What this chapter is about
Every living organism, from a tiny bacterium to a giant elephant, is made up of cells. This chapter introduces you to the cell as the basic structural and functional unit of life. You will learn how scientists discovered cells, why they are considered the fundamental building blocks of all organisms, and how cells are organised within living bodies.
The chapter explores the differences between plant and animal cells, the various parts that make up a cell (called organelles), and the specific jobs each part performs. You will understand how the nucleus controls cell activities, how mitochondria provide energy, and how the cell membrane regulates what enters and leaves the cell.
After studying this chapter, you should be able to describe the structure of a typical cell, identify the functions of different cell organelles, distinguish between prokaryotic and eukaryotic cells, and explain why cells are essential for life. This knowledge forms the foundation for understanding tissues, organs and how living organisms function.
Key ideas
- A cell is the smallest unit of life that can carry out all life processes independently. All living organisms are made of one or more cells.
- Cells were first observed by Robert Hooke in 1665 when he examined thin slices of cork under a microscope and saw small box-like compartments.
- Organisms with only one cell are called unicellular (like amoeba and bacteria), while those with many cells are called multicellular (like humans and mango trees).
- Prokaryotic cells lack a well-defined nucleus and membrane-bound organelles (found in bacteria), while eukaryotic cells have a true nucleus with a nuclear membrane (found in plants, animals, fungi).
- The cell membrane is a selectively permeable boundary that controls movement of substances into and out of the cell.
- The nucleus contains chromosomes made of DNA, which carries genetic information and controls all cell activities.
- Different organelles perform specific functions: mitochondria produce energy, ribosomes make proteins, chloroplasts carry out photosynthesis in plant cells, and vacuoles store materials.
Formulas and facts to remember
Cell theory states: All living organisms are composed of cells; cells are the basic units of life; all cells arise from pre-existing cells.
Cell membrane: A thin, flexible, selectively permeable layer made of lipids and proteins that forms the outer boundary of all cells.
Cytoplasm: The jelly-like substance between the cell membrane and nucleus where organelles are suspended and many chemical reactions occur.
Nucleus: The control centre of the cell containing DNA; bounded by a double-layered nuclear membrane with pores.
Mitochondria: Called the powerhouse of the cell; produces energy in the form of ATP through cellular respiration.
Chloroplast: Found only in plant cells; contains chlorophyll and is the site of photosynthesis.
Endoplasmic reticulum: A network of membranes; rough ER has ribosomes and makes proteins; smooth ER makes lipids.
Plant cells differ from animal cells: Plant cells have a cell wall, chloroplasts, and a large central vacuole; animal cells lack these but have small vacuoles and sometimes centrioles.
Worked examples
Example 1: Identifying cell types
A student observes two cells under a microscope. Cell A has a rigid outer layer, green disc-shaped structures, and one large central space filled with fluid. Cell B has no rigid outer layer, no green structures, and several small fluid-filled spaces. Identify which is a plant cell and which is an animal cell.
Solution:
- Cell A has a rigid outer layer → this is the cell wall, found only in plant cells
- Cell A has green disc-shaped structures → these are chloroplasts containing chlorophyll
- Cell A has one large central space → this is the large central vacuole typical of plant cells
- Cell B lacks all these features and has small vacuoles → characteristic of animal cells
- Therefore, Cell A is a plant cell and Cell B is an animal cell.
Example 2: Understanding cell size
A bacterial cell is about 2 micrometres (2 μm) in length. A human red blood cell is about 7 μm in diameter. How many times larger is the red blood cell compared to the bacterium?
Solution:
- Size of bacterium = 2 μm
- Size of red blood cell = 7 μm
- Comparison = 7 ÷ 2 = 3.5 times
- The human red blood cell is about 3.5 times larger than the bacterial cell in diameter.
Example 3: Function matching
Ravi wants to understand why a plant cell placed in sunlight can make its own food while an animal cell cannot. Explain using your knowledge of organelles.
Solution:
- Plant cells contain chloroplasts, which have a green pigment called chlorophyll
- Chlorophyll captures light energy from the sun
- Using this energy, carbon dioxide and water are converted into glucose (food) through photosynthesis
- Animal cells do not have chloroplasts
- Without chloroplasts, animal cells cannot capture light energy and therefore cannot make their own food
- Animals must obtain food by eating plants or other animals.
Common mistakes
Thinking the cell wall and cell membrane are the same thing → The cell wall is a rigid outer layer in plant cells only, while the cell membrane is a flexible inner boundary present in all cells.
Believing all cells look and function identically → Cells vary greatly in size, shape and function depending on their role (nerve cells are long, red blood cells are disc-shaped, muscle cells can contract).
Confusing prokaryotic and eukaryotic by just size → The key difference is organisation: prokaryotes lack a nuclear membrane and membrane-bound organelles, not just their size.
Thinking mitochondria are found only in animal cells → Mitochondria are present in both plant and animal cells; only chloroplasts are exclusive to plant cells.
Assuming the nucleus is the largest organelle in all cells → In mature plant cells, the central vacuole often occupies most of the cell volume and pushes the nucleus to one side.
Quick revision
- Cell is the structural and functional unit of life; discovered by Robert Hooke in 1665.
- Prokaryotic cells (bacteria) lack nuclear membrane; eukaryotic cells (plants, animals) have a true nucleus.
- Plant cells have cell wall, chloroplasts and large central vacuole; animal cells do not.
- Nucleus controls cell activities; mitochondria produce energy; ribosomes make proteins.
- Cell membrane is selectively permeable — controls what enters and leaves the cell.
- Cell theory: all living things are made of cells, cells are basic units of life, cells come from existing cells.