Transport across cell membrane

►Concentration of substances is different in ECF and ICF

►This difference is essential for survival

►Cell membrane ensures concentration difference between ECF & ICF

►Cell membrane is a selectively permeable barrier between ICF and ECF

Modes of transport

►Passive transport (diffusion)

►Active transport

►Vesicular transport

Passive transport (diffusion)

►Transport of molecules from high to low concentration due to their kinetic movement without using energy

Types

►Simple diffusion

►Facilitated diffusion

►Simple diffusion -net diffusion of molecules through cell membrane without using carrier proteins

►Through interstices of lipid bilayer

►Lipid soluble substances (non polar)

►O2 , CO2 , N2 , Fatty acids, steroid hormones

►Through protein channels

►Water & water soluble substances (polar)

►Urea, ions (Na+, K+, Ca++)

►Rate of diffusion increases in direct proportion with the concentration difference

►Attributes of protein channels

►Selective permeability

►Size of the channel

►Charge on the channel

►Gating

►Voltage gating

►Chemical gating (ligand gating)

►Facilitated diffusion -diffusion of molecules through cell membrane with help of carrier proteins (carrier mediated diffusion)

►Follows maximum rate of diffusion (Vmax)

►Due to dependence upon carrier protein

►Attributes of carrier proteins

►Specificity

►Presence of receptors

►Saturation

►Vmax

►Facilitated diffusion

►Glucose

►Insulin increases number of carrier proteins (transporters) for glucose

►Amino acids

►Factors affecting net diffusion

►Concentration difference

►Temperature

►Surface area

►Mass of diffusing substance

►Diffusion distance

►Pressure difference

►Membrane electrical potential

Osmosis

►Net simple diffusion of water across a semi-permeable membrane

►Occurs at

►Cell membranes

►Capillary membranes

Osmotic pressure

►Pressure required to stop osmosis

►More concentrated a solution the more osmotic pressure it will exert

Osmoles

-number of osmotically active particles in a solution

►One osmole is equal to one mole of a non dissociated molecule

►1 mole of glucose=1 osmole

►1 mole of NaCl =2 osmoles

►1 mole of Na2SO4= 3 osmoles

Osmolarity

►Osmoles per liter of solution

►ECF osmolarity (mainly due to Na+)

►281 mOsm/L

►Why to regulate ECF osmolarity?

►To maintain osmotic equilibrium between ICF & ECF

►Important for cell survival

►Hyperosmolar coma (diabetes mellitus)

Active transport

►Movement of substances againstconcentration gradient using energy with the help of carrier proteins

►Types (on the basis how energy is derived)

1) Primary active transport

►Energy derived directly by cleaving ATP

2) Secondary active transport

►Energy derived by concentration gradient

Primary active transport

►Active transport of a molecule by directly cleaving ATP for the use of energy

►Carrier proteins involved

►Called pumps

►Have ATPase activity

►Can break ATP to release energy

►Examples

►Na+, K+, Ca++, H+, I–

Sodium-potassium ATPase pump

►A carrier protein

►Present in all the cells

►A complex of 2 globular proteins

►Possesses ATPase activity on inside

►3 receptors for Na+on inside

►2 receptors for K+on outside

►Pumps 3 Na+outwards for 2 K+inwards

Functions:

►To establish Na+& K+concentration gradients between ICF and ECF

►Development of electrical potential across cell membrane

►Cell volume regulation

►Ca++ATPase pump

►Pumps Ca++out

Located at

►Cell membrane

►Intracellular membranes (sarcoplasmic reticulum of muscle cells)

►H+ATPase pump

►Pumps H+out of the cells

Located at

►Cell membranes of

►Tubular cells (intercalated cells) of late distal tubules of kidney

►Parietal cells of stomach

Secondary active transport

►Active transport of a molecule by using energy stored in the form of concentration gradient of some other molecule

►Mediated by carrier proteins

►Carrier proteins do not have ATPase activity

►ATP can not be cleaved to liberate energy

►Na+is generally utilized for secondary active transport of other molecules

►Na+concentration gradient is established in first place by primary active transport

►Na+while moving downhill into the cell moves other molecules uphill

►Types

1)Co-transport (symport)

►Na+ and molecule to be transported move in the same direction (into the cell)

2)Counter transport (antiport)

►Na+and molecule to be transported move in opposite direction (Na+always inwards)

►Co-transport

►Na+ -glucose co-transport

►Renal proximal tubular cells

►Na+ -amino acids co-transport

►Renal proximal tubular cells

►Counter transport

►Na+ -Ca++counter transport

►In all the cells

►Na+ -H+counter transport

►Renal proximal tubular cells

Vesicular transport

►Movements of substances across cell membrane in vesicles using energy

►Large molecules are transported

►Types

►Endocytosis

►Phagocytosis

►Pinocytosis

►Exocytosis

►Transcytosis

►Pinocytosis

►‘Cell drinking’

►Macromolecules, ECF fluid and solutes

►Cell membrane invaginates → pinocytic vesicle

►Phagocytosis

►‘Cell eating’

►Large particles, bacteria, dead cells

►Function of tissue macrophages and white blood cells

►Cell membrane evaginates by pseudopodia → phagocytic vesicle

►Exocytosis

►Reversal of endocytosis

Transcytosis

►Movement of materials from one side of the cell to other using vesicular transport

►No contact of vesicle with lysosomes

►First endocytosis and then exocytosis

►Transfer of proteins through capillary endothelial cells