Revision: Basic Biology >> Cell - The Structural and Functional Unit of Life [For Revision Only] Biology (English Medium) ICSE Class 10 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.

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.

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

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

The part of the cytoplasm other than the organelles (mitochondria, etc.) is called cytosol, which constitutes mainly the liquid medium. 

Cytosol is the fluid portion of the cytoplasm inside a cell, excluding organelles. It is a gel-like substance composed mainly of water, salts, and proteins, where many cellular processes such as metabolism and signal transduction occur.

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.

The non-living parts of a cell, i.e., vacuoles and granules, are called cell inclusions.

Organelles are the specialised and membrane-bound, living structures in a cell concerned with definite functions.

The living parts of a cell which consist of cytoplasm, nucleus and other living bodies are collectively called protoplasm or protoplast.

• 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.

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.

• The cell membrane (plasma membrane) is a thin, flexible, living outer boundary that separates the cell from the external environment.
• It is mainly composed of lipids and proteins, with phospholipids arranged in a bilayer (hydrophilic heads outward and hydrophobic tails inward).
• The membrane also contains cholesterol and carbohydrates, and the protein–lipid ratio varies in different cells (e.g., RBC membrane has ~52% protein and ~40% lipids).
• According to the Fluid Mosaic Model (Singer and Nicolson, 1972), the membrane is dynamic, with proteins moving within the lipid bilayer.
• Membrane proteins are of two types: integral proteins (embedded in the membrane) and peripheral proteins (present on the surface).
• The cell membrane is selectively permeable, allowing certain substances to pass while restricting others.
• It regulates the movement of ions and molecules, maintains cell shape, and helps in communication between the cell and its environment.

• Non-living rigid structure - Covers the plasma membrane in plants and fungi, giving the cell its shape.
• Functions - Protects from mechanical damage and infection, aids cell-to-cell interaction, and blocks undesirable macromolecules.
• Primary Wall - Present in young plant cells; capable of growth.
• Secondary Wall - Forms on the inner side as the cell matures.
• Middle Lamella - Made of calcium pectate; holds neighbouring cells together.
• Plasmodesmata - Channels connecting the cytoplasm of neighbouring cells through the cell wall.
• Composition - Algae: cellulose, galactans, mannans, calcium carbonate. Other plants: cellulose, hemicellulose, pectins, proteins.

1. Cytoplasm lies between the cell membrane and the nucleus, excluding the nucleus itself.
2. It is a semi-liquid medium containing water, solutes, and various organelles.
3. The cytosol is the fluid part of the cytoplasm, excluding organelles.
4. Site of all metabolic activities, including enzyme-catalyzed reactions.
5. It supports the functions of organelles and is the medium for glycolysis (anaerobic respiration phase).

• 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.

1. Organelles are membrane-bound, living structures within a cell, each with specific functions—like organs in the body.
2. Protoplasm includes the cytoplasm, nucleus, and all living parts of the cell; protoplast refers to a plant cell without its cell wall.
3. Most organelles are common to both plant and animal cells, except for chloroplasts and cell wall (plant only) and centrosome with centrioles (animal only).
4. Major organelles include nucleus, nucleolus, mitochondria, endoplasmic reticulum, ribosomes, Golgi bodies, and lysosomes.
5. Non-living components, such as vacuoles and granules, are called cell inclusions.

1. ER is a network of double-membraned tubules that extend between the plasma and nuclear membranes.
2. It occurs in two forms: Rough ER (RER) with ribosomes and Smooth ER (SER) without ribosomes.
3. RER is involved in protein synthesis and transport because it contains attached ribosomes.
4. SER is responsible for fat (lipid) synthesis and transport.
5. ER provides a supportive internal framework within the cell.

• 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. The Golgi apparatus, also called dictyosomes in plant cells, is found as stacks of flattened membrane sacs.
2. It consists of cisternae, vesicles, and vacuoles.
3. Involved in the synthesis and secretion of enzymes, hormones, and other substances.
4. Plays a role in the packaging and transport of cellular products.
5. In animal cells, it helps form the acrosome of sperm cells.

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. Lysosomes are membranous sacs formed from the Golgi body.
2. They contain around 40 different types of digestive enzymes.
3. Responsible for intracellular digestion and destruction of foreign substances.
4. Help break down worn-out organelles—hence known as "suicide bags".
5. Aid in digesting cartilage during bone formation.

• 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.

• 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.

1. Vacuoles are clear spaces containing water or dissolved substances, enclosed by a membrane called the tonoplast.
2. Plant cells have fewer but larger vacuoles, while animal cells have smaller ones.
3. Vacuoles store water, food, pigments, and waste, and help maintain cell turgidity.
4. Some vacuoles contain pigments like anthocyanins (violet-blue).
5. Granules are non-living particles (starch, glycogen, fats) that serve as food reserves for the cell.