Cell : The Unit of Life

The One Thing

The cell is the basic structural and functional unit of life. Its membranes, genetic material, and specialised organelles operate together to maintain structure, energy flow, information transfer, exchange, and reproduction.

Reactions, Processes and Experiments

Key Terms

• Cell theory: The theory that all living organisms are made of cells, the cell is the basic unit of life, and new cells arise from pre-existing cells.
• Prokaryotic cell: A generally small and simple cell without a membrane-bound nucleus or membrane-bound organelles; bacteria are examples.
• Eukaryotic cell: A cell containing a distinct membrane-bound nucleus and specialised membrane-bound organelles; plants, animals, fungi, and protists have eukaryotic cells.
• Plasma membrane: A selectively permeable boundary made mainly of lipids and proteins that controls movement of substances into and out of the cell.
• Cell wall: A rigid outer covering found in plant cells, fungi, and many prokaryotes. The plant cell wall is mainly made of cellulose and provides support and protection.
• Cytoplasm: The living material between the plasma membrane and nucleus, containing the cytosol and cell organelles.
• Nucleus: The membrane-bound control centre of a eukaryotic cell that contains chromosomes and regulates gene expression and cellular activities.
• Nucleolus: A dense region inside the nucleus where ribosomal RNA is produced and ribosomal subunits begin to form.
• Chromatin: The DNA-protein material in the nucleus that condenses to form visible chromosomes during cell division.
• Endoplasmic reticulum: A membrane network involved in the synthesis, modification, and transport of cell products.
• Rough endoplasmic reticulum: Endoplasmic reticulum bearing ribosomes; it mainly synthesises and transports proteins.
• Smooth endoplasmic reticulum: Endoplasmic reticulum without ribosomes; it helps synthesise lipids, detoxify substances, and store calcium ions in some cells.
• Ribosome: A non-membranous particle made of ribosomal RNA and proteins that carries out protein synthesis.
• Golgi apparatus: A stack of flattened membrane sacs that modifies, sorts, packages, and secretes proteins and lipids.
• Lysosome: A membrane-bound sac containing hydrolytic enzymes that digest worn-out cell parts, foreign materials, and macromolecules.
• Vacuole: A membrane-bound storage compartment; plant cells usually have a large central vacuole surrounded by the tonoplast.
• Mitochondrion: A double-membrane organelle that produces most cellular ATP through aerobic respiration and contains its own DNA and ribosomes.
• Plastid: A plant-cell organelle involved in photosynthesis, storage, or pigmentation; major types include chloroplasts, chromoplasts, and leucoplasts.
• Chloroplast: A green plastid containing chlorophyll that performs photosynthesis; its internal membranes form grana embedded in stroma.
• Peroxisome: A small membrane-bound organelle containing oxidative enzymes that break down fatty acids and help detoxify harmful compounds.
• Cytoskeleton: A network of protein fibres that maintains cell shape, enables movement, and helps position organelles and transport materials.
• Cilia: Short, numerous projections that help move the cell or substances across the cell surface.
• Flagellum: A longer, usually fewer projection that helps a cell move, as in sperm cells and some unicellular organisms.
• Centrosome: A region near the nucleus of many animal cells containing a pair of centrioles; it helps organise spindle fibres during cell division.
• Cell junctions: Specialised connections between cells; plant cells have plasmodesmata, while animal cells have tight junctions, anchoring junctions, and gap junctions.
• Fluid mosaic model: A model describing the plasma membrane as a flexible lipid bilayer with proteins embedded in or attached to it.
• Nucleoid: The irregular region in a prokaryotic cell containing its main, usually circular DNA molecule; it is not surrounded by a membrane.
• Mesosome: A term historically used for membrane infoldings described in some prokaryotic cells; such structures are not generally accepted as natural features in modern interpretations.
• Capsule or slime layer: An external covering that may occur in prokaryotic cells.
• Pili or fimbriae: Surface structures that may occur in prokaryotic cells.
• Plasmid: A small extra-chromosomal DNA molecule often found in prokaryotic cells.
• Cristae: Folds of the inner mitochondrial membrane that increase its surface area.
• Matrix: The internal compartment of a mitochondrion containing enzymes for important respiratory reactions.
• Thylakoid: An internal membrane structure in a chloroplast.
• Grana: Stacks of thylakoids in chloroplasts.
• Stroma lamellae: Membranes that connect grana.
• ATP: Adenosine triphosphate, the main immediate energy currency of the cell.
• Endomembrane system: The coordinated system comprising the endoplasmic reticulum, Golgi apparatus, lysosomes, and vacuoles.
• Surface-area-to-volume ratio: The relationship affecting how efficiently a cell exchanges materials with its surroundings.

Easily Confused

• Prokaryotic cells and eukaryotic cells: Prokaryotic cells lack a membrane-bound nucleus and membrane-bound organelles, whereas eukaryotic cells possess them.
• Nucleoid and nucleus: A nucleoid contains prokaryotic DNA without a surrounding membrane; a nucleus contains eukaryotic chromosomes within a membrane-bound compartment.
• Rough and smooth endoplasmic reticulum: Rough endoplasmic reticulum bears ribosomes and mainly synthesises proteins; smooth endoplasmic reticulum lacks ribosomes and is involved in lipid synthesis, detoxification, and calcium-ion storage.
• Cilia and flagella: Cilia are short and numerous, whereas flagella are longer and usually fewer.
• Cilium or flagellum and basal body: A cilium or flagellum typically has a 9 + 2 arrangement, whereas its basal body generally has a 9 + 0 arrangement.
• Cell wall and plasma membrane: The cell wall is a rigid supporting outer covering; the plasma membrane is selectively permeable and regulates exchange.
• Primary wall and secondary wall: The primary wall is capable of growth; the secondary wall is deposited inside the primary wall in some mature plant cells.
• Grana and stroma lamellae: Grana are stacks of thylakoids; stroma lamellae connect the grana.
• 70S and 80S ribosomes: Prokaryotic ribosomes and the ribosomes of mitochondria and chloroplasts are 70S, whereas eukaryotic cytoplasmic ribosomes are 80S; the designations describe sedimentation behaviour and are not simple additions of subunit values.
• Plant and animal cells: Plant cells generally have a cell wall, plastids, and a large central vacuole; animal cells lack a cell wall and plastids and usually have smaller vacuoles.
• Mesosomes and accepted prokaryotic structures: Mesosomes are a historical description of membrane infoldings and are not generally accepted as natural features in modern interpretations.
• Mitochondria and chloroplasts: Mitochondria produce most cellular ATP through aerobic respiration, whereas chloroplasts perform photosynthesis.

What Gets Asked

• Historical development of cell theory: Questions may ask who first observed and named cells, who observed living cells and microorganisms, or the contributions of Matthias Schleiden, Theodor Schwann, and Rudolf Virchow. The common error is omitting the specific contribution or replacing the named historical examples with a general statement.
• Cell comparison questions: Questions may require comparison of prokaryotic and eukaryotic cells or plant and animal cells. Marks are lost by failing to state the presence or absence of the membrane-bound nucleus, membrane-bound organelles, cell wall, plastids, or large central vacuole.
• Organelle structure–function questions: Questions may ask how cristae support mitochondrial energy production or how grana support photosynthesis. The specific structural feature must be linked to its function.
• Membrane questions: Questions may require description of the fluid mosaic model, phospholipid structure, or selective permeability. Marks are lost by reversing hydrophilic heads and hydrophobic tails or by describing the plasma membrane as completely permeable.
• Ribosome questions: Questions may ask for the subunit composition of 70S and 80S ribosomes. The 70S and 80S values must not be treated as simple arithmetic sums of the subunit values.
• Cell-size and endosymbiosis questions: Questions may ask why cell size is limited or why mitochondria and chloroplasts support the endosymbiotic explanation. Marks are lost by omitting the lower surface-area-to-volume ratio, or the double membranes, circular DNA, and prokaryote-like ribosomes.