Types of Cells (HSC SSCE Biology): Revision Notes

Types of Cells

Introduction to cells

Living organisms are remarkable biological systems whose secrets have fascinated scientists for centuries. The development of microscopy technology revolutionised our understanding of life by revealing the cell as the fundamental unit of all living organisms.

Whilst all cells share certain basic similarities, they differ significantly in their structure, function, and organisation. Some organisms consist of a single cell that must perform all life functions independently. Other organisms are composed of many specialised cells, each with specific structures and roles, working together to maintain the organism's overall function.

Classification of cells

Despite the enormous diversity of cells in nature, all cells can be classified into two fundamental types: prokaryotic cells and eukaryotic cells. Prokaryotic cells are considered 'primitive' because they have a much simpler internal organisation compared to eukaryotic cells. Interestingly, prokaryotic cells vastly outnumber eukaryotic cells on Earth.

Prokaryotic cells

Definition and characteristics

The term 'prokaryotic' comes from the Greek words pro (meaning 'before') and karyon (meaning 'nucleus'). This name reflects the fact that these cells existed before the evolution of cells with a true nucleus.

Prokaryotic cells are remarkably small, typically measuring between $0.1$ and $5.0$ micrometres ($\mu$m) in diameter. This is approximately 10 to 100 times smaller than most eukaryotic cells.

Essential components

All prokaryotic cells contain four fundamental structures:

1. Cell membrane - Controls the passage of materials entering and leaving the cell
2. Cytoplasm - A gel-like fluid where chemical reactions occur
3. Ribosomes - Structures that synthesise proteins
4. Genetic material - DNA that carries hereditary information

Additional structures

Some prokaryotic cells possess additional features that provide advantages in certain environments:

• Cell wall - A protective outer layer that provides structural support and shape. The composition varies depending on the type of prokaryotic cell.
• Capsule - An outer layer made of complex carbohydrates that offers extra protection.
• Pili (singular: pilus) - Hair-like projections on the cell surface that enable the cell to attach to surfaces or other cells.
• Flagella (singular: flagellum) - Whip-like tails that propel the cell through liquid environments.

Organisation and examples

Most prokaryotic organisms are unicellular, meaning they consist of just one cell that performs all necessary life functions. However, some bacterial species form colonies where multiple prokaryotic cells cluster together and cooperate.

Prokaryotic organisms are divided into two major groups:

Bacteria - Found in diverse environments worldwide, bacteria can be either beneficial (such as those in our digestive system) or harmful (disease-causing pathogens).

Archaea - These unicellular organisms thrive in extreme environments, including hydrothermal vents on the ocean floor and hot springs. They were once thought to be bacteria but are now recognised as a distinct group.

Eukaryotic cells

Definition and characteristics

The term 'eukaryotic' derives from the Greek words eu (meaning 'true' or 'proper') and karyon (meaning 'nucleus'), indicating these cells possess a true nucleus.

Eukaryotic cells are substantially larger and more complex than prokaryotic cells. They typically range from $10$ to $100$ micrometres ($\mu$m) in size—approximately 10 to 100 times larger than prokaryotic cells.

Key features

The defining characteristic of eukaryotic cells is the presence of a membrane-bound nucleus containing the cell's genetic material. This nucleus is surrounded by a double membrane that separates the DNA from the rest of the cell's contents.

Organisation and examples

Organisms containing eukaryotic cells, known as eukaryotes, can be either unicellular or multicellular.

Unicellular eukaryotes include:

• Paramecium - A ciliated protozoan found in freshwater
• Amoeba - A shape-changing organism that moves using pseudopodia
• Euglena - A photosynthetic organism with a flagellum for movement

Multicellular eukaryotes include all plants and animals. These organisms are composed of many different types of specialised eukaryotic cells, each designed for particular functions within the organism.

Comparing prokaryotic and eukaryotic cells

Understanding the differences and similarities between these two cell types is essential for grasping cellular biology. The table below summarises the key distinctions:

Characteristic	Prokaryotic Cells	Eukaryotic Cells
Membrane-bound nucleus	Absent	Present
Membrane-bound organelles	Absent	Present
Cellular organisation	Primarily unicellular	Unicellular or multicellular
Size	$0.1$ - $5.0$ $\mu$m	$10$ - $100$ $\mu$m
Genetic material organisation	DNA in cytoplasm (chromosome and plasmids)	DNA enclosed within nucleus
Examples	Bacteria, Archaea	Plants, animals, fungi, protists

Observing cells with microscopy

Light microscopes are valuable tools for examining both prokaryotic and eukaryotic cells. However, they have limitations in the level of detail they can reveal.

Using a light microscope, students can typically observe:

• Cell membranes
• Nuclei (in eukaryotic cells)
• Cytoplasm
• Chloroplasts (in plant cells)
• Cell walls (in plant cells and bacteria)

Smaller structures require more powerful electron microscopes to visualise clearly. When preparing specimens for microscopy, stains such as methylene blue are often used to make cellular structures more visible against the background.

Calculating total magnification

When using a compound light microscope, the total magnification is determined by multiplying the magnification of the eyepiece (ocular lens) by the magnification of the objective lens currently in use:

$\text{Total magnification} = \text{Ocular lens magnification} \times \text{Objective lens magnification}$

The following table shows common magnification combinations:

Ocular Lens Magnification	Objective Lens Magnification	Total Magnification
$10\times$	$20\times$	$200\times$
$10\times$	$40\times$	$400\times$
$12\times$	$40\times$	$480\times$

Remember!