# Eukaryotic cell ultrastructure

> CIE A-Level Biology · Unit 1: Cell Structure
> Source: https://www.owlsprep.com/study/cie-9700-u1-eukaryotic-cell-ultrastructure/

This sub-topic covers the fine detailed ultrastructure of eukaryotic cells, including the structure and function of all major membrane-bound and non-membrane bound organelles, and key differences between animal, plant, fungal and prokaryotic cells.

**Prerequisites:** [Basic cell structure and light microscopy](https://www.owlsprep.com/study/cie-9700-u1-basic-cell-structure/)

## Learning objectives

- Identify key eukaryotic organelles from electron micrographs of cell ultrastructure
- Relate the structure of each organelle to its specific function in the cell
- Distinguish between animal, plant, and fungal eukaryotic cell ultrastructure
- Compare key differences between eukaryotic and prokaryotic cell ultrastructure

## What is eukaryotic cell ultrastructure?

Eukaryotic cells are larger than prokaryotic cells, and contain a range of specialised membrane-bound organelles and other sub-cellular structures. Ultrastructure refers to the detailed fine structure of these structures, which can only be resolved using an electron microscope with magnification greater than 100,000×.

**Ultrastructure** — The fine, detailed structure of biological cells and tissues that can only be observed using an electron microscope, which has higher resolving power than a standard light microscope.

*Example:* The internal cristae of mitochondria can only be seen as part of eukaryotic cell ultrastructure.

**Worked example:** Explain why eukaryotic cell ultrastructure cannot be observed with a standard light microscope.

1. The maximum resolution of a standard light microscope is approximately 0.2 μm.
2. Most eukaryotic organelles (e.g. ribosomes, lysosomes, mitochondrial cristae) are smaller than 0.2 μm.
3. Light microscopes cannot resolve the fine detail of these small structures, so they can only be visualised using an electron microscope.

## Structure and function of major eukaryotic organelles

All eukaryotic cells share core organelles, each adapted to carry out a specific role. CIE exam questions almost always require you to link an organelle's structure directly to its function to gain full marks.

**Organelle** — A specialised sub-cellular structure that performs a specific function within the cell; most eukaryotic organelles are membrane-bound, separating their internal environment from the cytoplasm.

**Worked example:** Explain how the structure of a mitochondrion is adapted to its role in aerobic respiration.

1. Mitochondria are the site of aerobic respiration, which produces ATP (the cell's energy currency) for cellular processes.
2. Mitochondria have a double membrane: the outer membrane is permeable to small molecules like pyruvate, the starting molecule for aerobic respiration.
3. The inner membrane is folded into finger-like projections called cristae, which greatly increases its surface area.
4. The increased surface area provides more space for the electron transport chain and ATP synthase enzymes that produce ATP via oxidative phosphorylation.
5. The internal gel-like matrix contains all the enzymes required for the Krebs cycle stage of aerobic respiration.

> **tip**
>
> Always explicitly connect a structural feature to its functional benefit in exam answers. Don't just list features – explain why they matter.

## Comparing animal, plant and fungal eukaryotic cells

While all eukaryotic cells share core features, there are key structural differences between the main kingdoms that are regularly tested in CIE exams.

| Feature | Animal cell | Plant cell | Fungal cell |
| --- | --- | --- | --- |
| Cell wall | Absent | Present (cellulose) | Present (chitin) |
| Chloroplasts | Absent | Present (photosynthetic cells) | Absent |
| Large permanent vacuole | Absent | Present | Present |
| Centrioles | Present | Absent (most higher plants) | Absent (most fungi) |
| Storage carbohydrate | Glycogen | Starch | Glycogen |

**Worked example:** An unknown eukaryotic cell has a cell wall, 80S ribosomes, and no chloroplasts. Identify what type of eukaryotic cell this is, with reasoning.

1. Eliminate animal cells first: animal cells never have a cell wall, so this cannot be an animal cell.
2. Eliminate photosynthetic plant cells: photosynthetic plant cells with a cell wall and large vacuole always contain chloroplasts.
3. Fungal cells are eukaryotic, have a cell wall made of chitin, do not photosynthesise so have no chloroplasts, and have 80S ribosomes.
4. The unknown cell matches all features of a fungal eukaryotic cell.

## Eukaryotes vs prokaryotes: key ultrastructure differences

CIE exams regularly ask to compare the ultrastructure of eukaryotic and prokaryotic cells, so it is important to memorise the key distinguishing features.

**Worked example:** Give three key differences between the ultrastructure of a prokaryotic cell and a eukaryotic animal cell.

1. 1. Prokaryotic cells have no membrane-bound nucleus, with circular DNA free in the cytoplasm. Eukaryotic cells have a membrane-bound nucleus containing linear chromosomes.
2. 2. Prokaryotic cells have smaller 70S ribosomes, while eukaryotic cells have larger 80S ribosomes.
3. 3. Prokaryotic cells have no membrane-bound organelles (e.g. no mitochondria, no Golgi apparatus), while eukaryotic cells contain specialised membrane-bound organelles.

**Check your understanding**

Check your understanding:

1. Which of the following features is unique to eukaryotic cells?

   - 70S ribosomes
   - Cell wall
   - Membrane-bound nucleus
   - Circular DNA

   *Why:* Membrane-bound nuclei are only found in eukaryotes. 70S ribosomes and circular DNA are unique to prokaryotes, and cell walls are found in both prokaryotes and many eukaryotes (plants, fungi).

## Common pitfalls

- **Wrong:** Confusing 70S and 80S ribosome size in eukaryotes vs prokaryotes
  - Why it fails: Students often associate eukaryotes with smaller cells and mix up the ribosome sizes
  - Correct: Remember: Eukaryotes are larger, so they have larger 80S ribosomes. Prokaryotes are smaller, so they have smaller 70S ribosomes.
- **Wrong:** Claiming plant cells do not have mitochondria because they have chloroplasts
  - Why it fails: Students incorrectly assume only photosynthesis occurs in plant cells, forgetting all cells need energy
  - Correct: All aerobic eukaryotic plant cells have mitochondria to produce ATP via aerobic respiration, even non-photosynthetic cells.
- **Wrong:** Mixing up the function of rough and smooth endoplasmic reticulum
  - Why it fails: Students often forget which type of ER synthesises which molecule
  - Correct: Rough ER has bound ribosomes, so it synthesises proteins. Smooth ER has no ribosomes, so it synthesises lipids.
- **Wrong:** Claiming all eukaryotic cells have a nucleus
  - Why it fails: Students forget that some specialised eukaryotic cells lose their nucleus during development
  - Correct: Most eukaryotic cells have a nucleus, but exceptions include mature mammalian red blood cells, which lose their nucleus to carry more oxygen.
- **Wrong:** Confusing the cell wall with the cell surface membrane in plant cells
  - Why it fails: Students mix up the location and properties of the two outer structures
  - Correct: The cell wall is located outside the selectively permeable cell surface membrane, and is freely permeable to most small molecules.

## Cheatsheet

| Organelle | Key structure | Core function |
| --- | --- | --- |
| Nucleus | Double envelope, linear DNA, nucleolus | Stores genetic material, makes ribosomes |
| Mitochondrion | Double membrane, cristae, matrix | Aerobic respiration, produces ATP |
| Rough ER | Membrane network with bound ribosomes | Synthesis and transport of proteins |
| Smooth ER | Membrane network no ribosomes | Synthesis of lipids and steroids |
| Golgi apparatus | Stack of flattened membrane sacs | Modifies and packages proteins for secretion |
| Lysosome | Single-membrane vesicle | Contains digestive enzymes for breakdown |
| Ribosome (eukaryote) | Non-membrane bound, 80S | Site of protein synthesis (translation) |
| Chloroplast | Double membrane, grana, stroma | Site of photosynthesis in plant cells |
| Large central vacuole | Tonoplast membrane, cell sap | Maintains turgor, stores nutrients |

## What's next

Understanding eukaryotic cell ultrastructure is the foundation for all subsequent topics in CIE A-Level Biology, from cell division to membrane transport and cell signalling. You will build on this knowledge when studying how organelles work together to produce and secrete proteins, how cells divide for growth and reproduction, and how prokaryotic cells are adapted to their unique environments. This knowledge is also essential for interpreting electron micrographs, which is a regularly assessed skill in both multiple-choice and structured questions across all CIE A-Level Biology papers.

- [Prokaryotic cell structure](https://www.owlsprep.com/study/cie-9700-u1-prokaryotic-cell-structure/)
- [Virus structure](https://www.owlsprep.com/study/cie-9700-u1-virus-structure/)
- [Comparison of eukaryotic and prokaryotic cells](https://www.owlsprep.com/study/cie-9700-u1-comparison-of-eukaryotic-and-prokaryotic/)

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