Photosynthesis & Cellular Respiration
Ms. Terkper's Digital Classroom — Themes: Energy, Matter & Systems
Focusing Questions
"How does light energy from the environment enter living systems? How is the energy from light converted to chemical potential in organic matter? How is the energy in organic matter released for use by living systems? How do humans in their application of technologies impact photosynthesis and cellular respiration?"
General Outcomes
Key Concepts
The Central Theme
Photosynthesis and cellular respiration are complementary processes. Photosynthesis converts light energy into chemical potential energy stored in glucose. Cellular respiration releases that stored energy as ATP for cellular work. Together they form the biochemical engine driving all life on Earth — and together they are the key link between the sun's energy and the biosphere.
Builds on Bio 20 Unit A
This unit zooms into the molecular mechanisms behind the carbon and oxygen cycling explored in Unit A. The equations you studied there (photosynthesis and cellular respiration) are now unpacked stage by stage: where each reaction happens in the cell, which molecules carry energy, and how ATP is actually synthesized.
Pigments & Light Absorption
Why Do Leaves Look Green?
Photosynthetic pigments absorb certain wavelengths of light and reflect others. The reflected wavelengths are what we see. Chlorophyll a and b absorb red and blue-violet light most strongly but reflect green light — which is why leaves appear green. Multiple pigments working together capture a broader range of the light spectrum.
Absorption Spectrum vs Action Spectrum
Absorption Spectrum
Shows which wavelengths of light are absorbed by a pigment (measured by spectrophotometry). Each pigment has a unique absorption spectrum. Chlorophyll a has peaks at ~430 nm (violet-blue) and ~680 nm (red). The green region (~550 nm) is minimally absorbed.
Action Spectrum
Shows which wavelengths of light drive photosynthesis most effectively (measured by O2 production or CO2 uptake at different wavelengths). The action spectrum closely matches the combined absorption spectra of all pigments, confirming it is light absorption that drives photosynthesis. First measured by T.W. Engelmann (1882) using algae and motile aerobic bacteria.
Paper Chromatography
Chromatography separates pigments by their different solubilities in a solvent. Each pigment travels a characteristic distance. The Rf (reference flow) value = distance pigment travels / distance solvent travels. Rf values are consistent for a given pigment in a given solvent system.
(chromatogram)
Rf Values & Pigment Identification
| Pigment | Colour | Rf Value | Function |
|---|---|---|---|
| Run the chromatography to see results | |||
Rf Formula
Rf values are consistent for a given pigment in a given solvent. Non-polar pigments (carotenoids) travel farther in non-polar solvents. Polar pigments (chlorophylls) travel less far. Rf values are always between 0 and 1.
Photosynthesis
Cellular Respiration
| Stage | Location | O2 Required? | Direct ATP | NADH Produced | FADH Produced | Net ATP (via ETC) |
|---|---|---|---|---|---|---|
| Glycolysis | Cytoplasm | No | 2 ATP | 2 NADH | — | ~6 ATP |
| Pyruvate Oxidation | Mitochondrial matrix | Yes | 0 | 2 NADH | — | ~6 ATP |
| Krebs Cycle | Mitochondrial matrix | Yes | 2 ATP | 6 NADH | 2 FADH | ~24 ATP |
| Aerobic Total | — | Yes | 4 ATP direct | 10 NADH | 2 FADH | ~36–38 ATP total |
| Anaerobic (Fermentation) | Cytoplasm only | No | 2 ATP only | — | — | 2 ATP total |
ATP: The Cell's Energy Currency
What is ATP?
Adenosine triphosphate (ATP) is the universal energy currency of all living cells. It is a nucleotide with three phosphate groups. The bond between the second and third phosphate is high-energy. When hydrolyzed (ATP → ADP + Pi), approximately 30.5 kJ/mol of free energy is released, which is coupled to endergonic cellular reactions.
Why ATP and not glucose directly?
ATP provides small, precisely-controlled packets of energy. Cells cannot couple the enormous energy release of burning glucose directly to cellular reactions — it would be like using a firehose to fill a glass. ATP acts as a currency: glucose energy is converted to many ATP molecules, each dispensed in controlled amounts to specific reactions as needed.
Roles of ATP in Cellular Metabolism — Click to Expand
Factors Affecting Photosynthesis Rate
The rate of photosynthesis is limited by the least-available resource or least-favourable condition. This is Blackman's Law of Limiting Factors (1905): the rate of any process is limited by the factor that is most limiting at that moment. Adjust the sliders below to explore how each factor affects photosynthesis rate.
Limiting Factor
Calculating...
Overall Photosynthesis Rate
| Factor | Effect on Photosynthesis | Effect on Cellular Respiration | Mechanism |
|---|---|---|---|
| Light intensity | Increases rate up to saturation point; then plateaus (limited by CO2/enzyme capacity) | Indirect (provides glucose substrate via photosynthesis) | More photons excite more chlorophyll molecules; drives electron flow in light reactions |
| CO2 concentration | Increases rate (substrate for Calvin cycle); saturation at very high levels | No direct effect (product of respiration) | CO2 is the carbon source reduced to glucose by RuBisCO enzyme in Calvin cycle |
| Temperature | Increases rate up to ~25–35°C optimum; drops sharply above (enzyme denaturation) | Increases rate up to ~37°C optimum; drops above (denaturation of mitochondrial enzymes) | Temperature affects enzyme kinetics. Too hot = denaturation of RuBisCO, pyruvate dehydrogenase, etc. |
| Water availability | Low water = stomata close, reducing CO2 entry; also reactant in light reactions | Indirect effect via substrate availability | Stomatal closure prevents CO2/O2 exchange; leaf temperature rises; water is split in photolysis |
| O2 concentration | High O2 causes photorespiration (competitive inhibition of RuBisCO by O2) | Required for aerobic pathway; low O2 triggers anaerobic fermentation | RuBisCO can react with O2 instead of CO2 (photorespiration), wasting energy; C4 plants suppress this |
Applications of Cellular Biochemistry
Photosynthesis is the biological basis of all agricultural production. Improving photosynthetic efficiency could dramatically increase crop yields.
- C4 crops: Maize (corn), sugarcane, and sorghum use C4 photosynthesis, concentrating CO2 and suppressing photorespiration — up to 2× more efficient than C3 crops in hot, sunny conditions. Alberta researchers are working to engineer C4 traits into wheat.
- Greenhouses: CO2 enrichment (raising concentration from ~420 ppm to 1,000–1,500 ppm) increases growth rate of C3 plants by 20–40%.
- Photoperiodism: Day length is controlled in greenhouses to trigger flowering in short-day or long-day crops. Chrysanthemums, poinsettias (short-day); wheat, barley (long-day).
- Alberta connection: Canola (Brassica napus), a C3 crop, is Alberta's most valuable crop. Photosynthesis research at University of Alberta aims to improve canola's water-use efficiency under drought conditions.
Some industrial by-products interfere with cellular respiration at specific points in the electron transport chain or Krebs cycle.
- Cyanide (HCN): Binds irreversibly to cytochrome c oxidase (Complex IV of ETC), blocking electron transfer to O2. ATP production stops. Fatal within minutes. Industrial source: gold mining, electroplating, plastic manufacturing. Alberta connection: historical use in gold mining in the Rocky Mountain foothills.
- Hydrogen sulfide (H2S): Also inhibits cytochrome c oxidase. Produced by oil and gas extraction, pulp mills, and sewage. Workers in confined spaces near H2S sources at extreme risk. Alberta oil sands and gas processing facilities require H2S monitoring.
- Carbon monoxide (CO): Binds haemoglobin (reducing O2 transport) and also inhibits mitochondrial cytochrome oxidase.
- Herbicides (e.g., DCMU / diuron): Block the electron acceptor Q in Photosystem II, halting light-dependent reactions. No NADPH or ATP produced; photosynthesis stops. Used commercially to selectively kill weeds.
Anaerobic fermentation is harnessed industrially for food production, bioenergy, and bioremediation.
- Bread making: Yeast fermentation (alcohol fermentation) produces CO2 that leavens bread; ethanol evaporates during baking.
- Beer/wine: Yeast ferments glucose → ethanol + CO2. Alcohol concentration rises until yeast are killed by their own ethanol (~15%).
- Yogurt/cheese: Lactic acid bacteria ferment lactose → lactic acid, lowering pH and curdling milk proteins. No CO2 produced (lactic acid fermentation).
- Methane biogas: Anaerobic bacteria in oxygen-depleted environments (swamps, landfills, manure lagoons) produce CH4 from organic matter. Alberta feedlots use biogas digesters to capture methane from manure for electricity generation, reducing greenhouse gas emissions.
- Aerobic/anaerobic fitness: Muscle cells use aerobic respiration for sustained activity. When O2 delivery is insufficient (intense exercise), lactic acid fermentation supplements ATP production, causing the "burn."
| Herbicide Class | Site of Action | Mechanism | Example | Concern |
|---|---|---|---|---|
| Photosystem II inhibitors | Thylakoid membrane (PS II) | Block plastoquinone binding site; halt electron transport from PS II; no ATP or NADPH produced | Atrazine, DCMU | Atrazine widely detected in Alberta groundwater; disrupts amphibian hormones at sub-lethal concentrations |
| Photosystem I inhibitors | PS I electron acceptors | Divert electrons from Fd to O2, producing reactive oxygen species (ROS) that destroy cell membranes | Paraquat | Extremely toxic to humans and animals; has caused many accidental deaths |
| Carotenoid biosynthesis inhibitors | Carotenoid synthesis pathway | Block synthesis of protective carotenoids; unprotected chlorophyll destroyed by excess light energy (photobleaching) | Fluridone | Used in aquatic weed control; can affect non-target aquatic plants |
| Amino acid synthesis inhibitors | EPSPS enzyme (shikimate pathway) | Block synthesis of aromatic amino acids (tyrosine, tryptophan, phenylalanine) needed for protein synthesis | Glyphosate (Roundup) | Most widely used herbicide in Alberta agriculture; concerns about gut microbiome effects in mammals |
Indigenous Knowledge & Plant Productivity
Traditional Knowledge of Plant Productivity & Cellular Processes
Alberta's First Nations, Métis and Inuit peoples have long understood the importance of plant productivity to human sustainability — encoding observations about photosynthesis, plant growth, and seasonal energy cycles in practices and language that predate Western biochemistry by thousands of years.
"The Trees Are the Lungs of Mother Earth"
This Aboriginal metaphor, referenced directly in the Alberta Biology 20 curriculum (Outcome 20-C2.3s), captures a profound ecological truth: forests absorb CO2 through photosynthesis and release O2 through cellular respiration, functioning as the respiratory system of the planet. The metaphor also implies reciprocity — the forest breathes for all living things, requiring that humans reciprocate through stewardship.
This understanding aligns precisely with the complementary relationship between photosynthesis and cellular respiration — the core of Bio 20 Unit C. The metaphor also predates by centuries the quantitative carbon flux measurements that modern ecologists now use to confirm that forests are net carbon sinks.
Cree Ecological Knowledge of Plant Productivity
Cree Elders maintained oral records of which plant species indicated healthy vs. poor soil conditions — essentially describing soil nitrogen and phosphorus levels that influence photosynthesis rates. Knowledge of seasonal plant phenology (timing of leaf emergence, flower set, fruit production) encoded information about light availability and temperature effects on photosynthesis across Alberta's boreal landscape.
Blackfoot Fire Ecology & Nutrient Cycling
Controlled grassland burns practiced by Blackfoot peoples released locked carbon from dead plant material back to the atmosphere as CO2, and returned mineral nutrients to the soil in available form — a practical understanding of how cellular respiration (decomposition) and photosynthesis are linked through carbon cycling. Fresh growth after burns showed higher photosynthetic rates due to increased light availability and nutrient release.
Aboriginal Plant Productivity Knowledge & Human Sustainability
Plains peoples harvested plants at precise phenological stages (maximum sugar/starch content) — without knowing the biochemistry, they were harvesting at maximum photosynthetic accumulation: post-growing-season, before cellular respiration depleted storage organs.
Mixing rendered bison fat with dried berries and meat created a high-energy food that preserved glucose (as fat and carbohydrate) long-term. Essentially a form of biological energy storage analogous to the way plants store photosynthetic products.
Traditional medicine identified plants whose secondary metabolites (produced via photosynthetic carbon) had healing properties. Many of these compounds (salicylates, tannins, alkaloids) are synthesized from photosynthetic intermediates and are now confirmed bioactive by Western biochemistry.
Indigenous agriculture (Three Sisters: corn, beans, squash) maximized canopy photosynthesis by stacking vertical light-capture zones: tall corn captures high light; squash spreads at ground level. The arrangement maximized community-level photosynthetic yield per unit area.
Interactive Practice & Review
Knowledge Check Quiz
10 questions covering photosynthesis, cellular respiration, and ATP.
Bio 20 — Unit C
Stage Match
Match each process on the left to where it occurs in the cell on the right.
Vocabulary Flashcards
Click card to flip. Navigate all 20 terms.