5.1.1 Chloroplast Structures & their Functions: Biology AQA A level Revision

Chloroplast DNA

Chloroplast DNA, or cpDNA, is a small circular molecule found within chloroplasts. It encodes a portion of the proteins required for the photosynthetic machinery, as well as some RNA molecules involved in protein synthesis. Chloroplast DNA replicates independently of the nuclear genome and is essential for the proper functioning of chloroplasts. For example, chloroplast DNA is involved in the synthesis of proteins essential for photosynthesis, which are produced at the 70S ribosomes in the stroma.

Chloroplasts

Chloroplasts are specialised organelles found in the cells of plants and algae. They are responsible for conducting photosynthesis, the process by which light energy is converted into chemical energy in the form of glucose. Within chloroplasts, various pigments, including chlorophyll, absorb light energy, initiating the biochemical reactions necessary for glucose synthesis. For example, chloroplasts contain chlorophyll and other pigments responsible for capturing light energy.

Grana

Grana are stacks of thylakoid membranes found within chloroplasts. The organisation of thylakoids into grana allows for efficient light capture and energy transduction during photosynthesis. This structural arrangement facilitates the flow of electrons through the photosynthetic electron transport chain, leading to the generation of ATP and NADPH. For example, the arrangement of grana allows for efficient energy transfer during photosynthesis.

Stroma lamellae

Stroma lamellae are membranous structures that connect adjacent grana within chloroplasts. These connections provide continuity between thylakoid membranes while allowing for the diffusion of molecules, such as electrons and protons, between grana. Stroma lamellae play a vital role in coordinating the light-dependent reactions across the entire chloroplast. For example, stroma lamellae facilitate communication and transportation of molecules between grana.

Double-membrane envelope

The double-membrane envelope surrounding chloroplasts provides a barrier that separates the interior of the organelle from the surrounding cytoplasm. This structure plays a crucial role in regulating the passage of molecules in and out of the chloroplast, ensuring the proper functioning of photosynthesis while protecting the internal components from external influences. For example, the double-membrane structure protects the contents of the chloroplast and regulates what enters and exits the organelle.

Light-dependent reactions

The light-dependent reactions of photosynthesis occur in the thylakoid membranes of chloroplasts. These reactions harness the energy of photons to drive the synthesis of ATP and NADPH, which are used to power the light-independent reactions. Key processes in the light-dependent reactions include the absorption of light by chlorophyll, the splitting of water molecules (photolysis), and the transfer of electrons through the photosynthetic electron transport chain. For example, photolysis, the splitting of water molecules, is a key step in the light-dependent reactions.

Light-independent reactions

The light-independent reactions, also known as the Calvin cycle, take place in the stroma of chloroplasts. These reactions utilise the ATP and NADPH produced during the light-dependent reactions to convert carbon dioxide into carbohydrates, such as glucose. The Calvin cycle consists of a series of enzymatic reactions that fix carbon dioxide and reduce it to form sugars, which serve as energy storage molecules for the plant. For example, the Calvin cycle produces glucose from carbon dioxide in the stroma.

Stroma

The stroma is a semi-fluid matrix that fills the interior of chloroplasts. It contains enzymes, DNA, ribosomes, and other necessary molecules for the synthesis of glucose during the light-independent reactions of photosynthesis. The stroma serves as the site for various metabolic pathways, including the Calvin cycle, which converts carbon dioxide into carbohydrates. For example, carbon fixation, a key step in the Calvin cycle, takes place in the stroma.

Thylakoids

Thylakoids are membrane-bound compartments within chloroplasts where the light-dependent reactions of photosynthesis occur. These flattened sacs are arranged in stacks called grana and contain chlorophyll and other pigment molecules. Thylakoids provide a large surface area for the absorption of light energy and the assembly of the photosystems involved in the conversion of light into chemical energy. For example, thylakoids house the photosystems responsible for capturing light energy during the light-dependent reactions.