Class 9 BiologyChapter 2NCERT Exploration, Grade 9
Cell: The Building Block of Life
From hot springs in Ladakh to the trillions of cells in you.
- 6 topic pages
- 20 interactives
- 29 quick checks
- 29 flashcards
- 18 practice questions
Think it over
- Every living thing is made of cells, and every cell came from another cell. So where did the very first one come from?
- People lived for thousands of years without knowing cells existed. What had to be invented before anyone could find out?
- A brick is the unit of a wall. In what sense is a cell the unit of a living thing, and in what ways is it much more than a brick?
Where it may have begun
Picture a bitterly cold valley in Ladakh with springs of water close to boiling. These are the hot springs of the Puga Valley, and they may resemble the early Earth of about 3.5 billion years ago. Scientists broadly agree that life began in water; some suspect it began not in the open ocean but in small pools whose conditions kept shifting, much like these springs. Today they are home mostly to thermophiles: heat-loving bacteria, each a single cell.
Around these springs, calcium carbonate builds up fast, as a team from the Birbal Sahni Institute of Palaeosciences, Lucknow, found. Layers like these could have sheltered the first organic molecules from harsh radiation and other extremes, and possibly helped the very first protective membrane to form. That membrane is the boundary that marks out a cell.
From cell to organ system
A bacterium or a yeast lives its whole life as one cell. A mango tree, a fish or a person is built from millions of cells that share out the work. Either way, the cell is where life happens: the basic unit of structure and function.
- CellThe basic unit of structure and function.
- TissueA group of similar cells doing a similar job.
- OrganDifferent tissues organised together, such as the lungs.
- Organ systemOrgans working as a team: nasal passages, trachea and lungs make the respiratory system.
The chapter, section by section
- NCERT 2.1
How do we study cells?
- From about 25 cm away, the human eye can tell two points apart only if they are at least 0.1 mm apart. This is its limit of resolution.
- Most cells are far smaller than that, so we need lenses. A light microscope uses an objective lens and an eyepiece; their powers multiply.
- NCERT 2.2 and 2.2.1
The cell membrane and osmosis
- Every cell is wrapped in a cell membrane (plasma membrane). It is selectively permeable: some substances pass, others do not.
- Osmosis is the movement of water through a selectively permeable membrane, from a dilute solution to a concentrated one.
- NCERT 2.2.2, 2.3 and Activity 2.4
Cell wall, and prokaryotic and eukaryotic cells
- Plant cells have a rigid but permeable cell wall outside the cell membrane, mainly made of cellulose. Fungi and bacteria have walls too.
- In concentrated sugar solution a plant cell keeps its shape while its contents shrink away from the wall; an animal cell just shrinks.
- NCERT 2.3.1
Cell organelles: the factory inside a cell
- Eukaryotic cells divide their work among organelles, so many life processes run side by side, like departments of a factory.
- The nucleus holds the DNA in chromosomes; the nucleolus makes ribosome parts. Ribosomes make proteins.
- NCERT 2.4 and 2.4.1
Cell division: mitosis and meiosis
- Bodies grow because cells divide, not only because they get bigger; a cell has a size limit it cannot pass.
- Onion root tips divide all the time, so a squashed tip shows cells in different stages of division (Activity 2.5).
- NCERT 2.5 and 2.5.1
Cell theory and the life of a cell
- Schleiden (1838): all plants are made of cells. Schwann (1839): all animals are too. Virchow (1855): new cells come only from existing cells.
- Cell theory: all living things are made of one or more cells; the cell is the basic unit of structure and function; all cells come from pre-existing cells.
The chapter at a glance
- The cell is the basic unit of structure and function in every living thing.
- Prokaryotic cells have no well-defined nucleus (their DNA lies in the nucleoid) and no membrane-bound organelles.
- Eukaryotic cells are larger and more complex, with a membrane-bound nucleus and many membrane-bound organelles.
- Every cell has a cell membrane; plant, fungal and bacterial cells also have a cell wall outside it.
- Chromosomes in the nucleus are made of DNA and proteins and carry genetic information.
- The cytoplasm of eukaryotic cells holds organelles, each with its own job: nucleus, ER, Golgi apparatus, ribosomes, lysosomes, mitochondria.
- Plant cells also have plastids: chloroplasts, chromoplasts and leucoplasts.
- Mitosis makes two cells identical to the parent; meiosis divides twice to make four cells with half the chromosomes.
- Normal cells grow under control, work and die on time; cancer cells lose control and form tumours.