Movement of water molecule in and out of cells

• Water


  • It is the medium in which all metabolic reactions take place in cells
  • Between 70% to 95% of the mass of a cell is water
  • 71% of the Earth's surface is covered in water, it is a major habitat for organisms
• Chemicals


  Easily dissolve in water
• Biochemical reactions


  1. Can occur in cytoplasm of cells

  2. Food molecules are reacting with enzymes well in liquid medium
• Sweat


  Salt and water
• Urine


  Urea and water
• Excretory products


  Easily removing from the body as they are in liquid medium
• Dissolved substances


  1. Can be transported around organisms

  2. In animals via the blood and lymph systems

  3. In plants via the xylem and phloem
• Concentration gradient


  The idea of concentrations and gradients within them is important when understanding the movement of substances across cell membranes

  
• Concentration


  • The more particles there are in a certain volume, the more concentrated those particles are
  • A solution with a low solute concentration has a high water concentration, and a high water potential
  • A solution with a High solute concentration has a low water concentration, and a low water potential
  • Pure water has the highest water potential

  
• Concentration gradient


  1. Going from high to low concentration is going down the concentration gradient

  2. Going from low to high concentration is going against the concentration gradient
• Processes of movement of molecules


  • Diffusion
  • Osmosis
  • Active transport
• Diffusion


  • Dissolved or gaseous substances have to pass through the cell membrane to get into or out of a cell
  • Diffusion occurs when particles spread
  • Particles diffuse down a concentration gradient, from an area of high concentration to an area of low concentration
  • Molecules move down a concentration gradient, as a result of their random movement
• Diffusion in living organisms


  • The cell is surrounded by a cell membrane, which can restrict the free movement of the molecules
  • The cell membrane is a partially permeable membrane – this means it allows some molecules to cross easily, but others with difficulty or not at all
  • Diffusion helps living organisms to obtain many of their requirements, get rid of many of their waste products, and carry out gas exchange for respiration
• Osmosis


  The net movement of water molecules from a region of higher water potential (dilute solution) to a region of lower water potential (concentrated solution), through a partially permeable membrane
• Active Transport


  The movement of particles through a cell membrane from a region of lower concentration to a region of higher concentration using energy from respiration
• Active Transport in Animals


  1. Food molecules (such as the sugar glucose) can be absorbed across the wall of the small intestine by diffusion, but this is dependent on a concentration gradient existing between the lumen of the intestine and the bloodstream

  2. Active transport allows molecules such as glucose to be transported into the bloodstream from the lumen of the small intestine (the gut) when the concentration of sugar molecules in the blood is higher

  3. The active uptake of glucose by epithelial cells in kidney tubules in the kidney nephron allows for the reabsorption of glucose back into the blood so that none is lost in the urine
• Active Transport in Plants


  1. Root hair cells lining the surface of plant roots need to move minerals such as magnesium ions from a region of lower concentration (the very dilute solution of minerals in the soil surrounding the roots) to a region of higher concentration (inside the cytoplasm of the cell)

  2. Mineral ions are needed by plants to function, such as magnesium ions required to make chlorophyll and nitrate ions needed for amino acid and protein synthesis
• Factors that Influence Diffusion


  • Surface area to volume ratio
  • Diffusion distance
  • Temperature
  • Concentration gradient

  
• Surface area to volume ratio

  The bigger a cell or structure is, the smaller its surface area to volume ratio is, slowing down the rate at which substances can move across its surface

  
• Diffusion distance


  The smaller the distance molecules have to travel the faster transport will occur
• Temperature


  The higher the temperature, the faster molecules move as they have more energy, resulting in more collisions against the cell membrane and therefore a faster rate of movement across them
• Concentration gradient


  The greater the difference in concentration on either side of the membrane, the faster movement across it will occur
• Isotonic


  The two solutions have the same concentration of solutes; hence the same amount of water moves into the cell as moves out
• Hypotonic


  The water outside the cell has less solute, and therefore more free water with the result that water moves into the cell at a greater rate than it moves out
• Hypertonic


  The water outside the cell has more solute, and therefore less free water with the result that water moves out of the cell at a greater rate than it moves in
• Osmosis


  1. Water moves from a hypotonic (less solute = more water) solution to a hypertonic (more solute = less water) through a selectively permeable membrane

  2. Water will diffuse across a selectively permeable membrane until the concentrations are the same on both sides (i.e., isotonic)

  3. Osmosis depends only on the number of solutes

  
• Osmosis in plants


  • Osmosis enables the plant to absorb nutrients from the soil; the soil water is hypotonic to the root cells
  • Osmosis also makes the cells turgid (swollen) and gives rigidity to the plant
  • When water leaves the cells the cytoplasm shrinks away from the wall and collapses; the cell is plasmolyzed
  • Plant cells are normally turgid (swollen full of water) which provides strength to plants
  • Plant cells have a cell wall to stop them bursting when turgid
  • When plant cells start to lose water, they become flaccid and lose their strength and start to wilt

  
• Osmosis in plant cells

  1. A plant cell in a hypotonic solution will absorb water by osmosis, swell up and become turgid

  2. A plant cell in a hypertonic solution will lose water by osmosis, decrease in volume and become flaccid

  3. A plant cell in a very hypertonic solution will shrink so much that the membrane and cytoplasm split away from the cell wall, becoming plasmolyzed

  
• Turgor pressure


  • The pressure exerted by fluid in a cell that presses the cell membrane against the cell wall, making living plant tissue rigid
  • Loss of turgor, resulting from the loss of water from plant cells, causes flowers and leaves to wilt

  
• Experiment 1: Investigate the factors that influence diffusion


  1. Surface area: Use cubes of different sizes in a dye solution and measure the depth of dye diffusion

  2. Temperature: Observe the rate of dye or tea diffusion in hot and cold water

  3. Concentration gradient and distance: Use a long glass tube with litmus paper squares to observe the diffusion of ammonia vapour

  
• Cutting potato cubes and measuring dye diffusion

  1. Cut each of the cubes in half

  2. Measure the depth to which the dye has diffused
• Investigating temperature effects on dye diffusion


  1. Set up two beakers with equal volumes of hot water and iced water

  2. Add a few grains of potassium permanganate to each beaker

  3. Observe how quickly the dissolved dye spreads through the water in each beaker

  
• Investigating concentration gradient and distance effects on ammonia diffusion


  1. Use a wide glass tube that is at least 30 cm long and corked at one end

  2. Push squares of moist red litmus paper into the tube, so that they stick to the side

  3. Close the open end of the tube with a cork carrying a plug of cotton wool saturated with a strong solution of ammonia

  4. Start a stopwatch

  5. Observe and record the time when each square of litmus starts to turn blue

  6. Repeat the experiment using a dilute solution of ammonia

  7. Plot both sets of results on a graph, labelling each plot line

  
• Investigating the effects of immersing plant tissues in solutions of different concentrations


  1. Push a No.4 or No.5 cork borer into a large potato and cut potato tubers with same diameter

  2. Cut them accurately to the same length

  3. Label two test tubes A and B and place a potato cylinder in each

  4. Cover the potato tissue in tube A with water; cover the tissue in B with a 20% sugar solution

  5. Leave the tubes for 24 hours

  6. Remove the cylinder from tube A and measure its length; notice also whether it is firm or flabby

  7. Repeat this for the potato in tube B but rinse it in water before measuring it
• Investigating the effects of varying the concentration of sucrose solution on potato tissue


  1. Prepare six potato cylinders with same length

  2. Label six test tubes with the concentration of sucrose solution in them and place them in a test-tube rack

  3. Add the same volume of the correct sucrose solution to each test tube

  4. Weigh a cylinder of potato, record its mass and place it in the first test tube; repeat until all the test tubes have been set up

  5. Leave the tubes for at least 30 minutes

  6. Remove the potato cylinder from the first tube, surface dry the potato and reweigh it; repeat this for the other potato cylinders

  7. Calculate the change in mass and the percentage change in mass for each cylinder

  8. Plot the results on a graph with sucrose concentration on the horizontal axis and percentage change in mass on the vertical axis
• Investigating osmosis using dialysis tubing

  1. Take a 20 cm length of dialysis tubing that has been soaked in water and tie a knot tightly at one end

  2. Place 3 cm3 of a strong sugar solution in the tubing using a plastic syringe and add a small amount of coloured dye

  3. Fit the tubing over the end of a length of capillary tubing and hold it in place with an elastic band

  4. Push the capillary tubing into the dialysis tubing until the sugar solution enters the capillary

  5. Clamp the capillary tubing so that the dialysis tubing is totally covered by the water in the beaker

  6. Watch the level of liquid in the capillary tubing over the next 10–15 minutes

  
• Investigating partial permeability using dialysis tubing

  1. Take a 15 cm length of dialysis tubing that has been soaked in water and tie a knot tightly at one end

  2. Use a dropping pipette to partly fill the tubing with 1% starch solution

  3. Put the tubing in a test tube and hold it in place with an elastic band

  4. Rinse the tubing and test tube under the tap to remove all traces of starch solution from the outside of the dialysis tube

  5. Fill the test tube with water and add a few drops of iodine solution to colour the water yellow

  6. Leave for 10–15 minutes
• Investigating osmosis and turgor using dialysis tubing


  1. Take a 20 cm length of dialysis tubing that has been soaked in water and tie a knot tightly at one end

  2. Place 3 cm3 of a strong sugar solution in the tubing using a plastic syringe and then knot the open end of the tube

  3. Place the tubing in a test tube of water for 30–45 minutes

  4. Remove the dialysis tubing from the water and note any changes in how it looks or feels

  
• Investigating plasmolysis using rhubarb epidermis


  1. Peel a small piece of epidermis (the outer layer of cells) from a red area of a rhubarb stalk

  2. Place the epidermis on a slide with a drop of water and cover with a cover-slip

  3. Put the slide on a microscope stage and find a small group of cells

  4. Place a 30% solution of sugar at one edge of the cover-slip with a pipette

  5. Move the solution under the cover-slip by placing a piece of blotting paper on the opposite side

  6. Study the cells you identified under the microscope and watch for any changes in their appearance