transport across the cell membrane...active transport some molecules (ions) move across the cell...
TRANSCRIPT
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