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AQA Biology Topic 3, Structure and function of blood vessels, Evidence…
AQA Biology Topic 3
Exchange
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Multicellular organisms
- As the size of the organism, and its surface area to volume ratio increases, the need for specialised exchange surfaces arises.
- In single celled organisms, the substances can easily enter the cell as the distance that needs to be crossed is short.
- Multicellular organisms require specialised exchange surfaces for efficient gas exchange of carbon dioxide and oxygen.
Ventilation and gas exchange in fish, insects and plants
Fish
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Gill structure
- Bony fish have four pairs of gills, each gill is supported by a gill arch.
- Along each Gill arch there are multiple projections called gill filaments.
- The gill filaments have lamellae along them which participate in gas exchange.
- Blood and water flow across the lamellae in a counter current direction.
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- This ensures a steep diffusion gradient is maintained and the maximum amount of oxygen is diffused in to the blood.
- The projections are held apart by water, explaining why fish cannot survive for long out of water.
Ventilation
- Ventilation is required to maintain a continuous unidirectional flow.
- Ventilation begins with the fish opening its mouth followed by lowering the floor of buccal cavity. Allowing water to flowing in.
- The fish then closes its mouth, causing the buccal cavity floor to raise, thus increasing the pressure.
- Water is forced over the gill filaments by the difference in pressure between the mouth cavity and opercular cavity. Operculum acts as a valve.
Terrestrial insects
- Insects do not have a transport system therefore oxygen needs to be transported directly to tissues undergoing respiration.
- Insects use the tracheal system. Spiracles, small openings of tubes, either bigger trachea or smaller tracheoles, which run through supplying the required gases.
- Gases move in and out through diffusion, mass transport as a result of muscle contraction and as a result of volume changes in the tracheoles.
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Plants
- Plants are adapted to efficient gas exchange through many adaptations in their leaves.
- Leaves have lots of stomata which means no cell is far from the stomata, reducing diffusion distance.
- Leaves also possess air spaces to allow gases to move around the leaf and come into easy contact with photosynthesising mesophyll cells.
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Ventilation
Inspiration
- External intercostal muscles contract whereas the internal muscles relax. The ribcage is raised upwards.
- Diaphragm contracts and flattens causing the chest cavity (thorax) volume to increase.
- Air pressure is reduced, a gradient with atmospheric pressure is created. This forces air into lungs.
Expiration
- Internal intercostal muscles contract and the external muscles relax - lowering the ribcage.
- The diaphragm relaxes and raises upwards.
- Chest cavity (inside thorax) volume is decreased, therefore the pressure is increased which forces air out of the lungs
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Digestion and absorption
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Lipid absorption
- The products of lipid digestion are fatty acids, monoglycerides and glycerol.
- Bile salts emulsify lipids into smaller fatty acid droplets which have a higher surface area for lipase to work on.
- micelles are water-soluble vesicles formed of the fatty acids, glycerol, monoglycerides and bile salts. They deliver fatty acids, glycerol and monoglycerides to epithelial cells for absorption.
- Due to fatty acids and monoglycerides Non-polar nature they can diffuse across the cell surface membrane.
- Once in the cell, fatty acids and monoglycerides are modified back into triglycerides inside of the endoplasmic reticulum and Golgi apparatus.
- Triglycerides combine with proteins to form chylomicrons, these are released via a Golgi vesicle which fuses with membrane and releases via exocytosis.
- Chylomicron is then absorbed by the lacteal (Lymph vessel). Lymph carries away which eventually drains into capillaries.
Haemoglobin
Haemoglobin is a water-soluble globular protein which consists of two beta polypeptide chains and two alpha-helices. Each molecule forms a complex containing a haem group. It carries oxygen in blood as can bind to the haem (Fe2+) group. Each can carry four oxygen molecules.
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Dissociation curves
- Dissociation curves illustrate the change in haemoglobin saturation as partial pressure changes.
- The saturation of haemoglobin is affected by its affinity for oxygen. Therefore, if there is a high affinity if oxygen the haemoglobin will be highly saturated.
- Saturation can also have an affect on affinity. After the first oxygen molecules binding the the oxygen affinity for haemoglobin increases due to a change in shape, thus making it easier for oxygen to bind.
Shape of curve
- Initially the curve is shallow because it is hard for the first oxygen molecule to bind.
- Once the first has bound the shape is changed making it easier for the second and third molecules to bind, hence a steep increase. This is called positive cooperativity.
- The gradient flattens out again because the likelihood of the fourth oxygen finding a binding site is low.
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Transpiration
Transpiration is the process where plants absorb water through the roots, which then moves up through the plant and is released into the atmosphere as water vapour through pores in the leaves. Carbon dioxide enters, while water and oxygen exit through a leaf's stomata.
Xylem features
- Transport water and minerals, and also serve to provide structural support.
- They are long cylinders made of dead tissue with open ends, therefore they can form a continuous column.
- Xylem vessels also contain pits which enable water to move sideways between the vessels.
- They are thickened with a tough substance known as lignin, which is deposited in spiral patterns to enable the plant to remain flexible.
Transpiration stream
- The movement of water up the stem, enables processes such as photosynthesis, growth and elongation.
- The transpiration stream also supplies the required minerals.
- Transpiration involves osmosis, where water moves from the xylem to the mesophyll cells. Water is evaporated from the surface of the mesophyll cells and diffusion of water out through the stomata.
- Flow of water upwards through the xylem is maintained by the cohesion tension theory (hydrogen bonds between the xylem wall and other water molecules.
- Root pressure - action of endodermis moving minearlas up the xylem by acitve transport drives wtaer up the xylem.
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Xerophytes
- Plants adapted to live in dry conditions. They are adapted to minimise water loss.
- Adaptations include smaller leaves reducing surface area for water loss; densely packed mesophyll and thick waxy cuticle prevent water loss via evaporation.
- Xerophytes respond to low water availability by closing stomata to prevent water loss.
- Hairs and pits trap moist air , reducing the water potential gradient.
Translocation
Phloem vessel features:
- They're tubes made of living cells involved in translocation of nutrients to storage organs and growing parts of the plant.
- Consist of sieve tub elements and companion cells
- Sieve tube elements form a tube to transport sugars such as sucrose, in the dissolved form of sap.
- Companion cells are involved in ATP production for active process such as loading sucrose into sieve tubes.
- The cytoplasm of the sieve tube elements and companion cells is linked through structures known as plasmodesmata which are gaps between cell walls which allow communication and flow of substances such as minerals.
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Translocation - is an energy requiring process which serves as a means of transporting sucrose in the phloem which releases sucrose such as leaves and sinks
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Ringing Experiments
- Bark and phloem of leaf are removed leaving just the xylem. Overtime the tissues above swell with sucrose solution. Proving sucrose is transported in the phloem.
Tracer experiments
- Plants are grown in an environment that contains the radioactively labelled carbon dioxide (14Co2). They are then incorporated into the sugars produced via photosynthesis.
- The movement of sugars can be traced using autoradiography. Those areas that have been exposed to radiation from 14C in the sugars will appear black. These correspond to where the phloem is suggesting thats where sucrose is transported.
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