OCR Specification focus:
‘Identify chloroplast outer membrane, lamellae, grana, thylakoids, stroma and DNA, and state where each photosynthetic stage occurs.’
Chloroplasts are specialised organelles that capture light energy and convert it into chemical energy through photosynthesis, providing the foundation for almost all life on Earth.
Structure of the Chloroplast
General Overview
Chloroplasts are double-membraned organelles found in the cytoplasm of plant cells and photosynthetic protists. Each chloroplast contains internal membranes and fluid regions that compartmentalise photosynthetic reactions. Their structure is highly adapted to maximise light absorption and the conversion of energy into sugars.
Labeled overview of a chloroplast showing the double membrane envelope, stroma, and the internal thylakoid system organised into grana and stromal lamellae. This directly supports identifying the main structures and where photosynthetic reactions occur. The diagram also includes extra features (e.g. starch granules, plastoglobuli) not required by the syllabus—use them only as optional enrichment. Source.
Outer and Inner Membranes
The outer membrane is smooth and permeable to small molecules and ions. It allows the exchange of materials between the cytoplasm and the chloroplast’s internal environment.
The inner membrane, however, is selectively permeable and controls the passage of metabolites in and out of the chloroplast. Transport proteins embedded in the inner membrane regulate substances such as ATP, ADP, and phosphate.
Double membrane: Two concentric phospholipid bilayers surrounding an organelle, creating a controlled internal environment for metabolic reactions.
Between these two membranes lies the intermembrane space, a thin region that facilitates molecular exchange but plays little direct role in photosynthesis.
The Stroma
Inside the inner membrane is the stroma, a dense fluid containing enzymes, ribosomes, DNA, and dissolved ions. The stroma is the site of the light-independent reactions (Calvin cycle), where carbon dioxide is fixed into organic molecules.
The stroma also contains chloroplast DNA, which codes for some chloroplast proteins and ribosomes for their synthesis. However, many chloroplast proteins are encoded by nuclear DNA and imported post-translationally.
Stroma: The fluid-filled matrix of the chloroplast where light-independent reactions occur, containing enzymes, ribosomes, and DNA.
The enzymes in the stroma catalyse the conversion of carbon dioxide and hydrogen (from reduced NADP) into triose phosphate (TP), which can be used to form carbohydrates and other biomolecules.
Thylakoids
Suspended within the stroma are membrane-bound sacs called thylakoids. Each thylakoid is a flattened disc composed of a lipid bilayer containing embedded photosynthetic pigments, electron carriers, and ATP synthase complexes.
Thylakoids are arranged into interconnected stacks called grana (singular: granum) and are linked by unstacked regions known as intergranal lamellae or stromal lamellae.
Thylakoid: A flattened membrane sac within the chloroplast that contains chlorophyll and other pigments, forming the site of the light-dependent reactions.
The membranes of thylakoids form a separate internal compartment, the thylakoid lumen, which plays a crucial role in the proton gradient used for ATP synthesis during photosynthesis.
Grana
Grana are stacks of thylakoids joined together to maximise the surface area for light absorption. Each granum may contain up to 100 thylakoids.
The arrangement of grana allows chloroplasts to house large numbers of photosystems, electron transport chains, and ATP synthase enzymes. These structures collectively capture light energy and convert it into ATP and reduced NADP.
Electron micrograph of a chloroplast showing stacked grana (dark, coin-like layers) separated by lighter stroma; stromal lamellae connect the stacks. Use alongside the diagram to help learners recognise these structures in real cells. The image is unlabeled by design, so draw attention to the stacked vs unstacked regions during teaching. Source.
Granum (plural: grana): A stack of thylakoids in a chloroplast that increases the surface area for light-dependent reactions.
The organisation of grana enhances the efficiency of light capture and ensures close proximity between pigment molecules and the electron transport system.
Lamellae
The lamellae (also known as intergranal or stromal lamellae) are unstacked thylakoid membranes that connect different grana stacks, ensuring continuous membrane networks for efficient energy transfer. They also contain photosystem I and associated proteins but fewer photosystem II complexes compared to grana.
Lamellae: Membranous structures connecting grana, containing electron carriers and photosystem I, facilitating energy transfer across the chloroplast.
This distribution of photosystems allows the separation of different stages of the light reactions, ensuring electron flow between the photosystems and consistent energy conversion.
Chloroplast DNA and Ribosomes
Chloroplasts contain their own circular DNA and 70S ribosomes, similar to those found in prokaryotes. This supports the endosymbiotic theory, suggesting chloroplasts originated from photosynthetic bacteria engulfed by ancestral eukaryotic cells.
The DNA encodes proteins involved in photosynthesis and chloroplast maintenance. However, most chloroplast proteins are encoded by nuclear genes and imported into the organelle.
Endosymbiotic theory: The hypothesis that chloroplasts and mitochondria originated from free-living prokaryotes engulfed by ancestral eukaryotic cells.
Functional Sites of Photosynthetic Stages
Each structural region of the chloroplast is associated with a particular stage of photosynthesis, ensuring spatial separation and efficiency of the overall process.
Schematic of a chloroplast indicating light-dependent reactions on thylakoid membranes (within grana) and the Calvin cycle in the stroma. Arrows illustrate ATP/NADPH transfer from thylakoids to stroma, reinforcing the spatial logic of photosynthesis. The appearance of GA3P in the cycle is standard nomenclature and not essential beyond identifying the stroma as the site. Source.
Light-Dependent Reactions
Occur on the thylakoid membranes within the grana and lamellae.
Photosystems I and II absorb light energy, exciting electrons that travel along the electron transport chain (ETC).
Water is photolysed, releasing oxygen, protons, and electrons.
The ETC and ATP synthase in the thylakoid membrane drive photophosphorylation to produce ATP.
NADP is reduced to reduced NADP (NADPH) using electrons and protons.
Light-Independent Reactions
Occur in the stroma.
Use ATP and reduced NADP produced by the light-dependent reactions.
Involve the Calvin cycle, in which carbon dioxide is fixed by the enzyme RuBisCO into glycerate-3-phosphate (GP), then reduced to triose phosphate (TP).
RuBP is regenerated, enabling the cycle to continue.
Functional Adaptations of Chloroplast Structure
Large surface area from thylakoid membranes maximises light absorption.
Close proximity of pigments, electron carriers, and enzymes ensures rapid energy transfer.
Stroma enzymes operate optimally under stable internal conditions, enhancing carbon fixation efficiency.
DNA and ribosomes allow chloroplasts to synthesise essential proteins independently, enabling faster response to cellular demands.
The integration of chloroplast structures allows a seamless flow between the light-dependent and light-independent stages, ensuring that energy captured from sunlight is efficiently converted into chemical energy stored in organic molecules.
FAQ
Chloroplasts contain their own circular DNA and 70S ribosomes because they evolved from photosynthetic prokaryotes through endosymbiosis. This allows them to synthesise a small number of essential proteins, such as those involved in photosystem assembly and electron transport.
However, most chloroplast proteins are encoded by nuclear DNA and imported from the cytoplasm, meaning the chloroplast and nucleus cooperate closely to maintain efficient photosynthetic function.
Grana contain densely stacked thylakoids that host both photosystem II and photosystem I, enabling the light-dependent reactions. These stacks maximise the surface area for light absorption.
Lamellae (stromal or intergranal lamellae) connect grana stacks and mainly contain photosystem I and ATP synthase complexes. They allow electron flow between photosystems, maintaining the balance between cyclic and non-cyclic photophosphorylation.
Compartmentalisation ensures that photosynthetic reactions occur efficiently and without interference.
Thylakoid membranes confine the light-dependent reactions, allowing a proton gradient to form in the thylakoid lumen for ATP synthesis.
Stroma isolates the enzymes of the Calvin cycle, enabling controlled carbon fixation.
Double membranes separate chloroplast processes from cytoplasmic reactions, preserving optimal ionic and pH conditions for photosynthesis.
Chloroplast structure can vary according to light intensity and habitat.
Shade-adapted plants often have larger grana with more thylakoid stacking to maximise light capture.
Sun-adapted plants typically have smaller grana and a greater proportion of lamellae to enhance energy transfer and prevent photodamage.
In algae and some non-vascular plants, thylakoid organisation may be simpler, reflecting differences in their photosynthetic requirements.
The thylakoid lumen is the internal space enclosed by thylakoid membranes. During the light-dependent reactions, protons (H⁺ ions) accumulate in this lumen as water is photolysed.
This creates a proton gradient across the membrane. Protons flow back into the stroma through ATP synthase, driving the synthesis of ATP from ADP and phosphate — a process known as chemiosmosis.
The maintenance of this gradient is essential for efficient ATP production in photosynthesis.
Practice Questions
Question 1 (2 marks)
State the site of the light-dependent reactions and the site of the light-independent reactions in a chloroplast.
Mark scheme:
1 mark for identifying the light-dependent reactions occur on the thylakoid membranes (accept: grana or lamellae).
1 mark for identifying the light-independent reactions occur in the stroma.
Question 2 (5 marks)
Describe how the structure of the chloroplast is adapted to its role in photosynthesis.
Mark scheme:
Award up to 5 marks for the following points (maximum 1 mark per point):
Presence of grana (stacks of thylakoids) provides a large surface area for light absorption and the attachment of photosystems and electron carriers.
Thylakoid membranes contain ATP synthase and electron transport chains for the light-dependent reactions.
Stroma contains enzymes for the Calvin cycle (light-independent reactions).
Lamellae (intergranal membranes) connect grana, facilitating energy transfer and distribution of photosystems.
Chloroplast DNA and ribosomes allow synthesis of some chloroplast proteins needed for photosynthesis.
Double membrane regulates the movement of substances in and out of the chloroplast, maintaining an optimal internal environment for photosynthetic reactions.
