Biology - 5090

Cell Structure and Organisation


Cell Structure and Organisation

Plant and Animal Cell

Animal Cell

Plant Cell

No cell wall present
Cell wall made of cellulose
No central permanent vacuole present; many temporary vesicles present in the cytoplasm
A large central permanent vacuole present; no temporary vesicles present
No chloroplast present

Chloroplast present which is responsible for Photosynthesis

Figure (i) Animal and Plant Cell, Credit: Wikipedia

Cell Organelles

An organelle is a sac-like structure present in the cell, having a specific function.

1.      Cell Membrane

It is a partially permeable membrane, meaning that it allows only some molecules to pass through. It acts as a barrier between the cell and its surroundings. The semi-permeable membrane is made up of phospholipids and proteins. Some of those proteins present on the membrane work as enzymes and antigens.

2.      Cell Wall

It is a fully permeable membrane, allowing everything to pass through it. In plants, it is made up of a polysaccharide, known as cellulose, which helps the cell remain upright. The cell wall is rigid, which makes it withstand high water pressure for the maintenance of cell shape and turgidity. The cell wall is absent in animals. 

3.      Cytoplasm

It is the medium that holds the organelles. It contains the enzymes and substrates required for metabolic reactions to take place.

4.      Ribosomes

Ribosomes are the site of protein synthesis. Inside the cytoplasm, there are some extended membrane systems, known as Endoplasmic Reticulum. Ribosomes are either freely present in the cytoplasm or attached to the endoplasmic reticulum.

5.      Vesicles

Vesicles are membrane-bounded structures that contain proteins/enzymes/hormones, and they transport them in and out of the cell.

6.      Mitochondria

These double-membrane structures are the powerhouse of the cell. They have enzymes responsible for aerobic respiration (Glucose + Oxygen -> Carbon dioxide + Water + Energy). ATP is the molecule that stores energy.

Levels of Organisation

Figure (ii) Levels of Organization, Credit: Encyclopaedia Britannica

The cell is the basic structural and functional unit of life.

Tissues are all specialized cells that perform the same function.

Many tissues together performing the same function form an Organ.

Many organs together performing the same function is an Organ System.

Adaptations of Specialized Cells

1. Root hair cells 

The roots of plants have specialized cells whose cytoplasm extends outward and forms a hair-like extension known as root hair cell. This cell increases the surface area for absorption. Root hair cell needs a lot of energy, thus it has many mitochondria present in it to drag the mineral salts inside from the soil.

Figure (iii) Root hair cell, Credit:

2. Xylem vessels 

These do not have cytoplasm or any organelles in them for the maximum transport of water and mineral salts.  There are no cross walls present in adjacent cells, thus making a continuous column for water and mineral salt's transport. The walls of xylem vessels are lignified, making it rigid so it has no danger of collapse. Pits are present in lignified walls to remove a blockage caused by air.  Due to the lignifications, these are considered dead cells.

Figure (iv)Transport in the xylem of plants, Credit: Ms. Frost A world of biology

3. Red blood cells

They are biconcave, disc-shaped cells with a dent. The dent is present so it can squeeze through blood capillaries.  They have a special protein, known as Haemoglobin, which carries Oxygen. The nucleus is absent to increase the surface area for the transport of oxygen.

Figure (v) Red blood cell, Credit:

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