Topic 2

• Organelles


  Specialised subcellular structures found within living cells
• Levels of organisation


  • Cells
  • Tissues
  • Organs
  • Organ systems
• Respiratory organ system


  Contains the lungs (organ), which is made up of epithelial tissue consisting of epithelial cells
• Subcellular structures found only in plants


  • Chloroplasts
  • Permanent vacuole
  • Cell wall
• Chloroplasts


  Where photosynthesis takes place, providing food for the plant. Contains chlorophyll pigment (which makes it green) which harvests the light needed for photosynthesis.
• Permanent vacuole


  Contains cell sap. Found within the cytoplasm. Improves cell's rigidity.
• Cell wall


  Made from cellulose. Provides strength to the cell.
• Carbohydrates


  They are made of carbon, oxygen and hydrogen. They are polymers that break down into simple sugars.
• Proteins


  They are made of carbon, oxygen, hydrogen, sulfur, nitrogen and phosphorous. They are polymers that are broken down into its monomers: amino acids.
• Lipids


  Lipids (fats and oils) are made of carbon, oxygen and hydrogen. They are large polymers that are broken down into 3 fatty acids molecules and a glycerol molecule.
• Test for glucose


  1. Add the sample solution into a test tube

  2. Add drops of Benedict's solution into the test tube

  3. Heat in a water bath at 60-70°C for 5 minutes

  4. Brick red colour indicates glucose is present, blue colour indicates glucose is not present
• Test for starch


  1. Pipette the sample solution into wells or on a tile

  2. Add drops of iodine solution and leave for 1 minute

  3. Blue-black colour indicates starch is present, brown colour indicates starch is not present
• Test for protein


  1. Add the sample solution into a test tube

  2. Add drops of Biuret solution into the test tube

  3. Purple colour indicates protein is present, blue colour indicates protein is not present
• Test for fat


  1. Add 2cm3 of ethanol to the test solution

  2. Add 2cm3 of distilled water

  3. Milky white emulsion indicates fat is present, colourless solution indicates fat is not present
• Enzymes


  Biological catalysts (a substance that increases the rate of reaction without being used up). They are protein molecules and the shape of the enzyme is vital to its function. Each enzyme has its own uniquely shaped active site where the substrate binds.
• Lock and Key Hypothesis


  The shape of the substrate is complementary to the shape of the active site (enzyme specificity), so when they bond it forms an enzyme-substrate complex. Once bound, the reaction takes place and the products are released from the surface of the enzyme.
• Effect of temperature on enzymes


  The optimum is around 37°C (body temperature). The rate of reaction increases with an increase in temperature up to this optimum, but above this temperature it rapidly decreases and eventually the reaction stops. When the temperature becomes too hot, the bonds in the structure will break, changing the shape of the active site, so the substrate can no longer fit in. The enzyme is said to be denatured and can no longer work.
• Practical: investigate how enzyme activity can be affected by changes in temperature


  1. Starch solution is heated to set temperature

  2. Amylase is added

  3. Iodine is added to each well after a minute

  4. Measure the time it takes until the iodine stops turning blue-black (this means that starch is not present as amylase has broken the starch down into glucose)

  5. Repeat the test with different temperatures
• Diffusion


  The spreading out of the particles resulting in a net movement from an area of higher concentration to an area of lower concentration. It is a passive process as no energy is required. Molecules have to be small in order to be able to move across.
• Diffusion in single-celled organisms


  • They can use diffusion to transport molecules into their body from the air- this is because they have a relatively large surface area to volume ratio. Due to their low metabolic demands, diffusion across the surface of the organism is sufficient enough to meet its needs.
• Diffusion in multicellular organisms


  • The surface area to volume ratio is small so they cannot rely on diffusion alone. Instead, surfaces and organ systems have a number of adaptations that allows molecules to be transported in and out of cells. Examples include alveoli in the lungs, villi in the small intestines and root hair cells in plants.
• Factors affecting the rate of diffusion


  • Concentration gradient
  • Temperature
  • Surface area:volume ratio
  • Distance
• Osmosis


  The movement of water from a less concentrated solution to a more concentrated one through a partially permeable membrane. It is passive, as it does not use energy.
• Isotonic, hypertonic, hypotonic


  If the concentration of sugar in an external solution is the same as the internal, there will be no movement and the solution is said to be isotonic to the cell. If the concentration of sugar in external solution is higher than the internal, water moves out, and the solution is said to be hypertonic to the cell. If the concentration of sugar in external solution is lower than the internal, water moves in, and the solution is said to be hypotonic to the cell.
• Osmosis in animals


  • If the external solution is more dilute (higher water potential), it will move into animal cells causing them to burst. If the external solution is more concentrated (lower water potential), excess water will leave the cell causing it to become shrivelled.
• Osmosis in plants


  • If the external solution is more dilute, water will move into the cell and into the vacuole, causing it to swell, resulting in pressure called turgor (essential in keeping the leaves and stems of plants rigid). If the external solution is less dilute, water will move out of the cell and they will become soft. Eventually the cell membrane will move away from the cell wall (called plasmolysis) and it will die.
• Active transport


  The movement of particles from an area of lower concentration to an area of higher concentration, i.e. against the concentration gradient. This requires energy from respiration as it is working against the gradient, which is why it is called active.
• Active transport in root hair cells


  • They take up water and mineral ions (for healthy growth) from the soil. Mineral ions are usually in higher concentrations in the cells, meaning diffusion cannot take place. This requires energy from respiration to work.
• Active transport in the gut


  • Substances such as glucose and amino acids from your food have to move from your gut into your bloodstream. Sometimes there can be a lower concentration of sugar molecules in the gut than the blood, meaning diffusion cannot take place. Active transport is required to move the sugar to the blood against its concentration gradient.
• Practical: investigate diffusion in non-living systems


  1. Cut a 1cm3 cube of agar made of sodium hydroxide and phenolphthalein indicator

  2. Place cube in solution of hydrochloric acid

  3. Remove the cube and wash with water to stop further reaction

  4. Cut the cube in half and measure the distance that the acid has caused agar to become colourless from outside inwards

  5. Repeat the experiment two more times and calculate the mean

  6. Repeat with different concentrations of hydrochloric acid
• Practical: investigating osmosis in potatoes


  1. Place different sucrose solutions in beakers

  2. Cut potato cylinders and place in the sucrose solutions

  3. Measure the change in length of the potato cylinders after 30 minutes

  4. Plot a graph of sucrose concentration against change in length
• Substances moving from gut into bloodstream


  1. Substances such as glucose and amino acids from food have to move

  2. Sometimes there can be a lower concentration of sugar molecules in the gut than the blood, meaning diffusion cannot take place

  3. Active transport is required to move the sugar to the blood against its concentration gradient
• Photosynthesis


  The process of making glucose from sunlight in the leaves of the plant
• Photosynthesis is an endothermic reaction in which light energy is converted into chemical energy within the chloroplasts
• Photosynthesis


  carbon dioxide + water → glucose + oxygen
• Factors affecting photosynthesis


  • Temperature: With an increase in temperature, the rate of photosynthesis increases. However, as the reaction is controlled by enzymes, this trend only continues up to a certain temperature until the enzymes begin to denature and the rate of reaction decreases.
  • Light intensity: For most plants, the higher the light intensity, the rate of photosynthesis increases. As the distance between the light source and the plant increases, the light intensity decreases, i.e. It is inversely proportional to the square of the distance: light intensity ∝ 1/distance2
  • Carbon dioxide concentration: Carbon dioxide is also needed to make glucose (see equation). As the concentration of carbon dioxide increases, the rate of reaction increases.
• Leaf structures


  • Waxy cuticle: Helps to reduce water loss by evaporation and is a protective layer found at the top of the leaf
  • Upper epidermis: Very thin and transparent in order to let light in to the palisade mesophyll
  • Palisade mesophyll: Contain lots of chloroplasts so that photosynthesis can happen rapidly
  • Spongy mesophyll: Have lots of air spaces to allow gases to diffuse in and out of cells faster, as it increases the surface area to volume ratio
  • Lower epidermis: Contains guard cells and stomata (gaps)
  • Guard cell: Kidney-shaped cells that open and close the stomata by absorbing or losing water. When lots of water is available, the cells fill and open stomata
  • Stomata: Where gas exchange and loss of water by evaporation takes place - opens during the day and closes at night
• Magnesium


  Required for chlorophyll production. Deficiency causes leaves to turn yellow
• Nitrate


  Required to produce amino acids. Deficiency causes stunted growth and turns leaves yellow
• Components of a balanced human diet


  • Carbohydrates
  • Proteins
  • Lipids
  • Dietary fibre
  • Vitamins
  • Minerals
  • Water