Crossing membranes 2 (active)

Concentration Gradients
1. Cell's needs not always met by diffusion. Sometimes for cell to function properly, may need more of particular substance in cytoplasm than is present outside the cell.
  1. Other cases, cells may need to move materials into/out of cell more quickly than diffusion does.
2. Mg ions often v. short supply in soil. Plant cells need Mg ions to make chlorophyll. Plant cell must be able to move Mg, ions into cell against concentration gradient.
3. Active transport helps absorb glucose from our intestines.
Active Transport
1. Some carrier proteins found in membranes act as pumps. Proteins similar to protein carriers used for facilitated diffusion.
2. Shaped in way that fits (complementary to) molecule they carry. Carry larger/charged molecules + ions through membranes since they can't pass through lipid bilayer by diffusion.
3. Protein pumps differ significantly from proteins used in facilitated diffusion:
  1. Carry specific molecules one way across membrane.
  2. Carrying molecules across membrane, use metabolic energy in form of ATP.
  3. Can carry molecules in opposite direction to concentration gradient.
  4. Can carry molecules at much faster rate than by diffusion.
  5. Molecules can be accumulated either inside cells/organelles or outside cells.
Ensuring one-way flow
1. Energy used in pumping molecules across membrane by active transport changes shape of carrier protein. Shape change means specific molecule to be transported/pumped fits into carrier protein on 1 side of membrane only.
2. Calcium ions
  1. Muscle fibres can contract only if calcium ions are present. When muscle is stimulated to contract, calcium ions released from membrane-bound stores (specialised ER) where they're in v. high concentration.
  2. When muscle needs to relax, calcium ions pumped rapidly back into stores by many calcium ion pumps found on membrane of specialised ER.
Bulk Transport
1. Some cells need to move large quantities of material in/out. Endocytosis = involves bringing materials into cell + exocytosis = involves moving material out.
  1. Endo = inwards, exo = outwards.
2. Possible - membranes can easily fuse, separate or "pinch off". Requires energy in form of ATP. This case, energy used to move membranes around to form vesicles needed + move vesicles around cell.
3. Examples
  1. Hormones - pancreatic cells make insulin in large quantities. Insulin processed + packaged into vesicles in Golgi apparatus. Vesicles fuse with outer membrane to release insulin into blood.
  2. Plant cells, materials required to build cell wall are carried outside in vesicles.
  3. Some white blood cells engulf invading microorganisms by forming vesicle around them. Vesicle fuses with lysosomes so enzymes from lysosomes can digest microorganism = phagocytes.
    1. Phago = solid material, pino = liquid material.

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Crossing membranes 2 (active)

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	Created by anndjan21 about 10 years ago