Cell: The Unit of Life
ICSE Class 9 · Biology · Chapter 2
What this chapter covers
- The cell, a unit of life
- The cell theory
- Protoplasm, cytoplasm and nucleus
- Cell boundaries: cell wall and cell membrane
- Cell organelles and their functions
- Structure of a plant cell and an animal cell
- Prokaryotic and eukaryotic cells
- Differences between a plant cell and an animal cell
Key definitions
Protoplasm is the living substance of a cell, made up of the cytoplasm and the nucleus together, enclosed by the cell membrane.
Cytoplasm is the part of the protoplasm that lies between the cell membrane and the nuclear membrane, consisting of a fluid matrix in which the cell organelles are suspended.
ATP is the energy-carrying molecule of the cell, which supplies energy wherever it is needed for work such as muscle contraction or the making of proteins.
Key points
- The cell is the basic structural and functional unit of life; organisms may be unicellular or multicellular.
- Robert Hooke first saw cells in cork in 1665; Leeuwenhoek first saw living cells; Robert Brown discovered the nucleus.
- The cell theory (Schleiden, Schwann, Virchow) states that all organisms are made of cells, the cell is the unit of structure and function, and all cells come from pre-existing cells. Viruses are an exception.
- Protoplasm, the living substance of the cell, consists of the cytoplasm and the nucleus.
- The nucleus controls the cell and carries the hereditary material in its chromatin; the nucleolus makes ribosome parts.
- The cell membrane is living, thin and selectively permeable; the cell wall of plants is non-living, made of cellulose, rigid and freely permeable.
- Mitochondria release energy as ATP; the endoplasmic reticulum transports materials; ribosomes make proteins; Golgi bodies package and secrete.
- Plastids (chloroplasts, chromoplasts, leucoplasts) occur only in plant cells; lysosomes digest materials and are the suicide bags; the centrosome forms the spindle in animal cells; vacuoles store materials and keep plant cells turgid.
- Prokaryotic cells (bacteria, cyanobacteria) lack a true nucleus and membrane-bound organelles; eukaryotic cells (protists, fungi, plants, animals) have both.
- Plant cells differ from animal cells in having a cell wall, a large central vacuole and plastids, and in lacking a centrosome.
Questions answered
Define a cell, and explain why it is called both the structural and the functional unit of life.
A cell is the basic structural and functional unit of life. It is structural because every part of an organism is made of cells or their products, and functional because every life activity is carried out by cells.
State the cell theory and name the scientists who proposed it.
All living organisms are made of cells; the cell is the basic unit of structure and function; new cells arise only from pre-existing cells. Schleiden (1838) and Schwann (1839) proposed it, and Virchow (1855) added that every cell comes from a cell.
Why are viruses considered an exception to the cell theory?
A virus has no cytoplasm, cell membrane or organelles, so it is not a cell, and it can multiply only inside a living host cell.
Distinguish between protoplasm and cytoplasm.
Protoplasm is the whole living content of the cell, the cytoplasm and nucleus together. Cytoplasm is the part of the protoplasm between the cell membrane and the nuclear membrane, in which organelles lie.
Name the parts of the nucleus and give one function of the nucleolus.
Nuclear membrane, nucleoplasm, chromatin and nucleolus. The nucleolus makes the raw materials of ribosomes.
Give two differences between the cell wall and the cell membrane.
The cell wall is non-living, made of cellulose and freely permeable; the cell membrane is living, made of lipids and proteins and selectively permeable. The wall is found only in plants, fungi and bacteria; the membrane is found in all cells.
Why is the cell membrane called selectively permeable?
It allows some substances, such as water, oxygen and carbon dioxide, to pass freely while restricting or preventing the passage of others.
Why are mitochondria called the powerhouses of the cell?
Food is oxidised in them during aerobic respiration, and the energy released is stored as ATP, which the cell uses for all its work.
Past board exam questions
ICSE 2019 sample paper · 5 marks
(i) Draw a neat and labelled diagram of a plant cell. (ii) Based on the diagram drawn above in (i) give a suitable term for each of the following descriptions: 1. The sites of protein synthesis. 2. A cell organelle that is absent in a plant cell. 3. The supporting frame work of the cell. 4. The plastids that contain DNA and have the capacity to divide. 5. The structures that carry the hereditary units. 6. The cell organelle that regulates all the metabolic functions. 7. The structure that is semipermeable in nature.
Show answer
(i) Labelled diagram of a plant cell: a rectangular cell with an outer rigid cell wall, the cell membrane lining it, granular cytoplasm, a large central vacuole bounded by the tonoplast, a nucleus (with nuclear membrane, nucleolus and chromatin) pushed to one side, chloroplasts, mitochondria, endoplasmic reticulum with ribosomes, and Golgi bodies. Labels: cell wall, cell membrane, cytoplasm, vacuole, tonoplast, nucleus, nucleolus, chloroplast, mitochondrion, ribosomes, endoplasmic reticulum, Golgi body.
(ii) 1. Ribosomes
2. Centrosome (centrioles)
3. Endoplasmic reticulum (it forms the supporting framework of the cell)
4. Chloroplasts
5. Chromosomes
6. Nucleus
7. Cell membrane (plasma …