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. 

Section 2 g) Key Words

Alveoli: Tiny air-sacs in the lungs surrounded by capillaries in which gas exchange takes place.

Breathing: The process of inhaling and exhaling air so the body can perform gas exchange.

Bronchioles: The many small tubes held open by rings of cartilage inside the lungs.

Bronchus: The two medium sized tubes that connect the lungs to the windpipe.

Capillaries: The smallest type of blood vessel.

Cilia: A small, hair-like structure found on ciliated cells. Used to move mucus.

Ciliated epithelial cells: Cells found in the trachea, brochus and bronchioles that move mucus around and prevent pathogens and infection from reaching the lungs.

Diaphragm: The large muscle found beneath the lungs (when you have the hiccups, your diaphragm is spasming) that contracts and relaxes to change the volume of the chest and makes breathing possible.

Expiration: Expelling air from the lungs.

Gas exchange: The process by which carbon dioxide is excreted and oxygen is taken in.

Goblet cells: Mucus-producing cells found amongst ciliated cells.

Guard cells: Cells found in the lower epidermis of the leaf, they become turgid to open the stomata and flaccid to close them.

Haemoglobin: A red, iron-based protein that bonds with oxygen (forming oxyhaemoglobin) to transport it around the body for respiration in cells.

Hydrogen carbonate indicator: A sensitive indicator that can show very slight changes in carbon dioxide levels in the surroundings. It starts off red/orange, and when exposed to high concentrations of CO2 it becomes yellow, while when exposed to low concentrations, it becomes purple.

Inspiration: Taking air into the lungs.

Intercostal muscles: Muscles found inbetween the ribs that relax and contract to change the volume of the chest and allow breathing to take place.

Photosynthesis: The process by which plants produce glucose and oxygen from carbon dioxide and water, with the help of sunlight.

Pleural membrane: The membrane found between the lungs and the ribcage. Contains pleural cavity fluid which allows the lungs to move easily, while keeping them in place.

Respiration: The process by which energy is released from glucose.

Spirometer: A machine that measures the air capacity of the lungs.

Stomata: The holes found in the bottom of the leaf that allow gas to flow in and out.

Trachea/windpipe: A large tube held open by large rings of cartilage, allows the passage of air from the mouth into the bronchus, bronchioles and lungs.

Ventilation: The process of breathing.

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.