Why cells break down glucose
Every move you make, from blinking to sprinting, runs on energy your cells pull out of food. They get it by taking food molecules such as the sugar glucose apart, one small step at a time.
Getting energy out of food
Sugars, fats, and proteins are organic molecules, the carbon-containing molecules found in all living things. Their chemical bonds store a lot of energy. Cells release that energy through cellular respiration, the chemical reactions a cell uses to get energy from organic molecules and to give off the waste products. Its main purpose is to make ATP, the molecule that powers most of the cell’s activities.
When the process uses oxygen, it’s called aerobic respiration. The cell takes in O₂, breaks down organic molecules, and releases carbon dioxide (CO₂). That’s why you breathe: you inhale the oxygen your cells need and exhale the CO₂ they make. So respiration has two meanings: breathing, and the cellular process that breathing supplies.
For glucose, the overall reaction is:
C₆H₁₂O₆ (glucose) + 6 O₂ → 6 CO₂ + 6 H₂O
Energy released in small steps
The reaction releases a large amount of free energy, the energy available to do work. In a cell, work means jobs like building molecules, moving substances across membranes, and contracting muscles.
Chemists write the change in free energy as ΔG. For glucose, ΔG = −685 kcal/mol, and the minus sign means energy is given off. A reaction that releases free energy is exergonic.
A wood fire also combines fuel with oxygen, but it releases its energy all at once as heat and light, and none of it is captured. Cells work far more carefully. They take glucose apart in many small steps, each run by an enzyme, a protein that speeds up one particular chemical reaction.
Some of the energy still escapes as heat, but much of it is captured in three energy intermediates, molecules that store energy briefly until the cell uses it: ATP, NADH, and FADH₂.
ATP, the cell’s energy currency
ATP (adenosine triphosphate) carries a short chain of three phosphate groups. Breaking off the last phosphate releases energy the cell can spend on work. What’s left is ADP (adenosine diphosphate, with two phosphates) and a loose phosphate, written Pᵢ (inorganic phosphate).
ATP works like cash: the cell earns it by breaking down food and spends it constantly, so it has to keep making more. Cellular respiration does this by reattaching Pᵢ to ADP. Adding a phosphate group to a molecule is called phosphorylation, so making ATP from ADP is a phosphorylation.
Energy travels with electrons
Much of the energy in glucose is held by its electrons, the tiny particles with a negative charge found in every atom. As glucose is taken apart, enzymes pull electrons off it and hand them to other molecules.
Electrons can’t float around loose, so whenever one molecule is oxidized, another is reduced at the same moment. The paired transfer is a redox reaction (short for reduction-oxidation).
“Reduction” sounds like losing something, which trips people up. The name comes from electric charge: each electron a molecule gains makes its charge more negative, so the charge is reduced.
NADH and FADH₂ carry electrons
Cells don’t let the electrons from glucose wander off. They load them onto carrier molecules. The main carrier starts out as NAD⁺. When it picks up two high-energy electrons and a hydrogen ion (H⁺), it becomes NADH.
In redox terms, NAD⁺ is reduced to NADH, while the molecule that supplied the electrons is oxidized. A second carrier, FAD, is loaded in a similar way and becomes FADH₂.
Later, NADH and FADH₂ give their electrons away, which oxidizes them back to NAD⁺ and FAD, ready for another load. The high-energy electrons they deliver power the final stage of respiration, the one that makes most of the cell’s ATP.
Practice questions
1 of 2
An enzyme removes two electrons and an H⁺ from an organic molecule and passes them to NAD⁺, forming NADH. Which statement describes this reaction?
2 of 2
Much of the energy released from glucose is captured in three energy intermediates. Which three?