The autotrophic mode of nutrition requires (a) carbon dioxide and water. (c) sunlight. (b) chlorophyll. (d) all of the above.

Photosynthesis is an essential biological process used by autotrophic organisms, such as plants, to produce their own food. During photosynthesis, plants convert sunlight into chemical energy. This process involves taking in carbon dioxide from the air and water from the soil and transforming these raw materials into glucose and oxygen. The overall equation for photosynthesis is \[6CO_2 + 6H_2O + \text{light energy} \rightarrow C_6H_{12}O_6 + 6O_2\] Photosynthesis is vital not only for plant growth but also for life on Earth, as it produces the oxygen we breathe and the glucose that serves as an energy source for plants and organisms consuming plants.

Chlorophyll is a green pigment found in the chloroplasts of plant cells. It plays a crucial role in photosynthesis by absorbing light energy, primarily from the blue and red wavelengths of the light spectrum, and converting it into chemical energy through a series of reactions. There are different types of chlorophyll, with chlorophyll-a being the most common and essential for oxygenic photosynthesis. The green color of chlorophyll is what gives plants their distinctive green hue.

Chlorophyll is essential for the initiation of the light-dependent reactions in photosynthesis. Without chlorophyll, plants would not be able to efficiently capture light energy, making it impossible for them to produce the food and energy they need to grow.

Carbon dioxide (CO2) is a key component of photosynthesis, serving as one of the raw materials that plants need to create glucose. Plants absorb carbon dioxide from the atmosphere through small openings in their leaves called stomata. Inside the chloroplasts of plant cells, carbon dioxide is combined with water to produce glucose and oxygen in a process driven by light energy. The uptake of CO2 is not only vital for the plant's nourishment but also helps regulate atmospheric CO2 levels, playing a role in the Earth's carbon cycle.

Sunlight provides the energy necessary for photosynthesis. When sunlight hits the chlorophyll molecules in the chloroplasts of plant cells, it initiates the light-dependent reactions of photosynthesis. This energy is used to split water molecules into oxygen, protons, and electrons and to generate energy-rich molecules like ATP and NADPH. The ATP and NADPH produced in these reactions are then used in the Calvin cycle, which takes place in the stroma of the chloroplast, to convert carbon dioxide into glucose. Adequate sunlight exposure is crucial for the efficient production of glucose and oxygen.

Water (H2O) is another fundamental ingredient in the process of photosynthesis. It is absorbed by plants through their roots from the soil and transported to the leaves via the xylem vessels. In the chloroplasts, water undergoes a photolysis reaction, where light energy splits water molecules into oxygen, protons, and electrons. This reaction not only releases oxygen as a byproduct, which is essential for aerobic life forms, but also provides the electrons and protons needed to produce ATP and NADPH during the light-dependent reactions of photosynthesis. Thus, a regular supply of water is indispensable for the continuity of photosynthesis.