Revision: Cell: Structure and Function >> Cell : the Unit of Life Biology Science (English Medium) Class 11 CBSE

A prokaryotic cell is a simple, single-celled organism that lacks a membrane-bound nucleus and other membrane-bound organelles. Its genetic material is located in the nucleoid region of the cytoplasm.
Examples: Bacteria and Archaea.

Living organisms made up of only a single cell are called unicellular organisms.
Examples: Amoeba, Paramecium, bacteria, Chlamydomonas, etc.

Majority of living organisms are made up of millions or billions of cells and are called multicellular organisms.
Examples: Volvox, rose, mosquito, mango, humans, etc.

Protoplasm or living matter is a complex semifluid mass of various biochemicals that are often compartmentalized to perform different functions of life.

Nucleoplasm, also known as nuclear sap or karyoplasm, is the gel-like fluid inside the nucleus of a cell. It surrounds and supports the nucleolus and chromatin, helping to maintain the shape and structure of the nucleus and enabling the movement of materials within it.

The nucleoplasm contains a network of dark-coloured fibres called chromatin fibres.

1. All living organisms are made up of cells, the basic units of life.
2. Cells carry out vital functions necessary for the survival and activity of an organism.
3. Organisms typically begin as a single cell, which multiplies through repeated divisions.
4. Cells differentiate to perform specific roles, such as support, secretion, and other life functions.
5. All cells arise from pre-existing cells and share similar chemical composition and metabolic processes.

1. The number of cells in an organism increases with its size—from single-celled organisms like Amoeba to humans with approximately 37.2 trillion cells.
2. Humans have about 200 different cell types, including 100 billion nerve cells in the brain and 25 trillion red blood cells.
3. Cells are microscopic in size; the smallest include bacteria and red blood cells, while the largest is the ostrich egg (a single cell).
4. Cell shapes vary according to function—e.g., biconcave red blood cells for oxygen transport, amoeboid white blood cells for mobility.
5. Specialized cell shapes support specific roles—nerve cells are long for impulse conduction; guard cells are bean-shaped to regulate stomatal openings.

1. The cell membrane (plasma membrane) is the outermost layer in animal cells and lies next to the cell wall in plant cells.
2. It is a thin, flexible, living membrane made of lipoproteins with fine pores.
3. It is semi-permeable, allowing selective substances to pass while blocking others.
4. It separates the cell’s contents from the external environment.
5. It regulates entry of solutes and ions and maintains cell shape (especially in animal cells).

1. The cell wall is the outermost non-living layer in plant cells, surrounding the plasma membrane.
2. It is rigid, non-living, and primarily made of cellulose.
3. It is freely permeable, allowing free movement of substances in solution.
4. It provides rigidity and a definite shape to the plant cell.
5. It protects the cell and supports structural strength.

1. Mitochondria are double-walled organelles, usually sausage-shaped, with inner folds called cristae.
2. They have their own DNA and ribosomes, enabling some self-functioning.
3. Known as the powerhouse of the cell, they produce ATP through aerobic respiration.
4. Responsible for the release of energy from pyruvic acid formed in the cytoplasm.
5. Involved in the synthesis of respiratory enzymes.

1. Plastids are present only in plant cells and are of several types—chloroplasts, leucoplasts, and chromoplasts.
2. They are double-membraned organelles with a proteinaceous matrix and contain DNA.
3. Chloroplasts (green) contain chlorophyll in thylakoids and perform photosynthesis.
4. Leucoplasts are colourless, store starch, and have no pigment.
5. Chromoplasts are variously coloured, contain pigments like xanthophyll and carotene, and help in pollination by attracting pollinators.

1. Ribosomes are small granules, found either free in the cytoplasm or attached to the endoplasmic reticulum.
2. They are single-walled, dense, spherical structures composed mainly of RNA.
3. Not membrane-bound, unlike most organelles.
4. Found in both prokaryotic and eukaryotic cells.
5. Primary function: Protein synthesis.

1. The centrosome is found only in animal cells and is located near the nucleus.
2. It consists of one or two centrioles surrounded by microtubules.
3. It is the region that surrounds the centrioles.
4. Initiates and regulates cell division.
5. Helps form spindle fibres during cell division, aided by asters.

1. The nucleus is the largest, spherical organelle located centrally in the cytoplasm, enclosed by a double-layered membrane with pores.
2. It contains nucleoplasm, one or more nucleoli, and a network of chromatin fibres.
3. Nucleolus produces ribosomes and assists in protein synthesis by forming and storing RNA.
4. Chromatin fibres (made of DNA) condense into chromosomes during cell division and carry hereditary information.
5. The nucleus controls all cell functions, and its removal leads to cell death.