◯ A single DNA loop
The main genetic material is one long loop of DNA. It is not enclosed in a nucleus.
Cell Structure » Eukaryotes and Prokaryotes
What you'll learn this session
Study time: 30 minutes
AQA spec: 4.1.1.1
- The difference between eukaryotic and prokaryotic cells
- What is inside a bacterial cell, including the DNA loop and plasmids
- How big cells are, and the prefixes centi, milli, micro and nano
- How to use standard form and order of magnitude
Two kinds of cell
All living things are made of cells. Cells come in two main types, and the main difference is that eukaryotic cells keep their genetic material in a nucleus and prokaryotic cells do not.
Eukaryotic cells are the cells that make up plants and animals. Each one has a cell membrane, cytoplasm and genetic material that is enclosed in a nucleus.
Prokaryotic cells are bacterial cells. They are much smaller than plant and animal cells. They have cytoplasm and a cell membrane, and these are surrounded by a cell wall. Their genetic material is not enclosed in a nucleus.
Key terms:
- Eukaryotic cell: a cell whose genetic material is enclosed in a nucleus. Plant and animal cells are eukaryotic.
- Prokaryotic cell: a cell with no nucleus. Bacterial cells are prokaryotic.
- Cell membrane: the thin layer around the outside of the cell.
- Cytoplasm: the jelly-like substance inside the cell.
- Nucleus: the part of a eukaryotic cell that encloses the genetic material.
- Cell wall: the outer layer that surrounds the cell membrane and cytoplasm in a bacterial cell.
Inside a bacterial cell
A bacterial cell is simple compared with an animal cell. Its genetic material is not wrapped in a nucleus. It comes in two forms:
○ Plasmids
There may be one or more small rings of DNA called plasmids. A cell can have none, one or many.
Key terms:
- DNA loop: the single circular piece of DNA that makes up most of a bacterial cell's genetic material.
- Plasmid: a small ring of DNA found in a bacterial cell, in addition to the DNA loop.
Comparing the two cell types
● Eukaryotic (plant and animal)
Cell membrane and cytoplasm. Genetic material enclosed in a nucleus. Much larger than bacterial cells.
■ Prokaryotic (bacteria)
Cytoplasm and cell membrane inside a cell wall. No nucleus. A single DNA loop and maybe plasmids. Much smaller.
Common mistakes
Saying that bacteria have "no DNA". They do have DNA. It is just not inside a nucleus. Another mistake is writing that plasmids are the main genetic material. The main genetic material is the single DNA loop, and plasmids are extra small rings.
The size of cells
Cells are tiny, so we need very small units to measure them. Most animal cells are a few tens of micrometres across. Bacterial cells are much smaller, often only a few micrometres long. This is why you need a microscope to see them.
These prefixes help us to write sizes in a sensible way:
- centi (cm) means one hundredth. There are 100 cm in 1 m.
- milli (mm) means one thousandth. There are 1000 mm in 1 m.
- micro (µm) means one millionth. There are 1000 µm in 1 mm.
- nano (nm) means one thousand millionth. There are 1000 nm in 1 µm.
Each step down from millimetre to micrometre to nanometre makes the unit 1000 times smaller. To change to a smaller unit, multiply by 1000. To change to a bigger unit, divide by 1000.
Worked example
A cell is 0.05 mm long. How long is it in micrometres?
There are 1000 µm in 1 mm, so multiply by 1000.
0.05 × 1000 = 50 µm.
Standard form
Very small numbers have lots of zeros, which is easy to get wrong. Standard form is a shorter way to write them: A × 10n, where A is a number between 1 and 10, and n is a whole number (the power of ten).
- Large numbers have a positive power: 4 500 000 = 4.5 × 106
- Small numbers have a negative power: 0.000032 = 3.2 × 10-5
Worked example
A bacterial cell is 2 µm long. Write this in metres in standard form.
1 µm = 0.000001 m = 1 × 10-6 m.
2 µm = 2 × 10-6 m.
Worked example
A cell is 25 µm long. Write this in metres in standard form.
25 × 10-6 m = 2.5 × 10-5 m.
The number in front must be between 1 and 10, so 25 becomes 2.5 and the power goes up by one.
Order of magnitude
An order of magnitude is a size or difference written as a power of ten. One order of magnitude bigger means about 10 times bigger. Two orders of magnitude means about 100 times bigger. This lets you compare sizes quickly without exact numbers.
Worked example
A plant cell is about 100 µm long. A bacterial cell is about 1 µm long. How many orders of magnitude bigger is the plant cell?
100 µm ÷ 1 µm = 100 times bigger.
100 = 102, so the plant cell is two orders of magnitude bigger.
Key terms:
- Standard form: a way of writing very large or very small numbers as A × 10n, where A is between 1 and 10.
- Order of magnitude: a size or difference written as a power of ten.
Common mistakes
Getting the direction wrong when you change units. Going from mm to µm gives a bigger number (multiply by 1000), because µm are smaller units. Also, writing 25 × 10-6 and calling it standard form. The front number must be between 1 and 10.
Exam-style question
A bacterial cell and an animal cell are compared.
(a) State two features that a bacterial cell has that an animal cell does not have. [2 marks]
(b) A bacterial cell is 3 µm long. Write this length in metres in standard form. [2 marks]
(c) The animal cell is 30 µm long. How many orders of magnitude bigger is the animal cell than the bacterial cell? [2 marks]
Model answer
(a) Any two from: a cell wall (1); a single DNA loop (1); plasmids (1).
(b) 3 µm = 3 × 0.000001 m (1) = 3 × 10-6 m (1).
(c) 30 ÷ 3 = 10 times bigger (1), so one order of magnitude (1).
Exam tip
In part (b), check that the front number is between 1 and 10 and that the power is negative, because a micrometre is a very small unit.
Access the full course and all the resources like custom timetables and progress tracking.
Join Today