Section 4 - Respiration and Gas Exchange

Question 1

How do plants exchange gases by diffusion (3)

Answer 1

• When a plant is photosynthesising it uses up CO2, so there’s hardly any inside the leaf. Meaning more CO2 diffuses into the leaf (area of high conc to low)
• At the same time, oxygen is being made as a waste product of photosyenthesis
• Some is used up in respiration, and the rest diffuses out through the stomata (area of high conc to low conc)

Question 2

How does the net exchange of gases depend on light intensity (2)

Answer 2

• Photosynthesis only happens during the day (no light at night) however plants respire all the time, to get energy to survive
• During the day, plants make more oxygen by photosynthesis than they use in respiration

Question 3

During the day, what happens to the net exchange of gases (3)

Answer 3

• During the day, there is light, meaning plants can photosyenthesise. They also respire day and night, to get the energy they need to survive
• During the day, plants make more oxygen by photosyenthesis than they use in respiration. So, in daylight, they release oxygen.
• They also use up more carbon dioxide than they produce, so they take in carbon dioxide.

Question 4

At night, what happens to the net exchange of gases (2)

Answer 4

• At night, there is no light, meaning plants can’t photosyenthesise. However they can respire at night.
• At night, plants take oxygen and release carbon dioxide

Question 5

How are leaves adapted for efficent gas exchange (6)

Answer 5

• Leaves are broad, so there’s a large surface area for diffusion
• Leaves are thin, meaning gases only have to travel a short distance to reach the cells where they’re needed
• Air spaces: Let gases like carbon dioxide and oxygen move easily between cells, also increases surface area for gas exchange
• Stomata : Let gases like carbon dioxide and oxygen diffuse out, also allow water to transpire.
• Stomata close in the dark, so they don’t let carbon dioxide in, and don’t allow water to escape
• Opening and closing of the stomata is controlled by guard cells

Question 6

How does hydrogen-carbonate indicator show changes in carbon dioxide concentration (3)

Answer 6

Question 7

Describe an experiment to show how light affects gas exchange (7)

Answer 7

• Add the same volume of hydrogen-carbonate indicator to 4 boiling tubes
• Put three same sized leaves into 3 of the tubes and seal with a rubber bung
• Keep the fourth empty as a control, still seal with a rubber bung
• Completly wrap one tube in aluminium foil, and a second tube in gauze
• Place all the tubes in bright light : this will let plenty of light to the uncovered leaf, a little light to the gauze, and no light to the foil.
• Leave for an hour, and check colour of the indicator :

Control - No Change

Darkened Tube - Yellow (more carbon dioxide) no photosyenthesis

Shaded Tube - No Change; because respiration and photosyenthesis cancel each other out

Well lit - Purple (less carbon dioxide) more photosyenthesis than respiration

Question 8

Describe how a human breathes in (3)

Answer 8

• Intercostal muscles contract
• diaphragm contract
• Thorax volume increases
• Decrease in pressure
• draws air in

Question 9

Describe how a human breathes out (3)

Answer 9

• Intercostal muscles and diaphragm relax
• Thorax volume decreases
• Air is forced out

Question 10

Describe an investigation of the effect of exercise on breathing rate (5)

Answer 10

• Sit still for 5 minutes, then for one minute, count the number of breaths you take
• 4 munutes of running, stop, count breathes for a minute
• Get 3 other people to do the same (compare results)
• Excercise increases breathing rate
• Control : time spent exercising, temperature, same exercise type

Question 11

Why does exercise increase your breathing rate (3)

Answer 11

• Muscles respire more during exercise
• They need to be supplied with more oxygen
• And have more carbon dioxide removed

Question 12

Explain giw alveoli carry out gas exchange in the body (6)

Answer 12

• Lungs contain millions of little air sacs called alveoli, where gas exchange happens
• Blood passing next to the alveoli contains lots of carbon dioxide and little oxygen
• Oxygen diffuses out of the alveolus into the blood, carbon dioxide diffuses out of the blood into the alvelous to be breathed out
• When the blood reaches body cells oxygen is released from red blood cells, and diffuses into body cells
• At the same time, carbon dioxide diffuses out of body cells, into blood, then carried back to the lungs

Question 13

How are alveoli specialised for gas exchange (5)

Answer 13

• Huge number of microscopic alveoli gives the lungs an enormous surface area
• There’s a moist lining for gases to dissolve in
• Thin walls, one cell thick, so gas doesn’t have far to diffuse
• Great blood supply, to maintain a high conc gradient
• Permeable walls, so gases can diffuse across easily.

Question 14

Explain how smocking tobacco can affect your lungs and circulatory system (5)

Answer 14

• Damages walls of the alveoli, reducing surface area for gas exchange, leading to diseases like emphysema.
• Tar in cigs damages the cilia in your lungs and trachea. Makes chest infections more likely
• Tar irritates the bronchi and bronchioles, mucus produced which cannot be ceared by damaged cilia - this causes smoker’s cough and chronic bronchitis
• Carbon monoxide reduces amount of oxygen the blood can carry. Blood pressure increases, damaging artery walls, blood clots more likely.
• Carcinogens, which are chemicals that lead to cancer