topic 2: structures and functions in living organisms

• Levels of organisation


  • Organelles
  • Cells
  • Tissues
  • Organs
  • Organ systems
• Nucleus


  Contains the genetic material, which codes for a particular protein. Enclosed in a nuclear membrane.
• Cytoplasm


  Liquid substance in which chemical reactions occur. Contains enzymes (biological catalysts, i.e. proteins that speed up the rate of reaction). Organelles are found in it.
• Cell membrane


  Contains receptor molecules to identify and selectively control what enters and leaves the cell
• Mitochondria


  Where aerobic respiration reactions occur, providing energy for the cell
• Ribosomes


  Where protein synthesis occurs. Found on a structure called the rough endoplasmic reticulum.
• 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
• 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
• 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
• 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
• 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 the reaction takes place and the products are released from the surface of the enzyme.
• As temperature increases


  The rate of reaction increases up to the optimum temperature of around 37°C, 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.
• As pH changes


  If the pH is too high or too low, the forces that hold the amino acid chains that make up the protein will be affected, 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.
• Diffusion


  The spreading out of particles resulting in a net movement from an area of higher concentration to an area of lower concentration
• Diffusion is a passive process as no energy is required
• Molecules that can move by diffusion

  • Oxygen
  • Glucose
  • Amino acids
  • Water
• Larger molecules such as starch and proteins cannot move by diffusion
• Single-celled organisms


  • Can use diffusion to transport molecules into their body from the air due to their relatively large surface area to volume ratio
  • Their low metabolic demands mean diffusion across the surface is sufficient to meet their needs
• Multicellular organisms


  • Cannot rely on diffusion alone due to their small surface area to volume ratio
  • Have adaptations to allow molecules to be transported in and out of cells, e.g. alveoli, villi, root hair cells
• Factors affecting 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
• Osmosis is a passive process that does not use energy
• Osmotic conditions


  • Isotonic (no net movement)
  • Hypertonic (water moves out)
  • Hypotonic (water moves in)
• Osmosis in animals


  • If external solution is more dilute, it will move into cells causing them to burst
  • If external solution is more concentrated, excess water will leave the cell causing it to shrivel
• Osmosis in plants


  • If external solution is more dilute, water will move into cell and vacuole causing turgor pressure
  • If external solution is less dilute, water will move out and the cell will become soft, eventually leading to plasmolysis and death
• Active transport


  The movement of particles from an area of lower concentration to an area of higher concentration, against the concentration gradient
• Active transport requires energy from respiration as it is working against the gradient
• Active transport in root hair cells

  • They take up water and mineral ions from the soil, which are usually in higher concentrations in the cells than the surrounding soil
• Active transport in the gut

  • Substances like glucose and amino acids have to move from the gut into the bloodstream, even if there is a lower concentration in the gut than the blood
• Practical: investigate diffusion in non-living systems

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

  2. Place cube in hydrochloric acid solution

  3. Remove cube, wash, and cut in half to measure distance acid has diffused

  4. Repeat with different acid concentrations
• Practical: investigating osmosis in potatoes

  1. Place potato strips in different sucrose solutions including 0% control

  2. Measure mass changes after 20 minutes

  3. Repeat with potato strips of similar masses
• Photosynthesis


  The process of making glucose from sunlight in the leaves of the plant, an endothermic reaction converting light energy into chemical energy