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13.1.113 · Photosynthesis

Chloroplasts — Cambridge AS & A Level Biology 9700 Revision Notes & Topical Questions

5 min read 8 parts Lesson 2 of 13 1 quiz

📚 Cambridge AS & A Level Biology 9700 Hub / Topical Revision
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13.1.1 Chloroplasts — CAMBRIDGE AS & A LEVEL BIOLOGY 9700 — REVISION NOTES notes: Describe the structure of the chloroplast and relate each part to its role in photosynthesis.Site of photosynthesis — chloroplasts are the organelles in plant cells where photosynthesis occurs.Doubl…

  • Double membrane
  • Phospholipid bilayer
  • 70S ribosomes
  • Loop of DNA
  • Starch grains
  • Thylakoids

Part 1 of 6Chloroplast envelope

Site of photosynthesis
  • Site of photosynthesis chloroplasts are the organelles in plant cells where photosynthesis occurs.
Double membrane
  • Double membrane each chloroplast is surrounded by an envelope of two membranes.
Phospholipid bilayer
  • Phospholipid bilayer each envelope membrane is a phospholipid bilayer.

Part 2 of 6The stroma

The stroma
Fluid
  • Fluid the stroma fills the chloroplast and surrounds the thylakoids.
Function
  • Function site of the light-independent stage of photosynthesis.
Contents
  • CO2, sugars, enzymes and other dissolved molecules.
70S ribosomes
  • 70S ribosomes make the proteins coded by chloroplast DNA.
Loop of DNA
  • Loop of DNA codes for some chloroplast proteins.
  • Other chloroplast proteins are coded by nuclear DNA.
Starch grains
  • Starch grains store the sugars formed in photosynthesis as starch.

Part 3 of 6Thylakoids and grana

Thylakoids and grana
Thylakoids
  • Thylakoids a separate system of flattened, fluid-filled sacs in the stroma.
Membrane contents
  • Membrane contents pigments, enzymes and electron carriers for the light-dependent reactions.
Function
  • Function site of the light-dependent stage of photosynthesis.
Grana
  • Grana stacks of thylakoids.
    • (singular granum)
Large surface area
  • Large surface area the grana membranes allow many light-dependent reactions.
Pigment arrangement
  • Pigment arrangement many pigment molecules are arranged to capture as much light as needed.

Part 4 of 6Stroma lamellae

Stroma lamellae
Structure
  • Structure membranous channels that connect the grana.
Function
  • Function keep the stacks connected but distanced from each other.

Part 5 of 6Compartmentalisation

RegionStage of photosynthesisReason
Thylakoid membranes and spaceLight-dependentHold pigments, electron carriers and ATP synthase
StromaLight-independentHold Calvin cycle enzymes and CO2
  • Separation
    • compartments separate the two sets of reactions so each has its own conditions.

Part 6 of 6More from the specification

More from the specification
Photosystem Excitation and Chlorophyll Pigments
  • Photons absorbed by accessory pigments in light-harvesting complexes transfer excitation energy to reaction centre chlorophyll a molecules within photosystem II (P680) and photosystem I (P700), releasing high-energy electrons.
Non-Cyclic Photophosphorylation and Electron Transport
  • High-energy electrons are passed along a membrane-bound electron transport chain from photosystem II to photosystem I, pumping hydrogen ions across the thylakoid membrane to drive ATP synthesis via chemiosmosis.
Photolysis of Water at PSII
  • An enzyme complex associated with photosystem II catalyses the photolysis of water, supplying replacement electrons for P680 and releasing protons and oxygen.
    • (2H2O → 4H+ + 4e- + O2)
Reduction of NADP
  • Electrons exiting photosystem I are accepted by ferredoxin and passed to NADP+ along with stroma protons to form reduced NADP (NADPH), supplying reducing power for carbohydrate synthesis in the stroma.
Cyclic photophosphorylation
  • ATP formation using only PSI with electrons returning to it.
Non-cyclic photophosphorylation
  • ATP formation using PSII and PSI with electrons ending in NADP.
Triose phosphate
  • Three-carbon sugar phosphate made from GP.
Photosystem
  • A cluster of pigments and proteins that harvest light.
Photophosphorylation
  • Making ATP using light energy, an electron transport chain and chemiosmosis.
Reduced
  • Having gained electrons
Chlorophyll
  • A green pigment in chloroplasts that transfers energy from light into chemical energy.
Photolysis
  • Splitting of water by light at PSII.

Key terms

TermMeaning
ChloroplastOrganelle where photosynthesis occurs.
StromaFluid inside the chloroplast where the Calvin cycle occurs.
ThylakoidFlattened membrane sac that holds photosynthetic pigments.
GranumStack of thylakoids.
Stroma lamellaeMembrane channels linking grana.
Starch grainStore of starch in the stroma.

Avoid these mistakes

1
Saying the grana are in the stroma only — thylakoids form grana, and the stroma is the fluid around them.
2
Mixing up the sites of the two stages — light-dependent in thylakoids, light-independent in stroma.
3
Forgetting the chloroplast has its own DNA and 70S ribosomes — it makes some of its own proteins.
4
Calling the sacs stroma — the sacs are thylakoids.

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