Topic 5 - Energy Flow, Ecosystems and the Environment

Photosynthesis converts light energy into chemical energy. Light energy splits strong bonds in water, the released hydrogen is combined with carbon dioxide to form glucose, and oxygen is released to the atmosphere.

Glucose stores chemical energy in its bonds and can fuel respiration or be converted into other biological molecules. Water supplies hydrogen and oxygen; carbon dioxide supplies carbon, so the overall process is more than “plants take in carbon dioxide”.

A leaf in light can use the products of water splitting and carbon dioxide fixation to build glucose; the oxygen released is a waste product of the photosynthetic pathway, not the source of glucose carbon.

The overall reaction summarises coupled stages and does not mean every step happens directly in sunlight. Light-dependent and light-independent reactions use different inputs and locations.

Photophosphorylation uses light-driven electron energy to add inorganic phosphate to ADP and make ATP. ATP hydrolysis then releases a small, usable energy supply for processes such as active transport, synthesis and movement.

ATP is an energy-transfer molecule rather than a long-term store: ADP + Pi can be regenerated to ATP during respiration or photosynthesis, then ATPase-catalysed hydrolysis returns ADP + Pi.

During photosynthesis, ATP made on the thylakoid membrane supplies energy for Calvin-cycle reactions in the stroma. The ATP is not consumed as the carbon skeleton; it powers the chemical steps.

ATP hydrolysis releases usable energy, not “energy from a phosphate bond” in isolation. Photophosphorylation and oxidative phosphorylation are different ways of making ATP.

Light excites chlorophyll electrons in the thylakoid membrane. Their movement through electron carriers drives proton pumping and chemiosmosis to make ATP; non-cyclic flow also reduces NADP, while photolysis of water replaces electrons and releases oxygen.

Non-cyclic photophosphorylation uses photosystems II and I and produces ATP, reduced NADP and oxygen. Cyclic photophosphorylation uses photosystem I and returns electrons to the chain, producing ATP but not reduced NADP or oxygen.

In non-cyclic flow, excited electrons leave photosystem II, water supplies replacement electrons, protons accumulate in the thylakoid space, and ATP synthase uses their return flow to phosphorylate ADP.

“Light-independent” does not mean the Calvin cycle can run without ATP and reduced NADP. Also keep photosystem II’s name separate from its order in the pathway: it acts before photosystem I.

In the light-independent reactions, rubisco combines carbon dioxide with RuBP. The unstable six-carbon product splits into GP; ATP and reduced NADP reduce GP to GALP, and most GALP is used with ATP to regenerate RuBP.

Carbon fixation moves carbon dioxide into an organic molecule. Some GALP leaves the cycle to form glucose and other molecules; the rest keeps the cycle running, so the carbon input and regeneration steps are linked.

For each turn, one carbon dioxide is fixed and only a fraction of a glucose molecule is produced; six turns are needed for one six-carbon glucose, while five-sixths of GALP is recycled to RuBP.

The light-independent reactions do not use light directly, but they depend on ATP and reduced NADP from the light-dependent reactions. GP, GALP and RuBP are different intermediates, not interchangeable names.

Chloroplasts organise photosynthesis by separating reactions. The envelope contains the organelle, the stroma provides enzymes for carbon fixation, and thylakoid membranes hold photosystems, electron carriers and ATP synthase.

Grana provide extensive membrane area for absorbing light and building proton gradients, while the small thylakoid space allows a gradient to form quickly. The stroma is the right compartment for the Calvin cycle because its enzymes and substrates are outside the thylakoid lumen.

A defect in ATP synthase would affect ATP production at the thylakoid membrane first; a defect in a stromal Calvin-cycle enzyme would impair carbon fixation even if light absorption continued.

Photosystems are embedded in thylakoid membranes, not freely dissolved in the stroma. Structure supports function, but it does not mean every chloroplast component performs a photosynthetic reaction.

An absorption spectrum shows which wavelengths a pigment absorbs. An action spectrum shows how effective different wavelengths are at driving photosynthesis, usually inferred from oxygen production or another rate measure.

The two spectra answer different questions: absorption describes light capture, whereas action describes the biological response. A peak in action can reflect combined pigments and electron-transfer efficiency rather than one pigment alone.

If photosynthetic rate peaks under blue and red light but falls under green light, the action spectrum indicates effectiveness; a pigment absorption spectrum helps explain why those wavelengths are captured differently.

An absorption peak is not automatically a photosynthesis-rate peak. Keep wavelength, pigment absorbance and measured rate as separate variables.

Paper chromatography separates photosynthetic pigments because they differ in solubility in the solvent and attraction to the paper. Each band can be compared using its Rf value: distance travelled by pigment divided by distance travelled by the solvent front.

Keep the origin above the solvent, mark the solvent front quickly, and compare Rf values only when solvent, paper and conditions are comparable. Different bands indicate different pigments or pigment mixtures.

If a pigment travels 3.0 cm while the solvent front travels 6.0 cm, its Rf is 0.50. A second sample with the same Rf under the same conditions may contain the same pigment, but the result is comparative rather than absolute identification.

Rf has no units and should lie between 0 and 1. A changed solvent or a smeared band prevents direct comparison; Rf alone is not proof of molecular identity.

To investigate how light intensity, wavelength, temperature or carbon-dioxide availability affects photosynthesis rate, vary one independent variable and measure a response such as oxygen production or a colour change.

Use a controlled distance or filter for light, keep temperature stable, standardise plant material and CO2 concentration, repeat measurements and use a rate over a defined interval rather than a final amount alone.

Moving a lamp closer may increase oxygen bubbles at first, but heat from the lamp can become a second variable. A water bath or heat shield helps separate light intensity from temperature.

A plateau does not mean photosynthesis has stopped; another factor may have become limiting. Bubble counts are a proxy and need consistent bubble size, timing and calibration.

Gross primary productivity (GPP) is the rate at which producers store chemical energy during photosynthesis. Net primary productivity (NPP) is what remains in new plant biomass after respiratory losses: NPP = GPP − R.

Match each value to GPP, NPP or R before calculating, keep area/volume and time units consistent, and rearrange the equation when the unknown is GPP or R.

If GPP is 17,500 kJ m⁻² yr⁻¹ and respiration is 14,000 kJ m⁻² yr⁻¹, NPP is 3,500 kJ m⁻² yr⁻¹. That remainder is the energy available to higher trophic levels.

NPP is not total photosynthetic capture and is not simply plant mass. It is a rate, so the units include area or volume and time; aquatic systems commonly use volume.

Only part of the energy or biomass entering a trophic level becomes new tissue. Transfer efficiency is useful output divided by input, multiplied by 100; losses include respiration heat, egestion, excretion and uneaten material.

For energy, first subtract losses to find net productivity. For biomass, use dry mass and ensure intake and transferred biomass share units before applying the percentage formula.

A consumer receives 10,000 kJ m⁻² yr⁻¹ and loses 9,000 kJ, so its net productivity is 1,000 kJ and efficiency is 10%. A dry-mass calculation uses the same ratio logic but a different measured input.

The familiar “10% rule” is not a fixed law. Efficiency depends on organism, food quality and conditions; biomass transfer is not the same as counting the wet mass of organisms.

A habitat is where an organism lives; a population is all members of one species there; a community is interacting populations; an ecosystem is the community interacting with the non-living parts of its habitat.

These levels are nested, so the correct term depends on what is included. Ecosystems have energy flow and nutrient cycling, but they are not sealed boxes: organisms and materials can move between them.

A pond’s frogs form a population; frogs, fish and pond plants form a community; adding water chemistry, light and sediment makes the pond an ecosystem.

Do not call every group of organisms a population, and do not treat an ecosystem as self-contained. Scale and the stated interactions decide the term.

Biotic factors are effects of living organisms, such as predation, competition, disease and food availability. Abiotic factors are non-living conditions, such as temperature, pH, salinity, light, humidity and oxygen concentration.

A species is abundant only where the combined conditions permit survival and reproduction. One factor can limit a population even when another factor is favourable.

A plant may be restricted to moist, low-salinity soil (abiotic) while herbivory or competition from another plant (biotic) further changes its abundance within that range.

Listing a factor does not show its effect. Link the factor to a mechanism and distinguish correlation in a habitat survey from a controlled causal test.

A niche is the role of a species in its habitat: what it uses, where and when it feeds, and how it interacts with other organisms. Distribution and abundance reflect how well that role fits local conditions.

Two species with identical resource use would compete directly. Small differences in feeding time, food or location can reduce overlap and allow coexistence, but similar niches still tend to lower abundance.

Two birds may eat insects in the same tree yet coexist if one feeds in the canopy by day and the other on the trunk at dusk; the distinction is ecological, not just geographical.

“Same habitat” does not mean “same niche”. Do not infer competitive exclusion from a single observation without comparing resources and population evidence.

Use sampling when a habitat is too large or complex to census. Random sampling estimates abundance without choosing attractive sites; systematic sampling at fixed intervals, often along a transect, tests how distribution changes across a gradient.

Use quadrats for sessile organisms and record presence, frequency, abundance or percentage cover. Match quadrat size and number to the organism and habitat, and repeat enough sites to estimate variation.

To test whether plant cover changes away from a river, place a transect across the gradient and use quadrats at fixed distances. In a uniform meadow, random coordinates reduce selection bias.

A large sample is not automatically representative. State the target population, sampling frame, measurement and possible bias before generalising.

Succession is ecosystem change over time. Pioneer species colonise bare ground, alter soil and microclimate, and are followed by species requiring deeper, more fertile or wetter conditions until a relatively stable community forms.

Each stage changes the abiotic environment and the competition landscape: decomposition builds nutrients, roots stabilise soil, and later vegetation can shade earlier pioneers. Secondary succession starts with soil already present.

On bare rock, lichens and mosses help form soil; grasses and shrubs follow, then trees. Mowing or grazing can hold a stable plagioclimax instead of allowing the expected later community.

A climax community is not necessarily the most diverse and is not a universal endpoint. Describe the local conditions and human disturbance before predicting the sequence.

Evidence for climate change can come from instrumental temperature and carbon-dioxide records, biological indicators such as pollen, and natural archives such as tree rings. Each proxy measures a different variable and has its own uncertainty.

Agreement among independent records strengthens a conclusion; disagreement may reveal dating, calibration or spatial limits rather than immediately falsifying the trend. Evidence for change is distinct from evidence for a particular cause.

A rising instrumental temperature record alongside higher atmospheric CO2 and a dated shift in pollen composition supports a changing climate, but each record must be aligned in time and location.

A proxy is not a direct thermometer. Do not treat correlation alone as proof of causation, and do not extend a local record beyond its sampled range without justification.

Earth’s surface absorbs solar energy and emits infrared radiation. Greenhouse gases absorb some of that outgoing radiation and re-radiate it, reducing heat loss to space; higher concentrations increase the warming influence.

The natural greenhouse effect makes Earth habitable. Anthropogenic climate change refers to additional warming linked to human-driven increases in gases such as carbon dioxide and methane from fossil-fuel use, land-use change and agriculture.

Deforestation removes a carbon sink while fossil-fuel combustion adds carbon dioxide. Landfill and ruminants add methane; each pathway changes atmospheric composition through a different mechanism.

The greenhouse effect is not the same as ozone depletion, and correlation alone does not prove causation. Separate the physical mechanism from evidence about human sources.

Atmospheric carbon dioxide falls only when removal from the atmosphere exceeds additions over the relevant time period. Carbon sinks include forests, soils, peat and oceans, while combustion and land-use change add carbon.

Protecting or restoring a sink changes a flow or storage; it does not make emissions disappear. A strategy should be judged by permanence, scale, leakage and whether it reduces the net atmospheric increase.

Reforestation can increase carbon stored in biomass and soil, but drought, fire or later clearing can return carbon to the atmosphere. Cutting fossil-fuel combustion reduces the source flow directly.

“Carbon neutral” is not automatically permanent removal, and a local increase in biomass does not prove a global atmospheric decline. Track the stock–flow balance.

Extrapolating existing data allows models to project how climate variables may change under specified emissions and human-activity scenarios. The output is conditional on the assumptions, not a guaranteed forecast.

Models support planning, such as flood defences or emissions reduction, by showing how outcomes differ between pathways. Uncertainty comes from future behaviour, incomplete processes, feedbacks and possible tipping points.

A high-emissions pathway may produce more warming than rapid mitigation, but the comparison is meaningful only if both scenarios and their assumptions are stated.

A model range does not mean scientists know nothing, and one scenario is not a prediction of what must happen. Do not treat extrapolation beyond observed conditions as certainty.

Warming changes rainfall, extreme-weather patterns, ocean conditions and seasonal timing. Species may shift poleward or upslope, flower or breed earlier, lose water access, or face extinction when movement or adaptation is too slow.

A climate effect propagates through linked systems: altered temperature or rainfall changes habitat suitability, food timing and competition. Sea level rises through thermal expansion and ice loss, adding a physical pressure to coastal systems.

If plants flower earlier but a migratory bird arrives on its old schedule, the bird may miss peak food availability. A seasonal mismatch is a mechanism, not just a correlation.

Impacts vary by species and location. “Climate change causes extinction” is too broad without specifying the changed condition, exposure and biological response.

Raising temperature usually increases enzyme rate because molecules move faster and collide successfully more often, up to an optimum. Above that point, bonds maintaining tertiary structure break, the active site changes and the rate falls.

Different organisms and enzymes have different optima. Temperature therefore affects metabolism, photosynthesis, development and species distributions through specific enzyme-controlled processes rather than a single universal threshold.

Warmer water may speed cyanobacterial photosynthesis and bloom growth, while excessive heat can increase photorespiration or reduce hatch rates when developmental enzymes or proteins are damaged.

An optimum is not always 35 °C, and slower activity at low temperature is not denaturation. Identify whether the change is reversible slowing or permanent loss of active-site shape.

To test temperature effects on development, place comparable seeds or eggs at different temperatures while keeping light, water, pH, oxygen, food, age and time controlled. Measure growth or hatching rate rather than a final count alone.

Use replicates, calculate an average rate, record the temperature reached, and look for an optimum rather than assuming a straight trend. Living organisms require humane handling and suitable end conditions.

Seedling growth rate can be calculated as average height change divided by days; brine-shrimp hatching rate is hatched eggs divided by hours. Both may rise to an optimum then fall.

A difference between treatments is not causal if another variable changed with temperature. Small sample sizes, uneven starting material or poor welfare can undermine the conclusion.

Variation exists within a population. Random mutation, meiosis and fertilisation create heritable differences; selection pressures then make some phenotypes more likely to survive and reproduce.

Individuals do not evolve because they need to. Selection acts on existing variation: survivors pass advantageous alleles to offspring, so allele frequency changes over generations.

If foxes more often catch visible white rabbits, better-camouflaged brown rabbits leave more offspring and the brown-fur allele becomes more common in the population.

An acquired injury is not inherited, and “survival of the fittest” means reproductive success in a stated environment, not strength or intention.

Speciation is the formation of a new species when populations become isolated, gene flow falls, allele frequencies diverge and the populations can no longer produce fertile offspring together.

Geographical barriers produce allopatric speciation. Sympatric speciation occurs without a physical barrier when timing, behaviour or reproductive structures prevent interbreeding.

A mountain range separates two tree populations. Different selection pressures and genetic drift change allele frequencies until hybrids are no longer fertile; this is a mechanism, not a single sudden mutation.

Different appearance alone does not prove separate species. Identify the isolation mechanism and the reproductive outcome before claiming speciation.

A scientific conclusion becomes more credible when independent evidence supports it and other experts test the methods, data and reasoning. Fossils, DNA/protein sequences and observed resistance can each inform evolutionary explanations.

Peer review and replication do not make a result infallible; they expose weaknesses, refine claims and help separate a robust pattern from a single striking observation.

Similar gene sequences in species can support common ancestry, while antibiotic resistance shows natural selection in real time. The strength comes from the different evidence lines agreeing.

“Peer reviewed” is not a guarantee that a claim is true, and one correlation is not a complete causal explanation. State what the evidence actually supports.

Conservation protects biodiversity for the future. In-situ protection keeps organisms in functioning habitats; ex-situ methods such as seed banks, zoos and captive breeding provide a reserve when wild populations are unsafe.

A conservation method should be judged by genetic diversity, habitat needs, reintroduction feasibility and the people who manage or depend on the site. Small captive samples can lose genetic variation and may not represent wild populations.

A seed bank can preserve crop alleles for later restoration, but species with non-freezable seeds need living collections or tissue culture; a zoo programme succeeds only if release habitat and behaviour are suitable.

Ex-situ storage is not a substitute for protecting habitat, and “conserved” does not mean recovery is guaranteed. Match the method to the biological risk.