NCERT.UNIT 8.CELL:- THE UNIT OF LIFE

EUKARYOTIC CELL

EUKARYOTIC CELLS

They have a well-organized membrane-bound nucleus and organelles.

The presence of membranes gives clear compartmentalization of cytoplasm.

– Their genetic material is organized into chromosomes. – They have complex locomotory & cytoskeletal structures.

PLANT CELL AND ANIMAL CELL

PLANT CELL AND ANIMAL CELL

Cell organelles in Eukaryotic cells

1. Cell Membrane

– Chemical studies on human RBCs show that cell membrane is composed of a lipid bilayer, protein & carbohydrate.

– Lipids (mainly phosphoglycerides) have outer polar head and the inner hydrophobic tails.

-So the non-polar tail of saturated hydrocarbons is protected from the aqueous environment.

– Ratio of protein and lipid varies in different cells.

E.g. In human RBC, membrane has 52% protein,40% lipids, and 8% carbohydrates

– Based on the ease of extraction, membrane proteins are 2 types:

o Integral proteins: Partially or totally buried in membrane.

o Peripheral proteins: Lie on the surface of the membrane.

– Fluid mosaic model of cell membrane:

FLUID MOSAIC MODEL

– Fluid mosaic model : Proposed by Singer & Nicolson (1972).

According to this, the quasi-fluid nature of lipid enables the lateral movement of proteins within the overall bilayer.

This ability to move within the membrane is measured as its fluidity.

Functions:

o Transport of the molecules. The membrane is selectively permeable to some molecules present on either side of it.

o Due to the fluid nature, the plasma membrane can help in cell growth, the formation of intercellular junctions, secretion, endocytosis, cell division, etc.

Types of Transport

1. Passive transport:

-It is the movement of molecules across the membrane along the concentration gradient (i.e., from higher concentration to the lower) without the expenditure of energy.

It is 2 types:

a. Simple diffusion: It is the movement of neutral solutes across the membrane. b. Osmosis: It is the movement of water by diffusion across the membrane.

Polar molecules cannot pass through the non-polar lipid bilayer. So they require membrane carrier protein for transport.

2. Active transport: It is the movement of molecules across the membrane against the concentration gradient (i.e. from lower to the higher concentration) with the expenditure of energy (ATP is utilized). E.g. Na+/K+ pump.

TYPES OF TRANSPORT – ACTIVE AND PASSIVE TRANSPORT

2. Cell Wall

– It is a non-living rigid structure found outer to the plasma membrane of fungi and plants.

– Cell wall of Algae is made of cellulose, galactans, mannans and minerals like CaCO3.

– In other plants, it consists of cellulose, hemicellulose, pectins and proteins

-The cell wall of a young plant cell (primary wall) is capable of growth.

-It gradually diminishes as the ell matures and the secondary wall is formed on the inner side (towards membrane)

-The middle lamella is a layer containing calcium pectate which glues the neighbouring cell together .

-Cell wall and middle lamella may be transversed by plasmodesmata. it connects the cytoplasm of neighbouringcells.

Functions

a) It gives shape to cell

b)Protects cell from mechanical damage and infection

c) It helps in cell to cell interaction

d)Acts as barrier to undesirable macromolecules

to be continued…. next page…

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