unit membrane

8/2/2019 Unit Membrane

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Structure of Cell

Membrane

Surabhi

Roll No. - 39



Thin, pliable, elastic structure enveloping

the cell body

It is semipermeable allowing selective

passage to certain substances through it &

separating ECF from ICF

Approximate thickness : 7.5-10 nm



Lamellar / Sandwich Model – Danielli & Davson

in 1935 proposed that CM is made up of a lipid layersandwiched between two layers of proteins.



Theories for structure of Cell

Membrane Unit Membrane Concept proposed by Robertson in 1957

on basis of electron microscopic studies. According to him all

the cellular and organellar membranes are structurally andfunctionally similar.



All these models failed to explain theelasticity and selective permeability of the

Cell Membrane



Fluid Mosaic Model –

-Most accepted theory

-Proposed by Singer & Nicholson in 1972

-Membrane is formed as ‘Protein Iceberg’ ina sea of phospholipids.

-Phospholipids are arranged in a bilayer

Theories for structure of

Cell Membrane



• The hydrophilic phosphate end of outer layer faces the

ECF & those of inner layer faces the ICF.• The hydrophilic fatty acid part attach to each other in

the center of membrane

• The layer globular protein molecules are interspersed in

the lipid film.• The entire surface of the cell often has a loose

carbohydrate coat called ‘Glycocalyx’



• Major lipids present is

phospholipid such as

Phosphatidyl choline and

Phosphatidyl

ethanolamine.

• In eukaryotes, Cell

membrane also contains

various

glycosphyngolipids,sphingomyelin and

cholesterol.



Functions of Lipids –

• Forms a semipermeable membrane.

• Allows fat soluble substances like O2, CO2 and alcohol to

pass through it.

• Cholesterol determines degree of permeability of the

bilayer to water soluble substances and controls fluidity

of membrane.



• Globular mass floating in lipid bilayer most of which

occur as glycoproteins (arginine, lysine rich)

• Two types of proteins occur-

1) Integral Proteins2) Peripheral Proteins



• Integral Proteins

- Protrude all the way through the membrane

- Some proteins remain confined in the lipid bilayer

while some extend from outside to inside, i.e. , are

TRANSMEMBRANOUS, e.g., Cell adhesion proteins, cell

junction and transport proteins.

- Tightly bound to the membrane



• Peripheral Proteins

- partially embedded in the outer & inner

surfaces of membrane

- loosely bound with integral proteins or

lipid layer & dissociate readily



Pumps – The integralproteins serve as

pumps actively

transporting ions

across the membrane.



Carriers - Proteins like permeases & translocases

transport substances down electrochemical gradient by

facilitated diffusion.



Channels - Provide passagefor water and water soluble

substances.



Receptors – These function as receptors that bindligands or messenger molecules initiating physiological

changes inside the cell.

Enzymes – Proteins also function as enzymes catalysing

reactions at the surfaces of membrane. Cell adhesion molecules – Anchor cells to their

neighbours and basal lamina.



•Occur in combination with protein or lipid asglycoprotein and glycolipid

• The ‘glyco’ portion protrude to the outside of the cell,

dangling outside from surface

• Proteoglycans remain loosely attached to outer surface

thus coating entire outside surface



Functions of Carbohydrates –

• Repel negatively charged particles

• Acts as receptor substances for hormones

• Play role in immune reaction

• Oligosaccharides are involved in cell to cell recognition

as in fertilisation.



Protective – Forms outermost boundary of cell organelles

Digestive – Takes in food and excrete waste products

Selective Permeability – Helps in maintaining difference

in composition of ECF and ICF

Insulating Properties – Has a high insulating valve and

can stand without breaking down about one million volts

per meter

Links adjacent cells together by junctional complexes

(gap junctions, desmosomes, etc.) to form tissues



~Thank You~Any Query ???

unit membrane

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