1.1 Cell Structure and Function

Cambridge O Level Biology (5090)

https://cambridgeassistant.com

2026 Syllabus Objectives

By the end of this topic, you should be able to:

  1. Examine animal and plant cells under a microscope using staining techniques such as methylene blue or iodine solution
  2. Draw diagrams to represent what you observe when looking at animal and plant cells
  3. Identify the following structures in an animal cell: ribosomes, mitochondria, nucleus, cytoplasm, and cell membrane
  4. Identify the following structures in a plant cell: ribosomes, mitochondria, chloroplasts, nucleus, sap vacuole, cytoplasm, cell membrane, and cellulose cell wall
  5. Describe the structure of a bacterial cell, including: ribosomes, circular DNA and plasmids, cytoplasm, cell membrane, and cell wall
  6. Describe the functions of all the structures listed above in animal, plant, and bacterial cells

Main Content

https://cambridgeassistant.com

Section 1: Using a Microscope to Examine Cells

Cells are incredibly tiny — so small that you cannot see them with the naked eye. To look at cells, we use a microscope, which is a scientific instrument that uses lenses to magnify (make bigger) objects that are too small to see normally.

Why Do We Use Stains?

Most cells are nearly transparent (see-through), which makes it very hard to spot their internal parts under a microscope. To solve this, we add stains — coloured dyes that attach to different parts of the cell and make them visible.

Two common stains used in school practicals are:

  • Methylene blue — a blue dye used to stain animal cells (for example, cheek cells from inside your mouth). It colours the nucleus a deep blue, making it easy to see.
  • Iodine solution — an orange-brown dye used to stain plant cells (for example, cells from an onion). It turns the nucleus and starch grains in the cell a blue-black colour.

How to Make a Temporary Slide (Practical Steps)

A temporary stain means the stain is not permanent — it is just for looking at cells during a single session.

For animal cells (e.g. cheek cells):

  1. Gently scrape the inside of your cheek with a cotton swab.
  2. Smear the material onto a glass slide.
  3. Add a drop of methylene blue stain.
  4. Place a coverslip gently on top (to protect the slide and flatten the sample).
  5. View under the microscope, starting with the lowest magnification and increasing gradually.

For plant cells (e.g. onion cells):

  1. Peel a thin, single layer of skin from the inside of an onion.
  2. Place it flat on a glass slide.
  3. Add a drop of iodine solution.
  4. Place a coverslip gently on top.
  5. View under the microscope.

Sign in to view full notes