2. TYPES OF MOVEMENT
•The plasma membrane is a barrier, preventing movement in and out of the cell of large molecules, ions, etc. However, cells need molecules to cross the membrane sometimes, for excretion and absorption of nutrients.
•There are four main types of movement across the plasma membrane: simple diffusion, facilitated diffusion, osmosis and active transport
3. SIMPLE DIFFUSION
•Diffusion is the passive movement of particles from a region of high concentration to a region of law concentration
•Simple diffusion across the plasma membrane can only occur with small, non-polar substances:
•Oxygen (it is polar, but small enough to cross the membrane)
4. FACILITATED DIFFUSION
•Same as simple diffusion, but requires a channel protein
•It is used for larger, polar molecules which cannot cross the membrane alone
•The channel protein shields the molecules from the hydrophobic phospholipid tails
•Osmosis is the passive movement of water molecules, across a partially permeable membrane, from a region of lower solute concentration to a region of higher solute concentration.
•The plasma membrane has pores
called aquaporinswhich allow for the
movement of water
6. ACTIVE TRANSPORT
•Active transport occurs against the concentration gradient, so it is movement of a molecule from a region of low concentration to a region of high concentration
•The process uses protein pumps and ATP (energy)
•Active transport is highly selective. For example, the sodium-potassium pump, particularly in nerve cells
•The vesicle membrane fuses with the plasma membrane, and its contents are secreted. The vesicle fuses with the plasma membrane, and its contents are expelled.
•A vesicle is formed when the plasma membrane infolds, then breaks off. Part of the membrane is pulled inwards, and a droplet of fluid is enclosed when it is pinched off.
•They can then move the contents through the cytoplasm. The continuity of the plasma membrane is not disrupted.
10. NA-K CHANNELS AT AXONS
•The axons of neurons have specialised versions of the sodium-potassium pumps which are used to create electrochemical phenomena
•The sodium–potassium pumps are used for active transport and the potassium channels are for facilitated diffusion in axons.
• Tissues or organs to be used in medical procedures must be bathed in a
solution with the same osmolarity as the cytoplasm to prevent osmosis.
• Hypotonic – The outside solution has a lower concentration than the cell,
causing it to swell
• Hypertonic – The outside solution has a higher
concentration that the cell, causing to shrink
with water loss
• Isotonic – Same concentration in both the cell and solution