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# Gas Exchange

Gas exchange is the physical process by which gases move passively by diffusion across a surface. Oxygen is required in all organisms to release energy in the form of ATP during respiration.

## The process of gas exchange

The gases are transferred between the organism’s internal and external environments. The transfer occurs passively (no energy required), down the concentration gradient. The transfer of gases includes the exchange of oxygen and carbon dioxide during the respiration and photosynthesis processes.

### Diffusion and solubility of gases

The diffusion rate of gasses depends on the surface area, concentration gradient, permeability and thickness of the membrane. Diffusion of gases will happen between two close surfaces. For example, this occurs between the alveoli and capillaries in the lung.

Membrane permeability: ability of substances to move passively through the membrane.

Concentration gradient: difference in the concentration of a substance between two media.

#### Partial pressure and gas exchange

Partial pressure refers to the pressure exerted by a particular gas in the mixture of gases and is used to predict the movement of gases. The gas will move from a high partial pressure area to a lower partial pressure area because the higher the difference between the two environments’ partial pressures, the faster the movement of gases.

So why do gases move down their partial pressure gradient?

Gas will move in random directions due to heat energy and depending on what it crashes into. Gas has equal probability to move in any direction. In case of gas trapped in an enclosed space, such as a container, the spreading out is referred to as diffusion. Imagine that the molecules are on one side of the container, they will move towards the side where there are no gas molecules. This is like in normal diffusion: movement of substance from higher to lower concentration (down the gradient).

Because molecules are in a gas phase, the concentration gradient is exactly the same as partial pressure gradient (assuming that the temperature is not at a gradient). So, gas will move down the gradient i.e. down the partial pressure gradient.

### Mammalian lung in the exchange process

The trachea is a flexible airway supported by the cartilage rings, which prevents the trachea from collapsing when the air pressure inside falls while breathing in. The trachea is divided into two divisions called bronchi. Bronchi then split into a series of bronchioles ending in alveoli - minute air-sacs. The alveolar membrane is the gas exchange surface.

#### Gas exchange in the lung

When you inhale oxygen, it enters the lungs and travels to the alveoli. Cells lining alveoli and capillaries carrying blood are in close contact with each other. The barrier thickness averages to 1 micron - that’s 1/10 000 of a centimetre! Oxygen passes through the barrier into the blood in capillaries. In turn, carbon dioxide passes into the alveoli from the bloodstream and is exhaled.

Oxygen travels from the lungs to the pulmonary veins, which take oxygen to the left side of the heart, where it is pumped to the rest of the body. Deoxygenated blood returns to the right side of the heart and is pumped through the pulmonary artery back to the lungs.

Fig. 1 - The human gas exchange system

1. Large surface area - larger area allows for more effective gas exchange.
2. Alveoli walls are only one cell thick - this means they are in extremely close contact with the capillaries.
3. The alveolar walls are moist - the layer of moisture allows the gases to dissolve more quickly.
4. Alveoli have a good blood supply - due to close contact with capillaries making the gas exchange quicker and more efficient.

Fig. 2 - Movement of gases between alveoli and capillaries

#### Gas exchange process in single-celled organisms

Single-celled organisms have a large surface-to-volume ratio allowing efficient diffusion of gasses. Oxygen diffuses into the cell, and carbon dioxide leaves the cell, followed by respiration. The membrane is entirely permeable to facilitate this exchange. The distance between the internal and the external environments is small enough, and the surface area is large enough for the cell’s needs. No specialised structures are required for the gas exchange.

Fig. 3 - Simple gas exchange in a single-celled organism

### Gas exchange process in insects

Insects have an internal network of tubes called tracheae which divide into smaller tracheoles (the end tubes). There is only a short diffusion pathway from tracheoles - the oxygen is brought directly into the respiring tissues.

Gasses move in three ways:

1. Along the diffusion gradient - oxygen is used up during cell respiration, making the concentration fall and the gradient increase, which causes oxygen to diffuse from the atmosphere. In turn, carbon dioxide increases and diffuses down the concentration gradient - into the atmosphere.
2. Mass transport - muscle contraction can squeeze the air in and out, causing a mass movement, which speeds up the process of exchange.
3. The ends of tracheoles are filled with water - during major activities, muscles can start respiring anaerobically, as well as aerobically. Anaerobic respiration produces lactate which lowers the water potential of the muscle cells. Water will begin moving down the gradient by osmosis into the muscle cells. The water in the tracheoles will reduce in volume, and the final gas diffusion will be in the gas phase rather than liquid. The diffusion will be more rapid.

### Gas exchange process in fish

Fish have gills to facilitate gas exchange due to their small surface area to volume ratio (similar to mammals’ lungs). Gills are made up of gill filaments. Gill filaments are stacked (imagine stacking your notebooks, just like that). They contain gill lamellae which are at a right angle to the filaments. They increase the surface area for gas exchange.

Did you notice that the movement of water over gills and blood flow is in opposite directions in Figure 6? This is the countercurrent exchange system.

#### Countercurrent exchange system

The countercurrent exchange system allows the blood to be well-loaded with oxygen when it meets water. Diffusion will occur, and oxygen will move into the blood. Blood with little oxygen will meet water which has the most oxygen. Take a look at Figure 7; what if the flow was parallel instead of countercurrent? The blood would absorb a much lower percentage of 50% of available oxygen, which is why fish prefer to stick to the countercurrent flow, wouldn’t you?

### Gas exchange process in the leaf of a plant

Gas exchange in the leaf occurs in the gas phase quicker than in water. Living cells are in close contact with the source of oxygen and carbon dioxide (air).

Leaves, as the main gas exchange surfaces, have adaptations for rapid diffusion:

• Stomata - tiny pores on the leaf’s surface, which allows close air contact with the cells.
• Interconnecting air-spaces throughout mesophyll (the internal ground tissue located between the two epidermal cell layers of the leaf) - gases can quickly contact the mesophyll.
• Large surface area

## Factors affecting the rate of diffusion of gases

• The membrane thickness - the thinner the membranes, the faster the diffusion will be. For example, the barrier between alveoli and capillaries is only one cell thick in the lungs.
• The membrane surface area - larger the surface area, more gas exchange can occur. For example, fish contain stacks of gill filaments and gill lamellae, increasing the surface area for exchange.
• The pressure difference across the membrane - more pressure, more gases will diffuse. For example, mass transport in insects, muscle contractions can push gases.
• High diffusion gradient - for example, countercurrent exchange system in fish.

## Gas Exchange - Key takeaways

• Gas exchange is the physical process by which gases move passively by diffusion across a surface.

• Living organisms have developed different adaptations to facilitate efficient gas exchange to survive.

• For example, mammals have a lung system, fish have gills with the countercurrent flow system, and single-celled organisms rely on the diffusion of gasses in and out of the cell.

• Factors affecting the rate of gas exchange are membrane thickness over which gases have to diffuse, the surface area of the membrane, the pressure difference across the membrane and the steepness of the diffusion gradient.

Gas exchange is the physical process by which gases move passively by diffusion across a surface.

Gas exchange will take place across a surface. Depending on the organism, this surface will differ. Let’s use lungs in mammals as an example. Alveoli (the end air-sacs in the lungs) are the gas exchange surface.

Alveoli have certain adaptations to facilitate efficient gas exchange.

- Large surface area

- Alveoli walls are only one cell thick which allows them to be in close contact with surrounding capillaries.

- The layer of moisture in the alveoli allows gases to diffuse more quickly.

- Alveoli have a good blood supply due to close contact with capillaries making the gas exchange quicker and more efficient.

The trachea, a flexible airway supported by the cartilage rings which prevents the trachea from collapsing when air pressure inside falls when breathing in. The trachea is divided into two divisions called bronchi. Bronchi then divides into a series of bronchioles, ending in alveoli - minute air-sacs. Alveoli have a large surface area and are in close contact with capillaries. This allows rapid gas exchange between the bloodstream and the lungs.

Gas exchange allows the organisms to replenish the oxygen and eliminate the carbon dioxide. In the case of plants during photosynthesis, it allows release of oxygen and replenishment of carbon dioxide (plants also need oxygen, just like animals, for respiration). Oxygen is used to release energy as ATP during respiration.

## Final Gas Exchange Quiz

Question

Why will fish use a countercurrent flow system for gas exchange, rather than a parallel one?

The countercurrent exchange system means blood is already well-loaded with oxygen when it meets water. This is because diffusion will occur, and oxygen will move into the blood. Blood with little oxygen will meet water which has the most oxygen. If fish used a parallel exchange system, only 50% of oxygen would diffuse into the blood (lower diffusion gradient). The system would be less efficient.

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Question

What is the gas exchange surface in the mammalian lungs?

Alveoli is the exchange surface of the lungs. They are in close contact with capillaries - this is where the gas exchange takes place.

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Insects use active transport for gas exchange. True or false?

False.

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What is the main adaptation of single-celled organisms for gas exchange?

Large surface area.

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What are the small pores on the surface of the leaf called and how are they important?

Stomata. They allow close air contact with the cells. Gas exchange can take place at a more rapid rate.

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Gas exchange is more rapid in water. True or false?

False.

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Single-celled organisms do not require a specialised gaseous exchange system. Why is this?

The distance between the external and the internal environments is small enough and the surface area is large enough to facilitate an efficient gaseous exchange.

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What are the three factors affecting the rate of gaseous exchange?

Surface area, concentration gradient and permeability of the membrane.

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Question

FIll in the blanks about the partial pressure. Partial pressure is the pressure _______ by a particular gas in the mixture of gases. It is used to predict the ________ of gases. The gas will move from an area of a _______ partial pressure to an area of a _________ partial pressure. The higher the _________, the faster the diffusion.

Partial pressure is the pressure exerted by a particular gas in the mixture of gases. It is used to predict the movement of gases. The gas will move from an area of a high partial pressure to an area of a lower partial pressure. The higher the pressure, the faster the diffusion.

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Insects have an internal network of tubes called bronchi which divide into smaller bronchioles. True or False?

False

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What are the three ways that gases can move in insects?

2. Mass transport

3. The water in the tracheoles will reduce in volume and the final gas diffusion will be in the gas phase, rather than liquid. The diffusion will be more rapid.

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Question

Stem of a plant is the main gas exchange surface. True or False?

False

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What are the three adaptations that facilitate efficient gas exchange in plants?

Stomata, air-spaces throughout mesophyll, leaves have a large surface area.

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Why do we require gas exchange?

We need oxygen for aerobic respiration, to release energy in the form of ATP.

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Question

Fill in the blanks about gas exchange in fish. Fish have _______ to facilitate gas exchange. They are made up of _________ __________. These structures are stacked and contain _________ __________ which increase the surface area for gas exchange.

Fish have gills to facilitate gas exchange. They are made up of gill filaments. These structures are stacked and contain gill lamellae which increase the surface area for gas exchange.

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The lungs are specifically adapted for __________.

Gas exchange

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Why do humans need oxygen?

Oxygen is used in aerobic respiration to generate ATP.

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Why do humans need to expire carbon dioxide?

Carbon dioxide is produced as a by-product of aerobic respiration. Carbon dioxide needs to be removed as its buildup is toxic to cells.

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What is the trachea?

The trachea is the main airway of the lung. It is lined with ciliated epithelium and contains C shaped rings of cartilage that prevent it from collapsing.

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The trachea splits into two _________.

Bronchi

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Each bronchus branches off to _________.

Bronchioles

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The bronchioles end in ___________.

Alveoli or air sacs

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What are the 3 important adaptations of the human gas-exchange surface?

The three most important adaptations are short diffusion distance, large surface area, and a maintained steep concentration gradient.

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Question

Give two examples of COPD.

Chronic bronchitis, and emphysema.

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What are the symptoms of lung cancer?

Some symptoms of cancer include persistent cough, coughing up blood, weight loss, back pain, and difficulty breathing.

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Name some risk factors for COPD.

• Smoking (smoking is also known to increase the risk of developing lung cancer).
• Air pollution.
• Genetics (some people are genetically more likely to develop lung disease while some other people are less likely).
• Infections (frequent chest infections are known to increase the risk of developing COPD).
• Occupation (working with harmful chemicals and gases also increase the risk of COPD and lung cancer)

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Question

Where does gas exchange occur in the human respiratory system?

Gas exchange occurs in the alveoli of the lungs.

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It’s a triad of risk factors predisposing the development of asthma. They include a family history of asthma, allergies such as hay fever, and eczema.

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Is asthma a type of COPD?

No

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What diseases are included in COPD?

Emphysema and chronic bronchitis

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Water loss mostly occurs by evaporation in humans. How do plants mostly lose water?

By transpiration.

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Give two examples on why water is important in plants.

Photosynthesis and movement of ions.

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The body of an insect is covered by a waxy cuticle that is made of cellulose. True or False?

False.

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Fill in the blanks about spiracles in insects.

Spiracles are small openings in the ________, at the body ________ of an insect. They are usually found around the ________ area. Oxygen and carbon dioxide are ________ at the spiracles. Water loss will occur when the spiracle is _______. In order to limit the water loss, when the insect is resting, it will ______ its spiracles. Due to the lowered activity, the need for _______ is reduced. Spirales have _______ which will _______ and ________. Spiracles also have _______ that will trap the water ________. This maintains the ________.  Preventing water loss will ensure that the gas exchange surfaces will remain _______.

Spiracles are small openings in the tracheae, at the body surface of an insect. They are usually found around the abdominal area. Oxygen and carbon dioxide are exchanged at the spiracles. Water loss will occur when the spiracle is open. In order to limit water loss, when the insect is resting, it will close its spiracles. Due to the lowered activity, the need for oxygen is reduced. Spiracles also have valves which will open and close. Spiracles also have hairs that will trap the water vapour. This maintains the humidity. Preventing water loss will ensure that the gas exchange surfaces will remain moist.

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Question

What would happen if the insect’s cuticle was utterly impermeable?

Water loss will be reduced, however, the gas exchange would not take place and the insect will die.

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Plants transport water and ions via phloem. True or False?

False.

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Question

Give examples of four adaptations in xerophytes to limit water loss.

Small SA:Vol ratio, thick cuticle, rolled leaves, CAM physiology. (any other adaptation).

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Define a halophyte.

Halophyte is a plant that has adapted to live in salty conditions.

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Question

Explain how rolling of the leaves will reduce water loss from the plant. Give an example of a plant that rolls its leaves.

Rolling of the leaves will trap water vapour and create a high water potential. When the water potential is the same in both internal and external environments, therefore, there will be no loss of water. Plants such as Marram grass have this adaptation.

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What is meant by CAM physiology in plants?

CAM is a carbon fixation pathway which has evolved to reduce water loss. A plant using CAM, will photosynthesise during the day and exchange gases during the night.

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Why would a halophyte alter its flowering season? Give an example of a cue to flower.

Halophytes live in salty conditions. Flowering will require extra energy which is used for salt exclusion etc. A halophyte will flower during the rainy season, when the salt content in the soil is lower.

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Fill in the blanks about water loss control in humans.

_______ regulate the amount of water and ions ________ from the body. This process is referred to as ___________. __________ that modulate __________ are found in the _________ of the brain. The_________ gland, which is attached to the __________, will control how much _________ is produced by the kidneys and water _________ in the bloodstream. It will control this by releasing ___________.

Kidneys regulate the amount of water and ions removed/excreted from the body. This process is referred to as osmoregulation. Osmoreceptors that modulate osmolality are found in the hypothalamus of the brain. The pituitary gland, which is attached to the hypothalamus, will control how much urine is produced by the kidneys and water concentration/content in the bloodstream. It will control this by releasing hormones.

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What is an example of a symptom that you need to drink more water?

Dry mouth.

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What is the most external surface of the insect’s cuticle called?

Epicuticle.

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Large surface area to volume ratio is great for reducing water loss in an insect. True or False?

False.

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Question

What substances can individuals smoke?

Tobacco, cannabis, methamphetamine, heroin.

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Identify the 2 hormones that nicotine triggers the release of.

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What are the main causes of smoking?

The main causes of smoking include anxiety, stress, depression, media influence and peer influence.

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What fertility complications can tobacco smoke trigger?

Males can have reduced sperm motility, sperm count and increase sperm DNA mutations.

In females, tobacco smoke increases the likelihood of an ectopic pregnancy and interrupts the egg maturation process.

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Briefly describe ectopic pregnancies.

Ectopic pregnancies occur when a fertilised egg implants in tissues outside of the uterus and this includes the fallopian tubes, ovaries or the cervix.

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