CELL MEMBRANE DR.IMRANA EHSAN

Structure and function of cell components (i) Carbohydrates (ii) Lipids (iii) Proteins (iv) Nucleic Acids (v) Membranes (vi) Cytoskeleton

The plasma membrane is the boundary that separates the living cell from its nonliving surroundings The plasma membrane exhibits selective permeability, allowing some substances to cross it more easily than others

Cell membranes Many organelles contain or are enclosed by membranes, including ◦Plasma membrane ◦Mitochondria ◦Chloroplasts ◦Nuclear envelope ◦Endoplasmic reticulum ◦Golgi apparatus ◦Lysosomes ◦Microbodies (aka perioxisomes)

Cellular membranes are fluid mosaics of lipids and proteins Phospholipids are the most abundant lipid in the plasma membrane Phospholipids are amphipathic molecules, containing hydrophobic and hydrophilic regions The fluid mosaic model states that a membrane is a fluid structure with a “mosaic” of various proteins embedded in it.

Structure of cell membrane

The Fluidity of Membranes Phospholipids in the plasma membrane can move within the bilayer Most of the lipids, and some proteins, drift laterally Rarely does a molecule flip-flop transversely across the membrane

As temperatures cool, membranes switch from a fluid state to a solid state The temperature at which a membrane solidifies depends on the types of lipids Membranes rich in unsaturated fatty acids are more fluid than those rich in saturated fatty acids Membranes must be fluid to work properly; they are usually about as fluid as salad oil

As temperatures cool, membranes switch from a fluid state to a solid state The temperature at which a membrane solidifies depends on the types of lipids Membranes rich in unsaturated fatty acids are more fluid than those rich in saturated fatty acids Membranes must be fluid to work properly; they are usually about as fluid as salad oil

Role of Cell Membranes Based on the functions of the above organelles / membranes write down some of the roles of the membranes in the cells (Leave some space as we will be adding to this list)

Structure of Cell membranes Fluid Mosaic Model

Electron micrograph of cell membrane

Structure of Cell membranes Lipids ◦Remember lipids have hydrophilic head and hydrophobic tails ◦To avoid the hydrophobic tails coming in contact with water a continuous bilayer sphere is formed. ◦Therefore hydrophobic interactions hold membranes together

Structure of Cell Membranes Proteins ◦Integral proteins (intrinsic)  Proteins that are embedded in the membrane  They are held in place by hydrophobic interactions (integral proteins have hydrophobic groups on their outer surface) ◦Peripheral proteins (extrinsic)  Proteins attached to the surface of the membrane (often forming non-covalent bonds with integral proteins)

Proteins cont…

Function of membranes macromolecules ◦Phospholipid bilayer  Forms boundary to isolate cell contents from environment  Restricts passage of hydrophilic substances across the membrane ◦Cholesterol  Increases bilayer strength, flexibility  Reduces membrane fluidity  Reduces permeability to water-soluble substances

Some proteins in the plasma membrane can drift within the bilayer Proteins are much larger than lipids and move more slowly

Function of Membrane Proteins Attachment ◦Cytoskeleton ◦Extracellular matrix Intercellular Junctions ◦Plasmodesmata ◦Tight Junctions ◦Gap Junctions ◦Desmosomes

Function of Membrane Proteins Transport ◦Carrier Proteins ◦Channel Proteins Receptors Enzymes Cell to Cell recognition

Function of membrane protiens Integral (Membrane-spanning or intrinsic) Can span membrane several times Either move around or are kept in place by cytoskeleton proteins Allows for cell polarity Associated (peripheral or extrinsic) Loosely bound to membrane Enzymes and structural proteins

Learning Activities Use all resources to find out more about the functions of membrane proteins. Based on the functions of proteins, add to your list on the role of membranes in cells