Revision: Class 11 >> Cell The Unit of Life NEET (UG) Cell The Unit of Life

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.

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.

• 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.

• 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.

• Cells are the basic structural and functional units of life, and all living organisms are made up of cells.
• Prokaryotic cells lack a true nucleus and membrane-bound organelles, while eukaryotic cells have a well-defined nucleus and membrane-bound organelles.
• The cytoplasm is a semi-fluid matrix present in all cells and is the main site of cellular activities.
• Eukaryotic cells contain organelles such as the ER, the Golgi apparatus, mitochondria, lysosomes, and vacuoles, whereas prokaryotic cells lack these structures.
• Ribosomes are non-membrane-bound organelles present in all cells and are responsible for protein synthesis.
• Cell size and shape vary widely, such as mycoplasma being the smallest cell, ostrich egg being the largest single cell, and nerve cells being the longest.
• Important contributors include Leeuwenhoek (who first observed cells) and Robert Brown (who discovered the nucleus), and the invention of the microscope made cell study possible.

• Prokaryotes include bacteria, blue-green algae, mycoplasma and PPLO. They are generally smaller and multiply faster than eukaryotic cells.
• Most prokaryotes have a cell wall, except mycoplasma. They lack a well-defined nucleus as genetic material is not enclosed by a nuclear membrane.
• Four basic shapes of bacteria: bacillus (rod-like), coccus (spherical), vibrio (comma-shaped) and spirillum (spiral).
• Many bacteria have plasmids, which are small circular DNA outside the main genomic DNA and can confer unique traits like antibiotic resistance.
• Prokaryotes lack membrane-bound organelles except ribosomes and have unique cell inclusions and mesosomes (infoldings of the cell membrane).

• The prokaryotic cell envelope has three layers: glycocalyx (outermost; slime layer or capsule), cell wall (shape and support) and plasma membrane (selectively permeable).
• Gram-positive bacteria have a thick peptidoglycan layer, no outer membrane and appear purple. Gram-negative bacteria have a thin peptidoglycan layer, an outer membrane with lipopolysaccharides, appear pink/red and are more antibiotic-resistant.
• Mesosomes are plasma membrane infoldings that assist in cell wall formation, DNA replication, respiration and secretion.
• Flagella (filament + hook + basal body) help in motility. Pili and fimbriae help bacteria attach to surfaces but do not contribute to motility.
• Chromatophores contain pigments and are found in prokaryotes like cyanobacteria.

• Ribosomes were first observed by George Palade (1953). They are non-membrane-bound, composed of RNA and proteins, and are the site of protein synthesis.
• Eukaryotic ribosomes: 80S (60S + 40S subunits); Prokaryotic ribosomes: 70S (50S + 30S subunits). 'S' = sedimentation coefficient.
• Multiple ribosomes on a single mRNA form polysomes, which translate mRNA into proteins.
• Inclusion bodies are non-membrane-bound reserve materials in prokaryotic cytoplasm (e.g. phosphate granules, glycogen granules). Gas vacuoles are found in blue-green, purple and green photosynthetic bacteria.

• Eukaryotic cells are found in protists, plants, animals, and fungi.
• They have a well-defined nucleus enclosed by a nuclear envelope.
• The cytoplasm shows compartmentalisation due to membrane-bound organelles like mitochondria, the ER, and the Golgi apparatus.
• Genetic material is organised into chromosomes inside the nucleus.
• Plant cells have a cell wall, plastids, and a large vacuole, while animal cells lack these but contain centrioles.

• 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.

• The cell membrane is a flexible, dynamic bilayer of phospholipids with hydrophilic heads facing outward and hydrophobic tails facing inward.
• Integral proteins span the bilayer (transport and communication); peripheral proteins are on the surface (signalling and cell recognition).
• Cholesterol regulates membrane fluidity and stability by preventing fatty acid chains from packing too closely.
• Glycoproteins and glycolipids on the extracellular surface are involved in cell recognition, adhesion and signalling.
• Passive transport (diffusion, osmosis) requires no energy. Active transport moves substances against the concentration gradient and requires ATP (e.g. Na⁺/K⁺ pump).

• 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.

• 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.

• 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. 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.

• 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.

• 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.

• 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.

• 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.

• Chromatin is the thread-like material present in the nucleus.
• It is made up of DNA (40%) and histone proteins (60%).
• DNA carries genetic information, while histones help package and organize DNA.
• Chromatin condenses to form chromosomes during cell division.
• It remains as long, thin fibres when the cell is not dividing.