Eukaryotic Cell Ultrastructure: Chloroplasts, Plasma Membrane and Centrioles

This lesson completes the detailed tour of eukaryotic organelles by examining chloroplasts (the site of photosynthesis in plants and algae), the plasma membrane, and the centrioles. These structures are vital for energy capture, controlled exchange with the environment, and cell division. You should be able to describe each in precise ultrastructural terms and link structure to function, as required by OCR 2.1.1 (e)(iii).

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Chloroplasts

Chloroplasts are the site of photosynthesis in plants and algae. They are typically 2–10 µm long and shaped like flattened discs (biconvex lenses). A typical plant mesophyll cell contains 20–100 chloroplasts, which move within the cell to optimise light capture.

Ultrastructure

• Double membrane (chloroplast envelope) — the outer membrane is permeable to small molecules; the inner membrane is more selective and contains transport proteins.
• Intermembrane space — small region between outer and inner membranes.
• Stroma — the fluid-filled interior enclosed by the inner membrane, analogous to the mitochondrial matrix. Contains:
  • Enzymes of the light-independent reactions (Calvin cycle), including RuBisCO (the most abundant enzyme on Earth).
  • Chloroplast DNA — a small, circular molecule.
  • 70S ribosomes — synthesise some chloroplast proteins.
  • Starch grains — stored carbohydrate.
  • Lipid droplets — plastoglobuli.
• Thylakoids — a system of flattened membrane-bound sacs within the stroma. This is where the light-dependent reactions take place.
• Grana (singular: granum) — stacks of thylakoids, typically 10–20 thylakoids high. A typical chloroplast contains 40–60 grana.
• Lamellae (intergranal lamellae) — thylakoid membranes that interconnect grana, ensuring the interior thylakoid spaces form a continuous compartment.
• Thylakoid lumen — the space inside the thylakoids, where H⁺ ions accumulate during the light-dependent reactions to form a proton gradient.

The Photosynthetic Pigments

Embedded in the thylakoid membranes are the photosystems (PS I and PS II), each containing chlorophyll a, chlorophyll b, carotenes, and xanthophylls. Chlorophyll absorbs light energy, which drives the excitation of electrons and ultimately the synthesis of ATP and reduced NADP.

Structural Adaptations Relating to Function

• Many stacked thylakoids in grana → large surface area for light absorption by chlorophyll and for the electron transport chain.
• Stroma contains concentrated enzymes for the Calvin cycle → efficient fixation of CO₂ into glucose.
• Double membrane → selective entry of substrates and separation of stroma from cytoplasm.
• Own DNA and ribosomes → rapid synthesis of proteins needed for photosynthesis (consistent with endosymbiotic origin).
• Plastoglobuli and starch grains → storage of lipid and carbohydrate within the organelle.

Chloroplasts vs Mitochondria

Chloroplasts share several features with mitochondria, both being thought to have originated from prokaryotic endosymbionts:

Feature	Mitochondria	Chloroplasts
Number of membranes	Double	Double (plus thylakoid system)
Internal folded membrane	Cristae	Thylakoids arranged in grana
DNA	Circular, own	Circular, own
Ribosomes	70S	70S
Role	Respiration (ATP production)	Photosynthesis (glucose production)
Distribution	Nearly all eukaryotes	Plants, algae, some protists

Exam Tip: When answering questions about chloroplast structure and photosynthesis, always distinguish between the light-dependent reactions (on thylakoid membranes) and the light-independent reactions / Calvin cycle (in the stroma). Locate each stage precisely in the organelle.

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The Plasma Membrane (Cell Surface Membrane)

Every eukaryotic cell is bounded by a plasma membrane — a phospholipid bilayer with embedded proteins acting as a selectively permeable boundary. It is 7–10 nm thick and typically appears as a thin line under light microscopy, or as a trilaminar "railway track" structure under TEM.

The Fluid Mosaic Model

The accepted model of membrane structure is the fluid mosaic model proposed by Singer and Nicolson in 1972:

• Fluid: phospholipids and proteins move laterally within the membrane (though not usually flipping between leaflets without assistance).
• Mosaic: proteins are scattered throughout the bilayer like tiles in a mosaic.

Components