B.3.1 Gas Exchange

the process of exchanging gases between the orginsm and their surrounding enviroment

Plants absorb carbon dioxide and release oxygen,
animals absorb oxygen and release carbon dioxide,
Terrestrial organisms exchange gasses with the air,
aquatic organisms exchange gasses sit the water.

Gas exchange is only rapid enough if it occurs over a large surface and the distance it has to diffuse across is short.

Unicellular organisms have a large SA:V ratio and the distance between the center of the organisms and the exterior environment is small, they can therefore use their external structure for gas exchange.

In larger organisms, the SA:V ratio is smaller, and distance between the center and exterior is greater. A specialized gas-exchange structure is therefore needed, such as the alveoli in lungs, or spongy mesophyll in leaves.

Gas exchange surfaces share 4 properties:

Permeable -> oxygen and carbon dioxide can diffuse across freely

Large SA -> Total surface area is large in relation to the volume of the organism

Moisture -> Surface is covered by a film of moisture in terrestrial organisms to gasses can dissolve

Thin -> Gasses must only diffuse a short distance, in most cases through only a single layer of cells

Diffusion even outs concentration gradients, which could slow or stop gas exchange (not good!), so they need to be maintained. Diffusion only happens if there's a concentration gradient

Blood flow - In large organisms, blood flows through dense capillary networks in the organs specialized for gas exchange. Due to aerobic respiration, the blood has a low concentration of oxygen and a high concentration of carbon dioxide, allowing for diffusion of oxygen in and carbon dioxide out. The continual blood flow helps maintain the concentration gradient.

Lungs and ventilation: Mammals expel air from alveoli by exhaling, and replace it by inhaling fresh air, allowing concentration gradient of gases. This prevents the oxygen concentration from dropping too low for diffusion from the air into the blood, and prevents carbon dioxide concentration getting so high that it can't diffuse out of the blood. Rate of ventilation is adjusted according to carbon dioxide concentration in the blood.

Gills and ventilation: Fish take in fresh (oxygenated) water in through their mouths and pump it over their gills and out through the gill slits. This one-way flow of water in combination with blood flow in the opposite direction ensures that oxygen concentration in the water adjacent to the gills remains high and carbon dioxide is low, so that the gasses diffuse into the blood.

lungs: alveoli

Air is drawn into lungs through the trachea (windpipe) and then the left and right bronchi. In each lung the bronchus (singular of bronchi) branch off into bronchioles, and alveolar ducts branch off from the bronchioles, each leading to a group of 5-6 alveoli (air-sacs). Alveoli are surrounded by a dense capillary network full of blood.

surfactant: reduces surface tension, prevents aveoli walls from sticking and collasping during expiration
bronchioles: Air moves through bronchioles
capillary bed: flow of blood throught the capillary maintains a concentration gradient for diffusing gases
Aveoli: air sacs that provide lots of surface area for gas exchange.

see slide 12

inspiration: Air is drawn into the lungs as muscle contractions lower the pressure in the thorax to below atmospheric pressure until lung pressure is at atmospheric pressure (inspiration).


exhalation: muscle contractions than increasing pressure in the lungs until it is above atmospheric pressure, so air is forced out of the lungs, helped by the recoil of the elastic lung fibers that are stretched during inspiration.

This process is ventilation.

They are inversely proportional. A smaller volume means an increase in pressure, while a larger volume means a decrease in pressure.

inspiration: v increase, p deacreas
expiration: v deacrease, p increases

Inspiration:External intercostal muscles contract- ribs move up and out, internal intercostal muscles relax. Diaphragm contracts and flattens → volume in thoracic cavity increases and pressure drops to below atmospheric pressure. Air in the atmosphere moves from higher pressure in the atmosphere to lower pressure in the lungs.

Expsiration: Internal intercostal muscles contract- ribs move in and down; External intercostal muscles relax, Diaphragm relaxes and moves up into the thoracic cavity → volume in thoracic cavity decrease and pressure increases to above atmospheric pressure. Air in the lungs moves from higher pressure in the lungs to lower pressure in the atmosphere.

Ventilation rate: number of inspiration-expsiration cycles in a minutereathing frequency
Tidal volume: Amount of air that moves in and out of the lungs during a normal breath (genrally 0.5dm3 for adult)
Vital capacity: Maximum volume of air that can be breathed in or out of the lungs in one forced breath(Inspiratory reserve volume + tidal volume + expiratory reserve volume)
Inspiratory reserve: amount of air that can be forcefully inspired after a normal tidal volume inspiration
Expirtatory reserve: Amount of air that can be forcefully expired after a normal tidal volume expiration

spirometer is a device that detects the changes in ventilation and presents the data on a digital display

Carbon dioxide diffuses into the leaves, and oxygen diffuses out of the leaves.

waxy cuticle - Waterproof, no evaporation. Not permeable well to gasses so guard cells.
Epidermis - secretes waxy cuticle, surface area, contains guard cells
Stoma(ta if plural) - Pore in guard cells that lets CO2 in and O2 out.(Open lets transpiration of water to occur)
guard cells - cells in epidermis that change shape to open or close stoma pore.
air spaces - Where CO2 and O2 diffuse .
spongy mesophyll - Large SA for gas exchange.
Veins - Water gets to leaves using xylem vessels in the veins.

It focuses on outlining the different tissues and their relative proportions within the specimen, rather than depicting the cellular structure

see slide 13 for diagram

The loss of water vapor from the leaf and stem of a plant

Evaporation is the process of surface water absorbing heat energy from the sun or from warm air and changing from a liquid to a gas. Transpiration is when water evaporates through plants.

Temp: Positive correlation -> At high temps more evap (more transpir), + warmer air can hold more water vapor before becoming saturated

Humidity: Negative correlation -> Higher Humid = less concentration gradient for water in air spaces, lower diffusion of water, no transpiration

Wind speed: positive correlation -- wind speed increases = transpiration rate increases = steeper concentration gradient bc Higher wind speeds remove water vapor from around the leaves creates concentration gradient between the leaf and the surrounding air = more water to evaporate from the leaf, thus increasing transpiration

Light intensity: positve correlation -> light intensity increases = the transpiration rate increases, Increased light intensity = increases the rate of photosynthesis, the stomata open to allow gas exchange; when this happens, water vapour diffuses out of the stomata

When no photosynthesis closed so no water can escape/no water lose via transpiration, when can do photosynthesis open so that carbon dioxide can diffuse in.

the number of stomata per unit area on a leaf surface.
Stomatal density (mm-2) = Mean number of stomata/Area of field of view

can be measured by taking a leaf cast of the upper and lower epidermis of the leaf.
The number of stomata can then be counted by viewing the cast using a microscope.
The number of stomata can be counted and the stomatal density calculated using the formula:

Stomatal density (mm-2) = Mean number of stomata/Area of field of vie