2.4 Transport Across Membranes

How are materials transported into/out of of the cell?

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Depends…on?

1. Properties

   1. Size (big, small)
   2. Polarity (p, np,ionic)

2. Concentration gradient

   1. Diffuse along the gradient (high → low)
   2. diffusion pumped against the gradient (low → high)

Diffusion:

• Movement of substance from an area of high concentration to low concentration
• Equilibrium: reached when particles arr uniformly distributed

Dynamic Equilibrium:

• state at which there is uniform distribution and zero not movement of particles
• Concentration gradient: concentration between 2 regions
• Large difference → Large gradient → faster rate of diffusion
• Particles continue to move even after equilibrium

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Permeability:

• ability of membrane to allow particles through
• Permeable: allows particles through
• Impermeable: allows NO particles through
• Selectively permeable: allows SOME particles through based on: size & charge
• Cell membranes are: selectively permeable

Passive Transport: (high → low)

• Movement of material across a cell membrane along its concentration gradient without the expenditure of energy (no ATP)

  Simple Diffusion

  • unassisted movement of small molecules
  • What CAN cross: O2. CO2, N2 (Small hydrophobic that are soluble on the interior) & H2O, Glycerol (Small polars that fit between lipids molecules and polarity isnt strong enough to repel)
  • What CANT cross: glucose, amino acids (large polar - too large to fit in lipids) & Ions (repelled by hydrophobic interior due to charge)

  Facilitated Diffusion: Protein helping it

  • Assisted movement of small molecules
  • Assisted by integral membrane proteins: %%Transport proteins%%
  • Channel proteins:
    • Hydrophobic pathway
    • Allows IONS to diffuse
    • Voltage gated: open/closed in response to separation of charge (potential difference/voltage)
    • Channel doesn’t open unless gradient is large enough OR closes when gradient is too large
  • Carrier proteins:
    • Protein binds specific solute and changes conformation, moving molecule along gradient

Osmosis:

• Diffusion across semipermeable membrane
• Water diffuses in the direction of HIGHER SOLUTE CONCENTRATION - hate salt, no salt, away from salt. SALT SUCKS.
• HYPOtonic: Lower solute concentration
  • Concentration is greatest inside cell
  • Water floods in causing it to swell and possibly burst (Plasmolysis)
• HYPERtonic: Higher solute concentration
  • Concentration is greatest outside cell
  • Water flows out and causes cell to shrink (CRENATION)
• ISOtonic: Equal solute concentration
  • Water flows in and out freely (Dynamic Equilibrium)

Active Transport: (low → high)

• movement of substances across the cell membrane AGAINST their concentration gradient (from low to high)
• Requires energy in the form of ATP
• uses pumps

Primary Active Transport: (1 ion)

• Against gradient, costs ATP, uses transport proteins
• Concentration is high outside the cell and low inside
• Ions move towards high concentration, like w/ like
• Energy is require to move the pump
• 2 gradients
  • Chemical gradient: diff concentrations
  • Electrical gradient: separation of charge
• ATP → ADP after use
  • Adenosine TriPhosphate
  • High energy bonds
  • When phosphate is removed, energy is released

Secondary Active Transport: (2 ions)

• Against gradient, uses transport proteins, uses energy released from Active Transport from ATP (ATP→ ADP)
• Driving ion + Secondary ion
• Energy released by the diffusion of a driving ion to transport a secondary ion against its gradient
  • Symport: Same direction
  • Antiport: Opposing directions

Bulk Transport:

• Large molecules across membrane
• Transporter in Vesicles
• Portions of membrane are exchanged between plasma membrane and endomembrane system (Golgi, ER)

Exocytosis: Leaving

• Removal of materials from cytosol to ECF (Secretory proteins & waste)
• Endo-m loses membrane
• Plasma gains membrane

Endocytosis: Entering

• Into cytoplasm
• Endo-m gains membrane
• Plasma loses membrane
  • Bulk phase endocytosis: Pinocytosis:
  • Cell drinking
  • Membrane indents to form vesicle containing water and dissolved solutes
  • Non-specific
  • Receptor meditated endocytosis
  • Molecules bind to specific receptors on surface
  • Region w/ proteins indents to form vesicle
  • Vesicle may fuse with lysosome whose enzymes digest the contents
  • Phagocytosis
  • Cell engulfs another cell by extending membrane around it (Pseudopods)
  • Common function of Macrophages

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Proton pumps move hydrogen from cytosol to inside the nucleus of the cell

All primary active transport pumps move NEGATIVELY CHARGED IONS across membranes

Secondary active transport pumps use the concentration gradient of an ion, established by the primary pump, as their energy source