Revision: Cell: Structure and Function >> Cell - The Unit of Life Biology (Theory) ISC (Science) ISC Class 11 CISCE

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.

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.

• Cell theory was proposed by Matthias Schleiden (1838) and Theodor Schwann (1839), stating that all plants and animals are made up of cells.
• Rudolf Virchow (1855) further expanded the theory by stating that new cells arise from pre-existing cells (“Omnis cellula e cellula”).
• According to modern cell theory, all living organisms are composed of cells and their products.
• The cell is the basic structural and functional unit of life.
• Cells arise only from pre-existing cells through cell division.
• The theory established that cells are the fundamental building blocks of all living organisms and highlighted the importance of cell division.
• Early cell theory had limitations as it did not explain cell formation, which was later resolved by Virchow.

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

• The endomembrane system includes endoplasmic reticulum (ER), Golgi apparatus, lysosomes, and vacuoles, which work together in a coordinated manner.
• Endoplasmic Reticulum (ER) is a network of membranous tubules that divides the cell into luminal and extraluminal compartments.
• Rough ER (RER) has ribosomes and is mainly involved in protein synthesis and secretion, while Smooth ER (SER) lacks ribosomes and helps in lipid and steroid synthesis.
• The Golgi apparatus consists of stacked cisternae and modifies, packages, and transports materials received from the ER.
• The Golgi has a cis (forming) face that receives materials and a trans (maturing) face that sends them to their destination.
• Lysosomes are membrane-bound vesicles containing digestive enzymes and perform intracellular digestion, hence called “suicidal bags.”
• Vacuoles are membrane-bound sacs that store water, waste, and other substances, and in plants, they help maintain turgor pressure.

• Structure - Double membrane-bound organelle. The outer membrane is smooth; the inner membrane has infoldings called cristae. Inner space is called the matrix.
• Shape & Size - Sausage-shaped or cylindrical. Diameter: 0.2–1.0 µm; Length: 1.0–4.1 µm.
• Function - Site of aerobic respiration; produces energy as ATP. Called the 'Powerhouse of the Cell'.
• Matrix Contents - Contains circular DNA, RNA molecules, and 70S ribosomes for protein synthesis.
• Reproduction - Divides by fission.

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.

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

• Meaning - Small, membrane-bound vesicles containing various enzymes. Present in both plant and animal cells.
• Structure - Minute, membrane-enclosed organelles that are part of the complex internal structure of eukaryotic cells.
• Function - House specific enzymatic reactions and allow specialised biochemical processes within cells.
• Importance - Contribute to cellular metabolism and help in the compartmentalisation of reactions inside the cell.
• Variety - Different types may exist depending on the cell type and function. Play a crucial role in the internal organisation of the cell.

• Meaning - An elaborate network of filamentous proteinaceous structures present throughout the cytoplasm.
• Composition - Made entirely of proteins.
• Components - Three types: Microtubules, Microfilaments, and Intermediate filaments.
• Functions - Provides mechanical support, maintains cell shape, and is involved in cell motility.
• Nature - Dynamic structure; can be reorganised based on cellular needs. Works with other cell components to carry out various cellular processes.

• Meaning - Hair-like outgrowths of the cell membrane found in both prokaryotes and eukaryotes.
• Difference - Cilia are shorter and work like oars to move the surrounding fluid. Flagella are longer and are responsible for cell movement.
• Structure - Covered with plasma membrane; core is called axoneme with a 9+2 arrangement (9 doublets of peripheral microtubules + 1 pair of central microtubules).
• Origin - Both cilia and flagella emerge from centriole-like structures called basal bodies.
• Function - Critical for cellular motility and fluid movement in various organisms.

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

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