
• Nerve cells

  • Long axon for carrying impulses, Many dendrites for branched connections, Many mitochondria at nerve endings
• Axon

  Long, enabling impulses to be carried along long distances
• Dendrites

  Extensions from the cell body that allow branched connections to form with other nerve cells
• Nerve endings

  Have many mitochondria supplying energy to make neurotransmitters, allowing impulses to be passed from one cell to another
• Muscle cells are specialised to contract quickly to move bones (striated muscle) or simply to squeeze (smooth muscle)
• Myosin and actin

  Special proteins that slide over each other, causing muscle contraction
• Mitochondria in muscle cells


  Provide energy from respiration for contraction
• Glycogen in muscle cells


  Stored chemical used in respiration by mitochondria
• Specialised cells in plants

  • Root hair cells
  • Xylem cells
  • Phloem cells
• Root hair cells

  Specialised to take up water by osmosis and mineral ions by active transport from the soil
• Xylem cells

  Specialised to transport water and mineral ions up the plant from roots to shoots
• Phloem cells

  Specialised to carry the products of photosynthesis (food) to all parts of the plant
• Stem cells

  Undifferentiated cells that can undergo division to produce many more similar cells
• Embryonic stem cells


  Formed when an egg and sperm cell fuse to form a zygote, can differentiate into any type of cell in the body
• Adult stem cells

  Found in bone marrow, can form many types of cells including blood cells
• Meristems in plants


  Found in root and shoot tips, can differentiate into any type of plant throughout the life of the plant
• Challenges of size: Multicellular organisms have a small surface area to volume ratio compared to the amount of substances
• Desirable features of a plant

  • Disease resistance
  • Research
  • Saving a rare plant from extinction
• Exchange systems

  multicellular organisms have a small surface area to volume ratio compared to the amount of substances they need to exchange
• Surface area to volume ratio

  The size of the surface area of the organism compared to its volume
• Calculating surface area to volume ratio

  Calculated by finding the volume (length x width x height) and the surface area (length x width), and writing the ratio in the smallest whole numbers
• If surface area to volume ratio is large


  The organism is less likely to require specialised exchange surfaces and a transport system because the rate of diffusion is sufficient in supplying and removing the necessary gases