The Fundamental Unit of Life

The One Thing

The cell is the basic structural and functional unit of life, in which the plasma membrane, nucleus, cytoplasm, and specialized organelles work together to perform essential life processes. Cell structures are adapted to their particular functions, and plant and animal cells share a common plan while differing in features related to their modes of life.

Reactions, Processes and Experiments

Key Terms

• Cell: The smallest unit of life capable of performing essential life processes.
• Cell theory: The idea that all living organisms are made of cells, the cell is the basic unit of life, and new cells arise from pre-existing cells.
• Plasma membrane: A thin, living, flexible boundary around the cell that regulates the movement of substances.
• Selective permeability: The property of allowing some substances to pass through the plasma membrane while restricting others.
• Diffusion: The movement of particles from a region of higher concentration to a region of lower concentration.
• Osmosis: The movement of water through a selectively permeable membrane from a region of higher water concentration to a region of lower water concentration.
• Cell wall: A rigid, non-living outer layer found in plant cells, mainly made of cellulose, which provides shape, support, and protection.
• Cytoplasm: The jelly-like substance between the plasma membrane and nucleus in which many cell activities and chemical reactions occur.
• Nucleus: A membrane-bound organelle that contains chromosomes and controls the activities of the cell.
• Chromosomes: Thread-like structures in the nucleus made of DNA and proteins that carry hereditary information.
• Genes: Functional units of DNA that carry information for inherited characteristics.
• Nucleoid: The region containing genetic material in a prokaryotic cell; it is not surrounded by a nuclear membrane.
• Prokaryotic cell: A cell without a membrane-bound nucleus or membrane-bound organelles, such as a bacterial cell.
• Eukaryotic cell: A cell with a well-defined nucleus and membrane-bound organelles, such as a plant or animal cell.
• Endoplasmic reticulum: A network of membranes involved in the transport and manufacture of substances within the cell.
• Rough endoplasmic reticulum: Endoplasmic reticulum with ribosomes attached; it helps make and transport proteins.
• Smooth endoplasmic reticulum: Endoplasmic reticulum without ribosomes; it helps make fats and supports detoxification in some cells.
• Ribosomes: Tiny structures that manufacture proteins.
• Golgi apparatus: A stack of membrane-bound sacs that modifies, sorts, packages, and transports cell products.
• Lysosomes: Small sacs containing digestive enzymes that break down waste materials, worn-out organelles, and foreign substances.
• Mitochondria: Organelles that release energy from food during cellular respiration and are often called the powerhouses of the cell.
• ATP: Adenosine triphosphate, the energy-rich molecule used by cells for various activities.
• Plastids: Organelles found mainly in plant cells that help in photosynthesis, storage, or colouration.
• Chloroplast: A green plastid containing chlorophyll that carries out photosynthesis.
• Vacuole: A membrane-bound storage space containing cell sap, water, food, or waste materials; plant cells usually have a large central vacuole.
• Cell division: The process by which a cell produces new cells for growth, repair, or reproduction.
• Plasmolysis: The shrinking of the cell contents away from the cell wall when a plant cell loses water in a concentrated solution.
• Turgidity: The firm condition of a plant cell caused by the entry of water into the cell.

Easily Confused

• Diffusion and osmosis: Diffusion is the movement of particles from higher to lower concentration, whereas osmosis specifically concerns the movement of water through a selectively permeable membrane.
• Hypotonic, hypertonic and isotonic solutions: Water tends to enter a cell in a hypotonic solution, leave it in a hypertonic solution, and show no net movement in an isotonic solution.
• Plasmolysis and turgidity: Plasmolysis occurs when a plant cell loses water and its contents shrink away from the cell wall; turgidity occurs when water enters and the plant cell becomes firm.
• Cell wall and plasma membrane: The cell wall is a rigid, non-living cellulose layer outside the plasma membrane in plant cells, whereas the plasma membrane is a living, selectively permeable boundary found around cells.
• Prokaryotic and eukaryotic cells: Prokaryotic cells lack a membrane-bound nucleus and membrane-bound organelles, whereas eukaryotic cells possess them.
• Rough and smooth endoplasmic reticulum: Rough endoplasmic reticulum has attached ribosomes and helps make and transport proteins; smooth endoplasmic reticulum lacks ribosomes and helps make fats and support detoxification.
• Chromosomes and genes: Chromosomes are DNA-and-protein structures carrying hereditary information; genes are functional units of DNA that carry information for inherited characteristics.
• Plant and animal cells: Plant cells have a cell wall, chloroplasts, and generally a large central vacuole; animal cells lack a cell wall and chloroplasts and usually have smaller vacuoles or no prominent vacuoles.
• Nucleus and nucleoid: The nucleus is membrane-bound, whereas the nucleoid is the genetic-material region of a prokaryotic cell and is not surrounded by a nuclear membrane.

What Gets Asked

• Questions may ask for the cell theory or for the significance of Robert Hooke’s observation of small compartments in cork in 1665. Marks are lost by omitting that Hooke observed cork compartments and called them cells.
• Questions may require the functions of the plasma membrane, cytoplasm, nucleus, mitochondria, chloroplasts, vacuoles, ribosomes, endoplasmic reticulum, Golgi apparatus, and lysosomes. Marks are lost when an organelle is named without its specific function.
• Questions may ask students to compare plant and animal cells. Marks are lost by failing to identify the cell wall, chloroplasts, and large central vacuole as plant-cell features, or by incorrectly assigning them to animal cells.
• Questions may test diffusion, osmosis, or energy-dependent transport. Marks are lost by describing osmosis without referring to water and a selectively permeable membrane.
• Questions may present hypotonic, hypertonic, or isotonic conditions and ask what happens to a cell. Marks are lost by reversing the direction of water movement or confusing plasmolysis with turgidity.
• Questions may distinguish prokaryotic from eukaryotic cells. Marks are lost by failing to state that prokaryotic cells lack a membrane-bound nucleus and membrane-bound organelles, while eukaryotic cells possess them.