2.1.1(g)-(h) - Eukaryotic cell ultrastructure and micrographs
This lesson builds a usable map of eukaryotic cell components and then shows how to interpret electron micrographs without guessing. The key habit is structure-function reasoning: identify what is visible, then connect it to a precise function.
Ultrastructure Meaning
The diagram compares an animal cell with a photosynthetic plant cell. Relative positions vary in real cells; it is a model for locating components, not a micrograph.
[DIAGRAM: eukaryotic_components: Lesson 015: Eukaryotic cell components; asset_slug: 015_m02_1_1_eukaryotic_cell_ultrastructure_and_photomicrographs__diagram_01; recommended_method: drawn_biology; description: Labelled animal and photosynthetic plant cells, with double nuclear envelopes, rough ER, Golgi, mitochondria and plant wall and chloroplasts.]
Ultrastructure means cell detail that is usually seen with an electron microscope rather than a light microscope. It includes small organelles and membrane systems that cannot be resolved clearly with ordinary light microscopy.
Eukaryotic cells contain membrane-bound organelles. Plant and animal cells share many components, but plants also have structures such as a cellulose cell wall and chloroplasts. Some structures, such as ribosomes, are not membrane-bound but are still part of the required eukaryotic cell content.
Use the structural features in the next table to distinguish the components. A name and function alone are not enough to recognise an organelle in an image.
Organelle Functions
Structure makes the function possible. Nuclear pores allow RNA and proteins to move between nucleus and cytoplasm while the DNA remains enclosed. Rough ER provides a compartment for folding proteins; Golgi sacs modify and sort cargo. Lysosomal membranes keep hydrolytic enzymes separate from the cytoplasm. Mitochondrial cristae provide a large membrane area for ATP production, while chloroplast thylakoids provide membrane area for the light-dependent reactions of photosynthesis. A cell that secretes protein therefore needs a different abundance of organelles from a photosynthetic cell.
Plant cells also contain mitochondria: respiration is needed even when photosynthesis cannot occur. Chloroplasts occur in photosynthetic plant tissues, not every plant cell. Fungal cell walls contain chitin rather than cellulose. Centrioles, ribosomes and the nucleolus have no surrounding membrane; an organelle need not be membrane-bound.
Learn each component by asking what job it makes possible.
| Component | Structural feature | Outline function |
|---|---|---|
| Nucleus | Contains chromatin within a nuclear envelope | Contains DNA and controls gene expression/cell activities |
| Nucleolus | Dense region of RNA and protein inside the nucleus; no surrounding membrane | Makes ribosomal RNA and assembles ribosome subunits |
| Nuclear envelope | Two membranes with nuclear pores | Separates the nucleus from cytoplasm and allows controlled exchange |
| Rough ER | Membrane sacs with ribosomes on their outer surface | Folds and transports proteins made on attached ribosomes |
| Smooth ER | Membrane tubules without attached ribosomes | Makes lipids and helps with detoxification in some cells |
| Golgi apparatus | Stack of flattened membrane sacs, with associated vesicles | Modifies, sorts and packages proteins or lipids into vesicles |
| Ribosomes | Small RNA-protein particles with two subunits; not membrane-bound | Site of protein synthesis |
| Mitochondria | Double membrane; inner membrane folded into cristae around a matrix | Site of aerobic respiration and ATP production |
| Lysosomes | Single-membrane compartments containing digestive enzymes | Breakdown of material inside the cell |
| Chloroplasts | Double envelope; internal thylakoid membranes, often stacked into grana, in a stroma | Site of photosynthesis in suitable plant cells |
| Plasma membrane | Phospholipid bilayer containing proteins | Controls movement into and out of the cell |
| Centrioles | Cylinders made of microtubules | Involved in spindle organisation during cell division in animal cells |
| Cell wall | Cellulose fibres outside the plasma membrane in plant cells | Provides support and shape |
| Flagella | Longer membrane-covered projections containing microtubules in eukaryotes | Can move a cell |
| Cilia | Shorter membrane-covered projections containing microtubules in eukaryotes | Can move fluid or material over a cell surface |
For a one-mark organelle identification, give one clear name. Two contradictory names usually lose the precision that mark schemes expect.
Membranes And Non-Membranes
Many eukaryotic organelles are membrane-bound, including the nucleus, ER, Golgi apparatus, mitochondria, lysosomes and chloroplasts. Membranes separate reactions into compartments and help control conditions inside each organelle.
Ribosomes are not membrane-bound. A common error is to call every visible dot in an electron micrograph a vesicle or a lysosome. Ribosomes are small dense particles and may appear free in the cytoplasm or attached to rough ER.
The plasma membrane is not just a boundary line. It controls movement into and out of the cell. The plant cell wall lies outside the plasma membrane and gives support; it is not the membrane controlling exchange.
Interpreting Photomicrographs
Photomicrographs are microscope images. In this lesson, the important OCR skill is interpreting TEM and SEM images.
[DIAGRAM: photomicrograph_interpretation_cues: Lesson 015: Photomicrograph cues for OCR electron images - diagram 02; asset_slug: 015_m02_1_1_eukaryotic_cell_ultrastructure_and_photomicrographs__diagram_02; recommended_method: drawn_biology; description: Schematic mitochondrial cristae and chloroplast grana and lamellae, showing diagnostic internal features visible in suitable thin sections. Shapes and positions vary in actual micrographs.]
This is a schematic cue sheet, not a genuine electron micrograph. Use it to compare image types; identify real organelles only from the evidence in the actual image or supplied description.
TEM images show internal structure in thin sections. A mitochondrion often appears as an oval or elongated structure with internal folds called cristae. A lysosome can look like a small vesicle-like body, but absence of cristae alone does not establish that identification. Other vesicles can also lack internal folds, and a section may miss a structure. Additional evidence, such as digestive-enzyme function, supports a lysosome identification.
SEM images show surface features. They may show surface projections such as cilia, but they do not show the internal arrangement of organelles.
Electron micrographs are prepared evidence. Sections may cut through only part of an organelle, false colour may be added, and absence from one image is not proof that the cell lacks a structure.
Scale And Standard Form
Cell images often use very small measurements. Exact organelle sizes are not a recall requirement here, but you must be comfortable reading ordinary and standard form values when a question supplies them.
For example:
because one micrometre is metres.
Using A Scale Bar
A TEM scale bar is labelled . A mitochondrion is three scale-bar lengths long.
Actual length:
This is 3.0 micrometres.
Quick Recap
Identify the image type and visible diagnostic features before naming a component or linking it to function. Interpret the scale, preparation and limits of the evidence.