Revision notes for Edexcel A A Level Biology Eukaryotic and prokaryotic cell ultrastructure. Open the guide for explanations and worked examples. Written against the Edexcel A A Level Biology (9BN0) specification, so the content matches what's examinable rather than general Biology background.
Eukaryotic and prokaryotic cell ultrastructure
What you'll learn
- What ultrastructure means, and why electron microscopes are needed to study it.
- The main structures found in eukaryotic cells, including animal and plant cells.
- The main structures found in prokaryotic cells, such as bacteria.
- How to compare cells and calculate actual size from magnification.
Starting point: cells, scale and microscopy
A cell is the basic unit of life. Inside cells are organelles, which are specialised subcellular structures with particular functions. Some organelles are surrounded by membranes, while others are not.
A micrograph is an image produced using a microscope. In this topic, you often interpret micrographs to identify cell structures.
Definition
Ultrastructure
Ultrastructure means the fine internal detail of cells and organelles that is too small to resolve clearly with a light microscope, so it is usually studied using electron microscopy.
Cell structures are very small, so you need to be comfortable with units. Cell sizes are usually measured in micrometres, where 1 μm=1×10−6 m1\ \mu\text{m}=1\times10^{-6}\ \text{m}1 μm=1×10−6 m. Smaller structures, such as membranes and ribosomes, may be measured in nanometres, where 1 nm=1×10−9 m1\ \text{nm}=1\times10^{-9}\ \text{m}1 nm=1×10−9 m.
Magnification and resolution
Magnification tells you how many times larger an image is than the real object. Resolution is the ability to distinguish two close points as separate.
A light microscope can show whole cells and some larger organelles, but an electron microscope uses a beam of electrons and has much higher resolution, so it can reveal ultrastructure. A transmission electron microscope, or TEM, shows internal structures in thin sections. A scanning electron microscope, or SEM, shows surface detail.
M=image sizeactual sizeM=\frac{\text{image size}}{\text{actual size}}M=actual sizeimage sizewhere MMM is magnification. Magnification has no units because it is a ratio.
Example
Calculating actual bacterial cell length
A bacterium measures 36 mm long on an electron micrograph. The magnification is 6000. Find its actual length in micrometres.
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Convert the image size into micrometres so the final answer is in micrometres:
36 mm=36×1000 μm=36 000 μm36\ \text{mm}=36\times1000\ \mu\text{m}=36\,000\ \mu\text{m}36 mm=36×1000 μm=36000 μm -
Rearrange the magnification equation to make actual size the subject:
actual size=image sizeM\text{actual size}=\frac{\text{image size}}{M}actual size=Mimage size -
Substitute the values, carrying the unit through the calculation:
actual size=36 000 μm6000=6.0 μm\text{actual size}=\frac{36\,000\ \mu\text{m}}{6000}=6.0\ \mu\text{m}actual size=600036000 μm=6.0 μm -
Compare with biological sense: 6.0 µm is a little larger than the usual 1 to 5 µm range for bacteria, but still plausible for a relatively long bacterial cell or if the capsule was included in the measurement.
Common Mistake
Magnification is not resolution
Making an image larger does not automatically reveal more detail. Resolution determines whether tiny structures can actually be distinguished.
Eukaryotic cells: compartmentalised cells
Definition
Eukaryotic cell
A eukaryotic cell is a cell with DNA enclosed inside a nucleus and with membrane-bound organelles in the cytoplasm.
Eukaryotic cells include animal cells, plant cells, fungi and protoctists. Protoctists are mostly unicellular eukaryotes, such as Amoeba.
The nucleus contains the cell’s genetic material. DNA, or deoxyribonucleic acid, is the molecule that stores genetic instructions. In eukaryotic cells, DNA is arranged as linear chromosomes inside the nucleus. The nuclear envelope is the double membrane around the nucleus, and nuclear pores allow substances such as messenger RNA to leave the nucleus. The nucleolus makes ribosomal RNA and helps assemble ribosome subunits.
The cytoplasm is the fluid-filled region of the cell where many metabolic reactions occur. The plasma membrane, also called the cell-surface membrane, controls movement of substances into and out of the cell.
Ribosomes are small structures made of RNA and protein; they are the site of protein synthesis. Eukaryotic cytoplasmic ribosomes are described as 80S ribosomes. The S value is a sedimentation value, not a length measurement.
Several eukaryotic organelles are part of the cell’s internal membrane system. The rough endoplasmic reticulum, or rough ER, has ribosomes on its surface and is involved in making and transporting proteins. The smooth endoplasmic reticulum, or smooth ER, lacks ribosomes and is involved in lipid synthesis and detoxification. The Golgi apparatus modifies and packages proteins into vesicles, which are small membrane-bound sacs used for transport. Lysosomes contain digestive enzymes that break down unwanted material.
Mitochondria are the site of aerobic respiration, where energy is transferred to ATP. Plant cells also contain chloroplasts, which carry out photosynthesis. Inside chloroplasts are stacks of membrane discs called grana, where light-dependent reactions occur.
Plant cells have a cellulose cell wall, which strengthens the cell and helps maintain shape. They also usually have a large permanent vacuole, a fluid-filled space surrounded by a membrane called the tonoplast. The vacuole helps maintain turgor pressure, keeping the plant cell firm.
Animal cells do not have a cellulose cell wall or chloroplasts. They often contain centrioles, which help organise microtubules during cell division.
The diagram pulls these eukaryotic structures together for an animal cell and a plant cell.
Key Idea
Compartmentalisation
Eukaryotic cells use membrane-bound organelles to separate different reactions, allowing many processes to happen efficiently in controlled conditions at the same time.
Prokaryotic cells: small cells without a nucleus
Definition
Prokaryotic cell
A prokaryotic cell is a cell with no nucleus and no membrane-bound organelles; its DNA is free in the cytoplasm.
Bacteria are prokaryotes. They are usually smaller than eukaryotic cells, often about 1 to 5 µm long.
A bacterial cell has a plasma membrane and cytoplasm, but it does not have a nucleus, mitochondria, chloroplasts, rough ER or Golgi apparatus.
Instead of a nucleus, a bacterium has a loop of circular chromosomal DNA in a nucleoid region. The nucleoid is not surrounded by a membrane. Many bacteria also contain plasmids, which are small extra loops of DNA. Plasmids may carry genes for useful traits, such as antibiotic resistance.
Prokaryotes have 70S ribosomes, which are smaller than eukaryotic 80S cytoplasmic ribosomes. Most bacterial cells have a cell wall made of peptidoglycan, also called murein. Some have a capsule or slime layer outside the cell wall, which can help protect the cell or help it attach to surfaces.
Some bacteria have pili, which are short hair-like structures involved in attachment and sometimes DNA transfer. Some have one or more flagella, which are long structures used for movement.
This diagram shows the key features you should be able to recognise in a bacterial prokaryotic cell.
Common Mistake
No nucleus does not mean no DNA
Prokaryotic cells still contain DNA. The difference is that their DNA is not enclosed inside a nucleus.
Comparing eukaryotic and prokaryotic cells
| Feature | Eukaryotic cells | Prokaryotic cells |
|---|---|---|
| Typical size | About 10 to 100 µm | About 1 to 5 µm |
| Nucleus | Present | Absent |
| DNA | Linear chromosomes in nucleus | Circular DNA in cytoplasm; plasmids may be present |
| Membrane-bound organelles | Present | Absent |
| Ribosomes | 80S in cytoplasm | 70S |
| Cell wall | Plants have cellulose; fungi have chitin; animals have none | Bacteria have peptidoglycan |
| Examples | Animals, plants, fungi, protoctists | Bacteria |
Common Mistake
Ribosome evidence has exceptions
Mitochondria and chloroplasts contain their own circular DNA and 70S-like ribosomes. To classify a whole cell, focus on whether there is a nucleus and whether membrane-bound organelles are present.
Example
Classifying an unknown cell
An electron micrograph shows a cell that is 2.4 µm long. It has a cell wall, many ribosomes, circular DNA in the cytoplasm and no mitochondria or nucleus. Decide whether it is eukaryotic or prokaryotic.
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Compare the size: 2.4 µm fits the usual bacterial range, while most eukaryotic cells are larger. Size supports “prokaryotic”, but it is not enough on its own.
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Use the DNA evidence: circular DNA in the cytoplasm means the DNA is not enclosed by a nuclear envelope. This strongly supports “prokaryotic”.
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Check for membrane-bound organelles: no mitochondria and no nucleus are consistent with a prokaryotic cell. The cell wall alone is not decisive because plants, fungi and bacteria can all have cell walls.
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Conclude that the cell is prokaryotic, most likely bacterial, because it lacks a nucleus and membrane-bound organelles and has circular DNA in the cytoplasm.
Tip
Fast comparison test
If you are stuck, ask: “Is the DNA inside a nucleus?” If yes, it is eukaryotic. If no, and there are no membrane-bound organelles, it is prokaryotic.
Exam technique
In the exam
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Use nucleus and membrane-bound organelles as the decisive eukaryote/prokaryote distinction, not just the presence of a cell wall.
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In magnification questions, convert units before substituting into the equation, and remember that magnification has no unit.
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For structure-function questions, name the structure precisely and link it to its role, such as rough ER having ribosomes for protein synthesis and transport.
Self review
Check yourself
- Which structures would you expect in a plant cell but not an animal cell?
- Why can electron microscopes reveal ultrastructure that light microscopes cannot?
- A cell has circular DNA, plasmids and 70S ribosomes but no nucleus — what type of cell is it, and why?
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