CHAPTER 3 BIOLOGY

• Cell membrane regulates the materials pass in and out of the cell.
• Materials may pass in and out of cells by:

  1. Diffusion
  2. Osmosis
  3. Active Transport
• The energy for diffusion comes from the kinetic energy of random movement of particles and ions. Cells do not have to use energy for diffusion.
• Definition of diffusion.

  • Net movement of particles
  • From a region of high concentration to a region of low concentration
  • Down concentration gradient
  • As a result of their random movement
• Concentration Gradient is the difference in concentration between two regions.
• Importance of diffusion to living organisms

  Help living organisms to:

  • obtain their requirements ( nutrients )
  • get rid of waste products
  • gas exchange for respiration
• Cell membrane

  Regulates the materials pass in and out of the cell
• Materials may pass in and out of cells by

  • Diffusion
  • Osmosis
  • Active transport
• The process of diffusion

  1. Particles in liquid or gas move continuously and randomly

  2. Particles collide and spread out

  3. The energy for diffusion comes from the kinetic energy of random movement of particles and ions

  4. Cells do not have to use energy for diffusion
• Diffusion


  Net movement of particles from a region of high concentration to a region of low concentration down the concentration gradient as a result of their random movement
• Diffusion across a permeable membrane

  • Potassium iodide diffuses across the membrane to the right side down the concentration gradient (high to low)
  • Copper sulfate diffuses across the membrane to the left side down the concentration gradient (high to low)

  
• Eventually, equal concentrations (equilibrium) of the particles at both sides
• Importance of diffusion to living organisms

  • Obtain their requirements
  • Get rid of waste products
  • Gas exchange for respiration
• Examples of diffusion in living organisms

  • Nutrients digested in small intestine are absorbed into the blood by diffusion
  • Plants absorb carbon dioxide from the surroundings by diffusion
  • Oxygen produced in photosynthesis diffuses out of the plant into the air
  • Gaseous exchange by diffusion in the alveoli
  • Mineral ions from the soil solution are absorbed by plant roots by diffusion and active transport

  
• Factors affecting diffusion

  • Surface area to volume ratio
  • Distance
  • Temperature
  • Concentration gradient

  
• Surface area to volume ratio

  The smaller a cell, the larger its surface area to volume ratio, the faster the rate of diffusion
• Distance

  The smaller the distance, the faster the rate of diffusion
• Temperature

  The higher the temperature, the faster molecules move as they have more energy, more collisions, faster rate of diffusion
• Concentration Gradient

  The greater the difference in concentration, the faster rate of diffusion

  
• Water as a solvent in

  • Digestion: breakdown of food
  • Excretion: transport waste products to excretory organs
  • Transport: transport dissolved substances in the body
• Partially permeable membrane

  Cell membrane is partially permeable, not all particles can diffuse through cell membrane, particles that are too big or have the wrong electrical charge cannot pass through
• Example of partially permeable membrane

  • Cell membrane is permeable to water but impermeable to protein
• Osmosis

  The diffusion of water molecules from a region of higher water potential to a region of lower water potential, down a water potential gradient, through a partially permeable membrane
• Water potential

  Dilute solution: high water potential

  Concentrated solution: low water potential

  Water potential gradient= difference in water potential
• Plant cells and osmosis

  • Plant cells in hypertonic solution (lower water potential): water leaves the cell by osmosis, cell becomes flaccid / plasmolysed
  • Plant cells in hypotonic solution (higher water potential): water enters cell by osmosis, cell becomes turgid
  • Plant cells in isotonic solution (equal water potential): no net movement of water, cytoplasm just presses against cell wall
• Turgor pressure


  Pressure exerted by the water in vacuole, important to maintain the shape of soft tissues in plants and to cause the opening and closing of stomata

  When there is high rate of evaporation of water from the cells, the plant cells lose turgidity and it wilts

  
• Animal cells and osmosis

  • Animal cells in hypotonic solution (higher water potential): cell takes in water, cell swells and bursts
  • Animal cells in isotonic solution (equal water potential): cell is in equilibrium
  • Animal cells in hypertonic solution (lower water potential): cell loses water, cell shrinks and becomes crenated

  
• 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
• Examples of active transport


  • Uptake of ions by plant root hairs
  • Uptake of glucose by microvilli of epithelial cells in small intestine
  • Uptake of glucose from kidney tubules into blood capillaries
• Active transport requires carrier protein in cell membrane
• Zone A : concentrated sucrose solution ( LOWER WATER POTENTIAL )

  Zone B : dilute sucrose solution (HIGHER WATER POTENTIAL )

  Water move across the partially permeable membrane from Zone B to Zone A, down water potential gradient by osmosis

  
• After OSMOSIS

  A) rise in level of water solution

  
• The surrounding solution has a higher water potential than the solution in the visking tubing.

  Water enters the visking tubing by osmosis. The visking tubing swells and becomes firm and turgid.

  
• Water leaves the visking tubing by osmosis. The visking tubing shrinks and becomes soft.

  
• Plant cells in HYPERTONIC solution (LOWER WATER POTENTIAL)

  • Cell in solution of lower water potential than cell contents
  • Water leaves the cell by osmosis
  • Vacuole smaller
  • Cytoplasm pulls away from cell wall
  • Cell becomes flaccid / plasmolysed
• Plant cells in HYPOTONIC solution ( HIGHER WATER POTENTIAL )

  • Cell in solution of higher water potential than the cell contents
  • Water enters cell by osmosis
  • Vacuole bigger
  • Cytoplasm pushes against cell wall
  • Cell becomes turgid
• Plant cells in ISOTONIC solution (EQUAL WATER POTENTIAL )

  • Cell in solution of equal water potential
  • No net movement of water (NET = 0)
  • Cytoplasm just presses against cell wall
• Animal cells in HYPOTONIC SOLUTION ( HIGHER WATER POTENTIAL)

  • Cell in a solution of higher water potential
  • Cell takes in water
  • Cell swells and bursts
  • HAEMOLYSIS -> burst of RED BLOOD CELL
• Animal cells in ISOTONIC SOLUTION ( EQUAL WATER POTENTIAL)

  • Cell in a solution of same water potential as the cell contents
  • Cell is in EQUILIBRIUM
• Animal cells in HYPERTONIC SOLUTION (LOWER WATER POTENTIAL)

  • Cells in a solution of lower water potential
  • Cell loses water
  • Cell shrinks and becomes crenated
  • This process is called CRENATION