• Thin plasma membrane • Separates the cell content from extracellular environment• Semi-permeable• Consist of a bilayer of phospholipids and some protein molecules• The individual phospholipids and some protein molecules move around within their layer • The protein molecules are scattered in the membrane giving a mosaic pattern to the surface of the membrane

• Polar hydrophilic heads of the phospholipid are in contact with the aqueous surrounding and form hydrogen bonds with water molecules• Nonpolar hydrophobic tails are repelled by water.The hydrophobic tails of both layers face inwards and are attracted to one another

•There are intrinsic proteins embedded in the membrane.•There are extrinsic proteins on the outer and inner surface of the membrane•The hydrophobic amino acids interact with the water hydrophobic fatty acid chains to expel water• The hydrophobic part of the protein is attracted to the aqueous surrounding• Some of the proteins and phospholipids conjugate with short oligosaccharides to form glycoproteins and glycolipids on the outer surface• Oligosaccharides is important for cell recognition• Glycocalyx surrounds the cells

• Cholesterol molecules are found between phospholipids molecules

it stabilises the membrane structure

it regulates the membrane fluidity• The membrane is supported by an intracellular protein filaments which acts as cytoskeleton

Membrane Fluidity • The viscosity of the lipid bilayer of a cell membrane• The viscosity of the membrane can affect the rotation and diffusion of proteins and other bio-molecules within the membrane,hence it affects the functions of the molecules

How To Increase Membrane Fluidity ?

•The phospholipid molecules and some protein molecules can move laterally or change places and thus give the membrane fluidity•Unsaturated fatty acids have double bonds which form kinks

•Increase the temperature

Functions Of Membranes

•Forms a protective barrier between cell contents and the external environment• Provide attachment to the cytoskeleton filaments which gives support and maintain cell shapes•Forms membranous organelles•Electron carrier protein• Enzymes

•Cell surface receptor proteins are involved in signal transduction

Detects chemical signals on the outside of the cell Transmit the detection inside the cell

•Selectively permeable and regulates transport of substances in and out of the cell

• Glycoproteins act as antigens• Glycolipids act as recognition sites for specific chemicals• Cell adhesion proteins

• Folding of cell membranes enables phagocytosis pinacytosis the increase of surface area for absorption

To obtain nutrients or to engulf and destroy foreign particles

• Cholesterol stabilises the membrane structure and reduce the entry or exit of polar molecules

Movement of SubstancesSubstances Across Membrane

Substances move in and out by the following process

• Diffusion• Facilitated Diffusion• Osmosis• Active Transport•Endocytosis• Exocytosis

Importance• Supply cell with oxygen• Regulate the pH and solute concentration• Excrete toxic waste substances• Secrete useful substances

Passive Transport• Movement of ions and molecules down their• Concentration gradient

Diffusion • Net movement down the concentration gradient• Does not require any energy ATP• Permeable to very small uncharged molecules• Not permeable to charged ions• Rate of diffusion : Concentration gradient

Surface area Temperature Size and nature of moleculesDistance over which diffusion takes place

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