4 Metabolism and Energy
Follow how enzymes organize metabolism, how cells transfer energy through ATP and electron carriers, and how respiration, fermentation, and photosynthesis transform energy.
and pathway control
Cells need energy to build molecules, move substances, communicate, and maintain internal conditions. is the coordinated network of reactions that supports these tasks. Those reactions do not all do the same kind of work: catabolic pathways break molecules down and release usable energy, while anabolic pathways build molecules and require energy.
A links individual reactions into an ordered sequence. Because each step can depend on a different , controlling enzymes allows a cell to adjust a pathway to its needs. For example, when enough of a pathway’s end product has accumulated, it may inhibit an earlier and slow further production.
Takeaway: is organized into pathways, and regulation helps balance the breakdown and construction of molecules.
How enzymes support reactions
An is a biological catalyst, usually a protein, that lowers the activation energy needed for a reaction to begin. The reactant an acts on is its substrate. The substrate binds at the ’s active site, where the reaction takes place; the products then leave, and the can be used again.
Enzymes speed reactions but do not change the reaction’s overall energy balance and are not used up in the process. Their activity can be affected by temperature, pH, substrate availability, and regulatory molecules. In a pathway that breaks down a sugar, for instance, enzymes catalyze successive steps without being consumed.
Takeaway: Enzymes make specific reactions proceed more readily, while cellular conditions and regulatory signals help determine their activity.
and electron transfer
Energy is transferred between cellular processes rather than created. is a short-term energy carrier: when it is hydrolyzed to ADP and inorganic phosphate, released free energy can be coupled to a process that needs energy. -powered phosphorylation, for example, can help activate a molecule for a later reaction. Cells continually regenerate using energy from food or, in photosynthetic organisms, light.
Electrons also transfer energy. In , a molecule that loses electrons is oxidized, and a molecule that gains electrons is reduced. Carriers such as NADH and NADPH temporarily hold high-energy electrons and deliver them to other reactions. In respiration, electrons from food help drive production; in , light energizes electrons that help make and NADPH.
Takeaway: transfers usable energy between processes, while electron carriers connect reactions that move high-energy electrons.
and production
extracts energy from organic molecules, such as glucose, and transfers some of that energy to . In eukaryotic cells, it begins with in the cytoplasm. splits one glucose molecule into two pyruvate molecules and yields a net two and two NADH.
When oxygen is available, pyruvate is further broken down. Reactions in the mitochondrial matrix release carbon dioxide and transfer electrons to carriers. The electrons then pass through an electron transport chain in the inner mitochondrial membrane. Their energy helps pump protons across the membrane, creating a gradient. As protons flow back through synthase, the uses the gradient’s energy to make . Oxygen is the final electron acceptor and combines with electrons and protons to form water.
Respiration releases energy in steps, capturing some in carriers and while some is dissipated as heat. It is not a direct conversion of glucose into . Plants, like animals and fungi, also carry out to supply for cellular work.
Takeaway: Respiration transfers energy from organic molecules to through linked reactions, electron carriers, and a proton gradient.
when oxygen is unavailable
When oxygen is unavailable, regenerates NAD⁺ from NADH, allowing to continue. itself makes no additional , so the net yield remains the two produced by per glucose.
In lactic acid , pyruvate is converted to lactate. This occurs in some bacteria and in animal cells under conditions where oxygen supply cannot meet demand. In alcoholic , used by yeasts and some other microbes, pyruvate-derived products become ethanol and carbon dioxide.
is not the same as anaerobic respiration. Anaerobic respiration uses an electron transport chain with a final electron acceptor other than oxygen; does not use an electron transport chain.
Takeaway: enables to continue without oxygen by restoring NAD⁺, not by adding another source of .
: capturing light and fixing carbon
captures light energy and stores it in chemical form. In plants and algae, it takes place in chloroplasts. Its two linked stages make and use energy carriers to support the incorporation of carbon dioxide into organic molecules.
The occur in thylakoid membranes. Pigments absorb light, which energizes electrons. Water supplies replacement electrons, and splitting water releases oxygen. Electron transfers help establish a proton gradient that powers synthase, while energized electrons help form NADPH.
The takes place in the stroma. It uses and NADPH to incorporate carbon dioxide into organic molecules. The RuBisCO helps catalyze carbon fixation, and some resulting carbon compounds can be used to make sugars and other biomolecules.
Takeaway: provide and NADPH; the uses them to support carbon fixation and the production of organic molecules.
How and respiration connect
and connect the cycling of matter with different energy transformations. uses carbon dioxide and water to build energy-rich organic molecules and releases oxygen. Respiration uses organic molecules and oxygen, transferring some of their energy to and releasing carbon dioxide and water.
The two processes are not simply opposites in how they handle energy. stores captured light energy in chemical bonds. Respiration transfers energy from organic molecules to , with some energy dissipated as heat. Together, these processes link the movement of carbon and oxygen through organisms with the transfer of energy needed for cellular work.
Overall takeaway: Enzymes organize , and electron carriers transfer energy, respiration mobilizes energy stored in organic molecules, sustains without oxygen, and stores light energy in chemical form.