GCSE Biology: This first section will cover the following core topics: -

  • A very Basic overview of cell theory and the important people involved.

  • Microscopy

  • Simple magnification calculations

  • Components of a bacterial cell

  • The nucleus of the cell

  • Function of the components of a plant cell

  • Function of the components of an animal cell

  • How plant and animal cells can be studied with a light microscope

Cell Theory: Organization of Life

So, it's important that you know first of all that "All organisms are composed of cells".

Robert Hooke: By Rita Greer - The original is an oil painting on board by Rita Greer, history painter, 2004. This was digitized by Rita and sent via email to the Department of Engineering Science, Oxford University, where it was subsequently uploaded to Wikimedia., FAL, https://commons.wikimedia.org/w/index.php?curid=7667243

By Rita Greer - The original is an oil painting on board by Rita Greer, history painter, 2004. This was digitized by Rita and sent via email to the Department of Engineering Science, Oxford University, where it was subsequently uploaded to Wikimedia., FAL, https://commons.wikimedia.org/w/index.php?curid=7667243

GCSE Biology: Cell Biology

What is a cell?

Cells can be described as "the basic units life".


Cells were 'discovered' by Robert Hooke in England in 1665. Hooke was using one of the first microscopes. Another important dude in the realm of cell biology was Anton van Leeuwenhoek. He used microscopes capable of magnifying 500 times, and discovered an amazing world of single-celled life in a drop of pond water.

Anton van Leeuwenhoeks microscope - capable of magnifying 500 times!

What Anton van Leeuwenhoek saw through his microscope

In 1839, two German biologists, Matthias Schleiden and Theodor Schwann summarised a large number of observations by themselves and others. They concluded that all living organisms consist of cells. Their conclusion forms the basis of what has come to be known as cell theory.

Matthias Schleiden

Theodor Schwann


Of all the techniques used in biology microscopy is probably the most important. The vast majority of living organisms are too small to be seen in any detail with the human eye, and cells and their organelles can only be seen with the aid of a microscope. 

A Light (or Optical) microscope uses light waves


is how much bigger a sample appears to be under the microscope than it is in real life.

Resolution: Is the ability to distinguish between two points on an image i.e. the amount of detail or clarity. The resolving power of a particular microscope depends on the wavelength or form of radiation used.

Whilst you are going to be most familiar with the light microscope (since this is the type you'll use in school and college (and most likely have to label up!) Another much more powerful microscope is The Electron Microscope. Electron microscopes are categorised in 2 types: -

  • A scanning electron microscope (SEM) and a

  • Transmission electron microscope (TEM).


As their name suggests Electron microscopes use a focused beam of electrons (instead of light).

It was in the 1930s that Electron microscopes were made. By Focusing a beam of electrons (instead of light), the electron microscopes (EM) were able to achieve a much higher resolution power.



Electron microscopes achieved better magnification and higher resolutions because electrons have a smaller wavelength than light.

The Electron Microscope

Microscopy: Simple magnification calculations

When Working out simple microscopy calculation use the "IAM" Triangle

When you need to work out Magnification:


If you are given the Image size and the Actual size you need to work out the Magnification (how much bigger the image size is compared to its Actual size). Use the "IAM" Triangle like this: -

Cover the "M" and use Image size divided by the Actual size.

When you need to work out the image size (i.e. the diagram/picture):


If you are given the Actual size of an object and the Magnification used, you are being asked to work out the Image size (this could be the diagram /photograph measured).

Use the "IAM" Triangle like this: -

Cover the "I" and use Actual size given Multiplied by the Magnification given.

When you need to work out the Actual size of the object (e.g. cell):


If you are given the Image size (or asked to measure it!) (e.g. a photograph or drawing) and you are given the Magnification used, you are being asked to work out the Actual size of something.

Use the "IAM" Triangle like this.

Cover the "A" and use

image size given

divided by the

Magnification given.

Here is a common example using the IAM triangle and very commonly asked question in both GCSE and A-Level Biology!


The diagram below shows Escherichia coli (E. coli) bacterium at a magnification of 20 000x.


Common questions at GCSE (and A-Level!) are:


"What is the actual length of the bacterium from A to B in micrometers (µm)?"


Remember to be awarded full marks you must always show your working out!


So how do you answer this very common (GCSE biology and A-Level Biology) question?


1. Measure the distance from A to B in mm


2. Use the IAM Triangle to calculate the actual length...

(Image) 150mm / (Magnification) 20000 = (Actual size) 0.0075 mm


3. Convert millimeters (mm) to micrometers (µm). Remember millimetres (mm) and micrometers (µm) differ by a factor of 1000, that is 1µm is 1000 times smaller than a 1 mm!

So, to convert  millimetres (mm) into micrometers (µm) just multiply by 1000 like this: -


0.0075mm x 1000 = 7.5µm (micrometers)

Answer: 7.5µm

So, there it is your step by step 'working out' and your final answer must be clearly stated (or shown as it is above.

GCSE Cell Biology: Prokaryotes (bacterial cells) Vs Eukaryotes (plant and animal cells)

Did you know that ALL Organisms can be categorised as either Prokaryotic or Eukaryotic?

So, What do these names mean?

and What is  the main difference between a prokaryotic cell (bacteria) and a Eukaryotic cell (plant, animal, fungal cell)?

What do the terms Prokaryote and Eukaryote mean?

Well 'Pro' means "before" and 'Kary' is from the Greek referring to 'Nut' ("meaning the nucleus of a cell looked a bit like a darkly stained nut").


So, Prokaryote means Before Nut, or more literally for our use in cell biology "Before Nucleus". This simply means that Prokaryotes DO NOT HAVE a (True) NUCLEUS.

'Eu' means True. Which quite simply means that Eukaryotes have a TRUE 'Nut' - or Biologically speaking Eukaryotes have a TRUE NUCLEUS.

So what is the main difference between a Prokaryotic cell and Eukaryotic cell?

Prokaryotes DO NOT have a 'True' Nucleus whereas

Eukaryotes HAVE a True, well defined Nucleus.

Cells: Structure and Function

Cell Membrane
Cell Wall

You have to know the structure of a prokaryotic cell and the structure and function of a Eukaryotic cell (you must be able to compare and contrast the differences between prokaryotic & eukaryotic cells too).


Eukaryotic cells are more complex than prokaryotic cells, and include all animal and Plant cells. Eukaryotic cells have different Organelles, e.g True nucleus, larger ribosomes, and mitochondria… on the other hand Prokaryotes (bacteria) are smaller and simpler than Eukaryotes with no membrane bound organelles.

Eukaryotes have membrane bound organelles, where important cellular functions take place. You’ll need to know the structure and function of a generalised plant cell, a generalised animal cell and a few examples of specialised animal cells like sperm, liver and muscle.

Eukaryotes have a true well defined nucleus.


The Nucleus

The nucleus contains the genetic information of the cell in multiple strands of DNA and protein (chromosomes). The nucleus contains genes that control the eukaryotic cell.


The nucleus: contains DNA. Key functions are – replication, cell division, & protein synthesis. 

In a eukaryotic cell its DNA is linear & attached to proteins called histones.


Ribosomes: “Site of protein synthesis” - Small & dense structure NO membrane.


Mitochondria are found in almost all eukaryotic cells and are responsible for generating most of the ATP (energy currency).

Specialised cells


sperm cell, red blood cell, muscle cell and liver cell.


These specialised cells are very active and require lots of energy. so, they have many mitochondria which provides the ATP (energy currency) generated from cellular respiration.

Plant cells:


Chloroplasts are the sites of photosynthesis and are found only in the cells of plants and green algae - which have chloroplasts. Chloroplasts are where photosynthesis occurs, and chloroplasts contain a green substance called chlorophyll.


Inside the plant cell is a permanent vacuole containing cell sap – a weak solution of sugars and salts.


The cell wall a tough, flexible, (sometimes fairly rigid) layer, located external to the cell membrane.

Prokaryotes (bacterial cells)

So you know what a Prokaryote is (a cell without a true nucleus).

Did you know that Prokaryotes are Bacteria?


Biologist like to classify and categorise things... and a Prokaryotic cell is just the biological classification for all bacterial cells. So, all bacteria are prokaryotes and all prokaryotes are bacteria! 


Bacteria (prokaryotes) are single celled organisms.


For example: Escherichia coli (E. coli) is a species of bacteria that is commonly found living in the intestines of people and animals. E. coli is a single cell, which is "rod" shaped. Under certain conditions E. coli can become pathogenic (which means it has the potential to cause disease).


So, a pathogen is an organism that can cause disease - and the pathogenic strains of E. coli cause food poisoning and diarrhoea (sometimes called travellers diarrhoea)

Recording of E. coli bacterial cells when viewed under the light microscope