Exchange - Chapter 6

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  • Exchange
    • Fish
      • Structure of gills
        • Made up of gill filaments, stacked in a 'pile' on top of each other
          • At right angles to the filaments are gill lamellae, which increase the SA of the gills.
            • Flow of water over the gill lamellae, and the blood within them, are in opposite directions.
              • Counter-current flow
              • Ensures max gas exchange
      • Counter- current exchange principle
        • Blood that is already loaded with oxygen meets water with a higher [O2], and so diffusion of oxygen from water to blood takes place.
        • blood with little oxygen meets the water, which has a relatively same [O2], thus diffusion of oxygen takes place.
        • Diffusion gradient is maintained across the entire width of the gill lamellae.
          • 80% O2 available is absorbed.
          • If blood:water flow was in the same direction, only 50% of O2 would be absorbed
    • Leaf of a plant
      • Photo- synthesis and respiration can use each other's "waste" gases
        • Reduces gas exchange with the external air
        • Volumes or types of gas exchange change, depending on the rate of photo-synthesis and respiration.
        • Most CO2 for photo-synthesis is sourced from the external air
          • Similarly, O2 from photo-synthesis is mostly diffused out, instead of being used for respiration
        • During night (or when its dark), oxygen is still taken up, as respiration never stops.
          • Photo-synthesis stops when there is no sunlight.
      • Structure of plant leaf
        • Diffusion takes place in the gaseous phase, therefore more rapid
        • Air spaces within the leaf has a large surface area.
        • Adaptations for gas exchange
          • Small pores, called stomata; no cell is far from a stomata, therefore diffusion pathway is short
          • Numerous inter-connecting air-spaces throughout mesophyll
          • Large SA of mesophyll cells for rapid diffusoin
        • Stomata
          • Minute pores that occur mostly. but not exclusively on leaves, specifically on the underside of the leaf.
          • Surrounded by guard cells
            • Open/close stomata pore.
              • Control rate of gaseous exchange
                • Prevent water loss via evaporation
                  • Controlled by closing stomata when water loss would be excessive
    • Limiting water loss
      • Insects
        • Permeable surface and large surface area are contradictory to conserving water.
        • Adaptations
          • Small surface area to volume ratio (SA:VOL)
          • Waterproof coverings over body surfaces (waterproof cuticle)
          • Spiracles; opening of spiracles happens mostly when insect is resting.
      • Plants
        • Photo-synthesis needs a large surface area to capture sunlight
        • Waterproof coverings (over most of insect), and ability to control stomata.
        • Limit water loss via transpiration (there plants are called xerophytes)
          • Adaptations
            • Thick (waxy) cuticle; e.g. holly,
            • Rolling up of leaves,
              • Protects lower epidermis (where stomata are usually found)
              • Still air build up within rolled up leaf, air becomes saturated with water vapour, increasing its water potential, thus conservation of water.
              • Example: Marram grass
            • Hairy Leaves
              • Traps still, moist air next to the leaf surface.
                • Water potential gradient between inside of leaf and outside is reduced, therefore less water loss.
                  • Example: Heather Plant
            • Stomata in pits or grooves
              • Trap still, moist air near the leaf and reduces water potential gradient.
                • Example: Pine trees.
            • Reduced SA:VOL of the leaf.
              • Leaves that are small and roughly circular in cross-section, rather than broad or flat, rate of water loss can be considerably reduced
                • Example: Pine needles
        • Adaptations
          • Thick (waxy) cuticle; e.g. holly,
          • Rolling up of leaves,
            • Protects lower epidermis (where stomata are usually found)
            • Still air build up within rolled up leaf, air becomes saturated with water vapour, increasing its water potential, thus conservation of water.
            • Example: Marram grass
          • Hairy Leaves
            • Traps still, moist air next to the leaf surface.
              • Water potential gradient between inside of leaf and outside is reduced, therefore less water loss.
                • Example: Heather Plant
          • Stomata in pits or grooves
            • Trap still, moist air near the leaf and reduces water potential gradient.
              • Example: Pine trees.
          • Reduced SA:VOL of the leaf.
            • Leaves that are small and roughly circular in cross-section, rather than broad or flat, rate of water loss can be considerably reduced
              • Example: Pine needles
    • Human gas-exchange system
      • Structure and function
        • Lungs
          • Series of highly branched tubules, called bronchioles, which end in tiny air sacs called alveoli.
        • Trachea
          • Flexible airway that's supported by cartilage, which prevents trachea from collapsing (as pressure falls within when breathing in).
            • Trachea walls made of muscle, lined with ciliated epithelium and goblet cells.
        • Bronchi
          • Two divisions leading to each lung
            • Produce mucus to trap dirt particles, have cilia that move dirt-laden mucus to the throat.
          • Larger bronchi are supported by cartilage, although reduced as bronchi get smaller.
        • Bronchioles
          • Series of branching subdivisions of the bronchi
            • Walls are made of muscle lined with epithelial cells.
              • Muscles allow them to constrict to control flow of air in/out of alveoli.
        • Alveoli
          • Minute air-sacks, at end of bronchioles
            • Between alveoli, there are some collagen and elastic fibres.
            • Lined with epithelium.
            • Elastic fibres allow alveoli to stretch when they fill with air.
              • Spring back during breathing out in order to expel the carbon dioxide-rich air.
                • Alveoli membrane is the gas-exchange surface.
      • Mechanism of breathing
        • Breathing is also known as ventilation
        • When pressure of atmosphere is greater than in the lungs, air is forced into the lungs.
          • Called inspiration (inhalation)
            • Step - by - step process:
              • 1) External intercostal muscles contract, internal intercostal muscles relax.
                • 2) Ribs are pulled upwards and outwards, increasing volume of thorax.
                  • 3) Diaphragm muscles contract, causing it to flatten, which also increases volume of thorax.
                    • 4) Increased volume of thorax results in reduction of pressure in the lungs.
                      • 5) Atmospheric pressure greater than pulmonary pressure, so air is forced into the lungs.
                    • Untitled
        • When air pressure in lungs is greater than the atmosphere, air is forced out.
          • Called expiration (exhalation)
            • Step - by - step process:
              • 1) internal intercostal muscles contract, wile the external intercostal muscles relax.
                • 2) Ribs move downwards and inwards, decreasing the volume of the thorax.
                  • 3) Diaphragm muscles relax, pushed up again by contents of abdomen. Volume of thorax therefore decreases.
                    • 4) Decreased volume of the thorax increases the pressure in the lungs.
                      • 5) Pulmonary pressure is now greater than that of the atmosphere, air is forced out of the lungs.
            • Passive process
        • Pressure changes brought about by three sets of muscles
          • Diaphragm
          • Internal intercostal muscles - contraction = expiration
          • External intercostal muscles - contraction = inspiration
        • During normal breathing, the recoil of the elastic tissue in lungs us the main cause of air being force out.
          • Only under strenuous conditions such as exercise do various muscles play a vital part.
      • Exchange of gases in lungs
        • Efficient transfer, surfaces must be thin, partially permeable and have a large surface area.
        • Role of alveoli in gas exchange
          • 300 million alveoli in each human lung
          • Each alveolus is lined with epithelial cells.
            • Around each alveolus is a network of pulmonary capillaries.
              • Red blood cells are flattened against the thin capillary walls.
          • Capillaries have walls consisting of one cell thickness.
        • Diffusion of gasses is rapid because:
          • Red blood cells are slowed as they pass through the pulmonary capillaries, allowing more time for diffusion.
            • Distance between alveolar air and red blood cells is reduced as the red blood cells are flattened against the capillary walls
              • Walls of alveoli and capillaries are very thin, therefore distance of diffusion pathway is very short.
                • Alveoli and pulmonary capillaries have very large total surface area.
                  • Breathing movements constantly ventilate the lungs, action of the heart constantly circulates blood around the alveoli.
                    • These ensure that a steep concentration gradient of the gases to be exchanged is maintained.
                      • Blood flow through pulmonary capillaries maintains a concentration gradient.
    • Enzyme and digestion
      • Digestive system
        • Oesophagus
          • Carries food from mouth to stomach
        • Stomach
          • Muscular sac, with inner layer that produces enzymes
            • Mainly digest protein, has glands that produce enzymes to help digest proteins
        • Ileum
          • Long muscular tube
          • Food further digested by enzymes produced by its walls and the glands that pour their secretions into it
            • Inner walls folded into villi.
              • Gives them a large surface area
                • Surface areas of the villi is further increased by millions of tiny projections, called microvilli.
                  • Helps (adaptation) absorb products of digestion into the bloodstream.
            • Surface areas of the villi is further increased by millions of tiny projections, called microvilli.
              • Helps (adaptation) absorb products of digestion into the bloodstream.
        • Large intestine
          • Absorbs water
            • Most water absorbed is from the secretions of the many digestive glands.
        • Rectum
          • Final section of intestine
            • Faeces are stored here
              • Removed via egestion.
        • Salivary glands
          • Located near the mouth
            • Pass secretions via a duct into the mouth
              • Secretion contain amylase, which hydrolyses starch into maltose.
        • Pancreas
          • Large gland located behind the stomach.
            • Produces secretion known as pancreatic juice.
              • Contains protrases to hydrolyse proteins
                • Lipase to hydrolyse lipids
                  • Amylase to hydrolyse starch.
      • Stages of digestion
        • Physcial breakdown
          • Carried out by structures such as teeth
            • Provides a larger surface area for Chemical digestion.
        • Chemical digestions
          • Hydrolyses large, insoluble molecules into smaller, soluble ones.
            • Carried out by enzymes.
          • One enzyme hydrolyses a large molecule into smaller sections
            • Additional enzymes then hydrolyse the molecule even more,
      • Types of digestion (2)
        • Carbohydrate digestion
          • Many enzymes involved in breaking down large molecules into their monomers
            • Usually produced in different parts of the digestive system.
          • Amylase is produced in the mouth and pancreas
            • Hydrolyses the alternate glycosidic bonds of the starch molecule to produce the dissacharide maltose.
              • Hydrolysed into the mono-saccharide alpha-glucose by a second enzyme.
                • Second enzyme is the disaccheride maltase
                  • Produced by lining of the ileum.
          • Process takes place as follows:
            • 1) Saliva enters the mouth through the salivary glands and is thoroughly mixed with the food chewing.
              • 2) Saliva contains salivary amylase; starts hydrolysing any starch.
                • Also contains mineral salts that help to maintain the pH at around neutral (optimum pH)
                • 3) Food enters stomach, where the conditions are acidic; this denatures the amylase.
                  • 4) After, food has passed through the small intestine, where it mixes with the secretion from the pancrease
                    • 5) Contains pancreatic amylase; continues to hydrolyse any remaining starch to maltose.
                      • Alkaline salts are produced by pancreas and intestinal wall to maintain H around neutral.
                      • 6) Muscles in intestine push the food along the ileum; epithelial lining produces maltase.
                        • Not released into lumen of the ileum, but is part of the cell-surface membranes of epithelial cells.
                        • 7) Maltose hydrolyses the maltose from the starch breakdown into the alpha- glucose.
          • Sucrose/ Sucrase
            • Hydrolyses single glycosidic bond in the sucrose molecule.
            • Produces two mono-saccharides glucose and fructose.
          • Lactose/ Lactose
            • Hydrolyses single glycosidic bond in the lactose molecule.
            • Hydrolysis produces the two mono-saccharides glucose and galactose.
        • Protein digestion
          • Endo-peptidases
            • Hydrolyse peptide bonds between amino acids in the central region of the protein molecule
          • Exo-peptidases
            • Hydrolyses peptide bonds on the terminal amino acids of the peptide molecules formed by endo-peptidases
          • Dipeptidases
            • hydrolyses bonds between two amino acids of a dipeptide
              • Dipeptides are membrane -bound, being part of the cell-surface membrane of the epithelial cells lining the ileum.
      • Types of digestion (1)
        • Lipases
          • Produced by the pancreas
          • Hydrolyses lipids.
            • Hydrolyse the ester bond found in triglycerides to form fatty acid molecule attached.
            • Split up into tiny droplets called micelles by bile salts (produced by the liver).
              • Process is called emulsification
                • Increases the surface area of the lipids so that the action of lipases is sped up
      • Absorption of products of digestion
        • Structure of ileum
          • Wall of ileum is folded and possesses finger-like projections called villi
            • Think walls, line with epithelial cells on the other side of which  is a rich network of blood capillaries.
              • They increase surface area, therefore accelerate the rate of absorption.
          • Villi are situated at the interface between the lumen and the other tissues inside the body.
            • Increase the efficiency of absorption in  the following ways:
              • Increase surface area for diffusion
              • Very thin walled, reducing the distance over which diffusion takes place.
              • Contain muscle and so are able to move; helps to maintain diffusion gradients because their environment mixes the contents of the ileum.
              • Ensures product of digestion is absorbed by the villi.
              • They're well supplied with blood vessels so that blood can carry away absorbed molecules and hence maintain a diffusion gradient.
              • Epithelial cells lining the villi possess microvilli.
                • Finger-like projections of the cell-surface membrane that further increase the surface area for absorption
      • Absorbing different molecules
        • Amino acids and mono-saccherides
          • Method of absorption is mainly diffusion and co-transport
        • Triglycerides
          • Mono-saccherides and fatty acids remain associated with the bile salts that initially emulsified the lipid droplets
            • Called micelles
            • Through the movement of other material, Micelles come into contact with the epithelial cells lining the villi of the ileum.
          • Through the movement of other material, Micelles come into contact with the epithelial cells lining the villi of the ileum.
          • Micelles break-down, releasing mono-glycerides and fatty acids.
            • Non-polar, so normally diffuse across cell-membrane into the epithelial cell.
          • Transported to the endoplasmic reticulum where they recombined to form triglycerides.
            • Starting at the ER and continuing to the Golgi apparatus, the triglycerides associate with cholesterol and lipoproteins
              • Forms structures called chylomicrons
                • Special particles adapted for the transport of lipids.
                • Move out of epithelial cells by exocytosis.
                  • Enter lymphatic capillaries called lacateals
                    • Found at the centre of each villus.
                    • Pass via lymphatic vessels, into the blood stream.
                      • Triglycerides in the chylomicrons are hydrolysed by an enzyme in the endothelial cells of blood capillaries
                        • From here they diffuse into cells

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