(3) Breathing and gas exchange

What is the role of diffusion in gas exchange (plants) ?

When plants photosynthesise, they use up CO2 from the atmosphere and produce O2 as a waste product - CO2 diffuses into the leaf down a concentration gradient, as CO2 is being used up therefore there is a lower concentration of CO2 inside the leaf then outside - this means that the CO2 can be used in photosynthesis. Similarly, the O2 produced diffuses out of the leaf down a concentration gradient. This diffusion occurs through stomata on the lower epidermis of a leaf.

The reverse occurs with respiration.

Describe the net movement of gases in respiration (plants)

Net movement of O2 into the leaf, and net movement of CO2 out of a leaf

Describe the net movement of gases in photosynthesis

Net movement of CO2 into the leaf, and net movement of O2 out of the leaf

List the adaptations of the leaf for gas exchange

- Leaves are broad, so there's a large surface area for diffusion. -Leaves are thin, so there is a short diffusion distance for the gases to reach the cells where they are needed. -There are air spaces inside the leaf. This allows gases to move easily between cells and also increases the surface area for gas exchange. -Lots of stomata in the lower epidermis which allow Carbon dioxide and Oxygen to diffuse in and out. -Stomata close when it is dark as photosynthesis can't happen at night so Carbon dioxide doesn't need to be let in. When the stomata are closed, water can't escape so it prevents the plant from drying out. Stomata also close when supplies of water from the roots start to dry up this stops the plant from photosynthesising but if the stomata didn't close, the plant would dry up and die. -The opening and closing of stomata is controlled by the cells that surround them (guard cells) Guard cells change their shape and volume by absorbing water to open the stomata and releasing water to close the stomata

What is the role of the stomata in gas exchange?

Allow CO2 and O2 to diffuse in and out of the leaf

Where are the stomata found?

Mostly on the lower epidermis of plants' leaves

Although some are also located on the upper

What controls the size of the stomata?

The volume of the guard cells

Describe the process by which a stoma opens

The guard cells fill with water and become turgid, and move apart from each other, opening the stoma

What is the net movement of gases in the day (plants)?

Net production of O2, and net intake of CO2 - both respiration and photosynthesis occur, however photosynthesis is producing O2 at a greater rate than respiration is using it

What is the net movement of gases in the night (plants)?

Net intake of O2 and net production of CO2, as respiration must still occur to produce energy for the plant's metabolic processes to keep it alive, however no photosynthesis can occur as this requires light

When does photosynthesis occur?

During the day when sunlight is available

When does respiration occur and why (plants) ?

All the time - needed to produce energy for the plant's metabolic processes to keep the plant alive

Describe the steps in the practical to investigate the effect of light on the net gas exchange from a leaf

1. Place the same volume of hydrogen carbonate indicator in for boiling tubes

2. Add the same size, same species, same age and healthy looking leaves to 3 of the tubes, and seal with a rubber bung (trap the leaf stem with this to prevent it falling down into the solution) - leave one tube with no leaf - this is the control

3. Wrap one tube in aluminium foil, another in gauze, and leave the other tube and the control uncovered

4. Leave all tubes in bright light for one hour

What should the results of the practical to investigate the effect of light on gas exchange be?

Tube 1, the control, should remain orange - no photosynthesis or respiration should have occurred therefore CO2 concentration remains normal

Tube 2, in foil, should turn from orange to yellow - no light, therefore no photosynthesis should have taken place, therefore only respiration occurred, producing CO2 and increasing CO2 concentration in the tube

Tube 3, in gauze, should have remained orange - a little photosynthesis and respiration should have occurred, therefore roughly equal amounts of CO2 will be taken up and produced by the leaf, so the CO2 concentration in the tube won't change much

Tube 4, uncovered, should have turned from orange to purple - there will be some respiration, but photosynthesis at a far greater rate, therefore there would be a net uptake of CO2 by the leaf, lowering CO2 concentration in the tube

Describe the colour changes of hydrogencarbonate indicator

Orange in normal CO2 concentration
Yellow in high CO2 concentration
Purple in low CO2 concentration

What must we keep the same in the experiment to measure the effect of light on gas exchange in plants?

Same size of leaf
Same species of leaf
Same age of leaf
Same health of leaf
Same volume of hydrogen carbonate indicator
Same temperature

What indicator do we use in the experiment to measure the effect of light intensity on gas exchange in leaves?

Hydrogencarbonate indicator

What does net exchange of gases depend on?

Light intensity

What is the thorax?

- The top part of your body.
- Separated from lower part of body by diaphragm.

What are the lungs surrounded by?

- Pleural membrane.
- Protected by ribcage.

Describe the journey of air that is breathed in.

- Through trachea.
- Splits bronchi - each going into each lung.
- Bronchi splits into bronchioles.
- Bronchioles into alveoli where gas exchange takes place.

What does the trachea contain?

- Rings of cartilage or gristle.

Describe the pleural membrane.

- Continous envelope around lungs forming airtight seal.
- Pleural cavity filled with pleural fluid between the two membranes.

What is the diaphragm?

- Muscular sheet of tissue.
- Separates thorax from abdomen.

What are the intercostal muscles?

- Muscles which run between the ribs.
- Two sets of muscles.

What happens when breathing in?

- Intercostal muscles contract.
- Diaphragm contracts and flattens.
- Thorax volume increases.
- Pressure decreases, drawing air in.

What happens when breathing out?

- Intercostal muscles relax.
- Diaphragm relaxes and forms dome shape.
- Thorax volume decreases.
- Pressure increases, forcing air out.

Define respiration.

- Chemical oxidation reaction that releases energy from foods such as glucose.

Define breathing.

- The mechanism that moves air into and out of the lungs, allowing gas exchange to take place.

Describe the airways and trachea.

- Lined with cells to keep them clean.
- Some secrete mucus (sticky liquid).
- Others are covered cilia: with tiny hair-like structures.
- Cilia beat backwards and forwards, sweeping mucus and trapped particles out towards the mouth.
- Dirt and bacteria are prevented from entering the lungs.

What does mucus do?

- Traps particles of dirt or bacteria that are breathed in.

Describe the overall trend in gases in the atmospheric air and breathed out.

- Nitrogen: remains the same.
- Oxygen: less exhaled than atmospheric.
- Carbon dioxide: more exhaled than atmospheric.
- Others: remains the same.

What do the lungs contain millions of?

- Alveoli.
- Where gas exchange takes place.

Describe the blood that passes near the alveoli.

- Blood in capillaries passing next to alveoli has just returned to the lungs from the rest of the body.
- Blood is deoxygenated, contains lots of CO2 and little O2.

Describe gas exchange.

- Oxygen diffuses out of alveolus to blood. (high->low)
- Carbon dioxide diffuses out of blood to alveolus. (high->low).

What happens when oxygenated blood reaches the body cells?

- Oxygen is released from red blood cells and diffuses into body cells. (high->low)
- Simultaneously, carbon dioxide diffuses out of body cells into blood to be carried to lungs.

Define diffusion.

The net movement of particles from a region of higher concentration to a region of lower concentration down a concentration gradient.

How are alveoli adapted for gas exchange?

- Large number of alveoli gives lungs huge SA.
- Higher SA:V means higher diffusion rate: faster gas exchange.
- Moist lining for gases to dissolve in.
- Thin walls -one cell thick- short diffusion distance, higher diffusion rate.
- Great blood supply to maintain high concentration gradient.
- Permeable walls: gases diffuse across easily.

Why is gas exchange needed?

- Respiration (oxygen).
- For removal of waste carbon dioxide from the blood.

What is smoking associated with?

- Lung cancer.
- Bronchitis.
- Emphysema

How does smoking affect cilia?

- Tar damages cilia in lungs and trachea.
- Increases change of chest infections.
- Mucus not swept away from lungs, remains to block air passages.

How is bronchitis formed?

- Lung disease.
- Irritation of bronchial tree and infections from bacteria in mucus.
- Tar irritates lining of airways (bronchi/ bronchioles).
- Stimulates cells to secrete more mucus (can't be cleared with cilia.)
- Blocks normal air flow.

What is smoker's cough?

- Sticky mucus blocking airways.

How is emphysema formed?

- Lung disease with no cure.
- Smoke damages walls of alveoli.
- Walls break down and fuse together.
- Forms enlarged and irregular air spaces.
- Reduces SA for gas exchange: becomes inefficient.
- Reduces oxygen concentration in blood.

How does smoking cause cancer?

- Tobacco smoke contains carcinogens.

What are carcinogens?

- Chemicals that can lead to cancer.
- Increases chance of getting lung cancer.

How does carbon monoxide affect the body?

- CO binds with haemoglobin much stronger than oxygen does.
- Forms carboxyhaemoglobin which is permanent.
- Blood carries less oxygen around the body.
- Leads to heart disease.

- Body increases heart rate to make up for decrease in oxygen, this increases blood pressure.
- Increased blood pressure damages artery walls, increases chance of blood clots.
- Increases risk of coronary heart disease.

Describe the test for carbon dioxide.

- Limewater is a colourless solution.
- Goes cloudy with carbon dioxide.

Describe the method for investigating release of CO2 in breath.

- Set up 2 boiling tubes with equal volumes of limewater.
- Put mouth around mouthpiece, breathe in/out several times.
- Breathing in, air from room is drawn through tubes. Air contains little carbon dioxide, limewater colourless.
- Breathing out, exhaled air bubbles through other test tube. Air contains CO2 produced in respiration so limewater cloudy.

Describe the results from the CO2 breath experiment.

- Since one test tube (A) remains clear, we know CO2 in exhaled air was from respiration.
- Wasn't inhaled through A because if so, then A would've gone cloudy.

Describe the method for the exercise effect practical.

- Sit still for 5 mins, then count number of breaths taken in in 1 min. Wait a min then count again .
- Now do 4 mins of vigorous exercise (eg. running). Immediately after count breaths for 1 min and record.
- Repeat until you get mean of results and steady values.

Describe the results seen from the exercise experiment.

- Breathing rate increased with exercise.
- Muscles respire more during exercise (need more energy) thus need to be supplied with more oxygen and have more carbon dioxide removed.
- Breathing rate increases.

Describe the control variables in the exercise experiment.

- Time spent exercising using stop watch.
- Temperature of room using AC or thermostat.
- Volume of water in between.
- Food intake.