Question

You would recollect from the previous unit that the mesophyll cells in the leaves, have a large number of chloroplasts. Usually, the chloroplasts align themselves along the walls of the mesophyll cells, such that they get the optimum quantity of the incident light. When do you think the chloroplasts will be aligned with their flat surfaces parallel to the walls? when would they be perpendicular to the incident light?

Answer 1

Hint: The position of the chloroplast depends on the intensity of the light as the main function of the chloroplast is to absorb the sunlight and use it accordingly depending on the requirement. Chloroplasts of the plants are known to be membrane-bound plastids.

Complete answer:
The chloroplast of the plant cell, specifically the chloroplast of leaf cells aligns parallel to the walls of mesophyll cells i.e mainly palisade cells when the intensity of the sunshine is minimum, in order to get more and more sunshine. This type of condition is commonly known as Epistrophe.
And when the intensity of the sunshine is maximum, at that time the chloroplast gets aligned perpendicular to the mesophyll cells to avoid photooxidation of chlorophyll molecules. This type of condition is named Parastrophe.
Chloroplasts are free-moving, membrane-bound plastids. It is present in photosynthetic organisms like plants.
- They can be normally found in the mesophyll cells of the leaves of the plant. They are known to be plano-convex or bi-convex shaped in higher plants.
-Chloroplast consists of two membranes i.e an inner and outer membrane to form an envelope, with an empty intermediate space present in between. The inner membrane helps in the regulation of the passage of materials in and out of the chloroplast.
-The single thylakoid discs are stacked on top of each other known as a granum (plural: grana).
-Stroma is the matrix that is colorless and surrounds the grana in the chloroplast. It is known as the site for Calvin cycle reactions and chloroplast DNA replication. It is alkaline and aqueous.
-The grana are connected to each other by lamellae or stroma thylakoids.
-Certain plants contain an organelle commonly known as the peripheral reticulum that is
an additional set of membranous tubules.

Note:
Some varieties of the algae for instance glaucophytes have a peptidoglycan layer which is present between their inner and outer chloroplast membranes.
- In some of the ${ C }_{ 4 }$ plants and some algae, the chloroplasts found contain free-floating thylakoids.
- The thylakoid membrane is also known as a site for ATP synthesis.