6 - Exchange

• What is the relationship between the size of an object and its surface area to volume ratio?

  The larger an organism is, the smaller its surface area compared to its volume and the larger the distance from the middle to the outside
• Why is it sufficient for small organisms such as amoeba to exchange gases across their surface, rather than requiring a special exchange system (like the lungs)?

  Small organisms have a very large surface area in comparison to their volume. This means that there is a big surface for the exchange of substances, but also there is a smaller distance from the outside of the organism to the middle of it. As a result, the very small organism can simply exchange substances across its surface by diffusion
• What are some examples of adaptations that larger organisms have to meet the demands that help make the exchange across surfaces more efficient?

  1. Villi and microvilli
  2. Alveoli and bronchioles
  3. Spiracles and tracheoles
  4. Gill filaments and lamellae
  5. Thin wide leaves
  6. Many capillaries
• What occurs during inspiration in terms of the external/internal intercostal muscles and diaphragm?

  • External IM - Contract to pull ribs up & out
  • Internal IM - Relax
  • Diaphragm - Contracts to move down & flattens
  • Air pressure in lungs - Initially drops. As air moves in it rises above atmospheric pressure
  • Lung volume - increases
  • Movement of Air - Air moves in, as atmospheric pressure is higher than that in the thorax

  
• What occurs during inspiration in terms of the external/internal intercostal muscles and diaphragm?

  • External IM - Relax
  • Internal IM - Contract to pull the rib down and in
  • Diaphragm - Relaxes to move up and dome
  • Air pressure in lungs - Initially greater than atmospheric pressure. Drops as air moves out
  • Lung volume - decreases
  • Movement of Air - Air moves out as pressure in thorax is higher than that of atmosphere

  
• What are some key structures in the gas exchange system?

  • alveoli
  • bronchioles
  • bronchi
  • trachea
  • lungs

  
• What are the three features that all gas exchange surfaces have to increase the rate of exchange (Ficks Law)?

  1. Large surface area
  2. Short diffusion distance
  3. Maintained concentration gradient
• Describe the adaptations the alveoli have to increase the efficiency of gas exchange?

  1. Alveoli are tiny air sacs, and there are 300 million in each human lung which creates a very large surface area for gas exchange (diffusion).
  2. The alveolar epithelial cells are very thin, to minimise diffusion distance.
  3. Each alveolus is surrounded by a network of capillaries to remove exchanged gases, and therefore maintains a concentration gradient.

  
• State the adaptations the fish gills have to increase the efficiency of gas exchange?

  1. To create a large surface area:volume for diffusion, there are many gill filaments covered in many gill lamellae.
  2. There is a short diffusion distance due to a capillary network in every lamella and each gill lamella is very thin.
  3. The concentration gradient is maintained by the countercurrent flow mechanism.
• What are the key components of fish gills?

  • Gill filaments
  • Capillaries
  • Gill lamellae

  
• Why is countercurrent flow important for oxygen absorption in fish?

  This is when water flows over the gills in the opposite direction to the flow of blood in the capillaries.

  
  

  This ensures that the diffusion gradient is maintained across the entire lamella.

  
• What are the 3 key components of the tracheal system for insects?

  • Spiracles
  • Trachea
  • Tracheoles
• What adaptions does the tracheal system have to increase efficiency of gas exchange?

  1. A large number of fine tracheoles - large surface area
  2. The walls of the tracheoles are very thin and there is a short distance between spiracles and tracheoles - a short diffusion pathway
  3. The use of oxygen and the production of carbon dioxide set up steep diffusion gradients.
• How is the tracheal system involved in ventilation & how are gases exchanged (first 2 methods - diffusion & abdominal muscles)?

  1. Gas can exchange by diffusion, as when cells respire, they use up oxygen and produce carbon dioxide, creating a concentration gradient from the tracheoles to the atmosphere.
  2. The second method of gas exchange is mass transport, in which an insect contract and relaxes its abdominal muscles to move gases en masse.
• How is the tracheal system involved in ventilation & how are gases exchanged (final method - in flight)?

  
  

  3. The final method is when the insect is in flight, where the muscle cells start to respire anaerobically to produce lactic acid. This lowers the water potential of the cells, and therefore water moves from the tracheoles into the cells by osmosis. This decreases the volume in the tracheoles and as a result, more air from the atmosphere is drawn in
• What are the three adaptations insects have to prevent water loss across their gas exchange surface?

  1. Insects have a small surface area to volume ratio where water can evaporate from.
  2. Insects have a waterproof exoskeleton.
  3. Spiracles, where gases enter and water can evaporate from, can open and close to reduce water loss.
• What are the stages of gas exchange in plants?

  The palisade mesophyll is the site of photosynthesis, where lots of oxygen is produced and carbon dioxide is used up. This creates a concentration gradient. Therefore, oxygen will travel through the air spaces in the spongy mesophyll and diffuse out of the stomata created by guard cells. Carbon dioxide will diffuse in through the stomata.
• What adaptations do plants have to reduce water loss (including xerophytes)?

  1. Stomata close at night when photosynthesis wouldn’t be occurring.
  2. Xerophytic plants are adapted to survive in environments with limited water. Adaptations include sunken stomata, fewer stomata, rolled leaves and tiny hairs around the stomata. Waxy future, needle-like leaves to reduce SA sp less evaporation. Deep root system to reach more water
• What is digestion?

  Large biological molecules are hydrolysed into smaller molecules that can be absorbed across cell membranes.
• Name the three locations carbohydrates are digested?

  1. Mouth
  2. Duodenum
  3. Ileum
• Which two enzymes hydrolyse carbohydrates, where are they produced and what are the products?

  • Name - Amylases
  • Location - pancreas and salivary glands
  • Products - hydrolyses polysaccharides into the disaccharide maltose by breaking the glycosidic bonds.
  • Name - Membrane-bound disaccharidases
  • Location - small intestines
  • Products - Sucrase and lactase are membrane-bound enzymes that hydrolyse sucrose and lactose into monosaccharides.
• Three enzymes are involved in digesting proteins. Describe the role of each?

  1. Endopeptidases - hydrolyse peptide bonds between amino acids in the middle of a polymer chain.
  2. Exopeptidases - hydrolyse peptide bonds between amino acids at the end of a polymer chain.
  3. Membrane-bound dipeptidases - hydrolyse peptide bonds between two amino acids.
• Where does protein digestion occur?

  Digestion starts in the stomach, continues in the duodenum and is fully digested in the ileum.
• Which two molecules are needed to digest lipids?

  1. Lipase
  2. Bile Salts
• Describe the digestion of lipids?

  Lipase is produced in the pancreas and it can hydrolyse the ester bond in triglycerides to form monoglycerides and fatty acids.

  
  

  Bile salts are produced in the liver and can emulsify lipids to form tiny droplets and micelles. This increases the surface area for lipase to act on.
• What is the difference between the physical and chemical digestion of lipids?

  • Physical is emulsification & micelle formation (bile salts)
  • Chemical is the lipase hydrolysing lipids into glycerol and fatty acids (some monoglycerides).
• What adaptations of the ileum take place to maximise absorption?

  The ileum wall is covered in villi, which have thin walls surrounded by a network of capillaries and epithelial cells have even smaller microvilli.

  
  

  These features maximise absorption by increasing the surface area, decreasing the diffusion distance and maintaining a concentration gradient

  
• Which two molecules are absorbed by co-transport?

  1. Glucose
  2. Amino Acids
• Co-transport stages in absorption?

  1. Sodium ions are actively transported out of the epithelial cell into the blood in the capillary.
  2. This reduces the sodium ion concentration in the epithelial cell.
  3. Sodium ions can then diffuse from the lumen down their concentration gradient into the epithelial cell.
  4. The protein the sodium ions diffuse through is a co-transporter protein, so glucose/amino acids also attach and are transported into the epithelial cell against their concentration gradient.
  5. Glucose/amino acids then move by facilitated diffusion from the epithelial cell to the blood.

  
• What are micelles?

  Micelles are water-soluble vesicles formed of fatty acids, glycerol, monoglycerides and bile salts.
• Why can micelles simply diffuse across the cell surface membrane?

  When the micelles encounter the ileum epithelial cells, due to the non-polar nature of the fatty acids and monoglycerides, they can simply diffuse across the cell surface membrane to enter the cells of the epithelial cells
• What happens to the micelles once they have been absorbed into the cells?

  Once in the cell, these will be modified back into triglycerides inside of the endoplasmic reticulum and Golgi body.
• What are the stages of lipid absorption?

  1. Fatty acids (& monoglycerides) leave micelles and enter epithelial cells
  2. Fatty acids link to form triglycerides
  3. Fatty globules combine with proteins forming chylomicrons (inside Golgi Apparatus)
  4. Chylomicrons are extruded from epithelial cell and enter a lacteal
  5. Lymph in the lacteal transports chylomicrons away from intestine