ATP is a nucleotide used as a short-term energy currency

A nucleotide is a monomer of nucleic acids that contains a nitrogenous base, a pentose sugar and one or more phosphate groups. ATP, adenosine triphosphate, is a nucleotide that contains adenine, ribose and three phosphate groups. The structure matters. ATP is not just “energy” floating around; it is a small, soluble molecule that carries usable chemical potential energy from one reaction site to another inside the cell.

Calling ATP the cell’s “energy currency” works well as shorthand, as long as you don’t push the comparison too far. Cells don’t keep huge reserves of ATP like money in a bank. Instead, they keep a small amount turning over very rapidly. ATP suits this role because it is water-soluble, stable enough in the near-neutral pH of cytoplasm, and too charged to pass freely through the hydrophobic core of membranes. That last feature is useful: ATP stays where the cell can control it.

The terminal phosphate can be removed and reattached readily. When ATP is hydrolysed to ADP and inorganic phosphate, the energy released is small enough to be useful for many cell tasks, rather than mostly being wasted as heat. That “small packet” size is what makes ATP so effective; cells can couple ATP hydrolysis to individual jobs.

ATP powers work inside cells

ATP supplies energy for processes that would otherwise run too slowly or be energetically unfavourable. Anabolism is metabolism that builds larger molecules from smaller molecules. During anabolic reactions, ATP helps form covalent bonds as monomers join to make macromolecules. DNA replication, transcription and translation all rely on ATP or closely related nucleotide energy transfers.

Cells also need ATP for active transport, which is membrane transport that moves substances against their concentration gradient using energy. As ATP is used, a pump protein changes shape, moving ions or other particles from the lower-concentration side to the higher-concentration side. Sodium-potassium pumps and proton pumps are classic examples of this same principle.

A third major use is movement. ATP powers whole-cell movement, movement of organelles and vesicles, and movement of chromosomes during cell division. Muscle contraction shows this especially clearly: actin and myosin filaments slide past each other through repeated ATP-powered shape changes.

ATP hydrolysis releases energy; ATP synthesis requires energy

Hydrolysis is a chemical reaction that splits a bond by adding water. In ATP hydrolysis, adenosine triphosphate is converted to ADP, adenosine diphosphate, which is a nucleotide with adenine, ribose and two phosphate groups, and inorganic phosphate. This overall change releases energy, which the cell can couple to cellular work.

Phosphorylation is the addition of a phosphate group to a molecule. ATP sometimes passes its terminal phosphate straight to another molecule, for example a membrane pump protein or a metabolic intermediate. Once phosphorylated, the molecule is often less stable or has a different shape, allowing it to do work or react further.

To make ATP again, ADP and phosphate have to be joined. That step needs an input of energy. In this topic, the energy mainly comes from cell respiration, although living organisms can also regenerate ATP using light energy in photosynthesis or chemical energy in chemosynthesis.

Don’t picture a cell keeping a large stockpile of ATP. ATP is being hydrolysed and resynthesized all the time. If regeneration stops, processes such as nerve impulse transmission, active transport and muscle contraction fail very quickly. Like all energy transfers in metabolism, the ATP cycle is not perfectly efficient; some energy is dispersed as heat.

Cell respiration is not breathing

Cell respiration is a metabolic process in cells that releases energy from carbon compounds and uses it to synthesize ATP. In many cells, the main respiratory substrates are glucose and fatty acids, although cells can also use many other organic molecules, including some amino acids once their nitrogen-containing groups have been removed.

A respiratory substrate is an organic molecule that is oxidized in cell respiration to provide energy for ATP synthesis. Controlled oxidation is the key idea here. Cells don’t burn glucose in a single step; instead, they release energy through a sequence of enzyme-catalysed reactions, allowing a useful proportion of that energy to be transferred to ATP.

Gas exchange is the movement of respiratory gases between an organism or cell and its surroundings by diffusion or ventilation-linked diffusion. It connects to cell respiration, but it isn’t the same thing. Gas exchange brings in oxygen and removes carbon dioxide, while cell respiration is the intracellular metabolic system that uses respiratory substrates to make ATP. In humans, breathing and blood circulation support gas exchange, but the reactions of cell respiration take place inside cells.

Aerobic and anaerobic respiration in humans

Aerobic cell respiration is cell respiration that uses oxygen as the final electron acceptor and yields a relatively large amount of ATP. In humans, it starts in the cytoplasm and then continues in mitochondria, so the aerobic pathway needs mitochondria. Using glucose as the substrate, the word equation is:

glucose + oxygen → carbon dioxide + water

Anaerobic cell respiration is cell respiration that produces ATP without using oxygen. In human cells, it takes place in the cytoplasm, uses carbohydrate as the substrate, and releases lactate as the waste product. With glucose as the substrate, the word equation is:

glucose → lactate

Learn the comparison below as connected ideas rather than as separate facts.

Key differences between aerobic and anaerobic respiration in humans.

During short, intense activity, human muscle can use anaerobic respiration to supply ATP quickly when oxygen delivery can't keep up with demand. The cost is lactate accumulation. After vigorous exercise, the body needs extra oxygen to process the lactate; this extra requirement is often called oxygen debt.

Measuring respiration rate

Rate is the amount of change in a measured variable per unit time. In a respirometer measuring oxygen uptake, r = ΔV / Δt, where r is respiration rate as oxygen volume used per time (m³ s⁻¹), ΔV is the change in oxygen volume (m³) and Δt is the time interval (s). School data often uses cm³ min⁻¹ or mm³ min⁻¹. That’s fine, as long as the units stay attached throughout.

You can estimate respiration rate from oxygen uptake, carbon dioxide production, or the loss of a respiratory substrate such as glucose. For aerobic respiration, oxygen uptake is usually the neatest measurement. For anaerobic respiration in yeast, carbon dioxide production can be tracked directly, for example by measuring gas volume collected, or indirectly, by measuring the loss of mass as carbon dioxide escapes from a fermenting flask.

Respirometers and oxygen uptake

A respirometer is an apparatus that measures respiration rate by detecting gas-volume changes caused by a respiring organism or tissue in a sealed system. A typical set-up has living material, a carbon dioxide absorber such as potassium hydroxide, and a capillary tube with a movable fluid marker. The alkali absorbs carbon dioxide, so any fall in gas volume is mainly due to oxygen uptake.

Control temperature and pressure carefully, because gases expand and contract when physical conditions change, not only when respiration occurs. A thermostatically controlled water bath makes the results more reliable by keeping temperature constant. A second, matched tube without respiring tissue can be used as a control for pressure and temperature changes that are not related to oxygen uptake.

Variables that affect respiration rate

Common variables include temperature, type or mass of organism or tissue, activity level, availability of oxygen, and type or concentration of respiratory substrate. In a well-designed investigation, change one independent variable while keeping the others constant. The dependent variable is the calculated respiration rate.

With raw or secondary data, convert distance moved in a capillary to volume if the tube diameter is known, calculate rates, then compare means or draw a graph. For example, a rising line in oxygen uptake against temperature shows that respiration rate increases over that range. It does not prove the trend continues forever, because enzymes and living tissues have tolerance limits.

NAD links oxidation to later ATP production

Oxidation is a chemical process in which a substance loses electrons. Reduction is a chemical process in which a substance gains electrons. These two happen together in redox reactions, where electrons move from one substance to another.

In cell respiration, many oxidations are dehydrogenation reactions, meaning hydrogen atoms are removed from a substrate. The substrate is oxidized because each hydrogen atom includes an electron. Watch the detail here: removing H⁺ alone would not take away an electron, so it would not oxidize the substrate.

NAD, nicotinamide adenine dinucleotide, is a coenzyme that carries hydrogen and electrons during cell respiration. NAD takes hydrogen from oxidized substrates and becomes reduced NAD. In simplified IB wording:

NAD + hydrogen → reduced NAD

More precisely, NAD⁺ accepts two electrons and one proton, while another proton is released. The key biological idea is that NAD can be reduced and then oxidized again later, so it carries energy from earlier stages of respiration to later stages.

Glycolysis splits glucose in the cytoplasm

Glycolysis is a cytoplasmic metabolic pathway that converts one glucose molecule into two pyruvate molecules through enzyme-catalysed steps. A different enzyme catalyses each step. You don’t need to memorize the intermediate names, but you do need to understand the logic of the pathway.

First comes phosphorylation. ATP adds phosphate groups to molecules derived from glucose. This makes the sugar more reactive and prepares it for splitting. Because ATP is used early on, glycolysis is described as having an “investment” phase.

Next is lysis. The six-carbon phosphorylated sugar splits into two three-carbon molecules. From here onwards, every event happens twice for each original glucose molecule.

Oxidation then takes place. Hydrogen is removed from the three-carbon molecules and accepted by NAD, forming reduced NAD. At this stage, glycolysis starts moving energy into a carrier molecule.

Finally, ATP is formed. Phosphate groups are transferred to ADP to make ATP. Four ATP molecules are produced in this later phase, but two were used at the start, giving a net yield of two ATP per glucose. Per glucose, the overall products of glycolysis are two pyruvate, two reduced NAD and a net gain of two ATP.

Lactate formation keeps glycolysis running without oxygen

Glycolysis needs a supply of NAD. Once all the NAD has been reduced, the oxidation step in glycolysis stops, so ATP production by glycolysis stops too. In human anaerobic respiration, pyruvate deals with this by accepting hydrogen from reduced NAD.

Lactate fermentation is anaerobic cell respiration in which pyruvate is reduced to lactate, regenerating NAD so glycolysis can continue. As pyruvate becomes lactate, reduced NAD is oxidized back to NAD. This happens in the cytoplasm.

The ATP yield stays at the glycolysis yield: two ATP per glucose. Lactate formation doesn’t directly add any extra ATP; it regenerates NAD. That small trick lets muscle cells keep making some ATP when the oxygen supply is too low for aerobic respiration.

Yeast regenerates NAD by making ethanol and carbon dioxide

Yeast is a unicellular fungus that can use sugars as respiratory substrates. A facultative anaerobe is an organism that can respire aerobically when oxygen is available and anaerobically when oxygen is unavailable. Yeast makes a useful example here: its anaerobic pathway starts with the same glycolysis stage as in humans, but it regenerates NAD by a different route.

Ethanol fermentation is anaerobic cell respiration in which pyruvate is converted to ethanol and carbon dioxide, regenerating NAD. First, pyruvate is decarboxylated to form ethanal, which releases carbon dioxide. Ethanal then accepts hydrogen from reduced NAD and becomes ethanol, while NAD is regenerated.

This is why yeast matters in both baking and brewing. In bread dough, carbon dioxide bubbles get trapped in the sticky dough and make it rise; ethanol evaporates during baking. In brewing, ethanol is usually the desired product, while carbon dioxide escapes or is retained depending on the drink. Fermentation stops when sugar runs out or when ethanol becomes toxic to the yeast.

The comparison with humans is straightforward. Both humans and yeast use glycolysis, and both must regenerate NAD. Human anaerobic respiration uses pyruvate to make lactate; yeast uses pyruvate to make ethanol and carbon dioxide.

The link reaction connects glycolysis to the Krebs cycle

The link reaction is the aerobic respiration step that converts pyruvate into an acetyl group attached to coenzyme A in the mitochondrial matrix. Glycolysis produces pyruvate in the cytoplasm, and the cell then transports it into the mitochondrion. In eukaryotic cells, the link reaction takes place in the matrix.

During the link reaction, pyruvate, a three-carbon molecule, is decarboxylated: carbon dioxide is removed. The remaining two-carbon acetyl group is oxidized as hydrogen/electrons are removed, which reduces NAD. The acetyl group is then passed to coenzyme A, a coenzyme that carries acetyl groups into the Krebs cycle, forming acetyl-CoA.

Carbohydrate and lipid breakdown can both feed in at this point. Carbohydrates can be metabolized through glycolysis to pyruvate and then to acetyl groups. Fatty acids are broken down into two-carbon acetyl groups carried by coenzyme A. This makes acetyl-CoA a crossroads in aerobic respiration.

The Krebs cycle oxidizes acetyl groups and regenerates oxaloacetate

The Krebs cycle is a cyclic metabolic pathway in the mitochondrial matrix that oxidizes acetyl groups to carbon dioxide while producing reduced electron carriers and ATP. For this section, the named intermediates are citrate, a six-carbon organic acid formed when an acetyl group joins oxaloacetate, and oxaloacetate, a four-carbon organic acid regenerated at the end of the Krebs cycle.

An acetyl group from acetyl-CoA transfers to oxaloacetate, forming citrate. Enzymes then convert citrate through a series of reactions back into oxaloacetate. Since oxaloacetate is made again, the pathway runs as a cycle rather than a straight line.

In one turn of the Krebs cycle, two decarboxylations release two carbon dioxide molecules. Four oxidations also take place. These are dehydrogenation reactions, so hydrogen is removed and accepted mainly by NAD to form reduced NAD. ATP is produced as well. Much of the energy released from the acetyl group is carried by the reduced NAD towards the electron transport chain.

For each acetyl group that enters the cycle, the products are carbon dioxide, reduced NAD, and ATP, and oxaloacetate is regenerated. One glucose gives two pyruvates and therefore two acetyl groups, so the Krebs cycle turns twice per glucose molecule.

Reduced NAD delivers high-energy electrons to the inner mitochondrial membrane

The electron transport chain is a series of electron carriers in the inner mitochondrial membrane that pass electrons along in redox reactions. Reduced NAD gives a pair of electrons to the first carrier in the chain. When reduced NAD loses electrons and hydrogen, it becomes oxidized back to NAD.

Reduced NAD is produced in glycolysis, the link reaction and the Krebs cycle. This is useful because the energy released from oxidizing glucose is not captured all at once as ATP. A large share is stored for a short time in reduced NAD, then passed to the electron transport chain.

The first electron carrier is reduced when it accepts electrons from reduced NAD. It is then oxidized again as it passes those electrons to the next carrier. Step by step, energy moves along the chain instead of being released in one uncontrolled burst.

Electron flow is coupled to proton pumping

As electrons move along the electron transport chain, the carriers release energy. Cells use that energy to pump protons from the mitochondrial matrix into the intermembrane space, across the inner mitochondrial membrane. You don’t need to learn the names of the protein complexes here; focus on the direction and the consequence.

A proton gradient is a difference in proton concentration across a membrane. In mitochondria, the intermembrane space ends up with a higher proton concentration than the matrix. Energy is stored because protons have been moved against their concentration gradient.

The inner mitochondrial membrane matters because it acts as the barrier that lets the gradient build. If protons could simply diffuse back through any part of the membrane, the gradient would collapse and ATP synthesis by chemiosmosis would stop.

ATP synthase uses the proton gradient to phosphorylate ADP

Chemiosmosis is the process in which the diffusion of protons down an electrochemical gradient through ATP synthase provides energy for ATP synthesis. ATP synthase is a membrane enzyme that catalyses phosphorylation of ADP to ATP using energy from proton flow. In mitochondria, ATP synthase sits in the inner mitochondrial membrane.

Protons pass from the intermembrane space back into the matrix through ATP synthase. As they move, energy is released and used to phosphorylate ADP. Put simply: the gradient runs down, and ATP is made.

ATP synthase has a membrane region for proton movement and a catalytic region that projects into the matrix, where ADP and phosphate are joined. It’s a neat structure-function example: proton flow drives the membrane part, while the matrix-facing part catalyses ATP formation.

Cristae are folds of the inner mitochondrial membrane. They increase the surface area available for electron transport chains and ATP synthase. More inner membrane gives more sites for chemiosmosis, which is why active cells often contain many mitochondria with extensive cristae.

Oxygen keeps electrons moving

A terminal electron acceptor is the final substance that receives electrons at the end of an electron transport chain. In aerobic cell respiration, oxygen is the terminal electron acceptor. It takes electrons from the final carrier in the chain and protons from the mitochondrial matrix, producing metabolic water.

So oxygen is not just “used somewhere” in respiration; it is needed right at the end of the electron transport chain. Without oxygen, electrons cannot leave the final carrier. The chain backs up. Carriers stay reduced, reduced NAD cannot pass on electrons, and NAD is no longer available for the link reaction and Krebs cycle. These aerobic stages then stop.

Glycolysis can still carry on anaerobically if NAD is regenerated in the cytoplasm, but it produces far less ATP. Oxygen keeps electron flow going, allows proton pumping to continue, maintains chemiosmosis, and gives a high ATP yield.