Alcohol Still Burns Because Anaerobic Respiration Gives Up Too Early

Yeast respiring without oxygen turns sugar into alcohol and carbon dioxide, and releases a little energy in the process.

Now notice something about that alcohol: it burns. Pour it out, put a flame to it, and it gives off a great deal of heat.

So the yeast finished with the sugar and left behind a product that still contains plenty of energy. It took a small share and walked away from the rest.

With oxygen available, the same sugar is broken down completely — all the way to carbon dioxide and water, neither of which will burn at all. Nothing is left to extract, and the energy released is far greater.

That single comparison explains why aerobic respiration yields so much more than anaerobic, and it is a far better answer than "because oxygen is present". The oxygen matters because of what it makes possible: the complete oxidation of the food.

Respiration happens in two stages. The first, glycolysis, needs no oxygen and is where anaerobic respiration stops. The second, the Krebs cycle, needs oxygen and is where most of the energy actually comes out.

This page covers the second part of the ICSE Class 9 Biology chapter on plant physiology: what respiration is with its two stages, aerobic against anaerobic respiration with balanced equations, how a plant exchanges gases without lungs, and the experiment that shows respiration produces heat.

Respiration is the process by which food, usually glucose, is oxidised inside living cells to release energy, which is stored as ATP for the cell to use.

It goes on in every living cell, all the time, day and night. A plant does not switch respiration off in the light, and a seed in a packet is doing it very slowly.

It is a catabolic process, because a large molecule is broken into smaller ones, and an exothermic one, because energy is released. Photosynthesis is the exact opposite — anabolic and endothermic — and the two together account for the whole energy economy of a plant.

Aerobic respiration happens in two stages.

Glycolysis — the splitting of sugar.

- Where: in the cytoplasm
- Oxygen: not needed
- What happens: one molecule of glucose, which has six carbon atoms, is broken into two molecules of pyruvic acid, each with three carbon atoms
- Energy released: a small amount
- Significance: it is the common first step of both aerobic and anaerobic respiration, and because it needs no oxygen it can keep a cell supplied when oxygen runs short

Krebs cycle — also called the citric acid cycle.

- Where: in the mitochondria
- Oxygen: needed
- What happens: the pyruvic acid is completely oxidised to carbon dioxide and water
- Energy released: a large amount
- Significance: this is where most of the energy of the glucose is actually recovered

Check the carbon arithmetic, because it is the quickest way to see that glycolysis only goes halfway. Glucose has six carbon atoms. Two pyruvic acid molecules account for $2\times 3=6$ — so every carbon atom is still there, locked in pyruvic acid, and not one has been released as carbon dioxide yet.

So after glycolysis the food has been split but not oxidised. The carbon is all still in an organic molecule, and the energy is still mostly in it too. That is why glycolysis releases so little, and why the second stage matters so much.

And it explains why the two stages are in different places. Glycolysis, needing no oxygen, happens loose in the cytoplasm. The Krebs cycle, needing oxygen and a great deal of enzyme machinery, happens inside the mitochondria — on the folded cristae that the cell chapter described as carrying the respiratory enzymes. A cell with no mitochondria can perform glycolysis and nothing more, which is exactly the position of a yeast cell deprived of air.

Aerobic respiration breaks glucose down completely with oxygen; anaerobic respiration breaks it down incompletely without oxygen.

Aerobic respiration.

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

- Oxygen: required
- Breakdown: complete
- Products: carbon dioxide and water
- Energy: a large amount
- Site: cytoplasm and mitochondria
- Occurs in: almost all living cells, most of the time

Check the balance. Carbon $6=6$; hydrogen $12=12$; oxygen $6+12=18$ on the left against $12+6=18$ on the right.

Anaerobic respiration in plants and yeast.

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

- Oxygen: not required
- Breakdown: incomplete
- Products: ethyl alcohol and carbon dioxide
- Energy: a small amount
- Site: cytoplasm only
- Occurs in: yeast, in waterlogged roots, in germinating seeds sown too deep, and in the deeper tissues of a fleshy fruit

Check the balance. Two molecules of ${\text{C}}_2{\text{H}}_5\text{OH}$ give carbon $4$, hydrogen $12$ and oxygen $2$; two of ${\text{CO}}_2$ give carbon $2$ and oxygen $4$. Totals: carbon $4+2=6$; hydrogen $12$; oxygen $2+4=6$. All three match the glucose on the left.

Anaerobic respiration in yeast is called fermentation, and it is used in making bread, in leavening idli and dosa batter and in brewing. The bubbles that make a dough rise and give idli batter its froth are carbon dioxide from yeast or bacteria respiring anaerobically in the mixture.

Now the reason for the energy difference, which is the examinable point. It is not simply that oxygen is absent. It is that the glucose is only partly broken down.

Look at the products. Aerobic respiration ends with carbon dioxide and water — and neither of those will burn, because there is no energy left in them to release. Anaerobic respiration ends with ethyl alcohol — and alcohol burns readily, which proves it still holds a great deal of unreleased energy.

So anaerobic respiration stops early and abandons most of the energy inside its own product. The alcohol is not waste in the sense that water is waste; it is unfinished fuel. A question asking why anaerobic respiration releases less energy should be answered on the incomplete breakdown, and the burning of alcohol is the evidence.

One important difference from animals. In an animal muscle working without enough oxygen, the product is lactic acid, not alcohol — and that is what causes cramp and muscle fatigue. Plants and yeast make alcohol; animal muscle makes lactic acid, and a question about the product of anaerobic respiration must be answered for the right organism.

By simple diffusion, at every surface, over a very short distance — so it needs no lungs and no transport system at all.

An animal has a problem a plant does not. Its cells are packed in bulk, many of them far from the outside, so it needs lungs to take in oxygen and blood to carry it to cells that cannot reach the air themselves.

A plant is built quite differently. Its living cells are arranged in thin sheets and layers with air spaces between them, so almost every cell is close to air already. Each cell exchanges gases directly with the air immediately around it, and nothing has to be carried anywhere.

The surfaces used.

- Stomata — tiny pores in the epidermis of the leaves and young stems, each guarded by two guard cells that open and close it. Most gaseous exchange happens here
- Lenticels — small openings in the bark of woody stems, where the epidermis has been replaced by corky tissue
- The general surface of young roots and root hairs, which take oxygen from the air held in the spaces between soil particles

The rate of respiration in a plant is slow, because a plant does not move, does not maintain a body temperature and needs far less energy than an animal of the same mass. So diffusion is quite fast enough to supply it.

Now what happens by day and by night, and this is where the misconceptions live.

Respiration goes on continuously — day and night, without a pause. What changes is whether photosynthesis is also happening.

- In bright light, photosynthesis is going on as well, and it is the faster of the two. It uses up carbon dioxide and produces oxygen in greater quantity than respiration consumes and produces. So the net exchange at the leaf surface is oxygen out and carbon dioxide in
- At night, only respiration occurs. So the net exchange is oxygen in and carbon dioxide out
- At a particular low light intensity the two processes exactly balance and there is no net exchange at all. This is the compensation point

So the common statement that "plants give out carbon dioxide at night and oxygen during the day" is describing the net exchange and not the processes. A plant releases carbon dioxide from respiration all day as well — it is simply used up inside the leaf by photosynthesis before it can get out.

That distinction is worth being precise about, because it decides a whole class of questions. Respiration does not stop in the light, slow down in the light or wait for night. A question asking whether a plant respires during the day is answered yes, always — and the reason nothing appears to come out is that photosynthesis in the same cells is consuming it faster than respiration makes it.

Experiment to show that carbon dioxide is released during respiration. Place some germinating seeds in a conical flask and fit a cork carrying a bent delivery tube whose other end dips into a test tube of lime water. Keep the whole apparatus in the dark, so that no photosynthesis can occur. After some hours the lime water turns milky, showing that carbon dioxide has been given off.

Set up a second flask with seeds that have been boiled, and its lime water stays clear — proving the gas came from the living activity of the seeds and not from the apparatus.

Why germinating seeds are chosen and why the dark is essential. Germinating seeds respire very rapidly, so a measurable amount of carbon dioxide appears quickly, and they contain no chlorophyll to complicate matters. And in the light a green plant's photosynthesis would consume the carbon dioxide as fast as respiration released it, so the lime water might never turn milky at all. The darkness is not a convenience — without it the experiment could fail while the conclusion was still true.

To show that oxygen is used up, the same flask is fitted with a small vessel of potassium hydroxide solution to absorb the carbon dioxide as it forms, and connected to a narrow graduated tube containing a drop of coloured liquid. As oxygen is consumed and the carbon dioxide is absorbed rather than replacing it, the pressure inside falls and the coloured drop moves in towards the flask.

Put germinating seeds in an insulated flask with a thermometer, compare them with boiled seeds treated the same way, and the living seeds warm up.

The apparatus. Take two vacuum flasks, labelled A and B.

- In flask A place germinating seeds that have been washed in a disinfectant
- In flask B place an equal quantity of seeds that have been boiled — and therefore killed — and then washed in the same disinfectant
- Plug both flasks with cotton wool carrying a thermometer, with the bulb pushed down among the seeds
- Note the initial temperature in both, and leave them for a day or two

Result. The temperature in flask A rises noticeably. In flask B it stays where it was.

Conclusion. The living germinating seeds in A are respiring, and respiration has released heat. So respiration is an exothermic process.

Now the reason for each feature of the design, which is where the marks are.

Why a vacuum flask. The heat produced by a handful of seeds is small. A vacuum flask prevents it escaping to the surroundings, so the temperature inside rises enough for a thermometer to register it. In an ordinary beaker the heat would be lost as fast as it was made and nothing would be detected.

Why the boiled seeds in B. They are the control. Boiling kills them, so they cannot respire. B contains the same quantity of the same seeds in the same flask with the same thermometer, and the only difference is that they are dead. Its failure to warm up is what proves the rise in A came from living activity rather than from the flask, the cotton wool or the weather.

Why both lots are washed in disinfectant, and this is the cleverest part of the design. Dead moist seeds are an excellent food for bacteria and fungi. If B were not disinfected, moulds would grow on the dead seeds, and those moulds would respire — and flask B would warm up too.

The experiment would then show no difference between the flasks, and it would appear to prove that respiration produces no heat at all. The conclusion would be exactly wrong, and nothing in the readings would reveal the mistake.

So the disinfectant is what keeps the control valid. A control has to differ from the test in one respect only, and a dead flask growing mould differs in two — it has no living seeds, but it does have living moulds. The control must be dead and must stay dead, and that is the single most instructive detail in this chapter.

One further precaution sometimes specified. The flasks are kept inverted or loosely plugged, so that the carbon dioxide produced — which is heavier than air — does not collect around the seeds and suffocate them. A seed surrounded by carbon dioxide would be forced into anaerobic respiration, which releases far less heat.

And there is a practical version of this experiment in every grain store. Damp grain stored in bulk heats up, sometimes enough to spoil or even to catch fire, because the grain is respiring and the heat cannot escape from the middle of a large heap. That is flask A on an industrial scale, and it is why grain is dried before storage — a dry seed respires so slowly that it generates almost no heat at all.