Question

Describe the ultrastructure of chloroplast.

Answer 1

Hint: The chloroplasts are the cell organelles that are found in plant cells. The first description of chloroplast was first given by Hugo von Mohl in 1837. He described chloroplasts are discrete bodies within the green plant cells. By the time of 1884, Eduard Strasburger gave the term chloroplasts for these discrete bodies.

Complete answer:
The chloroplasts are the cell organelles that are present in the cells of plants and many photosynthetic algae. Chloroplast absorbs the sunlight and conducts the process of photosynthesis for the generation of food from sunlight and carbon dioxide. It is responsible for preparation of food in the form of starch. These are abundantly present in leaves of plants where the maximum photosynthesis takes place.
Chloroplasts are mostly lens shaped and have about 3-10 micrometers of diameter and 1-3 micrometers thick. These can also vary in shape depending on the type of organism. For example, green algae spirogyra has ribbon-like spiral shaped chloroplasts and Chlamydomonas has a cup shaped chloroplast. Chloroplast is a double membrane bound structure which is believed to be in endosymbiont relationship with eukaryotes.
Its two membranes are –outer membrane and inner membrane, also an intermembrane space exists between both these membranes. A semi gel-like fluid is enclosed inside the inner membrane called a stroma. The outer chloroplast membrane is semi permeable, as it allows few molecules and ions to diffuse through it but restricts the entry of large protein molecules across it. Talking about inner chloroplast membrane, it is the place where the regulation of passing of molecules takes place. In addition, inner chloroplast membrane is the site of synthesis of fatty acids, lipids and carotenoids.

Stroma is the gel like fluid present in chloroplast. It consists of proteins, ions and other organic molecules dissolved in. Moreover, a circular DNA, ribosomes, starch granules, many soluble proteins and thylakoid system are observed floating in it. Thylakoids are flat sac-like structures that are arranged in stacks which are collectively called as granum. These grana are connected with other stacks with the structure like lamellae. The light reaction takes place inside the thylakoids where the photosynthetic pigment ‘chlorophyll’ is present. The sunlight is absorbed by thylakoids and light reaction is carried out.

Note:
The chloroplasts contain their own circular DNA and ribosomes. By evolutionary evidence, it is believed that mitochondria as well as chloroplast existed as unicellular organisms. But due to the onset of changing environment both of these organelles came into endosymbiont relationship with eukaryotic cells.