Movement of substances

• Diffusion

  • Passive movement of molecules from a region of higher concentration to a region of lower concentration
  • Steeper concentration gradient = faster rate of diffusion
• Factors affecting rate of diffusion

  • Particle size - Smaller particle = faster rate of diffusion
  • Membrane - Thinner membrane = faster rate of diffusion
  • Distance - Shorter distance = faster rate of diffusion
  • Surface Area - Greater surface area = faster rate of diffusion
  • Temperature - Increase in temperature = Greater kinetic energy = faster rate of diffusion
  • Partially permeable membrane = allows certain molecules to diffuse into cytoplasm but not others
• Osmosis

  • Passive movement of water molecules from a region of higher water potential to a region of lower water potential
  • Only water molecules are able to diffuse across partially permeable membrane in both directions
  • Dynamic equilibrium reached
• What happens when animal cell placed in hypotonic solution

  • Cytoplasm has lower water potential then surroundings
  • Water diffuses from surroundings into cell by osmosis
  • Cell swells, causing it to burst due to lack of cell wall
• What happens when animal cell placed in hypertonic solution

  • Cytoplasm has greater water potential then surroundings
  • Water diffuses out of cell into surroundings by osmosis, causing it to shrink in size
  • Cell crenates, where little spikes appear on cell surface membrane
  • Cell will eventually become dehydrated, causing it to die
• What happens when plant cell placed in hypotonic

  • Cell sap has lower water potential surroundings
  • Water diffuses into cytoplasm by osmosis
  • Vacuole increases in size, pushing cell contents against cell wall
  • Cell enlarges + becomes turgid
  • Cell walls' inelastic trait prevents over expansion of vacuole by exerting opposing pressure as water enters cell
  • This prevents the entry of more water, preventing the cell from bursting
• What happens when cell placed in hypertonic solution

  • Cell sap has greater water potential than surroundings
  • Water diffuses out of cytoplasm by osmosis
  • Vacuole decreases in size
  • Plasmolysis occurs, causing the shrinkage of cell membrane and cytoplasm away from cell wall
• Water potential

  • Tendency of water to move from one place to another
• Turgor pressure

  • Pressure of cell contents against cell wall
  • Determined by water content in vacuole from osmotic pressure
• Similarities between diffusion and osmosis

  1. Passive processes
  2. Down a concentration gradient
• Differences between diffusion and osmosis

  1. Diffusion
  2. Any substance
  3. Does not need membrane
  4. Movement of molecules/ions
  5. Osmosis
  6. Only water
  7. Needs partially permeable membrane
  8. Movement of water molecules
• Facilitated diffusion

  1. Spontaneous passage of molecules and ions, bound to specific carrier proteins, facilitated by channel proteins, down a concentration gradient
  2. Passive transport
  3. Assisted by carrier and channel proteins
  4. Charged ions like potassium and sodium cannot diffuse easily across non-polar centre of phospholipid bilayer
• Facilitated diffusion - Channel proteins

  • Channel proteins open up spaces across membrane, allowing the entry/exit of substances
  • Each channel protein only allows one type of ion to pass through
• Facilitated diffusion - Carrier proteins

  • Allows the diffusion of larger polar molecules across membrane
  • Particular molecule binds itself to specific binding site
  • Causes carrier proteins to undergo change in shape, delivering the molecule across the membrane
  • Two types
  • Passive transport
  • Substrate attachment brings about conformational change
  • Active transport
  • Needs ATP to bring about conformational change
• Similarities between passive and facilitated diffusion

  1. Equilibrium reached when concentration equal
  2. Does not need ATP
• Differences between passive and facilitated diffusion

  1. Passive
  2. Depends on centration gradient
  3. Similar molecules diffuses at similar rate
  4. In both directions
  5. Facilitated
  6. Specific molecules diffuses faster
  7. Via carrier proteins
  8. Faster in one direction
• Active transport

  • Movement of substances through plasma membrane against concentration gradient
  • Requires energy in the form of ATP
  • From regions of low concentration to regions of high concentration
• Factors affecting active transport

  1. Temperature
  2. Oxygen concentration
  3. Presence of poison
• Endocytosis

  • Cellular uptake of macromolecules and substances by localized regions of plasma membrane
  • Pinched off to form intracellular vesicles + active process
  • Two types
  • Phagocytosis
  • Pinocytosis
• Phagocytosis

  • Solid material taken into cell by vesicles
  • Lysosomes fuses with vesicles, releasing enzymes into it
  • Enzymes digests material and products are absorbed into cytoplasm
• Pinocytosis

  • Liquid material taken into cell
  • Many small vesicles formed
• Exocytosis

  • Cellular secretion of macromolecules by fusion of vesicles with plasma membrane
  • Useful secretion
  • Passage of molecules out of cell
  • Active process
• Limitation of surface area on cell

  • Too big
  • Volume increases more than surface area
  • Cell cannot get nutrients in and wastes out fast enough
  • Too small
  • Organelles cannot fit and function correctly