Introduction
Every living organism — from the tiniest bacterium to a giant whale — is made of cells. The cell is the structural and functional unit of life. Understanding cells is the foundation of biology. In this chapter you will learn about cell discovery, cell types, cell organelles, and the differences between plant and animal cells.
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Key Concepts
1. Discovery of the Cell
- Robert Hooke (1665) first observed cells in a thin slice of cork using a compound microscope. He described the tiny compartments as "cells" (from Latin: cellula = small room).
- Anton van Leeuwenhoek was the first to observe living cells (bacteria and protozoa).
- Cell Theory (Schleiden, Schwann, Virchow):
1. All living organisms are made of cells.
2. The cell is the basic unit of life.
3. All cells arise from pre-existing cells.
2. Types of Cells
| Feature | Prokaryotic | Eukaryotic |
|---|---|---|
| Nucleus | Absent (nucleoid region) | Present (membrane-bound) |
| Size | 1-10 micrometres | 10-100 micrometres |
| Examples | Bacteria, Archaea | Plants, Animals, Fungi |
| Membrane-bound organelles | Absent | Present |
- 3. Cell Organelles (Eukaryotic)
- Cell membrane (plasma membrane): Selectively permeable; controls what enters and leaves the cell. Made of phospholipid bilayer.
- Cell wall (plants, fungi, bacteria): Provides rigidity and support. Made of cellulose in plants.
- Nucleus: Control centre; contains DNA. Surrounded by the nuclear membrane (envelope). Contains nucleolus (site of ribosome synthesis).
- Mitochondria: "Powerhouse of the cell." Site of cellular respiration; produces ATP energy.
- Chloroplasts (plant cells only): Site of photosynthesis. Contains green pigment chlorophyll.
- Endoplasmic Reticulum (ER): Rough ER (has ribosomes, makes proteins); Smooth ER (makes lipids, detoxifies).
- Golgi apparatus: Packages and ships proteins and lipids to their destinations.
- Lysosomes: Contain digestive enzymes; break down waste and foreign particles. Called "suicide bags."
- Vacuoles: Storage organelles. Plants have a large central vacuole for water and structural support.
- Ribosomes: Tiny organelles (no membrane); site of protein synthesis. Found in all cells.
- Centrosome/Centrioles (animal cells): Involved in cell division.
4. Differences: Plant vs Animal Cells
| Feature | Plant Cell | Animal Cell |
|---|---|---|
| Cell wall | Present (cellulose) | Absent |
| Chloroplasts | Present | Absent |
| Large vacuole | Present | Small or absent |
| Centrioles | Absent | Present |
| Shape | Regular, rectangular | Irregular |
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Worked Examples
Why is the cell called the structural and functional unit of life?
Cells carry out all life processes — respiration, reproduction, nutrition, excretion — independently. They form the physical structure of all tissues and organs. Hence they are both structural (building blocks) and functional (carry out life processes) units.
A student observes a cell with a cell wall, a large central vacuole, and chloroplasts. Is it a plant or animal cell? Explain.
It is a plant cell. All three features — cell wall, large central vacuole, and chloroplasts — are present in plant cells but absent in animal cells.
Why are mitochondria called the powerhouse of the cell?
Mitochondria carry out aerobic cellular respiration, breaking down glucose to produce ATP (adenosine triphosphate), the energy currency of the cell. This ATP powers all other cellular activities.
Differentiate between prokaryotic and eukaryotic cells with one example each.
Prokaryotic cells lack a membrane-bound nucleus; DNA floats freely in the cytoplasm (example: Escherichia coli). Eukaryotic cells have a well-defined nucleus enclosed in a nuclear membrane (example: human liver cell).
What happens if the cell membrane is destroyed?
The cell membrane controls the selective entry and exit of substances. Without it, toxic substances would enter freely, essential molecules would leak out, and the cell would die. It is essential for maintaining homeostasis.
Why do plant cells not need centrioles?
Centrioles form the mitotic spindle in animal cells during cell division. Plant cells form a spindle using other proteins and do not need centrioles. This is a structural difference related to how the two cell types divide.
A cell has many mitochondria and many ribosomes. What type of cell is it likely to be?
A cell with many mitochondria needs a lot of energy (e.g., a muscle cell). Many ribosomes indicate high levels of protein synthesis. This describes an active muscle cell or a secretory gland cell.
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Common mistakes
- Confusing cell wall (rigid, outer layer) with cell membrane (thin, inner layer present in all cells).
- Stating that only plant cells have mitochondria — both plant and animal cells have mitochondria.
- Thinking ribosomes are only in eukaryotic cells — ribosomes are present in all living cells, including prokaryotes.
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Summary
The cell is the fundamental unit of life. Prokaryotic cells are simple and lack membrane-bound organelles. Eukaryotic cells are complex, with a true nucleus and specialised organelles. Plant and animal cells share many features but differ in the presence of cell walls, chloroplasts, and central vacuoles. Each organelle plays a distinct and vital role in keeping the cell alive.