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. 

Section 2 f) Summary

Respiration is the chemical breakdown of sugars to release energy in living cells.
This is very important, without it we wouldn't have the energy for any of the other life processes. Notable everyday energy-requiring processes include:

• nerve impulses
• cell division
• muscle contraction
• homeostasis and thermoregulation
• protein synthesis
• active transport

Aerobic respiration is releasing energy from glucose by reacting it with oxygen:

Glucose + Oxygen --> Carbon dioxide + Water (+ Energy)

C6H12O6 + 6O2 --> 6CO2 + 6H2O (+ Energy)

Anaerobic respiration is releasing energy from glucose without oxygen. It releases different toxic by-products in plants and animals, and less energy than aerobic respiration:

Plants: Glucose --> Carbon dioxide + Ethanol (+ Some energy)

Animals: Glucose --> Carbon dioxide + Lactic acid (+ Some energy)

The rate of respiration is affected by enzymes and their optimum temperature and pH. 

We can investigate respiration through the following experiments:

Rising dough (yeast):

1. Place dough into oiled measuring cylinders, and record their height. 
2. Place each measuring cylinder into a water bath (regular intervals of 10℃), with three cylinders in each bath so an average for each can be found.
3. Measure the height of the dough every 10 minutes, for 30 minutes.
4. Record the average heights in a table, then find the average percentage change in height. 

Seeds:

1. Fill one vacuum flask with living seeds, and thee other with surface-sterilised seeds. Place a thermometer in each and seal the top with cotton wool.
2. Record starting temp, then wait 30 minutes. 
3. The flask containing the living seeds will have increased in temperature, and the one with dead seeds will not. 

Heat is a by-product of the use of the energy for respiration. 

Humans: 

You can see that we produce carbon dioxide by blowing into a straw in a boiling tube containing lime water - it goes cloudy - or hydrogen carbonate indicator - it goes from red to yellow. 

Section 2 f) Key Words

Aerobic respiration: Respiration including oxygen. Releases more energy, and has less toxic by-products.
Glucose + Oxygen --> Carbon dioxide + Water

Anaerobic respiration: Respiration without oxygen. Releases less energy than aerobic respiration, and has toxic by-products, e.g. lactic acid in animals, ethanol in plants and yeast.
Glucose --> Carbon dioxide + Lactic acid
Glucose --> Carbon dioxide + Ethanol

Respiration: The chemical breakdown of sugar molecules in living cells to release energy

Section 2 f) Specification

2.33 understand that the process of respiration releases energy in living organisms

Respiration is the process by which living organisms release energy from sugar.

2.34 describe the differences between aerobic and anaerobic respiration

Aerobic respiration is respiration that occurs with oxygen, and anaerobic is without. Anaerobic respiration produces significantly less energy than aerobic respiration, and usually has a toxic by-product (lactic acid in humans, alcohol in yeast or plants, etc.)

2.35 write the word equation and the balanced chemical symbol equation for aerobic respiration in living organisms

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

2.36 write the word equation for anaerobic respiration in plants and in animals

Plants: Glucose --> Carbon dioxide + Ethanol (+some energy)
Animals: Glucose --> Carbon dioxide + Lactic acid (+some energy)

2.37 describe experiments to investigate the evolution of carbon dioxide and heat from respiring seeds or other suitable living organisms.

Rising dough (yeast):

1. Place dough into oiled measuring cylinders, and record their height. 
2. Place each measuring cylinder into a water bath (regular intervals of 10℃), with three cylinders in each bath so an average for each can be found.
3. Measure the height of the dough every 10 minutes, for 30 minutes.
4. Record the average heights in a table, then find the average percentage change in height. 

Seeds:

1. Fill one vacuum flask with living seeds, and thee other with surface-sterilised seeds. Place a thermometer in each and seal the top with cotton wool.
2. Record starting temp, then wait 30 minutes. 
3. The flask containing the living seeds will have increased in temperature, and the one with dead seeds will not. 

Heat is a by-product of the use of the energy for respiration. 

Humans: 

You can see that we produce carbon dioxide by blowing into a straw in a boiling tube containing lime water - it goes cloudy - or hydrogen carbonate indicator - it goes from red to yellow. 

Section 2 e) Summary

Flowering Plants
Plants don't need to eat, they get all of their nutrients by absorbing mineral ions from the soil, and carrying out photosynthesis.
Photosynthesis is the process by which plants generate glucose (which is stored as starch) using sunlight:

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

They are able to do this because their cells contain organelles called chloroplasts, which contain a green pigment known as chlorophyll. Chlorophyll is the chemical that carries out photosynthesis.

Factors affecting photosynthesis:

• Temperature: As temp. increases, the particles move faster and the rate of photosynthesis is increased, but after reaching optimum temperature for the enzymes they begin to denature at any higher temp. and the rate of photosynthesis drops steeply.
• Light intensity: More intense light means more photosynthesis, but only up until a certain point as the number of chloroplasts is limited.
• Carbon dioxide concentration: Higher concentrations of CO2 mean more photosynthesis, but only up until a certain point as the number of chloroplasts is limited.
• Chlorophyll: Variagated leaves will photosynthesize less than single coloured leaves, lighter coloured leaves will photosynthesize less than darker ones due to the number of chloroplasts available to carry out photosynthesis

The diagram below shows the structure of a leaf

 Each layer has a different function:
The waxy cuticle protects the cell from damage, and prevents water loss.
The upper epidermis is thin and clear, and provides a layer of protection.
The palisade mesophyll is made up of column-shaped palisade cells, which are densly packed with chloroplasts to maximise absorption of light, and therefore photosynthesis.
The spongy mesophyll contains air pockets to increase diffusion in gas exchange and photosynthesis.
The lower epidermis contains stomata and guard cells which control photosynthesis by opening during the day, and closing at night to minimise water loss through evapotranspiration.
The lower wax cuticle provides protection to the underside of the leaf.

Plants don't just need glucose, though. They require mineral ions, which they can absorb from the soil using active transport and diffusion.

A variety of different experiments can be done to test different parts of a flowering plant's nutrition.

Experiment 1: Oxygen and water plants

1. Choose the variable you wish to change (temp, light intensity, etc.) 
2. Place containers of water and water plants of about the same size in the different conditions. 
3. Count the bubbles that are formed in a certain length of time. 

Experiment 2: Starch and light intensity

1. Put 3 leaves from the same plant of similar sizes in different light conditions: One in a dark room, one in direct sunlight, and one in shaded light. Leave for 48 hours.
2. Test leaves for starch by boiling each in water for 1 minute, then placing in ethanol, then returning it to the water and finally spreading on a petri dish. Add iodine solution to see which leaves test positive for starch 

Experiment 3: Chlorophyll
Test variagated leaves for starch. You can see the green parts test positive for starch while the white parts do not.

Experiment 4: Carbon dioxide
Place a plant in a sealed plastic bag with a container of sodalime (which removes CO2), then test for starch.

Experiment 5: Carbon dioxide
Place water plants in different light levels for 12 hours, with hydrogencarbonate indicator. At the start, indicator should be red, then change to purple for low levels of CO2 (In sunlight) and change to yellow for high levels of CO2 (In darkness)

Experiment 6: Mineral ions
Place cuttings of the same plant into different mineral ion solutions: One with a complete ion solution, each of the others missing one ion and one containing just water. Place them together in controlled conditions (light, temp, etc.), then after a week or two check the plants to see any changes.

Humans

Humans gain nutrients from eating food, which has to pass through the digestive tract.
The essential nutrients are:

These are represented in the correct proportions by the eatwell plate:

This gives a good indication of what proportions of food we should be eating, but the total energy intake varies from person to person. Generally, you need more energy if:

• You have more body mass
• You are a man
• You are pregnant
• You are active
• You are a teen. After puberty, energy requirements gradually decrease, and children require less energy than adults. 

Below is a diagram of the digestive system. This is the system within your body where food is digested and nutrients absorbed. But how does this work?

1. First, food is ingested through the mouth.

It is mechanically digested, by chewing, and chemically digested, by salivary amylase. The teeth break up the food to increase its surface area (aiding chemical digestion later on) and to make it easier to swallow in food bolus.
The food is then swallowed, (the epiglottus closes over the trachea to avoid food from falling into the lung) and it travels down the oesophagus through peristalsis: waves of contraction and relaxation of circular and longitudinal muscles. Peristalsis pushes the bolus of food into the stomach.

2. The stomach is a large muscular bag that contracts and relaxes to churn the food. The food is held here for 2-4 hours, during this time mixing with gastric juice (a combination of HCl and pepsin, a form of protease), which breaks down protein in the food. 

The stomach is lined with mucus-producing goblet cells, which helps to prevent the highly acidic HCl from damaging the stomach. Food then passes through into the duodenum.

3. The duodenum is the first part of the small intestine, it is where digestion is completed. Digestive enzymes (Carbohydrases, proteases and lipases) are secreted in pancreatic juice, from the pancreas. 

Bile is also released after being made in the liver and stored in the gall bladder. It emulsifies lipids to increase their surface area. The fully digested nutrients are then transported to the ileum.

4. The ileum is where absorption takes place. The surface of the intestine is folded into tiny villi, which increase the surface area for maximum absorption. It can take place passively, through diffusion, or via active transport. 

The villi contain capillaries and lacteals (lymph vessels) which absorb digested lipids, amino acids and glucose. After being absorbed, a process called assimilation takes place, where the nutrients are used or stored by the body. 

The above diagram is of two villi. 

Villi are small, hair-like protrusions in the lining of the small intestine. Their shape increases the surface area, helping to absorb nutrients more quickly. Each villus is covered in micro-villi, which further increase the surface area.
The walls of the villi are just one cell thick to decrease the distance and increase the rate of absorption.
Villi each contain a lacteal, a vessel connected to the lymphatic system. This absorbs fatty acids and glycerol, then transports them away from the small intestine.
Each villus contains a network of capillaries connected to blood vessels. Glucose and amino acids are absorbed into the bloodstream through them.

5. The remaining material is now passed on to the large intestines, the colon. Here, water and mineral ions are reabsorbed. The leftover undigested food, bacteria, etc. (faeces) is stored in the rectum, then egested via the anus.
The table below shows the most important digestive enzymes to know in this course:

Experiments can be done to determine the energy content of foods:

1. Take a food sample, and light it on fire. Hold beneath a quantity of water with a thermometer in it. 
2. If the sample goes out, quickly relight it. 
3. Note down the temperature rise

Use this equation to calculate the energy content:

energy transferred (J) = mass of water (g) × 4.2 (J/g°C) × temperature increase (°C)