MYP Biology 4.4 Gas Exchange Notes

What Is Gas Exchange?

1. Gas exchange is the movement of oxygen into an organism and carbon dioxide out of it.
2. All living cells need oxygen for aerobic respiration and must remove carbon dioxide to maintain healthy internal conditions.
3. Gas exchange happens wherever gases cross a specialized surface, such as alveoli, gills, tracheoles, or stomata.

What Makes an Effective Gas Exchange Surface?

1. Effective gas exchange surfaces share structural features that maximise the rate at which gases can move across them.
2. Across organisms, these adaptations tend to follow the same principles:
   1. Large surface area: Allows more gas to diffuse across at the same time.
   2. Very thin barriers: Reduces the distance gases must travel.
   3. Moist surfaces: Allows gases to dissolve before crossing membranes.
   4. Steep concentration gradients: Maintained by blood flow or ventilation to keep gases moving in the right direction.

How Does Gas Exchange Occur in Human Lungs?

1. Human lungs exchange gases in tiny air sacs called alveoli that sit next to an extensive capillary network.
2. Oxygen moves from the air inside each alveolus into the blood, while carbon dioxide moves from the blood into the alveolus to be exhaled.
3. Key structural features:
   1. Millions of alveoli provide a large surface area
   2. Alveolar and capillary walls are each one cell thick
   3. Moist surfaces allow gases to dissolve
   4. Blood flow and ventilation maintain strong concentration gradients

How Do Fish Exchange Gases in Water?

1. Fish exchange gases using gills, which are specialized structures adapted to extract oxygen from water.
2. Gills contain thin filaments covered in lamellae, greatly increasing surface area.
3. Blood flows through the gills in the opposite direction to the flow of water, maintaining a gradient that maximizes oxygen uptake.
4. Key structural features:
   1. Thin filaments for short diffusion distance
   2. Large surface area from many lamellae
   3. Continuous water flow across the gills
   4. Counter-current flow to maintain steep gradients

How Do Insects Exchange Gases Without Using Blood?

1. Insects exchange gases using a tracheal system that delivers air directly to their tissues.
2. Air enters through openings called spiracles and travels through branching tubes (tracheae and tracheoles) that reach individual cells.
3. Oxygen moves directly into tissues, and carbon dioxide moves directly out, bypassing the circulatory system entirely.
4. Key structural features:
   1. Spiracles regulate gas entry and exit
   2. Tracheae transport air deep into the body
   3. Tracheoles reach individual cells for direct gas exchange

How Do Plants Exchange Gases?

1. Plants exchange gases through stomata on their leaves and air spaces within mesophyll tissue.
2. Carbon dioxide enters for photosynthesis, while oxygen exits as a by-product.
3. During respiration, the direction of movement reverses depending on concentrations inside the leaf.
4. Key structural features:
   1. Stomata allow gases to enter and exit
   2. Guard cells regulate stomatal opening
   3. Spongy mesophyll provides air spaces for gas movement
   4. Moist cell surfaces enable gas diffusion

Why Is Gas Exchange Essential for Respiration?

1. Gas exchange supplies the oxygen needed for aerobic respiration and removes carbon dioxide produced during the process.
2. Without efficient gas exchange, oxygen levels fall, carbon dioxide accumulates, and cells cannot release enough energy to function.