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AQA Biology Topic 3, Structure and function of blood vessels, Evidence…

- - - - Surface should be thin to ensure the exchange distance is short.
        
        Requires a good blood supply to maintain a steep gradient, e.g alveoli.
  - - - 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.
- - - - Fish have a small surface area to volume ratio for gas exchange.
        
        They have an impermeable membrane so gases cannot diffuse through their skin.
      - The need for gills?
    - - 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.
        
        Counter current flow
        
        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 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.
  - - - 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.
  - - - 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.
- - - - Lungs are surrounded by the ribcage which serve to protect them.
        
        A lubricating substance is secreted to prevent friction between the rib cage and lungs during inflation and deflation.
        
        External and internal intercostal muscles between ribs contract to raise and lower the ribcage respectively.
        
        Diaphragm separates the lungs from abdomen area.
  - - - Trachea and bronchi are similar in structure with the exception of size - bronchi are narrower. They are composed of several layers to make a thick wall. The wall is mostly cartilage in c-rings. Inside surface of the cartilage is a layer of glandular and connective tissue, elastic fibres, smooth muscle and blood vessels. The inner layer is a epithelial layer composed of ciliated epithelium and goblet cells>
        
        The Bronchioles are narrower than bronchi. Only the larger bronchioles contain cartilage. Their wall is made of smooth muscle and elastic fibres. Smallest bronchioles have alveoli clusters at the ends.
  - - - The constant blood supply by capillaries means a steep concentration gradient is maintained.
        
        There are a large number of alveoli, creating a large surface area.
  - - - Ciliated epithelium - present in bronchi, bronchioles and trachea, involved in moving mucus along to prevent lung infection. Moves it towards throat where it can be swallowed.
        
        Goblet cells - cells present in the trachea, bronchi involved in mucus secretion to trap bacteria and dust to reduce risk of infection with the help of lysozymes which digest bacteria.
        
        Smooth muscle - ability to contract enables them to play a role in constricting the airway, thus controlling the flow of air to and from the alveoli.
        
        Elastic fibres - stretch when we exhale and recoil when we inhale thus controlling the flow of air.
- - - - 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.
  - - - 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
- - - - A person breathes into an airtight chamber, thus causing it to move up and down, leaving a trace on a graph.
  - - - Breathing rate - the number of breaths per minute, can be calculated from the spirometer trace.
        
        Tidal volume - the volume of air we breathe in and out at rest.
        
        Residual volume air left behind in lungs following breath.
        
        Inspiratory reserve volume is the volume of air that can be inhaled on top of the tidal volume.
        
        Expiratory reserve volume is the volume of air that can be exhaled on top of tidal volume.
        
        forced expiration volume maximum amount of air exhaled (expiratory vital capacity).
- - - - Amylases in the mouth digest larger polymers.
        
        Maltases in the ileum break down monosaccharides.
        
        Sucrases and Lactases break down the disaccharides sucrose and lactose
    - - Endopeptidases - hydrolyse peptide bonds between specific amino acids in the middle of a polypeptide.
        
        Exopeptidases - hydrolyse the bonds at end of polypeptides.
        
        Dipeptidases break dipeptides into amino acids.
  - - - For every sodium ion that is transported into the cell, an amino acid is transported in. This occurs via facilitated diffusion through co-transport protein.
        
        Amino acids diffuse across the epithelial cell and pass into the capillaries via facilitated diffusion through a channel protein.
        
        Concentration gradient of sodium ions between lumen of ileum and epithelial cell is maintained by active transport of sodium into blood via sodium-potassium pump.
  - - - Sodium ions and glucose molecules are co-transported into the epithelial cells via facilitated diffusion.
        
        The glucose molecules diffuse across the epithelial cell and enter the capillary at the other end of the cell by facilitated diffusion.
        
        The concentration gradient of sodium ions is maintained by actively transporting sodium ions out of the epithelial cells into the blood.
  - - - 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.
- - - - The greater the concentration of dissolved oxygen in cells the greater the partial pressure.
        
        As Partial pressure increases, the affinity of haemoglobin for oxygen increases, henceforth oxygen binds to the haemoglobin in a process known as loading.
        
        During respiration, oxygen is used up and therefore the partial pressure decreases, thus decreasing the affinity for oxygen for haemoglobin.
        
        As a result of that, oxygen is released in respiring tissues where it is needed. This is known as unloading, the haemoglobin returns to the lungs where it binds again.
  - - - 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.
        
        Affect of carbon dioxide concentration
        
        Affinity of haemoglobin for oxygen is also affected by the partial pressure of carbon dioxide.
        
        Carbon dioxide is released by respiring cells which require oxygen for the process to occur.
        
        In the presence of carbon dioxide, the affinity of haemoglobin for oxygen decreases, thus causing it to be released. This is known as the bohr effect. Carbon dioxide creates slightly acidic conditions which change the shape of the haemoglobin protein, making it easier for oxygen to be released.
    - - Fetal Haemoglobin has a higher affinity for oxygen (compared to adult haemoglobin) to allow the foetus to survive at low partial pressures. The foetus needs to be better at absorbing oxygen because, by the time oxygen reaches the placenta, oxygen saturation of the blood is reduced.
- - - - Suitable Medium - in mammals the transport medium is blood. Water based so substances can easily dissolve into it.
        
        System to move the medium - animals often have a pump known as a heart to maintain pressure differences throughout the body.
        
        Mechanisms to control flow around the body - valves are used to stop backflow.
        
        Closed system for vessels - in mammals the system is closed and branched to reach all parts of the body.
        
        The system in mammals is a closed double circulatory system. The heart pumps blood to the lungs and rest of body.
  - - - Atrium - thin walled and elastic, the atrium can stretch when filled with blood.
        
        Ventricle - thick muscular wall to pump blood around the body or to the lungs.
        
        Two separate pumps are required to maintain blood pressure throughout the whole body.
      - Chambers of heart
    - - Between the atria and the ventricles there are a set of valves. Left atrioventricular valve (Bicuspid) and the Right atrioventricular valve (Tricuspid).
        
        Between the ventricles and arteries there is a set of valves. Left semi-lunar valve and the Right semi-lunar valve.
    - - Aorta - connected to the left ventricle and carries oxygenated blood to all parts of the body.
        
        Pulmonary artery - connected to the right ventricle and carries deoxygenated blood to the lungs.
        
        Pulmonary vein - connected to left atrium and brings oxygenated blood back from the lungs.
        
        Vena cava - connected to the right atrium and brings deoxygenated blood back from the tissues.
  - - - Atrial systole - atria contract forcing blood into the ventricles.
        
        Ventricular systole - contraction of ventricles causes the atrioventricular valves to close and the semi lunar valve open. Thus allowing blood to leave.
  - - - Hydrostatic pressure is created when blood is pumped along the arteries, into the arterioles and then capillaries. The pressure forces blood fluid out of the capillaries. Only substances which are small enough to escape through the gap, these are components of tissue fluid - includes dissolved nutrients such as amino acids, fatty acids, ions, glucose and oxygen.
        
        The fluid is also acted on by hydrostatic pressure which pushes some of the fluid back into the capillaries. Both contain solutes so both have a negative water potential. The water potential of the tissue fluid is less negative. This causes water to move down the water potential gradient out of the tissue fluid via osmosis.
        
        The remaining tissue fluid which is not pushed back into the capillaries is carried back via the lymphatic system. The lymphatic system contains:
        
        Lymph fluid - similar to tissue fluid but contains less oxygen and nutrients as its main purpose is to carry waste products.
        
        Lymph nodes - filter out bacteria and foreign material from fluid with the help of lymphocytes which destroy pathogens as a part of the immune system.
- - - - 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.
  - - - 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.
  - - - Measuring the movement of a meniscus or a Bubble can be used to determine the rate of transpiration.
        
        The rate of transpiration depends on: number of leaves, position of stomata, waxy cuticle, amount of light present, humidity and air/water availability.
  - - - 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.
- - - - 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.
  - - - Facilitated diffusion allows returning H+ Ions to bring sucrose molecules into the companion cells, causing the concentration of sucrose in companion cell to increase.
        
        Sucrose diffuses out of companion cells down the concentration gradient into sieve tube elements through link known as plasmodesmata.
        
        As sucrose enters the sieve tube element, the water potential inside the tube is reduced. Therefore, water enters the sieve tube element from the xylem, increasing the **hydrostatic pressure.
        
        As a result water moves down the sieve tube elements from and area of high hydrostatic pressure to and area of low.
        
        Sucrose is removed from the sieve tube elements by diffusion or active transport into surrounding cells, increasing water potential. Means water diffuses back into xylem reducing the pressure at the sink.
  - - - The concentration of sucrose is higher in the leaves than the root (sink).
        
        Increase in sucrose levels in leaves is followed by similar increase in sucrose concentration in phloem.
        
        Lack of oxygen inhibits translocation of sucrose in the phloem.
  - - - 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.
- - - - Apoplast pathway where the water moves through the water filled spaces between cellulose molecules in the cell wall. In this pathway, water doesn't pass through any plasma membranes therefore it can carry dissolved mineral ions and salts.
- - - - Venules - larger than capillaries but smaller then veins.
        
        Veins - carry blood from the body to the heart, contain a wide lumen to maximise blood flow. They are thin walled as blood is under a low pressure and contain valves to prevent backflow of blood. Weak pulse means there is little elastic tissue or smooth muscle.