Class 9, Science, Chapter-5, Lecture-1, Structure of Cell (Notes)

Structure of Cell:

1. Cell membrane (Plasma membrane)
2. Protoplasm
   (i) Nucleus
   (ii) Cytoplasm

Plasma Membrane:

The living flexible selectively permeable outermost covering of the cell that separates the protoplasm from the external environment is termed as the plasma membrane or the cell membrane or PLASMALEMMA.
Structure: Bi-layered, Thickness = 7 nm (70 Å)
Chemical Composition: Phospholipids, Proteins, Carbohydrates (Cholesterol and Polysaccharides)

Functions:

1. Provides shape to the cell.
2. Protects the protoplasm of the cell
3. Selectively allows entry and exit of chemicals
4. Its flexibility helps in endocytosis

Cell Wall:

The non-living rigid permeable covering present outside the plasma membrane of a plant cell is called the cell wall.
Structure: Uni-layered
Chemical Composition: Cellulose, Hemicellulose, Pectin

Functions:

1. Provides structural strength to plant cells.
2. Protects the plant cell from foreign materials and pathogens
3. prevents bursting of cells due to osmotic pressure in hypotonic solutions
4. Helps plants withstand environmental changes.

Differences Between

Cell membrane	Cell wall
is Living	is non-living
is flexible	is rigid
is selectively permeable	is permeable
is bi-layered	is uni-layered
is made up of phospholipids and proteins	is made up of cellulose
is present in all cells	is present only in plant cells, fungi and bacteria.
provides shape	provides strength
is digestible by human being	is indigestible by human being

Plasma membrane is called a selectively permeable membrane.

Justification:
The plasma membrane selectively allows the entry and exit of some materials in and out of the cell but prevents movement of some other materials.
So, plasma membrane is called a selectively permeable membrane.

DIFFUSION:

The spontaneous movement of a solute from a region of higher concentration to a region of lower concentration is called diffusion.

MECHANISM OF GASEOUS EXCHANGE IN CELLS:

Oxygen present inside a cell is consumed in respiration and carbon-dioxide is produced as cellular waste. Consumption of oxygen leads to lower concentration of O₂ inside than that outside. O₂ moves from outside (region of higher concentration) to inside (region of lower concentration).
Accumulation of CO₂ inside the cell leads to higher concentration of CO₂ inside than that outside. CO₂ moves from inside (region of higher concentration) to outside (region of lower concentration).
Thus gaseous exchange takes place between the cell and its external environment.