Form and Function connects biological structures to the processes they enable, from molecules and membranes to tissues, organs, transport systems and ecological adaptations.
Published Concept evidence in Form and Function repeatedly asks students to connect structure with function across molecules, membranes, cells and exchange systems. The strongest pattern is explaining how specific features enable transport, support, movement, regulation or adaptation.
Carbohydrates and lipids connect carbon chemistry, condensation and hydrolysis reactions, molecular structure, solubility, membrane formation, and energy storage functions in organisms.
Carbon has four electrons available for bonding and can form four strong covalent bonds. It bonds to carbon and to elements such as H, O, N, S and P, so a stable skeleton can carry many different groups.
| Structural choice | New possibility |
|---|---|
| chain, branch or ring | different overall shapes |
| single or double bond | different geometry and reactivity |
| attached functional group | characteristic chemical behaviour |
The carbon skeleton provides a stable framework; the arrangement of bonds and functional groups determines how the molecule behaves.
In a condensation reaction, an –OH from one molecule and an –H from another leave as H₂O. A new covalent bond joins the remaining parts.
| Subunits joined | Larger molecule | Linkage formed |
|---|---|---|
| monosaccharides | polysaccharide | glycosidic |
| amino acids | polypeptide | peptide |
| nucleotides | nucleic acid | phosphodiester |
Each joining event releases one water molecule. Repeating the reaction builds a macromolecule; the covalent linkage, not the released water, holds its subunits together.
Each hydrogen bond is weak, but many acting in parallel give high tensile strength. Cellulose therefore resists stretching and helps a water-filled plant cell maintain its shape without bursting.
Short carbohydrate chains attached to membrane proteins and lipids project from the outer membrane surface. Together they form the glycocalyx: a chemically varied cell-surface coat.
The exposed carbohydrate patterns act as:
ABO antigens differ in their terminal surface sugars. A recipient's antibodies can bind an unfamiliar antigen and agglutinate donor red cells, showing that small carbohydrate differences can have whole-organism consequences.
A fatty-acid carboxyl group reacts with a glycerol hydroxyl group. Condensation releases water and forms an ester bond. Glycerol has three hydroxyl groups, so it provides three attachment positions.
| Product | Components attached to glycerol | Overall character |
|---|---|---|
| triglyceride | 3 fatty acids | almost entirely hydrophobic |
| phospholipid | 2 fatty acids + ionised phosphate group | amphipathic: polar head, non-polar tails |
Glycerol is an alcohol, not a fatty acid. Replacing one fatty acid with phosphate changes not only the parts list but the molecule's behaviour in water.
| Fatty acid | C=C double bonds | Typical tail shape |
|---|---|---|
| saturated | 0 | relatively straight |
| monounsaturated | 1 | one cis kink |
| polyunsaturated | 2 or more | several kinks |
More cis double bonds prevent neighbouring tails from approaching closely. Intermolecular attractions are weaker, so less thermal energy is needed to separate the molecules and the melting point falls.
At the same temperature, tightly packed saturated lipids are more likely to be solid; more unsaturated lipids are more likely to remain fluid.
| Storage property | Triglyceride consequence |
|---|---|
| highly reduced C–H-rich molecules | more than twice the energy per unit mass of carbohydrate |
| insoluble in water | stored without lowering water potential |
| oxidised completely | yields ATP, carbon dioxide and substantial metabolic water |
Adipose triglyceride also provides:
Triglyceride is compact long-term fuel, but it is mobilised less rapidly than branched glycogen and its oxidation requires more oxygen. Storage suitability depends on the biological timescale and demand.
A phospholipid is amphipathic: its ionised phosphate head is hydrophilic, while its two hydrocarbon tails are hydrophobic.
In water, phospholipids arrange so that:
Self-assembly lowers the exposure of non-polar tails to water. The resulting stable bilayer has a hydrophobic core that restricts many ions and polar solutes, forming the basic boundary of a cell.
Proteins connect amino acid structure, peptide-bond formation, dietary requirements, folding levels, R-group chemistry, denaturation, and functional protein shapes in cells.
Every protein amino acid has an α-carbon bonded to an amine group, a carboxyl group, a hydrogen atom and a variable R-group. The shared groups form peptide bonds; the R-group changes charge, polarity, size and reactivity. Proteins contain C, H, O and N, and usually S in sulfur-containing R-groups.
The carboxyl group of one amino acid loses OH and the amine group of another loses H. Water is released, and the remaining carbon and nitrogen form a covalent C–N peptide bond.
Two residues form one peptide bond and release one water molecule; joining residues forms peptide bonds and releases waters. Ribosomes repeat this reaction to assemble a chain with N- and C-termini.
| Change | Molecular effect | Consequence |
|---|---|---|
| High temperature | weak interactions break | fold and binding surfaces, including active sites, change |
| Extreme pH | R-group charges change | ionic attractions and H-bonds break |
Denaturation changes conformation and function but usually leaves the amino-acid sequence intact. Refolding requires the original interactions to reform before aggregation.
| Pattern | Arrangement | Stabilizing basis |
|---|---|---|
| α-helix | one segment coils | repeated backbone C=O···H–N bonds |
| β-sheet | strands align | repeated H-bonds between backbone segments |
Secondary structure is local backbone geometry; R-groups project away. These motifs can combine into domains such as coiled coils or β-sandwiches. The complete 3D shape of one chain is tertiary structure.
Tertiary structure is the unique overall three-dimensional fold of one polypeptide. Folding brings residues that may be far apart in the primary sequence close together, allowing their R-groups to stabilize the same compact shape.
Many weak interactions cooperate across the folded chain, while disulfide bridges provide stronger cross-links. Peptide bonds still belong to the primary backbone; they are not the interactions that define tertiary folding.
There is no universal rule that polar residues are always “outside” and non-polar residues are always “inside.” Ask: inside or outside relative to which chemical environment?
Quaternary structure arranges two or more polypeptide chains. A conjugated protein also contains a required non-protein prosthetic group.
| Protein | Assembly | Extra group? |
|---|---|---|
| Insulin | A + B chains | none |
| Collagen | three-chain helix | none |
| Haemoglobin | 2α + 2β | four Fe-containing haem groups |
Count chains for quaternary structure; check separately for a prosthetic group.
Membranes and membrane transport link bilayer structure, membrane proteins, selective permeability, gradients, and vesicle movement to cellular control; Students connect membrane structure to transport mechanisms across cellular boundaries.
A phospholipid has a hydrophilic phosphate head and two hydrophobic fatty-acid tails. In water, these interactions drive spontaneous bilayer formation.
heads face water + tails hide from water → continuous bilayer → exposed edges close → sealed membrane or vesicle
Diffusion is passive net movement from higher to lower concentration, driven by random kinetic motion. Particles continue moving both ways at equilibrium, but net movement becomes zero.
Steeper gradient + larger surface area + higher temperature → faster diffusion. Greater distance or membrane thickness → slower diffusion.
Integral proteins enter the hydrophobic bilayer and may span it; peripheral proteins attach to one membrane surface or to another protein.
Integral: channels, carriers, pumps, receptors, enzymes.
Peripheral: signalling partners, enzymes, cytoskeletal anchors and scaffolds.
Osmosis is passive net movement of water across a partially permeable membrane from higher water potential to lower water potential.
Lower solute concentration → higher water potential.
Higher solute concentration → lower water potential.
Aquaporins raise the rate but do not change the direction.
A channel is a hydrophilic pore that permits selected ions or molecules to diffuse down an electrochemical gradient without ATP.
Pore diameter + charge + chemical lining determine selectivity. Some channels remain open; gated channels change between closed and open states.
A pump binds a specific solute and couples ATP hydrolysis to a shape change that moves it against its concentration or electrochemical gradient.
bind on one side → ATP transfers energy, often by phosphorylation → conformation changes → solute released uphill → pump resets
Choose a transport route from particle chemistry and gradient direction: the bilayer sorts by size and polarity; proteins add molecular recognition; ATP permits uphill movement.
small non-polar + downhill → simple diffusion
ion/polar + downhill → channel or carrier
solute + uphill → pump or coupled transporter
very large cargo → vesicle
The fluid mosaic model describes laterally mobile lipids and many proteins arranged asymmetrically in a phospholipid bilayer.
freeze fracture → particles within membrane faces
protein extraction → integral vs peripheral attachment
fluorescent cell fusion → labelled proteins spread laterally
Saturated tails are straight and pack tightly; cis-unsaturated tails contain kinks and pack loosely. More unsaturation generally raises membrane fluidity.
Cooling → organisms can increase unsaturated tails to resist stiffening. Warming → more saturated/longer tails can restrain excess fluidity. This is homeoviscous adaptation.
Cholesterol inserts between animal-membrane phospholipids: its polar –OH aligns near the heads while its hydrophobic rings lie among the tails.
Low temperature: prevents tight packing and stiffening.
High temperature: restrains phospholipid movement and excess fluidity.
Also lowers permeability to small water-soluble molecules.
Because a bilayer is fluid and self-sealing, it can curve, bud and fuse while keeping aqueous contents separated from the cytosol.
Endocytosis: membrane invaginates and pinches off inward.
Exocytosis: an internal vesicle fuses and releases contents outside while adding membrane.
Gated ion channels are selective pores whose probability of opening changes with a signal. Once open, ions move passively down their electrochemical gradients.
Ligand-gated: chemical binding changes shape.
Voltage-gated: membrane potential changes opening.
Mechanical/other gates respond to deformation or intracellular signals.
For each ATP hydrolysed, the sodium–potassium pump moves 3 Na⁺ out and 2 K⁺ in, both against their gradients.
Unequal exchange is electrogenic and helps maintain low cytosolic Na⁺, high cytosolic K⁺ and the gradients used in nerve signalling and cotransport.
In intestinal and kidney epithelia, SGLT cotransports Na⁺ downhill with glucose uphill across the apical membrane. The Na⁺ gradient supplies the immediate energy.
basolateral Na⁺/K⁺ pump uses ATP → low intracellular Na⁺ → apical SGLT imports Na⁺ + glucose → GLUT lets glucose leave downhill
Cell-adhesion molecules connect cells to one another or to extracellular matrix, linking membrane recognition with tissue strength, sealing, communication and signalling.
cadherins → cell–cell anchoring
integrins → cell–matrix attachment and signalling
tight junctions → seal
gap junctions → direct small-molecule/ion communication
Organelles and compartmentalization explain how eukaryotic cells divide work among specialized structures, controlled internal spaces, and directed vesicle transport routes.
An organelle is a distinct cellular subunit adapted for a particular function. Membranes create local conditions, concentrate enzymes and substrates, and isolate incompatible reactions.
boundary → controlled composition → efficient reaction conditions; vesicles and transport proteins then connect the separate spaces into one coordinated cell
The nuclear envelope keeps DNA and transcription in the nucleus while translation occurs on cytoplasmic ribosomes. This separation permits pre-mRNA processing before mature mRNA reaches a ribosome.
DNA transcription → 5′ cap, intron removal and poly-A tail → mature mRNA export through a nuclear pore → several ribosomes can translate it as a polysome
The matrix contains enzymes for the link reaction and Krebs cycle. The inner membrane carries electron-transfer chains and ATP synthase; cristae enlarge its area, while the narrow intermembrane space permits rapid H⁺ accumulation.
reduced carriers donate electrons → inner-membrane complexes pump H⁺ → electrochemical gradient forms → H⁺ returns through ATP synthase → ATP is produced
Thylakoid membranes contain chlorophyll, electron carriers and ATP synthase. Their lumen accumulates H⁺; the surrounding stroma contains enzymes for the Calvin cycle. Grana increase thylakoid membrane area.
light excites electrons → H⁺ accumulates in thylakoid lumen → ATP and reduced NADP form at the membrane → both enter the stroma and support carbon fixation
The nucleus has inner and outer membranes separated by a perinuclear space. The outer membrane is continuous with rough ER; nuclear pore complexes selectively control traffic between nucleoplasm and cytoplasm.
out: processed mRNA, tRNA and assembled ribosomal subunits
in: histones, polymerases, transcription factors and other nuclear proteins
lamina: supports nuclear shape and organizes chromatin
All cytosolic ribosomes begin translation free. A signal peptide on the growing polypeptide can redirect the ribosome to rough ER, where translation continues into the ER lumen or membrane.
The Golgi is a polarized stack of flattened cisternae. The cis face receives ER vesicles; different cisternae modify proteins and lipids; the trans face sorts cargo into vesicles for distinct destinations.
rough ER → transport vesicle → cis Golgi → sequential modification through cisternae → trans Golgi sorting → lysosome, plasma membrane or secretion
A vesicle buds with selected membrane and soluble cargo, travels to a target, and fuses so that cargo enters the correct compartment without exposure to cytosol.
cargo selection → membrane curvature and budding → scission → uncoating → target recognition → fusion; clathrin helps form selected coated buds during receptor-mediated endocytosis
Cell specialization links selective gene expression, stem cell potency, cell size, exchange surfaces, and specialized tissues to biological function in multicellular organisms.
| Potency | Typical developmental source | Possible descendants |
|---|---|---|
| totipotent | zygote and first few divisions | all body cells and extraembryonic tissues such as placenta |
| pluripotent | inner cell mass of the blastocyst | all body cell types, but not placenta or a totipotent cell |
| multipotent | adult tissue stem-cell niches | a restricted family of cells in one tissue or organ |
Development usually moves from broad potential toward commitment: totipotent → pluripotent → multipotent → specialized. A narrower potency does not mean the cell is less useful; it means fewer fates remain available.
Together these features speed oxygen loading and unloading while allowing the cell to bring haemoglobin close to tissues throughout the capillary network.
| Cell type | Structure | Main contribution |
|---|---|---|
| type I pneumocyte | flattened and extremely thin; covers most exchange surface | minimizes the air-to-blood diffusion distance |
| type II pneumocyte | cuboidal; lamellar bodies and secretory machinery | releases surfactant, lowering surface tension so alveoli resist collapse |
Tight junctions between epithelial cells limit tissue-fluid leakage into the alveolar air space. A dense capillary supply keeps blood close to the thin type I surface.
One cell type cannot be maximally thin and strongly secretory at the same time. Division of labour lets the alveolus preserve a short diffusion path, a dry surface and low surface tension.
At an intercalated disc, strong junctions hold adjacent cells together during forceful contraction, while gap junctions provide low-resistance routes for depolarization to pass from cell to cell.
Electrical coupling spreads excitation rapidly and mechanical coupling prevents separation, so many cardiac cells contract as a coordinated wall.
Gas exchange links animal ventilation, alveolar diffusion, leaf stomata, transpiration and haemoglobin affinity to oxygen supply and carbon dioxide removal.
Fick’s law predicts faster gas transfer with greater surface area and partial-pressure difference, and with a thinner diffusion barrier. A moist, permeable surface lets gases dissolve before crossing.
For every claimed adaptation, name the changed variable: area, gradient, distance or permeability. Then connect it to faster O₂ or CO₂ diffusion.
Air moves because thoracic volume changes alter lung pressure. Inspiration is active: diaphragm and external intercostals contract, increasing volume and lowering pressure below atmospheric.
inspiration: muscles contract → thorax expands → pressure falls → air enters
quiet expiration: muscles relax + elastic recoil → volume falls → pressure rises → air leaves
CO₂ enters and water vapour exits mainly through stomata. Guard cells change pore width, balancing photosynthetic demand against dehydration risk.
CO₂: atmosphere → stoma → substomatal air space → moist mesophyll wall → photosynthetic cell
H₂O: xylem → mesophyll wall → evaporation → air spaces → diffusion through stoma
Leaf tissue distribution places palisade cells near light, spongy mesophyll beside interconnected air spaces, stomata in epidermis and vascular bundles within diffusion range of cells.
upper epidermis/cuticle → light entry + water conservation
palisade → chloroplast-rich light capture
spongy mesophyll → moist exchange area + air spaces
xylem/phloem → water supply + assimilate export
Stomatal density = number of stomata counted ÷ sampled epidermal area. It estimates potential pore abundance, while aperture determines how open those pores are at a given moment.
clear nail-varnish impression → peel and mount → calibrate field area → count stomata in random fields → calculate each density → repeat and report variation
Haemoglobin has four haem-containing subunits and binds O₂ reversibly. Binding one O₂ promotes a higher-affinity conformation, producing the sigmoid oxygen-dissociation curve.
low pO₂: initial binding is difficult
middle range: small pO₂ change causes large saturation change
high pO₂: near plateau, loading remains high despite modest fluctuation
Curve height gives percentage saturation at a chosen pO₂. A left-shifted curve has higher affinity; a right-shifted curve has lower affinity and releases more oxygen at the same tissue pO₂.
choose pO₂ on x-axis → draw vertically to each curve → read saturation horizontally → subtract saturations → explain the difference using affinity and biological context
Transport systems move materials through animal blood vessels, plant xylem and phloem, and heart-driven circuits using specialised structures and pressure gradients.
Capillaries are narrow, highly branched exchange vessels whose structure maximizes contact with tissues while minimizing diffusion distance.
Branching produces a very large total surface area. A lumen only slightly wider than a red blood cell brings blood close to the wall, and a one-cell-thick endothelium provides a short path for diffusion.
Veins use large lumens, valves and skeletal-muscle contractions to return blood to the heart despite low pressure.
A wide lumen reduces resistance, valves stop reverse flow and muscle compression raises local pressure. Breathing movements can also help draw venous blood toward the chest.
Mature xylem vessel elements form dead, hollow, lignified tubes adapted to carry water under tension with little resistance.
Lignin thickens and waterproofs the wall and prevents collapse under negative pressure. Unlignified pits allow water to enter or leave laterally, bypass a blockage, and move between xylem and surrounding tissue.
A transverse section of a young dicot root has an outer epidermis, a broad cortex and a central vascular cylinder containing xylem and phloem.
Root hairs extend from epidermal cells to increase absorption area. The endodermis forms the inner boundary of the cortex, surrounding the central vascular tissue and controlling entry to xylem.
Tissue fluid is formed when hydrostatic pressure at the arteriole end of a capillary forces some plasma out through the capillary wall.
Small solutes and water leave, but cells and most plasma proteins remain in the blood. As pressure falls and osmotic effects change along the capillary, some fluid is reabsorbed.
Tissue fluid is the extracellular liquid that bathes body cells and provides the immediate route for exchange between blood plasma and cell membranes.
Oxygen and nutrients diffuse from tissue fluid into cells, while carbon dioxide and other metabolic wastes diffuse out. After passing cells, tissue fluid tends to contain less oxygen and more carbon dioxide than the plasma that initially supplied it.
Excess tissue fluid enters blind-ended lymph capillaries and is returned as lymph to the blood circulation.
Larger lymph ducts use one-way valves, smooth-muscle contraction and compression by body movement to move lymph at low pressure. The ducts eventually empty into large veins near the heart.
Muscle and motility connect sarcomere contraction, antagonistic muscles, skeletons, synovial joints and locomotor adaptations to movement across biological scales efficiently.
Movement changes position or shape; locomotion moves the whole organism from one place to another. A sessile organism remains attached yet may move body parts or grow directionally.
Locomotion can improve access to food, escape from predators, mate finding and migration. The benefit must outweigh its metabolic cost and exposure to risk.
A sarcomere runs from Z line to Z line. During contraction, myosin pulls thin actin filaments toward the centre, increasing overlap and bringing Z lines closer; neither filament shortens.
Ca²⁺ permits cross-bridge formation, while ATP controls detachment and re-cocking. Repeated myosin power strokes slide actin toward the sarcomere centre.
1 Ca²⁺ binds troponin; tropomyosin moves
2 energized myosin–ADP–Pi binds actin
3 Pi/ADP release drives the power stroke
4 ATP binding detaches myosin
5 ATP hydrolysis re-cocks the head
Muscles pull; they do not push. A tendon transmits tension to a rigid skeletal element, a joint acts as fulcrum, and an antagonistic muscle produces the reverse movement.
At the elbow: joint = fulcrum, biceps force = effort, forearm/hand load = resistance. Attachment close to the joint sacrifices force advantage for speed and range.
External and internal intercostals have opposing fibre directions. Their contraction moves ribs differently, changing thoracic volume and therefore lung pressure.
external intercostals contract → ribs up/out → volume rises → pressure falls → inspiration
internal intercostals contract in forced expiration → ribs down/in → volume falls → pressure rises → air exits
Water is dense, so marine mammals combine a streamlined body with controlled thrust and rapid surface ventilation. They remain air-breathing mammals.
| Feature | Mechanical or respiratory effect |
|---|---|
| tapered body and reduced projections | reduces drag in dense water |
| forelimb flippers | steer and stabilize the body |
| horizontal tail flukes driven by axial muscles | generate up-down thrust |
| dorsal blowhole | permits rapid ventilation with little emergence |
Adaptation to environment explains how abiotic conditions, tolerance ranges, habitats and biomes shape organism survival, distribution and specialised traits over time.
A habitat is the physical and biological environment where an organism lives. Describe location together with resources, abiotic conditions and interactions.
A microhabitat is a smaller local environment with distinct conditions. A niche goes further by describing how a species uses resources and interacts within its habitat.
An adaptation is an inherited feature that increases survival or reproduction under particular conditions. Explain it as challenge → feature → physical effect → fitness benefit.
marram: rolled leaf + hairs + sunken stomata → humid trapped air → smaller water-vapour gradient
mangrove: aerial roots and salt exclusion/excretion → aeration and ion balance in saline, anoxic mud
For one environmental factor, a species has a critical minimum and maximum. Performance is highest near an optimum and declines through zones of physiological stress toward either limit.
choose an x-value → locate performance on the curve → classify optimum, stress or intolerance → remember another factor may still prevent the species occurring
Reef-building corals depend on warm, shallow, clear, well-lit seawater with suitable salinity and carbonate chemistry. Photosynthetic symbionts supply much of the energy supporting calcification.
greater depth or turbidity → less light
thermal stress → bleaching risk
lower pH → harder CaCO₃ deposition
sediment/nutrient disturbance → damaged or shaded coral
Long-term temperature and rainfall are the main predictors of terrestrial biomes. Seasonality and insolation refine the prediction because they change water availability, growing season and productivity.
warm + wet year-round → tropical forest
warm + very dry → hot desert
cold + short growing season → tundra
seasonal/moderate rain → grassland or temperate forest, depending on water balance
Tropical rainforests are warm and wet, but light, space and mineral nutrients are unevenly distributed through their vertical layers. Adaptations improve access to these resources or reduce exposure to predators.
Pitcher plants still photosynthesize for carbon and energy; captured animals mainly supply scarce mineral nutrients such as nitrogen.
Ecological niches connect species roles, nutrition modes, adaptations and competition to how organisms use resources and interact within communities over time.
A species' niche is its role in a community: the conditions it tolerates, resources it uses, times and places it is active, and interactions that affect survival and reproduction.
Their habitat overlaps, but diet, feeding position and nesting behaviour separate their niches and reduce competition.
A mixotroph combines autotrophic and heterotrophic nutrition. The two contributions need not be equal or simultaneous; conditions can shift which route dominates.
In light, Euglena can photosynthesize in chloroplasts. It can also take in bacteria by phagocytosis and digest them in food vacuoles. Facultative mixotrophs shift route as light, prey or nutrients change; obligate mixotrophs require both contributions.
A forest canopy intercepts most incoming light, creating a steep vertical gradient. Plant form determines how a species reaches light and which costs it accepts.
A strangler fig can surround and eventually kill its host, while an ordinary epiphyte uses support without necessarily harming it. Position changes access to light, but also water, minerals and pollinators.
The fundamental niche is the range of conditions and resources a species could use from its adaptations and tolerance limits. The realized niche is the range it actually occupies after competition, predation, mutualism and dispersal act.
Observe the focal species with the competitor present → remove the competitor while keeping abiotic conditions comparable → observe expansion into previously unoccupied space → infer that competition had narrowed the realized niche.
A realized niche is often narrower than the fundamental niche, but beneficial interactions can also make occupancy possible. Absence alone does not reveal which interaction or dispersal barrier is responsible.
