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5.1.5Unit 5: Respiration, Internal Environment, Coordination and Gene Technology

Chloroplasts — IAL Edexcel Linear Revision Notes & Topical Questions

5 min read 5 parts Lesson 12 of 36 Unit quiz

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Part 1 of 5Structure

5.1.5 Chloroplasts

Chloroplasts are the organelles in which photosynthesis takes place.

Envelope
  • Two phospholipid membranes enclose the chloroplast and control what enters and leaves.
Stroma
  • The fluid in the chloroplast contains enzymes such as rubisco, sugars, ribosomes and DNA.
Thylakoids
  • Flattened membrane sacs enclosing a fluid space, where the light-dependent stage occurs.
Grana
  • Stacks of thylakoids give a large membrane area for pigments and electron carriers.
Lamellae
  • Membrane extensions link grana to each other while keeping the stacks apart.
Size
  • A chloroplast is only a few micrometres long, so the cell can contain many of them.

Part 2 of 5Structure Linked to Function

Structure Linked to Function
Large area
  • Many thylakoid membranes carry many pigment molecules and increase light absorption.
ATP synthase
  • The enzyme is embedded in the thylakoid membranes, where the proton gradient forms.
Compartments
  • The thylakoid space is small, so a steep proton gradient builds up quickly.
Stroma enzymes
  • Enzymes of the Calvin cycle are kept near the ATP and reduced NADP made on the thylakoids.
Selective envelope
  • The envelope controls entry of carbon dioxide, water, and the products leaving the chloroplast.

Part 3 of 5Own Genetic Material

Chloroplasts can make some of their own proteins.

1
The stroma contains a loop of DNA, and DNA is transcribed in the chloroplast.
2
Ribosomes in the stroma translate the messenger RNA into some chloroplast proteins.
3
These proteins include parts of the photosynthetic machinery in the thylakoids.
4
Other proteins are made from nuclear genes in the cytoplasm and imported into the chloroplast.

Part 4 of 5Other Contents

Starch grains
  • These store carbohydrate produced by photosynthesis during bright periods.
Lipid droplets
  • These may be found in the stroma as another storage product.
Pigments
  • Chlorophyll a and b and carotenoids are held in photosystems on the thylakoids.
Distribution
  • Chloroplasts are found mainly in palisade and spongy mesophyll cells near the leaf surface.

Part 5 of 5Exam Checklist & Tips

5.1.5 – Key Points for the Exam

  • Chloroplast The organelle in plant cells where photosynthesis occurs.
  • Granum A stack of thylakoids in a chloroplast.
  • Stroma The fluid matrix of a chloroplast containing the enzymes of the Calvin cycle.
  • Thylakoid membrane The membrane carrying photosystems, electron carriers and ATP synthase.
  • Lamella A membrane linking one granum to the next.

Common Mistakes

  • Putting the light-dependent stage in the stroma; it is in the thylakoid membranes.
  • Describing the chloroplast as having a single membrane; it has an envelope of two.
  • Mixing up the terms grana and thylakoids; grana are stacks of thylakoids.
  • Forgetting to link each structure to a function when asked to explain.

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