Topic 2 - movement in and out of cells

• Diffusion


  Net movement of particles from an area of high concentration to an area of low concentration
• Diffusion


  • Happens Because of random movement of individual particles
  • Occurs until they are in equilibrium
  • Can only occur in liquids & gases
• The concentration gradient is the difference between the concentrations inside and outside the cell.
• factors affecting diffusion rate:

  concentration gradient (bigger difference = faster rate)

  Temperature

  Surface area

  Diffusion distance
• Concentration differences


  • Bigger the difference: faster the rate
  • Difference is called the concentration gradient
• Concentration gradient


  The greater the difference in concentration, the faster the rate (speed) of diffusion will be
• Higher temperature


  Faster diffusion rate due to more kinetic energy meaning more diffusion will occur
• Kinetic energy


  More kinetic energy = particles move around faster
• Larger surface area


  Faster diffusion rate (cilia can increase surface area)
• Shorter diffusion distance


  Faster diffusion rate
• Where speed of diffusion across cells is important, the cells are often squashed to make the distance through cytoplasm shorter
• In the cross section of an air sac (alveoli) in the lung, cells are squashed to make the distance for oxygen to travel into the blood shorter
• Diffusion in living systems

  • Oxygen required for respiration, passes through cell membranes by diffusion
  • Glucose needed for respiration, diffuses into cells from the blood
  • Carbon dioxide, a waste product of respiration, diffuses out of cells & back into blood
  • In the lungs, oxygen diffuses through the alveolar walls & into the surrounding capillaries
• Alvecli


  Where gas exchange takes place in mammals (plants have stomata)
• Alvecli


  • Gas exchange takes place between blood and the atmosphere
  • Elastic - can fill up more, more gas, more diffusion, high concentration
  • Thin - one cell thick, easier diffusion
• Villi


  Tiny outgrowths from the surface of some tissues and organs which serve to increase the surface area
• Villi


  • large surface area, make intestinal wall more absorbent
• Placenta


  Designed to maximise diffusion, to provide nutrients, gas exchange, oxygen, protection from bacteria, waste (CO2), antibodies for the baby
• Placenta

  • Has villi - increased surface area
• Large organisms have a smaller surface area to volume ratio and smaller organisms have a larger surface area to volume ratio
• Diffusion in small organisms happens at a much faster rate than larger ones
• SA:VOL Experiment

  If the cells were real, then the bigger cell wouldn’t have received what it needs to all parts of the cell.

  therefore, it would need a bigger surface to rely on diffusion (i.e. villi) or respiratory circulation
• SA:VOL


  For a cube:

  SA= L x W x 6

  VOL = L x W x H
• Osmosis


  A special type of movement involving water (or small enough particles) through a partially permeable membrane
• Osmosis


  • Has to move through a partially permeable membrane
  • Involves the net movement of water molecules from an area of high water concentration to an area of low water concentration through a partially permeable membrane
• Dilute solution


  A solution with a high concentration of water (e.g. dilute glass of squash)
• Concentrated solution


  A solution with a low concentration of water
• Dilute solution

  High concentration of water
• Concentrated solution

  Low concentration of water
• OSMOSIS IN POTATO TISSUE

  Independent variable: salt solution percentage (0-4%)

  dependent variable: % change in mass

  control variables: size of potato, mass of potato, time in water, same temp., etc.

  % change in mass can be better than just a change in mass as it is a valid comparison
• When a solution has a lower concentration of water than cells the solution is hypertonic
• When a solution has the same concentration of water as the cells, the solution is isotonic - very little osmosis happens
• Hypotonic (normal for plant cell)

  When a solution has a high concentration of water than cells the solution is hypotonic

  
• Hypertonic 


  when a solution has a lower concentration of water than cells, it is hypertonic

  
• Isotonic (normal for animal cell)

  when a solution has the same concentration of water as the cells (very little osmosis happens)

  
• Hypertonic solution


  When animal cells are placed in a hypertonic solution, they become shrivelled or crenated as they lose their water. The tissue fluid is hypertonic, with a higher solute concentration inside the cells than outside, so water moves out by osmosis.
• Isotonic solution


  When animal cells are placed in an isotonic solution, no osmosis occurs as there is equilibrium. The water concentration in the tissue fluid is the same as inside the cells.
• Hypotonic solution


  When animal cells are placed in a hypotonic solution, too much water enters the cells. The tissue fluid is hypotonic, with a lower solute concentration inside the cells than outside, so water moves into the cells by osmosis. The cells eventually burst.
• Plant cells (hypotonic)


  • Have a cell wall to prevent them bursting
  • When water enters, the cell membrane pushes up against the cell wall and becomes turgid
  • Turgid cells give the plant support and keep the stems upright
• What happens when plant cells lose water (isotonic)


  1. Cell membrane no longer pushes against the cell wall

  2. Cells are no longer firm and turgid, they are now flaccid

  3. Plant stems will wilt