9.2 Cell Structure and Function

Taxonomic Hierarchy and the Three Domains

Biologists organize life into a nested hierarchy of ranks. From broadest to most specific, the ranks are Domain, Kingdom, Phylum, Class, Order, Family, Genus, and Species. A useful mnemonic for TExES candidates is "Did King Philip Come Over For Good Soup?" Each rank narrows a group of organisms; species is the most specific standard rank, and members of a species share the closest common ancestry.

The three-domain system, proposed by Carl Woese in 1990, sorts life by comparisons of ribosomal RNA:

• Bacteria — single-celled prokaryotes with cell walls containing peptidoglycan; examples include Escherichia coli and the cyanobacteria.
• Archaea — single-celled prokaryotes with unique ether-linked membrane lipids and no peptidoglycan; methanogens, halophiles, and thermophiles often thrive in environments once considered extreme.
• Eukarya — organisms whose cells contain a true nucleus; this domain contains the four eukaryotic kingdoms.

The six-kingdom system divides Eukarya further into Protista (protozoa, algae, slime molds — mostly unicellular eukaryotes), Fungi (molds, yeasts, mushrooms — heterotrophic by absorption, chitin cell walls), Plantae (autotrophic, cellulose cell walls, chloroplasts), and Animalia (multicellular heterotrophs without cell walls, motile at some stage). Texas 4-8 teachers should be able to justify each kingdom's placement using cell structure, nutrition mode, and body organization.

Constructing and Using a Dichotomous Key

A dichotomous key is a tool that identifies organisms through a sequence of two-choice steps. Each step offers a pair of mutually exclusive descriptions; the user chooses the one that matches the organism and is directed to another step or to a name. The final step names the species. To build one for a classroom set of organisms, list every observable trait for each organism, choose the most variable trait for step 1, split organisms into two groups, then pick a trait that separates each subgroup, and continue until every organism stands alone.

Worked example — identifying four common Texas trees:

Students practice observing, classifying, and defending identifications with evidence — exactly what Competency 011 asks a teacher to facilitate.

Cell Theory and Cell Types

Modern cell theory has three tenets: (1) all living things are made of one or more cells, (2) the cell is the basic unit of structure and function, and (3) all cells arise from pre-existing cells. The theory rests on Schleiden, Schwann, and Virchow's 19th-century work, with later refinements from electron microscopy and molecular biology.

Prokaryotic cells (Bacteria, Archaea) lack a membrane-bound nucleus and membrane-bound organelles. Their DNA is a single circular chromosome in the nucleoid region, and many also carry small loops of DNA called plasmids. Ribosomes are smaller (70S). Cell-wall composition differs between the two prokaryotic domains: Bacteria use peptidoglycan, Archaea use pseudopeptidoglycan or protein walls.

Eukaryotic cells (Eukarya) have a true nucleus bounded by a nuclear envelope, membrane-bound organelles, and linear chromosomes. Cytoplasmic ribosomes are 80S, but mitochondria and chloroplasts contain 70S ribosomes — strong evidence for the endosymbiotic origin of these organelles from ancestral prokaryotes.

Plant vs Animal Cells

Both cell types share the nucleus, mitochondria, endoplasmic reticulum, Golgi apparatus, ribosomes, cytoskeleton, and plasma membrane. They differ in ways the TExES framework expects candidates to articulate:

The cell wall resists turgor pressure; chloroplasts convert light to chemical energy; the large central vacuole stores water and maintains turgor. Animal cells use centrioles to organize spindle fibers during mitosis and rely on lysosomes to digest worn components.

Organelle Structure and Function

Structure complements function at the organelle level. The table below summarizes the organelles a TExES 4-8 teacher must know and the structural features that enable each function.

The extensive cristae folds multiply inner-membrane surface area and host the electron-transport-chain proteins; thylakoid stacks carry the light-dependent reactions of photosynthesis; Golgi cisternae process secreted proteins in sequence from the cis face to the trans face.

The Cell Membrane and Selective Permeability

The plasma membrane is a phospholipid bilayer with embedded proteins. Each phospholipid has a hydrophilic phosphate head and two hydrophobic fatty-acid tails. In the bilayer, heads face the watery cytoplasm and extracellular fluid while tails cluster inward, forming a flexible, self-sealing barrier. Selective permeability lets small, nonpolar molecules such as O2 and CO2 cross freely; ions, large polar molecules, and proteins require transport proteins or vesicular transport. Passive transport (diffusion, osmosis, facilitated diffusion) moves substances down their gradient without energy input; active transport uses ATP to move substances against their gradient.

The Four Biomolecules

All living things build four classes of organic macromolecules, each assembled from distinct monomers by dehydration synthesis and broken apart by hydrolysis.

Organic compounds always contain carbon, typically with carbon-hydrogen bonds; inorganic compounds such as water, carbon dioxide, and mineral salts usually lack C-H bonds. Water's polarity makes it the universal solvent and a reactant in hydrolysis; CO2 is fixed into organic carbon during photosynthesis. Students should distinguish organic macromolecules from the water and minerals that support metabolism but are not built into polymers.