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# Chapter 2: Cells: the basic units of life

## 2.1 Overview (ESG4M)

### Introduction (ESG4N)

THE BASIC UNITS OF LIFE

Overview

A brief overview of the history of microscopy needs to be taught. This should include the progression from the use of lenses, light microscopy and then electron microscopy. The development of microscopy over the years has enabled people to view cells and then with the introduction of the electron microscope the structures within cells. All this led to the cell theory.

This section deals with the ultra-structure of cells, including cell wall, cell membrane, nucleus, cytoplasm, and various organelles such as chloroplast, mitochondria, ER, vacuoles, Golgi bodies and centrosomes.

Transport across membranes is also covered briefly. Learners will need to understand the movement of molecules by diffusion, osmosis and active transport.

 Key Concepts: Cells are considered to be the basic units of life. Cells are microscopic and we use microscopes to view them. Cells differ in size, shape and structure and these are adapted to their specific functions within the tissue. The cell is made up of smaller structures known as organelles, namely: nucleus cytoplasm mitochondria vacuoles Golgi body plastids vacuole, lysosomes, vesicles We will learn the structure and function of each of these organelles.

"In the year of 1657 I discovered very small living creatures in rain water." — Antonie van Leeuwenhoek, the Father of Microbiology, on discovering cells. Together with Robert Hooke's discoveries, van Leeuwenhoek's findings laid the foundations of microbiology.

• The invention of microscopes allowed us to see life at the microscopic level.
• Cell walls are present in plants, bacteria and fungi and provide a rigid support structure.
• Cell membranes are semi-permeable and have a fluid rather than a fixed structure. Substances move across them by diffusion, osmosis, facilitated transport and active transport.
• Cells contain organelles with structures adapted to perform specific functions within the cell.
• Cells differ in size, shape and structure in order to carry out specialised functions. Cells with similar structures and functions associate to form tissues.
• Plant and animal cells differ in many crucial ways.

The previous chapter discussed the various organic molecules that make up living organisms. In this chapter we will look at the cellular level of organisation of living things.

atom$$\rightarrow$$molecule$$\rightarrow$$cell$$\rightarrow$$tissue$$\rightarrow$$organ$$\rightarrow$$system$$\rightarrow$$organism$$\rightarrow$$ecosystem