03 Cell Structure and Organization

03 Cell Theory and the Structure–Function Principle

Cells are the fundamental units of biological organization. Although cells vary in size, shape, and function, every cell has a plasma membrane, genetic material, ribosomes, and cytoplasm.

The central principles are:

1. All organisms are composed of one or more cells.

2. The cell is the basic structural and functional unit of life.

3. All cells arise from pre-existing cells.

These ideas developed as microscopy improved. Observations of cork compartments, living microorganisms, plant cells, animal cells, and cell reproduction gradually established the modern framework. Viruses are generally considered acellular because they lack cellular organization and cannot reproduce independently of a host cell.

Structure and function

Cellular form is closely related to cellular function:

• Red blood cells have a thin, flexible shape that supports gas exchange and movement through capillaries.

• Neurons have long extensions that transmit electrical signals over distances.

• Muscle cells contain many mitochondria because contraction requires substantial ATP.

• Leaf mesophyll cells contain chloroplasts that capture light energy.

• Some bacteria have flagella for movement or capsules for protection and attachment.

Takeaway: Cell biology repeatedly connects what a structure looks like with what the cell must do.

03 Microscopy: Magnification, Resolution, and Scale

Microscopy makes cells visible by enlarging specimens and resolving details that are close together. Three concepts must be distinguished:

• Magnification enlarges an object's apparent size.

• Resolution determines whether two nearby structures can be seen as separate.

• Contrast is the difference in brightness or color between a specimen and its background.

Magnification without improved resolution produces a larger but blurry image. In a compound light microscope:

For example, a $10\times$ eyepiece used with a $40\times$ objective produces $400\times$ total magnification.

Light microscopy

Light microscopes use visible light and glass lenses. Bright-field microscopy often uses stains to increase contrast, while phase-contrast microscopy can reveal many unstained living cells. Fluorescence microscopy uses fluorescent dyes or proteins to identify specific molecules or organelles. Confocal microscopy collects optical sections that can be combined into a three-dimensional reconstruction.

Electron microscopy

Electron microscopes use electron beams, whose shorter wavelengths provide much greater resolving power than visible light. Conventional electron microscopy requires prepared, nonliving specimens.

• Transmission electron microscopy, or TEM, reveals internal ultrastructure in thin sections.

• Scanning electron microscopy, or SEM, reveals detailed surface features with an apparent three-dimensional quality.

Preparation steps such as fixation, dehydration, staining, sectioning, and coating can create artifacts. Microscopy images must therefore be interpreted in relation to both the instrument and the specimen preparation.

Scale

Cellular dimensions are commonly expressed in micrometres and nanometres:

• $1\,\text{m} = 10^6\,\mu\text{m}$

• $1\,\mu\text{m} = 10^3\,\text{nm}$

Approximate examples include a plasma membrane thickness of $5\text{–}10\,\text{nm}$, a ribosome diameter of about $20\text{–}30\,\text{nm}$, a bacterium measuring about $0.5\text{–}5\,\mu\text{m}$, and a typical animal or plant cell measuring about $10\text{–}100\,\mu\text{m}$. These are general ranges rather than fixed limits.

Takeaway: A trustworthy microscopic observation depends on magnification, resolution, contrast, scale, and specimen preparation.

03 Prokaryotic Cells and Cellular Exchange

Prokaryotic cells include organisms in the domains Bacteria and Archaea. They usually consist of a single cell, although some form colonies or multicellular-like communities. They lack a membrane-bound nucleus and the extensive membrane-bound organelles characteristic of eukaryotes, but they are internally organized rather than structurally simple.

Important structures include:

• The plasma membrane forms a selective boundary and can participate in energy conversion.

• The cytoplasm is the aqueous interior where many metabolic reactions occur.

• The nucleoid contains the main chromosome, usually a circular DNA molecule.

• Plasmids are small DNA molecules that may carry accessory genes, including antibiotic-resistance genes.

• Ribosomes carry out protein synthesis.

• A cell wall provides shape and protection against mechanical stress and osmotic lysis.

• Capsules or slime layers can reduce drying, promote attachment, or help cells evade host defenses.

• Pili or fimbriae support attachment, and specialized pili can participate in DNA transfer.

• Flagella provide movement for many prokaryotes.

Bacterial cell walls commonly contain peptidoglycan. Archaeal walls lack bacterial peptidoglycan and may contain different polymers. Archaeal membrane lipids also have distinctive ether-linked structures.

Surface-area-to-volume ratio

As a cell becomes larger, volume increases faster than surface area. The surface-area-to-volume ratio therefore decreases. Because nutrients, gases, and wastes must cross the plasma membrane, a low ratio can limit exchange with the environment. This helps explain why many prokaryotes are small and why larger cells use folds, internal compartments, or branching shapes to improve exchange efficiency.

Takeaway: Prokaryotic organization lacks a nucleus but still includes regulated transport, specialized structures, protein complexes, and spatially organized metabolism.

03 Eukaryotic Cells: Membranes and Organelles

Eukaryotic cells occur in animals, plants, fungi, protists, and many algae. Their nucleus and membrane-bound organelles divide cellular work into specialized compartments.

Boundary and internal fluid

The plasma membrane is described by the fluid mosaic model. A phospholipid has a polar, hydrophilic head and two nonpolar, hydrophobic tails. In water, phospholipids form a bilayer with heads facing aqueous environments and tails forming the interior. Integral proteins can form channels, carriers, receptors, enzymes, or anchors. Peripheral proteins attach to a surface or to other proteins. Cholesterol helps regulate membrane fluidity and stability in animal cells, while carbohydrate chains support recognition and adhesion.

The cytoplasm includes the cytosol, organelles, cytoskeletal elements, and dissolved materials between the plasma membrane and nuclear envelope. The cytosol is the fluid portion, where many metabolic reactions occur.

Nucleus and ribosomes

The nucleus stores most eukaryotic DNA inside a double nuclear envelope. Nuclear pores regulate movement between the nucleus and cytoplasm. DNA associated with proteins forms chromatin. The nucleolus produces ribosomal RNA and begins assembling ribosomal subunits.

Ribosomes translate messenger RNA into polypeptides. Free ribosomes generally produce proteins used in the cytosol or certain organelles. Ribosomes attached to rough endoplasmic reticulum produce proteins destined for secretion, membranes, or the endomembrane system.

Endomembrane system

• Rough endoplasmic reticulum synthesizes and begins processing proteins.

• Smooth endoplasmic reticulum contributes to lipid synthesis, detoxification, carbohydrate metabolism, and calcium-ion storage.

• The Golgi apparatus modifies, sorts, and packages proteins and lipids received from the endoplasmic reticulum.

• Lysosomes digest macromolecules, worn-out organelles, and material taken into the cell.

• Peroxisomes carry out oxidation reactions, including fatty-acid breakdown and detoxification; catalase can break down hydrogen peroxide.

• Vesicles transport, store, or digest materials.

• Vacuoles are larger storage compartments. Plant cells often have a central vacuole that stores water and solutes, contributes to internal pressure, and helps maintain shape.

Takeaway: Eukaryotic compartmentalization allows different chemical processes to occur efficiently and under controlled conditions.

03 Energy Conversion, the Cytoskeleton, and Cell Connections

Several organelles specialize in energy conversion, support, movement, and communication.

Mitochondria and chloroplasts

Mitochondria are double-membrane organelles that carry out much of the aerobic production of ATP. Their inner membrane forms folds called cristae, which increase surface area for electron transport and ATP synthesis. The matrix contains several reactions of cellular respiration. Mitochondria have their own DNA and ribosomes and divide independently of the nuclear envelope.

Chloroplasts occur in plants and many algae. Chlorophyll and other pigments absorb light energy for photosynthesis. Flattened thylakoids may be arranged in stacks called grana, and the surrounding fluid is the stroma. Chloroplasts also contain their own DNA and ribosomes and are surrounded by two membranes. These features support the endosymbiotic theory for both mitochondria and chloroplasts.

Other plastids include chromoplasts, which store pigments; leucoplasts, which store substances such as starch; and amyloplasts, which specialize in starch storage.

Cytoskeleton

The cytoskeleton is a network of protein fibers that supports cell shape, positions organelles, enables movement, and assists cell division.

• Microfilaments, composed primarily of actin, support the cell cortex and participate in movement and contraction.

• Intermediate filaments provide tensile strength and stabilize cellular structures.

• Microtubules, composed of tubulin, organize organelle movement, form the spindle during cell division, and contribute to cilia and flagella.

Cell walls and junctions

Plant cell walls are composed primarily of cellulose, fungal cell walls contain chitin and other polymers, and bacterial cell walls generally contain peptidoglycan. Archaeal cell walls have different chemical compositions. Animal cells lack cell walls.

Plant cells communicate through plasmodesmata, channels that cross adjacent cell walls. Animal cells use tight junctions, anchoring junctions, and gap junctions to seal, attach, and communicate between cells.

Takeaway: Organelles and structural fibers work together to convert energy, move materials, maintain shape, divide cells, and coordinate neighboring cells.

03 Comparing Prokaryotic and Eukaryotic Cells

The two cellular plans share essential features but differ in organization and compartmentalization.

Prokaryotic cells

• The nucleus is absent; DNA is located in a nucleoid region.

• DNA is usually organized as one main circular chromosome, although plasmids may also occur.

• Membrane-bound organelles are absent in the typical textbook sense.

• Ribosomes and a plasma membrane are present.

• A cell wall is common but chemically variable.

• Typical dimensions are approximately $0.1\text{–}5\,\mu\text{m}$.

• Cell division usually occurs by binary fission.

• Examples include Bacteria and Archaea.

Eukaryotic cells

• A nucleus is present, and DNA is enclosed by a nuclear envelope.

• DNA is usually organized as multiple linear chromosomes associated with histones.

• Membrane-bound organelles, including the endoplasmic reticulum, Golgi apparatus, and mitochondria, are present.

• Ribosomes and a plasma membrane are present.

• Cell walls occur in plants and fungi but not in animals.

• Common dimensions are approximately $10\text{–}100\,\mu\text{m}$.

• Mitosis produces new cells, while meiosis produces gametes or other reproductive cells.

• Examples include animals, plants, fungi, and protists.

A comparison should not treat eukaryotic cells as simply “more advanced.” Prokaryotes perform diverse metabolisms and occupy nearly every environment. Eukaryotes achieve extensive specialization through compartmentalization and, in many cases, multicellularity.

Interpreting cell form

When identifying a cell, use several lines of evidence rather than one visible feature. A nucleus, membrane-bound organelles, cell walls, plastids, cell size, and overall arrangement can help distinguish cell types, but a structure that is not visible may still be present. It may be too small, transparent, unstained, or outside the focal plane.

Takeaway: The most useful distinction is not size alone; it is whether the cell has a membrane-bound nucleus and internal membrane-bound compartments.

03 Inquiry-Based Microscopy and Cell Evidence

Microscopy investigations connect visible evidence with models of cell structure and function. A strong investigation includes a question, hypothesis, controlled variables, repeated observations, recorded data, and an evidence-based conclusion.

Comparing onion and cheek cells

Onion epidermal cells generally appear rectangular and regularly arranged because plant cells have rigid cell walls. Cheek cells are more irregular and show a plasma membrane and stained nucleus but no cell wall. Both cell types share a plasma membrane, cytoplasm, nucleus, and ribosomes, while plant cells additionally have a cell wall, a large vacuole, and—depending on the tissue—plastids.

Begin with the lowest-power objective, center and focus the specimen, then increase magnification. Record cell shape, arrangement, visible boundaries, nuclei, and approximate dimensions from several fields of view.

Estimating cell size

A calibrated field of view or stage micrometer can provide an estimate. If four cells span a field with a diameter of $1{,}000\,\mu\text{m}$, the average length is approximately:

The estimate includes uncertainty because cells may be irregularly shaped or not aligned across the field.

Osmosis in plant cells

In a hypertonic solution, water leaves a plant cell and plasmolysis may occur. In a hypotonic environment, water enters the cell, but the cell wall limits expansion. A water-treated control helps determine whether the observed change resulted from the external solute concentration.

Cytoplasmic streaming

To test whether temperature affects streaming rate, vary temperature as the independent variable and measure movement as distance per unit time or chloroplast movements per minute. Keep light intensity, specimen type, observation time, magnification, and sample thickness constant. A moderate temperature increase may increase movement, but excessive heat can damage membranes and proteins, so the relationship may not be linear.

Prepared bacterial slides

Bright-field microscopy may reveal bacterial shape and arrangement, such as cocci, bacilli, or spirilla in pairs, chains, or clusters. It generally cannot directly resolve ribosomes, membrane layers, or detailed nucleoid organization.

Takeaway: A good conclusion separates what was directly observed, what was inferred from established biology, and what remains uncertain.

03 Evaluating Microscopy Evidence

Microscopy data are observations rather than complete pictures of a cell. Interpret them by considering the limits of the instrument and the design of the investigation.

• Sampling: One field of view may not represent the entire specimen.

• Preparation artifacts: Drying, staining, fixation, or sectioning can alter cell shape.

• Resolution limits: Structures below the resolving power cannot be distinguished directly.

• Focal depth: Three-dimensional cells may require focusing through multiple planes.

• Scale bars: Apparent size is meaningful only when the image scale is known.

• Color: Micrograph colors may represent stains, fluorescent labels, or image processing rather than natural colors.

• Controls: A control sample helps identify whether a treatment caused a change.

• Replication: Repeated observations improve confidence and reveal biological variation.

A careful investigation also distinguishes correlation from causation. For example, if streaming appears faster at a higher temperature, repeated trials and controlled conditions are needed before concluding that temperature caused the difference. Means, graphs, and uncertainty estimates make patterns easier to evaluate.

Final takeaway: Understanding cells requires combining structural models with appropriately cautious interpretation of evidence. Cell theory provides the organizing framework; microscopy reveals selected features; and comparisons of membranes, organelles, and cellular plans explain how form supports function.