Can Life Run Without Oxygen?

Every cell needs a steady supply of energy, to move, to grow, to build and to repair. It gets it from food, mostly from sugar (glucose). You know the reaction from Grade 8, where it happens in the mitochondria Grade 8: Mitochondria:

$$\text{glucose} + \text{oxygen} \longrightarrow \text{carbon dioxide} + \text{water} + \text{energy}$$

$$\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \longrightarrow 6\text{CO}_2 + 6\text{H}_2\text{O} + \text{energy}$$

In Grade 9 you met it again as an exothermic reaction, one that gives out energy, like burning Grade 9: Exothermic reaction. Respiration and burning sugar in a flame start and end with the same substances, and give out the same total energy: about 2,800 kJ for every 180 g of glucose. The difference is that the cell releases it in many small steps, and catches part of it in a ready-to-use energy carrier, ATP, instead of losing it all as a flash of heat.

Respiration that uses oxygen is called aerobic respiration (aero = air). It breaks the sugar down completely, to carbon dioxide and water: small molecules with almost no chemical energy left in them. That is why it releases so much. (How the cell does it, step by step, and how it uses the energy to build itself, is the topic How Does Food Become Energy, and Become You?)

But what if there is no oxygen?

Without oxygen, a cell can still break glucose apart, but only part of the way. The products are still fairly big molecules with plenty of energy left in them. Releasing energy from food without oxygen is called anaerobic respiration (an = without). When the products are things like alcohol or lactic acid, it is also called fermentation. (Biologists sometimes keep the name “anaerobic respiration” for microbes that use another substance in place of oxygen, such as the sulphate behind the rotten-egg smell of mud. In school, and in this topic, it means any way of releasing energy from food without oxygen.)

There are three kinds you can meet in Nepal in a single day.

Yeast, the single-celled fungus in marcha and in bread dough, makes ethanol (the alcohol in jaand and raksi) and carbon dioxide. You met this reaction in Grade 10 Grade 10: Ethyl alcohol:

$$\text{C}_6\text{H}_{12}\text{O}_6 \longrightarrow 2\text{C}_2\text{H}_5\text{OH} + 2\text{CO}_2 + \text{a little energy}$$

Lactic acid bacteria in milk turn it into curd (dahi); in mustard and radish leaves packed tightly into a sealed jar, they make gundruk. They make lactic acid, which gives curd and gundruk their sour taste:

$$\text{C}_6\text{H}_{12}\text{O}_6 \longrightarrow 2\text{C}_3\text{H}_6\text{O}_3 + \text{a little energy}$$

Your own muscle cells do the same as the bacteria in curd when they run short of oxygen: they make lactic acid.

Yeast is a good example of a living thing that can do both. With plenty of air, it uses oxygen, breaks its sugar down completely, and grows and multiplies quickly. Without air, it ferments. That is why the pot of jaand is closed: the yeast keeps making alcohol, and other microbes that need air, which would turn the alcohol into vinegar, can’t grow.

Remember that every 180 g of glucose holds about 2,800 kJ. With oxygen, the cell releases all of it. Without oxygen, where does it go?

The answer is that most of it is still in the products. Ethanol burns: that is why raksi catches fire, and why ethanol is used in spirit lamps. Lactic acid can be burned too, and so can methane, the gas that some microbes make. Burn the products, and you get back almost all the energy that was in the sugar.

That small share is all the cell has to live on. A cell catches the energy it releases in ATP. With oxygen, one glucose molecule gives the cell about 30 ATP; without oxygen, only 2. So to get the same energy, a fermenting cell must get through about 15 times as much sugar. That is why Pasteur saw yeast without air eat sugar so fast, and why a fermenting pot gets warm.

Stand up and run up three flights of stairs as fast as you safely can. At the top you are gasping, and you keep breathing hard for a minute or two after you stop. Why, if you have stopped using the energy?

Your muscles can switch from resting to full effort in a second. Your breathing and heart can’t: they take a minute or more to speed up, and even at their fastest they can deliver only so much oxygen. While oxygen runs short, the muscles make up the difference without oxygen: first from a small store of a ready-made energy chemical, then by turning glucose into lactic acid. The shortfall is called the oxygen deficit.

When you stop, your breathing stays fast for a while. The extra oxygen you take in is used to refill the muscles’ energy store and to deal with the lactic acid: much of it is used as fuel with oxygen, by the heart and other muscles, and some is turned back into glucose by the liver. This extra oxygen after exercise used to be called the oxygen debt.

How much of a race runs without oxygen depends on how long and hard it is. In a 100 m sprint, most of the energy is made without oxygen. In an all-out effort lasting a little over a minute, roughly half. In a 5,000 m race or a long walk up to a hill village, almost all of it comes with oxygen, because those muscles work only as hard as oxygen can keep up with.

Your body runs short of oxygen only for minutes. But large parts of the living world have no oxygen at all, all the time. In each of them, life goes on without it.

Why would a place have no oxygen? Air is about 21% oxygen, but water holds very little: a liter of water at 20 °C holds only about 9 mg of oxygen, while a liter of air holds about 280 mg, some 30 times as much. And oxygen moves through water about 10,000 times more slowly than through air. So wherever there is water and a lot of food for microbes, the microbes use up the oxygen faster than it can come in, and it runs out.

• Flooded paddy fields. A few days after a field is flooded for planting, the soil below the top few millimeters has no oxygen.
• The mud at the bottom of ponds, lakes and rivers, and swamps and marshes. When you stir it, it smells of rotten eggs: hydrogen sulphide, made by microbes without oxygen.
• Guts. Your large intestine has almost no oxygen; its bacteria ferment the fiber you can’t digest. A buffalo’s or cow’s rumen, its huge first stomach, is a fermenting tank where microbes break down grass and straw without oxygen, making acids and methane. The buffalo then absorbs the acids and uses them, with oxygen, for most of its energy.
• A biogas digester and the inside of a heap of rubbish or dung, as you know from Grade 9 Grade 9: Biogas.
• Deep in the soil and in the sediments under the sea, sometimes hundreds of meters down.

The living things in these places are of three kinds:

Kind	With oxygen	Without oxygen	Examples
need oxygen	live and respire aerobically	die	you as a whole, rice plants, most animals
can do either	respire aerobically and grow fast	ferment and grow slowly	yeast, E. coli in your gut, your muscle cells for a short time
can’t stand oxygen	are harmed or killed	live and respire anaerobically	the microbes that make methane; Clostridium, which causes tetanus

The microbes that make methane are not even bacteria: they belong to a separate group of single-celled life, the archaea, and most of them are killed by even a little oxygen.

You saw in Grade 9 how an ecosystem works Grade 9: Ecosystem: producers capture the Sun’s energy in food; consumers eat it; decomposers break down dead bodies and wastes Grade 9: Decomposers; materials go round and round, and energy flows through. Now you can say more about that last step, because it happens in two different ways.

With oxygen, as on the floor of a dry forest, decomposers break dead matter down completely, to carbon dioxide, water and minerals. A single microbe can break its food all the way down. The carbon goes back into the air within months or a few years, and the energy is all released, ending up as heat.

Without oxygen, as in a flooded field or pond mud, no single microbe can finish the job. It takes a team. Some microbes ferment the dead matter into acids, alcohols and hydrogen, taking a little energy for themselves. Others use those products, taking a little more. The last in line, the methane makers, turn what is left into methane and carbon dioxide. Each gets only a small share of the energy, so they all grow slowly, and decay is slow.

This has three big results for an ecosystem.

1. Dead matter builds up. Where decay is slow, dead plants pile up faster than they rot. Over thousands of years, wet places store huge amounts of carbon as peat, and long ago swamps buried the plants that became coal.
2. Energy leaves as methane. Most of the energy in the dead matter is still there, in the methane. If the methane bubbles up through water where there is oxygen, other microbes, methane eaters, use it as food, with oxygen, and the energy is released there. If it escapes into the air, the energy leaves the ecosystem, and the methane traps heat in the atmosphere: molecule for molecule, it warms the Earth much more than carbon dioxide does, though it lasts only about a decade in the air.
3. Matter still cycles. Carbon in methane is not lost: in the air, methane slowly reacts with oxygen to form carbon dioxide, which plants take in again. Without oxygen the carbon cycle runs more slowly, and by a different road.

Rice plants live with their roots in mud with no oxygen. They manage because they have air channels running from their leaves down their stems to their roots, carrying oxygen down. The same channels carry methane up, and it leaves through tiny pores low on the stems: most of the methane from a paddy field escapes through the rice plants. Rice fields produce roughly a tenth of all the methane that human activity releases. Farmers who drain their fields for a few days now and then during the season, letting air back into the soil, cut the methane by about half (results vary from field to field), usually with the same harvest.

• How Does Food Become Energy, and Become You? Aerobic respiration close up: how a cell releases the energy in food with oxygen, and uses it, and the food’s atoms, to build itself.
• Where Does a Fallen Leaf Go? The whole carbon cycle: how carbon moves between the air, living things, the soil and the sea, and how people are changing it.
• How Many Tigers Can Chitwan Hold? How much life an ecosystem can hold, and what limits it.