Cells and Organisms

The One Thing

Cells are the basic structural and functional units of all living organisms, and their specialized structures enable the processes required for life. In multicellular organisms, coordinated cellular activities produce tissues, organs, organ systems, growth, development, and homeostasis.

Reactions, Processes and Experiments

Key Terms

• 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 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 organelles; plants, animals, fungi, and protists have eukaryotic cells.
• Plasma membrane: A selectively permeable boundary that controls movement of substances into and out of the cell.
• Cell wall: A rigid outer layer that provides support and protection; it is present in plants, fungi, and many prokaryotes.
• Nucleus: The organelle that contains most of the cell’s genetic material and regulates cellular activities.
• Cytoplasm: The region between the plasma membrane and nucleus where many metabolic reactions occur.
• Mitochondrion: The organelle where aerobic cellular respiration produces most of the cell’s ATP.
• Chloroplast: A plant-cell organelle containing chlorophyll where photosynthesis occurs.
• Ribosome: A structure made of ribosomal RNA and proteins that synthesizes proteins.
• Endoplasmic reticulum: A membrane network involved in synthesis and transport; rough ER has ribosomes and smooth ER is involved in lipid synthesis and detoxification.
• Golgi apparatus: An organelle that modifies, sorts, and packages proteins and lipids for transport.
• Lysosome: A vesicle containing digestive enzymes that breaks down waste, foreign material, and worn-out cell parts.
• Vacuole: A membrane-bound sac used for storage; plant cells commonly contain a large central vacuole that helps maintain turgor.
• 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 higher water potential to lower water potential.
• Active transport: Movement of substances across a membrane against a concentration gradient using cellular energy, usually ATP.
• Cell cycle: The sequence of events through which a cell grows, duplicates its genetic material, and divides.
• Interphase: The period of growth and preparation between cell divisions, including G1, S, and G2 phases.
• Mitosis: Nuclear division that produces two genetically similar daughter nuclei, supporting growth, repair, and asexual reproduction.
• Meiosis: A specialized division that produces haploid reproductive cells and increases genetic variation.
• Differentiation: The process by which unspecialized cells develop distinct structures and functions.
• Tissue: A group of similar cells working together to perform a particular function.
• Organ: A structure made of different tissues that work together for a specific function.
• Organ system: A group of organs coordinating to perform major functions in a multicellular organism.
• Homeostasis: The maintenance of relatively stable internal conditions despite changes in the external environment.
• Metabolism: The complete set of chemical reactions occurring in a cell or organism, including anabolism and catabolism.
• Anabolism: Energy-requiring reactions that build complex molecules from simpler substances.
• Catabolism: Reactions that break complex molecules into simpler substances and often release energy.
• Cell signaling: Communication between cells through chemical signals, receptors, and intracellular responses.
• ATP: The main immediate energy currency of cells, powering processes such as active transport, biosynthesis, and movement.
• Enzyme: A biological catalyst that lowers activation energy and increases the rate of a cellular reaction without being consumed.
• Stem cell: A relatively unspecialized cell capable of self-renewal and differentiation into specialized cell types.
• Fluid mosaic model: A model describing the plasma membrane as a flexible phospholipid bilayer containing proteins, carbohydrates, and cholesterol.
• Hydrophilic phosphate head: The water-attracting part of a phospholipid.
• Hydrophobic fatty-acid tails: The water-repelling parts of a phospholipid that face inward in the membrane bilayer.
• Surface-area-to-volume relationship: The relationship affecting exchange efficiency as a cell grows; volume increases faster than surface area, making exchange less efficient.
• G1 phase: The cell-cycle phase mainly associated with cell growth.
• S phase: The cell-cycle phase in which DNA replication occurs.
• G2 phase: The cell-cycle phase involving preparation for cell division.
• M phase: The cell-cycle phase including nuclear division and cytokinesis.
• Cytokinesis: The division of the cytoplasm following nuclear division.
• Checkpoint: A control point in the cell cycle that helps prevent division of cells with damaged or incompletely copied DNA.
• Gene expression: The regulated use of genetic information, contributing to cell development and differentiation.
• Feedback system: A regulatory mechanism that responds to changes in a system; negative feedback counteracts changes to help restore stability.

Easily Confused

• Prokaryotic cells and eukaryotic cells: Prokaryotic cells lack a membrane-bound nucleus and membrane-bound organelles, whereas eukaryotic cells contain them.
• Plant cells and animal cells: Plant cells usually have a cell wall, chloroplasts, and a large central vacuole; animal cells lack a cell wall and chloroplasts.
• Diffusion and osmosis: Diffusion concerns the movement of particles down a concentration gradient, whereas osmosis specifically concerns water moving through a selectively permeable membrane from higher water potential to lower water potential.
• Diffusion and active transport: Diffusion does not require ATP and occurs down a concentration gradient; active transport requires cellular energy and moves substances against a concentration gradient.
• Mitosis and meiosis: Mitosis maintains the chromosome number and produces genetically similar nuclei; meiosis halves the chromosome number and increases genetic variation.
• Cell growth and cell development: Growth involves increases in cell size and mass, whereas development involves changes in form, function, and organization.
• Rough ER and smooth ER: Rough ER has ribosomes; smooth ER is involved in lipid synthesis and detoxification.
• Anabolism and catabolism: Anabolism builds complex molecules and requires energy; catabolism breaks complex molecules down and often releases energy.
• Cell wall and plasma membrane: The cell wall provides rigid support and protection, whereas the plasma membrane is selectively permeable and controls movement of substances.
• Nucleus and ribosome: The nucleus contains most of the cell’s genetic material and regulates activities; the ribosome synthesizes proteins using messenger RNA information.
• Tissue and organ: A tissue is a group of similar cells performing a particular function; an organ contains different tissues working together.
• Cell signaling and homeostasis: Cell signaling describes communication through chemical signals and receptors; homeostasis is the maintenance of stable internal conditions, often using feedback systems.

What Gets Asked

• Identify structures in typical eukaryotic cells: Questions may require identification of the plasma membrane, cytoplasm, nucleus, mitochondria, ribosomes, endoplasmic reticulum, Golgi apparatus, or specialized plant structures. Marks are lost by attributing chloroplasts, a cell wall, or a large central vacuole to animal cells.
• Compare prokaryotic and eukaryotic cells: The required distinction is the presence or absence of a membrane-bound nucleus and membrane-bound organelles, not simply whether the cell is “simple” or “complex.”
• Explain membrane transport: Questions may distinguish diffusion, osmosis, and active transport. The key slips are confusing particle movement with water movement, or omitting the ATP requirement and movement against a concentration gradient in active transport.
• Describe the cell cycle and cell division: Questions may ask for the roles of G1, S, G2, and M, or the stages of mitosis. Marks are lost by placing DNA replication outside S phase, omitting cytokinesis, or confusing chromosome maintenance in mitosis with chromosome-number reduction in meiosis.
• Use the equations for cellular respiration and photosynthesis: The required equations are C6H12O6 + 6O2 -> 6CO2 + 6H2O + energy in the form of ATP. and 6CO2 + 6H2O + light energy -> C6H12O6 + 6O2. A common error is reversing the reactants and products or failing to associate respiration with ATP production and photosynthesis with light energy.
• Explain organization and regulation in multicellular organisms: Questions may require the sequence cell -> tissue -> organ -> organ system -> organism, or an explanation of differentiation, cell signaling, and homeostasis. Marks are lost by treating specialized cells as independent rather than coordinated components of tissues, organs, and organ systems.