Structural Organization of Chloroplasts - Page-by-Page Notes

ENVELOPE OR OUTER MEMBRANE
- Semi-porous; permeable to small molecules/ions; restricts larger proteins.
- Barrier between organelle and cytoplasm.
- Lipid composition (outer membrane): roughly $46\%\text{ galactolipids}$ and $7\%\text{ sulfolipids}$ (note: other lipids make up the rest).

Primary functions of the outer membrane:
- Import of nuclear-encoded proteins.
- Diffusion of low-molecular-weight compounds and ions.
- Site for lipid biosynthesis.

INTERMEMBRANE SPACE
- Very thin: $10-20\,\text{nm}$ between outer and inner membranes.
- Generally considered to have no specific function.

INNER MEMBRANE
- Forms border to the stroma; regulates transport in/out of the chloroplast.
- Also site for synthesis of fatty acids, lipids, and carotenoids.
- Highly specialized with transport proteins.
- Composition (spinach chloroplasts): $\approx 16\%\text{ phospholipids}, \; 79\%\text{ galactolipids}, \; 5\%\text{ sulfolipids}.$

Other inner membrane roles:
- Synthesis of various metabolites.
- Involvement in chloroplast organelle division.
Note: inner membrane is heavier than the outer membrane.

STROMA
- Alkaline, aqueous, protein-rich fluid inside the inner membrane.
- Contains chloroplast DNA, chloroplast ribosomes, thylakoid system, starch granules, and many proteins.
- Structure: outer membrane + inner membrane network forming grana; grana connected by lamellae.

STROMA functions
- Hosts carbon fixation enzymes; involved in chloroplast stress responses and inter-organelle signaling.
- Site for both light-dependent and light-independent (Calvin cycle) reactions.
- Light-dependent products (ATP, NADPH) are produced in the thylakoids and stored in the stroma for the Calvin cycle.

LIGHT-INDEPENDENT REACTION (Calvin Cycle) in the stroma
- Takes CO2 and reduces it to carbohydrate using ATP and NADPH from the light reactions.
- Key steps: carbon fixation, reduction, regeneration of RuBP.
- End product: triose phosphates (e.g., G3P) used to synthesize sugars (glucose) or starch.

Calvin cycle details (high level)
- CO2 fixation and reduction powered by ATP and NADPH from light reactions.
- Overall purpose: convert inorganic CO2 into carbohydrate in the stroma.
- Link to Atmospheric CO2 capture via stomata and production of sugars.

CALVIN CYCLE PATHWAY (high-level map)
- Phase 1: Fixation of CO2 to form 3-phosphoglycerate (3-PGA).
- Phase 2: Reduction of 3-PGA to glyceraldehyde-3-phosphate (G3P) using NADPH.
- Phase 3: Regeneration of RuBP from G3P for continued CO2 fixation; net production of sugar precursors (e.g., G3P) used to form sucrose or starch.
- For 3 CO2, approx. consumption involves ATP and NADPH to drive the cycle; the G3P pool is used for carbohydrate synthesis.

THYLAKOID SYSTEM
- Suspended in the stroma as membranous sacs called thylakoids.
- Chlorophyll resides in thylakoids; light reactions occur here.
- Thylakoids are arranged into grana (stacks); each granum contains ~10-20 thylakoids.
- Grana models mentioned: various structural interpretations of stacking (grana with helical/paired arrangements).

THYLAKOID STRUCTURE
- Composed of lipid bilayers; encloses the thylakoid lumen (a large aqueous space).
- Two thylakoid types: granal thylakoids (in grana) and stromal thylakoids (in contact with stroma).
- Light-dependent reactions occur here, producing ATP and NADPH.

THYLAKOID & PHOTOSYSTEMS
- Light reactions require light energy to proceed.
- Photosystems: PSI (P700) and PSII (P680).
- Chlorophyll excitation drives electron flow through the electron transport chain, generating ATP and NADPH; water is split to release O2.

LIGHT DEPENDENT REACTIONS (summary)
- Overall equation (oxygenic photosynthesis):
  $2\,\text{H}<em>2\text{O} + 2\,\text{NADP}^+ + 3\,\text{ADP} + 3\,\text{P}</em>i \rightarrow \text{O}_2 + 2\,\text{NADPH} + 3\,\text{ATP}$
- Key players: ferredoxin-NADP+ reductase (FNR), NADP+/NADPH, ATP synthase, PS I (P700) and PS II (P680), plastoquinone (PQ), plastocyanin (PC), oxygen-evolving complex (OEC).
- Location: thylakoid membranes and lumen.

PERIPHERAL RETICULUM
- Some plants contain an additional set of membranous tubules from the inner membrane of the envelope.
- Tiny vesicles bud from the inner membrane and assemble into peripheral reticulum tubules.

PLASTOGLOBULI
- Spherical lipid-protein bodies (~ $45-60\,\text{nm}$ ) surrounded by a lipid monolayer.
- Contain enzymes and proteins for lipid synthesis/metabolism and various lipids (e.g., plastoquinone, vitamin E, carotenoids, chlorophyll).

RIBOSOMES
- Chloroplasts possess their own ribosomes (~two-thirds the size of cytoplasmic ribosomes).
- Size reference: ~ $17\,\text{nm}$ (chloroplast) vs ~ $25\,\text{nm}$ (cytoplasmic).
- They translate mRNAs transcribed from chloroplast DNA into proteins.

STARCH GRANULES
- Common in chloroplasts; occupy roughly $15\%$ of chloroplast volume.
- Form and grow during the day; consumed at night for respiration and sugar export.
- In mature chloroplasts, complete consumption of a starch granule is rare.

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Reminder: Key terms to review for quick recall: outer membrane, inner membrane, intermembrane space, stroma, thylakoids, grana, chloroplast DNA, ribosomes, plastoglobuli, peripheral reticulum, plastoglobuli, starch granules, photosystems, light-dependent reactions, Calvin cycle.