Why do you think it is a good idea to soak wilted lettuce in cool water before serving it?

Choice A rationale: Chlorophyll is the molecule that traps the sun's energy and initiates the process of photosynthesis. Photosynthesis is the process by which plants convert light energy into chemical energy, stored in the bonds of sugar molecules. Chlorophyll is a green pigment that is found in the thylakoid membranes of the chloroplasts, the organelles where photosynthesis occurs. Chlorophyll absorbs red and blue light from the sun and reflects green light, giving plants their characteristic color. Chlorophyll also transfers the light energy to electrons, which are then used to split water molecules and generate ATP and NADPH, the energy intermediates for the dark reaction of photosynthesis¹².

Choice B rationale: ATP is not the molecule that traps the sun's energy, but an energy molecule that is synthesized by the light reaction of photosynthesis. ATP stands for adenosine triphosphate, and it consists of a nitrogenous base, a sugar, and three phosphate groups. ATP stores energy in the bonds between the phosphate groups, and releases energy when one of the bonds is broken, forming ADP (adenosine diphosphate) and Pi (inorganic phosphate). ATP provides energy for the dark reaction of photosynthesis, which uses CO2 to produce glucose, a type of sugar³⁴.

Choice C rationale: Chloroplast is not the molecule that traps the sun's energy, but the organelle where photosynthesis occurs. Chloroplast is a membrane-bound structure that is found in the cytoplasm of plant cells. Chloroplast contains its own DNA and ribosomes, and can divide independently of the cell. Chloroplast has two main parts: the stroma, which is the fluid-filled space inside the chloroplast, and the thylakoid, which is a system of flattened sacs that contain chlorophyll and other pigments. The light reaction of photosynthesis takes place in the thylakoid, while the dark reaction takes place in the stroma⁵⁶.

Choice D rationale: Glyceraldehyde-3-phosphate is not the molecule that traps the sun's energy, but an intermediate molecule in the dark reaction of photosynthesis. Glyceraldehyde-3-phosphate, also known as G3P, is a three-carbon sugar that is formed from CO2 and energy intermediates from the light reaction. G3P can be converted to glucose, which is the main product of photosynthesis, or to other organic molecules, such as amino acids, lipids, and nucleotides. G3P can also be recycled to regenerate the five-carbon starter molecule called ribulose, which is needed for the dark reaction to continue⁷⁸.

Choice E rationale: Rubisco is not the molecule that traps the sun's energy, but an enzyme that catalyzes the first step of the dark reaction of photosynthesis. Rubisco stands for ribulose-1,5-bisphosphate carboxylase/oxygenase, and it is the most abundant protein on Earth. Rubisco combines CO2 with ribulose, a five-carbon sugar, to form a six-carbon intermediate that splits into two molecules of G3P. Rubisco is also responsible for a wasteful process called photorespiration, in which it binds O2 instead of CO2, resulting in the loss of carbon and energy⁹ .

Choice A rationale: The cell membrane did not change at all because of the protection of the cell wall is incorrect because the cell wall does not prevent the movement of water across the cell membrane. The cell wall only provides mechanical support and prevents the cell from bursting in a hypotonic solution.

Choice B rationale: The cells burst because the solution was hypotonic is incorrect because the solution was hypertonic, not hypotonic. A hypertonic solution has a lower concentration of water than the cell, so water moves out of the cell and into the solution by osmosis, causing the cell to shrink.

Choice C rationale: The cell membrane shrank, but the central vacuole remained full is incorrect because the central vacuole also loses water in a hypertonic solution. The central vacuole is a large organelle that stores water and other substances in the plant cell. When water leaves the cell, the central vacuole shrinks and the cell loses its turgor pressure.

Choice D rationale: The cell membrane shrank away from the cell wall because the solution was hypertonic is correct because this is what happens when a plant cell is placed in a hypertonic solution. The cell membrane and the cytoplasm contract and pull away from the cell wall, creating a gap between them. This is called plasmolysis and it can damage the cell.

Choice E rationale: The cell became turgid is incorrect because the cell becomes turgid in a hypotonic solution, not a hypertonic one. A hypotonic solution has a higher concentration of water than the cell, so water moves into the cell and out of the solution by osmosis, causing the cell to swell and press against the cell wall. This is called turgor and it helps the cell maintain its shape and rigidity.