Cells as the basic units of living organisms

1. Overview

All living organisms are built from cells, the basic units of life. There are two fundamental cell types: eukaryotic cells (with a true nucleus and membrane-bound organelles, as in plants and animals) and prokaryotic cells (smaller and simpler, lacking a nucleus, as in bacteria).

Plant and animal cells share the eukaryotic plan but differ in several structures. Bacterial cells are even more different again, and viruses are not cells at all. Whatever the cell type, the energy for energy-requiring processes comes from ATP made in respiration.

This topic also covers how we view cells — using photomicrographs, electron micrographs and biological drawings — and how to compare the main cell types accurately.

Key Definitions

• Cell: the basic structural and functional unit of all living organisms, bounded by a cell surface membrane.
• Eukaryotic cell: a cell that contains a true nucleus and other membrane-bound organelles, found in plants, animals, fungi and protoctists.
• Prokaryotic cell: a smaller, simpler cell lacking a nucleus and membrane-bound organelles, as found in bacteria.
• Organelle: a specialised structure within a cell that carries out a particular function, often surrounded by one or two membranes.
• Nuclear envelope: the double membrane that surrounds the nucleus of a eukaryotic cell and contains many pores.
• Magnification: the number of times larger an image is compared with the real size of the object.
• Resolution: the smallest distance between two points that can still be seen as separate, which sets the level of detail visible.
• Virus: a non-cellular infectious particle made of a nucleic acid core (DNA or RNA) surrounded by a protein capsid, sometimes with a phospholipid envelope.
• ATP: adenosine triphosphate, the energy-carrying molecule made in respiration and used to drive energy-requiring processes in cells.

Content

Viewing cells: micrographs and drawings

A photomicrograph is a photograph taken through a light microscope, while an electron micrograph is an image produced by an electron microscope. Because an electron beam has a far shorter wavelength than light, the electron microscope has a much higher resolution and reveals fine detail of organelles that a light microscope cannot show.

When you interpret these images, identify organelles by their characteristic appearance:

• the nucleus as a large rounded structure bounded by a double membrane (the nuclear envelope);
• mitochondria by their inner folds (cristae);
• chloroplasts by their stacks of membranes (grana) in plant cells.

A biological drawing is a clear line representation of what is seen. A good drawing should:

• use sharp, continuous, unbroken lines;
• show structures in correct proportion;
• carry no shading;
• have labels added with straight ruled lines that do not cross.

A plan drawing of a tissue section shows only the boundaries between tissues (for example, the outline of layers). It must not show individual cells or any shading.

Comparing plant and animal cells

Both plant and animal cells are eukaryotic, so both have a cell surface membrane, cytoplasm, a nucleus (bounded by a nuclear envelope), mitochondria, ribosomes, rough and smooth endoplasmic reticulum and a Golgi apparatus. The key differences are summarised in the table below.

Feature	Typical plant cell	Typical animal cell
Cell wall	present, made of cellulose	absent
Chloroplasts	present (in many cells)	absent
Large permanent vacuole	present, filled with cell sap and bounded by the tonoplast	absent (small temporary vacuoles only)
Shape	usually regular/fixed by the wall	usually rounded/irregular
Centrioles	absent in most flowering plants	present
Carbohydrate store	starch	glycogen

ATP as the energy currency

Cells cannot use the energy released in respiration directly. Instead, respiration transfers this energy to make ATP (adenosine triphosphate). ATP then acts as the cell's energy currency: when it is broken down it releases a usable amount of energy that drives energy-requiring processes, such as active transport, muscle contraction, cell division and the synthesis of large molecules. You should be able to state simply that cells use ATP from respiration for energy-requiring processes.

Structure of a prokaryotic cell

A typical bacterium is a prokaryotic cell with several distinctive features:

• it is usually unicellular;
• it is small, generally 1–5 µm in diameter (much smaller than a typical eukaryotic cell);
• it has a cell wall containing peptidoglycan (a mesh of polysaccharide chains cross-linked by short peptides), not cellulose;
• its genetic material is a single circular chromosome of DNA lying free in the cytoplasm — it is not enclosed in a nucleus and, unlike eukaryotic DNA, it is "naked" (not wound around or associated with histone proteins), which is the "no histones" point in the comparison table below;
• it has smaller 70S ribosomes (eukaryotes have larger 80S ribosomes);
• it shows an absence of organelles surrounded by double membranes — there is no nucleus, no mitochondria and no chloroplasts.

Some bacteria also have additional features such as small rings of DNA called plasmids, a protective capsule, or flagella for movement.

Comparing prokaryotic and eukaryotic cells

The table below sets a typical bacterium side by side with eukaryotic plant and animal cells.

Feature	Prokaryotic cell (bacterium)	Eukaryotic cell (plant/animal)
Size	generally 1–5 µm diameter	typically 10–100 µm
Nucleus	none; DNA free in cytoplasm	true nucleus bounded by a nuclear envelope
DNA	circular, no histones	linear, associated with histone proteins
Ribosomes	70S (smaller)	80S (larger)
Membrane-bound organelles	absent (no double-membrane organelles)	present (e.g. mitochondria; chloroplasts in plants)
Cell wall	present, contains peptidoglycan	cellulose in plants; none in animals

Viruses are non-cellular

Viruses are not cells — they have no cytoplasm, no cell surface membrane of their own and no ribosomes, and cannot carry out metabolism or reproduce on their own. Every virus consists of:

• a nucleic acid core, which is either DNA or RNA (never both);
• a protein coat called a capsid surrounding the core.

In addition, some viruses have an outer envelope made of phospholipids (derived from the membrane of the host cell they came from). Because viruses lack cellular machinery, they can only replicate inside a living host cell.

Worked example

Exam-style question: A student examines an electron micrograph and identifies a cell as prokaryotic. State two features visible in the micrograph that show the cell is prokaryotic rather than eukaryotic, and explain why each indicates a prokaryotic cell. [3]

Model answer:

• No nucleus / no nuclear envelope is visible; instead the DNA is a single loop lying free in the cytoplasm — eukaryotic cells enclose their DNA in a nucleus bounded by a nuclear envelope. (1)
• No membrane-bound organelles such as mitochondria are present — prokaryotes lack organelles surrounded by double membranes. (1)
• The cell is very small (about 1–5 µm), consistent with a typical bacterium rather than a much larger eukaryotic cell. (1)

Worked example

Exam-style question: On an electron micrograph, a mitochondrion measures 30 mm in length. The micrograph has a magnification of ×15 000. Calculate the actual length of the mitochondrion, in µm. [2]

Model answer:

• Rearrange the magnification formula to make actual size the subject: $\text{actual size} = \dfrac{\text{image size}}{\text{magnification}}$. (1)
• First convert the image size to the same scale you want the answer in. $30\ \text{mm} = 30\,000\ \mu\text{m}$. Then divide: $\dfrac{30\,000\ \mu\text{m}}{15\,000} = 2\ \mu\text{m}$. (1)

(Tip: if you forget to convert mm to µm, you would get $30 \div 15\,000 = 0.002\ \text{mm}$, which is the same length — $0.002\ \text{mm} = 2\ \mu\text{m}$. Always state the unit the question asks for.)

Worked example

Exam-style question: An electron micrograph carries a scale bar labelled 1 µm. On the page the scale bar measures 20 mm long, and a chloroplast in the same micrograph measures 60 mm long. Calculate the magnification of the micrograph and the actual length of the chloroplast, in µm. [3]

Model answer:

• The scale bar tells you that 20 mm on the page represents 1 µm of real length. Convert to the same unit: $20\ \text{mm} = 20\,000\ \mu\text{m}$, so $\text{magnification} = \dfrac{\text{image size}}{\text{actual size}} = \dfrac{20\,000\ \mu\text{m}}{1\ \mu\text{m}} = \times 20\,000$. (1)
• Rearrange for the chloroplast: $\text{actual size} = \dfrac{\text{image size}}{\text{magnification}}$. Convert the measured length: $60\ \text{mm} = 60\,000\ \mu\text{m}$. (1)
• $\text{actual length} = \dfrac{60\,000\ \mu\text{m}}{20\,000} = 3\ \mu\text{m}$. (1)

(Tip: measure the bar and the structure in the same units with a ruler, then divide the bar length into the structure rather than the other way round.)

Key Equations

This is mainly a qualitative topic. The one quantitative relationship you should know links magnification, image size and real (actual) size: $$\text{magnification} = \frac{\text{size of image}}{\text{actual size of object}}$$ Always convert both measurements to the same unit before dividing (for example, both in µm), and remember that magnification has no units. Useful conversions: $1\ \text{mm} = 1000\ \mu\text{m}$ and $1\ \mu\text{m} = 1000\ \text{nm}$.

Common Mistakes to Avoid

• Calling the double membrane around the nucleus a "nuclear membrane". When describing what is seen in an electron micrograph, use the precise term nuclear envelope for the double membrane (with pores) that surrounds the nucleus.
• Adding shading or drawing individual cells in a plan drawing. A plan (low-power) drawing shows only the boundaries between tissues, drawn with sharp continuous lines and no shading or cells.
• Saying prokaryotes have "no organelles". They lack organelles surrounded by double membranes (no nucleus, mitochondria or chloroplasts), but they do still have ribosomes (70S), which are organelles without a membrane.
• Mixing up ribosome sizes. Prokaryotes have 70S ribosomes and eukaryotes have 80S ribosomes — do not state them the wrong way round.
• Stating that a virus contains both DNA and RNA. A virus has either DNA or RNA as its nucleic acid core, never both, surrounded by a protein capsid.
• Describing viruses as cells or as "living cells". Viruses are non-cellular structures with no cytoplasm or ribosomes, so they cannot be classed as cells.
• Confusing peptidoglycan with cellulose. Bacterial cell walls contain peptidoglycan, whereas plant cell walls are made of cellulose — naming the wrong material loses the mark.
• Putting units inside the body of a results table. Write each unit once, in the column heading, after a forward slash (for example, "Length / mm" or "Time / s"). The cells of the table should then contain numbers only — repeating "mm" or "s" next to every value is penalised.
• Treating mesosomes as genuine organelles. Infoldings of the membrane sometimes drawn in older diagrams as "mesosomes" are now widely regarded as artefacts of how the cell was prepared for the electron microscope, not real structures — so do not rely on them when describing a bacterium.

Exam Tips

• For "compare" questions, write comparative sentences that mention both cells (e.g. "a plant cell has a cellulose cell wall whereas an animal cell has none"), rather than two separate lists.
• When labelling or interpreting micrographs, use the exact terms — nuclear envelope, cell surface membrane, 70S/80S ribosomes — as vague wording is penalised.
• Learn the prokaryote feature list precisely (1–5 µm, peptidoglycan wall, circular DNA, 70S ribosomes, no double-membrane organelles); these are quick, reliable marks.
• In magnification calculations, convert units first and show your working; quote the answer with no units for the magnification itself, and to a sensible number of significant figures.
• For biological drawings, use a sharp pencil, draw clean continuous lines with no shading, keep structures in proportion, and add ruled label lines that do not cross.
• If a question asks specifically about viruses, always mention the nucleic acid core (DNA or RNA) and the protein capsid, and add the phospholipid envelope only as a feature of some viruses.