Question

Find out the incorrect statement from the following: (a) Dark reaction depends on the product formed by light reaction. (b) In stroma, enzymatic reactions incorporates \(\mathrm{CO}_{2}\) into the plant leading to the synthesis of sugar. (c) Purple and green sulphur bacteria use \(\mathrm{H}_{2} \mathrm{~S}\) as hydrogen donor. (d) There is no division of labour in chloroplast.

Short answer

The incorrect statement is (d) There is no division of labour in chloroplast.

Step-by-step solution

Analyze statement (a)

The dark reaction of photosynthesis is indeed dependent on the products of the light reaction. During the light reaction, energy is captured from light and transformed into chemical energy, stored in NADPH and ATP. The dark reaction uses these products to fix carbon dioxide into carbohydrate. Therefore, this statement is correct.

Analyze statement (b)

In the stroma of the chloroplast, CO2 is incorporated into the plant through a series of enzymatic reactions leading to the formation of sugars. The sequence of reactions known as the Calvin cycle, uses the CO2 along with the ATP and NADPH produced in the light reactions to produce glucose. Therefore, this statement is correct.

Analyze statement (c)

Purple and green sulphur bacteria are photosynthetic bacteria that can use H2S, rather than H2O like most other photosynthetic organisms, as a reducing agent or 'hydrogen donor'. The oxidation of H2S allows these bacteria to fix carbon dioxide into biomass. Therefore, this statement is correct.

Analyze statement (d)

The last statement suggests that there is no division of labour within a chloroplast. This is incorrect. A chloroplast is divided into distinct compartments (like the stroma and thylakoids) each performing different functions, thus showing a clear division of labour. This statement is incorrect, making it the answer to the exercise.

Key concepts

Light Reaction

The light reaction is the first stage of photosynthesis. It occurs in the thylakoid membranes of the chloroplasts. When sunlight hits these membranes, it excites electrons in chlorophyll. This excitement leads to the splitting of water molecules into oxygen, protons, and electrons.
These high-energy electrons travel through a series of proteins called the electron transport chain. As they move, their energy is used to pump protons into the thylakoid space, creating a gradient. This energy-rich environment helps synthesize ATP and NADPH.
Both ATP and NADPH are essential, as they provide the energy and reducing power needed for the next phase of photosynthesis.

Dark Reaction

The dark reaction, also known as the Calvin Cycle, doesn’t require direct sunlight. It takes place in the stroma of the chloroplast. While it's called the "dark" reaction, it doesn’t actually occur only in darkness. It uses the ATP and NADPH produced in the light reactions to convert carbon dioxide into glucose.
The Calvin Cycle begins with the fixation of carbon dioxide. This process involves attaching \(\text{CO}_2\) to a 5-carbon sugar named ribulose bisphosphate (RuBP). The enzyme responsible for this is called Rubisco. Through a series of reactions, this compound is eventually converted into glucose, a sugar molecule that plants use for energy and growth.

Chloroplast Structure

Chloroplasts are specialized organelles found in plant cells. They are the sites of photosynthesis and have a unique structure. They are surrounded by a double membrane with an intermembrane space. Inside the chloroplast, there are two main parts: the stroma and the thylakoids.
The stroma is a fluid-filled space where the Calvin Cycle occurs. It contains enzymes important for the conversion of carbon dioxide into glucose. Thylakoids are stacked into grana, and it's here that the light reactions take place. These compartments work together, showing a clear division of labour in the chloroplast.
This division allows the plant to efficiently carry out photosynthesis, utilizing different processes in distinct areas.

Calvin Cycle

The Calvin Cycle is a series of biochemical reactions that occur in the stroma of the chloroplast. It forms the "dark" reaction of photosynthesis and is responsible for synthesizing glucose from carbon dioxide.
The cycle begins with carbon fixation. Rubisco, an important enzyme, catalyzes the reaction of \(\text{CO}_2\) with RuBP to form a 6-carbon compound that splits into two 3-carbon molecules. Through a series of steps, these molecules are converted, eventually forming glucose.
ATP and NADPH, produced in the light reactions, provide the energy and reducing power for these transformations. The Calvin Cycle is crucial, as it not only supplies energy in the form of glucose but also regenerates RuBP, enabling the process to continue.