Unit notes

Oxidation is loss of electrons; reduction is gain of electrons. Energy is transferred and transformed, not created. A concentration gradient can store potential energy. Track matter, electrons, and energy separately rather than treating all three as interchangeable.

An enzyme lowers activation energy by providing a reaction pathway that stabilizes the transition state. It increases the rate of approaching equilibrium but does not change the overall free-energy difference, the equilibrium constant, or which direction is thermodynamically favored under a specified set of conditions. Enzymes are not consumed by each catalytic cycle.

The active site binds suitable substrates and can change conformation during binding. Temperature increases may initially increase collision frequency, but excessive heat can disrupt protein structure. pH affects protonation and interactions. At fixed enzyme concentration, increasing substrate concentration eventually produces a plateau as active sites become occupied.

Competitive inhibition can involve competition for the active site. Allosteric regulation involves binding at a distinct site that changes function. Feedback inhibition occurs when a pathway product reduces an earlier step, preventing unnecessary production. Do not assume every inhibitor can be overcome by adding substrate.

ATP hydrolysis can be coupled to an otherwise unfavorable process so that the combined process is favorable. Breaking a bond requires energy; the net release from ATP hydrolysis reflects all bonds and interactions broken and formed. ATP is an immediate energy-transfer molecule, not an unlimited long-term energy store.

Glycolysis in the cytosol splits glucose into pyruvate, yielding a small net amount of ATP and reduced electron carriers. In aerobic eukaryotes, pyruvate oxidation and the citric acid cycle in the mitochondrial matrix release carbon dioxide and transfer more electrons to NADH and FADH₂. Carbon dioxide is not made by the electron transport chain itself.

Electron transfer along the inner mitochondrial membrane releases energy that supports proton pumping. Oxygen is the final electron acceptor and is reduced to water. Protons flowing through ATP synthase down their electrochemical gradient drive ATP formation. A membrane leak can allow electron transport while reducing ATP yield.

Glycolysis requires oxidized NAD⁺. When respiration cannot reoxidize NADH quickly enough, fermentation can transfer electrons to an organic acceptor and regenerate NAD⁺. This allows glycolysis to continue producing ATP by substrate-level phosphorylation. Fermentation itself does not add a large oxidative ATP yield.

In a closed experiment, gas production may indicate a pathway but must be interpreted in context. Some fermentation pathways release carbon dioxide, while lactate formation does not. Oxygen consumption is useful evidence for aerobic respiration but is not a direct count of all ATP molecules formed.

Light-dependent reactions in thylakoid membranes excite electrons and use electron transport to help produce ATP and NADPH. Water supplies replacement electrons and is the source of released oxygen. The Calvin cycle in the stroma uses ATP and NADPH to reduce carbon fixed from CO₂ into carbohydrate precursors.

A limiting-factor experiment can reveal whether light, carbon dioxide, temperature, or another resource constrains rate. Increasing a nonlimiting factor need not increase the rate. Photosynthetic organisms also respire; a measured net oxygen change reflects oxygen produced minus oxygen consumed during the measurement.

Glycolysis occurs in the cytosol and splits a six-carbon glucose into two three-carbon pyruvates. Some ATP is invested and more is produced, giving a net gain. Electrons are transferred to NAD⁺ to make NADH. Pyruvate oxidation and the citric acid cycle release carbon dioxide while transferring more electrons to carriers. Carbon dioxide therefore records a carbon transformation, not the direct use of oxygen in glycolysis.

At the inner mitochondrial membrane, electron transfer releases energy that drives proton movement from the matrix into the intermembrane space. Oxygen accepts electrons at the end of the chain and combines with protons to form water. Protons returning through ATP synthase drive ATP production. If oxygen is unavailable, electron carriers cannot be reoxidized normally and oxidative phosphorylation stops. Fermentation can regenerate NAD⁺ for glycolysis, but it does not replace the large ATP yield of an operating electron transport chain.

Light absorption excites electrons in photosystems. In the light reactions, water supplies replacement electrons and oxygen is released. Electron transport establishes a proton gradient across the thylakoid membrane. ATP synthase uses proton flow into the stroma; NADP⁺ is reduced to NADPH. The oxygen released by photosynthesis comes from water, not by splitting carbon dioxide into carbon and oxygen.

The Calvin cycle in the stroma uses ATP and NADPH to reduce carbon derived from carbon dioxide. Rubisco catalyzes carbon fixation to RuBP. Subsequent reactions generate a carbohydrate precursor while regenerating RuBP. The cycle does not directly capture light, but it depends on products of the light reactions. A plant kept in darkness may briefly use existing ATP and NADPH before those supplies become limiting; “not directly light dependent” does not mean “independent of light-reaction products.”

Initial rate is often preferable to an endpoint because substrate depletion and product accumulation change conditions during a long experiment. Vary one independent variable while controlling enzyme concentration, temperature, pH, reaction volume, and timing as appropriate. Include a no-enzyme or denatured-enzyme comparison when it helps distinguish catalysis from spontaneous reaction. Replicates estimate variability; a larger mean alone does not establish a reliable treatment difference.

A competitive inhibitor competes at the active site, so sufficient substrate can reduce its effect in the simplest model. Other inhibitors can alter activity at a different site. Avoid assuming every inhibitor follows one idealized curve. For a graph question, describe the observed change first, give a mechanism consistent with the data, and identify what extra evidence would distinguish alternative explanations.

A reaction can release free energy overall and still proceed slowly. Reactants must first reach a transition state: an unstable arrangement of atoms with higher free energy. Activation energy is the barrier between the reactants and this transition state. The free-energy change, ΔG, compares products with reactants; it does not measure the height of the barrier.

An exergonic reaction has negative ΔG under the stated conditions. An endergonic reaction has positive ΔG and needs coupling to a favorable process. Cells maintain local organization by taking in energy and releasing heat; they do not violate the tendency for the total entropy of a system and its surroundings to increase. Energy is transferred and transformed, not created by enzymes.

ATP hydrolysis can drive an unfavorable process when the reactions are mechanistically coupled, often through transfer of a phosphate group. Simply placing ATP near another reactant does not guarantee coupling. Breaking a bond requires energy; the net release comes from the complete reaction, including formation and stabilization of the products.

An enzyme is a catalyst with an active site whose chemical environment helps bind particular substrates and stabilize a transition state. Shape matters, but so do charge, polarity, and interactions with amino-acid side chains. Binding can change the enzyme’s conformation: induced fit is a dynamic interaction rather than a rigid lock.

Substrate binds, a reaction occurs, and products leave. The enzyme can participate again. Enzymes may orient substrates, strain bonds, or provide a useful local environment. They are not consumed as reactants, although enzymes can be damaged or degraded over time.

At a fixed enzyme concentration, increasing substrate initially increases the frequency of productive encounters. At high substrate concentration, most active sites are occupied much of the time. Rate approaches a maximum set by enzyme abundance and turnover. This does not mean the enzyme has stopped or that all substrate has been used up. Initial-rate measurements help avoid the changing conditions of a long reaction.

Within a tolerable range, warming often increases molecular motion and productive encounters. At sufficiently high temperature, interactions maintaining protein structure can be disrupted and the active site can change. The optimum is a measured property of that enzyme under the experimental conditions, not one universal temperature for life.

Changing pH alters protonation and charge of chemical groups. That can affect substrate binding, catalysis, and protein structure. A lower rate does not by itself prove total unfolding: the catalytic residues may simply be in less effective charge states. Enzymes adapted to different environments can have different pH-response curves.

A cold sample may recover activity when warmed because cooling need not destroy its structure. Some heat-induced changes may persist after cooling; others can be reversible. Measure recovery instead of assuming it. Keep substrate, enzyme concentration, reaction time, and assay conditions comparable when attributing an effect to temperature or pH.

Oxidation is loss of electrons and reduction is gain of electrons. The two occur together because electrons move from a donor to an acceptor. As fuel molecules are oxidized in respiration, carriers such as NAD⁺ become reduced to NADH. In common biochemical reactions, electron transfer is accompanied by hydrogen transfer.

NADH is not a permanent storage destination. It must transfer electrons onward so NAD⁺ becomes available for more oxidation reactions. If the pool of oxidized carriers runs low, earlier reactions can slow even while fuel remains available. This is why the final electron acceptor can affect events far upstream.

Carbon atoms eventually released as CO₂ and electrons transferred through carriers follow related but different routes. Oxygen consumed in aerobic respiration primarily accepts electrons and protons to form water at the end of the electron transport chain. Do not assume the oxygen gas is directly attached to each carbon to make CO₂.

Glycolysis takes place in the cytosol. Through a series of enzyme-catalyzed steps, one six-carbon glucose is converted into two three-carbon pyruvate molecules. The pathway invests two ATP and produces four ATP, giving a net gain of two ATP per glucose. It also reduces two NAD⁺ to two NADH.

The ATP made here comes from substrate-level phosphorylation: an enzyme transfers a phosphate group from a phosphorylated intermediate to ADP. This is different from ATP production powered by a proton gradient. Glycolysis does not directly require oxygen, although sustained glycolysis requires regeneration of NAD⁺.

Fermentation regenerates NAD⁺ by transferring electrons from NADH to an organic acceptor. Lactate fermentation reduces pyruvate; alcoholic fermentation releases CO₂ before reducing acetaldehyde. Fermentation itself does not add ATP to the glycolytic yield. It permits glycolysis to continue, which supplies the net ATP.

In eukaryotes, pyruvate oxidation and the citric acid cycle occur mainly in the mitochondrial matrix. Pyruvate oxidation forms acetyl-CoA, CO₂ and NADH. The cycle oxidizes the acetyl group, regenerates its starting acceptor, and transfers electrons to NADH and FADH₂. A small amount of ATP or GTP is also made directly. Much of the usable energy is still carried by the reduced carriers.

The inner mitochondrial membrane contains electron-transfer complexes. Energy released as electrons move through parts of the chain is coupled to pumping H⁺ from the matrix into the intermembrane space. Oxygen is the final electron acceptor and is reduced to water. The membrane separates the two compartments, allowing an electrochemical gradient to form.

H⁺ moving back into the matrix through ATP synthase powers ATP formation from ADP and phosphate. This is chemiosmosis; electron transport plus this process is oxidative phosphorylation. A leak lets some H⁺ return without powering ATP synthase. Electron flow and oxygen consumption can continue, or increase under some conditions, while less ATP is made per fuel molecule.

Photosynthetic pigments absorb some wavelengths more strongly than others. Absorbed light can excite electrons. An absorption spectrum measures light absorbed by a pigment; an action spectrum measures the effectiveness of wavelengths in producing a biological response, such as photosynthesis. They are related but not identical because multiple pigments and processes contribute.

In linear electron flow, photosystem II supplies excited electrons to an electron-transfer pathway. Oxidation of water replaces those electrons and releases oxygen and protons. Electrons reaching photosystem I are re-excited by light and ultimately reduce NADP⁺ to NADPH. The numbering reflects discovery history, not the order of linear flow.

Electron transfer and water oxidation help increase H⁺ concentration in the thylakoid lumen. ATP synthase allows H⁺ to return to the stroma, coupling that flow to ATP synthesis. ATP and NADPH provide energy and reducing power for carbon fixation; neither is itself the final sugar product.

Rubisco catalyzes incorporation of CO₂ into an organic acceptor, RuBP. The unstable product yields three-carbon molecules. Carbon fixation means conversion of inorganic carbon into an organic form; it does not mean the carbon has already become glucose.

ATP and NADPH support conversion of the products into a higher-energy three-carbon sugar phosphate, G3P. Some G3P can leave the cycle and contribute to synthesis of carbohydrates and other molecules. The remaining carbon is used to regenerate RuBP. Without regeneration, there would soon be no acceptor to continue fixation.

A net G3P containing three carbons requires fixation of three CO₂ in the standard cycle accounting, consuming nine ATP and six NADPH. Much more carbon circulates internally than exits on each round. The cycle occurs in the stroma and does not directly absorb light, but it depends on resources and regulation connected to the light reactions.

At low light, more light can increase the supply of ATP and NADPH. At higher light, another requirement can become limiting: available CO₂, enzyme capacity, temperature conditions, or other resources. A plateau alone does not identify which factor is limiting. Change one factor and test whether the plateau moves.

Closing stomata reduces water loss but can also reduce entry of CO₂. Rubisco can react with O₂ as well as CO₂. When oxygenation becomes more important, photorespiration consumes resources and reduces net carbon gain compared with productive carbon fixation. This is not the same process as mitochondrial respiration.

C₄ plants concentrate CO₂ near rubisco using a spatial separation of initial fixation and the Calvin cycle. CAM plants separate initial CO₂ uptake and later use in time, commonly opening stomata at night. These strategies can improve performance in particular environments, but they carry costs and do not mean that every C₄ or CAM plant always grows faster.

A rate is change in a measured quantity divided by elapsed time. On a product-versus-time graph, a steeper positive slope means faster product accumulation. Use an interval where the relationship is approximately linear to estimate an initial rate. Comparing endpoints after different durations confounds rate with time.

A plant can photosynthesize and respire at the same time. Measured net O₂ release reflects production by photosynthesis minus consumption by respiration and other oxygen-consuming processes. Zero net release can occur when opposing rates balance. It is not proof that both processes have stopped.

Two leaf samples may differ in mass or area. Expressing rate per unit area or mass can improve comparison, but the chosen normalization must match the biological question. Keep units attached throughout the calculation and report the interval used.

The independent variable is the factor deliberately changed; the dependent variable is the response measured. A control condition provides a meaningful baseline. For a dissolved inhibitor, a solvent-only control helps distinguish the inhibitor’s effect from the solvent’s effect. Hold enzyme amount, pH, temperature, substrate supply and timing comparable unless one is the intended variable.

Independent replicates help reveal variation across experimental units. Repeated readings from the same tube improve measurement detail but are not equivalent to independently prepared tubes. A larger sample does not repair a systematic confound. Random assignment can reduce systematic differences between treatment groups.

Error bars must be defined: they might show standard deviation, standard error or a confidence interval. Their appearance alone does not substitute for an appropriate statistical comparison. Describe trends and variation, and distinguish a supported conclusion from a mechanism that still needs testing.

Both organelles couple electron transfer to formation of a proton gradient across a membrane and use ATP synthase to harness return flow. This shared mechanism is evidence of a broadly useful biological strategy. It does not make the electron sources, terminal acceptors or locations identical.

Mitochondrial respiration receives electrons from reduced carriers produced during fuel oxidation and transfers them ultimately to oxygen. Linear photosynthetic electron flow begins with water as an electron source and ultimately reduces NADP⁺, with light supplying energy at the photosystems. Respiration pumps H⁺ toward the intermembrane space; photosynthetic processes accumulate H⁺ in the thylakoid lumen.

Carbon atoms can move repeatedly between CO₂ and organic molecules through biological and environmental processes. Energy enters, changes form, does work and is increasingly dispersed as heat. The ecosystem requires continued energy input. An organism also uses organic molecules as building materials, not solely as fuel.