Gas exchange

Question 1

Explain the surface area to volume ratio

Answer 1

• When an organism doubles in size, its volume (and O2 needs) is cubed, but is surface area is only squared
• As organism’s size increases, specialised gas exchange surface is needed to increase area available

Question 2

Characteristics of efficient gas exchange surface

Answer 2

• Large SA:Vol ratio
• Moist (allow gas to dissolve)
• Thin (short diffusion pathway)
• Gas permeable

Question 3

How does the size of unicellular organisms affect its gas exchange?

Answer 3

• Surface area is large enough to meet the organism’s needs so materials exchanged across thin permeable membrane
• Cytoplasm always moving=concentration gradient maintained

Question 4

How does the size of multicellular organisms affect its gas exchange?

Answer 4

• Surface area of body surface (for gas exchange) is insufficient for the organisms needs => evolved adaptations solve problems
• Active animals with fast metabolisms need more O2 than just the body surface would provide
• Have specialized gas exchange surface with ventilation system (ensuring constant conc. gradient is maintained)

Question 5

What’s the problem with terrestrial animals maintaining a moist respiratory surface, and how is it minimised?

Answer 5

Water loss: minimised by having internal gas exchange surfaces (lungs)

Question 6

How is a flatworm adapted for gas exchange?

Answer 6

Flattened body - reduce diffusion distance between surface and inside cells + increase surface area

Question 7

How is a earthworm adapted for gas exchange?

Answer 7

• Secrete mucus (maintain moist surface) + well developed capillary network under skin
• Low metabolic rate (reduce O2 needs)
• Network of blood vessels, transporting O2 via haemoglobin in blood (CO2 in blood plasma)

Question 8

How are amphibians adapted for gas exchange?

Answer 8

• Moist permeable skin with well developed capillary network under skin
• Lungs for when more active

Question 9

How are reptiles adapted for gas exchange?

Answer 9

Internal lungs - like amphibians but more complex with larger surface area

Question 10

How are birds adapted for gas exchange?

Answer 10

• High metabolic rate from flying=large O2 requirement

- Efficient ventilation system to increase concentration gradient across lung surface

Question 11

Describe the structure of a fish’s internal gas exchange surface? How is this an adaptation for gas exchange?

Answer 11

-Gills: vertical gill arches/bars have layers of filaments coming off them horizontally
Filaments contain lamellae at right angles to them
-Greatly increase the surface area for O2 and CO2 gas exchange

Question 12

What are the 2 ways fish ventilate their gills?

Answer 12

• Parallel flow: Cartilaginous fish (e.g. sharks)

- Counter-current flow: Bony fish (e.g. salmon)

Question 13

Describe Parallel flow

Answer 13

• Blood flows in same direction as water over gills
• Gas exchange only over part of filament surface (equilibrium is reached - reducing O2 absorption)
• Simple ventilation: open mouth while swimming allows water to pass over gills

Question 14

Describe Counter-current flow

Answer 14

• Blood flows in opposite direction to water over gills
• Diffusion maintained along entire length of filament (always higher O2 concentration in water than in meeting blood - no equilibrium)
• More efficient than parallel as higher O2 absorption
• Advanced ventilation

Question 15

Describe ventilation in bony fish

Answer 15

• As mouth opens floor of buccal cavity lowers (increased volume decreases the pressure, causing water to rush in + opercular valve to close)
• As mouth closes floor of buccal cavity rises (decreased volume increases the pressure, forcing the rush of water over gills + opercular valve to open

Question 16

Describe the parts of the human ventilation system

Answer 16

• Trachea branches into 2 bronchi (each entering a lung)

- These branch into finer bronchioles, further ending as alveoli (site of gas exchange)

Question 17

Describe human inspiration (active)

Answer 17

• External intercostal muscles and diaphragm contract, moving ribs up and out (pulling pleural membranes out) and diaphragm flat
• Pressure in pleural cavity reduces (from volume increase) => pulling of lung surface causes alveoli to expand
• Alveolar pressure below ATM=air sucked in

Question 18

Describe human expiration (passive)

Answer 18

• External intercostal muscles and diaphragm relax, moving ribs down and in (pushing pleural membranes in) and diaphragm up
• Pressure in pleural cavity increases (from volume decrease) => pushing on lung surface causes alveoli to contract
• Alveolar pressure above ATM=air forced out

Question 19

How are alveoli adapted for gas exchange?

Answer 19

• Large surface area + thin walls (1 cell thick)
• Short diffusion pathway+good blood supply (capillaries surround)
• Moist lining + permeable to gases)
• Collagen and elastic fibres allow expansion/recoil

Question 20

Describe gas exchange in insects

Answer 20

• Branched tracheae system with spiracle openings, lined with chitin (arranged in rings - allowing tracheae to expand/relax)
• Spiracles (on surface of organism) can close during inactivity+chitin helps to reduce water loss
• Tracheoles touch all tissue with fluid for (C)O2 exchange=no haemoglobin needed

Question 21

Describe the ventilation system in insects for gas exchange

Answer 21

• Muscles in thorax/abdomen contract and relax

- Rhythmic movements ventilate the tracheole tubes (keeping concentration gradient)

Question 22

Describe gas exchange in plants

Answer 22

• Need O2 for respiration + CO2 for photosynthesis (diffusion through leaves)
• Waxy cuticle (covering leaf surface) reduces water loss and diffusion of gases
• Stomata on most leaves’ underside open for gas exchange in day and close at night/drought to reduce water loss

Question 23

What is transpiration?

Answer 23

Evaporation of H2O (from leaves/any above ground parts) through stomata into the atmosphere
-Controlled by size of pore between guard cells

Question 24

Explain the stomatal opening mechanism

Answer 24

Guard cells produce ATP via photosynthesis (energy released used to actively transport potassium ions into guard cells)

• This triggers starch to convert into malate ions (soluble), so H2O diffuses in guard cells (lower ψ)
• Pore created between cells by outer wall stretching more than inner
• Reverse happens at night

Question 25

How are leaves adapted for gas exchange?

Answer 25

• Flat and thin (large SA for gas exchange and capture light)
• Many stomata allow gas exchange
• Spongy mesophyll cells (below palisade) are surrounded by air spaces - allow gases to diffuse up through pores

Question 26

What stops the alveoli collapsing during exhalation

Answer 26

surfactant

Question 27

What is the function of pleural fluid

Answer 27

forces the lungs to expand and acts as a cushion between the lungs and ribcage

Question 28

What are the gas concentrations of inspired air

Answer 28

Oxygen - 20%
Carbon dioxide - 0.04%
Nitrogen - 79%
water - Variable

Question 29

What are the gas concentrations of expired air

Answer 29

Oxygen - 16%
Carbon dioxide - 4%
Nitrogen - 79%
Water - saturated

Question 30

Total capacity definition

Answer 30

maximum volume of air the lungs can hold during deepest breathing

Question 31

Residual volume definition

Answer 31

volume of air remaining in the lungs after exhaling

Question 32

Vital capacity definition

Answer 32

Maximum usable lung volume

Question 33

Tidal volume definition

Answer 33

the volume of air that moves in and out of the lungs during normal breathing