Cytoplasm and Cell Organelles

The cytoplasm contains many cell organelles.

  1. Trap and release energy - mitochondria and chloroplasts
  2. Secretory or involved in synthesis and transport - Golgi, ribosomes and endoplasmic reticulum
  3. Organelles for motility - cilia and flagella
  4. Suicidal bags - lysosomes
  5. Nucleus which controls all activities of the cell, and carries the hereditary material

Mitochondria and Chloroplast

Mitochondria (found in plant and animal cells) are the energy releasers and the chloroplasts (found only in green plant cells) are the energy trappers.

Chloroplasts traps the solar energy locking it in a complex molecule (by photosynthesis), mitochondria releases the energy by breaking the complex molecule (by respiration).

Both, mitochondria and chloroplasts, contain their own DNA (genetic material) as well as their own RNA (for protein synthesis). Thus, they can self-duplicate to produce more of their own kind without the help of nucleus.

Mitochondria

They appear as tiny thread like structures under light microscope - approximately 0.5 - 1.00 μm (micrometer). Their number varies from usually a few hundred to a few thousand per cell. The smallest number is just one as in an alga.

Function: It oxidizes pyruvic acid (breakdown product of glucose) to release energy which gets stored in the from of ATP for ready use. This process is also called cellular respiration. That is why mitochondria are called the power house of a cell.

In cytoplasm glucose (C6H12O6), containing 6 atoms of carbon, enters cells. It is broken into two Pyruvic acid (C3H4O3) molecules containing 3 atoms of carbon. In mitochondria Acetyl-Co-A is oxidized into CO2, H2O and ATP.

Plastids

Plastids are found only in a plant cell. These may be colourless or coloured. Based on this fact, there are three types of plastids:

  1. Leucoplast - white or colourless
  2. Chromoplast - blue, red, yellow
  3. Chloroplast - green

Chloroplast

  • Found in all green plant cells in the cytoplasm.
  • Number 1 to 1008
  • Shape: Usually disc-shaped or laminate as in most plants. In some, ribbon-shaped as in an alga Spirogyra or cup-shaped as in another alga
    Chlamydomonas.
  • Inside of the chloroplast is filled with a fluid medium called stroma.

Function: Chloroplasts are the site of photosynthesis (production of sugar, from carbon dioxide and water in the presence of sunlight).

Endoplasmic reticulum (ER), golgi body and ribosomes

The Endoplasmic reticulum (ER) and Golgi body are single membrane bound structures. The membrane has the same structure (lipid-protein) as the plasma membrane but ribosomes do not have membranes.

Ribosomes are involved in synthesis of proteins in the cell, Golgi bodies in secreting and the ER in transporting and storing the products. These three organelles operate together. 

Microbodies

These are small sac-like structures bounded by the single membranes. These are of different kinds.

Lysosomes (lysis mean breaking down and soma means body)

Lysosomes are present in almost all animal cells and some non-green plant cells. They perform intracellular digestion. Lysosomes are called suicidal bags as enzymes contained in them can digest the cell’s own material when damaged or dead.

Function: They help in nutrition of the cell by digesting food, as they are rich in various hydrolyzing enzymes which enable them to digest almost all major chemical constituents of the living cell. When cells are old, diseased or injured, lysosomes attack their cell organelles and digest them.

Peroxisomes

They are found both in plant and animal cells. They participate in oxidation of substrates resulting in the formation of hydrogen peroxide.

Glyoxysomes

The microbodies are present in plant cells and morphologically similar to peroxisomes. They are found in the cell of yeast and certain fungi and oil rich seeds in plants. 

They contain enzymes of fatty acid metabolism involved in the conversion of lipids to carbohydrates during germination.

Cilia and Flagella

Some unicellular organisms like Paramecium and Euglenas wim in water with the help of cilia and flagella respectively. In multicellular organisms some living tissues (epithelial tissues) have cilia. They beat and create a current in the fluid in order to move in a given direction.

Cilia beat like tiny oars or pedals (as in a boat) and flagella bring about whiplash like movement. Both are made up of contractile protein tubulin in the form of microtubules. The arrangement of the microtubules is termed as 9 + 2, that is, two central microtubules and nine duplet sets surrounding them.

Centriole

It is present in all the animal cells (but not in Amoeba), located just outside the nucleus. It is cylindrical, 0.5 μm in length and without a membrane. It has 9 sets of peripheral triplet tubules but none in the centre (9 + 0). Each set has three tubules arranged at definite angles. It has its own DNA and RNA and therefore it is self duplicating.

Function: Centrioles are involved in cell division. They give orientation to the mitotic spindle which forms during cell division.