Cell:

Definition:
The smallest structural and functional unit of an organism, typically microscopic and consisting of cytoplasm and a nucleus enclosed in a membrane.
OR
The cell is the functional basic unit of life. It was discovered by Robert Hooke and is the functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life.

Cell Structure And Functions (Plant & Animal Cell)

1. Cell wall
- Most commonly found in plant cells
- Controls turgidity
- Extracellular structure surrounding plasma membrane
- Primary cell wall: extremely elastic
- Secondary cell wall: forms around primary cell wall after growth is complete

2. Plasma membrane
- Outer membrane of cell that controls cellular traffic
- Contains proteins (left, gray) that span through the membrane and allow passage of materials
- Proteins are surrounded by a phospholipid bi-layer.

3. Protoplasm
-colourless, semitransparent, viscous granular fluid present inside the cell is called as protoplasm.
-colloidal in nature and considered as physical basis of life.
-exhibits the active streaming movement called cyclosis
-it consists of water, protein, lipid or fats, carbohydrates, enzymes, salts, nucleic acid in a fixed proportion.
-it is divided into Nucleus and Cytoplasm

4. Cytoplasm
-Protoplasm outside the nucleus is called cytoplasm
-all the organelles are present in the cytoplasm like;

i) Endoplasmic reticulum
- Tubular network fused to nuclear membrane
- Goes through cytoplasm onto cell membrane
- Stores, separates, and serves as cell's transport system
- Smooth type: lacks ribosomes
- Rough type (pictured): ribosomes embedded in surface

ii) Golgi apparatus
- - Composed of numerous layers forming a sac
- Works as the distribution and shipping department of the cell’s chemical product.

iii) Lysosome
- - Digestive 'plant' for proteins, lipids, and carbohydrates
- Transports undigested material to cell membrane for removal
- Vary in shape depending on process being carried out
- Cell breaks down if lysosome explodes

iv) Mitochondria
- Second largest organelle with unique genetic structure
- Double-layered outer membrane with inner folds called cristae
- Energy-producing chemical reactions take place on cristae
- Controls level of water and other materials in cell
- Recycles and decomposes proteins, fats, and carbohydrates, and forms urea

v) Ribosomes
- Each cell contains thousands
- Miniature 'protein factories'
- Composes 25% of cell's mass
- Stationary type: embedded in rough endoplasmic reticulum
- Mobile type: injects proteins directly into cytoplasm

vi) Vacuoles
- Membrane-bound sacs for storage, digestion, and waste removal
- Contains water solution
- Contractile vacuoles for water removal (in unicellular organisms)

vii) Chloroplasts
- A plastid usually found in plant cells
- Contain green chlorophyll where photosynthesis takes place

viii) Cytoskeleton
- Composed of microtubules
- Supports cell and provides shape
- Aids movement of materials in and out of cells

ix) Centrioles
- Paired cylindrical organelles near nucleus
- Composed of nine tubes, each with three tubules
- Involved in cellular division
- Lie at right angles to each other

5. Nucleus:
-Control centre of the cell
-Serves as information processing and administrative centre of the cell
-Performs the following major functions:
a) stores the cell’s hereditary material, or DNA
b) coordinates the activities of the cell, which includes growth, metabolism, protein synthesis, and reproduction or cell division.

i) Nuclear membrane
- Surrounds nucleus
- Composed of two layers
- Numerous openings for nuclear traffic

ii) Nucleolus
- Spherical shape
- Visible when cell is not dividing
- Contains RNA for protein manufacture

iii) Chromosomes
- Usually in the form of chromatin
- Contains genetic information
- Composed of DNA
- Thicken for cellular division
- Set number per species (i.e. 23 pairs for human)

Difference Between Animal And Plant Cell:

1) Cell Wall OR Cell Membrane
- The outer boundary of plant cells is known as the cell wall, which is mainly composed of cellulose.
- It helps the plant cell to maintain its shape.
- The cell wall is absent in animal cell.
- In animal cell the outer covering is known as cell membrane which is composed of lipid bilayer, proteins and carbohydrates.

2) Plastids
- Plants cells also contain organelles which are known as plastids.
- The most common type of plastid is the chloroplast which contains chlorophyll.
- These organelles or plastids are totally absent in animal cell.

3) Vacuole
- The main difference between the vacuole of animal and plants is their size.
- The vacuole of plant cell is much bigger than that of animal cells.

4) Centrosomes
- In animal cells centrosomes are present, which are responsible for making some protein fibres which help in movement of chromosomes during division of cells.
- Centrioles are absent in the plant cells.

5) Lysosome
- Lysosomes are not evident in plants while they are prominent in animal cells.