Movement across membranes

• Net movement of water

  A) Concentrated

  B) Dilute

  C) High

  D) Low

  E) Low

  F) High

  G) Low

  H) High

  I) High

  J) Low

  K) Solute

  L) Water

  
• Movement across membranes

  • Membranes are a barrier to most substances
  • Selectively permeable
  • Allows materials to be concentrated in cells, excluded, or separated from outside environment
  • Eukaryotic also compartmentalise inside organelles
  • There are 5 main ways substances move across cell membranes:
  • Simple Diffusion
  • Facilitated Diffusion
  • Active Transport
  • Osmosis
  • Exo/Endocytosis
  • ATP in bulk transport:
  • Provide energy for vesicle movement along cytoskeleton microtubules (via motor proteins)
  • Help fuse vesicle and plasma membrane together
• Simple Diffusion

  • Particle Movement down concentration gradient (high to low)
  • Doesn’t require energy (passive process)
  • Hydrophobic phospholipid bilayer restricts what passes
  • Lipid-soluble molecules can diffuse or small molecules
  • H_2O - only a singular molecule; bulk done by osmosis
  • Lipid diffusion is uncontrolled
  • Also happens in liquids (dye into water)
  • Diffusion is affected by
  • Temperature
  • SA
  • Distance
  • Molecule size
  • Concentration gradient
• Facilitated Diffusion

  • Diffusion across a partially permeable membrane
  • Via transmembrane transport protein
  • Tend to be specific for one molecule
  • Complementary shape binding
  • Substances can only cross a membrane if it contains their appropriate protein
  • Passive process (no energy needed to move down concentration gradient)
• Active Transport

  • Requires energy/ATP (active process)
  • Transmembrane protein (highly specific) binds to substance on one side, changes shape, and releases it on other side
  • Allows cells to maintain internal concentration of small molecules different from immediate surroundings
  • Can move molecules against concentration gradient
• Endocytosis

  • Material engulfed by membrane enters cell in vesicle
  • Requires energy
  • Receptor-mediated:
  • Some integral proteins have receptors on surface
  • To recognise/take in hormones, cholesterol, etc
  • Phagocytosis “cellular eating”:
  • Phagocytes (leukocytes) are covered in receptors
  • Engulf large particles into vesicles
  • Phagosome is a phagocytic vacuole
  • Pinocytosis “cellular drinking”:
  • Most common form
  • Takes in dissolved molecules, in a vesicle
• Exocytosis

  • Material contained in small vesicles migrates to and fuses with cell membrane
  • Contents of vesicle (large cell-manufactured molecules) exit (e.g. hormones, neurotransmitters or waste)
  • Requires energy
• Osmosis

  • Down water potential gradient
  • Tonicity: Solution ability to cause cell to gain/lose water
  • No Cell Wall (e.g. Animal)
  • Isotonic solution: Solute concentration is same in and out, no net movement
  • Hypertonic solution: Solute concentration is greater inside, loses water; shrivel (cremate)
  • Hypotonic solution: Solute concentration is less inside, gain water; burst (lyse)
  • Cell Wall (e.g. Plant)
  • Hypotonic: Gain water; turgid (ideal)
  • Hypertonic/Isotonic: Lose water; soft, and flaccid, cell may lose so much membranes detach from wall (plasmolysed)
• Water Potential

  • Osmosis quantified using water potential
  • Ψ: tendency for water molecules to diffuse from one solution to another
  • Ψ = Solute Potential + Pressure Potential
  • Polar solute’s presence restricts water movement as water molecules, cluster around particles
  • Less ‘free’ water molecules able to diffuse to another solution to another
  • Pure Water Potential = 0kPa
  • All solutions have negative Ψ
  • More solute, more negative
  • All cell membranes are selectively permeable
  • Water diffuses in/out of organelles/cells by osmosis
  • Down water potential gradient