paper 1 - biology

• Organisms


  Can be Prokaryotes or Eukaryotes
• Vacuole


  Large, watery solution of sugar and salts.
• Chloroplast


  Where photosynthesis occurs, which makes food for the plan. They contain grubolence colled chloroph which webs the light needed for photosynthesis.
• Bacterial Cells Are Much Smaller
• Bacterial cell


  • Don't have chloropisats or mitochondela
  • Have a single circulse strand of DNA that floats freely in the cytoplasm
  • May also contain one or more kmall rings of DNA called plasmide
• Cells are Studied Using Microscopes
• Light microscopes


  Use light and lenses to form an image of a specimen and magnity it (make it ha bigger)
• Electron microscopes


  Use electrons instead of light to form an image. They have a much higher magnification and resolution than light microscopes.
• Magnification


  Image size / real size
• Using a light microscope

  Clip the slide onto the stage.

  1. Select the lowest-powered objective lens.
  2. Use the coarse adjustment knob to move the stage up to just below the objective lens.
  3. Look down the eyepiece and use the coarse adjustment knob to move the stage downwards until the image is roughly in focus.
  4. Adjust the focus with the fine adjustment knob.
  5. Swap to a higher-powered objective lens and refocus if needed.
• Cells differentiate to become specialised
• Examples of specialised cells

  • Sperm cells
  • Nerve cells
  • Muscle cells
  • Root hair cells
  • Phloem and xylem cells
• Chromosomes contain genetic information
• Chromosomes


  • Coiled up lengths of DNA molecules
  • Each chromosome carries many genes
  • Body cells normally have two copies of each chromosome
• The cell cycle


  1. Growth and DNA replication

  2. Mitosis
• Prokaryotic cells replicate by binary fission
• Binary fission


  1. The circular DNA and plasmid(s) replicate

  2. The cell gets bigger and the circular DNA strands move to opposite 'poles' (ends) of the cell

  3. The cytoplasm begins to divide and new cell wall begin to form

  4. The cytoplasm divides and two daughter cells are produced. Each daughter cell has one copy of the circular DNA, but can have a variable number of copies of the plasmid
• Bacteria can divide very quickly if given the right conditions (e.g. a warm environment and lots of nutrients). Some bacteria, such as E. coll, can take as little as 20 minutes to replicate in the right environment. However, if conditions become unfavourable, the cells will stop dividing and eventually begin to die.
• Using mean division time to find the number of bacteria in a population

  1. Make sure both times are in the same units

  2. Divide the total time that the bacteria are producing cells by the mean division time. This gives you the number of divisions

  3. Multiply 2 by itself for the number of divisions to find the number of cells
• Each cell divides to produce 2 cells, so the number of cells increases by a power of 2 for each division.
• Culturing microorganisms

  • Bacteria (and some other microorganisms) are grown (cultured) in a "culture medium" which contains the carbohydrates, minerals, proteins and vitamins they need to grow
  • The culture medium used can be a nutrient both solution or solid agat jelly
  • Bacteria grown on agar plates' will form visible colonies on the surface of the jelly, or will spread out to give an even covering of bacteria
• Making an agar plate

  1. Hot agar jelly is poured into shallow round plastic dishes called Petri dishes

  2. When the jelly's cooled and set, inoculating loops (wire loops) can be used to transfer microorganisms to the culture medium. Alternatively, a sterile dropping pipette and spreader can be used to get an even covering of bacteria
• In the lab at school, cultures of microorganisms are not kept above 25 °C because harmful pathogens are more likely to grow above this temperature.
• Investigating the effect of antibiotics on bacterial growth

  1. Place paper disos soaked in different types (or different concentrations) of antibiotics on an agar plate that has an even covering of bacteria. Leave some space between the discs

  2. The antibiotic should diffusé (soak) into the agar jelly. Antibiotic-resistant bacteria (i.e. bacteria that aren't affected by the antibiotic-see p.51) will continue to grow on the agar around the paper discs, but non-resistant strains will die. A clear area will be left where the bacteria have died-this is called an inhibition zone

  3. Make sure you use a control. This is a paper disc that has not been soaked in an antibiotic. Instead, soak it in sterile water

  4. Leave the plate for 48 hours at 25 °C

  5. The more effective the antibiotic is against the bacteria, the larger the inhibition zone will be
• Avoiding contamination when culturing microorganisms

  1. The Petri dishes and culture medium must be sterilised before use (e.g. by heating to a high temperature), to kill any unwanted microorganisms that may be lurking on them

  2. If an inoculating loop is used to transfer the bacteria to the culture medium, it should be sterilised first by passing it through a hot flame

  3. After transferring the bacteria, the lid of the Petri dish should be lightly taped on to stop microorganisms from the air getting in

  4. The Petri dish should be stored upside down-to stop drops of condensation falling onto the agar surface
• Calculating the sizes of the inhibition zones to compare results

  1. Divide the diameter of the inhibition zone by two to find the radius

  2. Use the equation area = πr^2 to calculate the area of the inhibition zone

  3. Repeat for the other inhibition zones

  4. Compare the sizes of the areas to determine the relative effectiveness of the antibiotics
• The equation area = πr^2 can also be used to calculate the area of a bacterial colony, by measuring the diameter of the colony.
• Embryonic stem cells


  Stem cells found in early human embryos that have the potential to turn into any type of cell
• Adult stem cells


  Stem cells found in certain places in adults, like bone marrow, that can only turn into certain cell types
• Potential uses of stem cells

  • Medicine already uses adult stem cells to cure disease, e.g. replacing faulty blood cells
  • Embryonic stem cells could be used to replace faulty cells in sick people, e.g. insulin-producing cells for diabetes, nerve cells for spinal injuries
  • Therapeutic cloning could produce stem cells with the same genetic information as the patient, reducing risk of rejection
  • Risks include stem cells becoming contaminated and passing on infections
• Arguments against stem cell research

  • Embryos are potential human lives and shouldn't be used for experiments
  • Curing existing patients is more important than the rights of embryos
  • Embryos used are often unwanted ones from fertility clinics that would be destroyed anyway
  • Scientists should focus on finding other sources of stem cells
• Plant stem cells

  Found in the meristems (parts of the plant where growth occurs), can differentiate into any type of plant cell and be used to produce clones of whole plants
• Diffusion


  The spreading out of particles from an area of higher concentration to an area of lower concentration
• Diffusion happens in both solutions and gases, as the particles in these substances are free to move about randomly.
• The bigger the concentration gradient (the difference in concentration), the faster the diffusion rate.
• A higher temperature will also give a faster diffusion rate because the particles have more energy and move around faster.
• Cell membranes


  • Hold the cell together but allow small dissolved substances to move in and out by diffusion
  • Only very small molecules like oxygen, glucose, amino acids and water can diffuse through the membrane
  • Particles flow through the membrane from higher to lower concentration, but move randomly in both directions
  • The larger the surface area of the membrane, the faster the diffusion rate
• Osmosis


  The movement of water molecules across a partially permeable membrane from a region of higher water concentration to a region of lower water concentration
• A partially permeable membrane only allows tiny molecules like water to pass through, while bigger molecules like sucrose cannot.
• The water molecules actually pass both ways across the membrane, but if there are more on one side, there is a net movement from that side.