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Transport across cell membrane by Dr. Tehmas

The document discusses membrane transport mechanisms that allow substances to move in and out of cells, emphasizing the roles of various proteins such as ion channels, ionophores, and water channels. It outlines different transport methods including passive diffusion, facilitated diffusion, and active transport, and details processes like exocytosis and endocytosis for macromolecules. The document also highlights the importance of specific proteins and energy requirements in these transport mechanisms.

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Cell Membrane Insights•
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TRANSPORT ACROSS MEMBRANE BY: Dr. Tehmas Ahmad Khan Demonstrator-Biochemistry Department
• All biological membranes allows transport of certain products inside and outside of cell. • For transport Certain integral proteins are required that are Classified as sunder: • Ion Channels • Ionophores • Water Channels (Aquaporin) • Gap Junction
ION CHANNELS • Pore like structures allowing transport of ions into or outside of cell. • All channels have gates, and are controlled by opening and closing • Types of Gates • Ligand gated channels • Voltage gated channels
PROPERTIES OF ION CHANNELS • Composed of transmembrane protein subunits. • Highly selective. • Well regulated by presence of “gates”. • Two main types of gates: Ligand-gated and voltage gated. • Activities are affected by certain drugs. • Mutations of genes encoding transmembrane proteins can cause diseases.
IONOPHORES • Ionophores contain hydrophilic centers that bind specific ions and are surrounded by peripheral hydrophobic regions. • Two Types: • Mobile ion carriers • Channel formers
WATER CHANNELS AND GAP JUNCTIONS • Water Channels are specialized channel proteins that only allow water inside cell. • Mutation leads to problems in Aquaporin-2 that leads to decreased water absorption and lead to nephrogenic diabetes insipidus. • Gap Junctions • specialized regions on their membranes for intercellular communications which are in close proximity.
TYPES OF TRANSPORT MECHANISMS
PASSIVE OR SIMPLE DIFFUSION • The solute passes from higher concentration to lower concentration till equilibrium is reached. • The process neither requires any carrier protein nor energy. • Slowest among all.
FACTORS AFFECTING NET DIFFUSION • Concentration gradient • Electrical potential • Hydrostatic pressure gradient • Temperature • Permeability coefficient
FACILITATED DIFFUSION • Movement of molecules from higher concentration to lower concentration via a carrier protein • The rate of diffusion is faster than simple diffusion • Example: D-fructose is absorbed from intestine by facilitated diffusion.
ACTIVE TRANSPORT • Against the concentration gradient and electrical gradient. • Requires energy • 40 per cent of the total energy requirement in a cell is utilised for active transport system • Requires the mediation of specific carrier or transport proteins.
TYPES OF TRANSPORT IN ACTIVE TRANSPORT • Uniport system Transport single ion in or out of cell • Co-transport system moves two or more molecules • Symport carries the two solutes in the same direction. • Antiport carries the two solutes in the opposite direction.
TRANSPORT OF MACRO MOLECULES • For transport of big molecules that cannot move into or out of cell via any of the previous mentioned methods. • It includes • Proteins • Hormones • immunoglobulins • low density lipoproteins (LDL) • viruses
EXOCYTOSIS • Substances created and packaged by Golgi complexes are moved out of cells via Exocytosis. • Also called reverse pinocytosis • Mechanism: Vesicles gets attached to the cell membrane  local influx of Ca++  leads to temporary membrane rupture  Substances released outside  vesicles membrane than integrates with the cell membrane.
ENDOCYTOSIS • Endocytic vesicles are formed when segments of plasma membrane invaginates enclosing a minute volume of extracellular fluid (ECF) and its contents. • These vesicles then fuses with lysosomes or other organelles. • Factors required for endocytosis. • ATP • Ca++ • Contractile elements
PHAGOCYTOSIS • Phagocytosis means To engulf something. • Large particles like Bacteria, viruses and cell debris are engulfed like this. • Cell extends its pseudopodia around particles and then forms a vesicle like structure that is called phagopolysome that later attaches to lysosome and is called phagolysosome. • This mechanism needs O2 and is called respiratory Burst.
PINOCYTOSIS • Cellular uptake of fluid and fluid contents. • Fluid phase pinocytosis: Nonselective process in which uptake of a solute by formation of small vesicles is simply proportionate to its concentration in the surrounding extracellular fluid (ECF). • Receptor mediated absorptive pinocytosis: 2 per cent of the external surface of plasma membrane are covered with receptors. Coming in contact specific fluid composition lead to invagination of cell membrane into pits that have calithrin molecule that upon contact signals the actin and myosin filaments to contact and the invaginated vesicle becomes endosome.
THANKS A LOT • Resources used: • Textbook of Medical Biochemistry by M.N.CHATTERJEA • Basic Medical Biochemistry by Lieberman and MARK • Textbook of biochemistry for medical students by Sreekumari and DM Vasudevan