50 Fun Facts About Cellular Respiration

Which nickname, popularized by a 1957 Scientific American article, do mitochondria carry for making most of our ATP?

Powerhouse of the cell was the title of Philip Siekevitz's article, and the phrase stuck. A single liver cell can hold more than 2,000 mitochondria.

Which poison jams the last enzyme of the electron transport chain, leaving cells unable to use oxygen?

Cyanide blocks complex IV, so cells suffocate with oxygen all around them. Victims can turn cherry red because their veins still carry the oxygen that the tissues could not use.

Which gas, released as yeast ferments the sugars in dough, makes a loaf of bread rise?

Carbon dioxide bubbles get trapped in the stretchy dough, leaving the air pockets seen in every slice. Bakers use the same yeast species that brews beer.

Which change after death happens because muscle fibers, starved of ATP, can no longer release their grip?

Rigor mortis sets in because muscle needs ATP to relax, not just to contract. It peaks about 13 hours after death and fades as the tissue starts to break down.

Which organ is only about 2% of human body weight yet burns roughly a quarter of the body's glucose?

Brain tissue also takes about a fifth of the oxygen the body uses. It runs mostly on glucose, which is why very low blood sugar can cause a blackout.

Which compound do human muscle cells make by fermentation when a hard sprint outruns their oxygen supply?

Lactic acid was long blamed for next-day muscle soreness, a theory now largely rejected. Chemists first isolated it in 1780 from sour milk.

Mitochondria carry their own DNA because they descend from which free-living microbes, engulfed by an ancient cell?

Bacteria swallowed by a larger cell became permanent lodgers. Mitochondria still multiply by splitting in two, as free-living microbes do, and keep a circular chromosome.

Which burrowing African rodent can survive 18 minutes without oxygen by fueling glycolysis with fructose?

Naked mole-rat colonies live in crowded tunnels where oxygen runs low. The animal can also carry on as normal for five hours in air that holds only 5% oxygen.

Which tissue, abundant in newborn babies and hibernating mammals, burns fuel to make heat instead of ATP?

Brown fat is crammed with mitochondria that let protons leak back across their inner membrane, turning the energy into warmth. That keeps a baby warm without shivering.

Which disease's cells often ferment glucose even when oxygen is plentiful, a quirk known as the Warburg effect?

Cancer cells take up glucose so greedily that PET scans find tumors by tracking a radioactive sugar. The effect is named for Otto Warburg, who studied tumor metabolism in the 1920s.

Oxygen collects the spent electrons at the end of the electron transport chain and becomes which molecule?

Water forms when oxygen takes on the electrons along with two protons. Without oxygen waiting at the end of the line, the whole chain backs up and stops.

Which process in plants is, in outline, the opposite of cellular respiration, building sugar instead of burning it?

Photosynthesis stores the sun's energy in sugar, and respiration releases it again. Plants do both, since their cells need ATP just as animal cells do.

Which human cell type has no mitochondria at all and so makes its ATP by glycolysis alone?

Red blood cells discard their mitochondria as they mature, so they use none of the oxygen they carry. They throw out the nucleus too, leaving more room for hemoglobin.

In which part of a cell, outside the mitochondria, do the ten enzyme steps of glycolysis take place?

Cytoplasm holds the whole pathway, which needs no oxygen and no organelle. Glycolysis turns up across the living world, a sign that it is an ancient way of making ATP.

ATP, the energy carrier that cellular respiration exists to make, is short for which chemical name?

Adenosine triphosphate is spent and rebuilt so fast that an average adult recycles around 50 kilograms of it every day.

When ATP gives up one phosphate group to power a cell's work, which molecule is left behind?

ADP is the flat battery to ATP's charged one. Cellular respiration recharges it by bolting a third phosphate group back on.

Which chemist overturned the idea that fermentation is mere decay by showing that living yeast makes the alcohol?

Louis Pasteur was drawn in when a Lille manufacturer asked why his beetroot alcohol kept souring. He later described fermentation as life without air.

Glycolysis splits each six-carbon glucose molecule into two molecules of which three-carbon compound?

Pyruvate sits at a crossroads: with oxygen it moves into the mitochondria to be burned completely, and without oxygen it is fermented instead.

Glycolysis invests ATP to split one glucose molecule, then earns back more, for a net gain of how many ATP?

A net gain of 2 ATP is all glycolysis offers, because it makes four but spends two getting started. The big payoff comes later, inside the mitochondria.

Which alcohol do yeast cells make when they ferment sugar without oxygen, as in brewing beer?

Ethanol is not only a yeast product: goldfish and carp make it too, to get energy when oxygen is scarce. Yeasts switch to it whenever oxygen is absent.

Which stage of cellular respiration, a loop of reactions that begins with citric acid, honors a 1953 Nobel laureate?

Krebs cycle honors Hans Krebs, whose first write-up was turned down by the journal Nature for lack of space. He had fled Nazi Germany for England four years earlier.

In which innermost compartment of a mitochondrion, home to the organelle's own DNA, does the citric acid cycle run?

Matrix is the name of that inner space, where nearly all the enzymes of the cycle float free. Only one of them sits elsewhere, fixed in the membrane that surrounds it.

Which folds of a mitochondrion's inner membrane multiply the surface available for the electron transport chain?

Cristae give the membrane its wrinkled look, and they are not fixed: the folds are remodeled with stress and nutrition. Cells hungry for ATP, such as muscle, pack in even more of them.

Which enzyme in the inner mitochondrial membrane, honored by a 1997 Nobel Prize, spins as protons pass through?

ATP synthase earned Paul Boyer and John Walker their share of the prize. Part of the enzyme turns like a rotor in the membrane while the rest stamps out fresh ATP.

Which term names the flow of protons down a gradient across a membrane, the push behind most ATP production?

Chemiosmosis works like a dam: pumps pile protons up on one side of a membrane, and their rush back does the work. Chloroplasts use the same trick, with light driving the pumps.

Which B vitamin, whose shortage causes pellagra, do cells need to build the main electron carrier of respiration?

Niacin is vitamin B3. Pellagra killed more than 100,000 Americans between 1906 and 1940, an epidemic tied to diets built on corn.

Which molecule, used up by glycolysis, does fermentation in muscle and yeast exist to regenerate?

NAD+ picks up electrons during glycolysis and must be emptied before it can be used again. Fermentation does that by dumping the electrons onto a waste product, so the sugar-splitting can go on.

Which molecule delivers two-carbon fragments from sugars, lipids and proteins alike into the citric acid cycle?

Acetyl-CoA is the hub of metabolism: whatever the fuel, its carbon arrives here before the final burn. Cells also use it as the starting material for cholesterol.

Cellular respiration breaks big molecules into smaller ones to release energy, making it which kind of pathway?

Catabolic pathways tear molecules down, while anabolic ones build them up, which is how muscle-building anabolic steroids got their name.

What do chemists call the loss of electrons that glucose undergoes as cellular respiration breaks it down?

Oxidation is the O in the student mnemonic OIL RIG, whose first three letters spell out the loss. The electrons that glucose gives up end their journey on oxygen.

Marathon runners hit the wall around mile 20 when muscles and liver run out of which stored carbohydrate?

Glycogen is a branched chain of glucose units, and an untrained body stores only about 380 grams of it. Cyclists call running out the bonk.

Which compound, a popular gym supplement, is stored in muscle with a phosphate attached to recharge ATP within seconds?

Creatine with its phosphate can top up ATP for the first five to eight seconds of an all-out effort. About 95% of the body's supply sits in skeletal muscle.

Which hormone from the pancreas tells muscle and liver cells to take in glucose from the blood after a meal?

Insulin takes its name from the Latin for island, after the islets of the pancreas that make it. It was first isolated in 1921, from a dog's pancreas, in Toronto.

Which word for oxygen-fueled exercise did physician Kenneth Cooper coin and use as the title of a 1968 book?

Aerobics was the work of a doctor who also devised a fitness test for the US Air Force. The word means with oxygen: exercise steady enough for the mitochondria to keep up.

Which illness, a risk of badly canned food, comes from a germ that grows only where oxygen is absent?

Botulism takes its name from the Latin for sausage, an early culprit. The germ's spores wake up in the airless inside of a sealed can, and its toxin is the one sold as Botox.

Which nectar-drinking birds have the highest metabolic rate, weight for weight, of any warm-blooded animal?

Hummingbirds save fuel at night by slipping into torpor, a state like hibernation that cuts their metabolic rate to one fifteenth of normal.

Which glowing animal's enzyme, luciferase, powers hygiene swab tests that light up wherever ATP is found?

Firefly luciferase needs ATP to make light, and every living cell contains ATP. A glowing swab therefore means that a wiped surface still carries microbes or food residue.

Which French chemist put a guinea pig in an ice calorimeter and concluded that respiration is a slow form of burning?

Antoine Lavoisier and Pierre-Simon Laplace measured how much ice the animal's body heat melted. He later used a colleague, Armand Seguin, as a human test subject.

Which American biologist argued that mitochondria began as free-living microbes, in a much-rejected 1967 paper?

Lynn Margulis saw the paper turned down by about fifteen journals before one took it. The idea won general acceptance in the 1980s, once the organelles' own DNA had been studied.

Which North American wetland plant melts the snow around its flowers with heat from its own cellular respiration?

Skunk cabbage runs its mitochondria so hard in early spring that the flower can sit 15 to 35 °C above the air around it. The warmth may also help spread the foul smell that draws pollinating insects.

Which weight-loss drug, banned by the end of the 1930s, makes mitochondria turn fuel into heat instead of ATP?

Dinitrophenol raises the body's energy use by 30 to 40 percent, and an overdose can push body temperature to a fatal 44 °C. Its effect was first noticed in poisoned munitions workers.

Which carrier of the electron transport chain, present in nearly all living things, is sold as a heart supplement?

Coenzyme Q10 is so widespread in nature that chemists also call it ubiquinone, after ubiquitous. It shuttles electrons from the first two complexes of the chain to the third.

Which small protein ferries electrons between complexes III and IV and helps trigger a cell's programmed death?

Cytochrome c has barely changed across plants, animals and fungi, which makes it a favorite for tracing how species are related. Its release from a mitochondrion is a signal for the cell to die.

Which British biochemist won a 1978 Nobel Prize for the theory that proton gradients power the making of ATP?

Peter Mitchell tested the theory in a private laboratory inside a Cornish mansion he had restored, having resigned from the University of Edinburgh amid opposition and ill health.

Which German chemist won a 1907 Nobel Prize for showing that juice pressed from dead yeast could still ferment sugar?

Eduard Buchner ground yeast with quartz until no living cell remained. That the juice still worked dealt a blow to vitalism, the belief that such chemistry needed a living organism.

Which enzyme runs the committed step of glycolysis and slows down when a cell already has plenty of ATP?

Phosphofructokinase acts as the pathway's throttle: ATP, the end product, jams it, while AMP, a sign that energy is running low, competes for the same site and frees it.

Which plant-derived fish poison and pesticide works by blocking complex I of the electron transport chain?

Rotenone was first purified from a derris plant in 1902, but people had crushed such plants into streams for centuries, because the stunned fish float to the surface.

Which fuel molecules, made by the liver from fatty acids during a fast, share a name with a popular low-carb diet?

Ketone bodies include acetone, the solvent in many nail polish removers, which is why the breath of someone deep in a fast can smell fruity.

Which oxygen-storing protein, abundant in the muscles of diving whales and seals, is the pigment that colors red meat?

Myoglobin grips oxygen more tightly than hemoglobin does, so muscle can pull the gas out of the blood and keep a reserve of its own.

Which sweet food begins as tree seeds that are heaped in fruit pulp to ferment for several days and then dried?

Chocolate owes its color to that step: heat building up in the fermenting heap drives the purple out of the cacao seeds. Some growers distill spirits from the pulp that drains away.