Movement in and out of cells

• Diffusion


  The movement of molecules from a region of its higher concentration to a region of its lower concentration
• Cell membrane


  Partially permeable membrane - allows some molecules to cross easily, but others with difficulty or not at all
• Simplest sort of selection

  Based on the size of the molecules
• Diffusion helps living organisms to

  • Obtain many of their requirements
  • Get rid of many of their waste products
  • Carry out gas exchange for respiration
• Energy for diffusion


  Comes from the kinetic energy of the random movement of molecules and ions (Brownian motion)
• 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
• Factors that influence diffusion

  • Surface area to volume ratio
  • Distance
  • Temperature
  • Concentration gradient
• The highly folded surface of the small intestine increases its surface area
• The smaller the distance molecules have to travel the faster transport will occur
• The higher the temperature, the faster molecules move as they have more energy
• The greater the difference in concentration on either side of the membrane, the faster movement across it will occur
• Water as a solvent

  Many substances are able to dissolve in it, making it incredibly useful and essential for all life on Earth
• Importance of water as a solvent

  • Dissolved substances can be easily transported around organisms

  Digested food molecules can be moved to cells all over the body

  Toxic substances and substances in excess can be removed from the body in urine

  Plays a role in ensuring metabolic reactions can happen in cells
• If the plant tissue gains mass, water must have moved into the plant tissue from the solution surrounding it by osmosis, as the solution surrounding the tissue is more dilute than the plant tissue (which is more concentrated)
• If plant tissue loses mass, water must have moved out of the plant tissue into the solution surrounding it by osmosis, as the solution surrounding the tissue is more concentrated than the plant tissue (which is more dilute)
• If there is no overall change in mass, there has been no net movement of water as the concentration in both the plant tissue and the solution surrounding it must be equal, though water will still be moving into and out of the plant tissue
• Dialysis tubing


  • It is a non-living partially permeable membrane made from cellulose
  • Pores in this membrane are small enough to prevent the passage of large molecules (such as sucrose) but allow smaller molecules (such as glucose and water) to pass through by diffusion and osmosis
• Dialysis tubing experiment


  1. Fill a section of dialysis tubing with concentrated sucrose solution

  2. Suspend the tubing in a boiling tube of water for a set period of time

  3. Note whether the water level outside the tubing decreases as water moves into the tubing via osmosis
• Water moves from a region of higher water potential (dilute solution) to a region of lower water potential (concentrated solution), through a partially permeable membrane
• Osmosis in plant tissues

  • When water moves into a plant cell, the vacuole gets bigger, pushing the cell membrane against the cell wall
  • Water entering the cell by osmosis makes the cell rigid and firm
  • This provides support and strength for the plant, making it stand upright with its leaves held out to catch sunlight
  • The pressure created by the cell wall stops too much water entering and prevents the cell from bursting
  • If plants do not receive enough water the cells cannot remain rigid and firm (turgid) and the plant wilts
• 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
• A dilute solution has a high water potential and a concentrated solution has a low water potential
• When plant cells are placed in a solution that has a higher water potential (dilute solution) than inside the cells, water moves into the plant cells via osmosis, making them turgid
• When plant cells are placed in a concentrated solution (with a lower water potential than inside the cells) water molecules will move out of the plant cells by osmosis, making them flaccid
• If plant cells become flaccid it can negatively affect the plant's ability to support itself
• If plant cells are plasmolysed, the cell membrane has pulled away from the cell wall
• Animal cells

  • They lose and gain water as a result of osmosis
  • They do not have a supporting cell wall, so the results on the cell are more severe
• What happens when an animal cell is placed in a strong sugar solution

  It will lose water by osmosis and become crenated (shrivelled up)
• What happens when an animal cell is placed in distilled water

  It will gain water by osmosis and, as it has no cell wall to create turgor pressure, will continue to do so until the cell membrane is stretched too far and it bursts
• Isotonic solution

  Equal water potential between red blood cell and solution, no net movement of water
• Hypotonic solution

  Red blood cells have lower water potential than solution, resulting in net movement of water in and cells swelling, may lyse (burst)
• Active transport

  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 across the cell membrane

  Carrier molecule picks up substance outside cell, uses energy to change shape and move substance through membrane, releases substance inside cell
• Energy is needed for active transport because particles are being moved against a concentration gradient, in the opposite direction from which they would naturally move (by diffusion)
• Importance of active transport

  • Uptake of glucose by epithelial cells in the villi of the small intestine
  • Uptake of ions from soil water by root hair cells in plants
• Active transport is vital for the movement of molecules or ions across membranes
• Carrier proteins embedded in the cell membrane are used to pick up specific molecules and take them through the cell membrane against their concentration gradient
• Small intestine


  A part of the digestive system where most digestion and absorption of nutrients takes place. The lumen of the small intestine is the cavity inside the small intestine where digested food products are found, including glucose, amino acids, and fatty acids and glycerol. Diffusion occurs across the walls of the small intestine, allowing these nutrients to enter the bloodstream and be transported to cells throughout the body.
• Lumen of the small intestine

  The cavity inside the small intestine where digested food products are found. Diffusion occurs across the walls of the small intestine, allowing nutrients to enter the bloodstream and be transported to cells throughout the body.
• Glucose


  A type of sugar that is a digested food product found in the lumen of the small intestine. Diffuses across the walls of the small intestine and is transported to cells throughout the body to be used for energy.