A student examines an electron micrograph of a chloroplast from a palisade mesophyll cell and identifies three structures, described in the table below. Structure Description P A stack of flattened, membrane-bound sacs Q A short strand of circular DNA lying free within the fluid…

Answer: ((a)) A stack of thylakoids such as structure P is called a granum (plural grana). Stacking many thylakoid discs together greatly increases the membrane surface area available, within a small volume of the chloroplast, for pigments, photosystems and electron carriers to be packed onto… ((b)) Because the chloroplast has its own DNA and ribosomes, it can synthesise some of the proteins it needs (for example, some polypeptides of the photosystems and the large subunit of the enzyme rubisco) itself, within the organelle, instead of relying entirely on proteins made in t… ((c)) During several hours in bright light, the light-dependent stage continuously supplies ATP and reduced NADP, allowing the Calvin cycle to keep producing triose phosphate; any triose phosphate not immediately needed to regenerate RuBP or exported from the chloroplast is converted…. Part (a): Grana and the efficiency of the light-dependent stage A stack of thylakoid sacs, such as structure P, is called a granum (plural: grana). Grana are connected to one another by unstacked regions of membrane called stroma lamellae.…

A2 Biology 9700: Chloroplast Structure and Function

A student examines an electron micrograph of a chloroplast from a palisade mesophyll cell and identifies three structures, described in the table below.

Structure	Description
P	A stack of flattened, membrane-bound sacs
Q	A short strand of circular DNA lying free within the fluid matrix, alongside the chloroplast's own ribosomes
R	A dense granule composed of an insoluble storage carbohydrate

(a) Give the name for a stack of sacs such as structure P, and explain how this stacked arrangement makes the light-dependent stage more efficient. [3]

(b) Suggest why it is significant for the chloroplast's role in photosynthesis that it contains its own DNA (structure Q) and its own ribosomes, rather than relying only on proteins made using the cell's nuclear DNA. [3]

(c) Structure R is present in much greater amounts in a chloroplast that has been photosynthesising in bright light for several hours than in a chloroplast that has been kept in darkness for the same length of time. Explain this difference. [3]

Part (a): Grana and the efficiency of the light-dependent stage

A stack of thylakoid sacs, such as structure P, is called a granum (plural: grana). Grana are connected to one another by unstacked regions of membrane called stroma lamellae.

Stacking the thylakoids together greatly increases the total membrane surface area that can be packed into a small volume of the chloroplast. This larger surface area allows many more pigment molecules, photosystems (PSI and PSII), electron carriers and ATP synthase molecules to be embedded within the same space than if the thylakoid membrane were left as a single, unstacked sheet. As a result, more light can be absorbed and more electron transport chains can operate simultaneously, so ATP and reduced NADP can be generated at a faster overall rate.

Part (b): Significance of chloroplast DNA and ribosomes

Structure Q (a short loop of DNA) and the chloroplast’s own 70S ribosomes allow the chloroplast to synthesise some of its own proteins internally, rather than depending entirely on proteins encoded by the nucleus and imported from the cytoplasm. Examples include some polypeptides of the photosystems and the large subunit of rubisco, the key enzyme of the Calvin cycle.

This ability reflects the chloroplast’s origin, according to the endosymbiotic theory, as a once free-living photosynthetic prokaryote that was engulfed by an ancestral eukaryotic cell. Retaining its own DNA and ribosomes means the chloroplast can divide by binary fission independently of the cell cycle and can produce some of its essential proteins directly, without being wholly dependent on the nucleus for every photosynthetic protein.

Part (c): Starch grains in light versus darkness

Starch grains such as structure R are a temporary carbohydrate store within the chloroplast, built up whenever photosynthesis is producing more triose phosphate (TP) than is immediately needed.

In bright light for several hours: the light-dependent stage continually supplies ATP and reduced NADP, so the Calvin cycle keeps running and keeps producing TP. Most TP is used to regenerate RuBP, but any surplus is converted into starch. Starch is used for storage because, being insoluble, it does not draw water into the chloroplast by osmosis (unlike a soluble sugar such as glucose), and it can be packed compactly as a granule.
In darkness: with no light energy available, the light-dependent stage cannot occur, so no ATP or reduced NADP is generated and the Calvin cycle halts. No new TP, and therefore no new starch, can be made. Meanwhile, existing starch reserves continue to be broken down (hydrolysed back to soluble sugars) to be exported from the cell or used in respiration, so the amount of starch present falls.

This is why a chloroplast from a leaf kept in bright light accumulates far more starch than one kept in darkness for the same length of time.

Final answers

(a) Structure P is a granum; stacking increases membrane surface area for pigments/photosystems/electron carriers, allowing faster ATP and reduced NADP production.
(b) Chloroplast DNA and ribosomes allow some photosynthetic proteins (e.g. part of rubisco) to be made within the organelle itself, reflecting its endosymbiotic origin and allowing some independence from the nucleus.
(c) Bright light keeps supplying ATP/reduced NADP so surplus triose phosphate is stored as starch; in darkness no new starch is made and existing starch is broken down, so starch content falls.