Chloroplasts Definition and Structure

Chloroplasts are plant organelles where photosynthesis is conducted. In this part of the leaves, chlorophyll captures the sunlight, converts it into energy, and stores it for different functions. The energy gets stored in ATP and NADH molecules and releases oxygen from water in cells of plants and animals. It then utilizes ATP and NADPH to form organic molecules from CO2. This process is known as the Calvin cycle. Chloroplast is responsible for many functions like synthesizing amino acids and fatty acids and the plants' immune response.

Structure of chloroplast

Chloroplast is a type of plastid. It is enveloped by two membranes and has a high concentration of chlorophyll. There is also another kind of plastids like leucoplast and chromoplast. However, both of these contain a minimal amount of chlorophyll and cannot carry out photosynthesis.

Chloroplasts are highly dynamic. They keep circulating within the plant cells. The behaviour of every chloroplast is affected by different environmental factors like colour, light, and intensity. In addition, chloroplast has DNA that is considered to be inherited.

The stroma present in the chloroplast takes up the maximum volume. It is vesicular in form and has a colourless centre. The shape of the cell varies from organism to organism. These cell organelles are generally found in the mesophyll of the leaves. There is an intermediate space that exists between the outer cell membrane and inner cell membrane. Inside the chloroplast, small-sized particles are present called thylakoids.

Thylakoid: It consists of grana that are arranged in a stack. A single stack usually contains 40-80 numbers of grana. These granas consist of chlorophyll that gives the plant its green colour.

Function

Chloroplast is the binding site for photosynthesis to occur. Food for plants is made in the form of sugars. Sunlight, carbon dioxide, and water are taken from the environment to produce oxygen and sugar. Its reactions take place in the cell membranes of thylakoids. Its process converts light energy into biological energy. Also, chloroplasts play an essential role in the defence mechanism of plants. The breakdown of water in sunlight, which is also called photolysis, also takes place in chloroplasts.

Calvin cycle

The dark reaction is also known as the light-independent reaction that occurs in the stroma. As a result of photophosphorylation, ATP is produced. Three rounds of the Calvin cycle are needed to make one molecule of G3P, which can exit the process and go forward to make glucose. Sixty-six molecules of glyceraldehyde-3-phosphate are formed. Out of that, eleven G-3-P molecules leave to make glucose molecules.

Fifty-five G-3-P molecules get recycled, which gives way for the regeneration of 33 RuBP acceptor molecules. Then 99 molecules of ATP get converted into 99 molecules of ADP. In this reduction step, NADPH is converted into NADP.

Location

Chloroplasts are located in the leaves of every green plant and eukaryotic algae in the living cells. All green plants have plastid, and chloroplasts are a kind of plastid. The chlorophyll located in the chloroplasts utilizes sunlight to convert solar energy into chemical form. Chloroplast contains three membrane layers. The outer membrane is semi-permeable and allows only small molecules to pass. The inner membrane is less porous, and the last membrane is the thylakoid membrane that looks like a series of flattened disks.

Role in photosynthesis

Six molecules of carbon dioxide, six molecules of water, and sunlight react together to give one molecule of glucose and six oxygen molecules.

Chlorophylls and carotenoids entrap sunlight to give adenosine triphosphate. ATP is considered to be the unit of energy in every organism. After this, the light-independent reactions are also known as the Calvin cycle. In this cycle, the electrons get carried by NADPH and convert all the inorganic carbon dioxide into the organic form of carbohydrates. The organic carbohydrates and other organic molecules that are not used are stored for later use.

Facts about chloroplasts

• Complex cells have more chloroplasts than simple cells.
• RuBisCo is the most abundant protein present in the chloroplast.
• Chlorophyll A is the most common kind, and it imparts green colour to the plants.
• They provide cells with amino acids.