CELL MEMBRANECELL MEMBRANE DR.IMRANA EHSAN

Structure and function Structure and function of cell componentsof 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 membranesCell 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 Cellular membranes are fluid mosaics of lipids and proteinsmosaics 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 membraneStructure of cell membrane

The Fluidity of MembranesThe 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 MembranesRole 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 membranesStructure of Cell membranesFluid Mosaic Model

Electron micrograph of cell Electron micrograph of cell membranemembrane

Structure of Cell membranesStructure 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 MembranesStructure 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…Proteins cont…

Function of membranes Function of membranes macromoleculesmacromolecules

◦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 ProteinsFunction of Membrane Proteins

Attachment◦Cytoskeleton◦Extracellular matrix

Intercellular Junctions◦Plasmodesmata◦Tight Junctions◦Gap Junctions◦Desmosomes

Function of Membrane ProteinsFunction of Membrane Proteins

Transport◦Carrier Proteins◦Channel Proteins

ReceptorsEnzymesCell to Cell recognition

Function of membrane protiensFunction 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 ActivitiesLearning 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