B3.1 Gas Exchange Flashcards
Properties of gas exchange surfaces
Large surface area - larger area for gas to diffuse across
Permeable - oxygen and carbon dioxide can diffuse across easily
Thin - so that there is a short distance across which gases need to move
Moist - this helps to dissolve gases before they diffuse across the exchange surface; for example, alveolar fluid into the alveoli
What circulatory system do mammals have?
A double circulatory system to ensure blood transported to respiring cells is highly oxygenated.
Adaptations of mammalian lungs for gas exchange
Trachea - ciliated and lined with mucus to trap or expel foreign particular matter like pathogens or dust
Right lung composed of three lobes and left only two due to position of heart
Bronchioles increase surface area and contain smooth muscle innervated by autonomic nervous system to allow for regulation of air flow.
Alveoli connected to dense network of capillary beds optimising exchange of gases with the blood
Inspiration
Diaphragm contracts and moves downwards
External intercostal muscles contract and internal relax
Rib cage moves up and outwards
Increases volume of thoracic cavity and decreases the pressure in the lungs
As a result air moves down it’s pressure gradient into the lungs
Expiration
Diaphragm relaxes moving up and inwards
External intercostal muscles relax and internal contract
Rib cage moves down and inwards
Decreased volume meaning increased pressure in thoracic cavity
Air moves down the pressure gradient out of the lungs
Factors affecting measurements of lung volumes
Age
Body composition
Sex
Respiratory diseases
Levels of physical activity
Foetal haemoglobin (HbF)
Dominant form of haemoglobin during foetal development and remains in infant until 6 months old and is gradually replaced by adult haemoglobin
Has a quarternary structure with 2 alpha and 2 gamma polypeptide chains, each contain a haem group that can bind reversibly to oxygen
Due to gamma polypeptide = HbF has a higher affinity for oxygen than adult haemoglobin
Can CO2 also be transferred via haemoglobin?
Yes, in small amounts. This CO2 binds to the allosteric site while oxygen binds in the haem group where it should be. This forms carbaminohaemoglobin
Bohr shift
The oxyhaemoglobin dissociation curve demonstrates the saturation of haemoglobin by oxygen under normal conditions
pH changes alter the affinity of haemoglobin for oxygen and hence alters the uptake and release of O2 by haemoglobin
Carbon dioxide lowers the pH of the blood (by forming carbonic acid), which causes haemoglobin to release its oxygen
This is known as the Bohr effect – a decrease in pH shifts the oxygen dissociation curve to the right
Cells with increased metabolism (i.e. respiring tissues) release greater amounts of carbon dioxide (product of cell respiration)
Hence haemoglobin is promoted to release its oxygen at the regions of greatest need (oxygen is an input of cell respiration)
Adaptations for gas exchange in leaves
Leaves are typically broad, flat and thin to maximise their surface area and optimise rates of photosynthesis
Most leaves are green due to the abundance of chlorophyll, however some leaves may have different colours (due to the presence of accessory pigments)
A waxy cuticle covers the epidermis to provide an impenetrable hydrophobic barrier (prevents water loss)
The lower epidermis is perforated with stomatal pores to facilitate the exchange of respiratory gases and water vapour
The cells of the palisade mesophyll are tightly packed and rich in chloroplasts (optimised for photosynthesis)
The cells of the spongy mesophyll are loosely packed between intercellular air spaces (maximising gas exchange)
The xylem functions to transport water and minerals from within the roots of the plant (via transpiration)
The phloem transports dissolved sugars produced by photosynthesis to other parts of the plant (as sap)
How is each part of the plant structured for maximum gas exchange and photosynthesis?
The waxy cuticle covers the exterior surface in order to prevent water loss from the leaf (except via stomata)
The palisade mesophyll is located on the upper half of the leaf (facing sunlight) to maximise light absorption
The spongy mesophyll is located on the lower half (near stomata) and contains air spaces for gas exchange
The stomata are on the underside of the leaf to prevent obstruction and maintain an open channel for gases
The vascular bundle is located centrally to allow for optimal access by all leaf tissue (palisade and spongy)
What are the stages of transpiration?
- Water EVAPORATES from the internal leaf cells through the STOMATA.
- Water passes from the XYLEM vessels to leaf cells due to OSMOSIS…
- …which pulls the water in that vessel upwards by a very small amount.
- Water enters XYLEM from root cortex to replace water which has moved upwards.
- Water enters ROOT HAIR CELLS by OSMOSIS to replace water which has entered the XYLEM.
Can you give a summary of transpiration?
Water leaves the plant by transpiration through the leaves.
This reduces the pressure at the top of the xylem vessels.
This creates a ‘transpiration pull’: water molecules are drawn up through the xylem as they move towards the area of lower water potential at the top of the plant.
One of the properties of water is that the molecules want to stick together. This cohesion* means that a column of water is drawn up the stem of the plant.
*Cohesion refers to the attraction of molecules for other molecules of the same kind, and water molecules have strong cohesive forces thanks to their ability to form hydrogen bonds with one another.
What is a transpiration pull?
Water molecules are drawn up through the xylem as they move towards the area of lower water potential at the top of the plant.
