B3.1 Gas exchange

Gas exchange

The process by which gases are exchanged between living organisms and their environment

Factors affecting the rate of diffusion

Size of the respiratory surface (larger is higher rate), concentration gradient (steeper is higher rate), and diffusion distance (shorter is higher rate)

Challenges of gas exchange in organsisms

An increase in size will result in a smaller SA:V ratio, and a greater diffusion distance. Large multicellular organisms cannot rely on diffusion alone, and require specialised organs

Trachea

A tube supported by rings of cartilage which help to support its shape and ensures it stays open while allowing it to move with the body. It carries air from the nose/mouth to the lungs and divides to form two bronchi, each leading to one lung

Bronchioles

Bronchioles branch off the two bronchi to form a network of narrow tubes. The walls of the bronchioles have smooth muscle which dilates or constricts to regulate air flow. Alveoli are at the end of bronchioles, each surrounded by a network of capillaries for blood supply

Surfactant

Lowers surface tension, prevents alveoli from collapsing and sticking together

Ventilation

Maintains the concentration gradient of oxygen and carbon dioxide between alveoli and the blood, involves inspiration and expiration

Inspiration

The diaphragm contracts and flattens and the external intercostal muscles contract, causing the ribcage to move upwards and outwards, increasing the volume of the thorax. Air pressure decreases until it is lower than atmospheric pressure, and air moves down the pressure gradient and enters the lungs

Passive expiration

The diaphragm relaxes and moves up and the external intercostal muscles relax, causing the ribcage to move downwards and inwards, decreasing the volume of the thorax. Air pressure increases which causes air to be forced out down its pressure gradient

Active expiration (blowing out candle)

The internal intercostal muscles contract, causing the ribcage to move downwards and inwards. Abdominal muscles contract to push organs upwards against the diaphragm, decreasing the volume of the thorax.

Spirometer

Apparatus used to measure ventilation rate, tidal volume, vital capacity, reserve volumes during inspiration and expiration

Tidal volume

The volume of air inhaled and exhaled during normal breathing

Inspiratory and expiratory reserve volumes

The extra volume of air that can be inhaled or exhaled when taking a extra deep breath

Vital capacity

The total amount of air exhaled after taking a deep breath, calculated by adding tidal volume, inspiratory reserve volume, and expiratory reserve volume together

Epidermis

Formed by a single layer of tightly packed cells, has lower and upper epidermis. It is covered by the waxy cuticle which forms an impermeable barrier

Lower epidermis

Contains tiny pores (stomata). Each stoma is surrounded by two guard cells which controls opening and closing. When water moves into the guard cells they become turgid which opens the stomata. When water is lost they become flaccid which closes the stomata.

Mesophyll tissue

Formed by parenchyma cells which contain chloroplasts. The palisade mesophyll forms a layer beneath the upper epidermis and has chloroplasts for photosynthesis. The spongy mesophyll contains air spaces for gas exchange

Vascular tissue

Vascular tissue is arranged in vascular bundles and is responsible for transporting substances around the plant. They form in the veins of leaves. The xylem transports water from roots to leaves and the phloem transports products of photosynthesis from leaves to other parts

Transpiration

The loss of water vapour from the stems and leaves of plants

Factors affecting the rate of transpiration - air movement

More air movement leads to increased rates of transpiration. When the air is still, water molecules accumulate outside the stomata, creating a local area of high humidity. Less water vapour diffuses out as the concentration gradient is less steep

Factors affecting the rate of transpiration - temperature

Higher temperatures lead to higher rates of transpiration as the kinetic energy of molecules increases. This means more water molecules evaporate out of the leaf

Factors affecting the rate of transpiration - light intensity

Higher light intensities increase the rate of transpiration up until a plateau. In the dark stomata close which reduces transpiration. In the light stomata open for photosynthesis which increases transpiration

Factors affecting the rate of transpiration - humidity

Higher humidity reduces the rate of transpiration as the concentration gradient between the inside and outside of the leaf decreases. At a certain level equilibrium is reached so there is no net loss

Potometer

Equipment used to measure the rate of water uptake to indicate the rate of transpiration

Stomatal density

Used to assess the plant’s likely response to dry weather or predict its behaviour in climates

Haemoglobin

Each haemoglobin molecule consists of four polypeptide subunits, at the center of each is an iron-containing haem group that oxygen binds to. After the first oxygen molecule binds, haemoglobin changes shape, which allows more oxygen to bind (cooperative binding)

Partial pressure

The pressure of each gas in a mixture of gases

Foetal haemoglobin

Foetal haemoglobin has a higher affinity for oxygen than adult. It can bind to oxygen at low pO2

Oxygen dissociation curve

Represents the percentage saturation of haemoglobin at different pO2. The curve for foetal haemoglobin shifts to the left, as foetal haemoglobin has a higher percentage saturation than adult

The Bohr shift (effect)

Changes in the oxygen dissociation curve as a result of carbon dioxide levels. When pCO2 is high, affinity for oxygen is reduced, as carbon dioxide lowers the pH of blood. The curve shifts to the right as carbon dioxide levels increase