B3.1 Gas exchange Flashcards
B3.1.1 Why is gas exchange a vital function for all organisms?
Gas exchange allows organisms to obtain the gases required for cellular processes such as aerobic respiration and photosynthesis, and remove waste gases produced in metabolic reactions. It occurs by diffusion.
B3.1.2 Properties of gas-exchange surfaces
- permeable—oxygen and carbon dioxide can diffuse across freely
- large—the total SA is large in relation to the organism’s volume
- moist—the surface is covered by a film of moisture in terrestrial organisms so gases can dissolve
- thin—the gases must diffuse only a short distance, in most cases through a single layers of cells.
B3.1.3 Maintenance of concentration gradients in small aerobically respiring organisms
In small, aerobically respiring organisms that use their outer surface for gas exchange, cell respiration maintains conc gradients. Oxygen is used and CO2 produced, meaning the oxygen concentration within the organism remains lower than outside and the carbon dioxide concentration higher.
B3.1.3 How do capillaries maintain concentration gradients for substance exchange?
This dense network of blood vessels means that there is much opportunity for substances to be exchanged between the surface and the blood.
B3.1.3 Maintenance of concentration gradients due to continuous blood flow
When substances enter the blood, the constant blood flow moves them away, keeping their concentration low near the exchange surface.
B3.1.3 How ventilation maintains concentration gradients in mammalian lungs
Inhale oxygen-rich air, keeping O₂ levels high. Exhale CO₂-rich air, keeping CO₂ levels low.
This maintains a steep gradient so O₂ diffuses into blood and CO₂ diffuses out.
B3.1.3 Maintenance of concentration gradients due to ventillation in fish/gills
Fish take in water, pump it over their gills, and out through the gill slits. The opposite flow of water and blood keeps oxygen high and CO2 low near the gills, oxygen moves into the blood and CO2 out.
B3.1.4 Journey of gas to mammalian lungs for gas exchange (nose/mouth -> bronchi)
Air enters respiratory system nose/ mouth -> passes through pharynx to trachea
- Trachea = ciliated + lined w/ mucus -> traps + expels foreign particulate matter.
Air travels down the trachea until it divides into two bronchi (sing: bronchus), connect to the lungs.
B3.1.4 Journey of gas to mammalian lungs for gas exchange (bronchi -> alveoli)
Bronchi divide -> bronchioles, increasing SA.
- Bronchioles have smooth muscle innervated by the autonomic nervous system, which tightens or relaxes to regulate airflow and breathing.
End = alveoli (air sacs), where gas exchange w/ bloodstream occurs
- Alveoli connected to dense capillary bed network – optimised exchange of gases w/ the blood
B3.1.5 Ventilation of the lungs - inspiration
Diaphragm contracts and moves downward, creating more space in the chest. External intercostal muscles contract (lifts the rib cage up and out), internal relax.
This causes the volume of the thoracic cavity to increase, which decreases lung pressure. Lower pressure inside = air to flow in from outside.
B3.1.5 Ventilation of the lungs - expiration
Diaphragm relaxes, moves upwards and inwards -> less space in chest. External intercostal muscles relax, internal contract (rib cage to moves down and in).
This causes the volume of the thoracic cavity to decrease, increasing the pressure. Air moves down its pressure gradient out of the lungs.
B3.1.6 Measurement of lung volumes
Tidal volume is the amount of fresh air inhaled and stale air exhaled with each breath. Ventilation rate is no. breaths per minute.
Vital capacity: Total air volume that can be exhaled after a maximum inhalation/inhaled after a maximum exhalation.
Inspiratory/expiratory reserve vol.: Extra air a person can inhale/exhale forcefully after a normal breath.
B3.1.6 Factors affecting human lung capacities
Age: Capacities increase until about 25 years old.
Body composition: Larger individuals tend to have larger lung capacities.
Biological sex: Males generally have larger lung capacities than females.
Respiratory disease: Conditions e.g. asthma can reduce it.
Physical activity: Regular exercise can increase.
B3.1.7 Adaptations for gas exchange in leaves - the leaves
Have a large surface area for CO₂ to enter and O₂ to leave. Moist surface allows gases to dissolve to diffuse in/out.
B3.1.7 Adaptations for gas exchange in leaves - Waxy Cuticle
Structure: Layer of wax secreted by epidermal cells.
Function: Waterproof barrier = reduces water loss. Thicker on upper surface + plants in dry habitats.
Gas Permeability: Low.
B3.1.7 Adaptations for gas exchange in leaves - Stomata and Guard Cells
Guard Cells: Lower epidermis is perforated with them, controlling stomatal openings to regulate gas exchange.
Stomata function: Pore that allows CO₂ and O₂ to pass through.
Behavior: Close at night (no photos. + gas exchange) or during water stress to conserve moisture.
B3.1.7 Why are stomata on the bottom?
To control water loss from evaporation, as water can be lost when the stomata open to allow gas exchange. The more sunlight hits an area, the more water it will lose.
B3.1.7 Adaptations for gas exchange in leaves - Air Spaces in Mesophyll
Stomata are connected to these air spaces.
Function: These air spaces allow gases to diffuse easily throughout the leaf (CO₂ in and O₂ out).
Moisture: Walls of mesophyll cells = moist, allows CO₂ to dissolve + diffuse into the cells, aiding photosynthesis.
B3.1.7 Adaptations for gas exchange in leaves - palisade mesophyll
Structure/function: The cells of the palisade mesophyll are tightly packed and rich in chloroplasts (optimised for photosynthesis).
B3.1.7 Adaptations for gas exchange in leaves - vascular bundle
Function: Xylem transports water + minerals from within the roots of the plant (via transpiration)
Phloem transports dissolved sugars produced by photosynthesis to other parts of the plant (as sap)
B3.1.8 Distribution of tissues in a leaf
Palisade mesophyll: upper half (facing sunlight) cells are tightly packed and rich in chloroplasts (optimised for photosynthesis).
Spongy mesophyll: lower half (nearer stomata), cells are loosely packed between intercellular air spaces (maximising gas exchange).
Vascular bundle: located centrally, allows for optimal access by all leaf tissue.
B3.1.9 Transpiration process
1) Water EVAPORATES from the internal leaf cells through the STOMATA.
2) Water passes from the XYLEM vessels to leaf cells due to OSMOSIS…
3) …which pulls the water in that vessel upwards by a very small amount.
4) Water enters XYLEM from root cortex to replace water which has moved upwards.
5) Water enters ROOT HAIR CELLS by OSMOSIS to replace water which has entered the XYLEM.
B3.1.9 Factors effecting transpiration (positive)
Temperature: higher temp = more energy available for evaporation.
Light: Brighter light -> guard cells open more to allow more CO2 in for photosynthesis.
Wind: Air movement removes water vapor, preventing saturation and maintaining the diffusion gradient around the leaf, speeding up transpiration.
B3.1.9 Factors effecting transpiration (negative)
Humidity: Higher humidity reduces water vapor concentration gradient, lowering diffusion. Transpiration stops if air is fully saturated.
B3.1.10 Calculating stomatal density
SD= N / A
SD = Stomatal density (no. of stomata per unit area)
N = No. of stomata counted in a given field of view
A = Area of the field of view (usually in mm² or µm²)
B3.1.11 What is haemoglobin, and what is its structure?
An oxygen transport protein in red blood cells. A haemoglobin molecule carries up to 4 oxygen molecules, its 4 haem groups that act as binding sites.
B3.1.11 What is haemoglobin’s function?
It binds oxygen reversibly and transports it from the lungs to respiring tissues, then CO2 from respiring tissues back to the lungs where it can be exhaled.
Haemoglobin + oxygen -> oxyhaemoglobin complex Haemoglobin - oxygen -> deoxyhaemoglobin
B3.1.11 What is cooperative binding in haemoglobin?
When one oxygen binds to haemoglobin, it increases the oxygen affinity of the remaining haem groups. Conversely, when an oxygen molecule dissociates, it decreases the affinity of the other haem groups.
B3.1.11 Adaptaions of foetal haemoglobin HbF
HbF is the main haemog. in developing foetuses, gradually replaced by adult haemog. by 6 months.
2 alpha + 2 gamma (instead of beta) PP chains -> each contain haem group. Gamma presence -> HbF has a higher affinity for oxygen than adult haemoglobin.
B3.1.12 What is the Bohr shift, and why is it important?
The Bohr shift is the reduction in haemoglobin’s affinity for oxygen due to increased carbon dioxide levels. This helps deliver more oxygen to actively respiring tissues where it is needed most.
B3.1.12 How does CO₂ affect haemoglobin and oxygen release?
CO₂ lowers blood pH by forming H⁺ (CO₂ + H₂O → H⁺ + HCO₃⁻), reducing haemoglobin’s oxygen affinity. CO₂ binds to haemoglobin (carbaminohaemoglobin), which stabilizes haemoglobin in a low-affinity state. This promotes oxygen release in tissues with high CO₂ levels, ensuring active cells receive more oxygen.
B3.1.12 What happens to haemoglobin in the lungs?
In the lungs, where CO₂ concentration is low, carbaminohaemoglobin converts back to haemoglobin, and pH rises. This restores haemoglobin’s high oxygen affinity, allowing full oxygen saturation before transport.
