Diffusion, Osmosis, and Active transport

• Diffusion


  Gradual movement of particles from places where there are lots of to spread out
• Net (overall) movement of particles

  From an area of higher concentration to an area of lower concentration
• If something moves from an area of higher concentration to an area of lower concentration


  It is said to have moved down a concentration gradient
• Diffusion happens in both liquids and gases
• That's because the particles in these substances are free to move about randomly
• Diffusion continues until the concentration of the diffusing particles is even in both areas
• Cell membranes are partially permeable


  Only very small molecules can move through them - things like glucose, amino acids, water and oxygen
• Big molecules like starch, sucrose and proteins can't fit through the membrane
• Factors that affect Diffusion:

  • High Temperature, diffuse fast
  • High Concentration Gradient, diffuse fast
  • Larger Surface Area, more rate of diffusion
• Osmosis


  The net movement of water molecules across a partially permeable membrane from a region of higher water concentration to a region of lower water concentration
• The water molecules actually pass both ways through the membrane during osmosis
• Water molecules move about randomly all the time
• There are more water molecules on one side than on the other


  There's a steady net flow of water into the region with fewer water molecules, i.e. into the stronger sugar solution
• The strong sugar solution gets more dilute


  The water acts like it's trying to "even up" the concentration either side of the membrane
• An animal cell is placed in a weak sugar solution


  Water will move into the cell by osmosis
• An animal cell is placed in a strong sugar solution

  The cell will shrink as water moves out
• An animal cell is placed in a weak sugar solution

  The cell will swell and eventually burst
• Active transport

  • Active transport is the movement of particles across a membrane against a concentration gradient (i.e. from an area of lower to an area of higher concentration) using energy transferred during respiration.
  • Works Against a concentration gradient.
• Active transport is a bit different from diffusion because particles are moved up a concentration gradient rather than down. The process also requires energy (unlike diffusion, which is a passive process).
• Cell membranes


  Partially permeable, allow cells to exchange substances with their environment
• Single-celled organisms


  • Substances can diffuse straight into and out of them across their cell membranes
• Multicellular organisms

  • Diffusion across outer surface is more difficult, specialised exchange surfaces are needed
• Gas exchange in plants

  CO2 and O2 move through stomata and cell membranes
• During photosynthesis


  Plants use up CO2 from the atmosphere and produce oxygen as a waste product
• Low CO2 inside leaf during photosynthesis

  More CO2 moves into the leaf by diffusion
• Stomata


  • Tiny pores on the lower surface of a leaf
  • Allow gases like CO2 and O2 to diffuse in and out
  • Allow water vapour to escape (transpiration)
• Gas diffusion in plants

  Gases diffuse between air spaces inside the leaf and the plant's cells through the cells' partially permeable outer membrane
• Root hairs


  Take in water and mineral ions
• Plants


  • Need water for photosynthesis (amongst other things)
  • Need mineral ions for growth
• How plants take in water and mineral ions

  1. Through their roots

  2. Roots have cells that grow into long 'hairs' which stick out into the soil
• Molecules crossing a membrane


  Can only do so when they're right next to it
• A large surface area means loads more molecules are close to the membrane
• Each branch of a root will be covered in millions of these microscopic hairs
• Root hairs


  • Give the plant a big surface area for absorbing water and mineral ions from the soil
• There's usually a higher concentration of water in the soil than there is inside the plant


  Water is drawn into the root hair cell across the partially permeable cell membrane by osmosis
• Plants need nitrogen to make proteins
• Nitrate ions (NO3-)


  Mineral ions that plants get nitrogen from
• How plants take in mineral ions

  1. Mineral ions move into root cells by active transport

  2. Concentration of mineral ions in the root hair cells is usually higher than in the soil

  3. Cells use molecules of ATP to provide energy to actively transport the ions through the cell membrane
• Phenolphthalein 

  • is a pH indicator.
  • it's pink in alkaline solutions and colourless in acidic solutions.
• Using phenolphthalein to investigate diffusion in agar jelly


  1. Make agar jelly with phenolphthalein and dilute sodium hydroxide

  2. Add dilute hydrochloric acid to a beaker

  3. Measure and cut cubes from the jelly

  4. Put the jelly cubes in the beaker of acid

  5. Time how long it takes for the cubes to turn colourless as the acid diffuses into the jelly and neutralises the sodium hydroxide