Definition

The cell is the smallest structural and functional unit of an organism, typically microscopic and consisting of cytoplasm and a nucleus enclosed in a membrane.

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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.

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Cell Structure And Functions 

1. Cell wall

– Most commonly found in plant cells

– Controls turgidity

– Extracellular structure surrounding plasma membrane

– Primary wall: extremely elastic

– Secondary wall: Forms around primary cell wall after growth is complete

2. Plasma membrane

– Outer membrane of a 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 bilayer.

3. Protoplasm

– colorless, semitransparent, viscous granular fluid present inside the cell is called protoplasm.

– colloidal in nature and considered as the 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 the cytoplasm

– All the organelles are present in the cytoplasm like

i) Endoplasmic reticulum

– Tubular network fused to the nuclear membrane

– Goes through cytoplasm onto the cell membrane

– Stores, separates and serves as cell’s transport system

– Smooth type: lacks ribosomes

– Rough type (pictured): ribosomes embedded in the 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 the cell membrane for removal

– Vary in shape depending on the 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 the 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 the rough endoplasmic reticulum

– Mobile type: injects proteins directly into the 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 the nucleus

– Composed of nine tubes, each with three tubules

– Involved in the cellular division

– Lie at right angles to each other

5. Nucleus

– Nucleus is the control center of the cell and serves as the information processing and administrative center.

–  Nucleus 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.

i) Nuclear membrane

– Surrounds nucleus

– Composed of two layers

– Numerous openings for nuclear traffic

ii) Nucleolus

– Spherical shape

– Visible when the 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 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.

– This wall is absent in animals and instead, an outer covering is present which is known as the cell membrane. It is composed of the lipid bilayer, proteins, and carbohydrates.

2) Plastids

– Plant 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 cells.

3) Vacuole

– The main difference between the vacuole of animals and plants is their size.

– The vacuole of plant cells is much bigger than that of animal cells.

4) Centrosomes

– In animal cells, centrosomes are present, which are responsible for making some protein fibers that help in the movement of chromosomes during the 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.

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