4.3 - Gas exchange

What are the three features of an efficient gas exchange surface?

• Large surface area (e.g. cristae in mitochondria)

• Thin/short diffusion distance (e.g. walls of capillaries → one cell thick)

• Steep concentration gradient (e.g. in alveoli)

What are the main features of an insect’s gas exchange system?

• Spiracles: openings on the body’s surface

• Tracheae: large tubes extending through all body tissues, supported by rings of chitin

• Tracheoles: smaller branches dividing off the tracheae, delivering oxygen directly to cells

How are insects adapted for gas exchange?

• Spiracles can be opened or closed by sphincters to regulate diffusion

• Muscles in the tracheae allow mass movement of air in & out → rings of chitin prevent tracheae from collapsing

• Highly branched tracheoles provide large surface area

How do insects reduce water loss?

• A rigid exoskeleton: waterproof covering over their body

• Small surface area to volume ratio: decreases area over which water is lost

• Sphincters: close spiracles when insect is inactive

• Some insects have spines/hairs around spiracles to increase humidity → reduces evaporation

What are the two features of a fish’s gas exchange system?

• Gills: made of filaments & supported by arches

• Lamellae: folds that cover the filaments → water passes over due to pressure from the floor of the mouth

How are fish adapted for gas exchange?

• Large surface area: gills made of numerous filaments & covered by lamellae

• Counter-current flow means water & blood flow in opposite directions → maintains high concentration gradient for diffusion of oxygen across entire length of lamellae

How is a unidirectional flow of water maintained over the gills of a fish?

1. The mouth opens (operculum is closed), meaning the buccal cavity floor is lowered

2. This increases the volume & decreases the pressure of the buccal cavity compared to the outside

3. Water rushes into the mouth down a pressure gradient, causing the opercular cavity to expand

4. The mouth closes & the buccal cavity floor is raised, which decreases the volume, so the pressure inside the buccal cavity is higher than the opercular cavity

5. This increase in pressure forces the opercula valves open, causing water to flow out over the gills & through the operculum (down a pressure gradient)

What is the main site of gas exchange in the lungs?

Alveoli

What are the two tubes leading to the lungs?

Bronchi

What are the narrow tubes which carry air from the bronchi to the alveoli?

Bronchioles

What is the airway that leads from the nose & mouth to the bronchi?

Tracheae

How are mammals adapted for gas exchange?

• Alveoli provide large surface area & short diffusion distance (one cell thick): maximises amount of oxygen absorbed from one breath

• Alveoli have plentiful supply of deoxygenated blood: maintains steep concentration gradient

Outline the process of inhalation in humans

(active energy-using process → uses ATP for muscle contraction)

1. Diaphragm contracts & flattens downwards & the intercostal muscles contract, pulling the ribs up & out

2. This increases the volume of the thorax & the lungs, & stretches the elastic-walled alveoli

3. This decreases the pressure in the alveoli below atmospheric, causing air to flow in down a pressure gradient

Outline the process of exhalation in humans

1. Diaphragm relaxes & curves upwards & the intercostal muscles relax, allowing ribs to move down & in

2. This decreases the volume of the thorax & the lungs, & allows the alveoli & bronchioles to shrink by elastic recoil

3. This increases the pressure of air in the alveoli above atmospheric, causing air to flow out down a pressure gradient

What are the equations for respiration & photosynthesis?

Respiration (occurs all the time):

• Glucose + Oxygen → Carbon dioxide + Water

• C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O

Photosynthesis (occurs only in daytime):

• Carbon dioxide + Water → Glucose + Oxygen

• 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

What are the structures of a leaf?

1. Waxy cuticle

2. Upper epidermis

3. Palisade mesophyll layer

4. Spongy mesophyll layer

5. Air space

6. Lower epidermis

7. Guard cells & stomata

How are leaves adapted for gas exchange?

• Large surface area & short diffusion distance: leaves are broad, thin & flat + spongy mesophyll layer

• Waxy cuticle is impermeable to gas: prevents excess water loss

• Lenticels (loosely arranged cells): allow gases to enter & leave

How do plants limit their water loss while still allowing gases to be exchanged?

• Stomata are regulated by guard cells, allowing them to open & close as needed

• Most stay closed to prevent water loss, while some open to let oxygen in

How is the stomata opened & closed?

• When the guard cells are flaccid, the stoma is closed

• To open the stoma, K⁺ ions are actively pumped into the guard cells, lowering water potential. So water diffuses in by osmosis → guard cell becomes turgid