organisation

• adaptations for efficient exchange surfaces

  A short distance required for diffusion:

  • the membranes of cells
  • the flattened shape of structures such as leaves
  • the walls of blood capillaries are one cell thick
  • the epithelia of alveoli in the respiratory system and the villi in the small intestine are only one cell thick
• An efficient blood supply to transport molecules to and from the exchange surface increases effective exchange. Examples of this include:

  • the network of blood capillaries that surrounds each alveolus in the lungs
  • the network of blood capillaries in each villus in the small intestine

  efficient blood supply means a steep concentration gradient can be maintained which increases the rate of exchange
• human gas exchange system

  The human lungs provide an exchange surface adapted for:

  • absorbing oxygen – needed for respiration – into the blood from the air
  • transferring carbon dioxide – produced by respiration – from the blood into the lungs then the air

  The lungs are enclosed within the chest (thorax). Air needs to be inhaled and brought into contact with the exchange surfaces within the lungs. This process is called ventilation.
• human gas exchange system

  The lungs are enclosed in the thorax, surrounded and protected by 12 pairs of ribs. The ribs are moved by two sets of intercostal muscles. There is a muscular diaphragm below the lungs. The lungs are sealed within two airtight pleural membranes. These wrap around the lungs and line the rib cage.
• human gas exchange system

  pleural membranes: Thin, moist membranes surrounding the lungs that make an airtight seal.
• The trachea (windpipe) branches into two bronchi .

  Rings of cartilage in the walls of the trachea help to keep it open as air is drawn in.

  The bronchi split into smaller branches and then into smaller tubes called bronchioles.

  Each bronchiole ends in a cluster of microscopic air sacs called alveoli.
• adaptations of aveoli (for effective gas exchange)

  • small size - each alveolus is a small sphere about 300 μm in diameter, giving it a larger surface area to volume ratio than larger structures
  • number - there are around 700 million alveoli – ie 350 million per lung
  • surface area: very high, the total surface area for aveoli is around 70m^2
• adaptations of aveoli (for effective gas exchange)

  • short diffusion path - the walls of the aveoli and of the capillaries are just one cell thick
  • lined with a thin film of moisture - Gases dissolve in this water, making the diffusion path even smaller.
  • steep concentration gradient - ventilation of the lungs and constant blood supply through the capillaries means gases are continually being removed
• inhaling

  intercostal muscles contract ---> ribs move upwards

  volume of thorax (chest) increases ---> pressure inside lungs decreases

  air is drawn in

  at the same time: diaphram also contracts, pulling downwards
• exhaling

  intercostal muscles relax ---> ribs move downwards

  volume of thorax (chest) decreases ---> pressure inside lungs increases

  air is breathed out

  at the same time: diaphram relaxes, moving upwards
• effective gas exchange in fish gills

  • the large surface area of the gills
  • the large surface area of the blood capillaries in each gill filament
  • the short distance required for diffusion – the outer layer of the gill filaments and the capillary walls are just one cell thick
  • the efficient ventilation of the gills with water - there is a counter current flow of water and blood
• we digest protiens, lipids and fats because

  • most of the molecules in food are too large to pass through the absorbing surface of the gut wall
  • the carbohydrates, proteins and lipids are reassembled in the form required, rather than other animal or plant versions
• Carbohydrates

  Source of energy, glucose is the main respiratory substrate

  carbohydrates consist of starch and sugars

  Starch: potatoes, rice and wheat products, bread, cereals and pasta. Sugars: fruit, smoothies, fizzy drinks, chocolate and sweets
• Proteins

  Growth and repair

  Meat, eggs, cheese, beans, nuts and seeds
• Lipids

  provide energy, make up part of cell membranes ---> essential for normal growth

  Butter and margarine, meat and processed meat, plant oils, oily fish, nuts and seeds
• Starch is a polymer of glucose. It must be broken down into glucose molecules – it is too large to pass through the gut.

  
• glucose - single sugar

  sucrose - double sugar
• Cellulose is also made up of glucose molecules.

  It makes up plant cell walls (fundamental part of our diet)

  It cannot be broken down by the digestive system, so is egested (shat out) from the gut.

  Once absorbed by the body, glucose molecules are transported to cells and:

  • used for respiration
  • reassembled into the storage form of carbohydrate in animals - glycogen
• In plant metabolism, the glucose produced by photosynthesis is converted into starch for storage, and cellulose, for cell wall synthesis.

  In humans and animals glucose is stored in glycogen. It is not converted into starch.
• Proteins are made up of amino acids.

  Proteins must first be broken down into amino acids to pass through the gut wall)

  Once inside the body, the amino acids are reassembled into the proteins the individual requires – the process of protein synthesis.

  Excess amino acids are broken down in the liver.
• amino acid - 1 molecule

  peptide - 2-20 amino acids, with a peptide bond

  protien - 50-2000 amino acids, folded in correct shape

  
• protien synthesis: The production of proteins from amino acids, which happens in the ribosomes of the cell.
• Lipids are esters of fatty acids and glycerol.

  Lipid molecules are too large to pass through the gut wall and must be digested first.

  In the body's cells, they are reassembled into the lipids the cell needs, for instance, for the cell membranes.
• ester: A type of organic compound formed in the reaction between an alcohol and a carboxylic acid.
• a lipid molecule consists of a glycerol molecule attached to three molecules of fatty acid

  
• food tests

  prepping the food (do this before any test, as it is how you get the food solution you test!!!)

  1. take the food sample, and grind it with distilled water using a mortar and pestle (we want to make a paste)
  2. transfer the paste to the beaker and add more distilled water, stir (so chemicals in food dissolve into water)
  3. filter the solution (to remove any suspended food particles)
• test for starch (carbohydrate food test)

  1. place 2cm^3 of food solution into a test tube
  2. add a few drops of iodine

  if iodine is present the solution will turn blue/black, if not it will remain orange
• test for sugars (carbohydrate food test)

  1. place 2cm^3 of food solution into a test tube
  2. add 10 drops of benedicts solution
  3. place test tube into a beaker containing hot water, from a kettle (water bath)
  4. leave for 5 mins

  if sugar is present it'll change colour, we can only approximate how much sugar is available based off the colour

  green (small amount of sugar) ---> yellow (increasing) ---> red (high)

  this test only works for reducing sugars (ie: glucose) an won't work on non- reducing sugars (ie: sucrose)
• test for protiens

  1. place 2cm^3 of food solution into a test tube
  2. add 2cm^3 of biurets solution

  if protien is present the biurets solution will turn purple/lilac, if it isn't present it will remain blue
• test for lipids

  1. when doing the food prep do not filter the solution! this is because lipid molecules can get stuck to the filter paper
  2. place 2cm^3 of the food solution into a test tube
  3. add a few drops of distilled water and ethanol
  4. gently shake the solution

  if lipids are present then a cloudy white emulsion forms

  remember: ethanol is highly flammable, so make sure there are no naked flames around when doing this experiment
• the human digestive system has 2 functions

  • break down complex food substances
  • provide very large surface area for maximum absorption of food
• the human digestive system

  Mouth

  Begins the digestion of carbohydrates

  Stomach

  Begins the digestion of protein; small molecules such as alcohol absorbed

  Small intestine - Duodenum

  Continues the digestion of carbohydrate and protein; begins the digestion of lipids

  Small intestine - Ileum

  Completes the digestion of carbohydrates and proteins into single sugars and amino acids; absorption of single sugars, amino acids and fatty acids and glycerol

  Large intestine

  Absorption of water; egestion of undigested food
• absorption in the gut

  the surface of the small intestine wall is folded and has small projections called villi

  the epithelial cells of the villi have projections called microvilli

  > increase surface area, allowing absorption

  villi also have a network of capillaries (constant flow of blood allows steep concentration gradient to be maintained)
• most digested food passess through the epithelial cells of the gut wall is transported by the blood to the liver

  lipids: pass through the gut wall and enter the lacteals
• villi and microvilli of the small intestine

  
• enzymes

  • enzymes are biological catalysts
  • the are also responsible for building up chemical molecules elsewhere in the body
  • they have a complex shape made up of protiens, when a substrate fits into the active site a chemical reaction takes place
  • the shape of the active sight is different for specific substrate molecule(s) - lock and key theory
• factors affecting enzymes - temperature

  the activity of enzymes decrease if temperature decreases

  the amount of successful collisions reduces, because the molecular movement decreases ---> reaction is slow

  if the temperature increases, the shape of the activation sight changes, reducing activity or stopping it from working. it has been denatured

  they work best/efficiently at an optimum temperature: human body temperature (37)
• how enzymes work

  protiens are chains of amino acids, joined end to end

  > chain is not straight, it can be twisted or folded, and amino acids in the chain can be attracted to or repel eachother

  each enzyme is made of these amino acids ---> each enzyme has a unique shape

  this structure is held together through weak forces between the amino acid molecules in the chain

  high temperature will break these forces

  > the enzyme (+ its active site) will change shape and the substrate will no longer fit

  reaction will be affected or will stop
• factors affecting enzymes - Ph

  changing the Ph of the surroundings will also change the shape of active sight

  many amino acids will carry a positive and negative charge

  within the enzyme amino acids can repel and attract, creating the (folded and twisted) shape

  changing the Ph will affect the charge of the amino acid molecules (shape of enzyme and active site are changed)

  extremes of Ph can also denature enzymes, changes are usually permanent
• enzymes

  enzymes work inside and outside of cells

  different parts of the digestive system produce different enxymes, which all have their own optimum Ph

  the optimum Ph in the stomach is produced by secretion of hydrochloric acid (stomach acid)

  the optimum Ph in the duodenum is produced by the secretion of sodium hydrocarbonate