2.2 Adaptations for gas exchange

Question 1

How does an organism’s size relate to its surface area to volume ratio?

Answer 1

The larger the organism, the lower the surface area to volume ratio.

Question 2

How does surface area to volume (SA/V) ratio affect transport of molecules?

Answer 2

The lower the SA/V ratio, the further the distance molecules must travel to reach all parts of the organism. Diffusion alone is not sufficient in organisms with small SA/V ratios.

Question 3

Why do larger organisms require mass transport and specialised gas exchange surfaces?

Answer 3

• Small SA/V ratio
• Diffusion insufficient to provide all cells with the required oxygen and to remove all carbon dioxide
• Large organisms more active than smaller organisms

Question 4

Name four features of an efficient gas exchange surface.

Answer 4

• Large surface area
• Short diffusion distance
• Steep diffusion gradient
• Ventilation mechanism

Question 5

Describe the gas exchange mechanism in the Amoeba.

Answer 5

• Unicellular organism with a large SA/V ratio
• Thin cell membrane provides short diffusion distance
• Simple diffusion across the cell surface membrane is sufficient to meet the demands of respiratory processes

Question 6

Describe the gas exchange mechanism in flatworms.

Answer 6

• Multicellular organisms with a relatively small SA/V ratio (in comparison to the Amoeba)
• However, flat structure provides a large surface area and reduces the diffusion distance
• Simple diffusion is sufficient to meet the demands of respiratory processes

Question 7

Describe the gas exchange mechanism in earthworms.

Answer 7

• Cylindrical, multicellular organisms with a relatively small SA/V ratio (in comparison to the flatworm)
• Slow moving and low metabolic rate ∴ require little oxygen
• Rely on external surface for gas exchange
• Circulatory system transports oxygen to the tissues and removes carbon dioxide, maintaining a steep diffusion gradient

Question 8

Define ventilation.

Answer 8

The movement of fresh air into a space and stale air out of a space to maintain a steep concentration gradient of oxygen and carbon dioxide.

Question 9

Name the organ of gaseous exchange in fish.

Answer 9

Gills

Question 10

What are gill filaments?

Answer 10

• Main site of gaseous exchange in fish, over which water flows
• They overlap to gain resistance to water flow - slows down water flow to maximise gaseous exchange
• Found in large stacks, known as gill plates, and have gill lamellae which provide a large surface area and good blood supply for exchange

Question 11

Explain the process of ventilation in bony fish.

Answer 11

• Buccal cavity volume increases and pressure decreases to enable water to flow in
• Contraction of the buccal cavity forces water across the gills
• Pressure in the gill cavity rises, opening the operculum.
Water leaves

Question 12

How is a steep diffusion gradient maintained across the entire gas exchange surface in bony fish?

Answer 12

Due to counter current flow.

Question 13

Define counter current flow.

Answer 13

Blood and water flow in opposite directions across the gill plate.

Question 14

How does counter current flow maintain a steep diffusion gradient? What is the advantage of this?

Answer 14

• Water is always next to blood of a lower oxygen concentration
• Keeps rate of diffusion constant and enables 80% of available oxygen to be absorbed

Question 15

What type of flow is exhibited in cartilaginous fish?

Answer 15

Parallel flow

Question 16

Define parallel flow.

Answer 16

Water and blood flow in the same direction across the gill plate.

Question 17

Compare counter current and parallel flow.

Answer 17

counter current / parallel flow

Blood and water flow in opposite directions across the gill plate/ Water and blood flow in the same direction across the gill plate

Steep diffusion gradient maintained, allowing diffusion of oxygen across the whole gill plate/ Diffusion gradient not maintained. diffusion of oxygen does not occur across the whole plate

High rate of diffusion/ lower rate of diffusion

More efficient - more oxygen absorbed into the blood/ Less efficient - less oxygen absorbed into the blood

Found in bony fish/ Found in cartilaginous fish, e.g. sharks

Question 18

Name and describe the main features of an insect’s gas transport system.

Answer 18

• Spiracles - small, external openings along the thorax and abdomen through which air enters, and air and water leave the gas exchange system

• Trachee - large tubes extending through all body tissues, supported by rings of chitin to prevent collapse

• Tracheoles - smaller branches dividing off the tracheae

Question 19

What is the main site of gas exchange in insects?

Answer 19

Tracheoles

Question 20

Describe the adaptations of the insect tracheal system to a terrestrial environment.

Answer 20

• Spiracles can be opened or closed to regulate diffusion
• Bodily contractions speed up the movement of air through the spiracles
• Highly branched tracheoles provide a large surface area
• Impermeable cuticle reduces water loss by evaporation

Question 21

Describe the ventilation of the tracheal system in insects.

Answer 21

• Expansion of the abdomen opens the thorax spiracles (through which air enters) and closes the abdominal spiracles
• Compression of the abdomen closes the thorax spiracles and opens the abdominal spiracles (through which air is expelled)

Question 22

Compare the gas exchange surface of an active and inactive amphibian.

Answer 22

• Active amphibian has simple lungs

• Inactive amphibian relies on its moist external surface for gas exchange

Question 23

How are mammals adapted for gas exchange?

Answer 23

Alveoli provide a large surface area and thin diffusion pathway, maximising the volume of oxygen absorbed from one breath. They also have a plentiful supply of deoxygenated blood, maintaining a steep concentration gradient.

Question 24

Describe the structure and function of the larynx.

Answer 24

A hollow, tubular structure located at the top of the trachea involved in breathing and phonation.

Question 25

Describe the trachea and its function in the mammalian gaseous exchange system.

Answer 25

• Primary airway, carries air from the nasal cavity down into the chest • Wide tube supported by C-shaped cartilage to keep the air passage open during pressure changes • Lined by ciliated epithelial cells which move mucus, produced by goblet cells, towards the back of the throat to be swallowed. This prevents lung infections

Question 26

Describe the structure of the bronchi.

Answer 26

• Divisions of the trachea that lead into the lungs • Narrower than the trachea • Supported by rings of cartilage and lined by ciliated epithelial cells and goblet cells

Question 27

What is the primary gaseous exchange surface in humans?

Answer 27

Alveoli

Question 28

Describe the alveoli in the mammalian gaseous exchange system.

Answer 28

• Mini air sacs, lined with epithelial cells • Walls one cell thick • Good blood supply to maintain a steep diffusion gradient • 300 million in each lung

Question 29

What are the pleural membranes?

Answer 29

Thin, moist layers of tissue surrounding the pleural cavity that reduce friction between the lungs and the inner chest wall.

Question 30

Define pleural cavity.

Answer 30

The space between the pleural membranes of the lungs and the inner chest wall.

Question 31

Describe ventilation in humans.

Answer 31

• The movement of fresh air into the lungs and stale air out of the lungs via inspiration and expiration • Via negative pressure breathing

Question 32

What are internal intercostal muscles?

Answer 32

A set of muscles found between the ribs on the inside that are involved in forced exhalation.

Question 33

What are external intercostal muscles?

Answer 33

A set of muscles found between the ribs on the outside that are involved in forced and quiet inhalation.

Question 34

Explain the process of inspiration and the changes that occur throughout the thorax.

Answer 34

• External intercostal muscles contract (while internal relax), raising the ribcage • Diaphragm contracts and flattens • Outer pleural membrane moves out, reducing pleural cavity pressure and pulling the inner membrane out • The alveoli expand. Alveolar pressure falls below air pressure so air moves into the trachea

Question 35

What is surfactant?

Answer 35

A fluid lining the surface of the alveoli that reduces surface tension and prevents collapse of the alveoli during exhalation.

Question 36

Describe the function of the waxy cuticle.

Answer 36

Reduces water loss from the leaf surface.

Question 37

Describe how the upper epidermis is adapted for photosynthesis.

Answer 37

• Layer of transparent cells allow light to strike the mesophyll tissue • Epidermal cells also synthesise the waxy cuticle, reducing water loss

Question 38

Where is the palisade mesophyll layer located?

Answer 38

Directly below the upper epidermis.

Question 39

How is the palisade mesophyll layer adapted for photosynthesis?

Answer 39

It receives the most light so contains the greatest concentration of chloroplasts.

Question 40

How is the spongy mesophyll layer adapted for photosynthesis?

Answer 40

• Contains air spaces that reduce the diffusion distance for carbon dioxide to reach the chloroplasts in the palisade layer • Contains some chloroplasts

Question 41

What is a vascular bundle?

Answer 41

The vascular system in dicotyledonous plants. It consists of two transport vessels, the xylem and the phloem.

Question 42

Why are vascular bundles important in photosynthesis?

Answer 42

They form a large network to deliver water and nutrients to photosynthetic tissues and remove glucose.

Question 43

Describe how the lower epidermis is adapted for photosynthesis.

Answer 43

It contains many stomata which enable the evaporation of water and inward diffusion of CO₂

Question 44

What are stomata?

Answer 44

Small holes found on leaves that can be opened or closed by guard cells to control gas exchange and water loss.

Question 45

Summarise the 'malate' theory.

Answer 45

The 'malate' theory states that the accumulation or loss of malate and K⁺ ions by guard cells results in changes in turgor pressure that open or close the stomata.

Question 46

By what mechanism do K⁺ ions enter guard cells?

Answer 46

Active transport

Question 47

How does the accumulation of K⁺ and malate ions affect guard cells?

Answer 47

• Lowers the water potential of guard cells • Water moves in by osmosis • Guard cells becomes turgid, opening the stomata

Question 48

Why is starch important for stomatal opening?

Answer 48

Starch is converted to malate ions.

Question 49

What are stomata?

Answer 49

Pores surrounded by 2 guard cells in the aerial parts of the leaf.

Question 50

What is the function of stomata?

Answer 50

They open to allow gas exchange (carbon dioxide to diffuse in, oxygen to diffuse out) for photosynthesis.

Question 51

How should be leaf be prepared to find the number of stomata?

Answer 51

Peel off the epidermal layer of the leaf, or make a replica of the leaf using PVA glue or nail polish. Observe using a microscope.

Question 52

How should the stomata distribution be estimated?

Answer 52

Count the number of stomata 3 different fields of view and take a mean. Find the area of the field of view. Distribution = mean number of stomata in each field of view / area of field of view

Question 53

How can the area of the field of view be calculated?

Answer 53

Calibrate the eyepiece graticule using a stage micrometer. Measure the diameter of the field of view. Use πr² to calculate the area.

Question 54

State factors that may affect stomatal distribution.

Answer 54

Habitat of plant Growth habit of plant Upper / lower epidermis of leaf

Question 55

Which structure should be labelled in the cross section?

Answer 55

Lumen Tunica interna, Tunica media, Tunica externa Endothelium

Question 56

State an observable differences between arteries and veins.

Answer 56

Arteries have a smaller lumen Arteries have a thicker tunica media The shape of the lumen of an artery is more regular Veins contain valves

Question 57

How is magnification calculated?

Answer 57

Magnification = size of image / size of object

Question 58

How is the magnification of the drawing calculated?

Answer 58

Calibrate the eyepiece graticule. Measure the actual width of the artery/vein using the eyepiece graticule. Measure the width of the artery/vein of the drawing. Calculate the magnification.

Question 59

What is a plant diagram?

Answer 59

A large diagram showing the relative shapes and proportions of tissues, but no cells.

Question 60

Which tissues should be labelled in a diagram of a leaf cross section?

Answer 60

Upper and lower epidermis, guard cells Palisade mesophyll, spongy mesophyll Xylem, phloem Cuticle, collenchyma, sclerenchyma

Question 61

What is the formula to calculate magnification?

Answer 61

Magnification = size of image / size of object

Question 62

How is the magnification of a drawing calculated?

Answer 62

Calibrate the eyepiece graticule using a stage micrometer. Measure the length using the eyepiece graticule. Measure the length of the image using a ruler. Calculate magnification using these values.