Biology Paper 1

• Cell Differentiation


  The process by which a cell changes to become specialised
• Cell Specialisation


  Most cells in an organism are specialised and the structure of the cell helps it carry out its function
• After becoming specialised, animal cells lose the ability to differentiate, but plant cells do not
• Sperm cells


  • Specialised for reproduction
  • Have a long tail
  • Streamlined head
  • Help it swim towards the egg
  • Contain lots of mitochondria for energy
  • Contain enzymes in the head to digest the egg cell membrane
• Nerve cells


  • Specialised for rapid signalling
  • Function is to carry electrical signals around the body
  • Long with branches at either end to connect to other nerve cells
• Muscle cells


  • Specialised for contraction
  • Function is to contract quickly
  • Long and contain lots of mitochondria to transfer energy needed for contraction
• Root hair cells


  • Specialised for absorbing water and minerals
  • Have long hairs that stick out into the soil
  • Gives the plant a big surface area to absorb water and mineral ions from the soil
• Phloem and Xylem cells


  • Specialised for transporting substances
  • Tubes that transport food and water around the plant
  • Xylem cells are hollow, phloem cells have very few subcellular structures to allow substances to flow through
• Stem cells


  Cells that are undifferentiated
• Stem cells


  • Can divide to produce lots more undifferentiated cells
  • Have the potential to turn into any type of cell
• Stem cells are found in early human embryos
• Adult stem cells


  Stem cells found in the bone marrow, but they can't turn into any type of cell
• Plants have stem cells in the meristems
• Meristems


  Where growth occurs in the plant and cells can differentiate into any type of cell
• Issues with stem cell research

  • Human embryos shouldn't be used for experiments as each one is a potential life
  • Obtaining stem cells from an embryo destroys it
  • Some think that suffering patients are more important than embryos
  • The embryos used in research are unwanted ones from fertility clinics, so they would just be destroyed anyway
  • Stem cell research is banned in some countries
  • People think scientists should concentrate on finding and developing other sources of stem cells
• Diffusion

  Particles move from an area of high concentration to an area of low concentration
• Diffusion is a natural tendency for stuff to spread out
• Diffusion


  • Takes place in both gases and liquids, as these particles are free to move about randomly
  • Takes place across a cell membrane
• Cell membrane


  Controls what goes in and out of the cell (as well as holding it together)
• Substances that can move in and out of the cell by diffusion

  • Small molecules such as oxygen, glucose, amino acids and water
  • Bigger molecules like starch and proteins can't fit through the membrane
• Diffusion of particles


  Particles flow through the cell membrane from where there is a higher concentration to where there's a lower concentration
• Rate of diffusion


  • Affected by the concentration gradient (the bigger the gradient, the faster the rate)
  • Affected by temperature (the higher the temperature, the faster the rate as particles have more energy and move faster)
  • Affected by surface area (the larger the surface area, the faster the rate as more particles can pass through at once)
• 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
• Partially permeable membrane


  • A membrane with small holes in it, where only tiny molecules like water can pass through
• Osmosis is a type of diffusion, where water molecules pass both ways through the membrane
• The solution surrounding a cell has a different concentration from the fluid inside the cell


  Water will either move into the cell or out of the cell, by osmosis
• Active transport


  The movement of particles against a concentration gradient using energy transferred from respiration
• Active transport in plant roots

  1. Root hair cells look like long hairs that stick out into the soil

  2. Each branch of a root has millions of microscopic hairs

  3. This gives the plant a large surface area for absorbing water and mineral ions from the soil

  4. Minerals have higher concentration in root hair cells than in the soil

  5. Diffusion cannot be used

  6. Active transport is used instead
• Active transport in the gut

  1. Used when there is a lower concentration of nutrients in the gut, but a higher concentration of nutrients in the blood

  2. When there is a higher concentration of glucose and amino acids in the gut, they diffuse naturally into the blood

  3. When there is a lower concentration of nutrients in the gut than in the blood, the concentration gradient is the wrong way and so active transport is needed

  4. Active transport allows nutrients to be taken in by the blood

  5. Glucose is taken into the bloodstream and then transported to cells where it is used for respiration
• Transport types


  • Diffusion
  • Osmosis
  • Active transport
• Diffusion


  • Movement of particles from an area of high concentration to an area of low concentration
  • Doesn't require energy
  • Only in gases and liquids
• Potato Experiment (required practical)

  1. Cut up a potato into identical cylinders and measure their masses

  2. Get some beakers with different sugar solutions, including pure water and a very concentrated sugar solution

  3. Place one potato cube/cylinder in each beaker and leave for 24 hours

  4. Take the cylinders out, dry them and measure the masses again

  5. Calculate the % change and plot the graphs
• Osmosis

  • Movement of water molecules across a partially permeable membrane from a region of higher water concentration to a region of lower water concentration
  • Doesn't require energy
• Active transport


  • Movement of particles against a concentration gradient
  • Requires energy
• Independent variable


  • Concentration of sugar solution
• Dependent variable


  • Potato mass
• Control variables


  • Amount of solution, mass of potato, time, temperature, same sugar
• On the graph, where the curve hits the x-axis, this is the sugar concentration of the potato
• Exchange of dissolved substances


  Living organisms need to be able to exchange dissolved substances with their environment
• Organism size and surface area


  • Affect how quickly exchange of dissolved substances can be done