Chapter 2: Cell: The Building Block of Life
Original notes — ready CBSE (NCERT, English Medium) • Class 9 • Science • Book: Exploration
Chapter 2: Cell: The Building Block of Life — EG's own notes, in simple words
Book: Exploration (CBSE, Class 9) | Chapter 2 | Medium: English
Diagram (labels are printed on the image): Diagram (labels are printed on the image): A plant cell, with its wall, nucleus, chloroplasts and large vacuole. A general diagram for study, not a figure copied from the book.
Looking at cells through a microscope. A general picture about the microscopic world of cells, not a book scene.
What the chapter is about
All living things are made of cells. Some organisms, like bacteria, have just one cell, while large organisms like a mango tree or a human being are made of trillions of cells working together. The cell is therefore called the basic structural and functional unit of life.
Cells were discovered when microscopes became powerful enough to see them. The cell theory states that all living things are made of cells, and that new cells arise from existing cells. A cell has a membrane that holds it together, cytoplasm in which life processes happen, and a nucleus that controls the cell in most kinds of cells.
Plant and animal cells share many parts, such as the nucleus, cytoplasm and mitochondria, but they also differ. Plant cells have a cell wall and chloroplasts and usually a large vacuole, while animal cells do not have a cell wall or chloroplasts. Cells can also divide: mitosis makes new body cells, and meiosis makes reproductive cells.
What you should learn
Notice what all cells share, how plant and animal cells differ, and what each major organelle does for the cell.
Key Terms
- Cell — The smallest unit of a living thing that can carry out life processes.
- Nucleus — The part of the cell that controls its activities and holds the genetic material.
- Mitochondria — The organelle that releases energy from food for the cell; often called the powerhouse of the cell.
- Chloroplast — The green organelle in plant cells where photosynthesis takes place.
- Cell membrane — The thin boundary that holds the cell together and controls what enters and leaves it.
Remember
- All living things are made of one or more cells.
- New cells arise from existing cells.
- Plant cells have a cell wall, chloroplasts and usually a large vacuole; animal cells do not.
- Mitochondria release energy; the nucleus controls the cell.
Exam Tip / Common Mistake
Common mistake: saying the cell wall and the cell membrane are the same thing. The cell membrane is present in all cells; the cell wall is an extra, rigid layer found in plant cells (and some others), outside the membrane.
Chart / Table: Plant cell vs animal cell
| Point | Detail |
|---|---|
| Cell wall | Present in plant cells; absent in animal cells |
| Chloroplasts | Present in plant cells; absent in animal cells |
| Vacuole | Large and central in plant cells; small or absent in animal cells |
| Nucleus | Present in both |
| Mitochondria | Present in both |
Practice set — try it yourself
- Why is the cell called the basic unit of life?
Hint: Think about what every living thing is made of. - Name three parts found in both plant and animal cells.
Hint: Think about the parts every cell needs to live. - What does the nucleus do in a cell?
Hint: Think about control and instructions. - Why do plant cells need chloroplasts but animal cells do not?
Hint: Think about how plants make their food. - How is mitosis different from meiosis?
Hint: Think about body cells and reproductive cells.
Questions with Answers
- What does the cell theory state?
The cell theory states that all living things are made of cells, and that new cells arise from existing cells. - What is the function of mitochondria?
Mitochondria release energy from food in the form the cell can use, which is why they are often called the powerhouse of the cell. - How is a plant cell different from an animal cell?
A plant cell has a cell wall, chloroplasts and usually a large central vacuole, while an animal cell has none of these. Both have a nucleus, cytoplasm, cell membrane and mitochondria. - What is the role of the cell membrane?
The cell membrane holds the cell together and controls which substances enter and leave the cell. - Why do cells divide?
Cells divide for growth, repair of damaged tissues, and reproduction. Mitosis produces new body cells, while meiosis produces reproductive cells.
How to use this page: Read the summary, say the Key Terms in your own words, then try the practice set without looking. Use the Questions with Answers to check yourself afterwards. Ask your teacher or parent to check your answers.