B2

• Diffusion


  The spreading out of the particles resulting in a net movement from an area of higher concentration to an area of lower concentration
• Diffusion is a passive process as no energy is required
• Molecules that can move across by diffusion

  • Oxygen
  • Glucose
  • Amino acids
  • Water
• Molecules that cannot move across by diffusion

  • Starch
  • Proteins
• Single-celled organisms


  • Can use diffusion to transport molecules into their body from the air
  • Have a relatively large surface area to volume ratio
  • Due to their low metabolic demands, diffusion across the surface of the organism is sufficient enough to meet its needs
• Multicellular organisms


  • Surface area to volume ratio is small so they cannot rely on diffusion alone
  • Surfaces and organ systems have a number of adaptations that allows molecules to be transported in and out of cells
• Examples of adaptations in multicellular organisms

  • Alveoli in the lungs
  • Villi in the small intestines
  • Root hair cells in plants
• Factors affecting the rate of diffusion

  • Concentration gradient (difference in concentrations): The greater the difference in concentration, the faster the rate of diffusion
  • Temperature: The greater the temperature, the greater the movement of particles, resulting in more collisions and therefore a faster rate of diffusion
  • Surface area of the membrane: The greater the surface area, the more space for particles to move through, resulting in a faster rate of diffusion
• Osmosis


  The movement of water from a less concentrated solution to a more concentrated one through a partially permeable membrane
• Osmosis is a passive process, as it does not use energy
• Isotonic


  If the concentration of sugar in an external solution is the same as the internal, there will be no movement
• Hypertonic


  If the concentration of sugar in external solution is higher than the internal, water moves out
• Hypotonic


  If the concentration of sugar in external solution is lower than the internal, water moves in
• Osmosis in animals


  • If the external solution is more dilute (higher water potential), it will move into animal cells causing them to burst
  • If the external solution is more concentrated (lower water potential), excess water will leave the cell causing it to become shrivelled
• Osmosis in plants


  • If the external solution is more dilute, water will move into the cell and into the vacuole, causing it to swell, resulting in pressure called turgor (essential in keeping the leaves and stems of plants rigid)
  • If the external solution is less dilute, water will move out of the cell and they will become soft. Eventually the cell membrane will move away from the cell wall (called plasmolysis) and it will die
• Active transport


  The movement of particles from an area of lower concentration to an area of higher concentration, i.e. against the concentration gradient
• Active transport requires energy from respiration as it is working against the gradient
• Active transport in root hair cells

  • They take up water and mineral ions (for healthy growth) from the soil
  • Mineral ions are usually in higher concentrations in the cells, meaning diffusion cannot take place
  • This requires energy from respiration to work
• Active transport in the gut

  • Substances such as glucose and amino acids from your food have to move from your gut into your bloodstream
  • Sometimes there can be a lower concentration of sugar molecules in the gut than the blood, meaning diffusion cannot take place
  • Active transport is required to move the sugar to the blood against its concentration gradient
• Mitosis


  A type of cell division where one cell divides to form two identical daughter cells
• Cell cycle


  A series of steps that the cell has to undergo in order to divide by mitosis
• Interphase


  The cell grows, organelles (such as ribosome and mitochondria) grow and increase in number, the synthesis of proteins occurs, all 46 chromosomes are replicated (forming the characteristic 'X' shape) and energy stores are increased
• Mitosis


  The chromosomes line up at the equator of the cell and spindle fibres pull each chromosome of the 'X' to either side of the cell
• Cytokinesis


  Two identical daughter cells form when the cytoplasm and cell membranes divide, each containing the same 46 chromosomes as the original cell
• Importance of mitosis in multicellular organisms

  • Growth and development
  • Replacing damaged cells
  • Asexual reproduction
• Differentiation


  A process that involves the cell gaining new sub-cellular structures in order for it to be suited to its role
• Stem cells


  Undifferentiated cells which can undergo division to produce many more similar cells, some of which will differentiate to have different functions
• Types of stem cells

  • Embryonic stem cells
  • Adult stem cells
  • Meristems in plants
• Embryonic stem cells


  • Form when an egg and sperm cell fuse to form a zygote
  • They can differentiate into any type of cell in the body
  • Scientists can clone these cells (though culturing them) and direct them to differentiate into almost any cell in the body
• Adult stem cells


  • If found in bone marrow they can form many types of cells (not any type, like embryonic stem cells can) including blood cells
• Meristems in plants


  • Found in root and shoot tips
  • They can differentiate into any type of plant, and have this ability throughout the life of the plant
  • They can be used to make clones of the plant
• Surface area to volume ratio

  The size of the surface area of the organism compared to its volume
• If the 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
• Adaptations in multicellular organisms to increase surface area to volume ratio

  • Having a large surface area
  • Increasing the number of exchange surfaces
  • Developing transport systems
• Arent plant


  Has certain desirable features (such as disease resistance), for research or to save a rare plant from extinction
• Challenges of size


  • Exchange systems (2.2a and b)
• Multicellular organisms have a small surface area to volume ratio compared to the amount of substances they need to exchange
• Calculating surface area to volume ratio

  Find the volume (length x width x height) and the surface area (length x width), and write the ratio in the smallest whole numbers
• Large surface area to volume ratio

  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
• Multicellular organisms have had to adapt to increase this ratio as much as possible