Chapter 6: Model of Cells – the Basic Unit of Life (Summary Sheet)

Part A · Cells as Models

1 The Basic Unit of Life

Every living thing is made of cells. A cell is the smallest unit that can carry out all the processes of life on its own.
Some living things are a single cell; most are made of many cells.

Type	What it means	Examples
Unicellular	One cell only — that single cell does everything: feeds, moves, grows and reproduces	Amoeba, Euglena, bacteria
Multicellular	Many cells, which are the basic building blocks of the organism and are organised into tissues, organs and systems	Plants and animals, including us

2 Cells Are Models

A "typical plant cell" and a "typical animal cell" are models: simple drawings that stand in for the real thing, which is far too small and too varied to draw exactly.
They are models used to represent the many different forms cells take, so we can describe them in a common way and predict what a new cell will be like.
Like every model, they are simplified and not exact — real cells come in many shapes, and not every cell has every part.

3 Better Tools, Better Knowledge

Cells are far too small to see with the eye alone, so for a long time nobody knew they existed.
The development of the microscope changed that: once cells could be magnified, they could be seen, described, compared and drawn.
Better microscopes then showed finer detail, which built more knowledge again — new structures, then how those structures work.
So this is a two-way street: advances in technology make new observations possible, and the new observations build knowledge.

Part B · The Parts of a Cell and What They Do

4 The Six Parts You Must Know

Part	Function	Found in
Nucleus	Controls the cell's activities; holds the genetic material (DNA); responsible for growth, repair and reproduction	Plant and animal
Cytoplasm	The jelly-like substance filling the cell, where many chemical reactions take place	Plant and animal
Cell membrane	A thin layer around the cell that controls what enters and leaves — partially permeable	Plant and animal
Vacuole	A space filled with cell sap (water and dissolved substances). In plants it presses outwards and keeps the plant firm	Both — one large one in plants, small ones in animals
Chloroplast	Contains chlorophyll, which traps light so the cell can make sugar by photosynthesis	Plant cells only
Cell wall	A rigid outer layer of cellulose that supports the plant and gives the cell its regular shape	Plant cells only

Anything inside a cell that does a particular job is called an organelle. The vacuole and the chloroplast are two of them.

5 The Nucleus and DNA

The nucleus holds the cell's genetic material, called DNA, packed as chromosomes.
That DNA carries the instructions for how the cell works and is passed down to the next generation when cells divide.
This is why the nucleus controls the cell's activities and the passing on of characteristics.
Writing DNA is enough — the full spelling of the name is not required.

6 Plant Cell or Animal Cell?

Feature	Plant cell	Animal cell
Cell wall	Present — rigid, gives a fixed shape	Absent
Chloroplast	Present (in green parts) — makes food by photosynthesis	Absent
Vacuoles	One large central vacuole	Several small ones
Shape	Regular, often rectangular, fixed	Irregular or rounded

7 How to Decide Which It Is

Look for the cell wall first: outer rigid layer = plant cell. No wall, shape round = animal cell.
Then look for chloroplasts: present in green plant cells, never in animal cells.
Then look at the vacuoles: one large one = plant; several small ones = animal.
Careful with the odd ones out: a cell can have chloroplasts without a cell wall — Euglena does, and it is not a plant. Always use the structures in front of you, not the name of the organism.

Part C · Using the Microscope

8 Using It Safely and Correctly

Carry the microscope with both hands — one on the arm, one supporting the base.
Start on the lowest-power objective lens, and focus with the coarse knob first, then sharpen with the fine knob.
View with the specimen lit from below, and use the diaphragm to control how much light comes through.
The image you see is inverted — upside down and reversed — so move the slide the opposite way to bring something into view.
Stain the sample first to make the structures show up, for example methylene blue for cells.
Take the slide off on the low-power lens, and put the microscope away on its lowest setting with the cover on.

Part D · From Cells to Whole Organisms

9 Levels of Organisation

In a multicellular organism, cells are not just piled up: they are organised in levels, each built from the one before.

Level	What it is	Example
Cell	The smallest unit that can carry out all the processes of life	A muscle cell
Tissue	A group of the same kind of cells working together on one job	Muscle tissue
Organ	Several different tissues working together on one job	The heart — muscle tissue plus nerve tissue
System	Different organs working together on related jobs	The circulatory system — heart and blood vessels
Organism	A whole living thing, made of systems working together	A human; a flowering plant

10 Division of Labour

Division of labour: The assignment of different functions to specialised structural units, leading to efficiency in the organism.
In short, different parts do different jobs instead of each part trying to do everything.
It works even at the cellular level: cells become specialised, each one shaped for its own task, so that it can perform a specific function more efficiently.
Inside a single cell the same idea runs again — the nucleus directs, the membrane controls what comes in, the chloroplast makes food.
The payoff: each job is done better and faster, because nothing has to be a generalist at everything.

11 Specialised Cells

Cell	How it is built for the job	Why that helps
Red blood cell	No nucleus, and a biconcave shape	Room for more haemoglobin to carry oxygen, and a large surface area to pick it up and release it quickly
Nerve cell	Long thin extensions from the cell body	Carries electrical impulses quickly over long distances, from the senses to the brain and back
Root hair cell	A long narrow projection, with no waxy layer	A large surface area to absorb water and mineral salts from the soil
Sperm cell	A tail for swimming, and a head packed with enzymes	Swims to the egg cell, and the enzymes let it get through the egg's outer layer

Notice the pattern in every row: the shape of the cell is the adaptation — name the shape, then say what it allows.

Part E · Exam Tips

12 Exam Tips: What Each Syllabus Pointer Asks For

The pointer	What earns the mark
Show an understanding of the functions of the different parts of a typical cell, including the nucleus which contains genetic material (DNA)	Give the part and its job, not just the name — and for the nucleus say it holds the DNA that is passed down to the next generation
Show an understanding that typical plant and animal cells are models used to represent their various forms	Say a cell drawing is a model — simplified, not exact — used to represent the many real forms and to predict what a cell can do
Recognise that in multicellular organisms cells are the basic building blocks organised into tissues, organs and systems	Give the levels in order — cell, tissue, organ, system — with one example at each level, and say each level is built from the one below
Explain the significance of the division of labour even at the cellular level	Say cells are specialised, so different jobs happen at the same time and each is done better — then give one cell that shows it
Identify, using the microscope safely and correctly, the different parts of a typical cell	Name the part you can see in the drawing or micrograph (wall, membrane, cytoplasm, nucleus, vacuole, chloroplast) and match the label to the right structure
Infer whether an organism is an animal or a plant, based on its cell structures	Look for the cell wall, chloroplast and one large vacuole — any of them means plant; and watch for the odd one out that has chloroplasts but no wall
Show an appreciation for the relationship between advances in technology and knowledge-building	Use the microscope as the example: better instruments made new observations possible, and those observations built the knowledge we have of cells

Two questions are asked about the same drawing: "name the part" and "state its function". Answer them separately — a label earns one mark, the job it does earns the next.