Gas Exchange (DP IB Biology)

The alveoli have thin walls to minimise diffusion distance, a good blood supply to maintain a steep concentration gradient and surfactant to increase the rate of diffusion.

Bronchioles are lined with smooth muscle that can dilate when more air is needed and constrict when an allergen is present, regulating airflow.

Groups of alveoli are found at the end of bronchioles.

Alveoli provide a large surface area for gas exchange and are surrounded by an extensive network of capillaries.

True.

Bronchi have walls strengthened with cartilage and a layer of smooth muscle that can contract or relax to change the diameter of the airways.

Surfactant lowers the surface tension in the alveoli, preventing them from collapsing and sticking together during expiration.

Deoxygenated blood enters the capillary beds from a branch of the pulmonary artery while oxygenated blood leaves via a branch of the pulmonary vein, maintaining the concentration gradient.

True.

Bronchioles branch off the two bronchi to form a network of narrow tubes.

Ventilation is the process of breathing in and out, which is essential for effective gas exchange in the lungs by replacing older air with fresh air and maintaining concentration gradients.

Inspiration is the process of breathing in, which causes the volume of the chest to increase and air pressure to decrease until it is lower than atmospheric pressure.

During inspiration, the diaphragm contracts and flattens, increasing chest volume.

External intercostal muscles contract during inspiration, causing the ribcage to move upwards and outwards, increasing chest volume.

Air moves into the lungs during inspiration because it flows down the pressure gradient from the higher pressure outside to the lower pressure inside the lungs.

False.

Expiration is mostly a passive process due to the recoil of the lungs after they have been stretched by inspiration.

In passive expiration, external intercostal muscles relax, the diaphragm relaxes and becomes dome-shaped, and elastic fibres in alveoli walls recoil, reducing lung volume.

Active expiration occurs when there is a need to expel excess air from the lungs, such as when blowing out a candle.

During active expiration, abdominal muscles contract to push organs upwards against the diaphragm, decreasing the volume of the chest cavity.

False.

During expiration, the volume of the chest decreases and pressure increases, causing air to be forced out down its pressure gradient.

A spirometer is an apparatus used to investigate ventilation, consisting of a water-filled chamber covered by a hinged plastic lid.

Tidal volume is the volume of air inhaled and exhaled during normal breathing.

Vital capacity (VC) is the total amount of air exhaled after taking a deep breath, calculated as:

VC = Tidal Volume + Inspiratory Reserve Volume + Expiratory Reserve Volume.

False.

Exercise leads to an increase in tidal volume as more air is moved in and out of the lungs.

Inspiratory reserve volume is the difference between the maximum inspiratory level and tidal volume.

Ventilation rate is determined by counting the number of inhalations or exhalations per minute on the spirometer trace.

The maximum expiratory level is the maximum volume of air that can be exhaled during expiration.

True.

Exercise causes an increase in ventilation rate as you take more breaths per minute.

Stomata are tiny pores in the lower epidermis of a leaf through which gas exchange occurs.

Transpiration is the loss of water vapour from leaves by evaporation through the stomata.

The waxy cuticle prevents gases and water vapour from leaving through the epidermis, ensuring that gas exchange occurs through stomata.

False.

Most stomata are found in the lower epidermis where the temperature is lower, reducing water loss.

Palisade mesophyll contains many chloroplasts for maximum photosynthesis, while spongy mesophyll contains large air spaces between cells for gas exchange.

Vascular bundles form the veins in leaves and are responsible for the transport of substances around the plant, with xylem transporting water and mineral ions, and phloem transporting products of photosynthesis.

Guard cells control gas exchange and water loss by opening or closing stomata.

More air movement leads to increased rates of transpiration by maintaining a steeper concentration gradient for water vapour diffusion.

Higher temperatures lead to higher rates of transpiration up to a point, as increased kinetic energy causes water molecules to evaporate faster.

Higher light intensities increase the rate of transpiration up to a point, as stomata open in light to enable gas exchange for photosynthesis.

After a point, the stomata may close to reduce water loss as temperatures rise.

Higher humidity levels reduce the rate of transpiration by decreasing the concentration gradient for water vapour diffusion.

A potometer is a piece of equipment used to measure the rate of water uptake in plants, which is used to represent the rate of transpiration.

Two types of potometer are:

1. Bubble potometer

2. Mass potometer

Two advantages of transpiration are:

1. It provides a means of cooling the plant via evaporation

2. It helps in the uptake of mineral ions through the transpiration stream

False.

The movement of water molecules during transpiration is not by osmosis, as it does not cross a cell membrane.

False.

Plan diagrams should not include individual cells, only tissue layers enclosed by lines should be present.

When labelling your plan diagram remember to: 

• Use a ruler to draw label lines, not freehand

• Avoid using arrowheads and make sure the label lines stop at the structure

• Make sure label lines do not cross each other

• Write all labels horizontally, not at different angles

False.

Plan diagrams cannot include shaded areas.

A plan diagram should be big enough to fill up at least half the available space.

Stomatal density is the number of stomata per unit of area.

Leaf cast is an impression of a leaf's surface created by applying and peeling off a layer of nail varnish.

False.

A coverslip is not required as a leaf cast isn't a biological sample, just an impression of one.

The ideal range of stomata to count in a single field of view is between 15 and 100.

Stomatal density = Mean number of stomata ÷ Area of field of view

Anomalous measurements are those that deviate from the expected measurements when counting stomata.

The purpose of using a stage micrometer is to measure the diameter of the field of view at the same magnification used for counting stomata.

Reliability is the level of trust that can be placed in numerical measurements, increased by repeating measurements and obtaining consistent results.

Haemoglobin is the molecule responsible for binding oxygen in our blood. It is a globular protein found in abundance in red blood cells.

Each haemoglobin molecule consists of four polypeptide subunits, with an iron-containing haem group at the centre of each subunit.

Each haem group can bind to one oxygen molecule, so each haemoglobin molecule can transport four oxygen molecules.

Cooperative binding is the phenomenon where the first oxygen molecule binds to haemoglobin with difficulty, but subsequent oxygen molecules bind more easily due to a change in the shape of the protein.

The affinity of haemoglobin for oxygen is high in areas with high partial pressures of oxygen, such as the lungs, and low in areas with low partial pressures of oxygen, such as respiring muscle cells.

Fetal haemoglobin has a higher affinity for oxygen than adult haemoglobin, allowing a fetus to obtain oxygen from its mother's blood at the placenta.

The oxygen dissociation curve shows the percentage saturation of haemoglobin at different partial pressures of oxygen. The curve for fetal haemoglobin lies to the left of the curve for adult haemoglobin.

After birth, a baby begins to produce adult haemoglobin, which gradually replaces the foetal haemoglobin.

Haemoglobin is an allosteric protein, meaning it can exist in multiple conformations due to its ability to change shape when oxygen binds to it.

The Bohr effect or Bohr shift refers to changes in the oxygen dissociation curve as a result of carbon dioxide levels in the blood.

When the partial pressure of carbon dioxide in the blood is high, haemoglobin's affinity for oxygen is reduced.

Carbon dioxide lowers the pH of the blood by combining with water to form carbonic acid, which then dissociates into hydrogen carbonate ions and hydrogen ions. The hydrogen ions bind to haemoglobin, causing the release of oxygen.

The oxygen dissociation curve shifts to the right when CO₂ levels increase, meaning that at any given partial pressure of oxygen, the percentage saturation of haemoglobin is lower at higher levels of CO₂.

The Bohr shift is a helpful change because it means that haemoglobin gives up its oxygen more readily in the respiring tissues where it is needed.

The oxygen dissociation curve shows the rate at which oxygen associates and dissociates with haemoglobin at different partial pressures of oxygen (pO₂).

Haemoglobin is said to be saturated when all of its oxygen binding sites are occupied by oxygen molecules.

When haemoglobin has a high affinity for oxygen, it binds easily and dissociates slowly. When it has a low affinity, it binds slowly and dissociates easily.

At low pO2, oxygen binds slowly to haemoglobin, so the saturation percentage is low.

At medium pO₂, oxygen binds more easily to haemoglobin, and saturation increases quickly.

At high pO₂, oxygen binds easily to haemoglobin, so haemoglobin can become saturated as blood passes through the lungs.

When read from right to left, the curve provides information about the rate at which haemoglobin dissociates with oxygen at different partial pressures of oxygen.

Due to the shape of the haemoglobin molecule, it is difficult for the first oxygen molecule to bind, explaining the relatively shallow curve at the bottom left of the graph.

As the haemoglobin molecule approaches saturation, it takes longer for the fourth oxygen molecule to bind due to the shortage of remaining binding sites.