All living organisms are made of cells

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Cell structure and microscopy opens Topic 3, and everything in this unit is examined on the Unit 2 paper, WBI12 (Cells, Development, Biodiversity and Conservation).

Its microscope core practical, CP5, is also examined as a method on the practical paper, WBI13.

In the three most recent WBI12 papers this unit earned about 12 marks a paper, with a magnification calculation on every one.

This first section asks why life is built from cells at all, and what every cell has in common.

How the marks work (WBI12)

Each marking point earns one mark, shown "(1)"; there are no letter codes. A header such as "three of the following points" sets how many points count.

"/" separates alternative wordings; ACCEPT, IGNORE and REJECT lines say which wordings count; "max 2" caps a group of points.

Only the first answers are marked: give exactly as many as asked, one per line. A wrong or contradictory statement cancels a right one.

A stated format (two significant figures, two decimal places, ": 1") is a mark: a correct value in the wrong form earns 1 of 2. Magnification has no unit: a unit on it is rejected.

Multiple choice is one mark: a cross in one box; to change, draw a line through it and cross another box.

Asterisked (*) 6-mark items are marked in levels (1-2, 3-4, 5-6): describing the information alone stays in Level 1; Level 3 uses all the information with linked biology.

Printed when used and learn it (3a)

Printed in the question when used: the volume of a sphere, $V = \frac{4}{3}\pi r^3$ . A surface area is usually given as data.

Learn it (used, never printed): magnification = size of image ÷ size of real object, and its two rearrangements; 1 mm = 1000 µm and 1 µm = 1000 nm; calibrating an eyepiece graticule (length per eyepiece unit = length on the stage micrometer ÷ number of eyepiece units); surface area to volume ratio written "n : 1"; percentage change = change ÷ original value × 100.

The idea: one unit of life, repeated

You need to know that all living organisms are made of cells, sharing some common features.

That sentence is the core of cell theory, built up in the nineteenth century as microscopes improved.

In modern form it has three parts:

1. All living organisms are made of one or more cells.

2. The cell is the smallest unit that can carry out all the processes of life (taking in materials, releasing energy, growing, responding, reproducing).

3. New cells arise only by the division of existing cells.

A bacterium is a single cell that does everything for itself.

A human is about 30 million million cells, specialised into different jobs but each still a complete living unit.

Viruses are not cells: they have no cytoplasm, no ribosomes and no metabolism of their own, and they can only reproduce inside a host cell.

That is why most biologists do not call them living organisms.

The common features every cell shares

Cells come in two basic designs, prokaryotic (bacteria) and eukaryotic (animals, plants, fungi and protoctists).

Both are taught in this unit.

Whatever the design, every cell has:

Common feature

What it does

a cell surface membrane

separates the cell from its surroundings and controls what enters and leaves

cytoplasm

the watery medium in which most of the cell's reactions happen

DNA as its genetic material

carries the genes that code for the cell's proteins

ribosomes

assemble proteins by translating mRNA

Cells also use ATP as their immediate source of energy.

These shared features are strong evidence that all life on Earth descends from a common ancestor: the genetic code, the ribosome and the membrane are too similar in every cell to have arisen separately many times.

Why cells are small: surface area to volume ratio

If cells are so useful, why are they so small?

Most animal cells are 10 to 30 µm across.

The reason is the surface area to volume ratio.

Everything a cell exchanges (oxygen, glucose, waste, heat) crosses its surface.

Everything that needs supplying is in its volume.

As a cell gets bigger, its volume grows faster than its surface area.

For a sphere of radius $r$ :

\text{surface area} = 4\pi r^2 \qquad \text{volume} = \frac{4}{3}\pi r^3 \qquad \frac{\text{surface area}}{\text{volume}} = \frac{3}{r}

So doubling the radius halves the ratio.

A large cell has too little membrane for its contents, and substances would take too long to diffuse to its centre.

The graph shows how fast the ratio falls as a spherical cell gets bigger.

Graph of surface area to volume ratio against diameter for spherical cells. The ratio equals 6 divided by the diameter, so it falls steeply as cells get bigger: 3.0 : 1 for a 2 µm cell and 0.30 : 1 for a 20 µm cell.

The steep left-hand part of the curve explains a lot of biology.

Bacteria, a few micrometres long, can rely on diffusion alone.

Larger cells that exchange a lot are flattened (red blood cells, cheek cells) or folded (the microvilli of gut cells) so their ratio stays high.

Large organisms solve the same problem at the next level up with gas exchange surfaces and transport systems, which is where Topic 2 started (notes unit 2a).

Going further (not examined). The ratio 3/r is in µm⁻¹ when r is in µm.

Exam questions write it as a ratio, "0.86 : 1", which has no unit.

Examined beyond the statement: the volume of a spherical cell

Your syllabus names no calculation here, but recent WBI12 papers set the volume of a spherical cell from a stated diameter (with the formula printed) and a surface area to volume ratio from a given surface area, 1 to 2 marks each.

The reports name three losses: using the diameter in place of the radius, squaring in place of cubing, and ignoring the stated decimal places.

Show the radius on its own line before you substitute.

Connections

The common features in the table return throughout the course: the cell surface membrane in Topic 2 (fluid mosaic model), ribosomes in protein synthesis (notes unit 2b) and ATP in respiration (Topic 7).

The next section shows how cells in a multicellular organism are organised into tissues and organs, and later sections compare the two cell designs in detail.

Key terms

Cell theory: all living organisms are made of cells, sharing some common features; the cell is the basic unit of life, and new cells arise only from existing cells.

Surface area to volume ratio: the surface area of a cell divided by its volume, written "n : 1"; it falls as a cell gets bigger.

Worked example 1: the volume of a spherical cell (direct use)

A lymphocyte is roughly spherical with a diameter of 7.0 µm.

Calculate its volume using the formula $V = \frac{4}{3}\pi r^3$ .

Give your answer to two decimal places.

(2)

1. radius = 7.0 ÷ 2 = 3.5 µm; $V = \frac{4}{3} \times \pi \times 3.5^3$ (1) radius used and cubed

2. $V = \frac{4}{3} \times \pi \times 42.875 = 179.594...$ = 179.59 µm³ (1) to 2 d.p.

Answer: 179.59 µm³.

Writing 179.6 or 180 is a correct value in the wrong form and earns 1 of 2.

Worked example 2: surface area to volume ratio (chained from Worked example 1)

The surface area of the same lymphocyte is 153.94 µm².

Calculate its surface area to volume ratio.

(1)

1. ratio = 153.94 ÷ 179.59 = 0.857... so 0.86 : 1 (1) surface area divided by volume, ": 1" kept

Answer: 0.86 : 1.

The order matters: "surface area to volume" puts surface area first.

1.17 : 1 (volume ÷ surface area) is the ratio the wrong way round and scores 0.

If Worked example 1 had been wrong, this mark would still be given for a correct division of the wrong volume (error carried forward).

Worked example 3: what the ratio means (exam style)

A spherical bacterium (coccus) has a diameter of 1.0 µm, a volume of 0.52 µm³ and a surface area of 3.14 µm².

(a) Calculate its surface area to volume ratio. (1)

(b) Explain why a cell of this size can rely on diffusion alone, but the lymphocyte in Worked example 1 is near the size limit for a cell that does. (2)

1. (a) 3.14 ÷ 0.52 = 6.04 so 6.0 : 1 (1) (3/r = 3 ÷ 0.5 = 6 confirms it)

2. (b) The bacterium has a much larger surface area to volume ratio (6.0 : 1 against 0.86 : 1, about seven times larger), so it has more membrane for exchange per unit volume of cytoplasm (1); the distance from the surface to the centre is also short (0.5 µm against 3.5 µm), so diffusion to all parts of the cell is fast (1).

Answer: (a) 6.0 : 1; (b) as above.

A weaker answer, "the bacterium is smaller so diffusion is faster", scores at most 1: it states the size, not the ratio, and gives no comparison with values.

Worked example 4: spotting the two traps

Two students calculated the volume of the 7.0 µm lymphocyte.

Student P wrote $\frac{4}{3}\pi \times 7.0^3 = 1436.76$ µm³.

Student Q wrote $\frac{4}{3}\pi \times 3.5^2 = 51.31$ µm³.

Give the error each student made.

(2)

1. Student P used the diameter in place of the radius, so the volume is $2^3 = 8$ times too big (1).

2. Student Q squared the radius in place of cubing it (1).

Answer: as above.

A quick sense check catches both: a 7 µm sphere fits inside a 7 µm cube of volume 343 µm³, so its volume must be a little over half of that (a sphere fills 52% of its cube).

1436.76 is far too big, and 51.31 far too small.

Exam tip

In a sphere-volume item, write the radius on its own line, then the substitution, then the answer to the decimal places in bold in the stem.

Each line is a marking point, and the radius line protects you from the commonest error in recent reports.

Common mistakes

Diameter for radius. The stem gives the diameter because a microscope measures across a cell. Halve it first.

Squaring, not cubing. Volume is three-dimensional, so the radius is cubed; r² gives an area. Recent reports name "r² for r³" in every series.

Ignoring the stated decimal places. "Give your answer to two decimal places" carries a mark; 179.6 for 179.59 loses it.

Saying a virus is a cell. It feels like the smallest living thing, but it has no cytoplasm, ribosomes or metabolism of its own.

Ratio the wrong way round. Surface area to volume is surface area ÷ volume, written with ": 1".

Check your understanding

1. Which structure is found in every living cell? (1)

A a nucleus
B mitochondria
C a cell wall
D ribosomes Total [1]

2. A spherical cell has a radius of 2.0 µm. Which is its volume? Use $V = \frac{4}{3}\pi r^3$ . (1)

A 16.76 µm³
B 33.51 µm³
C 268.08 µm³
D 50.27 µm³ Total [1]

3. A student says: "Bigger cells are better because they can hold more organelles." Explain why most cells are small. (3) Total [3]

4. A single-celled alga is spherical, with a diameter of 12 µm.
(a) Calculate its volume using $V = \frac{4}{3}\pi r^3$ . Give your answer to two decimal places. (2)
(b) The surface area of the alga is 452.39 µm². Calculate the surface area to volume ratio. (1)
(c) Suggest why this alga has many small projections from its cell surface membrane. (2) Total [5]

5. A biologist found a new single-celled organism in a hot spring. It has a membrane, cytoplasm, ribosomes and circular DNA, but no nucleus. Explain why this observation supports the idea that all living organisms are made of cells sharing some common features. (2) Total [2]

6. Explain why a virus is not considered to be a cell. (2) Total [2]

Answers

1. D (1). Every cell, prokaryotic or eukaryotic, makes proteins on ribosomes. A is wrong because prokaryotic cells have no nucleus (and mature red blood cells lose theirs). B is wrong because prokaryotic cells have no mitochondria. C is wrong because animal cells have no cell wall.

2. B (1). $\frac{4}{3} \times \pi \times 2.0^3 = 33.51$ µm³. A squares the radius. C uses the diameter (4.0) as the radius. D is the surface area, $4\pi r^2$ .

3. Three from: as a cell grows, its volume increases faster than its surface area / its surface area to volume ratio falls (1); exchange of substances (oxygen, glucose, waste) happens across the cell surface membrane (1); a large cell has too little surface for the needs of its volume, so exchange is too slow (1); the distance from the surface to the centre increases, so diffusion takes too long (1).

4. (a) r = 6.0 µm; $\frac{4}{3} \times \pi \times 6.0^3$ (1) = 904.78 µm³ (1). 904.8 or 905 earns 1 of 2. (b) 452.39 ÷ 904.78 = 0.50 : 1 (1); ecf from (a). (c) The projections increase the surface area (1) without much increase in volume, so the surface area to volume ratio rises and exchange is faster / more substances can be absorbed per unit time (1).

5. It has the common features found in all cells: a cell surface membrane, cytoplasm, ribosomes and DNA as genetic material (1); so even an organism from an extreme habitat, with no nucleus, is built of cells with the same basic features, suggesting common ancestry (1).

6. Two from: a virus has no cytoplasm / no cell surface membrane of its own (1); it has no ribosomes, so it cannot make its own proteins (1); it has no metabolism and can reproduce only inside a host cell (1).

All living organisms are made of cells

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