Practical revision

• Test for starch?


  Iodine - positive test results shows colour change from brown to blue-black.
• Test for reducing sugars


  Benedict's solution - positive test shows a colour change from blue to brick-red (must heat solution)
• Test for proteins?


  Biuret's test - positive test shows colour change from blue to purple.
• Test for lipids?


  Ethanol solution - emulsion forms on distilled water solution.
• Give some safety precautions


  Safety googles, tie hair back, cleaning spills, care with open flames
• Sources of error with food tests?


  Contamination, incomplete reactions, and inaccurate measurements, colour of benedicts and biurets solution may be unclear if concentration of tested molecule is too low.
• Test for vitamin C


  Add a few drops of DCIP to food sample - positive test will turn solution colourless.
• Cell membrane and cell wall


  Control what substances enter and exit the cell
• Molecules move into the cell


  For use in metabolic reactions and storage
• Waste products move out of the cell


  Into the blood to be excreted from the body
• Diffusion


  The net movement of particles from an area of high concentration to an area of low concentration down the concentration gradient, as a result of their random movement
• The energy for diffusion comes from the kinetic energy of the molecules
• Solutes and gases, such as carbon dioxide and oxygen, are able to diffuse in and out of cells across the cell membrane
• Diffusion is important as these substances are crucial to metabolic reactions which occur within the cell, for example respiration and photosynthesis
• Without these substances, the processes would not occur, and the cell would die
• Factors affecting rate of diffusion

  • Surface area
  • Temperature
  • Concentration gradient
  • Diffusion distance
• As the surface area increases


  The rate of diffusion increases
• As temperature increases


  The rate of diffusion increases
• As the concentration gradient increases


  The rate of diffusion increases
• A greater diffusion distance


  Slows the rate of diffusion
• Osmosis


  The net movement of water molecules through a partially permeable membrane
• Turgid cells


  Described as turgid when they are swollen due to a high-water content
• Turgor pressure


  The pressure on the cell wall from the cell membrane pushing upon it, resulting from the cell becoming turgid as water moves into the cell via osmosis
• Flaccid


  Occurs when water moves out of the cell via osmosis. The cell shrinks but the cell membrane does not peel away from the cell wall. If more water leaves the cell, it becomes plasmolyzed.
• Plasmolysis


  Occurs when there is too little water in cells. In plant cells, the cell membrane peels away from the cell wall.
• Water


  • Moves in and out of cells through the cell membrane via osmosis
  • Provides support for the cell structure through maintaining the turgor pressure
  • Provides a medium in which metabolic reactions occur
  • Has a high specific heat capacity, thus acts as a temperature buffer and maintains the optimum temperature for enzyme reactions
• Water particles diffuse


  From regions of high-water potential to regions of low water potential, i.e. they move from dilute solutions to concentrated solutions
• When the cell is more concentrated than the surrounding cells


  Water molecules diffuse into the cell via osmosis, making it turgid
• When the cell is less concentrated than the surrounding cells


  Water molecules will leave the cell, making it flaccid and leading to plasmolysis
• Active transport


  1. Movement of molecules against a concentration gradient using energy from respiration

  2. Molecules are actively transported through a cell membrane from regions of low concentrations to regions of higher concentration
• Carrier proteins


  • Facilitate active transport
  • Embedded in the cell membrane and allow passage through it
  • Molecules from the side with lower concentration bind to the carrier protein
  • The carrier protein then changes shape using energy from respiration, forcing the molecule to move through the membrane to the side with high concentration, where it is released
• Active transport


  • Uptake of ions by root hair cells - plants take up ions such as nitrates and magnesium from the soil via root hair cells. The concentration of ions in the root hair cell is greater than the concentration of ions in the soil. Energy from respiration is therefore used to transport ions into the cell against the concentration gradient.
• Uptake of glucose


  Glucose is taken up in the small intestine and kidney tubules
• Glucose movement


  1. Moves against the concentration gradient

  2. Through carrier proteins
• What colour is hydrogencarbonate indicator in high concentrations of carbon dioxide?

  Yellow
• What colour is hydrogen carbonate indicator at atmoshpheric levels of carbon dioxide?


  Red
• What colour is hydrogencabronate inidcator at lower and lowest concentrations of carbon dioxide?


  Lower - Magenta

  Lowest - Purple.