A biologist examines an electron micrograph of a palisade mesophyll cell taken from a leaf near the top of a sunflower plant. The cell contains a large nucleus with a clearly visible nucleolus, many chloroplasts packed close to the cell surface membrane, and a number of mitochon…

Answer: ((a)) The nucleolus manufactures ribosomal RNA (rRNA) and assembles ribosome subunits, which then pass out through the nuclear pores into the cytoplasm. ((b)) The chloroplast is surrounded by a double membrane (envelope) that controls the entry and exit of substances such as carbon dioxide and the products of photosynthesis. Internal thylakoid membranes are folded and stacked into grana, greatly increasing the surface area available f… ((c)) Photosynthesis only occurs in the light and does not directly supply usable ATP for cellular work, so the cell cannot rely on chloroplasts alone for a continuous energy supply. Mitochondria carry out aerobic respiration, releasing ATP for energy-requiring processes such as activ…. Part (a): Function of the nucleolus The nucleolus is a dense region inside the nucleus. Its function is to manufacture ribosomal RNA (rRNA) and use it to assemble the two subunits of ribosomes. These ribosome subunits then leave the nucleu…

AS Biology 9700: Organelle Structure and Function in a Palisade Cell

Part (a): Function of the nucleolus

The nucleolus is a dense region inside the nucleus. Its function is to manufacture ribosomal RNA (rRNA) and use it to assemble the two subunits of ribosomes. These ribosome subunits then leave the nucleus through the nuclear pores and enter the cytoplasm, where they join together to form complete, functional ribosomes.

Part (b): Chloroplast structure and photosynthesis

Each structural feature of a chloroplast can be linked directly to its role in photosynthesis:

A double membrane (envelope) surrounds the chloroplast, controlling the movement of substances such as carbon dioxide into the chloroplast and the products of photosynthesis out of it.
The internal membranes are folded and stacked into flattened discs called thylakoids, which are further stacked into grana. This stacking creates a very large surface area on which chlorophyll and other pigments, together with the proteins of the electron transport chain, can be packed, maximising light absorption and the rate of the light-dependent reactions.
The stroma, the fluid-filled space surrounding the grana, contains the enzymes needed to carry out the light-independent reactions (the Calvin cycle), where carbon dioxide is fixed into organic molecules. The stroma also contains the chloroplast’s own small circular DNA molecule and 70S ribosomes, allowing some chloroplast proteins to be synthesised within the organelle itself.

Part (c): Why the cell still needs mitochondria

Although a palisade mesophyll cell photosynthesises, photosynthesis produces glucose and other organic molecules rather than directly supplying usable ATP for the cell’s everyday activities, and it only occurs while light is available. Processes such as active transport (for example, actively loading sugars into the phloem for transport around the plant), protein synthesis and cell maintenance all require a continuous supply of ATP, including during the night when photosynthesis has stopped. Mitochondria carry out aerobic respiration, breaking down glucose (including glucose made by the chloroplasts) to release ATP, so the cell needs a substantial number of mitochondria to meet this ongoing energy demand.

Final answers

(a) The nucleolus makes ribosomal RNA and assembles ribosome subunits.
(b) Double membrane controls entry/exit of substances; thylakoids stacked into grana give a large surface area for the light-dependent reactions; stroma contains enzymes for the light-independent reactions plus the chloroplast’s own DNA and 70S ribosomes.
(c) Photosynthesis does not directly provide usable ATP and only occurs in the light, so mitochondria are needed to release ATP by aerobic respiration for energy-requiring processes at all times, including in darkness.