Plasma membrane

Structure and function of plasma membrane

1. Plasma Membrane Dr . Sonia Bajaj [email protected]

2. Plasma Membrane Plasma membrane is outer membrane of a cell, which is composed of two layers of phospholipids and embedded with proteins. It is a thin semi permeable membrane layer , which surrounds the cytoplasm and other constituents of the cell. The cell membrane also known as the cell membrane or cytoplasmic membrane , and historically referred to as the plasma lemma. Definition of semipermeable . : partially but not freely or wholly permeable; specifically : permeable to some usually small molecules but not to other usually larger particles. a  semipermeable  membrane. Role of the plasma membrane All cells are surrounded by a  plasma membrane . The  membrane  is composed of a phospholipid bilayer arranged back-to-back. The  membrane  is also covered in places with cholesterol molecules and proteins. The  plasma membrane  is selectively permeable and regulates which molecules are allowed to enter and exit the  cell.

3. Structure of plasma membrane   (I) Lipid Bilayer Hypothesis: In Overton (1895) suggested that the cell membranes contain lipids. This conclusion was based on the fact that fat solvents dissolved the membrane easily and fat soluble substances passed easily through the cell membranes. Gorter and Grendel (1925)’ were the first to suggest a possible structure of the cell membrane.

4. ( ii) Unit Membrane Model: In( 1950) J. David Robertson studied the cell membranes from electron micrographs of sectioned material. The unit membrane is 75 Å thick with a 35 Å thick phospholipid layer between two 20 Å thick protein layers. The plasma membrane surrounding the cell is thicker at the free surfaces of the cell than where it is in contact with other cells. In unit membrane model the protein layers are asymmetrical. On the outer surface it is mucoprotein while on the inner surface it is non- mucoprotein .

5.   (iii) Davson and Danielli : In ( 1935) The  model  describes a phospholipid bilayer that lies between two layers of globular proteins and It is trilaminar and lipoprotinious . In 1935, Davson and Danielli proposed that biological membranes  are made up of lipid bi-layers that are coated on both sides with thin sheets of protein and they simplified their  model  into the " pauci -molecular" theory.

6. ( iv) Fluid mosaic model: In (1972) ,   S. Jonathan Singer  and  Garth Nicolson  developed new ideas for membrane structure. Their proposal was the  fluid mosaic model , which is the dominant model now. It has two key features—a mosaic of proteins embedded in the membrane, and the membrane being a fluid bi-layer of lipids. The lipid bi-layer suggestion agrees with previous models but views proteins as globular entities embedded in the layer instead of thin sheets on the surface. According to the model, membrane proteins are in three classes based on how they are linked to the lipid bi-layer: Integral Proteins: Immersed in the bi-layer and held in place by the affinity of  hydrophobic  parts of the protein for the hydrophobic tails of  phospholipids  on interior of the layer. Peripheral proteins : More  hydrophilic , and thus are non- covalently  linked to the polar heads of phospholipids and other hydrophilic parts of other membrane proteins on the surface of the membrane. Lipid anchored proteins : Essentially hydrophilic, so, are also located on the surface of the membrane, and are covalently attached to lipid molecules embedded in the layer and have two parts 1."Head": hydrophilic → attracts and mixes with H 2 O 2.Two "fatty acid tails": hydrophobic  

7. Function of plasma membrane- P rotein Carrier (change shape for different molecules) for water-soluble molecules such as glucose Channels for ions (sodium and chloride ions) Pumps use energy to move water-soluble molecules and ions Adhesion molecules for holding cells to extracellular matrix Receptors enable hormones and nerve transmitters to bind to specific cells Recognition sites, which identify a cell as being of a particular type Enzymes, which speed up chemical reactions at the edge of the membrane Adhesion sites, which help some cells to stick together E.g. glycoprotein acts as a receptor and recognition site Diffusion Substances move down their conc. gradient until the conc. are in equilibrium Microvilli are extensions of the plasma membrane They increase the surface area of the membrane, therefore They accelerate the rate of diffusion

8. Facilitate diffusion Trans membrane proteins form a water-filled ion channel Allows the passage of ions (Ca2+, Na+, Cl -) down their conc. gradient //passive - no ATP required Some channels use a gate to regulate the flow of ions Selective permeability - Not all molecules can pass through selective channels How do molecules move across the membrane? Substrate (molecule to move across the membrane) binds to carrier protein Molecule changes shape Release of the molecule (product) at the other side of the membrane Example If you want to move a muscle a nerve impulse is sent to this muscle The nerve impulse triggers the release of a neurotransmitter Binding of the neurotransmitter to specific trans membrane proteins Opens channels that allow the passage of Na+ across the membrane In this specific case, the result is muscle contraction These Na+ channels can also be opened by a change in voltage

9. Osmosis Special term used for the diffusion of water through a differentially permeable cell membrane Water is polar and able to pass through the lipid bilayer Trans membrane proteins that form hydrophilic channels accelerate osmosis, but water is still able to get through membrane without them Osmosis generates pressure called osmotic pressure Water moves down its concentration gradient When pressure is equal on both sites net flow ceases (equilibrium) The pressure is said to be hydrostatic (water-stopping) Passive Transport- Uses energy from moving particles (Kinetic Energy) Active Transport Movement of solute against the conc. gradient, from low to high conc. Involves materials which will not move directly through the bilayer Molecules bind to specific carrier proteins / intrinsic proteins Involves ATP by cells (mitochondria) / respiration Direct Active Transport - transporters use hydrolysis to drive active transport Indirect Active Transport - transporters use energy already stored in gradient of a directly-pumped ion.

10. Endocytosis and Exocytosis Substances are transported across plasma membrane in bulk via small vesicles Endocytosis Part of the plasma membrane sinks into the cell Forms a vesicle with substances from outside Seals back onto the plasma membrane again Phagocytosis : endocytosis brings solid material into the cell Pinocytosis: endocytosis brings fluid materials into the cell Exocytosis Vesicle is formed in the cytoplasm Moves towards plasma membrane and fuses with plasma membrane Contents are pushed outside cell Insulin is secreted from cells in this way