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B. Form and function - organisms - Coggle Diagram
B. Form and function - organisms
Gas exchange as a vital function in all organisms
What is gas exchange?
Gas exchange - All organisms absorb one gas from the environment and release another
Difussion
Because the molecules move randomly, is a relatively slow process
Unicellular and other small organisms have a large surface for gas exchange.
In larger organisms, the surface area-to-volume ratio is smaller and the distance between the centre of an organism and the exterior is greater
A specialised gas - exchange surface is requires that is much larger than the outer surface.
Properties of gas - exchange surfaces
permeable - oxygen and carbon dioxide
large - the surface is covered by a film of moisture in terrestrial organisms so gases can dissolve
moist - the total surface area is large in relation to the volume of the organism
thin - the gases must diffuse only a short distance, in most cases through a single layers of cells
Maintenance of concentration gradients at exchange surfaces in animals
Difussion gases only happens if there are concentration gradients
In small, aerobically respiring organisms that use their outer surface for gas exchange, it is cell respiration that maintain concentration gradient
Cell respiration - a process continuously uses oxygen and produces carbon dioxide, so the oxygen concentration within the organism remains lower than outside and the carbon dioxide concentration remains higher
Due to aerobic respiration, this blood has a low concentration ofoxygen and a high concentration of carbon dioxide
Ventilation also helps to maintain concentration gradient
ventilation - the movement of water across gills
Mammals - periodically expel air from the alveoly by exhaling and then replace it by inhaling fresh air. This prevents the oxygen concentration from dropping too low for diffusion from the air to the blood and also prevents the carbon dioxide concentration from rising too high. The rate of ventilation is adjusted according to the carbon dioxide concentration of the blood
Fish - take in fresh water throught their mouth and pump it over their gills and then out through the gill slits. This one - way flow of water, combined with blood flow in the opposite direction, ensures that the oxygen concentration in the water adjacent to the gills remains high and the carbon dioxide concentration remains low.
B3.1.4 Adaptations of mammalian lungs for gas exchange
Introduction
All mammals use lungs for gas exchange (even marine species)
Air is drawn into the lungs throught the trachea and the left and right bronchi. In each lung, the bronchus branches repeatedly to form bronchioles. Alveolar ducts branch off from the bronchioles, each leading to a group of five or six alveol
Alveoli - are surrounded by a dense capillary network
The capillary wall is also extremely thin and consists of a single layer of cells
Air and blood are therefore a very short distance apart
The capillaries cover much of the surface of the alveoli but there are also some other cells, some of them have collagen fibres to strengthen the lung tissue and elastic fibres to help limit inhalation and cause passive exhalation.
An individual alveolus provides a small surface area for gas exchange, but because there are so many of them the total area is very large
Cells in the wall secrete a pulmonary surfactant. Its molecus have a structure like phospohlipids. They form a monolayer on the surface of the moisture lining the alveoli (hydrophobic tails facing the air). This reduces the surface tension and prevents the water from causing the sides of the alveoli to adhere when air is exhaled from the lung - so the lung doesn't collapse
B3.1.5 Ventilation of the lungs
Introduction - parts of the process that connects the lungs to the air outside the body: Nose, mouth, trachea, bronchi, bronchioles
The trachea and bronchi have cartilage in their walls to ensure they remain open
The bronchioles have smooth muscle fibres in their walls allowing the width of the airways to vary
Ventilation process
If particle os gas spread out to occupy a larger volume, the pressure of the gas becomes lower (also, conversely).If gas is free to move, it will always flow from regions of higher pressure to regions of lower pressure
During ventilation, muscle contractions cause the pressure inside the thorax to drop below atmospheric pressure. As a consequence air is drawn into the lungs from the athmosphere (inspitation) until the lung pressure has risen to atmospheric pressure. Other musclecontractions then cause pressure inside the thorax rise above atmospheric, so air is forced out from the lungs.
Muscle actions that cause inspiration and expiration page 260.
B3.1.6 Measurement of lung volumes
Tidal volume is the volume is the volume of fresh air that is inhaled and also the amount of stale air that is exhaled with each ventilation.
Ventialation rate is the number of times that air is drawn in or expelled per minute
Vital capacity is the total volume of air that can be exhaled after a maximum inhalation or total volume of air that can be inhaled after a maximum exhalation
Inspiratory reserve volume i the amount of air a person can inhlae forcefully after normal tidal volume inspiration
Expiratory reserve volume is the amount of air a person can exhale forcefully after a normal exhalation
B3.1.7 Adaptations for gas exchange in leaves
Introduction
Chloroplasts need a supply of carbon dioxide for photosynthesis, the oxygen produced during the process of photosynthesis must be removed. A large area of moist surface is required over which carbon dioxide can be absorbed and oxygen excreted - by the leaves
A challenge for plants is to avoid excessive loss of water from the surface, so leaves are adapted for both gas exchange and water conservation
The outer surface of the leaf is covered in a layer of wax, secreted by the epidermis cells. (called waxy cuticle)
Waxy cuticle - has a low permeability to gases, epidermis have guard cells (they change their shape and can open up a pore or close it)
The pore is called a stoma. and it allows carbon dioxide and oxygen to pass through. The guard cells usually close the stomata at night when photosynthesis is not occuring - also if a plant is suffering water stress
Stomata - connect the air outside to a network of air spaces in the spongy mesophyll of the leaf. Carbon dioxide and oxygen can diffuse through these air spaces. The walls of the spongy mespohyll cells provide a large total surface area for gas exchange - these walls are permenently moist, carbon dioxide in the air spaces can dissolve and then diffuse through the mesophyll cells.
Photosynthesis uses carbon dioxide, generation a concentration gradient form the air outside the leaf to the chloroplast. Photosynthesis raises the concentration of oxygen in chloroplasts, so it diffuses to the surfaces of spongy mesophyll cellsand then into the air spaces and out of the leaf
Loss of water by evaporation from the moist spongy mesphyll and usea o water in photosynthesis
B3.1.8 Distribution of tissues in a leaf - diagram made in paper
B.3.1.9 Transpiration as a consequence of gas exchange in a leaf
Exchange of oxygen and carbon dioxide only works effectively if the gas-exchange surface is moist. Water evaporates from a moist surface unless the air is already saturated
If the air is very humid and the number of water molecules evaporating is equal to number condensing - the air saturated with water vapour
no net loss from gas-exchange surface
The amount of water vapour that air can hold when it is saturated varies with temperature, because more energy avaiable to break hydrogen bonds between water molecules at higher temperatures
Transpiration - is the evaporation of water form plants - helps to maintaining moisture conditions in the environment
transpiration rates are affected by environmental factors
Humidity (negative correlation)
The higher the relative humidity of the air, the smaller the concentration gradient of water vapour between air spaces inside the leaf and the air outside, so the lower rate of diffusion. There is no transpiration if the air outside is saturated with water vapour.
Temperature (positive correlation)
At higher temperatures there is more energy available for evaporation. Also warmer air can hold more water vapour before becoming saturated
Guards cells can prevent nearly ll transpiration by closing their stomata - for example in the night when there is not photosynthesis
Disadvantage of closing the stomata in the daylight - little or no carbon dioxide can be absorbed, o the rate of photosynthesis is limited.
Control mechanisms in the guard cell allow the aperture of the stomata to be varied according to the carbon dioxide concentration inside the leaf
B3.1.10 Stomatal density
What is a stomatal density?
Is the number of stomata per unit area of leaf surface. The number of stomata in a known area must be counted (with a microscope)
Two techniques
A sample of epidermisis peeled of the leaf, small areas of epidermis are then mounted in water on a microscope slide and are examined
If the leaf is non hairy and smooth, apply colourless nail varnished, and when it is dry the nail varnished is peeled off, mounted on a miscroscope and examined - the stomata is clearly visible
Formula: Stomatal density = mean number of stomata divided by area of field of view
Potometer
B.3.1.11 Adaptations of foetal and adult haemoglobin for the transport of oxygen
What is a haemoglobin and how it is used?
Is the oxygen transport protein carried by blood cells,oxygen binds reversibly to haemoglobin. Each of the four subunits in a haemoglobin molecule has a haem group which acts as a binding site, so up to four molecules of oxygen can be transported per haemo molecule
Binding in a haemoglobin molecule
When oxygen binds to one haem group, conformational chnges are caused that increase the oxygen affinity in other haem groups - when an oxygen dissociates it causes conformational changes
Haemoglobin molecule fully saturated with four oxygen bound (the R state)
Unsaturated with no oxygen bound (the T state)
The oxygen saturation level of haemoglobin is positively correlated with oxygen concentration. The oxygen concentrations are given as partial pressures with kilopastals as the pressure units. ' help
Because of its cooperative binding, oxygen saturation of haemoglobin is not directly proportional to oxygen concentration.
Instead, it changes from fully staurated to unsaturated over a relatively narrow wangle of oxygen concentrations, this ensures that haemoglobin unloads oxygen oxygen very readily in a tissue where aerobic respiration has reduced the oxygen concentratopm
Without this adaptation, concentrations of oxygen could not be kept as high as they are in respiring tissues, potentially reducing the activity of muscle and other tissues
Humans produce different types of haemoglobin before and after birth. At birth, a baby stillhas red blood cells with foetal haemoglibin )it takes several months to be replaced with cells carrying adult haemoglobin.
Foetal haemoglobin has a stronger affinity for oxygen, is more saturated with oxygen than adult haemoglobin. During pregnancy a foetis obtains oxygen via the placenta.
B.3.1.12 Bohr shift
Introduction
Increased aerobic respiration in active tissues results in greater release of carbon dioxide into the blood
Increases in carbon dioxide concentration decrease the affinity of haemoglobin for oxygen
Two mechanisms cause the decrease in affinity
Carbon dioxide and water are converted in red blood cells into hydrogen ions and hydrogen carbonate ions
This reduces de pH of the blood, which reduces the affinity of haemoglobin for oxygen
This small pH difference is enough to promote oxygen binding to haemoglobin in the lungs and dissociation in active respiring tissues
Each of the four subunits of haemoglobin can react reversibly with carbon dioxide at the amino terminal of the polypeptide. The amine group is converted to carbamate and the haemoglobin becomes carbaminohaemoglobin.
This reaction reduces its affinty for oxygen, due to the high carbon dioxide concentration of actively respiring tissues when it is produced carbaminohaemoglobin it releases oxygen.
The reduction in the affinity of haemoglobin for oxygen in high carbon dioxide concentrations is known as the Bohr shift or Bohr effect, it helps to ensure that respiring tissues have enough oxygen when their need for oxygen is greatest
B3.1.13 Oxygen dissociation curves as a means of representing the affinity of haemoglobin for oxygen at different oxygen concentrations
Oxygen dissociation curves show the percentage oxygen saturation of haemoglobin at different oxygen concentrations
see figure 24 and 23 page 268