Eukaryotes and prokaryotes

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Every living thing is made of cells.

A cell is the smallest unit that can carry out all the processes of life: it takes in substances, releases energy, grows and copies itself.

Some organisms, such as bacteria, are just one cell.

You are made of tens of trillions of them.

Biologists sort all cells into two big groups, by one question: is the cell's genetic material enclosed in a nucleus or not?

Eukaryotic cells

You need to know that plant and animal cells (eukaryotic cells) have a cell membrane, cytoplasm and genetic material enclosed in a nucleus.

The cell membrane is the thin outer layer of the cell. It controls which substances enter and leave.

The cytoplasm is the jelly-like liquid inside the membrane, where most of the cell's chemical reactions take place.

The nucleus holds the cell's genetic material, its DNA, packed into chromosomes. A membrane surrounds the nucleus and keeps the DNA apart from the cytoplasm.

"Eu-" means true and "karyon" means kernel or nucleus, so a eukaryote has a "true nucleus".

Fungi (such as yeast) and single-celled protists are eukaryotic too: recent papers have asked students to pick out the eukaryotic features of a single-celled organism from a drawing.

Prokaryotic cells

You need to know that bacterial cells (prokaryotic cells) are much smaller in comparison.

They have cytoplasm and a cell membrane surrounded by a cell wall.

The genetic material is not enclosed in a nucleus.

It is a single DNA loop and there may be one or more small rings of DNA called plasmids.

"Pro-" means before: prokaryotes are "before a nucleus".

Their DNA lies free in the cytoplasm.

The diagram below shows a typical bacterium.

Each number matches the item with the same number in the list.

Diagram of a bacterial cell: a cell wall around a cell membrane and cytoplasm, a single loop of DNA lying free in the cytoplasm with no nucleus, and small rings of DNA called plasmids.

1. cell wall: surrounds the cell membrane and supports the cell

2. cell membrane: controls what enters and leaves, as in every cell

3. cytoplasm: where the cell's reactions happen

4. a single loop of DNA, not enclosed in a nucleus

5. a plasmid: one of the small rings of DNA

Plasmids are small, but they matter.

They often carry genes that help the bacterium survive, such as a gene for resistance to an antibiotic, and bacteria can pass plasmids to one another.

In the inheritance topic you will meet plasmids again as the vectors used to carry genes into bacteria in genetic engineering.

Comparing the two kinds of cell

Feature

Eukaryotic cell (plant or animal)

Prokaryotic cell (bacterium)

Size

larger: most are about 10 to 100 µm across

much smaller: most are about 1 to 5 µm long

Cell membrane

yes

Cytoplasm

yes

Genetic material

enclosed in a nucleus

a single DNA loop, free in the cytoplasm; there may be plasmids

Cell wall

plant cells (and algae, fungi); animal cells have none

yes

Notice what the two kinds of cell share: both have a cell membrane and cytoplasm, and both have DNA.

A cell wall cannot tell them apart either, because plant cells have one as well.

The feature that separates them is the nucleus.

When a question asks for evidence that a cell is eukaryotic, give a feature that a bacterium does not have: a nucleus, or mitochondria.

Saying a cell "has no chloroplasts" proves nothing, because animal cells and fungi have none either.

When you compare the two in an answer, write comparative sentences that name both cells: "A bacterial cell is much smaller than an animal cell"; "A plant cell has its DNA enclosed in a nucleus, but a bacterial cell has a single loop of DNA in the cytoplasm."

How big is a cell? Units and prefixes

Cells are far too small to measure in metres or even millimetres, so biologists use smaller units.

Each one is made by putting a prefix in front of "metre":

Prefix

Symbol

Meaning

So 1 metre is ...

centi

one hundredth ( $10^{-2}$ )

100 cm

milli

one thousandth ( $10^{-3}$ )

1000 mm

micro

one millionth ( $10^{-6}$ )

1 000 000 µm

nano

one thousand-millionth ( $10^{-9}$ )

1 000 000 000 nm

The three steps you use most are each a factor of 1000: 1 mm = 1000 µm and 1 µm = 1000 nm.

The ladder below shows them, with an example object for each size.

Going to a smaller unit, the number gets bigger, so you multiply.

A ladder of length units: metre, millimetre, micrometre and nanometre, each a thousand times smaller than the one before, with an example object for each and arrows showing multiply by a thousand to change to the smaller unit and divide by a thousand to change back.

Typical sizes to keep in your head: a plant cell about 50 to 100 µm long; an animal cell such as a cheek cell about 20 to 60 µm across; a nucleus about 5 to 10 µm; a bacterium about 1 to 5 µm; a virus about 20 to 300 nm.

A light microscope lets you see plant and animal cells and their nuclei; most of the smaller structures need an electron microscope (the microscopy section of this unit explains why).

Orders of magnitude and standard form

You should be able to demonstrate an understanding of the scale and size of cells and make order of magnitude calculations, including the use of standard form.

An order of magnitude is a factor of ten.

If one thing is 10 times bigger than another, it is one order of magnitude bigger; 100 times is two orders; 1000 times is three.

To find how many orders of magnitude apart two sizes are:

1. Put both sizes in the same unit.

2. Divide the larger by the smaller.

3. Count the powers of ten: $10 = 10^1$ (one order), $100 = 10^2$ (two orders), $1000 = 10^3$ (three orders).

Standard form writes very large or very small numbers as a number between 1 and 10 multiplied by a power of ten.

A bacterium 2 µm long is 0.000 002 m long, which is much easier to read as $2 \times 10^{-6}$ m.

Count how many places the decimal point moves: six places to the right to reach 2, so the power is $-6$ .

A negative power means a number smaller than 1.

Biology only: how bacteria multiply by binary fission, and how they are grown on agar plates, are in the culturing microorganisms sections later in this unit.

How the marks work

Each marking point earns one mark. The answers in these notes also show what to allow (other wordings that earn the mark), what to ignore (words that neither gain nor lose) and what not to accept.

"Any two from" means a longer list where any two correct points score. In a list answer, a wrong extra answer cancels a right one: "nucleus and cell wall" for two features only animal cells lack scores 0 for that pair.

A comparison must be comparative and name both things ("a bacterial cell is smaller than a plant cell"). Two sizes quoted side by side score nothing.

In a calculation, working earns marks even when the final answer slips. A unit written after a ratio or a count loses the mark; when the question says "Give the unit.", the unit itself earns a mark.

4-mark and 6-mark extended answers are placed in a level by how well the points are linked, and the top level needs a must-include point. Short sentences and bullet points are fine.

"It" or "they" is credited only when it is clear what you mean, so name the structure: "the nucleus contains DNA", not "it contains it".

Key terms

Eukaryotic cell: a cell with a cell membrane, cytoplasm and genetic material enclosed in a nucleus (plant and animal cells).

Prokaryotic cell: a cell whose genetic material is not enclosed in a nucleus (bacterial cells).

Plasmid: one of the small rings of DNA that may be found in a bacterial cell, as well as its single DNA loop.

Order of magnitude: a factor of ten; two sizes are one order of magnitude apart if one is ten times the other.

Worked example 1: matching cells to their features (Foundation)

Draw one line from each type of cell to a feature it has.

Type of cell

Feature

animal cell

DNA in a nucleus, with no cell wall

bacterial cell

DNA in a nucleus, with a cellulose cell wall

plant cell

a single loop of DNA and plasmids

chloroplasts and plasmids

[3]

Answer: animal cell to "DNA in a nucleus, with no cell wall" (1); bacterial cell to "a single loop of DNA and plasmids" (1); plant cell to "DNA in a nucleus, with a cellulose cell wall" (1).

"Chloroplasts and plasmids" is the spare box: no cell has both.

Two lines drawn from one left-hand box score 0 for that box, even if one of them is right.

Worked example 2: converting a size and writing it in standard form

A yeast cell is 6 µm across.

Give the width of the yeast cell in millimetres and in metres in standard form. [2]

1 mm = 1000 µm, so 6 µm = 6 ÷ 1000 = 0.006 mm (1)

1 m = 1 000 000 µm, so 6 µm = 0.000 006 m = $6 \times 10^{-6}$ m (1)

Check the direction: a millimetre is a bigger unit than a micrometre, so the number must get smaller.

If your answer in mm is bigger than 6, you multiplied when you should have divided.

Worked example 3: an order of magnitude calculation

A root cell is 50 µm long.

A bacterium living on the root is 0.5 µm long.

How many orders of magnitude longer is the root cell than the bacterium? [2]

50 ÷ 0.5 = 100 (1)

100 = $10^2$ , so the root cell is two orders of magnitude longer (1)

Both sizes were already in µm, so they could be divided straight away.

If one size had been in mm, you would convert it first.

Worked example 4: evidence that a cell is eukaryotic (model answers)

Chlamydomonas is a single-celled organism that lives in ponds.

Under a microscope it shows a nucleus, a cell wall, a large chloroplast and mitochondria.

Give two features that show Chlamydomonas is eukaryotic and not prokaryotic. [2]

Full-mark answer: It has a nucleus that encloses its DNA (1).

It has mitochondria (1).

A weaker answer: "It has a cell wall and it is a single cell."

This scores 0.

Bacteria also have a cell wall and are also single cells, so neither feature separates the two groups.

This question is the kind that both tiers sit: comparing cell types is a favourite opener on both papers.

Exam tip

When a question asks how a bacterial cell differs from a plant or animal cell, give the nucleus point in full: "its DNA is not enclosed in a nucleus; it is a single loop", and then plasmids.

Write "DNA" or "chromosomes", never "genetic information": that phrase does not get the mark.

Common mistakes

"Bacteria have no DNA because they have no nucleus." Every cell has DNA. In a bacterium it lies free in the cytoplasm as a single loop, often with plasmids as well.

"Bacteria have no cell membrane." They do, inside their cell wall. A list answer that adds this wrong idea loses a mark it had earned.

Using the cell wall to tell the groups apart. Plant cells and bacteria both have one; animal cells have none. The nucleus is the separating feature.

Dividing when you should multiply in a conversion. Going from mm to µm, multiply by 1000: the unit is smaller, so the number gets bigger.

"Genetic information" for DNA. Write DNA or chromosomes; "genetic information" never gets the mark, on either tier.

Check your understanding

1. Cells can be prokaryotic or eukaryotic.

1.1 Which type of cell is prokaryotic?

Bacterial cell ☐

Muscle cell ☐

Onion cell ☐

Tick (✓) one box. [1]

1.2 Where is the DNA found in a prokaryotic cell?

In a nucleus ☐

In the cytoplasm ☐

In the cell wall ☐

Tick (✓) one box. [1]

2. Which two structures are found in both bacterial cells and animal cells?

Cell membrane ☐

Chloroplast ☐

Cytoplasm ☐

Nucleus ☐

Plasmid ☐

Tick (✓) two boxes. [2]

3. Complete the sentences. Choose answers from the box.

cell wall

DNA

nucleus

plasmids

vacuole

In a plant cell the genetic material is enclosed in a ______.

A bacterial cell has a single loop of ______ and may also have small rings of DNA called ______. [3]

4. A cheek cell is 0.04 mm across.

4.1 Give the width of the cheek cell in micrometres (µm).

Width = ______ µm [1]

4.2 A virus is 40 nm across.

How many orders of magnitude wider is the cheek cell than the virus?

Complete the following steps.

Width of the cheek cell in nm = ______

Width of the cheek cell ÷ width of the virus = ______

Number of orders of magnitude = ______ [3]

5. Table 1 shows the lengths of three cells.

Cell

Length

2 × 10⁻⁶ m

8 × 10⁻⁵ m

1.5 × 10⁻³ m

5.1 Which cell is most likely to be a bacterium?

Give a reason for your answer. [2]

5.2 Give the length of cell B in micrometres. [1]

6. A student says: "A bacterial cell is a simple cell, so it does not need a cell membrane. The cell wall does the same job."

Explain why the student is wrong. [2]

7. This question is the kind that both tiers sit.

Paramecium is a single-celled organism that lives in ponds.

E. coli is a bacterium.

Give two similarities and two differences between the structure of a Paramecium cell and the structure of an E. coli cell.

Similarities ______

Differences ______ [4]

Answers

1. 1.1 Bacterial cell (1). Muscle and onion cells are eukaryotic: their DNA is in a nucleus.

1.2 In the cytoplasm (1).

It is a single loop there; there is no nucleus.

The cell wall holds no DNA.

2. Cell membrane (1). Cytoplasm (1). Chloroplasts are in some plant cells only; a nucleus is in animal cells only; plasmids are in bacteria only. A third tick cancels one mark.

3. nucleus (1); DNA (1); plasmids (1), in this order.

4. 4.1 0.04 × 1000 = 40 µm (1)

4.2 40 µm × 1000 = 40 000 nm (1); 40 000 ÷ 40 = 1000 (1); 1000 = $10^3$ , so three orders of magnitude (1).

Allow a correct number of orders of magnitude from the student's own value of the division.

5. 5.1 A (1); it is the smallest / 2 µm long, and bacteria are about 1 to 5 µm long (1). The reason must be comparative or give the size.

5.2 8 × 10⁻⁵ m = 0.000 08 m = 80 µm (1)

6. The cell membrane controls which substances enter and leave the cell (1). The cell wall does not do this: it supports and strengthens the cell, and the membrane lies inside it in every bacterial cell (1).

7. Similarities, any two from:

both have a cell membrane (1)
both have cytoplasm (1)
both have DNA / genetic material (1)
both are single cells (1)

Differences, any two from (each must name both cells or be clearly comparative):

the DNA of Paramecium is enclosed in a nucleus, but in E. coli it is not (1)

E. coli has a single loop of DNA / plasmids, but Paramecium does not (1)

E. coli is much smaller than Paramecium (1)

Paramecium has mitochondria, but E. coli does not (1)

E. coli has a cell wall, but Paramecium does not (1)

Do not accept "E. coli has no DNA" or "E. coli has no cell membrane": a wrong statement in the list cancels a right one.

Eukaryotes and prokaryotes

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