Membrane transport

MEMBRANE TRANSPORTPRESENTED BY IB SCREWED

www.ibscrewed.org

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