Cells are the starting point of the biology strand of Sec 2 IP Science. Later topics, such as how substances move in and out of organisms, how food is digested and absorbed, and how organisms reproduce, are all explained in terms of what happens inside cells. This topic covers the cell theory, using the light microscope and preparing simple slides, the structures of the model animal and plant cell, and how cells are organised into tissues, organs, organ systems and organisms.
Most marks in this topic depend on exact wording. You must pair each structure with its function, use terms such as “partially permeable” precisely, and identify plant and animal cells from reliable features. The use of the light microscope is an asterisked learning outcome. It is not required for G2 students but is fully examinable for G3 and IP students.
What these notes cover
- •Cell theory
- •The cell as the basic unit of life
- •Parts of the light microscope
- •Safe and correct use of the microscope
- •Preparing onion and cheek cell slides
- •Staining and lowering the cover slip
- •Structures of the model animal and plant cell
- •Comparing animal and plant cells
- •Levels of organisation
- •Identifying plant or animal tissue
- •Division of labour
- •Common mistakes and exam technique
Key definitions
- Cell theory
- All living things are made up of one or more cells, and the cell is the basic unit of structure and function of life.
- Partially permeable
- Allows some substances to pass through but not others. The cell membrane is partially permeable and controls the movement of substances into and out of the cell.
- Tissue
- A group of similar cells working together to carry out a particular function.
- Organ
- A structure made up of different tissues working together to carry out a particular function.
- Organ system
- A group of organs working together to carry out a major life process.
Key formulas
Total magnification
total magnification = eyepiece lens power × objective lens power
Only some schools teach this. It is not a learning outcome of this topic. Example: a × 10 eyepiece with a × 40 objective gives a total magnification of × 400.
Worked example
A cell is taken from the root of a bean plant and examined under the microscope. No chloroplasts can be seen. A student concludes that the cell must therefore be an animal cell. Discuss whether the student is correct.
- 1Give the verdict: the student is not correct. (B1)
- 2Explain the science: chloroplasts are found only in plant cells that carry out photosynthesis. Root cells grow underground, where no light reaches them, so they do not photosynthesise and have no chloroplasts, even though they are plant cells. (B1)
- 3State the evidence that would settle the question: the absence of chloroplasts is not valid evidence of an animal cell. The student should look for a cell wall and a large central vacuole. If these are present, the cell is a plant cell whether or not chloroplasts are seen. (B1)
Watch out: Many students give only the verdict and the explanation. They lose the final mark for stating what evidence would settle the question.
Common mistakes
Saying the cell wall controls what enters the cell
Wrong: The cell wall controls the movement of substances into and out of the cell.
Right: The cell membrane is partially permeable and controls the movement of substances into and out of the cell. The cell wall is fully permeable and provides protection and support.
Confusing chloroplast with chlorophyll
Wrong: Photosynthesis occurs in the chlorophyll.
Right: Photosynthesis occurs in the chloroplast. Light energy is absorbed by the chlorophyll inside it.
Muddling the levels of organisation
Wrong: An organ is a group of similar cells.
Right: A tissue is a group of similar cells working together. An organ is made up of different tissues working together. The order is cell → tissue → organ → organ system → organism.
Questions students ask
What is the cell theory?
The cell theory has two statements. All living things are made up of one or more cells, and the cell is the basic unit of structure and function of life. When asked to state the cell theory, write both statements, because one alone earns partial credit at best.
What is the difference between a plant cell and an animal cell?
Both have a cell membrane, cytoplasm and a nucleus. A plant cell also has a cell wall made of cellulose and a large central vacuole containing cell sap. Plant cells in green parts also have chloroplasts. The rigid cell wall gives plant cells a regular, roughly rectangular shape, while animal cells have an irregular outline.
How do you focus a light microscope correctly?
Start with the lowest-power objective and place the slide with the specimen over the aperture. Watching from the side, use the coarse adjustment knob to bring the objective close to the slide without touching it. Then look through the eyepiece and turn the coarse adjustment the other way until the specimen is roughly in focus. Sharpen the image with the fine adjustment knob before switching to a higher-power objective, and re-focus with the fine adjustment only.
Why do we stain cells before viewing them under a microscope?
Cells are mostly water, so their structures are transparent and colourless, and an unstained cell is almost invisible. Different parts of the cell absorb a stain unevenly, which increases the contrast so the structures can be seen and identified. Iodine is used for onion epidermis (plant cells) and methylene blue for cheek cells (animal cells).
In the full PDF
- •Labelled diagrams of the model animal cell and model plant cell, with the biological drawing rules
- •A full table of microscope parts and their functions, plus the complete safe-use sequence
- •Further worked examples with mark-scheme answers on air bubbles, microscope errors, levels of organisation and identifying plant tissue
- •Complete cell-to-organism chains for one animal and one plant example
- •The remaining common mistakes and all five exam technique tips, including weak versus full-mark answers
- •End-of-topic summaries and the identification checklist for unknown cells