Section 2 g) Specification

2.38 understand the role of diffusion in gas exchange

Oxygen must diffuse into the organism and carbon dioxide out during gas exchange. Because of this, the organs designed for gas exchange, such as leaves and alveoli have a large surface area and a thin lining to increase the rate of diffusion.

Flowering plants:

2.39 understand gas exchange (of carbon dioxide and oxygen) in relation to respiration and photosynthesis

Photosynthesis is the conversion of carbon dioxide and water into glucose and oxygen, taking in energy from the sun:
6CO2 + 6H2O --> C6H12O6 + 6O2
Gas exchange is the conversion of glucose and oxygen into water and carbon dioxide, releasing energy:
C6H12O6 + 6O2 --> 6CO2 + 6H2O

As you can see, gas exchange is the reverse of photosynthesis, and vise versa.

2.40 understand that respiration continues during the day and night, but that the net exchange of carbon dioxide and oxygen depends on the intensity of light

Respiration is not limited by the energy absorbed by the plant (sunlight), so it can continue during day and night. Photosynthesis is affected by the intensity of light it receives, it can only occur during the day. However, in plants, the exchange of gas is controlled by the opening and closing of the stomata. To prevent water loss, the guard cells become flaccid and close the stomata at night. This decreases the rate of respiration as well, and the net gas exchange decreases significantly

2.41 explain how the structure of the leaf is adapted for gas exchange

It has stomata and spongy mesophyll with air pockets that increase the surface area for gas exchange. Leaves are thin to decrease the diffusion distance, as well as wide and flat to increase surface area

2.42 describe the role of stomata in gas exchange

Stomata can open and close due to the turgidity of the guard cells, which become flaccid and close at night, preventing water loss when photosynthesis is not possible. The stomata allow gas to travel inside the plant, where there are air pockets that further increase the leaf's surface area and allow gas to be exchanged between the leaf and the atmosphere.

2.43 describe experiments to investigate the effect of light on net gas exchange from a leaf, using hydrogen-carbonate indicator

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. 

These results are caused by the ratio of the rate of photosynthesis compared to the rate of respiration. 

Humans:

2.44 describe the structure of the thorax, including the ribs, intercostal muscles, diaphragm, trachea, bronchi, bronchioles, alveoli and pleural membranes

The thorax is enclosed by the rib cage, which is made up of rows of ribs connected by intercostal muscles that contract and relax which breathing. Inside the rib cage are the lungs, which is where gas exchange takes place. The lungs are surrounded by pleural membrane, holding them in place, and pleural fluid, which allows the lungs to move easily. Air rushes in through the mouth, down the trachea or windpipe, then through the two bronchi and many bronchioles (muscular tubes held open by rings of cartilage). The air reaches the alveoli, tiny sacs of air surrounded by capillaries, where gas exchange takes place, and the oxygen diffuses into the bloodstream, and carbon dioxide out.

2.45 understand the role of the intercostal muscles and the diaphragm in ventilation

In humans, breathing happens in the thorax. In inhaling, the intercostal muscles between the ribs contract, the rib cage moves up, the diaphragm contracts and lowers, increasing the volume of the chest and decreasing the pressure. This causes air from the atmosphere to rush into the lungs and fill them up. In exhaling, the intercostal muscles relax and the ribcage moves downwards. The diaphragm relaxes and raises, decreasing the volume of the chest and increasing the pressure, causing air to rush out into the atmosphere

2.46 explain how alveoli are adapted for gas exchange by diffusion between air in the lungs and blood in capillaries

Alveoli have a wet surface for dissolving oxygen, large surface area, thin lining for easy diffusion, and a lot of capillaries surrounding them.

2.47 understand the biological consequences of smoking in relation to the lungs and the circulatory system, including coronary heart disease

In the trachea and bronchi, there are ciliated epithelial cells, which are specialised cells that move mucus to clean the lungs of pathogens.

Tar from cigarettes coats and paralyses the cilia of these cells, preventing them from moving the mucus. This increases risk of bacterial infection and is known as chronic bronchitis, or commonly smoker's cough.
Tar also damages the alveoli - they are fused together by the sticky substance and their surface area is decreased, limiting gas exchange and making breathing difficult. This is called emphysema.

Cigarettes, when smoked, release carbon monoxide. This is a dangerous and highly toxic substance as it binds with haemoglobin in the bloodstream, as oxygen does, but is not released, it causes the red blood cell to be useless as it can no longer carry the less reactive oxygen. This leads to an increased heart rate, as not enough oxygen is able to reach the body's cells.

Nicotine, the addictive substance in cigarettes, affects the central nervous system, increasing heart rate and narrowing the blood vessels. This causes high blood pressure, which can lead to coronary heart disease.

2.48 describe experiments to investigate the effect of exercise on breathing in humans

Measure the rate of breathing while stationary (breaths per minute) by counting the number of breaths in fifteen seconds, then multiplying by four.
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.
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.