Why Your Legs Ache After Running Too Fast
Learn how respiration differs from breathing, compare aerobic with anaerobic respiration, see the experiments proving germinating seeds respire, and contrast respiration with photosynthesis.
Why do your legs ache after a hard sprint?
Because your muscles ran out of oxygen and switched to anaerobic respiration, which produces lactic acid. That build-up is what you feel as cramp and ache.
The same cells can release energy two different ways, and the products are not the same. This page covers everything in the ICSE Class 7 Biology chapter's second part: respiration and breathing, aerobic and anaerobic respiration, respiration in plants, and how respiration compares with photosynthesis.
The same cells can release energy two different ways, and the products are not the same. This page covers everything in the ICSE Class 7 Biology chapter's second part: respiration and breathing, aerobic and anaerobic respiration, respiration in plants, and how respiration compares with photosynthesis.
How is respiration different from breathing?
Respiration is the chemical process inside cells in which food is broken down to release energy. Breathing is the physical process of taking air in and pushing it out.
So breathing merely delivers the oxygen; respiration is what uses it. Breathing happens in the lungs, respiration happens in every cell of the body — in the mitochondria, which is why they are called the powerhouses of the cell.
The word equation for aerobic respiration:
glucose plus oxygen gives carbon dioxide plus water plus energy.
And as a chemical equation:
The energy released is stored as ATP and used for every activity — moving, growing, thinking and keeping warm.
You can prove the products are there. Breathe out through a straw into limewater and it turns milky, showing carbon dioxide, and breathe onto a cool mirror and it mists, showing water vapour.
The distinction worth keeping sharp is that a plant respires without breathing in the way an animal does. Plants have no lungs at all, yet every plant cell respires day and night.
So breathing merely delivers the oxygen; respiration is what uses it. Breathing happens in the lungs, respiration happens in every cell of the body — in the mitochondria, which is why they are called the powerhouses of the cell.
The word equation for aerobic respiration:
glucose plus oxygen gives carbon dioxide plus water plus energy.
And as a chemical equation:
The energy released is stored as ATP and used for every activity — moving, growing, thinking and keeping warm.
You can prove the products are there. Breathe out through a straw into limewater and it turns milky, showing carbon dioxide, and breathe onto a cool mirror and it mists, showing water vapour.
The distinction worth keeping sharp is that a plant respires without breathing in the way an animal does. Plants have no lungs at all, yet every plant cell respires day and night.
What is the difference between aerobic and anaerobic respiration?
Aerobic respiration uses oxygen; anaerobic respiration releases energy without it. Three things differ.
Aerobic respiration
- Oxygen — required.
- End products — carbon dioxide and water.
- Energy — a large amount released, because glucose is broken down completely.
Anaerobic respiration
- Oxygen — not required.
- End products — depend on the organism.
- Energy — a small amount released, because glucose is only partly broken down.
Two examples are in the syllabus.
Fermentation in yeast. Yeast breaks glucose down into alcohol (ethanol) and carbon dioxide:
glucose gives alcohol plus carbon dioxide plus a little energy.
This is why dough for bhature or bread rises — the carbon dioxide from the yeast forms bubbles in it.
Lactic acid in muscles. During hard exercise the blood cannot deliver oxygen fast enough, so muscle cells break glucose down into lactic acid plus a little energy. The lactic acid accumulates and causes cramp, and it is cleared away once you rest and oxygen returns.
The key reason for the energy difference is completeness. Aerobic respiration finishes the job, leaving only carbon dioxide and water; anaerobic respiration stops partway, so much of the energy is still locked inside the alcohol or lactic acid it leaves behind.
Aerobic respiration
- Oxygen — required.
- End products — carbon dioxide and water.
- Energy — a large amount released, because glucose is broken down completely.
Anaerobic respiration
- Oxygen — not required.
- End products — depend on the organism.
- Energy — a small amount released, because glucose is only partly broken down.
Two examples are in the syllabus.
Fermentation in yeast. Yeast breaks glucose down into alcohol (ethanol) and carbon dioxide:
glucose gives alcohol plus carbon dioxide plus a little energy.
This is why dough for bhature or bread rises — the carbon dioxide from the yeast forms bubbles in it.
Lactic acid in muscles. During hard exercise the blood cannot deliver oxygen fast enough, so muscle cells break glucose down into lactic acid plus a little energy. The lactic acid accumulates and causes cramp, and it is cleared away once you rest and oxygen returns.
The key reason for the energy difference is completeness. Aerobic respiration finishes the job, leaving only carbon dioxide and water; anaerobic respiration stops partway, so much of the energy is still locked inside the alcohol or lactic acid it leaves behind.
How can you prove that a germinating seed respires?
By showing that it releases carbon dioxide and heat — both products of respiration.
Experiment for carbon dioxide. Soak gram or moong seeds until they germinate and place them in a conical flask. Hang a small test tube of potassium hydroxide solution inside, and seal the flask with a cork carrying a bent delivery tube dipping into a beaker of water. As the seeds respire, they release carbon dioxide, which the potassium hydroxide absorbs. That lowers the pressure inside, so water rises up the delivery tube. A flask of boiled (dead) seeds set up alongside as a control shows no rise.
Experiment for heat. Put germinating seeds in one vacuum flask and boiled, disinfected seeds in another, each with a thermometer through the stopper. After a day, the thermometer among the germinating seeds reads higher — respiration has released heat.
Plants exchange gases through two kinds of opening:
- Stomata — pores on the leaf surface, mainly on the underside.
- Lenticels — small openings in the bark of woody stems.
Roots take in oxygen dissolved in the water and air spaces of the soil, which is why a plant in waterlogged soil dies — its roots cannot respire.
The control is the part students omit, and it is the reason the experiment proves anything. Boiled seeds are dead and cannot respire, so any difference between the two flasks must be caused by the living seeds.
Experiment for carbon dioxide. Soak gram or moong seeds until they germinate and place them in a conical flask. Hang a small test tube of potassium hydroxide solution inside, and seal the flask with a cork carrying a bent delivery tube dipping into a beaker of water. As the seeds respire, they release carbon dioxide, which the potassium hydroxide absorbs. That lowers the pressure inside, so water rises up the delivery tube. A flask of boiled (dead) seeds set up alongside as a control shows no rise.
Experiment for heat. Put germinating seeds in one vacuum flask and boiled, disinfected seeds in another, each with a thermometer through the stopper. After a day, the thermometer among the germinating seeds reads higher — respiration has released heat.
Plants exchange gases through two kinds of opening:
- Stomata — pores on the leaf surface, mainly on the underside.
- Lenticels — small openings in the bark of woody stems.
Roots take in oxygen dissolved in the water and air spaces of the soil, which is why a plant in waterlogged soil dies — its roots cannot respire.
The control is the part students omit, and it is the reason the experiment proves anything. Boiled seeds are dead and cannot respire, so any difference between the two flasks must be caused by the living seeds.
How do photosynthesis and respiration compare in plants?
They are almost exact opposites, and a plant does both.
Time of occurrence. Photosynthesis happens only in sunlight, during the day. Respiration happens all the time, day and night.
Gas taken in. Photosynthesis takes in carbon dioxide; respiration takes in oxygen.
Gas given out. Photosynthesis gives out oxygen; respiration gives out carbon dioxide.
Food. Photosynthesis makes food (glucose); respiration breaks down food.
Energy. Photosynthesis stores energy from sunlight; respiration releases energy for use.
Where. Photosynthesis occurs in chloroplasts, in green cells only; respiration occurs in mitochondria, in every living cell.
During bright daylight photosynthesis runs faster than respiration, so a plant releases oxygen overall. At night only respiration continues, so it releases carbon dioxide.
That is the fact behind a common half-truth. It is said that one should not sleep under a tree at night because it gives out carbon dioxide — true that it does, but the amount is tiny compared with the open air around it. The chemistry is right; the alarm is not.
Time of occurrence. Photosynthesis happens only in sunlight, during the day. Respiration happens all the time, day and night.
Gas taken in. Photosynthesis takes in carbon dioxide; respiration takes in oxygen.
Gas given out. Photosynthesis gives out oxygen; respiration gives out carbon dioxide.
Food. Photosynthesis makes food (glucose); respiration breaks down food.
Energy. Photosynthesis stores energy from sunlight; respiration releases energy for use.
Where. Photosynthesis occurs in chloroplasts, in green cells only; respiration occurs in mitochondria, in every living cell.
During bright daylight photosynthesis runs faster than respiration, so a plant releases oxygen overall. At night only respiration continues, so it releases carbon dioxide.
That is the fact behind a common half-truth. It is said that one should not sleep under a tree at night because it gives out carbon dioxide — true that it does, but the amount is tiny compared with the open air around it. The chemistry is right; the alarm is not.
Exam tip
Exam tip: always setting up a control
Respiration answers lose marks in three predictable places.
In any experiment, name the control and say what it is — boiled seeds, which are dead and cannot respire. Without it the experiment proves nothing, and examiners award a mark for it.
When comparing respiration and breathing, use the words chemical and physical, and say respiration happens in the mitochondria of every cell.
For anaerobic respiration, name the end product for the right organism: alcohol and carbon dioxide in yeast, lactic acid in muscles. Swapping them is the commonest error.
And in comparison questions, answer point by point in the same order — oxygen, end products, energy — rather than in a paragraph.
Remember that lenticels are the openings in bark; writing "stomata" for a woody stem loses the mark.
In any experiment, name the control and say what it is — boiled seeds, which are dead and cannot respire. Without it the experiment proves nothing, and examiners award a mark for it.
When comparing respiration and breathing, use the words chemical and physical, and say respiration happens in the mitochondria of every cell.
For anaerobic respiration, name the end product for the right organism: alcohol and carbon dioxide in yeast, lactic acid in muscles. Swapping them is the commonest error.
And in comparison questions, answer point by point in the same order — oxygen, end products, energy — rather than in a paragraph.
Remember that lenticels are the openings in bark; writing "stomata" for a woody stem loses the mark.
Did you know
Why does dough left in a warm kitchen rise on its own?
Because the yeast in it is respiring anaerobically, and the gas it produces has nowhere to go.
Yeast cells feed on the sugar and starch in the dough. With little oxygen available inside that sticky mass, they ferment the sugar into alcohol and carbon dioxide. The gas collects as thousands of tiny bubbles trapped in the stretchy dough, and the whole mass swells.
Baking then does two things at once: the heat expands those bubbles further, and it drives off the alcohol — leaving the soft, holey texture behind.
Yeast cells feed on the sugar and starch in the dough. With little oxygen available inside that sticky mass, they ferment the sugar into alcohol and carbon dioxide. The gas collects as thousands of tiny bubbles trapped in the stretchy dough, and the whole mass swells.
Baking then does two things at once: the heat expands those bubbles further, and it drives off the alcohol — leaving the soft, holey texture behind.
Key takeaways
Respiration: quick revision
- Respiration is the chemical breakdown of food to release energy, in the mitochondria of every cell; breathing is the physical exchange of air.
- Aerobic respiration: , releasing a large amount of energy.
- Anaerobic respiration needs no oxygen and releases little energy — alcohol and carbon dioxide in yeast, lactic acid in muscles.
- Germinating seeds prove respiration by absorbing released carbon dioxide with potassium hydroxide, and by raising a thermometer's reading — each needing a control of boiled seeds.
- Plants exchange gases through stomata in leaves and lenticels in bark, and roots use oxygen from the soil.
- Photosynthesis happens only by day, takes in carbon dioxide, makes food and stores energy; respiration runs day and night, takes in oxygen, breaks down food and releases energy.
You will remember all of this far better after answering five questions on it than after reading it twice.
- Aerobic respiration: , releasing a large amount of energy.
- Anaerobic respiration needs no oxygen and releases little energy — alcohol and carbon dioxide in yeast, lactic acid in muscles.
- Germinating seeds prove respiration by absorbing released carbon dioxide with potassium hydroxide, and by raising a thermometer's reading — each needing a control of boiled seeds.
- Plants exchange gases through stomata in leaves and lenticels in bark, and roots use oxygen from the soil.
- Photosynthesis happens only by day, takes in carbon dioxide, makes food and stores energy; respiration runs day and night, takes in oxygen, breaks down food and releases energy.
You will remember all of this far better after answering five questions on it than after reading it twice.