IB MYP 4-5 Biology-Gas Exchange- Study Notes - New Syllabus
IB MYP 4-5 Biology-Gas Exchange- Study Notes – New syllabus
IB MYP 4-5 Biology-Gas Exchange- Study Notes – IB MYP 4-5 Biology – per latest IB MYP Biology Syllabus.
Key Concepts:
- Alveoli structure and function
- Gill lamellae in fish
- Stomata in plants
Gas Exchange – Alveoli Structure and Function
What Are Alveoli?
- Alveoli (singular: alveolus) are tiny air sacs found at the end of the bronchioles in the lungs. They are the site of gas exchange – where oxygen enters the blood and carbon dioxide is removed from it.
- Think of alveoli as microscopic balloons that inflate and deflate with each breath, allowing gases to move in and out of the bloodstream.
Structure of Alveoli
- Very small (about 0.2 mm in diameter) but present in millions, creating a large surface area.
- Surrounded by a dense network of capillaries (tiny blood vessels).
- Lined with a single layer of flattened epithelial cells for rapid diffusion.
- Moist lining helps dissolve gases to ease diffusion.
- Elastic fibers allow alveoli to stretch during inhalation and recoil during exhalation.
Function of Alveoli
The main function of alveoli is gas exchange:
Oxygen from inhaled air diffuses across the alveolar wall into nearby capillaries and enters the bloodstream.
Carbon dioxide diffuses from the blood into the alveoli to be exhaled.
This exchange occurs by simple diffusion, driven by concentration differences (high oxygen in alveoli, low in blood, high carbon dioxide in blood, low in alveoli).
Diffusion Process Clarification:
Gas exchange at the alveoli occurs by diffusion due to the difference in partial pressure of gases. Oxygen moves from the alveoli (high oxygen concentration) into the blood (low oxygen), while carbon dioxide moves from the blood (high carbon dioxide) into the alveoli (low carbon dioxide).
Blood Oxygen Saturation Relevance:
- Oxygen entering alveoli binds with hemoglobin in red blood cells forming oxyhemoglobin, which is essential for efficient oxygen transport.
Transport Function:
Red blood cells are adapted for gas transport with:
- Biconcave shape (large surface area for gas exchange),
- No nucleus (more space for hemoglobin),
- Flexible membrane (to pass through capillaries).
How the Alveoli Are Adapted for Efficient Gas Exchange
| Adaptation | Function/Benefit |
|---|---|
| Large surface area (due to millions of sacs) | Increases area for gas exchange |
| Thin walls (1 cell thick) | Short diffusion distance for gases |
| Rich blood supply (dense capillary network) | Maintains a concentration gradient for oxygen and CO₂ |
| Moist lining | Helps dissolve gases so they can diffuse more easily |
| Elastic walls | Allow expansion and recoil during breathing |
Importance of Alveoli
Alveoli are essential for respiration – without efficient gas exchange, cells would not get oxygen or be able to remove carbon dioxide.
Damage to alveoli (as seen in conditions like emphysema or COVID-19 pneumonia) can severely affect breathing and gas exchange efficiency.
Gill Lamellae in Fish
What Are Gills?
Gills are the specialized gas exchange organs in fish. Just like humans use alveoli in the lungs, fish use gill lamellae to absorb oxygen from water and expel carbon dioxide.
Structure of Fish Gills
- Fish typically have four pairs of gill arches on each side of their head.
- Each gill arch holds:
- Gill filaments – soft, feathery structures that increase surface area.
- Gill lamellae – thin, plate-like structures that sit on the filaments and are the actual site of gas exchange.
- Lamellae are like thin pages stacked on a book (the filament) – providing massive surface area for diffusion.
Structure of Gill Lamellae
- Very thin, ensuring a short diffusion distance between water and blood.
- Rich in capillaries for efficient exchange of gases.
- Moist surface helps dissolve gases before diffusion.
Function of Gill Lamellae
The main function of the lamellae is gas exchange:
- Oxygen from water diffuses into the blood inside the capillaries.
- Carbon dioxide from the blood diffuses out into the surrounding water.
Counter-Current Flow System
Fish use a counter-current system to increase efficiency:
- Water flows over the lamellae in one direction.
- Blood flows in the opposite direction through the capillaries.
- This maintains a concentration gradient along the entire lamella surface.
- More oxygen is absorbed than if water and blood flowed in the same direction.
Why Gill Lamellae Are So Effective
| Adaptation | Benefit |
|---|---|
| Large number of lamellae | Increases surface area for more gas exchange |
| Thin epithelial layer | Shortens diffusion path |
| Counter-current flow system | Maintains high diffusion gradient |
| Dense capillary network | Ensures quick transport of gases |
| Constant water movement over gills | Keeps fresh oxygenated water flowing |
Key Points:
Blood Oxygen Saturation Analogy: Like alveoli in humans, oxygen binds to hemoglobin in fish blood after diffusing across lamellae. Efficient oxygen uptake is critical at low oxygen concentrations in water.
Impact of Temperature: At lower water temperatures, oxygen solubility increases, but metabolic rate decreases, reducing oxygen demand important in cold aquatic environments.
Conclusion
Stomata in Plants
What Are Stomata?
Stomata (singular: stoma) are tiny pores found mostly on the underside of plant leaves. They play a key role in gas exchange – allowing carbon dioxide in, and oxygen and water vapor out.
Where Are Stomata Found?
- Mainly on the lower surface of leaves (to reduce water loss).
- Also found on green stems and some flower parts.
- Each stoma is surrounded by two guard cells.
Structure of a Stoma
| Part | Description |
|---|---|
| Stomatal pore | The opening that allows gas exchange. |
| Guard cells | Two curved cells that control the opening and closing of the stoma. |
| Chloroplasts | Present in guard cells (unlike other epidermal cells) – allow them to respond to light. |
Function of Stomata
Gas Exchange:
- Carbon dioxide (CO₂) enters for photosynthesis.
- Oxygen (O₂) produced during photosynthesis exits.
Transpiration:
- Water vapor escapes through stomata, helping cool the plant and pull water up from roots.
Maintaining Water Balance:
- Stomata open or close based on water availability to prevent dehydration.
Opening and Closing of Stomata
Guard cells change shape to open or close the stomata:
| Condition | What Happens |
|---|---|
| In light (daytime) | Guard cells absorb water, swell, and open stomata. |
| In darkness or drought | Guard cells lose water, shrink, and close stomata. |
This shape change is due to osmosis. When guard cells take in water, they become turgid and curve apart. When they lose water, they become flaccid and collapse together.
Adaptations to Minimize Water Loss
- Sunken stomata (in xerophytes) reduce evaporation.
- Fewer stomata on the upper surface (less sunlight exposure).
- Waxy cuticle on leaf surface adds a waterproof layer.
Key Points
No Stomata in Submerged Plants:
- Plants like pondweed do not have stomata because gas exchange occurs directly through the surface of submerged leaves, which are thin and permeable to gases.
Stomatal Density Relevance:
- Higher stomatal density (like in sunflowers) enables greater photosynthetic rates but may increase water loss.
- Aquatic plants (like water lilies) have stomata only on the upper surface since lower surfaces are submerged.
Environmental Adaptation Insight:
- Sunflowers grow in drier conditions and have stomata on both sides to balance gas exchange and water conservation.
- Water lilies live on water surfaces and only require upper stomatal openings for gas exchange.
Importance of Stomata
- Crucial for photosynthesis, respiration, and transpiration.
- Help maintain the plant’s internal water balance.
- Regulate intake of CO₂ and release of oxygen.