Chapter Summary and Practice

One coordinated living system

Cells are the basic structural and functional units of living organisms. Some organisms consist of a single cell, while multicellular organisms rely on cooperation among specialised cells. The cell theory says that organisms are composed of cells and their products and that new cells arise from pre-existing cells. These principles apply across enormous variation in shape and size, from very small bacteria to large egg cells and long neurons. A cell's specialised form supports its tasks but does not remove its membership in the shared cellular organisation of life.

Prokaryotes and eukaryotes

Prokaryotic cells have DNA in a nucleoid without a surrounding nuclear envelope. Many bacteria also contain plasmids carrying accessory genetic functions. Their cytoplasm contains ribosomes and often storage inclusions; their envelopes may include plasma membrane, wall and external glycocalyx. Flagella can provide motility, while pili or fimbriae often assist attachment. Shapes include cocci, bacilli, vibrios and spirilla. Prokaryotes lack the typical eukaryotic membrane-bound organelles, but they are fully capable of complex cellular functions.

Eukaryotes have nuclei and compartmentalised organelles. Plant and animal cells both contain cytoplasm, plasma membrane, ribosomes, nucleus, ER, Golgi and mitochondria. Typical plant cells have a wall, plastids and a prominent central vacuole. Typical animal cells lack the rigid plant wall and often have centrioles within centrosomes. Differences in developmental state create exceptions: some specialised mature eukaryotic cells lack nuclei or certain organelles.

The plasma membrane and the cell wall have different jobs

The plasma membrane consists mainly of a phospholipid bilayer with proteins and associated molecules. Polar heads face water; nonpolar tails face inward. Integral proteins may span the membrane and mediate selective transport. The fluid mosaic model explains lateral membrane movement and diverse protein functions. Plant cell walls are extracellular, cellulose-rich supporting structures. Primary walls expand with growth, secondary walls add strength and the pectin-rich middle lamella cements neighbours; plasmodesmata connect their cytoplasms.

Simple diffusion moves selected solutes downhill across the membrane without direct cellular energy input. Facilitated diffusion moves a substrate downhill through channels or carriers. Osmosis is water movement driven by water-potential differences. Active transport uses an energy source to move selected substances against a relevant gradient. The sodium–potassium ATPase typically exports three Na⁺ and imports two K⁺ ions per ATP consumed. Water uptake by plant cells generates turgor pressure resisted by the cell wall; water loss can cause plasmolysis.

The endomembrane system: synthesis, modification and storage

Rough ER contains bound ribosomes and handles synthesis or early processing of selected proteins. Smooth ER specialises in lipid and steroid-related reactions. The Golgi apparatus receives cargo through its cis side, modifies and sorts it through stacks of cisternae, and directs it from the trans side towards different destinations. Lysosomes digest macromolecules through specialised enzymes in acidic compartments. Vacuoles have broader roles: central plant vacuoles store sap and maintain turgor; contractile vacuoles expel excess water; food vacuoles help with intracellular digestion.

Energy-related organelles

Mitochondria have an outer and inner membrane, a narrow intermembrane space and an internal matrix. Folds of the inner membrane are cristae, which provide area for respiratory machinery and ATP synthesis. Mitochondria contain some DNA and ribosomes, but depend on many nuclear-encoded proteins. Plastids include green chloroplasts, colourful chromoplasts and colourless leucoplasts. Chloroplasts have a double envelope, fluid stroma and a thylakoid membrane system organised into grana and connecting lamellae. Light-linked processes use thylakoid membranes, while many carbon-fixation reactions occur in stroma.

Ribosomes, cytoskeleton and motile structures

Ribosomes translate messenger RNA into proteins. Typical prokaryotic cytosolic ribosomes are 70S (50S and 30S subunits), while typical eukaryotic cytosolic ribosomes are 80S (60S and 40S). The S unit reflects sedimentation, so numerical values are not additive. The cytoskeleton includes tubulin-based microtubules, actin microfilaments and intermediate filaments providing strength. Motor proteins interact with the cytoskeleton to move materials or generate cellular movement.

A typical motile cilium or flagellum has a 9+2 axoneme: nine peripheral doublets surrounding two central microtubules, with radial spokes and connecting complexes. ATP-driven dynein generates sliding that becomes bending. Centrioles, by contrast, have nine peripheral triplets in a cartwheel-like arrangement, without the central axonemal pair. Basal bodies organise projections, while centrosomes in many animal cells organise spindle microtubules during division. Bacterial flagella use a different rotating filament system.

Nucleus and hereditary structures

A nucleus has two envelope membranes separated by a narrow space and punctuated by nuclear pores for regulated transfer. Its nucleoplasm contains chromatin and nucleoli; nucleoli support ribosomal RNA synthesis and assembly. Chromatin condenses into recognisable chromosomes during division. A duplicated chromosome may have sister chromatids associated at a centromere, where kinetochores interact with spindle microtubules. Metacentric, submetacentric, acrocentric and telocentric are categories defined by the centromere's position. Microbodies such as peroxisomes contain oxidative enzymes and are distinct from lysosomes.

Important distinctions in a single view

Do not confuse the cell wall with the plasma membrane or the mitochondrial matrix with the chloroplast stroma. Cristae belong to mitochondria; thylakoids and grana belong to chloroplasts. The nucleolus is not itself membrane-bound, while the whole nucleus is enclosed by a double envelope. A centriole's nine-triplet architecture differs from a motile cilium's nine-doublet-plus-two plan. A 70S ribosome is not the sum of two ordinary numeric masses. Classic bacterial mesosomes are historically described membrane folds but later studies identify the classic profiles as specimen-preparation artefacts.

Mixed application and diagram practice

Use the following problems to build explanations instead of memorising isolated definitions. Where a diagram is requested, label both structure and functional compartments and show causal links. For comparison questions, state a measurable distinguishing criterion rather than relying only on colour, approximate size or a single example.

A good answer to a transport question identifies the molecule, gradient, membrane pathway and energy coupling. A good answer to an organelle question identifies membrane number, internal regions and its role. A good answer about chromosomes specifies centromere location and arm arrangement. These reasoning patterns apply to both familiar diagrams and unseen cellular micrographs.

Quick Quiz

Model Answers

1. Schleiden established plant cellular organisation, Schwann extended it to animals, and Virchow added that all new cells arise from pre-existing cells. The theory states organisms are cellular and cells continue from existing cells.

2. A prokaryotic cell shows envelope, cytoplasm, 70S ribosomes, nucleoid and possible plasmid. A typical eukaryotic cell includes nuclear envelope, 80S cytosolic ribosomes and membrane-bound organelles.

3. Simple diffusion crosses lipid down gradient, facilitated diffusion uses proteins down gradient, and active transport couples energy to uphill transport.

4. Wall gives external support, middle lamella connects neighbouring plant cell walls, and the inner plasma membrane regulates solute movement. Plasmodesmata span wall regions.

5. A targeted polypeptide is translated on ER-bound ribosomes, enters ER for folding and early modification, then traverses Golgi receiving, processing and shipping regions before secretion.

6. Lysosomes use acidic hydrolases to degrade macromolecules; central plant vacuoles mainly store materials and help maintain turgor with transport across the tonoplast.

7. Mitochondrion has outer/inner membranes, matrix and cristae with respiratory machinery. Chloroplast has double envelope, stroma, grana/thylakoids and stroma lamellae with photosynthetic machinery.

8. Common 70S prokaryotic cytosolic ribosomes have 50S and 30S subunits. Typical 80S eukaryotic cytosolic ribosomes have 60S and 40S subunits. S is sedimentation coefficient.

9. Microtubules: tubulin tracks/spindle; actin microfilaments: movement and force; intermediate filaments: tensile support.

10. Axoneme has nine peripheral doublets around central pair and functions in ciliary/flagellar movement; centriole has nine peripheral triplets around a hub, organises microtubules and may become basal body.

11. Nuclear envelope has two membranes and pores; nucleoplasm contains nucleoli and chromatin. Chromosome types: metacentric middle, submetacentric offset, acrocentric near terminal, telocentric terminal.

12. Historical accounts treated mesosomes as membrane invaginations performing many functions, but methodological studies show classic profiles largely reflect preparation artefacts. Other functional bacterial membranes are real and distinct.