Transport Across the Cell Membrane

SEM I (2012)

FACULTY OF VETERINARY MEDICINEUNIVERSITI MALAYSIA KELANTAN

Week IV Lecture (1stOctober 2012)

By: DR. ERKIHUN AKLILU W.G. (Course Coordinator)

Transport Across the Cell membrane

The plasma membrane and the membranes ofintracellular organelles

– Determine and regulate the movement of substancesacross the cell or its organelles

Transport Across the Cell membrane

Transport into and out of cells may occur by:

Simple and facilitated diffusion

Osmosis Active transport Endocytosis or Exocytosis

Simple and Facilitated Diffusion A passive mechanism

Driven by kinetic energy

Down the concentrationgradient

Only lipid-soluble substances(O2, CO2 and alcohol) candiffuse through the cellmembrane

Ions cannot diffuse freelythrough the lipid bilayer ofthe plasma membrane

Facilitated Diffusion

• Lipid-insoluble substances cross the cell membrane is via atransmembrane protein or proteins that form a channel throughthe membrane

Facilitated Diffusion Electrically gated or voltage gated

– Their permeability varies with the electrical potential across the cell membrane

– Permeability is regulated by the conformational changes of the membrane proteins

– Most channels are permeable only to a single specific ion or a small number of specific ions (e.g. Ca++ ---Cardiac cells)

Facilitated Diffusion

The rate and direction of passageof a charged ion through a channeldepends on two factors that mayact synergistically

– Concentration gradient– Electrical gradient– The number of carrier proteins

available in the membrane

Osmosis Is the movement of water across

membranes.

Like many solutes, water does notdiffuse freely through the lipid bilayer.

Water diffuses through water channelsformed by transmembrane proteins(aquaporins).

Higher solute concentration (osmoticpressure is the driving force.

Cellular volume is maintained throughosmosis.

Osmosis

• Administration of intravenous (IV)fluids to animals for problems suchas dehydration, anorexia, milk fever,and diarrhoea.

• Important in physiological (normal) functions such as:– Flow of blood and lymph– The excretion of wastes in the urine by

the kidneys– The digestion and absorption of food

The importance of the concepts of osmosis and osmotic pressure

Active Transport Some molecules (ions) move across

the cell membrane againstconcentration gradient

It is energy (ATP) dependentmovement of substances across the cellmembrane– E.g. sodium–potassium (Na–K)

pump

The Na–K pump, or Na–K–ATPase, isa component of the membrane of allcells, and it is always arranged in themembrane so that Na+ moves out ofthe cell and K+ moves into the cell.

Keeps a relatively higher and lowerintracellular [K+] and [Na+]respectively.

Primary active transport

Active Transport

Also requires a membrane proteincarrier and cellular energy

– E.g. The uptake of glucosefrom the lumen of the intestineand the lumen of renal tubulesby epithelial cells

A given transport proteintransports only specific ions ormolecules.

Secondary active transport

Endocytosis and Exocytosis

Endocytosis (into the cell)– The inward movement of materials

enclosed in a vesicle from cellmembrane

Exocytosis (out of the cell)– The fusion of a vesicle with the cell

membrane, releasing its contents tothe surroundings

Three Pathways of Endocytosis

Phagocytosis– Pseudopods engulf target particle and merge as a

vesicle

Bulk-phase endocytosis– Extracellular fluid is captured in a vesicle and brought

into the cell

Receptor-mediated endocytosis– Specific molecules bind to surface receptors, which are

then enclosed in an endocytic vesicle

Phagocytosis