Transport Across the cell membrane Dr Kalpana B

Transport Across the cell membrane Dr. Kalpana. B Assistant professor Physiology

• At the end of this class students should: • Classify passive transport processes. • List the difference between simple diffusion and facilitated diffusion. • Explain the primary and secondary active transport processes with examples. • Describe the structure and functions of Na+-K+ ATPase • Define Endocytosis and exocytosis with examples

• Cell membrane is selectively permeable to some molecules & ions. • Lipid or fat-soluble substances, e. g. O 2, CO 2, OH; enter directly into cell membrane through the lipid bilayer. • Water-soluble substances, e. g. ions, glucose, water; enter through proteins of the cell membrane.

Types of Cellular Transport • Passive Transport • Active Transport

Passive Transport cell doesn’t use energy 1. Diffusion 2. Facilitated Diffusion 3. Osmosis

Active Transport cell does use energy 1. Protein Pumps 2. Endocytosis 3. Exocytosis

PASSIVE TRANSPORT • cell uses no energy • molecules move randomly • Molecules spread out from an area of high concentration to an area of low concentration • (High Low)

Types of Passive Transport 1. Diffusion 2. Facilitated Diffusion – diffusion with the help of transport proteins 3. Osmosis – diffusion of water

What determines the rate of diffusion?

Factors determining diffusion • • • Distance-greater longer the time for diffusion Thickness Cross- sectional area-directly proportional Molecular size-permeability –falls rapidly with increase in mwt. Lipid solubility Diffusion coefficient-permeability*cross section. Temperature Solubility Concentration gradient

Diffusion through protein channels • Leaky channels • GATING MECHANISM IN PROTEIN CHANNELS Voltage gated channels Ligand gated channels

Facilitated Diffusion • carrier –mediated process • Eg glucose transport(GLUT) across intestinal epithelium. • Transport of glucose into RBCs , muscles and adipose tissue in presence of insulin. • More rapid • Carrier protein-conformational change-binding site alternatively exposed to ICF and ECF.

• Saturation-Rate of diffusion increases with increase in concentration gradient. • Plateau-binding sites on carrier protein are filled. • Specific. • Competitive.

Glucose transporter Glut 4 is the insulin-responsive glucose transporter

Osmosis ‘The diffusion of water from an area of high concentration of water molecules to an area of low concentration of water across a partially permeable membrane. ’ Or Movement of solvent across the membrane from low solute concentration into higher concentration of solute. Osmotic pressure-prevents the movement.

DILUTE SOLUTION CONCENTRATED SOLUTION Cell membrane partially permeable. Sugar molecule VERY Low conc. of water molecules. VERY High conc. of water molecules. Inside cell Outside cell

Tonicity • Osmolality of solution relative to plasma. • Osmolality of human plasma-290 mosm/L • Isotonic – solution with same osmolality as plasma • Hypertonic-greater osmolality • Hypotonic-lesser osmolality

Active Transport Primary active transport Secondary active transport Vesicular transport

Primary active transport Ø Energy is required directly to move a substance uphill Ø Hydrolysis of ATP provides the energy for primary active transport Ø Major primary active transport proteins found in cells are 1 Na+/K+ ATPase 2. Ca 2+ ATPase 3. H+ ATPase 4. H+/K+ ATPase

Sodium –potassium pump • Is present in all the cells of the body • Responsible for maintaining the Na+ and K+ • concentration difference across the cell membrane Establishment of negative electrical potential inside the cell

• The sodium-potassium pump (a membrane protein) exchanges three Na+ ions for two K+ ions.

Functions of Na+ -K+ pump 1. CONTROLLING THE CELL VOULUME By decreasing the intracellular hyperosmolarity. 2. Resting membrane potential 3. ELECTROGENIC ACTIVITY 4. Hormone Actions 1. Thyroxine , insulin. . -increase pump activity 2. Dopamine , digitalis –decrease the activity 3. Hypoxia and hypothermia inhibit the activity.

Secondary active transport In this type, transport of substances are coupled to the primary active transport Usually secondary transporters utilize sodium 1. SODIUM CO –TRANSPORT 2. SODIUM COUNTER TRANSPORT

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SODIUM CO –TRANSPORT • All solutes move in the same direction “to the inside of the cell” Examples • Na+– glucose Co transport • Na+– amino acid Co transport • In the intestinal tract, & kidney’s brush borders.

Counter transport (Antiport) • Na+ is moving to the interior causing other substance to move out. Examples • Ca 2+– Na+ exchange (present in many cell membranes) • Na+– H+ exchange in the kidney • Cl-– HCO 3 - exchange across RBCs.

Transporters Carrier proteins used in active transport include: -uniporters – move one molecule at a time -symporters – move two molecules in the same direction -antiporters – move two molecules in opposite directions 31

Transport of large molecules • Macromolecules that are too large move in or out of the cell with the aid of bubble like vesicles created from cell membrane Basic mechanisms are as follows • Endocytosis: Very large particles enter the cell The principal forms of endocytosis are pinocytosis and • Exocytosis phagocytosis

ENDOCYTOSIS is the case when a molecule causes the cell membrane to bulge inward, forming a vesicle. PHAGOCYTOSIS is the type of endocytosis where an entire cell is engulfed. PINOCYTOSIS is when the external fluid is engulfed. RECEPTOR-MEDIATED ENDOCYTOSIS occurs when the material to be transported binds to certain specific molecules in the membrane.

Endocytosis phagocytosis pinocytosis receptor-mediated endocytosis fuse with lysosome for digestion non-specific process triggered by molecular signal

• References • Text Book of medical physiology-AP Krishna • Physiology for B. Sc. Nursing –Dr A. K jain