5.4 Factors Affecting Cellular Respiration and Photosynthesis

Cellular respiration and photosynthesis don't run at a fixed speed. Their rates shift constantly based on environmental conditions and the availability of key molecules. Understanding what speeds these processes up, slows them down, or shuts them off entirely is central to understanding how organisms manage energy in real, changing environments.

Environmental Factors

Temperature and pH

Temperature and pH both affect these processes primarily through their effects on enzymes.

Temperature increases the kinetic energy of molecules, so reaction rates generally rise as temperature increases. But this only works up to an optimal temperature. Beyond that point, the heat disrupts hydrogen bonds and other weak interactions that hold enzymes in their functional shape. The enzyme denatures, losing its ability to catalyze reactions. At very low temperatures, molecular motion slows so much that enzyme-substrate collisions become rare, and reaction rates drop.

• Most plant enzymes for photosynthesis work best around 25–35°C.
• Human cellular respiration enzymes function optimally near 37°C (body temperature).

pH works similarly. Each enzyme has an optimal pH where its active site maintains the correct shape and charge to bind substrates. Shifting away from that optimum changes the ionization of amino acid side chains in the active site, weakening substrate binding. Extreme pH denatures enzymes entirely.

• Pepsin (a digestive enzyme) works best at pH ~2, while most metabolic enzymes in the cytoplasm function near pH 7.
• Cofactors and coenzymes can also be affected by pH changes, further disrupting enzyme function.

Light Intensity and Water Availability

Light intensity directly controls the rate of the light-dependent reactions of photosynthesis. More photons striking the photosystems means more water is split, more electrons flow through the electron transport chain, and more ATP and NADPH are produced.

• As light increases, the photosynthetic rate rises until it hits a light saturation point, where the Calvin cycle can't use ATP and NADPH any faster than they're being made.
• Below the saturation point, light is the limiting factor.
• Excessive light can damage chlorophyll and other pigments through photooxidation, actually reducing photosynthetic efficiency.

Water availability matters for both processes:

• In photosynthesis, water is the electron donor in the light-dependent reactions. Photolysis of $H_2O$ provides electrons to Photosystem II, releases $O_2$, and contributes $H^+$ ions for the proton gradient.
• In cellular respiration, water is a product of the electron transport chain (when $O_2$ accepts electrons and combines with $H^+$).
• During drought, plants close their stomata to prevent water loss. This also blocks $CO_2$ from entering the leaf, which starves the Calvin cycle and can trigger photorespiration (more on that below).

Substrate Availability

Oxygen and Carbon Dioxide Concentration

Oxygen is the final electron acceptor in the electron transport chain of aerobic respiration. Without it, electrons have nowhere to go, the chain stalls, and oxidative phosphorylation stops.

• Under hypoxic conditions (low $O_2$), cells switch to anaerobic pathways like lactic acid fermentation (in animals) or ethanol fermentation (in yeast). These pathways produce far less ATP per glucose molecule.
• Anoxia (complete oxygen depletion) can cause cell death because the cell cannot generate enough ATP to maintain essential functions.

Carbon dioxide is the carbon source for the Calvin cycle. The enzyme RuBisCO (ribulose bisphosphate carboxylase/oxygenase) fixes $CO_2$ onto the 5-carbon molecule RuBP to begin the cycle.

• Higher $CO_2$ concentrations generally increase the rate of carbon fixation, up to a saturation point.
• Low $CO_2$ limits glucose production and favors photorespiration (since RuBisCO is more likely to fix $O_2$ instead).
• Interestingly, elevated $CO_2$ can also cause partial stomatal closure in some plants, which reduces water loss through transpiration but may eventually limit gas exchange.

Organic Substrates for Cellular Respiration

Cells don't only burn glucose. The rate of cellular respiration depends on the availability of various organic fuel molecules:

• Glucose enters glycolysis directly.
• Fatty acids undergo beta-oxidation to produce acetyl-CoA, which feeds into the citric acid cycle. Fats yield more ATP per gram than carbohydrates.
• Amino acids can be deaminated and converted into intermediates of glycolysis or the citric acid cycle when other fuels are scarce.

When substrate availability drops, ATP production slows. This is why prolonged fasting shifts the body from burning glucose to burning fat and eventually protein.

For photosynthesis, the key substrates are $H_2O$, $CO_2$, and light energy. If any one of these is insufficient, it becomes the limiting factor that caps the overall rate, regardless of how abundant the others are.

Biological Processes

Enzyme Activity in Cellular Respiration and Photosynthesis

Enzymes make both processes possible by lowering the activation energy of each reaction step. Without them, these reactions would be far too slow to sustain life.

Key enzymes to know:

Enzyme-catalyzed reactions follow Michaelis-Menten kinetics: as substrate concentration rises, the reaction rate increases until all enzyme active sites are occupied ($V_{max}$). Beyond that point, adding more substrate has no effect because the enzyme is saturated.

Enzyme activity is also regulated through feedback inhibition and allosteric regulation. For example, high ATP levels inhibit PFK, slowing glycolysis when the cell already has plenty of energy.

Photorespiration and Its Impact on Photosynthetic Efficiency

Photorespiration happens when RuBisCO binds $O_2$ instead of $CO_2$. This is a significant problem because RuBisCO can't perfectly distinguish between the two molecules.

Here's how it unfolds:

1. RuBisCO fixes $O_2$ onto RuBP instead of $CO_2$.
2. This produces a 2-carbon compound (phosphoglycolate) instead of the usual two 3-carbon molecules (G3P).
3. The cell must spend ATP and NADPH to recycle phosphoglycolate, and previously fixed $CO_2$ is released in the process.
4. No glucose is produced from this pathway.

Photorespiration is most common under hot, dry conditions. When stomata close to conserve water, $CO_2$ inside the leaf drops while $O_2$ (from the light reactions) builds up. This shifts the $O_2/CO_2$ ratio in favor of RuBisCO binding oxygen.

C4 plants (like corn and sugarcane) have evolved a workaround. They use PEP carboxylase (which has no affinity for $O_2$) to initially fix $CO_2$ in mesophyll cells, then shuttle the 4-carbon product to bundle sheath cells where $CO_2$ is released and concentrated around RuBisCO. This spatial separation, organized through Kranz anatomy, virtually eliminates photorespiration.

CAM plants (like cacti and succulents) use a similar strategy but separate the steps by time rather than space: they open stomata at night to fix $CO_2$ and run the Calvin cycle during the day with stomata closed.

Though photorespiration seems wasteful, it may serve a protective role. By consuming excess ATP and NADPH, it can prevent damage to the photosynthetic machinery when light energy exceeds what the Calvin cycle can use (photoprotection). It also contributes to the synthesis of the amino acids glycine and serine.