membrane transport
TRANSCRIPT
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
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:
• Steroids
• Glycerol
• Oxygen (it is polar, but small enough to cross the membrane)
• Carbon dioxide
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
• 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 aquaporins which allow for the
movement of water
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
ENDOCYTOSIS AND EXOCYTOSIS
• The plasma membrane is very fluid and is capable of folding and
breaking off into vesicles.
EXOCYTOSIS
• 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.
ENDOCYTOSIS
• 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.
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.
OSMOLARITY
• 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