Cell Function

• Osmosis

  The movement of water across the cell membrane through special tiny protein channels called aquaporins to balance out the solute concentration
• Diffusion


  The process where molecules move across the cell membrane from an area of high concentration to an area of low concentration until equilibrium is reached
• Types of diffusion

  • Simple diffusion: The movement of small, uncharged molecules (e.g. O2, CO2) easily through the phospholipid bilayer
  • Facilitated diffusion: Involves the aid of transport proteins to allow the movement of large molecules and small charged molecules across the cell membrane
• Active Transport

  The movement of molecules across the cell membrane with the input of cellular energy (ATP)
• Passive Transport


  The movement of molecules across the cell membrane without the input of cellular energy
• Transport Processes

  1. Passive Transport (high to low concentration no need of energy)

  2. Active Transport (low to high concentration need ATP)
• Transport proteins

  • Channel proteins control the movement of specific ions (e.g. NA positive)
  • Carrier proteins control the movement of larger molecules (e.g glucose, amino acids) by binding to the molecules and changing shape
• Types of Active Transport

  • Carrier proteins
  • Vesicular transport
• Endocytosis


  Process of the cell membrane changing shape to enclose an extracellular molecule forming a membrane-bound vesicle that enters the cell (e.g. phagocytosis)
• Exocytosis


  Process by which a membrane-bound vesicle fuses with the cell membrane and releases the molecules outside the cell (e.g. removal of cell wastes)
• Carrier Proteins


  • Protein pumps used to transport small, charged molecules or large molecules across the membrane
• Vesicular Transport

  • Transport of large molecules across the membrane
• Factors that affect the movement of molecules include properties, concentration gradient, and SA:V ratio
• Properties affecting movement of molecules

  • Size (small vs large)
  • Electrical charge (charged vs neutral)
  • Solubility (water vs lipid)
• Osmosis in Animal Cells: Cells in unicellular eukaryotes are surrounded only by a cell membrane. Hypotonic solutions, such as fresh water, pose a problem because water moves into animal cells by osmosis
• Osmosis in Plant Cells

  1. Plant cells do not burst in hypotonic solutions due to large, fluid-filled vacuoles and firm semipermeable cell walls

  2. Plant cell vacuoles with high solute concentration prevent bursting as water moves into the vacuole by osmosis

  3. When a plant cell is in a hypertonic solution, water leaves the cell causing plasmolysis
• Types of molecules

  • Size (small vs large)
  • Electrical charge (charged vs neutral)
  • Solubility (water vs lipid)
• Water is very important to living things as it is the medium for biochemical reactions, helps maintain cell shape, bathes tissues, and transports materials
• Osmotic pressure


  The pressure created by water moving across a semipermeable membrane due to osmosis
• Osmosis in Animal Cells

  1. Cells in unicellular eukaryotes surrounded by a cell membrane face issues with hypotonic solutions causing cell swelling and potential bursting due to water moving in by osmosis

  2. Cells in most animals are bathed in isotonic extracellular fluid, allowing efficient cell function with no net movement of water into or out of cells

  3. Water concentration in animal cells needs to be constant to coordinate biochemical reactions
• Osmotic gradient

  The net movement of water across a semipermeable membrane from a diluted to concentrated solution along its own concentration gradient
• More water moving across the membrane

  Higher osmotic pressure created
• Types of external solutions affecting water movement across a cell membrane

  • Isotonic solution - equal number of solutes on each side, water moves equally in both directions
  • Hypotonic solution - lower solute concentration outside, water moves into the cell causing swelling
  • Hypertonic solution - higher solute concentration outside, water moves out of the cell causing shrinkage
• Lower difference in the concentration of substances

  Concentration gradient will be less steep, and diffusion will occur slower
• Small Surface Area to Volume Ratio

  • Larger distance between the centre of the cell and the outside environment creating a slower exchange rate
  • Larger cells have more energy needs but not enough surface area to meet those needs, making them less efficient
  • This could lead to the death of the cell or cell division
• Concentrated solution

  High amounts of solute, low amount of water
• Chemical Properties

  • Polarity and charge molecules have the biggest impact
  • Non-Polar and uncharged (neutral) molecules can easily pass through the phospholipid bilayer
  • Polar and charged molecules can’t pass through and need to use transport or channel proteins
• Surface Area

  • Total area of the cell membrane surrounding the cell
  • The amount of material which can be moved in and out of a cell depends on the surface area of the cell membrane available
• Permeability

  • The permeability of a cell membrane depends on the molecule
  • High permeability: Molecules that are small, uncharged, and lipid-soluble can move freely across the lipid bilayer
  • Low permeability: Molecules that are large, charged, and water-soluble are impermeable and need to pass through using proteins or vesicles
• Dilute solution


  Low amount of solute, high amount of water
• Advantage for cells to maintain steep diffusion gradients


  If rapid transport is required
• Volume

  • Space taken up by the internal contents of the cell
  • The amount of material that needs to be moved depends on the volume of the cell
• Greater difference in the concentration of substances


  Concentration gradient will be steeper, and diffusion will occur faster
• Concentration Gradient

  • The difference in the concentration of a solute between regions affects movement
  • Substances move naturally from high to low concentration
  • The rate of diffusion changes depending on the concentration gradient
• Physical Properties

  • Size has the biggest impact
  • Small size meaning it can easily pass through the phospholipid bilayer
  • Large size meaning it can’t pass through and needs the aid of transport proteins and energy to cross the membrane
  • Very large size needs vesicular transport
• Surface Area to Volume Ratio

  • Determines the efficiency of transport and exchange of materials across the cell membrane
  • Substances that enter cells must travel from the outside environment across the surface of the cell and then diffuse inwards until they reach the centre of the cell
• Large Surface Area to Volume Ratio


  • Smaller distance between the centre of the cell and the outside environment creating a faster exchange rate
  • Smaller cells are more efficient and perform at an optimum level of functioning
• Substances that need to move across the cell membrane

  • Nutrients
  • Oxygen
  • Carbon Dioxide
  • Minerals and Vitamins (sugars, amino acids, glycerol, fatty acids)
  • Water and ions
  • Wastes (uric acid, excess CO2)
  • Obtain energy
• Cell requirements to exist

  • Source of energy
  • Supply of matter
  • Removal of wastes
• All organisms require energy to maintain their metabolic processes