A2.2 Cell structure

Cell theory states that living organisms are made of cells, the cell is the basic unit of life, and new cells arise from existing cells.

The theory links observations at different scales: tissues are organized from cells, and cell processes explain organismal functions. Modern evidence adds that cells share chemical continuity and pass genetic information during division.

Use the three core claims:

• all organisms contain one or more cells
• cells are the smallest functional units
• cells come from pre-existing cells

A multicellular muscle works because each cell maintains membranes and ATP production; the tissue is not a separate unit replacing the cells.

Cell theory does not say cells are identical or that non-living particles are cells.

Microscope magnification enlarges an image, while resolution determines whether two nearby points can be distinguished as separate.

Total magnification is the product of objective and eyepiece magnification. Increasing magnification without improving resolution produces a larger but blurry image, so useful observation depends on light, optics and preparation.

For a microscope calculation:

• multiply objective by eyepiece
• record the scale or field of view
• judge resolution before claiming detail

A 10× eyepiece with a 40× objective gives 400× total magnification; it does not guarantee that two 0.2 μm structures become distinguishable.

A bigger image is not automatically a more informative image.

Different microscopy methods reveal different structures because they use different probes, wavelengths, vacuum conditions or sectioning methods.

Light microscopy can observe living cells with limited resolution; electron microscopy offers greater detail but usually requires fixed, prepared samples. Fluorescence and confocal methods add molecular or depth-specific information.

Match method to question:

• living dynamics: light or fluorescence
• surface detail: scanning electron microscopy
• internal ultrastructure: transmission electron microscopy
• selected molecules: fluorescent labels

A fluorescent antibody can locate one protein in a cell, whereas a transmission electron micrograph can show membrane layers but not identify that protein by itself.

A high-resolution image is not automatically evidence for the function of every structure shown.

All cells have a plasma membrane, cytoplasm, ribosomes and genetic material, even though their shapes and internal compartments differ.

The membrane separates the internal reaction space, cytoplasm contains soluble chemistry, ribosomes make polypeptides, and DNA or equivalent genetic material stores instructions. These shared features support the cell-theory definition of a cell.

When identifying a cell, look for:

• boundary and internal fluid
• genetic material
• ribosomes or ribosome-rich regions
• a method of protein synthesis

A prokaryotic cell lacks a nucleus but still has DNA, ribosomes, cytoplasm and a plasma membrane.

A nucleus is not a universal cell feature; it is a eukaryotic compartment.

Prokaryotic cells have no membrane-bound nucleus but contain DNA in a nucleoid region and often possess a cell wall, capsule, pili or plasmids.

Their small size and lack of internal membrane compartments affect how transcription and translation are organized. A plasma membrane supplies transport and energy functions, while ribosomes synthesize proteins in the cytoplasm.

Distinguish common features from variable ones:

• nucleoid DNA and 70S ribosomes are typical
• cell wall and capsule vary by group
• plasmids are extra DNA molecules, not the chromosome

A bacterium can carry an antibiotic-resistance plasmid while its main chromosome remains in the nucleoid.

“No nucleus” does not mean “no organized DNA” or “no internal membrane.”

Eukaryotic cells contain a nucleus and membrane-bound organelles that create specialized compartments for different processes.

The nucleus protects chromosomes; mitochondria make ATP through respiration; the rough ER and Golgi process proteins; lysosomes digest materials. Compartmentalization lets incompatible reactions occur in controlled conditions.

Identify an organelle by its job:

• nucleus: genome storage and transcription control
• mitochondrion: aerobic respiration
• rough ER/Golgi: protein processing and export
• lysosome: hydrolytic digestion

A secreted enzyme is synthesized on ribosomes attached to rough ER, modified through the Golgi, and transported in a vesicle to the membrane.

An organelle label alone is not an explanation; connect its structure and location to its function.

A unicellular organism carries out nutrition, respiration, waste removal, sensing, movement and reproduction within one cell.

Because no tissue divides the work, the cell must coordinate its membrane, cytoplasm, genetic material and energy systems. Surface-area-to-volume ratio and environmental exchange become especially important.

Ask how one cell meets each need:

• membrane transport for nutrients and gases
• metabolic pathways for energy
• contractile or ciliary structures for movement
• cell division for reproduction

A Paramecium uses cilia to move and feed, a food vacuole to digest material and contractile vacuoles to expel excess water.

A unicellular organism is not “less alive”; it is a complete organism at a smaller scale.

Plant, animal and fungal cells are eukaryotic but differ in walls, storage materials, organelles and typical shape.

Plants commonly have cellulose cell walls, chloroplasts and large vacuoles; fungi have chitin walls and often grow as hyphae; animals lack a cell wall and rely on extracellular matrices and flexible membranes.

Use combinations, not one feature:

• cellulose wall + chloroplast suggests plant
• chitin wall + hypha suggests fungus
• no wall + flexible outline suggests animal

A chloroplast and cellulose wall together support a plant-cell identification more strongly than a single visible vacuole.

No single feature is infallible: specialized cells can lack a typical organelle or have unusual shapes.

Some eukaryotic cells have unusual nuclear arrangements, such as many nuclei, no nucleus at maturity or nuclei distributed through a shared cytoplasm.

Atypical structure can result from incomplete cytokinesis, cell fusion or loss of the nucleus during differentiation. The cell may still function because organelles, cytoplasm and genetic control are arranged differently.

Explain the cause before naming the pattern:

• multinucleate: nuclei share one cytoplasm
• enucleate: mature cell lacks a nucleus
• syncytium: fused cells form a shared compartment

A mature mammalian red blood cell loses its nucleus to leave more space for haemoglobin, but it cannot divide or repair itself indefinitely.

“One cell, one nucleus” is not a universal rule for eukaryotes.

Cell identification in a micrograph should combine boundaries, organelles, scale and context rather than rely on one visual cue.

A nucleus, cell wall, chloroplast, cristae or bacterial nucleoid can support a classification, but section angle and preparation may hide structures. Scale bars prevent impossible size claims.

Use a defensible sequence:

• calibrate scale
• note boundary and compartments
• compare multiple features
• state uncertainty when evidence is incomplete

A thick cell wall, chloroplasts and a large vacuole together support a plant-cell interpretation even if the nucleus is not visible in the section.

A round outline alone cannot identify a cell type.

A biological drawing is a simplified record of visible structures, not an artistic copy of every shade and texture in a micrograph.

Use clear single lines, proportional shapes, no unnecessary shading, and labels with ruled lines. Include the scale or magnification so the drawing can be interpreted and evaluated.

A reliable drawing includes:

• a title and magnification/scale
• large, accurate outlines
• labels that end on structures
• only structures actually observed

If a chloroplast is visible as an oval with internal stacks, draw that feature at a larger scale rather than inventing details hidden by the section.

Adding textbook structures that are not visible turns an observation into an unsupported diagram.

Endosymbiosis proposes that mitochondria and chloroplasts descended from bacteria that were engulfed by an ancestral host cell.

Evidence includes double membranes, circular DNA, bacterial-sized ribosomes, division by binary fission and gene similarities to bacterial lineages. These features support ancestry but do not mean modern organelles are independent bacteria.

Check several lines of evidence:

• membranes and genetic material
• ribosome type and division
• sequence similarity
• dependence on the host cell

A chloroplast that divides within a plant cell and retains a small circular genome fits the endosymbiotic model better than a membrane-bound organelle with no bacterial features.

Endosymbiosis is an evolutionary explanation, not a claim that organelles currently live freely.

Cell differentiation produces specialized cell types mainly by changing which genes are expressed, not by giving each cell a completely different genome.

Signals and transcription factors open or silence particular genes. The resulting proteins alter structure and function, so cells with the same DNA can become neurons, muscle cells or secretory cells.

Trace the causal chain:

• signal or transcription factor
• gene expression change
• protein production
• specialized structure/function

A muscle precursor expresses contractile-protein genes, while a neuron expresses ion-channel and neurotransmitter genes; both retain the same genome.

Different cell types are not usually created by losing unrelated genes; selective expression is the key mechanism.

Multicellular organisms can specialize cells and coordinate them into tissues, allowing larger size, division of labour and more stable internal conditions.

Multicellularity also creates costs: cells must communicate, transport resources, prevent cheating and coordinate reproduction. The advantage appears when cooperation benefits the collective more than a solitary cell could achieve.

Evaluate a multicellularity claim by checking:

• specialization and coordination
• transport distance and surface area
• communication and control
• costs of dependence

A leaf can combine specialized epidermal, mesophyll and vascular cells to capture light, exchange gases and transport water more effectively than one unspecialized cell.

Being multicellular is not automatically advantageous; it requires mechanisms that keep specialized cells cooperating.