Cell Organelles

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Cell Organelles

1. Cell Membrane

Cells are enclosed by a thin film like membrane called plasma membrane, cytoplasmic membrane or plasmalemma.

Lies immediately outside the cytoplasm.

Structure was defined by Singer and Nicholson in “Fluid Mosaic Model”.

Selectively Permeable in Nature.

Endocytosis : Taking of substance inside the cell by the plasma membrane. It is of two types :

  • Phagocytosis : Cell Eating.
  • Pinocytosis : Cell drinking.
  • Exocytosis : Reverse of Endocytosis, i.e., material is removed from the cells including Reverse Pinocytosis.

2. Cytoplasm

It is a part of protoplasm lying between plasma membrane and nucleus, Jelly – like fluid.

Participates in the intracellular distribution of nutrients, metabolites and enzymes.

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3. Cell Wall

Cells of most fungi, prokaryotes (bacteria and blue – green algae) and plants (except gametes) are surrounded by the cell wall. It is absent in animals.

In true bacteria and cyanobacteria, cell wall is of peptidoglycan, in some fungi it is of chitin and in most of the algae and higher green plants it is of cellulose.

4. Nucleus

Discovered by Robert Brown.

Every eukaryotic cell consists of at least one, almost spherical, dense, highly specialized structure called nucleus.

Although, sieve tube element of mature phloem and RBCs of mammals don’t have nucleus.

Contains nucleoplasm (nuclear sap) which contains chromatin. (Chromatin is composed of DNA mainly)

Chromatin organizes itself into thread like structures called Chromosomes.

The function of chromosomes is to carry genetic information from one cell generation to another.

Nucleolus is also present which helps in the production of ribosomes.

Nucleus controls the metabolic activities of the cell by controlling the synthesis of enzymes.

5. Chromosomes

Waldeyer coined the term chromosome. Sutton and Boveri proved that chromosome is the physical basis of hereditary.

Each chromosome is made up of DNA and this DNA by replication gives rise to messenger RNA which carries the genetic information in the form of code.

This m – RNA comes out of the nuclear wall into the cytoplasm where it helps to form a particular kind of protein, needed by the cell or body.

Number is constant for a particular species. Diploid number (2n) of chromosomes are there in somatic cells (all body cells except sperms and ova) and haploid number (n) of chromosomes are there in gametes (sperms and ova).

In humans, the diploid number is 46. Of these, 23 are from egg cell and 23 from sperm cell.

6. Mitochondria

Powerhouse of the cell or energy converting organelles, as oxidation of ‘fuel’ occurs stepwise in these, resulting in the release of chemical energy.

This energy is stored as ATP

From mitochondria, ATP molecules are shifted to cytoplasm, which is the chief site of their utilization.

They are semi-autonomous organelles. They contain DNA, m – RNA, ribosomes and can synthesize some of their own proteins.

Each mitochondria is enclosed by a double-membraned envelope, outer and inner.

Fluid (called matrix) is there between these 2 layers. Inner membrane has many folds called cristae.

7. Endoplasmic Reticulum

Provides an increased surface area for various metabolic activities within the cell.

Provides mechanical support to the cytoplasm.

2 types: Rough and smooth.

Both Smooth Endoplasmic Reticulum and Rough Endoplasmic Reticulum form Passages for transport of secretory proteins, lipids and sterols.

Hough Endoplasmic Reticulum collects and stores the proteins synthesized by its surface bound ribosomes.

8. Golgi Complex

Main function is secretion, (secretion of bile by liver, of synovial fluid by synovial membrane, in the production of enamel of teeth by amenoblast and the formation of fatty yolk in eggs are some of the examples).

Also forms lysosomes.

Secretions are in the form of granules in cytoplasm (called Zymogen Granules).

9. Lysosomes

Also called Suicidal bags.

Contain powerful enzymes (acid hydrolases), rupture of lysosome membrane release these enzymes.

Digest worn – out or unnecessary parts of the cell, or even whole cells by process called ‘Autophagy’.

10. Ribosomes

Found on Endoplasmic Reticulum. Made of RNA and proteins in equal amounts.

Sites of protein synthesis (Ribosomes are inactive for protein synthesis, but after combining with mRNA form polyribosomes which play important role in protein synthesis).

11. Vacuoles

Found in plant cells only.

Membrane surrounding the vacuole is tonoplast.

Function : Regulation of water, in osmoregulation, in storage and in digestion.

12. Plastids

Found in plant cells only.

Are of different types :

  • Chloroplast : Green contains the pigment chlorophyll. Contains the matrix (fluid), stroma which has many flat membranous structures called thylakoids
  • Leucoplasts : Colorless, occur in large no in cells of fruits, seeds, etc. They store nutrients (e.g., amyloplasts of potato store starch).
  • Chromoplasts : Colored, containing fat soluble yellow, orange and red pigments (chiefly carotinoids). Found in flowers and fruits.

13. Centrosome

Found in animal cells and cells of lower plants (e.g., Algae) only.

The first indication that the cell is about to divide is generally given by the centrosome.

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