Chapter 3 — Cell: Structure and Function

Introduction
The discovery of the cell and the development of cell theory are central to understanding biology. Observations made over centuries, enabled by improvements in glass lenses and microscopy techniques, revealed that living organisms are constructed from repeating living units. Early workers described small compartments in plant tissues and pond water; later careful observations and experiments clarified their importance and led to general principles.

Early observations and techniques
Simple microscopes allowed naturalists to see structures such as cork cells and algae. As staining techniques and compound microscopes improved, scientists began to distinguish living from non-living structures, to observe nuclei and cell division, and to compare cells from different organisms. The ability to prepare thin sections, mount specimens, and use dyes enhanced contrast and made internal components visible.

Formulation of cell theory
From many observations and experiments, three central ideas were formulated: (1) All living organisms are composed of one or more cells; (2) The cell is the basic structural and functional unit of life; (3) All cells come from pre-existing cells by division. These points emphasise continuity of life, the organisational role of the cell, and the fact that life processes operate at the cellular level.

Clarifications and extensions
Cell theory does not address the origin of the first cell, which remains a question for evolutionary biology and abiogenesis. As microscopic and molecular methods advanced, the theory expanded to include the idea that the activity of an organism reflects the activities of its cells and their interactions. The discovery of organelles and molecular genetics placed the nucleus and DNA at the centre of heredity and function.

Significance and modern context
Cell theory underpins modern biology, medicine and biotechnology. Recognising cells as basic units helps explain how diseases develop, why antibiotics target bacteria, and how tissues regenerate. It also paved the way for cell culture techniques, microscopy advances such as electron microscopy, and molecular cell biology studying gene expression, organelle function and signalling. In short, cell theory ties together anatomy, physiology, genetics and ecology by focusing attention on the cell as the seat of life.

Overview
Life on Earth is broadly divided into organisms made of prokaryotic cells and those made of eukaryotic cells. Understanding the differences helps explain diversity, evolutionary relationships and how cellular structures determine functions. Prokaryotes include bacteria and archaea; eukaryotes include animals, plants, fungi and protists.

Prokaryotic cell organisation
Prokaryotes are generally small (1–10 µm) and simpler in internal structure. They lack a membrane-enclosed nucleus; their genetic material is usually a single circular DNA molecule located in a nucleoid region. Often they carry small circular plasmids that contain extra genes like antibiotic resistance. Prokaryotic cells lack membrane-bound organelles such as mitochondria and chloroplasts. They possess 70S ribosomes which synthesise proteins. Many have a rigid cell wall composed of peptidoglycan (in bacteria), a protective capsule, and external appendages such as flagella for movement and pili for attachment and gene transfer. Metabolic pathways occur in the cytoplasm or across the plasma membrane.

Eukaryotic cell complexity
Eukaryotic cells are larger (10–100 µm) and compartmentalised by membranes into organelles. A defining feature is the nucleus — the DNA is enclosed by a nuclear envelope with pores that regulate transport. Eukaryotes possess membrane-bound organelles like mitochondria, chloroplasts (in plants), endoplasmic reticulum, Golgi apparatus and lysosomes, each performing specialised tasks. Ribosomes in the cytoplasm and on RER are 80S, while mitochondria and chloroplasts have 70S-like ribosomes. Genetic material is organised into linear chromosomes associated with histone proteins, allowing complex regulation of gene expression.

Functional consequences
The compartmentalisation in eukaryotes allows separation of incompatible reactions and increased surface area for metabolic processes. Prokaryotes, despite their simplicity, demonstrate high efficiency and adaptability — some perform nitrogen fixation, photosynthesis, or live in extreme environments. The differences also provide antibiotic targets: many drugs exploit features unique to prokaryotes, like peptidoglycan synthesis or 70S ribosomes.

Evolutionary perspective
Evidence suggests eukaryotes evolved from prokaryotic ancestors; organelles like mitochondria and chloroplasts likely arose by endosymbiosis of ancient bacteria. Recognising similarities and differences between the two cell types helps explain cellular evolution and the emergence of complexity.

Introduction
Cells exhibit a wide variety of sizes and shapes that reflect their roles. Physical principles, especially the surface area to volume (SA:V) ratio, place limits on cell size and influence cellular adaptations. Understanding these limits explains why most cells are small and why specialised structures exist to increase surface area.

Size and shape diversity
Cell shapes include spherical (many single-celled organisms), cuboidal (epithelial cells), elongated (muscle fibres), flat (squamous epithelial cells) and highly specialised forms like the long thin axons of neurons or the biconcave red blood cells in humans. Shape often increases efficiency: a flattened shape shortens diffusion distances; elongation allows cell networks to span distances; branched neurons increase contact with many other cells.

Surface area to volume ratio explained
Surface area determines how much material can cross a cell's boundary per unit time, while volume determines the amount of metabolic activity and material required. Mathematically, as linear dimensions increase, volume increases faster (proportional to the cube of the linear dimension) than surface area (proportional to the square). Therefore SA:V decreases with increasing size. A small cell has a relatively large surface area available for exchange compared to its volume, enabling adequate transport of nutrients and removal of wastes. If a cell becomes too large, diffusion across the membrane becomes insufficient to meet metabolic demands.

Biological adaptations
To overcome SA:V limitations, cells use several strategies: remain small; become flat or elongated to increase surface area; form specialised membranes (microvilli on intestinal epithelial cells) or internal membranes (extensive cristae in mitochondria, thylakoids in chloroplasts) to provide more area for reactions; or organise into multicellular structures where tissues and organs handle transport collectively. Root hair cells in plants increase absorptive surface area; intestinal villi and microvilli increase nutrient uptake in animals.

Physiological consequences and examples
SA:V affects heat loss in organisms, metabolic rates, and rates of diffusion-limited processes. Rapidly metabolising cells often are small or packed with internal membranes. Examples include bacteria (small with high SA:V), neurons (long processes to keep cell bodies compact), and muscle fibres (multi-nucleated to support large volumes by distributing control). The SA:V principle also explains why multicellularity evolved: it allows organisms to grow large while keeping cells small and specialised.

Introduction
Microscopy is the window into the cellular world. Different microscopes reveal different levels of detail: the light microscope allows study of living cells and tissues with staining, while electron microscopes reveal fine ultrastructure using electron beams. Mastery of basic microscopy techniques and slide preparation is essential for practical biology.

Principles of light microscopy
Compound light microscopes use a system of lenses to magnify specimens and visible light to form images. Resolution — the ability to distinguish two close points — is limited by the wavelength of light, typically around 200 nm. Contrast is often low in unstained samples, so stains are used to colour specific structures. Magnification equals the product of the eyepiece and objective lens magnifications. Proper illumination using a condenser and diaphragm improves clarity.

Preparing specimens
Common methods include wet mounts for observing living cells in water, squash preparations for delicate tissues, and smears for blood and unicellular organisms. Fixation (using chemicals like formalin) preserves structure, while staining (methylene blue, iodine, safranin, or specialised stains) increases contrast and highlights organelles. Mounting media and coverslips protect the specimen and maintain a flat field for viewing.

Electron microscopy
Electron microscopes use electron beams with wavelengths far shorter than light, achieving much higher resolution. Transmission electron microscopy (TEM) views thin sections to reveal internal structures like membranes and ribosomes; scanning electron microscopy (SEM) produces detailed three-dimensional images of surfaces. Samples must be fixed, dehydrated, and often metal-coated; living specimens cannot be observed. Interpretation differs from light microscopy because images are greyscale electron density maps requiring careful labelling.

Practical skills and good practice
When using a light microscope, always start with the lowest power objective, focus using coarse and then fine adjustment, centre the area of interest, and increase magnification gradually. Record total magnification and include scale if measuring. Make clear, labelled drawings rather than artistic sketches: show relative sizes and positions. For cell counts and measurements, focus on replicates and accurate calibration using a stage micrometer. Safety and cleanliness (proper disposal of stains and biological material) are important in practical work.

Applications and interpretation
Microscopy supports identification of cell types, observation of cell division stages, and detection of abnormalities. It is essential in diagnostics (blood smears, identifying pathogens), histology, and research. Combining microscopy with staining and molecular probes (fluorescent dyes, antibodies) expands what can be seen and linked to function.

Overview
The plasma membrane is a complex, dynamic structure that defines the boundary of the cell and regulates exchange with the environment. The fluid mosaic model provides a conceptual framework describing a phospholipid bilayer interspersed with diverse proteins and other molecules that can move laterally.

Phospholipid bilayer and amphipathic nature
Phospholipid molecules have hydrophilic (water-attracting) heads and hydrophobic (water-repelling) fatty-acid tails. In aqueous environments they self-assemble into bilayers with hydrophobic tails inward and hydrophilic heads facing outward. This bilayer forms the basic barrier to free movement of polar or charged molecules while allowing hydrophobic substances to pass. The barrier is continuous and flexible, providing mechanical protection and support for membrane proteins.

Membrane proteins and their roles
Integral (intrinsic) proteins span the bilayer and serve as channels, carriers, anchors or receptors. Peripheral (extrinsic) proteins attach to the membrane surface and participate in signalling, structure, or enzymatic processes. Transport proteins facilitate facilitated diffusion and active transport; receptor proteins bind signalling molecules and initiate intracellular responses. Glycoproteins and glycolipids, with carbohydrate chains exposed on the outer surface, are important for cell recognition, adhesion and immune responses.

Membrane fluidity and factors affecting it
Membrane components are not fixed. Phospholipids and many proteins can move laterally within the plane of the membrane, giving it fluidity essential for functions like fusion, endocytosis and mobility of receptors. Cholesterol (in animal cells) inserts between phospholipids to modulate fluidity: at low temperatures it prevents tight packing of fatty acid tails, maintaining fluidity; at high temperatures it reduces excessive movement and stabilises the membrane. The degree of saturation of fatty acid tails also affects fluidity: unsaturated tails (with kinks) increase fluidity, saturated tails decrease it.

Asymmetry and microdomains
The two leaflets of the bilayer differ in composition. The outer leaflet often has more glycolipids and glycoproteins, while the inner leaflet has lipids that interact with cytoskeletal elements and signalling proteins. Membrane microdomains or lipid rafts are enriched in cholesterol and specific proteins and act as organising centres for signalling and trafficking.

Functional consequences
The fluid mosaic model explains selective permeability, dynamic rearrangement for endo- and exocytosis, distribution and recycling of receptors, and membrane-based signalling. Disruption of membrane composition affects cell signalling, transport and viability — a reason why temperature, lipids and some toxins have strong cellular effects.

Introduction
Transport across the plasma membrane is essential for nutrient uptake, waste removal, volume regulation and signal transduction. Mechanisms are broadly passive (no energy) or active (energy required). Each mechanism depends on membrane structure and specialised transport proteins.

Simple diffusion
Diffusion is the spontaneous movement of particles from regions of higher concentration to lower concentration due to random thermal motion. Nonpolar molecules like O2, CO2 and small lipids diffuse directly through the lipid bilayer until equilibrium is reached. Diffusion rate depends on concentration gradient, temperature, membrane permeability and surface area.

Osmosis and water potential
Osmosis is the net movement of water across a selectively permeable membrane from a region of higher water potential (lower solute concentration) to lower water potential (higher solute concentration). Terms: hypertonic (external solution has higher solute concentration), hypotonic (lower solute concentration), isotonic (equal). In plant cells, hypotonic surroundings cause turgor (cell becomes firm), while hypertonic causes plasmolysis (protoplast shrinks away from the cell wall). Animal cells in hypotonic solutions may swell and burst (lysis) because they lack a rigid cell wall.

Facilitated diffusion
Polar molecules and ions require protein assistance to cross membranes. Channel proteins form gated or ungated pores permitting passive flow of ions down electrochemical gradients. Carrier proteins bind specific molecules and undergo conformational changes to shuttle them across the membrane. Facilitated diffusion is selective and saturable — transport rate shows a maximum as carriers become fully occupied.

Active transport and pumps
Active transport moves substances against their concentration gradients using energy from ATP hydrolysis or from electrochemical gradients. Primary active transport uses ATP directly; an example is the Na+/K+ ATPase (sodium-potassium pump) that exports 3 Na+ ions and imports 2 K+ ions per ATP, creating essential ionic gradients. Secondary active transport uses the energy of an established gradient (e.g., Na+ gradient) to co-transport other molecules (symport or antiport), without direct ATP consumption at the transporter.

Bulk transport: endocytosis and exocytosis
Large particles, fluids or macromolecules are moved by vesicular transport. Endocytosis includes phagocytosis (cell eating — large particles or cells), pinocytosis (cell drinking — fluids) and receptor-mediated endocytosis (specific uptake via receptors). Vesicles fuse with lysosomes for digestion. Exocytosis is the fusion of secretory vesicles with the plasma membrane to release substances outside, critical for neurotransmitter release and hormone secretion.

Physiological examples and regulation
Kidney tubules use active transport to reabsorb ions and glucose. Neurons exploit ion channels and pumps to maintain resting membrane potential and to generate action potentials. Cells regulate transport by controlling channel opening, carrier expression, and pump activity in response to signals, ensuring homeostasis under changing conditions.

Overview
The cytoplasm and cytoskeleton form the living interior of the cell where metabolism and structural organisation occur. The cytoplasm provides a medium for biochemical reactions and organelle suspension, while the cytoskeleton gives mechanical strength, defines shape and provides pathways for intracellular transport.

Cytoplasm composition and functions
The cytoplasm consists of the cytosol — a complex, aqueous solution of ions, small molecules, proteins and macromolecular complexes — and the suspended organelles. Many metabolic pathways, like glycolysis and parts of lipid and amino acid metabolism, occur in the cytosol. Molecular crowding affects reaction kinetics and stabilises transient complexes. The cytosol also contains components of signalling pathways, storage granules, and structural elements that influence diffusion and reactions.

Cytoskeleton components and properties
The cytoskeleton is made of three main types of protein filaments, each with distinct structure and function. Microfilaments (actin filaments, ~7 nm diameter) are dynamic polymers of actin involved in cell movement, shape changes, and cytokinesis. They interact with motor proteins (myosins) for contraction and vesicle movement. Intermediate filaments (8–12 nm) provide tensile strength and structural integrity; they are less dynamic and vary in protein composition (keratins, vimentin, neurofilaments) depending on cell type. Microtubules (~25 nm) are hollow tubes of tubulin dimers that form tracks for organelle and vesicle transport, make up cilia and flagella, and assemble into the mitotic spindle to segregate chromosomes during cell division. Microtubules interact with motor proteins kinesin and dynein, which use ATP to move cargo directionally along microtubules.

Dynamic nature and regulation
The cytoskeleton is not static; it rapidly assembles and disassembles in response to signalling cues, enabling cells to migrate, change shape and divide. Nucleation factors, capping proteins, severing proteins and cross-linkers regulate filament length, branching and network architecture. Post-translational modifications of cytoskeletal proteins modulate interactions and function.

Functions in transport and cell processes
Intracellular transport uses cytoskeletal tracks: vesicles bud from ER or Golgi and move to destinations along microtubules or actin filaments. Axonal transport in neurons depends on microtubules. During mitosis, the cytoskeleton reorganises to form the spindle for chromosome segregation and the contractile ring for cytokinesis. The cytoskeleton also links to cell adhesion sites and transmits mechanical forces between cells and the extracellular matrix.

Clinical relevance
Defects in cytoskeletal proteins cause diseases: mutations in keratins cause skin fragility, dynein defects affect ciliary movement leading to respiratory and fertility problems, and altered cytoskeleton dynamics are implicated in cancer metastasis. Drugs targeting microtubules (e.g., colchicine, taxanes) are used therapeutically to disrupt cell division in cancer or reduce inflammation in gout.

Overview
The nucleus is the defining organelle of eukaryotic cells, housing genetic material and coordinating processes such as gene expression, DNA replication, and cell cycle control. Its organisation into compartments and associated structures supports precise regulation of cellular activities.

Nuclear envelope and transport
The nucleus is enveloped by a double membrane — the nuclear envelope — continuous with the endoplasmic reticulum. Nuclear pores puncture the envelope, forming large complexes that regulate selective bidirectional traffic: mRNA and ribosomal subunits exit to the cytoplasm, while proteins like transcription factors and histones are imported. Transport through nuclear pores often requires specific signal sequences and energy-dependent carriers (importins/exportins).

Chromatin organisation
DNA in the nucleus is wrapped around histone proteins forming nucleosomes, the basic unit of chromatin. Chromatin exists in more open euchromatin regions where active transcription occurs and denser heterochromatin where genes are largely inactive. Chromatin structure is dynamic and regulated by chemical modifications (acetylation, methylation) of histones and DNA methylation, which influence gene expression patterns and cell identity.

Chromosomes and cell division
During interphase DNA is relatively decondensed for access by transcriptional machinery. Before mitosis, chromatin condenses into discrete chromosomes, each composed of two sister chromatids joined at a centromere after DNA replication. Accurate segregation of chromosomes during mitosis depends on kinetochore attachment to spindle microtubules and checkpoint controls to prevent aneuploidy.

Nucleolus and ribosome assembly
The nucleolus is a dense nuclear region where ribosomal RNA (rRNA) genes are transcribed and ribosomal subunits are assembled. It appears prominent in cells with high protein synthesis demands. Ribosomal subunits are exported to the cytoplasm where they combine to form functional ribosomes.

Functions and regulation
The nucleus regulates gene expression by controlling transcription factor access, RNA processing (capping, splicing, polyadenylation) and export. DNA repair machinery and replication complexes operate in defined nuclear compartments. Nuclear architecture — the spatial arrangement of chromatin and nuclear bodies — contributes to efficient gene regulation. Abnormal nuclear structure and genomic instability are hallmarks of many diseases including cancer.

Overview
The endoplasmic reticulum (ER) and ribosomes form the central machinery for synthesis of proteins and lipids. The ER is an extensive membranous network continuous with the nuclear envelope and exists in rough and smooth forms that perform distinct but interconnected functions.

Rough endoplasmic reticulum (RER)
RER is studded with ribosomes on its cytoplasmic face. Ribosomes translate mRNA into polypeptides; those destined for secretion, for insertion into membranes, or for lysosomal localisation are synthesised by membrane-bound ribosomes and threaded into the RER lumen co-translationally. Inside the RER lumen, nascent polypeptides fold with the help of chaperone proteins and undergo initial modifications such as N-linked glycosylation and formation of disulfide bonds. Misfolded proteins are recognised and targeted for degradation by quality control systems to prevent accumulation of dysfunctional proteins.

Smooth endoplasmic reticulum (SER)
SER lacks ribosomes and specialises in lipid and steroid synthesis, metabolism of carbohydrates, detoxification of drugs and toxins (prominent in liver cells), and storage and regulated release of calcium ions (sarcoplasmic reticulum in muscle cells). Cells engaged in lipid production or detoxification have extensive SER networks.

Ribosomes and protein synthesis
Ribosomes are ribonucleoprotein complexes composed of large and small subunits. They read mRNA codons and, using tRNA molecules charged with amino acids, catalyse peptide bond formation to assemble polypeptides. Free ribosomes synthesise proteins that remain in the cytosol or are targeted to organelles such as mitochondria; membrane-bound ribosomes synthesise proteins entering the secretory pathway via the RER. Ribosome activity is a key determinant of cellular protein synthesis capacity and is regulated at levels of transcription, processing of rRNA, and availability of mRNA templates.

Secretory pathway integration
Proteins processed in the RER are packaged into transport vesicles that bud off and fuse with the cis face of the Golgi apparatus for further modification and sorting. The coordination between ER, vesicular traffic and Golgi ensures proteins reach their proper cellular or extracellular destinations. Disruption of ER function causes stress responses (unfolded protein response) that can alter cell fate.

Physiological examples
Pancreatic acinar cells have abundant RER to synthesise digestive enzymes; hepatocytes show extensive SER for detoxification and lipid metabolism; neurons rely on RER for membrane protein synthesis important for synaptic function. Understanding ER and ribosome function links gene expression to cellular activity and helps explain diseases arising from protein misfolding or secretory pathway defects.

Overview
The Golgi apparatus, lysosomes and peroxisomes form essential parts of the cell's processing, packaging and degradation systems. Together with the ER they manage the flow, modification and recycling of macromolecules.

Golgi apparatus structure and function
The Golgi consists of stacks of flattened membrane-bound cisternae with a distinct polarity: the cis face receives vesicles from the ER, while the trans face ships processed products to their destinations. As proteins and lipids pass through the Golgi cisternae, they undergo modifications such as trimming or addition of carbohydrate chains (glycosylation), sulfation and phosphorylation. The Golgi also sorts proteins into transport vesicles bearing specific molecular tags that determine whether the cargo goes to lysosomes, the plasma membrane, or is secreted extracellularly. The Golgi is therefore central to the secretory pathway, creating mature glycoproteins and glycolipids required for cell-surface identity and function.

Lysosomes: intracellular digestion
Lysosomes are membrane-bound organelles containing hydrolytic enzymes active at acidic pH. They digest macromolecules delivered via endocytosis, phagocytosis or autophagy (turnover of worn-out organelles). Lysosomal enzymes are synthesised in the RER, modified in the Golgi and targeted to lysosomes by specific tags (e.g., mannose-6-phosphate in many eukaryotes). Failure of lysosomal enzymes causes storage diseases in which substrates accumulate and disrupt cell function. Lysosomes also participate in programmed cell death pathways and immune responses.

Peroxisomes and oxidative metabolism
Peroxisomes are single-membrane organelles containing enzymes for oxidation reactions, notably catalase which decomposes hydrogen peroxide (H2O2), a harmful by-product of some oxidative processes. Peroxisomes oxidise long-chain fatty acids via beta-oxidation, assist in detoxification (e.g., alcohol metabolism), and in plants play roles in photorespiration. Peroxisomes arise from ER-derived vesicles and can multiply by growth and division. Their enzyme content is targeted by specific peroxisomal targeting signals in proteins.

Coordination and cellular economy
The ER, Golgi and vesicular trafficking network coordinate to ensure proteins are correctly folded, modified and delivered. Endocytosed material follows pathways to early endosomes and late endosomes, often ending in lysosomes for degradation. Recycled materials are returned to the plasma membrane or reused in biosynthesis. Cells regulate these pathways by signal-dependent sorting, vesicle coat proteins (COPI, COPII, clathrin), and fusion machinery (SNAREs).

Physiological examples
Secretory cells have prominent Golgi; macrophages use lysosomes to digest pathogens after phagocytosis; liver peroxisomes detoxify harmful metabolites. Defects in these organelles underlie many human diseases and are targets for therapeutic interventions.

Overview
Mitochondria are specialised organelles that convert chemical energy from nutrients into ATP, the cell’s energy currency. Their double-membrane structure and internal organisation are adapted to maximise ATP production through aerobic respiration.

Structural organisation
A mitochondrion has an outer membrane that encloses the organelle and an inner membrane folded into cristae which increase the surface area available for the electron transport chain and ATP synthase. The space enclosed by the inner membrane is the matrix, which contains enzymes for the link reaction and the Krebs cycle, as well as mitochondrial DNA (usually circular), ribosomes and storage granules. These features enable mitochondria to carry out many functions semi-autonomously, including some protein synthesis.

Stages of aerobic respiration (overview)
Respiration of glucose to ATP involves several linked stages. Glycolysis in the cytosol converts glucose to pyruvate, producing a small net yield of ATP and reduced NADH. Pyruvate enters mitochondria (in eukaryotes) and is converted to acetyl-CoA in the link reaction. Acetyl-CoA enters the Krebs cycle in the matrix, generating NADH and FADH2 by oxidising carbon skeletons and releasing CO2. The electron transport chain (ETC) located on the inner membrane accepts electrons from NADH and FADH2 and passes them through a series of carrier complexes to molecular oxygen. This electron flow drives proton pumping from the matrix to the intermembrane space, creating an electrochemical (proton) gradient. ATP synthase uses this proton-motive force to phosphorylate ADP to ATP — a process called oxidative phosphorylation.

Energy yield and efficiency
Aerobic respiration yields considerably more ATP per glucose molecule than anaerobic pathways. Typical estimates for total ATP yield per glucose are in the range of 30–32 ATP in eukaryotic cells, though exact numbers vary with shuttle systems and cellular conditions. Mitochondria are most abundant in cells with high energy demands such as cardiac muscle, skeletal muscle, and active neurons.

Physiological and evolutionary significance
Mitochondrial function is central to metabolism, heat production, and apoptotic pathways. Their own DNA and 70S-like ribosomes support the endosymbiotic theory: mitochondria originated from ancestral aerobic bacteria that entered into a symbiotic relationship with early eukaryotes. Mutations in mitochondrial DNA can cause metabolic disorders and contribute to ageing processes.

Overview
Chloroplasts are the photosynthetic organelles of plants and some protists, capturing light energy to synthesise organic molecules. Their internal membranes, pigments and enzymes are organised to convert solar energy into chemical energy and to fix carbon dioxide into carbohydrates.

Structural features
Chloroplasts are bounded by a double membrane and contain an internal system of thylakoid membranes arranged in stacks called grana, connected by stroma lamellae. Thylakoids enclose a lumen and host the photosynthetic pigment chlorophyll, accessory pigments and protein complexes necessary for light-dependent reactions. The stroma is the aqueous matrix that contains enzymes for the Calvin cycle, chloroplast DNA (usually circular), and 70S-like ribosomes. The arrangement of membranes provides large surface area for light capture and ATP synthesis.

Light-dependent reactions
These reactions occur on the thylakoid membranes where pigment-protein complexes (photosystems I and II) absorb light. Photons excite electrons that travel through an electron transport chain, generating a proton gradient across the thylakoid membrane. Photophosphorylation (cyclic or non-cyclic) produces ATP, and electrons reduce NADP+ to NADPH. Water is split at photosystem II to replace lost electrons, releasing O2 as a by-product.

Calvin cycle (light-independent reactions)
In the stroma, the ATP and NADPH produced by the light reactions drive the Calvin cycle, which fixes atmospheric CO2 into organic molecules. The enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO) catalyses the first step, combining CO2 with ribulose bisphosphate to form 3-phosphoglycerate, which is then converted to triose phosphates and ultimately to glucose and other carbohydrates. The cycle requires energy and reducing power and regenerates the CO2 acceptor molecule.

Adaptations and variations
Different plant groups have evolved mechanisms to reduce photorespiration and conserve water. C4 plants spatially separate initial CO2 fixation and the Calvin cycle, concentrating CO2 around RuBisCO. CAM plants temporally separate CO2 uptake (night) and fixation (day) to minimise water loss. Structural adaptations such as large surface area leaves, cuticle, and stomata regulate gas exchange and water loss.

Importance
Photosynthesis is the foundation of food chains, producing organic matter and oxygen. Chloroplasts' endosymbiotic origin explains their own DNA and protein-synthesising machinery. Understanding chloroplast structure and photosynthesis links cellular biology to ecology and agriculture, influencing crop yield and responses to climate change.

Overview
Protein synthesis links genetic information to cellular function. The process involves decoding DNA information into RNA (transcription) and translating mRNA into polypeptides (translation). Ribosomes are the molecular machines that catalyse peptide bond formation during translation.

Transcription: making RNA from DNA
Transcription begins when RNA polymerase binds to a promoter region of a gene, locally unwinds the DNA and synthesises a complementary RNA strand using ribonucleotides. In eukaryotes, transcription produces a primary transcript (pre-mRNA) that requires processing: a 5' cap and 3' poly-A tail are added for stability and export, and introns (non-coding regions) are removed by splicing to produce mature mRNA. Regulatory sequences and transcription factors control when and how much RNA is synthesised, allowing cell-specific gene expression.

RNA processing and export
Splicing is carried out by spliceosomes which precisely remove introns and join exons. Alternative splicing can generate multiple protein isoforms from a single gene. The mature mRNA is exported through nuclear pores to the cytoplasm where ribosomes translate it. Transfer RNAs (tRNAs) are charged with specific amino acids by aminoacyl-tRNA synthetases and deliver them to ribosomes according to codon–anticodon pairing.

Translation: decoding mRNA into protein
Translation occurs on ribosomes which have a small and a large subunit. The mRNA provides codons — triplets of nucleotides — each coding for a specific amino acid according to the genetic code. Translation initiation locates the start codon (AUG), sets the reading frame and assembles the initiation complex. During elongation, tRNAs bring amino acids to the A site of the ribosome, peptide bonds form in the peptidyl transferase centre, and the ribosome translocates along the mRNA. Termination occurs when a stop codon is encountered; release factors promote release of the newly synthesised polypeptide and ribosome subunit dissociation.

Genetic code properties
The code is read in non-overlapping triplets, is nearly universal across organisms, and is degenerate (most amino acids are coded by more than one codon). AUG codes for methionine and also functions as the start signal; UAA, UAG and UGA are stop codons.

Post-translational modification and targeting
Polypeptides often require folding (assisted by chaperones), cleavage, glycosylation, phosphorylation or other modifications to become functional. Signal sequences direct proteins to organelles (ER, mitochondria, chloroplasts) or for secretion. Quality control systems degrade misfolded proteins via ubiquitin–proteasome pathways. Errors in transcription, splicing or translation can have functional consequences and underlie many genetic disorders.

Overview
Plant cells possess specialised structures absent in animal cells: a rigid cell wall, large central vacuoles and diverse plastids. These features support structural integrity, storage, photosynthesis and other specialised functions necessary for plant life.

Cell wall composition and function
The primary cell wall is a composite of cellulose microfibrils embedded in a matrix of hemicellulose and pectins. Cellulose microfibrils provide tensile strength, hemicellulose connects fibrils and pectins contribute to wall porosity and adhesion between cells. In some cells a secondary wall forms inside the primary wall, rich in cellulose and often lignin, providing added rigidity for support and water conduction (as in xylem). The cell wall is porous allowing water and solutes to move via apoplastic pathways, and also defines cell shape, resists turgor pressure and protects against pathogens.

Vacuoles: storage and turgor
Central vacuoles occupy much of the mature plant cell volume and are bounded by a membrane called the tonoplast. Vacuoles store water, ions, sugars, pigments (anthocyanins), waste products and defensive compounds. By adjusting solute concentration, vacuoles generate turgor pressure that maintains rigidity in non-woody tissues and drives cell expansion during growth without requiring excessive new cytoplasm. Vacuoles also participate in degradation of macromolecules and can sequester toxic compounds to protect the cytoplasm.

Plastids: types and roles
Plastids are a family of organelles including chloroplasts, chromoplasts and leucoplasts. Chloroplasts contain chlorophyll and conduct photosynthesis. Chromoplasts synthesise and store pigments (carotenoids) responsible for colours in fruits and flowers, attracting pollinators and aiding seed dispersal. Leucoplasts (amyloplasts, elaioplasts, proteinoplasts) specialise in storage of starch, lipids and proteins in roots and seeds. Plastids can differentiate from one form to another, for example, chloroplasts may convert to chromoplasts during fruit ripening as chlorophyll is degraded and carotenoids accumulate.

Intercellular communication: plasmodesmata
Plasmodesmata are cytoplasmic channels crossing cell walls that connect adjacent plant cells, allowing direct exchange of water, ions, metabolites and small signalling molecules. These channels enable coordinated responses across tissues and are important during development and defence responses. The symplastic pathway (via plasmodesmata) complements the apoplastic movement through cell walls.

Physiological examples and adaptations
Root hair cells increase surface area for water uptake; guard cells around stomata change shape via turgor changes to regulate gas exchange; thickened lignified walls in xylem confer strength and efficient water transport. Together, cell wall, vacuole and plastid specialisations enable plants to maintain structural support, store resources and carry out photosynthesis efficiently.

Overview
Animal cells are embedded in an extracellular matrix (ECM) and connected by specialised junctions that provide mechanical stability, restrict or permit passage of substances, and enable communication. Together these features maintain tissue integrity and coordinate multicellular function.

Extracellular matrix composition and roles
The ECM is a complex mixture of fibrous proteins (collagen, elastin), glycoproteins (fibronectin, laminin), and proteoglycans (protein core with attached glycosaminoglycans). Cells secrete ECM components which assemble into a scaffold that supports tissue architecture, determines mechanical properties, and binds growth factors. The ECM influences cell behaviour by interacting with cell-surface receptors (integrins) that link the ECM to the cytoskeleton, affecting migration, differentiation and survival.

Types of cell junctions
Specialised junctions between animal cells include tight junctions, desmosomes and gap junctions. Tight junctions (zonula occludens) form continuous seals near the apical surfaces of epithelial cells, preventing free passage of molecules between cells and maintaining distinct compositions of apical and basal compartments. Desmosomes (macula adherens) are spot-like adhesions that mechanically link intermediate filaments of adjacent cells, providing resistance to mechanical stress, especially in skin and cardiac muscle. Gap junctions consist of connexin protein channels that connect the cytoplasm of adjacent cells, allowing rapid exchange of ions and small metabolites — vital for electrical coupling in heart and smooth muscle.

Basement membrane and tissue polarity
The basement membrane is a specialised ECM layer composed of type IV collagen, laminin and other molecules that underlies epithelial layers and separates them from connective tissue. It provides attachment, serves as a selective filter (e.g., in kidneys), and helps establish cell polarity by defining basal surfaces and influencing orientation of cell division and migration during development and repair.

Physiological importance and pathology
Cell junctions and ECM are central to development, wound healing and immune responses. Defective junctions lead to diseases such as blistering skin disorders (faulty desmosomes) or cardiac conduction problems (gap junction defects). Excessive ECM deposition causes fibrosis, while ECM degradation facilitates cancer cell invasion and metastasis. Understanding how cells adhere, communicate and respond to ECM signals underlies tissue engineering and regenerative medicine strategies.

Overview
The cell cycle is a coordinated series of events that leads to cell growth and division. It comprises interphase (G1, S, G2) when the cell grows and duplicates DNA, followed by mitosis and cytokinesis which partition replicated chromosomes and divide the cytoplasm to form two daughter cells. Proper regulation of the cycle preserves genomic integrity and allows controlled growth and tissue repair.

Phases of the cell cycle
Interphase includes G1 (gap 1), a phase of growth, biosynthesis and preparation for DNA replication; S phase (synthesis) when the entire genome is replicated producing sister chromatids; and G2 (gap 2) where the cell checks DNA replication and synthesises proteins required for mitosis. Cells may exit the cycle into a quiescent G0 state. Cell cycle checkpoints (G1/S, G2/M, spindle checkpoint) monitor conditions and can halt progression to allow repair or trigger apoptosis if damage is irreparable.

Mitosis stages

• Prophase: chromatin condenses into visible chromosomes, nucleolus disappears, centrosomes (microtubule organising centres) move apart and spindle microtubules begin to form.
• Prometaphase: the nuclear envelope breaks down, microtubules attach to kinetochores at centromeres, and chromosomes begin to move.
• Metaphase: chromosomes align at the metaphase plate equidistant from spindle poles, an arrangement checked by the spindle assembly checkpoint.
• Anaphase: sister chromatids separate and are pulled toward opposite poles as kinetochore microtubules shorten; polar microtubules push poles apart.
• Telophase: chromatids reach poles, nuclear envelopes re-form around each set, chromosomes decondense and nucleoli reappear.

Cytokinesis mechanisms
In animal cells, a contractile ring composed of actin and myosin filaments constricts the cell membrane to form a cleavage furrow that deepens until the cell splits. In plant cells, vesicles from the Golgi coalesce at the cell equator to form a cell plate that develops into a new cell wall separating the daughter cells. The mechanisms reflect differences in cell wall presence and cytoskeletal arrangement.

Biological significance and control
Mitosis ensures genetically identical daughter cells for growth, tissue maintenance and asexual reproduction. Tight control involves cyclins, cyclin-dependent kinases (CDKs), and checkpoint proteins (p53, retinoblastoma protein), which integrate internal and external signals. Dysregulation leads to uncontrolled proliferation and cancer; understanding these controls is fundamental to therapies that target cell division in tumours.

Overview
Multicellular organisms show division of labour: cells specialise to perform particular functions, group into tissues, and form organs and organ systems. Specialisation arises during development by differential gene expression, enabling complexity and efficient functioning of organisms.

Cell differentiation and gene expression
Differentiation is the process by which unspecialised cells become specialised through regulated expression of subsets of genes. Signals from neighbouring cells, extracellular matrix, and morphogen gradients guide cells to adopt specific fates. Epigenetic mechanisms (DNA methylation, histone modifications) lock in gene expression patterns, while transcription factors activate cell-type specific programmes.

Examples of specialised cells
Cells adapt form to function: erythrocytes in mammals lose nuclei and organelles to maximise space for haemoglobin and adopt a biconcave shape to facilitate gas exchange; neurons have long axons and dendrites for rapid long-distance signal transmission; muscle cells contain organised contractile proteins (actin and myosin) and can be elongated and multinucleated for force generation; epithelial cells show polarity and tight junctions for selective absorption or secretion.

Tissue types and their organisation
In animals, tissues are grouped into four fundamental types: epithelial (lining and barrier functions, selective transport), connective (support and binding — includes bone, blood, cartilage), muscular (contraction and movement) and nervous (signal generation and integration). In plants, tissues include meristematic (regions of active cell division) and permanent tissues: epidermal (protective), ground (photosynthesis, storage), and vascular (xylem and phloem for transport). Tissue organisation depends on cell–cell adhesion, ECM composition and intercellular communication.

Structure–function relationships
Tissues are organised to optimise function: intestinal epithelium forms villi and microvilli to increase absorptive surface area; xylem vessels have lignified walls and form continuous tubes for efficient water transport; phloem sieve tubes associate with companion cells to manage translocation of sugars. Organ-level functions emerge from integrated tissue activities — e.g., heart tissue combines contractile muscle, connective tissue and conductive pacemaker cells to pump blood effectively.

Clinical and applied relevance
Understanding cell specialisation informs regenerative medicine and stem cell therapy, as stem cells can differentiate into needed cell types for repair. Tissue engineering uses scaffolds and growth factors to recreate tissue architecture. Diseases often reflect dysfunction of particular cell types (e.g., beta-cell failure in diabetes, neuron loss in neurodegeneration), highlighting the importance of cell-specific biology.

Overview
Cells communicate to coordinate growth, metabolism, immunity and development. Communication uses chemical signals that bind receptors and trigger intracellular signal transduction pathways, converting an external cue into a specific cellular response. Understanding these pathways explains hormone action, neurotransmission and many physiological regulations.

Modes of cell signalling
Signalling can be endocrine (hormones secreted into blood acting at distant sites), paracrine (local mediators acting near the source), autocrine (cells responding to signals they release), juxtacrine (contact-dependent via membrane-bound ligands), and synaptic (neurotransmitters across synapses). Each mode uses different molecules and spatial scales, from local growth factors to systemic hormones.

Receptors and specificity
Receptors are proteins located on the cell surface or intracellularly. Water-soluble ligands (peptides, neurotransmitters) bind membrane receptors that initiate intracellular cascades; lipid-soluble ligands (steroid hormones) cross membranes and bind intracellular receptors that typically modulate gene transcription. Receptor specificity ensures that only cells expressing the correct receptor respond to a given signal, enabling targeted communication.

Signal transduction mechanisms
Ligand binding induces conformational changes in receptors that activate intracellular effectors. Common pathways include G-protein-coupled receptors (GPCRs) that activate G-proteins to regulate second messengers (cAMP, IP3, diacylglycerol), receptor tyrosine kinases (RTKs) that autophosphorylate and recruit signalling complexes, and ion channel-linked receptors that alter membrane potential. Second messengers amplify signals and spread them within the cell; kinases and phosphatases regulate protein activity by phosphorylation cycles. Crosstalk between pathways integrates multiple signals to produce context-dependent responses.

Outcomes and regulation
Responses include changes in enzyme activity, ion channel opening, cytoskeletal rearrangement, altered gene expression or secretion. Cells terminate signals by degrading ligands, receptor internalisation, second messenger degradation, or dephosphorylation. Feedback mechanisms and receptor desensitisation adjust sensitivity and prevent overstimulation.

Examples and physiological roles
Insulin signalling via RTKs stimulates glucose uptake by promoting GLUT4 transporter translocation in muscle and fat cells. Adrenaline acting through GPCRs increases heart rate and mobilises energy stores. Neurotransmitters acting on ionotropic receptors rapidly change membrane potential for synaptic transmission. Dysregulated signalling underlies many diseases: insulin resistance in type 2 diabetes, oncogenic RTK activation in cancers, and neurotransmitter imbalances in psychiatric disorders. Therapeutic drugs often target receptors or signalling enzymes to correct pathological states.

Overview
Cells are subject to damage, ageing and the need to be removed when defective. Controlled mechanisms like apoptosis remove cells without causing inflammation; uncontrolled death (necrosis) does cause inflammation. Cells possess repair pathways for DNA and proteins, and organisms have systems for tissue repair and replacement.

Apoptosis: programmed cell death
Apoptosis is a regulated, energy-dependent process characterised by cell shrinkage, chromatin condensation, DNA fragmentation, membrane blebbing and formation of apoptotic bodies that are phagocytosed by neighbouring cells or macrophages. It is essential during development (e.g., removal of interdigital tissue to form separate digits), immune system development (elimination of autoreactive lymphocytes), and tissue homeostasis. Molecularly, apoptosis is mediated by cascades of cysteine proteases called caspases, which cleave structural and regulatory proteins. Pathways include the intrinsic (mitochondrial) route, involving cytochrome c release and apoptosome formation, and the extrinsic (death receptor) route triggered by ligand binding to cell-surface death receptors.

Necrosis and inflammatory cell death
Necrosis results from acute injury (toxins, ischemia) leading to loss of membrane integrity, uncontrolled release of cellular contents and inflammation. Unlike apoptosis, necrosis is not a programmed process and can amplify tissue damage. Recent research recognises regulated necrosis pathways (necroptosis) that share signalling features with apoptosis but result in lytic death.

DNA repair mechanisms
Cells constantly face DNA damage from replication errors, UV light, chemical agents and reactive oxygen species. Multiple repair pathways correct different lesions: base excision repair removes damaged bases; nucleotide excision repair removes bulky adducts such as thymine dimers; mismatch repair corrects replication errors; homologous recombination and non-homologous end joining repair double-strand breaks. Efficient repair maintains genomic stability; failure to repair leads to mutations, cancer and senescence.

Cellular senescence and ageing
Cells can enter senescence — a permanent cell-cycle arrest — in response to telomere shortening, DNA damage or stress. Senescent cells alter tissue function through secreted factors (senescence-associated secretory phenotype) and accumulate with age, contributing to ageing phenotypes. Telomerase maintains telomere length in germ cells and some stem cells; its dysregulation is implicated in cancer.

Tissue repair and stem cells
Tissue repair involves inflammation, proliferation of progenitor cells, differentiation and remodelling. Adult stem cells reside in niches and supply new cells for renewal (e.g., intestinal epithelium, haematopoietic system). Growth factors and ECM remodelling direct repair. Inadequate repair or excessive fibrosis can impair organ function. Understanding apoptosis, repair and ageing informs therapies in cancer, degenerative diseases and regenerative medicine.