The Plasma Membrane

Cell Membranes The cell membrane/plasma membrane represents the barrier that separates the cell’s contents from the surrounding environment and controls what moves in and out In eukaryotic cells, membranes are also used to generate compartments within the cell, each with a specialised function e.g. golgi apparatus, endoplasmic reticulum, lysosomes etc.

Membrane functions Provides selectively permeable barriers Compartmentalisation Localises reactions in the cell Transport of solutes (active transport) Signal transduction – receptor proteins on the membrane surface recognise and respond to different stimulating molecules, enabling specific responses to be generated Cell to cell recognition – the external surface of the membrane represents the cell’s biochemical “personality”

Hydrophobic and hydrophilic interactions Cellular proteins influenced by hydrophobic or hydrophilic nature of protein structure R groups determine protein location in the cell Hydrophilic R groups will occur predominately at the surface of a soluble protein found in the cytoplasm In these proteins hydrophobic R groups may cluster to from a globular structure. Regions of hydrophobic R groups allow strong hydrophobic Interactions that hold integral proteins within the phospholipid bilayer

Types of Membrane Proteins Proteins approx. 50% of the mass of the plasma membrane and can be classified into different groups depending on their arrangement in the membrane and/or their function INTRINSIC - proteins may be embedded TRANSMEMBRANE – proteins run through completely EXTRINSIC – proteins may be on surface Glycoproteins – proteins may have carbohydrates attached

Functions of Membrane Proteins The main functions of these membrane proteins are as follows: Transport Cell recognition Receptor sites Enzymes Intercellular Junctions

TRANSPORT PROTEINS Transport non-diffusable Channel proteins – provide a ‘pore’ across the membrane through which molecules (usually small and charged) can diffuse Carrier proteins – these are more specific with binding sites for only one solute Both these proteins permit passive transport (with a concentration gradient this is called facilitated diffusion) To transport molecules against the concentration gradient, special types of the carrier proteins are needed. These harness energy to drive the transport process during active transport e.g. sodium- potassium pump

CELL RECOGNITION PROTEINS usually glycoproteins the carbohydrate chain of the glycoprotein projects out of the cell the immune system can recognise it’s own cells and organs e.g. ABO blood group antigens: A = glycoprotein antigen A O = no glycoprotein antigens

RECEPTOR PROTEINS These have a specific conformation (shape) that allows binding of a particular molecule (ligand) The binding of the ligand will then trigger a response in the cell

ENZYMES A protein that catalyses a specific reaction Some receptor proteins have enzymatic activity; the cytoplasmic portion of the protein catalyses a reaction in response to binding a ligand

The Cytoskeleton The eukaryotic cell is a 3D structure. It has a cytoskeleton anchored to proteins in the plasma membrane These proteins both maintain shape and allow movement The cytoskeleton is a dynamic structure, as the microfilaments and microtubules can depolymerise and repolymerise very easily MICROFILAMENTS INTERMEDIATE FILAMENTS MICROTUBULES