Topic 2 – Membrane transport mechanisms: free diffusion, osmosis, facillitated

diffusion, primary and secondary active transport, endo-exocytosis.

There are 4 primary ways that water and other molecules can cross into or out of cells.

The transport mechanism is divided into passive and active transport:

a. Passive transport: does not involve chemical energy. 4 main kinds of passive

transport: diffusion, facilitated diffusion, filtration and osmosis.

b. Active transport: requires the use of chemical energy to move molecules

"uphill" against a gradient - electrical or concentration gradient (collectively

termed as electrochemical gradient). Generally carried out by transport

proteins found in membrane. There are primary and secondary active

transports and also larger molecules such as starch can go across the cell

membrane by processes known as endocytosis and exocytosis.

Passive transport:

• Free diffusion - net movement of material from an area of high concentration

to an area with lower concentration. 2 types:

1. Non-channel mediated – lipids, gases (CO2, O2), water.

2. Channel mediated – ions, charged molecules.

Channels can be always opened and not specific such as pores or they

can be selective to specific molecule and are not

always opened (have gates), these can be chemical,

voltage or mechanical gated channgels.

• Osmosis - movement of water molecules across a

semipermeable membrane from a region of low solute

potential to an area of high solute potential. Done by

diffusing or via aquaporins.

• Facillitated diffusion - mediated by a carrier. rate of

diffusion is a function of concentration gradient, amount of carrier protein,

rate of associati/dissociati for nutrient and transporter. Examples -glucose,

amino-acids.

Active transport:

• Primary active transport –Uses 3Na+/2K+ ATPase.

Energy is directed coupled to movement. Another example:

Ca²+ pump

• Secondary active transport –energy is derived from

energy stored in the form of concentration differences in a

second solute which was created by primary active

transport. Can be uniporter, symporter or antiporter.

Examples: Na+/glucose symport; Na/Ca ; HCO3/Cl ; Na/H

c. Endocytosis –process whereby cells absorb molecules

from the outside by engulfing it with their cell

membrane. Types: phagocytosis and pinocytosis

d. Exocytosis - process by which a cell releases large

biomolecules through its membrane. Also have a role in

signalling and regulatory functions.

Examples:

Substances Type of Transport Examples

Na+ Facilitated Diffusion Na+ Channel

Na+ Secondary Active Transport Na+-K+-2Cl- Symporter, Na+-H+ Antiporter, Na+-Glucose Symporter, Na+-Amino Acid

Symporter, Na+-Pi Symporter, Na+-Lactate Symporter, Na+-Cl- Symporter.

Na+ Active Transport Na+-K+ -ATPase

K+ Facilitated Diffusion K+ Channel

K+ Secondary Active Transport Na+-K+-2Cl- Symporter.

K+ Active Transport Na+-K+ -ATPase, H+-K+ -ATPase

H+ Secondary Active Transport Na+-H+ Antiporter

H+ Active Transport H+-K+ -ATPase

Nutrients – Glucose,

Amino Acids,

Vitamins, etc.

Secondary Active Transport Na+-Nutrient Symporter

Cations – Mg2+,

Ca2+, etc. Secondary Active Transport Na+-Cation Symporter

Anions – Cl-, HCO3-,

etc. Secondary Active Transport Na+-Cl- Symporter, Na+-HCO3

- Symporter

Urea Facilitated Diffusion – Solvent

Drag

Channel Protein

Small Proteins Active Transport - Endocytosis Engulfed by epithelial cells of the PT.

Osmotic pressure – The pressure required to prevent the passage of water

through a semi-permeable membrane from a region of low concentration of

solutes to one of higher concentration, by osmosis.

Hydrostatic pressure - Hydrostatic pressure refers to the pressure that any fluid

in a confined space exerts.

o Mole is the SI base unit that measures an amount of substance. One mole = Avogadro's number (approximately

6.022×1023) entities.

o Molarity= the number of moles of a given substance per litre of solution

o Osmole is a non-SI unit of measurement that defines the number of moles or amount of substance that contribute to a

solution's osmotic pressure

o Osmolarity = osmoles of solute per litre of solution

o Osmolality = osmoles of solute per kilogram of solvent

o Osmolarity of body fluids = 300 mOsm. Osmosis is a colligative property of solutions - depend on the concentration of solute particles rather than their (solute)

individual properties