Basic Biology

The One Thing

Cells are the basic structural and functional units of life, and their specialised structures work together to maintain life. In multicellular organisms, cells divide in a controlled manner and become organised into tissues, organs, and organ systems.

Reactions, Processes and Experiments

Key Terms

• Cell: The smallest unit of life capable of performing essential life processes.
• Cell theory: The principle 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 simple cell without a true nucleus or membrane-bound organelles; its genetic material lies in a nucleoid region.
• Eukaryotic cell: A cell with a distinct nucleus enclosed by a nuclear membrane and usually containing membrane-bound organelles.
• Cell membrane: A thin, selectively permeable boundary that controls the movement of substances into and out of the cell.
• Cell wall: A rigid outer covering present in plant cells, mainly made of cellulose, that provides support and protection.
• Cytoplasm: The jelly-like substance between the cell membrane and nucleus where many chemical reactions occur.
• Nucleus: The control centre of a eukaryotic cell containing genetic material and regulating cell activities.
• Chromosome: A thread-like structure made of DNA and proteins that carries genes.
• Gene: A functional segment of DNA that carries information for a particular inherited characteristic.
• Mitochondrion: An organelle that releases energy from food during aerobic respiration and produces ATP.
• Ribosome: A small structure where proteins are made.
• Endoplasmic reticulum: A network of membranes involved in transporting materials; rough ER has ribosomes and helps make proteins, while smooth ER helps make lipids.
• Golgi apparatus: An organelle that modifies, sorts, and packages substances for transport or secretion.
• Lysosome: A membrane-bound sac containing digestive enzymes that break down waste materials and worn-out cell parts.
• Vacuole: A storage sac containing cell sap, water, dissolved substances, or waste; it is usually large and permanent in plant cells.
• Chloroplast: A plant-cell organelle containing chlorophyll where photosynthesis takes place.
• Plastid: A plant-cell organelle that may store food, provide colour, or carry out photosynthesis.
• Cell cycle: The series of stages through which a cell grows, copies its genetic material, and divides.
• Interphase: The period of cell growth and preparation before division, including DNA replication.
• Mitosis: A type of cell division producing two genetically identical daughter cells with the same chromosome number as the parent cell.
• Prophase: The stage of mitosis in which chromosomes become visible, spindle fibres form, and the nuclear membrane begins to disappear.
• Metaphase: The stage in which chromosomes arrange themselves at the equator of the cell.
• Anaphase: The stage in which sister chromatids separate and move to opposite poles.
• Telophase: The stage in which chromosomes reach the poles and new nuclear membranes form around them.
• Cytokinesis: The division of the cytoplasm to form separate daughter cells.
• Tissue: A group of similar cells working together to perform a specific function.
• Organ: A structure made of different tissues working together to carry out a major function.
• Organ system: A group of organs working together to perform a major life process.
• Organism: An individual living thing made of one or more cells.
• Unicellular organism: An organism made of a single cell that performs all necessary life functions.
• Multicellular organism: An organism made of many specialised cells that cooperate with one another.
• 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 dilute solution to a more concentrated solution.
• Active transport: The movement of substances against a concentration gradient using cellular energy.
• Magnification: The number of times larger an image appears compared with the actual object.
• ATP: The usable form of cellular energy produced by mitochondria during aerobic respiration.
• DNA: Genetic material that carries hereditary information.
• Nucleoid region: The region of a prokaryotic cell where its genetic material is located.

Easily Confused

• Prokaryotic cells and eukaryotic cells: Prokaryotic cells lack a true nucleus and membrane-bound organelles, whereas eukaryotic cells possess a nucleus enclosed by a nuclear membrane.
• Plant cells and animal cells: Plant cells generally have a cell wall, chloroplasts, and a large central vacuole; animal cells do not have a cell wall or chloroplasts and usually have smaller vacuoles.
• Cell wall and cell membrane: The cell wall is a rigid cellulose layer providing support, whereas the cell membrane is selectively permeable and controls movement of substances.
• Diffusion and osmosis: Diffusion may involve gases or dissolved particles, whereas osmosis specifically involves water moving through a selectively permeable membrane.
• Diffusion and active transport: Diffusion does not require cellular energy and occurs down a concentration gradient, whereas active transport requires ATP and can move substances against a concentration gradient.
• Mitosis and cytokinesis: Mitosis is nuclear division, whereas cytokinesis is division of the cytoplasm.
• Chromosomes and genes: Chromosomes are DNA-and-protein structures that carry genetic information, whereas genes are functional segments of DNA located on chromosomes.
• Interphase and mitosis: Interphase is the period of growth, normal cell activity, organelle duplication, and DNA replication; mitosis is the subsequent division of the nucleus.
• Tissue and organ: A tissue consists of similar cells performing a specific function, whereas an organ contains different tissues working together.
• Unicellular and multicellular organisms: A unicellular organism performs all life processes within one cell, whereas a multicellular organism contains many specialised cells that cooperate.
• Cell plate and cleavage furrow: Plant-cell cytokinesis occurs through cell-plate formation, whereas animal-cell cytokinesis occurs through membrane constriction and cleavage-furrow formation.
• Chloroplasts and mitochondria: Chloroplasts trap light energy for photosynthesis, whereas mitochondria release usable energy from food during aerobic respiration.
• Magnification and actual size: Magnification is the number of times larger an image appears; actual size is the real size of the object.

What Gets Asked

• Identify cell structures from their functions or descriptions. Marks depend on distinguishing structures such as mitochondria, chloroplasts, ribosomes, the nucleus, the Golgi apparatus, lysosomes, vacuoles, and the endoplasmic reticulum.
• Compare plant, animal, prokaryotic, and eukaryotic cells. Common errors include attributing a cell wall or chloroplasts to animal cells, or describing a prokaryotic cell as having a true nucleus.
• State and sequence the stages of the cell cycle and mitosis. The expected sequence is interphase followed by Prophase, Metaphase, Anaphase, and Telophase (PMAT), then cytokinesis; confusing anaphase with metaphase loses marks.
• Explain the importance and outcome of mitosis. A complete answer should state that one parent cell produces two genetically identical daughter cells with the same chromosome number, supporting growth, replacement of worn-out cells, healing, and asexual reproduction in some organisms.
• Describe cytokinesis in plant and animal cells. The specific distinction is cell-plate formation in plant cells versus cleavage-furrow formation in animal cells.
• Apply transport definitions and microscopy calculations. Transport questions test whether the substance is water, whether movement follows or opposes the concentration gradient, and whether ATP is required; calculation questions use Magnification = Size of image ÷ Actual size of object and require correct conversion using 1 millimetre = 1000 micrometres and 1 micrometre = 1000 nanometres.