Gas Exchange (Edexcel IGCSE Biology)

At night oxygen diffuses into plant cells while carbon dioxide diffuses out. This is because respiration is taking place at a higher rate than photosynthesis so oxygen is being used faster than it is produced, while carbon dioxide is produced faster than it is used.

During photosynthesis carbon dioxide diffuses into photosynthesising tissues, and oxygen diffuses out. This is because photosynthesising plant tissues are carrying out photosynthesis at a higher rate than respiration.

Gases involved in gas exchange at the leaf are:

• Carbon dioxide

• Oxygen

• Water vapour

The spongy mesophyll is the region of the leaf where gas exchange takes place. Gases are exchanged between the cells of the spongy mesophyll layer and the surrounding air spaces.

Some photosynthesis also occurs here.

True.

The air spaces increase contact between the cells of the leaf and the surrounding air. Gases can diffuse in and out of the spongy mesophyll cells from and into the air spaces.

Stomata are small pores present in the epidermis of leaves that allow the movement of gases into and out of leaves by diffusion.

False.

Being thin decreases the diffusion distance for gas exchange in leaves.

Adaptations of leaves for gas exchange include:

• They are thin, reducing the diffusion distance for gases

• They are large and flat, increasing their surface area

• Stomata allow gases to move in and out

• Air spaces around mesophyll cells increase the contact between cells of the leaf and the surrounding air

Stomata (singular stoma) are pores in the leaf epidermis through which gases can diffuse. Each stoma is surrounded by a pair of guard cells.

False.

Stomata are predominantly found on the lower epidermis of the leaf in most plants.

When stomata are open the rate of photosynthesis will increase because carbon dioxide can diffuse into the leaf. When stomata close photosynthesis will stop due to low carbon dioxide availability.

Stomata close due to low water availability or at low light intensity.

True.

Stomata open when there is plenty of water; this allows gases to be exchanged, increasing the rate of photosynthesis.

False.

Plants can only photosynthesise when they have access to light, but plant cells respire all the time.

Net diffusion is the overall movement of molecules when they move from an area of higher concentration to an area of lower concentration.

There is net diffusion of carbon dioxide into the plant and net diffusion of oxygen out of the plant during the day.

There is net movement of oxygen into the plant and net diffusion of carbon dioxide out of the plant during the night.

True.

During the daytime there is net diffusion of carbon dioxide into the plant. This is because light intensity is high and photosynthesis is occurring at a higher rate than respiration.

Gas exchange in plants is different during the night and day time because of changing light intensity. When light intensity is high during the day plants photosynthesise faster than they respire, but the reverse of this is true at night.

Hydrogen carbonate indicator is used to study the effect of light intensity on gas exchange in plants by indicating changes in carbon dioxide levels.

Purple hydrogen carbonate indicator shows that there is less carbon dioxide than atmospheric levels, suggesting that carbon dioxide is being used in photosynthesis faster than it is being produced in respiration.

Yellow hydrogen carbonate indicator shows an increase in carbon dioxide above atmospheric levels. This suggests that carbon dioxide is being produced faster than it is being used, therefore respiration is occurring at a higher rate than photosynthesis.

Orange/red hydrogen carbonate indicator shows that carbon dioxide remain at atmospheric levels, suggesting a balance between photosynthesis and respiration rates.

False.

When investigating the effect of light intensity on net gas exchange in plants using hydrogen-carbonate indicator a control tube is to show that any colour change occurs due to the effect of light intensity on gas exchange and not any other factor (e.g. that the indicator wouldn't just change colour on its own).

True.

The tube is wrapped in foil to block out light, so we would expect it to turn yellow. This indicate an increase in carbon dioxide above atmospheric levels, which occurs because respiration will be taking place faster than photosynthesis.

True.

A tube wrapped in gauze will be exposed to low light intensity, so would be expected to remain orange/red. This indicates no net change in carbon dioxide levels due to balanced photosynthesis and respiration.

The variable that is changed when investigating the effect of light intensity on net gas exchange in plants using hydrogen-carbonate indicator is the light intensity received by each tube.

This is part of the CORMMS criteria for planning an investigation.

When investigating the effect of light intensity on net gas exchange in plants using hydrogen-carbonate indicator the variable being measured is the colour of the hydrogen carbonate indicator after 30 minutes.

This is part of the CORMMS criteria for planning an investigation.

The factors that should be kept the same when investigating the effect of light intensity on net gas exchange in plants using hydrogen-carbonate indicator are:

• Volume of hydrogen carbonate indicator

• Number of leaves

• Temperature of the environment

This is part of the CORMMS criteria for planning an investigation.

True.

The lungs are the gas exchange surface in humans.

The trachea, or windpipe, connects the nose and mouth to the lungs.

Alveoli are tiny air sacs surrounded by a network of capillaries, serving as the site of gas exchange between the air and the blood.

The bronchi (singular bronchus) branch off the trachea and carry air into the right and left lungs. They contain mucus to trap dust and pathogens.

False.

The function of the ribs is to protect the lungs and heart, and to aid in ventilation.

Pleural membranes surround the lungs inside the ribs. They provide protection by reducing friction between the lungs and the ribs during breathing.

The intercostal muscles are located between and around the ribs. There are two layers of intercostal muscles; the external intercostals are outside the rib cage and the internal intercostals are inside the ribs.

False.

The diaphragm is a sheet of connective tissue located beneath the lungs. It is involved with changing the volume inside the thorax during breathing.

The function of the external intercostal muscles is to pull the ribcage up. This occurs when the external intercostal muscles contract during inhalation.

The diaphragm is a thin sheet of muscle that separates the chest cavity (thorax) from the abdomen and plays a crucial role in ventilation.

False.

During inhalation the volume of the chest cavity increases.

During exhalation the diaphragm relaxes and moves upwards into its domed shape, decreasing the volume of the chest cavity.

During exhalation the pressure inside the chest cavity increases as the ribs move down and in and the diaphragm moves up. This causes air to be forced out of the lungs.

The function of the internal intercostal muscles is to pull the ribcage down. This occurs when the internal intercostal muscles contract during forced exhalation.

The process that takes place in the alveoli is gas exchange.

False.

Alveoli and surrounding capillaries are surrounded by single layers of cells to minimise diffusion distance.

True.

Gases dissolve in the layer of moisture on the surface of the alveoli, aiding diffusion.

Alveoli are an efficient gas exchange surface because:

• They have a large total surface area

• They provide a short diffusion distance

• They have a good blood supply and good ventilation, maintaining a steep concentration gradient

• They are lined with a layer of moisture in which gases can dissolved

A steep concentration gradient for both oxygen and carbon dioxide is maintained in the alveoli as follows:

• Good ventilation means that a high concentration of oxygen and a low concentration of carbon dioxide is maintained inside the alveoli

• A good blood supply ensures that blood is constantly flowing; this means that the blood immediately next to each alveolus is always low in oxygen and high in carbon dioxide

Emphysema can affect the lungs as follows:

• Reduce the surface area of the alveoli, so reducing gas exchange

• Reduce the elasticity of the alveoli, meaning that breathing is less efficient

• Thicken the alveolar lining, increasing the diffusion distance for gas exchange

Carbon monoxide binds to haemoglobin and reduces the oxygen-carrying capacity of the blood.

False.

Smokers tend to have a higher incidence of lung infections than non-smokers. This is because tar can damage the cilia in the airways, leading to a build-up of mucus and pathogens which can cause infection.

A carcinogen is a substance that causes cancer. E.g. tar in cigarette smoke is known to be a risk factor in the development of lung cancer.

True.

Smoking is one of multiple risk factors in coronary heart disease. This is because smoking can raise blood pressure, increasing the chance of damage to the blood vessel walls.

True.

Exercise causes the frequency of breathing to increase in order to provide more oxygen for respiration.

Breathing rate is measured in breaths per minute. To calculate a student's breathing rate at rest, either:

• Count breaths over a minute

• Count breaths for 30 seconds and multiply by 2

• Count breaths for 15 seconds and multiple by 4

Repeat several times to obtain an average.

When investigating the effect of exercise on breathing rate, repeating the investigation for each student allows several measurements to be taken. This allows any anomalies to be identified and missed out when calculating a mean.

Variables that should be kept the same when investigating the effect of exercise on breathing rate include:

• The age and general fitness level of the students

• Students have all eaten similar meals at the same time prior to exercise

• The type of exercise

• The temperature of the room

This forms part of the CORMMS criteria for planning an investigation.

To replicate exercise intensity consistently, give students an exercise type where intensity can be measured, such as running at a certain speed on a treadmill or cycling with a specific power output on a watt bike.

When investigating the effect of exercise on breathing rate the effects of fatigue on results can be limited by ensuring that students take a period of rest in between each repeat.