Biology - B3.1 gas exchange

Gas exchange definition

The absorption of one gas from the environment and release of another

Gas exchange and surface area to volume ratio

Only rapid enough if the surface area is large enough and the diffusion distance is short

E.g. small organisms or unicellular organisms can use their outer surface for gas exchange

Larger organisms need a specialised gas exchange surface (e.g. gills or lungs)

Properties of gas-exchange surfaces

• Permeable (gas cross plasma membrane)

• Large surface

• Moist (dissolve before diffusing)

• Thin

Concentration gradient mammalian lungs

Diffusion only occurs if there is a concentration gradient

So, in the lungs, a concentration gradient is established, as the blood brought to the lungs always has a lower oxygen content than the air breathed in.

Definition ventilation

Describes the movement of air in and out of the lungs (breathing), including the movement of water across the gills

Concentration gradient in fish

Water flows across gills and blood flows in an opposite direction to the water making sure that the water (rich in oxygen) meets blood (low in oxygen)

Concentration gradient unicellular organisms

Active transport or passive transport across the outer surface, with the molecules retained in the contractile vacuole.

What parts of mammalian lungs have adaptations for efficient gas exchange?

• Branching bronchioles,

• Thin cells/type 1 pneumocytes

• Thin endothelial cells of the capillaries

• Extensive capillary beds around alveoli

• Shape and extensive number of alveoli

What adaptations are there to increase rate of gas exchange in mammalian lungs?

• Bronchioles branched to increase volume of air possible to enter and exit, also larger gas exchange surface

• Type 1 pneumocytes are thin cells to reduce diffusion distance

• The endothelial cells of the capillaries are thin to reduce the diffusion distance

• There are many capillaries around the alveoli to increase the surface area of gas exchange, also allows constant flow of blood to establish a constant concentration gradient

• Many alveoli and in broccoli shape to increase surface area

Type 2 pneumocytes

Type 2 pneumocytes release surfactant. These phospholipid type molecules form a monolayer above the moisture layer in the alveoli and reduce the surface tension to prevent implosion of lungs.

Breathing in and out fancy words

Inspiration and expiration

Main muscles involved in ventilation in humans

Intercostal muscles (in between the ribs), the diaphragm, the abdominal muscles

Pressure difference and breathing

To breath in, the pressure inside the lungs must be lower than outside which occurs with a larger volume.

Muscle movement during inspiration

Muscles cause the volume to become larger

• Diaphragm contracts and moves downwards

• Abdominal wall muscles relax

• External intercostal muscles contract, so the ribcage moves upwards and outwards

• Internal intercostal muscles relax

Muscle movement during expiration

Volume decreases inside the thorax so the pressure increases

• Diaphragm relaxes and moves upwards

• Abdominal muscles relax

• External intercostal muscles relax

• Internal intercostal muscles relax

Muscle movement during exercise expiration

Same as normal expiration (diaphragm and external intercostal muscles relax) but abdomen wall muscles and internal intercostal muscles contract

Spirometer

Tool used to measure lung volume, a graph produced depicting volume of air breathed in and out.

Definition tidal volume

The volume of fresh air that is inhaled/exhaled during a typical breath, usually around 0.5 litres

Definition vital capacity

The total volume of air that can be exhaled after a maximum inhalation

Definition inspiratory reserve volume

The volume of air a person can inhale forcefully after a normal inhalation

Definition expiratory reserve volume

The volume of air a person can exhale forcefully after a normal exhalation

Definition ventilation rate

the number of breaths per minute

Tidal volume with exercise vs at rest

At rest the tidal volume is higher because the amount you breath out/in is higher just at a slower rate

At exercise the ventilation rate increases, increase in frequency

• Bc when exercising need more oxygen to generate more ATP through aerobic respiration

How is the leaf structure adapted for gas exchange?

• Waxy cuticle

• Stomata and guard cells

• Palisade mesophyll

• Spongy mesophyll

• Vascular bundles

How is the waxy cuticle adapted for gas exchange?

• Impermeable waxy cuticle to stop evaporation out of the leaf, so water drips down to soil

• Waxy cuticle thicker on upper surface to prevent evaporation bc of higher temps and higher light intensity

How are stomata and guard cells adapted?

• Stomata are holes at regular intervals to obtain carbon dioxide at all parts of the leaf

• the guard cells prevent evaporation of water so close the stomata at low water levels and at higher temperatures

How is the palisade mesophyll adapted?

• Many chloroplasts and located at the top of the leaf for most light available and increase in photosynthesis rate

How is spongy mesophyll adapted?

• Less chloroplasts bc get less light and don’t need to do as much photosynthesis

• More spread out creating more air spaces to store/have CO2 which can access all cells

How are the vascular bundles adapted for gas exchange?

• They are bundles of xylem and phloem between cells

• Need them to bring water (xylem) and nutrients and sugars (phloem)

Plan diagram

Drawings that show the tissue distributions (layers) of structures, not individual cells, with labels obvi

Definition transpiration

Movement of water in plants, measured by the loss of water

Other def: continual loss of water from a leaf

Transpiration as a consequence of gas exchange in a leaf

CO2 enters the leaf for photosynthesis and water is lost bc it is a trade

CO2 enters through stomata and water is lost out of the stomata

Factors affecting rate of transpiration

• Temperature (increase in temp, increases transpiration)

• Humidity (increase in humidity, slower transpiration, more water outside, less strong concentration gradient)

• Wind (increase in wind, reduces humidity, increases transpiration)

• Light (bright light, stomata open, increase transpiration)

Stomatal density definition

Number of stomata per unit area of leaf structure

Stomatal density formula

stomatal density (mm^-2) = (average number of stomata) / area of field view (mm²)

What does a potometer measure

Rate of transpiration

How does a potometer work?

Measure the volume of water taken up or the movement of air bubble

For a stem of a plant

Can change internal factors like number of stomata, surface area etc. or external factors like wind etc.

Structure of erythrocytes

Red blood cells have plasmam membrane filled with haemoglobin molecules (no nucleus)

Haemoglobin molecule is 4 alpha helix polypeptides with 4 irons

To what does oxygen bind?

Binds to an iron atom within the haem group

Conformational shape haemoglobin

The protein changes shape with a change in pH or temperature and when oxygen binds to it

Definition cooperative binding

The conformational shape haemoglobin experiences when oxygen binds makes it easer for the next oxygen molecule to bind.

Oxygen molecules working together to make it progressively easier

Definition affinity and trend

The ease with which haemoglobin accepts oxygen

High affinity = easy binding

Trend: more oxygens, easier for next to bind

Dissociation def and trend

The ease with which oxygen is released from te haemoglobin

Trend: more oxygens, harder to release oxygen

Definition oxygen dissociation curve

Shows how “full” with oxygen a proportion of haemoglobin molecules are

y-axis: percentage of haemoglobin that is saturated with oxygen, if 25% most proteins have 1 oxygen binded

x-axis: partial pressure

Definition partial pressure

Measure of the individual pressure of one gas within a mixture of gases

Depends on concentration of that gas as well as its pressure

Oxygen in this case, so PO₂

Shape of the dissociation curve

Sigmoid shape because of the changing affinity when more oxygen is bound

At low PO₂ its a plateau bc its difficult to attach the first oxygen

When there are one or two oxygens it binds more easily so a steep gradient (25% to 50%)

Flattens at the top bc maximum number

Dissociation graph with parts of the body

Areas with high PO₂ (e.g. lungs) easily taken up more oxygen and at low PO₂ (tissues) release it more easily

What is different for fetal haemoglobin?

It is structured slightly differently with a higher affinity, easier to take up more oxygen

Achieved through gene expression

What are the two way in which carbon dioxide changes the affinity of haemoglobin?

1. Decrease in pH

2. Allosteric site binding

Lower affinity with conformational change

Way in which carbon dioxide reacts with water

Reacts with water to form hydrogen ions (H+ ions) and hydrogen carbonate ions (HCO3-), decrease pH because H+ makes more acidic, conformational change

CO₂ + H₂O —> H⁺ + HCO₃^-

Way in which carbon dioxide binds to haemoglobin

Combines with each haemoglobin to form carbaminohemoglobin, bind to allosteric site, causing conformational change

Haemoglobin +4CO₂ —> carbaminohemoglobin

Bohr shift

A shift in the oxygen dissociation curve depending on the partial pressure of carbon dioxide

High PCO₂ is a shift to the right because at a random PO₂ less oxygen bound to haemoglobin