B3.1 Gas exchange

In which organisms is gas exchange an important function in?

All

What happens to the SA:V as the size of an organism increases?

SA:V decreases

Problem when organisms increase in size

SA:V decreases → distance from the centre of an organism to its exterior increases

• So longer diffusion distance

Properties of gas-exchange surfaces (eg alveoli)

1. Thin tissue layer → reduces DD

2. Permeable to gases

3. Large SA:V

4. Moist → gases diffuse better (small, non-polar)

Diffusion

Passive net movement of particles from an area of high to low conc down a CG until dynamic equilibrium is reached

How to increase diffusion rate using CGs?

Steeper concentration gradient

• Larger difference

CG in unicellular organisms

• Consumes O2 during cell respiration

• So O2 conc in cell is always low

How do exchange surfaces maintain a CG?

1. Dense networks of blood vessels

2. Continuous blood flow

3. Ventilation:

   • With air for lungs

   • With water for gills

How does a dense network of blood vessels maintain a CG?

Remove O₂ + deliver CO₂ efficiently

How does a continuous blood flow maintain a CG?

Maintains low O₂ + high CO₂ in tissues

• Opposite in lungs/gills

How does ventilation maintain a CG?

• Lungs: fresh air brings in O₂ + removes CO₂.

• Gills: water brings in O₂ + carries away CO₂

  • Fish move O2 rich water thru gills

How do unicellular organisms gain substances eg oxygen?

Simple diffusion thru the plasma membrane

How do multicellular organisms gain substances eg oxygen?

Transport system delivers the substances to the cells via diffusion

Why can unicellular organisms only be a maximum size?

Takes too long for the substances to diffuse into the centre of the cells bc the distance from the exterior of the cell to the centre of the cell increases

What do larger organisms need for gas exchange- bc they can’t rely on simple diffusion?

Specialized gas exchange systems

Adaptations of mammal lungs for gas exchange (alveolar lungs)

1. Presence of surfactant

2. A branched network of bronchioles

3. Extensive capillary beds (rich blood supply to maintain CG)

4. High SA (alveoli)

Surfactant

Substance that reduces surface tension → prevents alveoli from collapsing

• + Provides moisture

What cells in the alveoli produce surfactant?

Type II pneumocytes

How do alveoli help with gas exchange?

Increase SA

How is a CG maintained for efficient gas exchange?

1. Capillaries (lots)→ moves blood w high O2 conc away, continuous blood flow

2. Ventillation

   • Inhale → increases CG of O2 betw alveoli + blood → diffuses into blood

   • Exhale → increases CG of CO2 betw alveoli + blood → CO2 diffuses out of blood + into alveolus

   • Movement of water thru gills → high O2 conc + low CO2 outside gills

Ventilation in mammals vs fish

• Lungs → air movement maintains CG of O₂ + CO₂.

• Gills → water flow maintains CG for dissolved gases

Respiration

Release of ATP energy from organic compounds (food)

What do capillaries provide?

A continuous supply of blood w low O2 conc + high CO2 conc to the alveoli

Gas exchange

Exchange of gases at cells + tissues thru diffusion

• Moves O₂ into cells for respiration + CO₂ out as waste.

• Occurs at the alveoli in the lungs + at respiring tissues

Ventilation

Movement of air in + out of the alveoli in the lungs

• Facilitates gas exchange

• Breathing

What does ventilation maintain CG of?

CG of O2 + CO2 betw air in alveoli + blood flowing in adjacent capillaries

2 stages of ventilation

• Inspiration- breathing in

• Expiration- breathing out

role of the diaphragm, intercostal muscles, abdominal muscles and ribs

What happens in inspiration?

• Diaphragm, intercostal muscles, abdominal muscles, ribs

1. Diaphragm contracts (flattens) + moves downwards

2. External intercostal muscles contract → ribcage move up + out.

3. Thorax volume increases → decreases the pressure in the lungs

4. Air passively moves from the surrounding air (high pressure) into the lungs (low pressure)

5. Ab muscles relax

What happens in expiration?

• Diaphragm, intercostal muscles, abdominal muscles, ribs

1. Ab muscles contract → push diaphragm upwards

2. External intercostal muscles relax, internal contract → ribcage move down + in.

3. Thorax volume decreases → increases the pressure in the lungs

4. High pressure in lungs moves air out of the lungs to surrounding air (lower pressure)

Inspiration vs expiration summary

What muscle pushes the diaphragm up?

Abdominal muscle- contract

What does the ribcage do?

Protect lungs

Ventilation rate

No of inhalations / exhalations per min

Tidal volume

Vol of air inhaled / exhaled in a normal breath

Inspiratory reserve volume

The additional vol of air that can be inhaled w maximum effort (after a normal breath)

Expiratory reserve volume

The additional vol of air that can be exhaled w maximum effort (after a normal breath)

Vital capacity

Max amt of air the lungs can hold

• Max vol exhaled after max inhalation

Vital capacity equation

TV + IVR + EVR

In a lab, what can be used to find vital capacity?

• Balloons

• Water displacement

Spirometer

• Instruments used to measure air capacity of the lung

• By inhaling + exhaling

• Digital

Pros of spirometers vs bell jars

Spirometer:

• Works for inhalation + exhalation

  • Bell jar only works for exhalation

• Digital

  • Bell jar prone to human error

Why is gas exchange important for respiration?

• Aerobic respiration relies on O2

• O2 taken into organisms by GE

Adaptations for gas exchange in leaves

1. Waxy cuticle

2. Epidermis

3. Air spaces

4. Spongy mesophyll

5. Stomatal guard cells

6. Veins

Explain the adaptations for gas exchange in leaves

1. Waxy cuticle

2. Epidermis

3. Air spaces

4. Spongy mesophyll

5. Stomatal guard cells

6. Veins

• Waxy cuticle: reduces water loss

• Epidermis: protective, transparent for light entry.

• Air spaces (spongy mesophyll): allow diffusion of gases

• Spongy mesophyll cells: moist → gas dissolves for diffusion.

• Guard cells & stomata: regulate opening for gas exchange.

• Veins (xylem + phloem): transport water for photosynthesis and sugars.

Stomata

Pores / openings for gas exchange + water loss

Label the tissues in a (dicotyledonous) leaf

1. Upper epidermis: protective, transparent.

2. Palisade mesophyll: many chloroplasts, photosynthesis.

3. Spongy mesophyll: gas exchange, some photosynthesis.

4. Lower epidermis: contains stomata + guard cells.

5. Veins (vascular bundles): xylem (water), phloem (sugars)

Name the parts of the leaf

find answer

What is transpiration a consequence of?

Gas exchange in a leaf

• Bc stomata are open

Transpiration

Loss of water vapour from the leaves via the stomata

Transpiration stream

The continuous flow of water thru the xylem from the roots to the leaf, against gravity

• Driven by the tension created by transpiration, cohesion of water molecules, and capillary action.

Water rises through xylem vessels bc of what 2 properties of water?

• Cohesion

• Adhesion

How does cohesion allow water to move up the xylem?

1. Cohesion provides an unbroken column of water in the xylem

2. Water molecules evaprate + diffuse → other water molecules move to replace them

3. Ceates tension in the xylem. 

4. Due to the cohesive nature water is pulled upwards.

Role of adhesion in the transpiration stream

Adhesion betw water molecules creates tension in cell walls after water has evaporated from the leaf

Role of cohesion in the transpiration stream

Cohesion betw water molecules maintains the continuity of the water column in xylem.

Structure of xylem that makes it suitable for transport

1. Lignin

   • Provide strength → xylem doesn’t collapse

2. Perforated

   • Water can move in/out of xylem

3. Dead, hollow cells with no end walls

   • No organelles → water flow is unobstructed

Factors affecting rate of transpiration

1. Temperature

2. Humidity

3. Air movement

4. Light intensity

How do the following factors affect transpiration

1. Temperature

2. Humidity

3. Air movement

4. Light intensity

1. Higher temp = more

   • More evaporation

2. Higher humidity = less

   • Water conc outside is high → less CG

3. Higher air movement = more

   • Removes air → increases CG

4. Higher light intensity = more

   • More stomata open for PS (allow more CO2 in)

Why do stomata need to be open in the day?

For GE for PS

What controls the opening + closing of stomata?

Guard cells

• Control water loss

What does a potometer measure?

Rate of transpiration

How to find transpiration rate using a potometer?

• Measure distance moved by an air bubble every min (w ruler)

• Indicates the rate of water uptake by the plant

Explain how a potometer works

1. Tube filled w water, connected to leaf (represents xylem)

2. Transpiration → water leaves plant thru leaf → pulls water up thru the tube

3. Air bubble is pulled along as well. Measure distance air bubble travelled

4. Multiply by SA of tube → to calc vol of water transpired

What do you need to do if you are doing another trial with a potometer?

1. Open reservoir of water

2. Pushes air bubble back

Precaution (not safety) when using a potometer

1. Don’t allow air to enter, cut shoot underwater

   • Ensures continuous column of water

2. Keep abiotic factor constant (eg LI)

   • Affects rate of transpiration

3. Keep screw clip closed

   • Prevents entry of water when measuring

1. Potometer airtight

2. Dry leaves

3. Cut shoot under water / slanted

4. Measure distance air bubble travels per (named) time interval
   OR
   Measure time for air bubble to travel known distance

5. Calc volume of water uptake

6. Maintain (named) constant conditions

Stomatal density

No. of stomata per unit area of leaf surface

• Eg per mm²

How to calc stomatal density

1. Use microscope to count no. of stomata in FoV

2. Calc radius of field of view

3. Calc area of field of view

4. Stomatal density = mean no. of stomata ÷ area of the FoV

How to increase reliability of quantitative data?

Repeat measurements

• Reduces random errors

In relation to stomatal density, what shows that it is neccesary to replicate trials?

Repeated counts of the no. of stomata visible in diff FoV (at high power) show the variability of biological material

2 diff methods to calc stomatal density

1. Peel epidermis, mount on slide

2. Paint LE w clear nail polish → dry → peel to make a cast. Study under microscope

Cons of measuring stomatal density with the nail polish method

1. Some plant species don’t have easily accessible stomata. Won’t create strong imprint

2. Solvent-based nail polish can destroy some of the cell structure

3. Water-based nail polish safer but dries slower

Compare stomatal density betw?

1. Diff species of plants

2. Same species grown in diff conditions