Gas exchange and SA:V

Question 1

Why are gas exchange systems necessary?

Answer 1

SA:V ratio too low, simple diffusion across outer surface only meets needs for inactive organism and also substances take too long to reach middle.

Question 2

Fick’s law

Answer 2

(SA * difference in concentration) / length of diffusion pathway

Question 3

General features of gas exchange surfaces

Answer 3

High SA:V
Thin for short diffusion distance.
Selectively permeable.
Movement of environment to maintain diffusion gradient or transport system.

Question 4

Gas exchange in single organism

Answer 4

Large SA:V ratio.

Oxygen enters and CO2 leaves via diffusion.

Question 5

Structure of gas exchange system in insects

Answer 5

Air enters through spiracles.
Tracheae supported by rings of chitin.
Divide into tracheoles that supply short diffusion pathways to any cell.

Question 6

How is a diffusion gradient used in the tracheoles

Answer 6

Respiring cells use oxygen so concentration falls in ends of tracheoles -> oxygen from atmosphere diffuses along tracheae and tracheoles to cells.
Respiring cells produce CO2 so conc. increases in ends of tracheoles -> CO2 diffuses along tracheoles and tracheae to atmosphere.

Question 7

How is mass transport used in gas exchange in insects

Answer 7

Contraction of muscles means tracheae can be squeezed so mass movements of air in and out speeds up exchange of respiratory gases.

Question 8

How does the fluid in the end of tracheoles increase gas exchange

Answer 8

Lactate produced during major activity in cells, which lowers water potential of cells. Water moves in from tracheoles into cells by osmosis. Decreased volume of water means air is drawn in. Final diffusion pathway is in gas phase so diffusion is more rapid. Increases rate of gas exchange but more water evaporation.

Question 9

How do insects reduce water loss

Answer 9

Spiracles at opening of tracheae can close to reduce water loss.
Waterproof cuticle that covers exoskeleton.
Small SA:V.

Question 10

Why are insects small

Answer 10

Tracheal system relies on diffusion. Short diffusion pathway required so insects need to be small.

Question 11

Adaptations of leaves for gas exchange

Answer 11

Many stomata so no cell is far from stoma (short diffusion pathway).
Many air spaces in mesophyll so gas can come in contact with cells.
Large surface area of mesophyll for rapid diffusion.

Question 12

Gas exchange in a leaf in the day vs the night

Answer 12

In day when photosynthesis is taking place, CO2 and oxygen out.
In night when no photosynthesis, O2 in, CO2 out.

Question 13

Structure of stomata and opening and closing function

Answer 13

Two guard cells surround stoma. Turgid guard cells open stoma and flaccid closes.

Question 14

How to limit water loss in insects

Answer 14

Small SA:V ratio - minimise surface water is lost on.
Waterproof cuticle on exoskeleton.
Spiracles can open and close to limit water loss when insect is resting.

Question 15

How to limit water loss in plants

Answer 15

Thick waxy cuticle.
Leaves roll up so high water potential area outside lower epidermis (stomata) = less water potential gradient.
Hairy leaves to reduce water potential gradient.
Stomata in pits to reduce water potential gradient.
Reduced SA:V ratio on leaves (pines).

Question 16

Why do mammals have need for gas exchange system

Answer 16

Large organisms, high volume of living cells.

High body temp and high metabolic/respiratory rates.

Question 17

Tracheae structure

Answer 17

Flexible airway with rings of cartilage to prevent low air pressure causing airway to collapse.
Walls made of muscle, lined with ciliated epithelium and goblet cells.

Question 18

Bronchi structure

Answer 18

Smaller divisions of tracheae, produce mucus and use cilia to waft mucus up. Supported by cartilage.

Question 19

Bronchioles structure

Answer 19

Walls made of muscle lined with epithelial cells. Muscles can contract to control movement of air in and out of alveoli.

Question 20

Alveoli structure

Answer 20

Tiny air sacks at end of bronchioles connected by collagen and elastic fibres so they can stretch when filled with air. Spring back to expel CO2.
Site of gas exchange.
Alveoli epithelium flat against capillary epithelium.

Question 21

Inspiration

Answer 21

External intercostal muscles contract and internal relax.
Diaphragm contracts, flattening.
Volume of thorax increases, reducing pressure in lungs.
Air flows from atmosphere to lungs due to lower pressure.

Question 22

Expiration

Answer 22

Internal intercostal muscles contract and external relax.
Diaphragm relaxes, arching.
Volume of thorax decreases, increasing pressure in lungs.
Air flows from lungs to atmosphere as pressure is higher in lungs.

Question 23

How is diffusion of gases maximised in alveoli

Answer 23

Red blood cells slowed as passing through capillaries to allow more time for diffusion.
RBCs flattened against capillary walls to reduce diffusion distance.
Walls of alveoli and capillaries are thin to reduce diffusion distance.
Alveoli and capillaries have large total SA.
Breathing movements ventilate lungs and heart circulates blood so steep conc gradient maintained.

Question 24

Structure of gills

Answer 24

Gill filaments that have gill lamellae to increase SA of gills.

Question 25

Countercurrent flow principle

Answer 25

Flow of water over gill lamellae is opposite direction of flow of blood.
Oxygenated blood meets water with max O2 conc first so diffusion happens. Water flow continues over to deoxygenated blood where water has lower O2 conc. Blood has even lower O2 conc so diffusion happens still.