Plasma Membrane

1 PPISMP SAINS 2014

By:o Sabila

Plasma Membrane• Biological membrane that encloses the

cytoplasm/protoplasm of the cell.• Separate the interior and external of the cell.• Provides shape & protection.• Semipermeable or selectively permeable.• Made of a phospholipid bilayer ;- Hydrophilic [water liking] face ouside.- Hydrophobic [water dreading] face inside.

Structure of Plasma Membrane

DANIELLI-DAVSON MODEL• Proposed by Danielli & Davson in 1953.• Made of phospholipid bilayer where;- the hydrophilic [polar] heads facing outside &

coated with protein.- Hydrophobic [non-polar] tails facing inwards,

sandwiched between two bilayer.- Structure is stable, static & rigid.

FLUID MOSAIC MODEL• Also known as Singer Model.• Proposed by S.Jonathan Singer & Garth Nicolson in

1972 based on their study by using freeze-fracture technique.

• In this model, membrane is a fluid system with proteins embedded in phospholipid bilayer.

• Composed of : - 2 type of protein [ Integral & peripheral ]

- Phospholipid - Cholesterol - 2 type carbohydrates [ glycoprotein & glycolipid ]

• The membrane is a dynamic fluid system with globular proteins embedded in the phospholipid bilayer mosaic structure.

• Phospholipid move through the membrane and flip flopping across it fluidity

• Protein do not coat the membrane surface.• Thickness of PM: 7.5 nm.• Phospholipid is the main component of PM.

• Composed of 2 protein ; Integral & peripheral proteins.

- Integral proteins > interspersed between phospholipids.> act as enzymes, pumps, carriers, channels & receptors.- Peripheral proteins > attached to the inner or

outer surface of the membrane.

• Composed of carbohydrates [glycoproteins & glycolipid]

- Glycoproteins ; formed when oligosaccharides attached to globular proteins.

- Glycolipid ; formed when oligosaccharides attached to phospholipids.

• Both act as – chemical receptors - cell recognition

that enable the cells to : (i) recognise each other.(ii) differentiate themselves from foreign cells.

• Composed of cholesterol – reduce the membrane flexibility & permeability to the substances.

- Increasing the fluidity.• PM are semi permeable.• Small uncharged polar molecules are able to

cross the PM freely.• Ions & large uncharged polar molecules such

as glucose are not allowed.

Functions of Plasma Membrane• Controls the inflow & out flow of materials to

maintain the internal environment of the cell.• Separate the cytoplasm from its external

environment.• Act as cell identity marker provided by

glycoprotein & glycolipids.• Act as receptor for chemicals.• Helps transporting ions and large, polar substances

into & out of the cell.• Act as attachment site for cytoskeleton that

maintain the cell shape.• Play a role in cell adhesion.