23 September 2014

Structure and Function of Cells : NCERT / CBSE

Structure and Function of Cells: Science Cbse / Ncert Revision Notes

Cell is the structural and functional unit of all living organisms. e.g. Alveolar cell is an example of single cell. A group of cells that have similar structure and function constitute a tissue. e.g.  A group of alveolar cells form alveolar tissue.

Tissues aggregate to form an organ. e.g. Alveolar tissues aggregate to form lungs, the organ. Many organs together constitute organ system. e.g. Nostrils, nasal cavity, trachea, bronchi, bronchioles, lungs together form the respiratory system, the organ system.

Organ systems aggregate to form an organism. e.g. Digestive system, respiratory system, circulatory system, muscular system, nervous system reproductive system etc. work together to form a complete organism.
Structure and Function of Cells, Cbse / Ncert Revision Notes, Science Class 8

Types of cells

Depending on the presence of nuclear membrane, cells can be of two types – Prokaryotic cells and eukaryotic cells.

Bacteria and blue-green algae that contain nucleoids are called prokaryotes. Organisms that have a well-organized nucleus are called eukaryotes.

Cell organelles
Different structures present inside the cell are called as cell organelles.

Cell wall: This is the structure present only in plant cells. It is made up of cellulose and pectin.

Cell membrane:

  • The cell membrane is a biological membrane that separates the interior of all cells from the outside environment.
  • The cell membrane is selectively permeable to ions and organic molecules and controls the movement of substances in and out of cells.
  • It consists of the phospholipid bilayer with embedded proteins.


  • It is a jelly-like fluid present between the nucleus and the cell membrane.
  • In a prokaryotic cell, the organelles are dispersed in the cytoplasm.
  • In a eukaryotic cell, the contents of the nucleus are separated from the cytoplasm. Organelles like chloroplasts, mitochondria, ribosomes, lysosomes, Golgi apparatus etc. are dispersed in the cytoplasm.


  • The nucleus is the control centre of the cell as it controls all its activities.
  • The nucleus is a membrane-enclosed organelle found in eukaryotic cells.
  • It contains the genetic material of the cell in the form of chromosomes.


  • A chromosome is an organized structure of DNA and protein found in cells. It is a single piece of coiled DNA containing many genes.
  • These are considered to be the vehicles of heredity as they carry genes. Genes are small points on the chromosome responsible for one character of an individual.
  • Genes on chromosomes help in the inheritance or transfer of characteristics from the parents to the offspring.
  • Chromosomes also contain DNA-bound proteins, which serve to package the DNA and control its functions.

Chloroplast:  Chloroplast is a type of plastid involved in the process of photosynthesis in plants. These are specialised organelles found in all higher plant cells. These organelles contain the green pigment chlorophyll which absorbs energy from the sunlight.

Vacuole: A membrane bound organelle present in both plant and animal cells offers buoyancy to the cell.  Vacuoles are filled with water containing inorganic and organic molecules, including enzymes, in solution.

Golgi apparatus: These are membrane bound sacs present in the cytoplasm of the cell. They are responsible for processing and packing of macromolecules synthesized in the cell.

Lysosomes: These are membrane enclosed organelles containing hydrolytic bags. They burst themselves when necessary to kill the foreign substances entering the cells.