IB Biology HL A2.2 Cell structure Notes

Cells are the basic structural and functional units of living organisms. “Structural” means bodies are built from cells. “Functional” means life processes happen in cells. Most cells are microscopic, but a single cell can still metabolize, respond, grow, and reproduce.

Microscope questions are usually workflow plus maths. Use light microscopes to prepare mounts, stain specimens, and observe cells: first prepare a mount, stain if needed, and focus the specimen. Then calibrate the eyepiece graticule with a stage micrometer so each eyepiece division has a real size. After that, use the triangle: magnification = image size / actual size. Keep units consistent before calculating.

Microscopes should be compared by resolution, not only magnification. Resolution decides whether detail can actually be separated. Use TEM for internal ultrastructure in thin sections, SEM for surface detail, cryogenic EM for fine near-native molecular detail, fluorescence to locate fluorescent molecules, and immunofluorescence when antibodies are used to label specific proteins or structures.

Compare what each method reveals instead of ranking them by magnification alone.

Every cell, prokaryotic or eukaryotic, needs the same four essentials: a plasma membrane, cytoplasm, DNA, and ribosomes. The membrane controls exchange, cytoplasm is the site of many metabolic reactions, DNA stores genetic information, and ribosomes make proteins. These are universal; membrane-bound organelles are not.

The same four essentials appear in every cell type.

To identify a prokaryote, look first for what is missing: no nucleus and no membrane-bound organelles. Then look for the positive evidence: cell wall, plasma membrane, cytoplasm, 70S ribosomes, naked circular DNA, and sometimes plasmids. E. coli, Bacillus, and Staphylococcus are bacterial examples.

Diagnose prokaryotes by what they lack and by how their DNA is arranged.

Eukaryotic cells are built around compartmentalization. They have a nucleus, 80S ribosomes, cytoskeleton, and organelles such as mitochondria, ER, Golgi apparatus, vesicles, lysosomes, and vacuoles. Plant cells may add chloroplasts, a large permanent vacuole, and a cellulose cell wall. The key idea is that compartments allow different functions in different spaces.

A eukaryotic cell is defined by compartmentalized cytoplasm around a nucleus.

Unicellular organisms are the strongest proof that one cell can be a whole organism. Amoeba, Chlamydomonas, and Escherichia coli each carry out life processes in one cell: nutrition, metabolism, response, excretion, homeostasis, growth, and reproduction. The examples matter because they turn cell theory from a definition into evidence.

Eukaryotic cell types are best compared feature by feature. Plant cells have cellulose walls, chloroplasts, and large permanent vacuoles. Fungal cells have a cell wall but no chloroplasts. Animal cells lack a cell wall and may have centrioles, cilia, flagella, lysosomes, and temporary vacuoles.

Atypical cells show that textbook diagrams are simplified models. Multinucleate fungal hyphae and striated muscle fibres do not fit the “one cell, one nucleus” picture. Red blood cells and phloem sieve tubes lack nuclei at maturity because of specialization. These cases limit simple cell theory wording, but they do not make the structures non-living or irrelevant.

Micrograph identification is evidence work. First name visible structures: nucleus, chloroplasts, mitochondria, ER, Golgi, vacuoles, ribosomes, or cell wall. Then use scale and context. A good answer sounds like “This is likely a plant cell because a cell wall, chloroplasts, and large vacuole are visible,” not “I think it looks plant-like.”

Biological drawing is controlled observation. Draw clear outlines from the electron micrograph, avoid shading, label only visible organelles, and include magnification or scale information. If the question asks for annotations, link each function to the structure you actually drew. The drawing should communicate evidence, not artistic detail. Annotate structures with functions where required, but only when the function belongs to a visible labelled structure.

Endosymbiosis is an evidence argument. The model says larger prokaryotic cells engulfed smaller prokaryotes, which later became mitochondria and chloroplasts. The evidence points in the same direction: these organelles divide by binary fission, contain circular naked DNA, have 70S ribosomes, are bacterial-sized, and have double membranes. DNA comparisons link chloroplasts to cyanobacteria and mitochondria to alphaproteobacteria including Rickettsiales.

Use this when a question asks for evidence for endosymbiotic theory.

Differentiation explains how multicellular organisms make specialized cells, tissues, and organs. Most body cells share the same genome, but they express different genes. Different gene expression produces different proteomes, and different proteins create different cell structures and functions.

Cell differentiation is caused by differences in gene expression, not by different DNA sets.

Multicellularity evolved repeatedly in fungi, algae, plants, and animals because it offers useful advantages, but it only works if cells solve three problems. They need adhesion to stay together, communication to coordinate, and differentiation to specialize. The payoffs are larger body size and division of labour between specialized cells.

The SL core is a practical chain. First, understand cells as structural and functional units. Then use microscopes correctly: prepare, stain, calibrate, measure, and choose a method based on resolution and the detail needed. Finally, identify cell types from visible evidence and draw only what the micrograph shows. This is how the topic turns from definitions into exam performance.

Use this for combined SL questions involving cell identification, microscopy, or biological drawing.

The HL extension asks how complex cell organization could arise and become useful. Endosymbiosis explains the origin of mitochondria and chloroplasts using bacterial evidence. Differentiation explains how cells with the same genome become specialized through different gene expression and proteomes. Multicellularity explains why adhesion, communication, and differentiation allowed larger bodies and division of labour.

Use this for HL questions asking for evidence, mechanisms, or advantages in complex cell evolution.