Lesson 1: What are cells?

Cells are the basic structural, functional, and biological units of all living organisms. Everything from a tiny bacterium to a large tree or human is made up of cells. They are often called the “building blocks of life”.

Organisms can be:

• Unicellular – made up of one single cell (e.g., bacteria, amoeba)
• Multicellular – made up of many cells that work together (e.g., humans, plants)

In multicellular organisms, cells are specialized. This means different types of cells perform different functions, such as muscle cells for movement, nerve cells for communication, and root hair cells in plants for absorption.

Functions of Cells

Cells perform essential life processes like:

• Respiration (energy release)
• Growth and repair
• Transport of substances
• Reproduction (division to form new cells)
• Response to environmental changes

Parts of a Typical Animal and Plant Cell

Every cell contains tiny structures called organelles, each with a specific role. The main organelles are:

1. Cell membrane

   • Thin, flexible layer surrounding the cell
   • Controls what enters and exits the cell (selectively permeable)
   • Found in both animal and plant cells

2. Cytoplasm

   • Jelly-like substance filling the cell
   • Site of most chemical reactions
   • Organelles are suspended in it

3. Nucleus

   • Contains the cell’s DNA (genetic material)
   • Controls the activities of the cell (like a brain)
   • Surrounded by a nuclear membrane

4. Mitochondria

   • Known as the “powerhouse of the cell”
   • Site of aerobic respiration (energy release from food)
   • More active cells (like muscle cells) have more mitochondria

5. Ribosomes

   • Tiny structures (can be free-floating or attached to ER)
   • Responsible for making proteins

6. Vacuoles

   • In plant cells: large, central, filled with cell sap (stores nutrients, waste, helps maintain pressure)
   • In animal cells: small, temporary, used for storage or transport

7. Cell wall (plant cells only)

   • Rigid outer layer made of cellulose
   • Provides shape and mechanical support
   • Fully permeable

8. Chloroplasts (plant cells only)

   • Contain chlorophyll (green pigment)
   • Site of photosynthesis – process by which plants make their own food
   • Not found in animal cells

Comparison of Animal and Plant Cells

Important Concepts to Remember

• Cells are microscopic – too small to see with the naked eye
• Specialized cells perform specific tasks (e.g., red blood cells carry oxygen, nerve cells transmit signals)
• Tissues are made up of similar cells working together
• Organs are made up of different tissues
• Organ systems are groups of organs working together (e.g., digestive system)

Cells are like tiny factories — each part (organelle) has a job, and they all work together to keep the organism alive. Understanding the parts of a cell and their functions is essential before learning about DNA, genes, and how life operates at the molecular level.