Cell: Structure and Function

The One Thing

The cell is the basic structural and functional unit of life, in which specialized organelles, biomolecules, membranes, transport mechanisms, and regulated cell division work together to maintain life. Cell structure is closely related to cell function, and the plasma membrane helps maintain a stable internal environment through selective transport.

Reactions, Processes and Experiments

Key Terms

• Cell theory: The cell is the basic unit of life; all organisms are made of cells, and new cells arise from pre-existing cells.
• Prokaryotic cell: A cell without a membrane-bound nucleus or membrane-bound organelles; bacteria are common examples.
• Eukaryotic cell: A cell containing a membrane-bound nucleus and specialized membrane-bound organelles.
• Plasma membrane: A selectively permeable boundary made mainly of lipids and proteins that regulates movement of substances into and out of the cell.
• Fluid mosaic model: The membrane consists of a flexible phospholipid bilayer containing proteins that can move within it.
• Cell wall: A rigid outer covering that provides support and protection; in plants it is mainly composed of cellulose.
• Nucleus: The membrane-bound organelle containing chromosomes and controlling cellular activities through genetic information.
• Mitochondrion: The organelle where aerobic respiration produces most of the cell's ATP.
• Chloroplast: A plant-cell organelle containing chlorophyll and carrying out photosynthesis.
• Endoplasmic reticulum: A membrane network; rough ER has ribosomes and helps synthesize proteins, while smooth ER helps synthesize lipids and detoxify substances.
• Ribosome: A non-membranous structure that synthesizes proteins from amino acids using information from messenger RNA.
• Golgi apparatus: A stack of flattened membranes that modifies, sorts, and packages proteins and lipids.
• Lysosome: A vesicle containing digestive enzymes that breaks down wastes, worn-out organelles, and foreign materials.
• Vacuole: A membrane-bound storage compartment; the large central vacuole of plant cells helps maintain turgor pressure.
• Cytoskeleton: A network of protein fibres that maintains cell shape, supports movement, and helps transport materials inside the cell.
• Carbohydrates: Organic compounds made mainly of carbon, hydrogen, and oxygen that provide energy and structural support.
• Proteins: Polymers of amino acids that function as enzymes, transporters, receptors, antibodies, and structural materials.
• Lipids: Water-insoluble molecules including fats, oils, phospholipids, and steroids that store energy and form membranes.
• Nucleic acids: DNA and RNA are polymers of nucleotides that store, transmit, and express genetic information.
• Enzyme: A biological catalyst, usually a protein, that speeds up a reaction by lowering its activation energy without being consumed.
• Diffusion: The net 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 higher water potential to lower water potential.
• Facilitated diffusion: Passive movement down a concentration gradient through specific membrane proteins.
• Active transport: Movement of substances against a concentration or electrochemical gradient using energy, usually ATP.
• Endocytosis: The process by which a cell takes in materials by forming vesicles from the plasma membrane.
• Exocytosis: The release of materials when intracellular vesicles fuse with the plasma membrane.
• Cell cycle: The sequence of events through which a cell grows, duplicates its DNA, and divides.
• Interphase: The phase between cell divisions, consisting of G1, S, and G2 stages.
• G1 phase: The cell grows and produces proteins and organelles.
• S phase: DNA replication occurs, so each chromosome forms two sister chromatids.
• G2 phase: The cell continues growing and prepares materials needed for division.
• Mitosis: Nuclear division producing two genetically similar daughter nuclei.
• Meiosis: A specialized division producing four genetically varied haploid cells, usually for sexual reproduction.
• Cytokinesis: Division of the cytoplasm after nuclear division.
• Cell-cycle checkpoints: Control points that verify cell size, DNA integrity, and chromosome attachment before progression.
• Nucleotide: A unit consisting of a nitrogenous base, a pentose sugar, and a phosphate group.
• Magnification: The ratio of image size to actual size, calculated as Magnification = image size ÷ actual size.
• Resolution: The ability to distinguish two very close points as separate; it is different from magnification.
• Surface-area-to-volume ratio: The ratio that limits cell size because it decreases as a cell becomes larger, reducing efficient exchange with the environment.
• Endosymbiotic origin: The explanation that mitochondria and chloroplasts originated from symbiotic organisms; both contain their own DNA and ribosomes.

Easily Confused

• Prokaryotic and eukaryotic cells: Prokaryotic cells lack a membrane-bound nucleus and membrane-bound organelles, whereas eukaryotic cells contain them.
• Plant and animal cells: Plant cells generally have a cell wall, chloroplasts, and a large central vacuole; animal cells lack a cell wall and chloroplasts.
• Rough and smooth endoplasmic reticulum: Rough ER has ribosomes and helps synthesize proteins; smooth ER helps synthesize lipids and detoxify substances.
• Diffusion and facilitated diffusion: Both are passive and occur down a concentration gradient, but facilitated diffusion requires specific membrane proteins.
• Facilitated diffusion and active transport: Facilitated diffusion does not directly require ATP and moves substances down a gradient; active transport uses energy to move substances against a concentration or electrochemical gradient.
• Endocytosis and exocytosis: Endocytosis brings materials into the cell in vesicles; exocytosis releases materials when vesicles fuse with the plasma membrane.
• Mitosis and meiosis: Mitosis produces two genetically similar daughter nuclei and maintains chromosome number; meiosis produces four genetically varied haploid cells and halves chromosome number.
• Interphase and mitosis: Interphase consists of G1, S, and G2 and includes growth and DNA replication; mitosis is nuclear division.
• Mitosis and cytokinesis: Mitosis divides the nucleus; cytokinesis divides the cytoplasm after nuclear division.
• Magnification and resolution: Magnification increases the apparent size of an image; resolution determines whether two close points can be distinguished as separate.
• Condensation and hydrolysis: Condensation forms larger molecules by removing water; hydrolysis breaks bonds by adding water.
• DNA and RNA: DNA is usually double-stranded and stores hereditary information; RNA is generally single-stranded and participates in gene expression.
• Cell wall and plasma membrane: The cell wall is a rigid supporting and protective outer covering; the plasma membrane is selectively permeable and regulates substance movement.
• Hypotonic, hypertonic, and isotonic solutions: Hypotonic solutions may cause animal cells to swell and plant cells to become turgid; hypertonic solutions cause water to leave cells and may cause plasmolysis in plants; isotonic solutions cause no net water movement.

What Gets Asked

• Identify organelles and link structure to function. Questions may require the functions of the nucleus, mitochondrion, chloroplast, endoplasmic reticulum, ribosome, Golgi apparatus, lysosome, vacuole, or cytoskeleton. Marks are lost by assigning rough ER’s protein-synthesis role to smooth ER, or by omitting that the nucleus contains chromosomes and nuclear pores.
• Compare prokaryotic, eukaryotic, plant, and animal cells. The specific distinctions concern membrane-bound nuclei and organelles, cell walls, chloroplasts, and large central vacuoles.
• Explain membrane structure and transport. Questions may involve the fluid mosaic model, hydrophilic phospholipid heads, hydrophobic tails, selective permeability, diffusion, osmosis, facilitated diffusion, active transport, endocytosis, or exocytosis. Marks are lost by stating that all transport requires ATP or by reversing the direction of water movement in osmosis.
• Interpret hypotonic, hypertonic, and isotonic conditions. The required observations include swelling and bursting of animal cells, turgidity in plant cells, plasmolysis, and no net water movement. Confusing the effects of hypertonic and hypotonic solutions costs marks.
• Describe biomolecules and their roles. Questions may ask for carbohydrate examples—glucose, starch, glycogen, and cellulose—or for the structures and functions of proteins, lipids, and nucleic acids. Marks may be lost by omitting peptide bonds in proteins, the membrane role of phospholipids, or the components of a nucleotide.
• Sequence and explain the cell cycle. Questions may test G1, S, G2, prophase, metaphase, anaphase, telophase, cytokinesis, meiosis, and cell-cycle checkpoints. Marks are lost by placing chromosome alignment in the wrong mitotic stage, omitting DNA replication before meiosis, or failing to distinguish nuclear division from cytoplasmic division.