exchange

• characteristics of exchange surfaces for effective exchange

  large surface area to volume ratio

  very thin - short diffusion pathway

  steep concentration gradient
• gas exchange in single-celled organisms

  diffusion
• gas exchange in insects

  internal network of tubes called tracheae - supported by strengthened rings to stop them collapsing

  tracheae divide into tracheoles

  tracheoles extend throughout the body tissues

  ends of tracheoles are filled with water

  pores on the surface of insects are called spiracles

  contraction of muscles in insects - called abdominal pumping - can sqeeze tracheae

  • this moves more air in and out of the spiracles quicker
  • this maintains a greater concentration gradient
• how does the ends of tracheoles being filled with water help gas exchange in insects?

  during periods of high activity some muscle cells surrounding the tracheoles respire anaerobically - producing lactate

  lactate lowers water potential in muscle cells

  so - water moves into muscle cells from tracheoles by osmosis

  so decrease in volume in tracheoles

  this draws air into the tracheoles - so the ends become filled with gas - this increases the rate of diffusion of gas

  diffusion occurs faster in a gas rather than a liquid
• why do insects usually keep spiracles shut?

  to reduce water loss
• how do insects reduce evaporation?

  they have a waterproof waxy cuticle all over their body

  they have tiny hairs around their spiracles
• main structures of fish gills

  gills - located behind the head

  gill filaments - what make up gills, stacked up in a pile - they increase the surface area for gas exchange

  gill lamellae - are at right angles to gill filaments - they further increase surface area for gas exchange
• counter-current flow

  water goes in through the mouth and is forced out over the gills

  blood flows in the opposite direction to the flow of water over the gill lamellae - maintains a steep concentration gradient for the diffusion of oxygen

  concentration of oxygen in the water is always higher than that in the blood - ensure as much oxygen diffuses into the blood as possible

  this maintains a steep concentration gradient throughtout the entire width of the lamellae
• role of cartilage in the trachea and bronchi

  trachea:

  • prevent the trachea from collapsing when air pressure decreases inside the lungs during breathing in

  bronchi:

  • prevent the bronchi from collapsing when air pressure decreases inside the lungs during breathing in
• Inhalation (breathing in)

  active process

  • external intercostal muscles contract and internal intercostal muscles relax
  • rib cage moves up and out
  • diaphragm contracts and flattens
  • volume in thoracic cavity increases
  • pressure in the thoracic cavity decreases
  • air moves from an area of higher pressure to an area of lower pressure along the pressure gradient - so moves into the lungs
• exhalation (breathing out)

  passive process

  • external intercostal muscles relax and internal intercostal muscles contract
  • ribs move down and in
  • diaphragm relaxes and domes
  • decreases volume in thoracic cavity
  • increases pressure in thoracic cavity
  • air moves from an area of higher pressure to an area of lower pressure along the pressure gradient - so moves out of the lungs
• alveoli structure

  surrounded by a network of capillaries

  squamous epithelium of the epithelium - CO2 and O2 diffuse across

  capillary endothelium - CO2 and O2 diffuse across

  both epithelium and endothelium are one cell thick so provide a short diffusion pathway
• movement of oxygen in ventilation

  moves down the trachea, bronchi and bronchioles into the alveoli down the pressure gradient

  oxygen diffuses along concentration gradient:

  • across the squamous epithelium of the alveolus
  • across the capillary endothelium into the capillary (where it then enters the red blood cells and binds to haemoglobin)
• movement of carbon dioxide in ventilation

  carbon dioxide diffuses along concentration gradient:

  • across the capillary endothelium out of the capillary
  • across the squamous epithelium of the alveolus into the alveoli

  moves out of the alveoli into the air down a pressure gradient
• main parts of human gas exchange system

  trachea -> bronchi -> bronchioles -> alveoli
• risk factor

  something that increases chance
• salivary glands

  produce and release saliva

  saliva contains amylase which hydrolyses starch into maltose
• oesophagus

  muscles contract and relax to push food down to the stomach
• stomach

  muscle tissue, that contracts and relaxes to churn up food

  stomach acid

  stores and digests food
• liver

  produce bile
• gall bladder

  stores bile and secretes it into the start of the small intestine
• pancreas

  produces pancreatic juices

  juices contain:

  • enzymes - proteases, lipases and amylase
• ileum (small intestine)

  produce and secrete enzymes to digest food

  absorbs soluble products of digestion into the blood stream
• large intestine 


  absorbs water into the blood
• physical digestion


  breaking large pieces into smaller pieces using structures:

  • teeth
  • stomach

  makes it possible to swallow food

  provides a large surface area for chemical digestion
• chemical digestion 


  hydrolysing large insoluble molecules into smaller soluble ones

  carried out by enzymes
• amylase 


  produced in the salivary glands and pancreas

  hydrolyses alternate glycosidic bonds in starch to produce maltose
• disaccharidases 


  hydrolyse the glycosidic bond in disaccharides
• maltase


  disaccharidase

  produced in the epithelial lining of the ileum

  hydrolyses the glycosidic bond in maltose to produce alpha glucose
• sucrase 


  disaccharidase

  hydrolyses the glycosidic bond in sucrose to produce glucose and fructose
• lactase 


  disaccharidase

  hydrolyses the glycosidic bond in lactose to produce glucose and galactose
• digestion of proteins 


  endopeptidases:

  • hydrolyse internal peptide bonds between specific amino acids

  exopeptidases:

  • hydrolyse peptide bonds between specific amino acids at the ends of a polypeptide. they remove terminal amino acids

  dipeptidases:

  • hydrolyse the peptide bond between the two amino acids in a dipeptide
  • are membrane bound - are in the cell membrane of cells lining the ileum
• carbohydrases hydrolyse carbohydrates, ultimately into monosaccharides
• lipases hydrolyse lipids into glycerol and fatty acids
• proteases hydrolyse proteins into amino acids
• emulsification


  bile salts break the lipids up into smaller droplets

  this increases the surface area to increase the rate of hydrolysis by lipase
• stages of lipid digestion

  lipids are mixed with biles salts

  emulsification

  lipases hydrolyse the ester bonds in triglycerides to form fatty acids and monoglycerides

  micelles are formed - monoglycerides and free fatty acids remain in association with the bile salts, the structures formed are micelles
• absorption of glucose and amino acids 


  absorbed by co-transport:

  • sodium ions are actively transported out of the ileum epithelial cell into the blood
  • this maintains a steep concentration gradient for sodium ions to enter epithelial cells from the lumen
  • sodium ions diffuse along their concentration gradient from the lumen into the epithelial cell and with it glucose/amino acid molecules
  • glucose/amino acids leave the epithelial cell by facilitated diffusion and enters the blood capillary
• absorption of monoglycerides and fatty acids

  micelles carry the monoglycerides and fatty acids to the ileum epithelial cells

  micelles break up and release fatty acids and monoglycerides allowing them to diffuse across the epithelial cell membrane
• what are micelles made up of?

  bile salts, fatty acids and monoglycerides