Section 2 g) Summary

Gas exchange is the process of taking in oxygen and excreting carbon dioxide in organisms. This process is made possible by diffusion, which allows oxygen to travel from an area of high concentration (the air in the atmosphere) to an area or low concentration (e.g. the cells of a leaf, the capillaries surrounding the alveoli, etc.)

Flowering Plants

In flowering plants, gas exchange must happen for both respiration and photosynthesis.
Photosynthesis is essentially the reverse reaction of respiration:

Photosynthesis
Carbon dioxide + Water (+ energy from sunlight) --> Glucose + Oxygen
6CO2 + 6H2O --> C6H12O6 + 6O2

Gas exchange/respiration
Glucose + Oxygen --> Carbon dioxide + Water (+ energy)
C6H12O6 + 6O2 --> 6CO2 + 6H2O

Photosynthesis, however, can only take place during the day, when there is sunlight. During the night, respiration still occurs, but is limited by the stomata closing and preventing gas exchange from taking place in order to preserve water at night.

Leaves are adapted for gas exchange as they have stomata and guard cells, as well as air spaces to increase the surface area for diffusion. The guard cells become turgid during the day, opening the stomata and allowing air to flow in and out. During the night, to reduce water loss, the guard cells become flaccid and close the stomata. The leaves themselves are thin to decrease distance, and flat to increase surface area to volume ratio.

Net gas exchange in plants is affected by light levels overall. This is because photosynthesis happens more than respiration does, so it greatly affects the gas levels. We can investigate this using hydrocarbonate indicator which detects changes in CO2 levels.

1. Choose 3 leaves of similar sizes from the same plant, and suspend in a sealed boiling tube containing hydrogen carbonate indicator.
2. Leave one tube in the light, cover another tube in foil so it is in total darkness, cover another in gauze so it receives dim light, then leave a control without a leaf in the light. 
3. The hydrogen carbonate indicator will start off red in all, but each will end up with different results. The one in darkness will be yellow, the one in dim light will stay orange/red, the one in light will become yellow, and the control will not change in colour. 

Humans

Gas exchange in humans happens through the process of breathing, or ventilation.
This process happens in the thorax of the body, and involves a range of different structures for everything to work.

This diagram depicts some of the most important structures in the thorax for ventilation. This is how breathing in, then breathing out, involves each of them:

1. The intercostal muscles contract, the ribs move up, and the diaphragm contracts and moves down. This increase the volume and decreases the pressure within the chest and allows air to rush into the lungs.
2. The air outside at atmospheric pressure rushes through the larynx, and trachea, held open by the rings of cartilage, and then half the air travels down one bronchus in to one lung, and the other down the other. The air enters the bronchioles and is split down the branches. 
3. When the air reaches the alveoli (which are moist to dissolve the oxygen, as well as thin to decrease the diffusion distance and with a large surface areaa to increase diffusion. Oxygen diffuses from the air into the bloodstream and capillaries surrounding and bonding with haemoglobin in the red blood cells to form oxyhaemoglobin, and be transported around the body for respiration. 
4. Carbon dioxide diffuses out of the blood stream and into the alveoli. At this point, the intercostal muscles relax, the ribs move down, and the diaphragm relaxes and moves up. The volume of the chest is decreased, and the pressure increased, causing air to rush out into the atomsphere.  

The pleural membrane and fluid surrounding the lungs holds them in place, but allows them to move easily.

The rate of breathing is affected by exercise, because when in movement more oxygen is required for respiration to power the cells and bodily tissues.
We can investigate the way exercise affects breathing through this experiment:

1. Measure the rate of breathing while stationary (breaths per minute) by counting the number of breaths in fifteen seconds, then multiplying by four.
2. Exercise at a sustained and measured level for 1 minute (perhaps by running on a treadmill, which allows you to control your speed), then measure the rate of breathing again. Repeat after two minutes of exercise, then three, then four, and so on.
3. Draw a graph based of the length of time exercised (on the x-axis) and the breathing rate (on the y-axis), and you will see that the breathing rate is proportional to the time exercised.

This can also be done by using a spirometer to measure the depth of breath.


Smoking

Smoking is very bad for the lungs, it creates a number of problems with the different contents found in cigarettes.

• Cilia and mucus: Tar damages and paralyses the cilia, preventing them from moving mucus out of the lungs (causing mucus buildup) and leading to infections. This is CHRONIC BRONCHITIS.
• Alveoli: Tar fuses the alveoli together, decreasing surface area and making it more difficult to breathe. This is EPHYSEMA.
• Lungs, mouth and throat cancer: Cigarettes contain many carcinogens that cause cells to mutate and cancer to form in the mouth, throat and lungs. This is CANCER.
• Bloodstream: Carbon monoxide in cigarettes is favoured by haemoglobin, so it fuses to the protein and prevents oxygen from getting around the body, increasing heart rate and blood pressure. This is CARBON MONOXIDE POISONING.

Nicotine, a substance found in cigarettes, is highly addictive and affects the central nervous system, causing problems with blood pressure and coronary heart disease.