Cell structure and organisation: Grade 12 Biology

By the end of this lesson you should be able to

Name the parts of plant and animal cells and state their functions
Explain how specialised cells are adapted to their functions
Describe the levels of organisation from cell to organism
Calculate magnification and actual size from drawings and micrographs

Cells: the units of life

All living things are made of cells. A cell is the smallest unit that can carry out the processes of life, and new cells come only from existing cells. Some organisms, such as bacteria, have just one cell. Plants and animals are made of millions of cells that work together.

Most cells are too small to see without a microscope, so you need to know the parts that a light microscope shows and what each part does.

Key ideas

Parts of plant and animal cells

Part	Found in	Function
Cell membrane	Plant and animal cells	Controls what enters and leaves the cell
Cytoplasm	Plant and animal cells	Jelly-like material where many chemical reactions take place
Nucleus	Plant and animal cells	Contains the genetic material (DNA) and controls the activities of the cell
Mitochondria	Plant and animal cells	Site of aerobic respiration, which releases energy
Ribosomes	Plant and animal cells	Make proteins
Cell wall	Plant cells only	Made of cellulose. Supports the cell and stops it bursting
Chloroplasts	Plant cells with green parts	Contain chlorophyll. Site of photosynthesis
Large permanent vacuole	Plant cells	Contains cell sap and helps keep the cell firm

The main differences are that plant cells have a cell wall, chloroplasts and a large permanent vacuole, and animal cells do not.

Specialised cells

In many-celled organisms, cells become specialised. Their shape and contents are adapted to do one job well.

Cell	Adaptation	How it helps
Red blood cell	Biconcave shape, no nucleus, full of haemoglobin	Large surface area and more room to carry oxygen
Root hair cell	Long thin extension, thin cell wall	Large surface area to absorb water and mineral ions
Palisade cell	Tall shape, many chloroplasts	Absorbs a lot of light for photosynthesis
Muscle cell	Many mitochondria, can contract	Releases energy for movement
Nerve cell	Very long fibre	Carries electrical impulses over long distances
Xylem vessel	Hollow tube with strong walls	Carries water up the plant and supports it

Levels of organisation

Cells do not work alone. They are arranged in levels:

cell → tissue → organ → organ system → organism

A tissue is a group of similar cells doing the same job, such as muscle tissue.
An organ is made of different tissues working together, such as the heart.
An organ system is a group of organs with a shared function, such as the circulatory system.

Example in a plant: palisade cell → palisade tissue → leaf → shoot system → whole plant.

Magnification

\text{magnification} = \frac{\text{size of the image}}{\text{actual size of the object}}

Rearranged: $\text{actual size} = \dfrac{\text{image size}}{\text{magnification}}$

Units: $1\text{ mm} = 1000\ \mu\text{m}$ . Measure both sizes in the same unit before dividing.

Example 1. A drawing of a cell is 60 mm long. The real cell is 0.2 mm long. Find the magnification.

\text{magnification} = \frac{60}{0.2} = 300

The magnification is $\times 300$ .

Example 2. A cell is 0.03 mm long. It is drawn at $\times 400$ . How long is the drawing?

\text{image size} = 0.03 \times 400 = 12 \text{ mm}

Example 3. An image is 45 mm long at a magnification of $\times 150$ . Find the actual length in micrometres.

\text{actual size} = \frac{45}{150} = 0.3 \text{ mm} = 0.3 \times 1000 = 300\ \mu\text{m}

Common mistakes

Mixing up the cell wall and the cell membrane. The wall is a strong cellulose layer found only in plants. The membrane is thin, is found in every cell, and controls what passes in and out.

Saying mitochondria “make energy”. Energy cannot be made. Mitochondria release energy from glucose during respiration.

Mixing units in magnification. Convert both measurements to the same unit first. A ruler measures in millimetres, but cell sizes are often given in micrometres.

Untidy biological drawings. Use a sharp pencil, draw clear single lines, do not shade, and label with ruled lines that do not cross.

Practice questions

Name the part of a cell where (a) photosynthesis takes place, (b) proteins are made, (c) the genetic material is found.
State two differences between a plant cell and an animal cell.
Explain why a red blood cell has no nucleus.
Describe how a root hair cell is adapted for absorbing water.
Arrange these from smallest to largest: organ, cell, organism, tissue, organ system.
A cell has an actual length of 0.05 mm. In a drawing it is 25 mm long. Calculate the magnification.
Convert 0.08 mm into micrometres.
Explain why muscle cells contain many mitochondria.

Show answers

(a) Chloroplast. (b) Ribosomes. (c) Nucleus.
Any two of: plant cells have a cell wall, chloroplasts and a large permanent vacuole, and animal cells do not.
There is more room for haemoglobin, so the cell can carry more oxygen.
It has a long, thin extension that gives a large surface area for absorbing water, and its thin cell wall lets water pass in easily.
Cell, tissue, organ, organ system, organism.
Magnification $= \dfrac{25}{0.05} = 500$ , so $\times 500$ .
$0.08 \times 1000 = 80\ \mu\text{m}$ .
Muscle contraction needs a lot of energy, and mitochondria release energy by aerobic respiration.

Summary

Plant and animal cells share a membrane, cytoplasm, nucleus, mitochondria and ribosomes. Plant cells also have a cell wall, chloroplasts and a large vacuole.
Specialised cells have features that suit their job.
Cells form tissues, tissues form organs, organs form organ systems, and organ systems form an organism.
Magnification is image size divided by actual size, using the same units.

Now practise this topic

Real questions show you what still needs work. Try a past paper under timed conditions, or take a short quiz.

Grade 12 past papers Quiz Bank Video lessons AI Study Assistant