Transport across the cell membrane

Transport across the cell membrane

Learning objectives

Body compartments

ECF and ICF Constituents

Lipid Bilayer: Barrier to water and water-soluble substances ions glucose H 2 O urea CO 2 O 2 N 2 halothane

Cell Membrane but, other molecules still get across! urea ions H 2 O glucose CO 2 O 2 N 2 halothane

Molecular Gradients Na + K + Mg 2+ Ca 2+ H + HCO 3 - Cl - SO 4 2- PO 3 - inside (in mm) 14 140 0.5 10-4 (ph 7.2) 10 5-15 2 75 outside (in mm) 142 4 1-2 1-2 (ph 7.4) 28 110 1 4 protein 40 5

Types of Transport Passive and Active Passive Simple Diffusion Facilitated Diffusion Active Primary Co-Transport Counter Transport Secondary

Simple Diffusion (a) lipid-soluble molecules move readily across the membrane (rate depends on lipid solubility) (b) water-soluble molecules cross via channels or pores (a) (b)

Fick's law of Diffusion Several factors affect the rate of net diffusion across the membrane The magnitude of the concentration gradient ( C) The permeability of the membrane to substance (P) Surface area of the membrane (A) Molecular weight of the substance (MW) Distance (thickness) X Net rate of Diffusion Q = C.P.A MW. X Restated= Q C.A.D P =Diffusion coefficient (D) X MW

Factors that affect net rate of diffusion. cont'd 1. Membrane electrical potential- Nernst Potential 1. Pressure difference across the membrane

Nernst Potential: At normal body temperature, the electrical difference that will balance a given concentration difference of univalent ionssuch as Na ions- can be determined from the formula called Nernst equation EMF (in millivolts) = ±61 log C1/C2

Diffusion Through Protein Channels and Gating of These Channels Pores Integral cell membrane proteins that form open tubes through the membrane and are always open Characteristics of protein channels: Selective Permeability( due to diameter, shape & charge on channel) Gating

Ion Channels Characteristics: Ungated determined by size, shape, distribution of charge, etc. Gated voltage (e.g. voltage-dependent Na + channels) chemically (e.g. nicotinic ACh receptor channels) in Na + Na + and other ions out

Selective permeability of protein channels

Gating of protein channels

Carrier mediated diffusion A carrier facilitates diffusion of substance to the other side downhill. The rate of diffusion approaches a maximum called Vmax, as conc of substance increases Facilitated Diffusion

Carrier mediated transport system display three important characteristics 1. Specificity - Transport selectivity, Defective transport system for cysteinean inherited disease -cysteine urea 2. Saturation Limited number of carrier binding sites- transport maximum (Tm) 3. Competition- Closely related substances may compete for a ride across the membrane on the same carrier

Osmosis The process of net movement of water caused by a concentration difference of water, across a selectively permeable membrane is called osmosis

Osmotic pressure The amount of pressure required to stop osmosis is called osmotic pressure of any particular solution Osmotic pressure is determined by number of particles per unit volume of fluid, not by the mass of particles 1 osmole is 1 gram molecular weight of osmotically active solute ( 180 g of glucose which is 1 g molecular weight of glucose is 1 Osmole

Osmolality Osmolality given in Osmoles One osmole is 1 gram mol. wt. of undissociated solute One osmole of Glucose (180 gm) Two osmoles NaCl (58.5 gm) (2 ions) Osmolality is Osmoles/1kg of solvent Osmolarity is Osmoles/1L of solution

Active Transport When a cell membrane moves molecules or ions uphill against the concentration gradient ( or uphill against an electrical or pressure gradient), the process is called active transport. Examples: Sodium, potassium, Calcium, hydrogen, chloride, urate,some amino acids etc

Energy is required for active transport

Active transport is divided into two types according to the source of the energy used Primary active transport Secondary active transport

Primary Active Transport Energy is derived directly from breakdown of ATP (Adenosine triphosphate) Sodium potassium pump Substances such as sodium, potassium, calcium, hydrogen, chloride and few other ions are transported by primary active transport

Na-K PUMP

Functions of sodium potassium pump Control volume of each cell Without this function, most cells would swell until they burst. Electrogenic nature of Na-K pump: It causes negativity inside and positivity outside- cause an electric potential Across the cell membrane

Primary active transport of Calcium ions Two Calcium pumps- one in cell membrane and other in intracellular organelle (sarcoplasmic reticulum in muscle cell) maintain very low intracellular calcium concentration Primary active transport of Hydrogen ions: Gastric glands of the stomach Late distal tubules and cortical collecting ducts of the kidney

Secondary Active Transport In secondary active transport, the energy stored due to an ion gradient is used to transport another substance. e.g A large gradient (storehouse of energy) for sodium is created when it moves to outside the cell membrane by primary active transport.

Types of active transport Co- transport Co transport of glucose and amino acid along with Sodium ions in Epithelial cells of the intestinal tract Renal tubular epithelial cells

Counter Transport Sodium- Calcium counter-transport occurs through all or almost all Cell membranes Sodium- Hydrogen counter transport in proximal tubules of kidneys

Substances must transfer through cellular sheets. Intestinal epithelium Epithelium of renal tubules Epithelium of all exocrine glands Epithelium of gall bladder Membrane of choroid plexus of brain Active transport through cellular sheets