S-Cool Revision Summary

S-Cool Revision Summary

Respiration is the reason that we need oxygen.

Respiration is the process of releasing energy from food. You could think of it as burning the fuel, glucose.

Glucose is the key molecule. It is made by plants in photosynthesis. In animals, although lots of different molecules are absorbed after digestion they are usually turned into glucose. In animals glucose is especially important as two organs can only use glucose as a fuel.

The two organs which use glucose as fuel are; the brain and the heart.

Respiration summarised in this equation:

glucose + oxygen > carbon dioxide + water + energy

Aerobic Respiration: involving oxygen

Anaerobic Respiration: without oxygen

Plants make their own food by photosynthesis. But they also use aerobic respiration to release energy from it.

So how can the plant get oxygen?

Oxygen from the air is able to diffuse into the leaves of plants through tiny air-holes called stomata. This is obvious in plants as they take in oxygen at night-time and give out the carbon dioxide produced by respiration.

Photosynthesis formula:

Carbon Dioxide + Water + Energy > Glucose + Oxygen

Aerobic respiration occurs in the opposite direction as photosynthesis. So in plants, photosynthesis is building up sugars at the same time as respiration is using them up.

We make two sets of breathing movements:

In called inspiration also called inhalation

Out called expiration or exhalation

We breathe in by using 2 lots of muscles. The diaphragm is a muscle sheet below the lungs, which flattens and pulls down as it contracts. Then between the ribs are intercostal muscles which act to pull the ribcage up and outwards.

Expiration, breathing out, is a bit simpler. Usually all we have to do is to let the two sets of muscles relax, so that the diaphragm is moved back up to its original position, and our ribcage falls back into its normal place. This pushes the air out of the lungs.

Breathing isn't just about making movements. It is about moving gas molecules.

The air that we breathe out has more carbon dioxide in it (4%) than we breathe in (0.04%). It has less oxygen too, the air we breathe in has about 21% oxygen whereas that which we breathe out has 16%.

The air enters the lungs down the trachea which branches into the right and left bronchi. Each bronchus then divides further into bronchioles. After about 20 branchings you reach the air sacs, the alveoli.

Each alveolus has a thin layer of epithelial cells separating the air from blood capillaries - a bit like the villi in the digestive system.

The alveoli have a similar job too, they are involved in moving molecules about.

Oxygen molecules diffuse from the alveoli into the blood stream, where there is a lower concentration of oxygen. The carbon dioxide diffuses the other way, from the high concentration in the blood to the alveoli.

Anaerobic respiration is an emergency system of reactions used by animal and plant cells when they cannot get enough oxygen to carry out aerobic respiration but still need to obtain energy to stay alive.

In Animals

When you sprint for a bus, your muscles use so much oxygen that you cannot supply it in time. So they cannot use aerobic respiration. Instead they use anaerobic respiration in the following way:

Glucose > Lactic Acid + Energy

This way of getting energy is not as efficient as aerobic respiration and it also leaves a poisonous chemical, lactic acid. This stops your muscles working and they get sore.

When you stop the lactic acid is slowly destroyed but that needs oxygen. The amount of oxygen you need for this is called the 'oxygen debt'.

In Plants

If the roots of a plant get waterlogged they start to run out of oxygen too. So they need to use a different form of anaerobic respiration. The one they use is this:

Glucose > Ethanol + Energy

It is also the reaction used by yeast cells when they make bread or alcoholic drinks.

Again, like lactic acid, ethanol is poisonous - that's why it affects brain cells! If there is too much ethanol produced it will kill the cells. So it must be got rid of by using oxygen in aerobic reactions.

Otherwise the plant cells get 'pickled'- permanently!