Eukaryotic Cell Ultrastructure: Chloroplasts, Plasma Membrane and Centrioles

Spec Mapping — OCR H420 Module 2.1.1 — Cell structure, content statements covering the ultrastructure of chloroplasts, the plasma membrane and centrioles, and relating each structure to function (refer to the official OCR H420 specification document for exact wording). Chloroplast architecture is the structural foundation for the photosynthesis content in Module 5.2 and is reliably examined synoptically.

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.

Three intellectual histories converge here. Lynn Margulis (1967) extended endosymbiotic theory from mitochondria to chloroplasts, arguing that chloroplasts are descendants of free-living cyanobacteria engulfed by an ancestral eukaryote — supported by 70S ribosomes, circular chloroplast DNA, and double membranes. Seymour Singer and Garth Nicolson (1972) proposed the fluid mosaic model of the plasma membrane based on freeze-fracture electron-microscopy evidence — a paradigm that displaced the earlier "sandwich" models and remains the textbook standard. Theodor Boveri (early 1900s) recognised the role of centrosomes in chromosome segregation; the 9 × 3 microtubule arrangement of centrioles was later resolved by electron microscopy.

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SVG cross-section: chloroplast with grana

The diagram shows four grana as stacks of thylakoid discs (drawn here as ~6 thylakoids per stack, but in vivo 10–20 per stack), interconnected by intergranal lamellae. The thylakoid lumens of all stacks are continuous, forming a single proton-accumulation compartment. Embedded in the thylakoid membranes (not separately visible at this scale) are PS I and PS II with their chlorophyll-containing antenna complexes — the photosynthetic machinery.

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

• Phospholipid bilayer — hydrophilic phosphate heads face outwards (towards water) and hydrophobic fatty acid tails face inwards (away from water). This bilayer is the structural backbone and acts as a barrier to polar and large molecules.
• Cholesterol (in animal cells only) — wedge-shaped molecules interspersed between phospholipids, affecting membrane fluidity. Cholesterol stabilises the membrane at high temperatures (reducing fluidity) and maintains flexibility at low temperatures (preventing solidification).
• Integral (intrinsic) proteins — span the bilayer fully. Include channel proteins, carrier proteins, and some receptor proteins.
• Peripheral (extrinsic) proteins — attached to one surface of the bilayer. Include some enzymes, cytoskeletal attachment sites, and parts of signalling systems.
• Glycoproteins and glycolipids — proteins or lipids with attached carbohydrate chains, exposed on the outer surface. Involved in cell recognition, cell adhesion, and receptor binding.

Functions of the Plasma Membrane

1. Selective barrier — controls entry and exit of substances via diffusion, osmosis, facilitated diffusion, active transport, endocytosis, and exocytosis.
2. Cell recognition — glycoproteins (e.g., ABO blood group antigens, MHC proteins) identify self vs non-self.
3. Cell signalling — hormone receptors detect chemical signals and trigger intracellular responses.
4. Cell adhesion — glycoproteins and glycolipids help cells stick to each other and to the extracellular matrix.
5. Site of enzyme activity — some membrane-bound enzymes (e.g., digestive enzymes on intestinal microvilli) function directly at the membrane.

Fluidity and Temperature

At low temperatures, membranes become less fluid (fatty acid chains pack tightly, decreasing movement). At high temperatures, fluidity increases and the membrane becomes leakier. Unsaturated fatty acids (with kinks in their tails) prevent tight packing, keeping membranes fluid at low temperatures — this is why plants and cold-water fish have high proportions of unsaturated lipids.

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Centrioles

Centrioles are cylindrical, non-membrane-bound organelles found in most animal cells and some protoctist and lower plant cells. They occur in pairs, at right angles to one another, within a region called the centrosome.

Structure

• Each centriole is composed of nine triplets of microtubules arranged in a ring — the classic "9 × 3" arrangement.
• Each centriole is roughly 0.4 µm long and 0.2 µm wide.
• A pair of centrioles forms a centrosome, the main microtubule organising centre (MTOC) of the animal cell.

Functions

1. Organise spindle formation during cell division — before mitosis, the pair of centrioles duplicates, and the two new pairs migrate to opposite poles of the cell. From them, spindle microtubules grow, attach to centromeres of chromosomes at the kinetochore, and pull sister chromatids apart during anaphase.
2. Organise microtubule arrays during interphase, including those of the cytoskeleton.
3. Form basal bodies of cilia and flagella — centrioles can become embedded at the base of these motile appendages and template the "9 + 2" microtubule arrangement of the axoneme.

Note on Plant Cells: Higher plant cells lack centrioles. They still form a spindle during mitosis, but its microtubules are organised from diffuse MTOCs distributed near the poles of the cell.

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The fluid mosaic model — Singer and Nicolson 1972

The currently accepted model of biological membranes was proposed by Seymour Jonathan Singer and Garth Nicolson in 1972, displacing earlier "sandwich" models that pictured proteins as flat layers on either side of the lipid bilayer. The Singer-Nicolson model has three key claims, paraphrased as:

1. Phospholipids self-assemble into a bilayer with hydrophobic tails inward and hydrophilic heads outward (the lipid mosaic).
2. Proteins are embedded within or attached to the bilayer, distributed asymmetrically and laterally mobile (the protein mosaic).
3. The membrane is fluid — lipids and proteins diffuse laterally on millisecond timescales, though "flip-flop" between leaflets is rare without specific flippase enzymes.

The evidence for this model came from freeze-fracture electron microscopy, which split the bilayer along its hydrophobic interior and revealed integral membrane proteins as bumps embedded in the membrane interior — not as a smooth protein layer. Singer and Nicolson's interpretation has been confirmed and refined by subsequent fluorescence-recovery-after-photobleaching (FRAP), single-particle tracking, and X-ray crystallography of integral membrane proteins.

Mark-scheme literacy: the named authors (Singer and Nicolson), the year (1972), the words "fluid mosaic", and the lateral-mobility evidence are all reliable mark-scheme bait in extended-response questions on membrane structure.

Membrane fluidity and temperature adaptation

Cell membranes must remain fluid enough for lateral diffusion of lipids and proteins, but not so fluid as to leak ions. Organisms adapt their lipid composition to temperature: