6 Cellular Energy Conversion
Follow how ATP links cellular work to energy conversion in respiration and photosynthesis, and compare the chemiosmosis used by mitochondria and chloroplasts.
: the cell’s usable energy carrier
Cells need usable energy for chemical reactions, transport, movement, and growth. Rather than relying on a long-term store of , cells continually transfer energy between molecules and regenerate as it is used.
(adenosine triphosphate) is a readily usable energy carrier. Hydrolysis converts to ADP and inorganic phosphate, releasing energy that can be coupled to cellular work:
For example, a muscle cell consumes as it contracts. Cellular processes then help convert ADP and inorganic phosphate back into . therefore links energy-releasing processes with work that requires energy.
Takeaway: is a reusable carrier that transfers energy to cellular processes.
How makes
transfers energy from organic molecules, especially glucose, to . A simplified summary of aerobic respiration is:
In eukaryotic cells, respiration occurs in stages and in different locations:
occurs in the cytosol. It splits one glucose molecule into two pyruvate molecules and produces a small amount of and the electron carrier NADH.
When oxygen is available, pyruvate enters the mitochondrion and is converted to acetyl-CoA. The occurs in the mitochondrial matrix. It releases carbon dioxide, transfers energy to NADH and FADH₂, and makes a small amount of directly.
During , NADH and FADH₂ deliver electrons to the electron transport chain in the inner mitochondrial membrane. Energy released as electrons pass along the chain pumps H⁺ into the intermembrane space. H⁺ then flows back into the matrix through , which uses that energy to make . At the end of the chain, oxygen accepts electrons and combines with H⁺ to form water.
produces most of the in aerobic respiration. The exact yield from one glucose molecule varies with cellular conditions and with how electrons enter mitochondrial pathways.
Example: A muscle cell uses when it contracts. As is consumed, respiration breaks down fuel and helps convert ADP and inorganic phosphate back into .
Takeaway: Respiration transfers energy from fuel to through a sequence of reactions, with electron transfer driving much of the production.
How stores light energy
captures light energy and stores it in the chemical bonds of carbohydrates. A simplified overall summary is:
In plants and algae, takes place in chloroplasts and has two closely linked stages:
Light-dependent reactions occur in the thylakoid membranes. Light energizes electrons, and water supplies replacement electrons. Splitting water releases oxygen. Electron transfer establishes an H⁺ gradient across the thylakoid membrane. As H⁺ flows through , is produced; electron transfer also produces NADPH.
The occurs in the stroma, the fluid surrounding the thylakoids. It uses and NADPH to incorporate carbon dioxide into organic molecules, including G3P, which can be used to build sugars and other carbohydrates.
made in the light-dependent reactions is used within the chloroplast to help build carbohydrates; it is not simply exported as the cell’s general-purpose supply. The carbohydrate products can later fuel .
Takeaway: Light energy supports the production of and NADPH, which help convert carbon dioxide into energy-rich organic molecules.
connects two energy-converting organelles
is the use of an H⁺ gradient across a membrane to drive production. In both mitochondria and chloroplasts, electron transfer helps establish the gradient, and H⁺ flowing through powers formation.
The two organelles use this shared mechanism in different ways:
Mitochondria: Chemical energy from fuel-derived electrons carried by NADH and FADH₂ helps establish an H⁺ gradient in the intermembrane space. The returning H⁺ flow drives production during aerobic respiration.
Chloroplasts: Light energy drives electron transfer, which establishes an H⁺ gradient in the thylakoid lumen. The returning H⁺ flow produces , and the light-dependent reactions also produce NADPH for carbohydrate synthesis.
Plants generally have both organelles. Chloroplasts capture light energy to build organic molecules, while mitochondria transfer energy from fuel to make for cellular work. and aerobic respiration are linked through matter: uses carbon dioxide and water to build carbohydrates and releases oxygen; aerobic respiration uses carbohydrates and oxygen and releases carbon dioxide and water. These summaries describe connected processes, not one reaction simply running backward.
Takeaway: Mitochondria and chloroplasts both use , but differ in their energy inputs and the roles of the they produce.