Photosyntheis and chloroplast structure
Laura Armstrong & Joe Wolfensohn
Teachers
Contents
Recall Questions
This topic requires prior knowledge of photosynthesis from GCSE and chloroplast structure from year 12. You can test your knowledge on these below.
Write the word equation and the balanced chemical equation for photosynthesis.
Carbon dioxide + water → Glucose + oxygen
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
(In the presence of light and chlorophyll)
Where in the leaf does most photosynthesis occur?
The palisade mesophyll cells.
Why is chlorophyll important for photosynthesis?
Chlorophyll absorbs light energy which is required for photosynthesis. Chlorophyll is an example of a photosynthetic pigment.
Topic Explainer Video
Check out this @JoeDoesBiology video that explains photosynthesis and chloroplast structure or read the full notes below. Once you've gone through the whole note, try out the practice questions!
If you want to finish off the second part of @LauraDoesBiology's explanation of chloroplasts, check out this video and once you've gone through the whole note, try out the practice questions!
Structure of the Chloroplast
Below is an image detailing the structure of the chloroplast:
Structure of the Chloroplast
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Outer membrane
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Inner membrane
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Thylakoids: Flattened sacs surrounded by the thylakoid membranes where the light-dependent reactions occur.
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Grana (singular: granum): Stacks of thylakoids, increasing surface area for light absorption.
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Stroma: Fluid-filled space where the light-independent reactions (the Calvin cycle) occur.
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Lamellae: Extensions connecting grana.
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Circular DNA and 70S Ribosomes: Allow the chloroplast to produce proteins and enzymes required for photosynthesis.
How the Chloroplast is Adapted for Photosynthesis
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Large surface area of thylakoid membranes for maximum light absorption.
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Chlorophyll pigments are arranged in photosystems embedded in the thylakoid membranes to optimise light absorption.
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Electron carriers and ATP synthase are embedded in the thylakoid membrane for ATP production.
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The stroma contains enzymes required for the Light independent reaction such as Rubisco.
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Chloroplast DNA and ribosomes allow for quick protein synthesis, for example, synthesisng enzymes required for photosynthesis.
Photosystems
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Photosystems are embedded in the thylakoid membranes- this is where light is absorbed and where the light dependent reaction takes place.
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They contain photosynthetic pigments.
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Chlorophyll A is the primary pigment, there are two different forms: P700 and P680 that absorb slightly different wavelengths of light.
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There are other photosynthetic pigments in the photosystems such as Chlorophyll B and accessory pigments such as carotenoids and xanthophylls.
Sites of the Stages of Photosynthesis
|
Stage |
Location |
Key Processes |
|---|---|---|
|
1. Light-dependent reaction |
Thylakoid membrane |
Photolysis of water, ATP synthesis, NADP reduction |
|
2. Light-independent reaction (Calvin Cycle) |
Stroma |
CO₂ fixation, GP → Triose Phosphate conversion, RuBP regeneration |
Comparison of Chloroplast and Mitochondrion
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Both of these organelles have a double membrane.
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They both have an internal membrane with a large surface area for metabolic reactions. Both of these internal membranes have electron carriers and ATP synthase enzymes.
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Both of these organelles have their own DNA and ribosomes.
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Note- the DNA is circular and is not associated with histone proteins, it does not contain introns.
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The ribosomes are 70S (smaller than the 80S ribosomes found in the cytoplasm and on the rough endoplasmic reticulum).
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|
Feature |
Chloroplast |
Mitochondrion |
|---|---|---|
|
Function |
Photosynthesis (produces glucose) |
Aerobic respiration (breaks down glucose) |
|
Energy source |
Light |
Glucose |
|
Site of ATP production |
Thylakoid membrane |
Inner mitochondrial membrane |
|
Electron carriers |
Present in thylakoid membrane |
Present in cristae |
|
Circular DNA and 70S ribosomes |
Present |
Present |
|
Pigments |
Chlorophyll |
No pigments |
Absorption Spectrum and Action Spectrum
- Absorption Spectrum: A graph showing the wavelengths of light absorbed by photosynthetic pigments.
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Chlorophyll absorbs red and blue light best, reflecting green light (which is why chlorophyll appears green).
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- Action Spectrum: A graph showing the rate of photosynthesis at different wavelengths of light.
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The action spectrum closely matches the absorption spectrum, proving that absorbed light drives photosynthesis and the more light absorbed, the faster the rate of photosynthesis.
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Explanation:
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The similarity between the absorption and action spectra suggests that chlorophyll is the main pigment responsible for photosynthesis.
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Peaks in both spectra align with wavelengths where photosynthesis is most efficient.
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Having a range of photosynthetic pigments means that a greater range of wavelengths of light can be absorbed and used in photosynthesis.
Key Terms
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Thylakoid: Membrane-bound compartments containing chlorophyll for light-dependent reactions.
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Grana: Stacks of thylakoids, increasing surface area for light absorption.
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Stroma: Fluid-filled space containing enzymes for the Light independent reactions.
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Photosystem: A protein-pigment complex that captures light energy.
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Electron Transport Chain: Series of proteins in the thylakoid membrane that transfer electrons to generate ATP.
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Absorption Spectrum: A graph showing how much light a pigment absorbs at different wavelengths.
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Action Spectrum: A graph showing the rate of photosynthesis at different wavelengths of light.
No answer provided.
Exam Tip
Ensure you can label and describe the chloroplast structure accurately. Many exam questions require linking the structure to its function. Be prepared to compare chloroplasts to mitochondria in terms of structure and their roles in metabolic processes.
No answer provided.
The electron micrograph shows part of a chloroplast.
a) Name the parts labelled A and B and, for each, describe one role in the process of photosynthesis.
A Name ___________________________________________________________
Role ______________________________________________________________
B Name ___________________________________________________________
Role ______________________________________________________________
(4 marks)
b) Name the main substance present in the part labelled C. (1 mark)
c) How is this substance formed? (1 mark)
a) A – granum / thylakoid
contain chlorophyll molecules to absorb light / the site of the light dependent reaction
B – stroma
contains enzymes for the light-independent reaction / the site of the light-independent reaction
(b) C – starch
(c) from glucose in condensation reactions
Practice Question
Try to answer the practice question from the TikTok on your own, then watch the video to see how well you did!