Structure and Function of Chloroplast in Plant Cell

In this structure and function of chloroplast in plant cell post we have briefly explained about chloroplast in plant cell, definition, components, functions and importance.

Structure and Function of Chloroplast in Plant Cell

The word chloroplast is derived from the Greek word chloros meaning “green” and plastes meaning “the one who forms”. Chloroplasts are organelles, specialized compartments, in plant and algal cells.

Their discovery inside plant cells is usually credited to Julius von Sachs (1832– 1897), an influential botanist and author of standard botanical textbooks -sometimes called “The Father of Plant Physiology”.

Definition

Chloroplast in plant cell is an organelle that contains the photosynthetic pigment chlorophyll that captures sunlight and converts it into useful energy, thereby, releasing oxygen from water. These are organelles present in plant cells and some eukaryotic organisms. Chloroplast in plant cell are the most important plastids.

It is the structure in a green plant cell in which photosynthesis occurs. Chloroplast is one of the three types of plastids. The chloroplasts take part in the process of photosynthesis and it is of great biological importance.

Animal cells do not have chloroplasts. All green plant take part in the process of photosynthesis which converts energy into sugars and the by-product of the process is oxygen that all animals breathe. This process happens in chloroplasts.

The distribution of chloroplast in plant cell is homogeneous in the cytoplasm of the cells and in certain cells chloroplasts become concentrated around the nucleus or just beneath the plasma membrane. A typical plant cell might contain about 50 chloroplasts per cell.

Chloroplast in Plant Cell

These are unique organelles and are said to have originated as endosymbiotic bacteria. They develop from colourless precursors, called Proplastids or Eoplasts. They are semi-autonomous in nature and arise from pre-existing chloroplast as they have their own machinery to synthesize the required proteins. 

This is very clear in algae, where one chloroplast divides into two during cell division. In higher plants, the division of chloroplast in plant cell is very difficult to observe as. The number of chloroplast in plant cell is very high. Still, some-times the dividing chloroplast is seen under the phase contrast microscope as in Spinach.

Structure

Chloroplast

Structure and Function of Chloroplast in Plant Cell

It has been found in higher plants are generally biconvex or planoconvex shaped. In different chloroplast in plant cell have different shapes, they vary from spheroid, filamentous saucer-shaped, discoid or ovoid shaped.

They are vesicular and have a colourless canter. Some chloroplast in plant cell are in shape of club, they have a thin middle zone and the ends are filled with chlorophyll. In algae a single huge chloroplast is seen that appears as a network, a spiral band.

The size of the chloroplast also varies from species to species and it is constant for a given cell type. In higher plants, the average size of chloroplast is 4-6 microns in diameter and 1-0 microns in thickness.

The chloroplast in plant cell are double membrane bound organelles and are the site of photosynthesis. The chloroplast in plant cell have a system of three membranes: The Outer Membrane, The Inner Membrane and The Thylakoid system.

The outer and the inner membrane of the chloroplast in plant cell enclose a semi gel like fluid known as the Stroma. This stroma makes up much of the volume of the chloroplast, the thylakoids system floats in the stroma.

Components of Chloroplast

Outer Membrane

It is a semi-porous membrane and is permeable to small molecules and ions, which diffuses easily. The outer membrane is not permeable to larger proteins.

Intermembrane Space

It is usually a thin intermembrane space about 10-20 nanometers and it is present between the outer and the inner membrane of the chloroplast in plant cell.

Inner Membrane

The inner membrane of the chloroplast in plant cell forms a border to the stroma. It regulates passage of materials in and out of the chloroplast. In addition of regulation activity, the fatty acids, lipids and carotenoids are synthesized in the inner chloroplast membrane.

Stroma

Stroma is an alkaline, aqueous fluid which is protein rich and is present within the inner membrane of the chloroplast. The space outside the thylakoid space is called the stroma. The chloroplast DNA, chloroplast ribosomes and the thylakoid system, starch granules and many proteins are found floating around the stroma.

Thylakoid System

It is suspended in the stroma. The Thylakoid system is a collection of membranous sacks called thylakoids. The chlorophyll is found in the thylakoids and is the sight for the process of light reactions of photosynthesis to happen. The thylakoids are arranged in stacks known as Grana. Each granum contains around 10-20 thylakoids.

Functions

1. In plants all the cells participate in plant immune response as they lack specialized immune cells. The chloroplast in plant cell with the nucleus and cell membrane and ER are the key organelles of pathogen defences.

2. The most important function of chloroplast in plant cell is absorption of light energy and conversion of it into biological energy, making food by the process of photosynthesis. Food is prepared in the form of sugars. The chloroplast in plant cell is very important as it is the cooking place for all the green plants.

3. During the process of photosynthesis sugar and oxygen are made using light energy, water, and carbon dioxide. Conversion of PGA (phosphoglyceric acid) into different sugars and store as starch.

4. Like the mitochondria, chloroplast in plant cell use the potential energy of the H+ ions or the hydrogen ion gradient to generate energy in the form of ATP.

5. Light reactions take place on the membranes of the thylakoids. Production of NADPH2 and evolution of oxygen through the process of photolysis of water.

6. The dark reactions also known as the Calvin cycle takes place in the stroma of chloroplast.

7. Enzymes for carbon dioxide fixation and other dark reactions are present in the stroma and the enzymes for light reactions are present in the thylakoids. Two separate ways for carbon dioxide fixation are observed in higher plants which are broadly classified into C3 and C4 plants.

8. Breaking of 6-carbon atom compound into two molecules of phosphoglyceric acid by the utilization of assimilatory powers (NADPH2 and ATP).

Further Readings

Reference

  1. https://www.britannica.com/science/chloroplast
  2. https://en.wikipedia.org/wiki/Chloroplast
  3. https://alevelbiology.co.uk/notes/chloroplasts-structure-and-functions/
  4. https://www.studocu.com/in/document/chloroplast-notes/
  5. https://www.springer.com/gp/chemistry/biochemistry