Movement into & out of Cells

• What is Diffusion in Biology?

  • Diffusion is the movement of molecules from a region of its higher concentration to a region of its lower concentration
  • Molecules move down a concentration gradient, as a result of their random movement
• Diffusion across the cell membrane

  A) cross

  B) higher concentration

  C) lower concentration

  
• • For living cells, the principle of the movement down a concentration gradient is the same, but 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
  • The simplest sort of selection is 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
• Where does the energy for diffusion come from?

  • All particles move randomly at all times
  • This is known as Brownian motion
  • The energy for diffusion comes from the kinetic energy of this random movement of molecules and ions

  
• Plants require oxygen for respiration at all times, and carbon dioxide for photosynthesis when conditions for photosynthesis are right
• The conditions needed for photosynthesis are:

  • Light

  • Carbon dioxide

  • Water

  • Chlorophyll
• Factors that Influence Diffusion:

  • 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
  • Many cells which are adapted for diffusion have increased surface area in some way - eg root hair cells in plants (which absorb water and mineral ions) and cells lining the ileum in animals (which absorb the products of digestion)

  
• Factors that Influence Diffusion:

  • Distance
  • The smaller the distance molecules have to travel the faster transport will occur
  • This is why blood capillaries and alveoli have walls which are only one cell thick, ensure the rate of diffusion across them is as fast as possible
• Factors that Influence Diffusion:

  • Temperature
  • The higher the temperature, the faster molecules move as they have more energy
  • This results in more collisions against the cell membrane and therefore a faster rate of movement across them
• Factors that Influence Diffusion:

  • Concentration Gradient
  • The greater the difference in concentration on either side of the membrane, the faster movement across it will occur
  • This is because on the side with the higher concentration, more random collisions against the membrane will occur
• Water is important for all living organisms as many substances are able to dissolve in it (it is a solvent)
• • Water is important as a solvent in the following situations within organisms:
  • Dissolved substances can be easily transported around organisms
  • Digested food molecules are in the alimentary canal but need to be moved to cells all over the body - without water as a solvent this would not be able to happen
  • Toxic substances such as urea and substances in excess of requirements such as salts can dissolve in water which makes them easy to remove from the body in urine
  • Water is also an important part of the cytoplasm and plays a role in ensuring metabolic reactions can happen as necessary in cells 
• • All cells are surrounded by a cell membrane which is partially permeable
• • Water can move in and out of cells by osmosis
• Osmosis is the diffusion of water molecules from a dilute solution (high concentration of water) to a more concentrated solution (low concentration of water) across a partially permeable membrane
• Through osmosis, water is moving down its concentration gradient
• The cell membrane is partially permeable which means it allows small molecules (like water) through but not larger molecules (like solute molecules)

  
• • 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
• • Cells getting water by osmosis is important for plants as the effect of all the cells in a plant being firm is to provide support and strength for the plant - making the plant 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 is 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
• • When plant cells are placed in a solution that has a higher water potential than inside the cells then water moves into the plant cells via osmosis
  • These water molecules push the cell membrane against the cell wall, increasing the turgor pressure in the cells which makes them turgid 
• Turgid


  When the cell is swollen or distended due to the absorption of water.

  
• Flaccid

  The state when the cell lacks turgor pressure and becomes soft or limp due to a loss of water.
• • 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 looked at underneath the microscope, the plant cells might be plasmolysed, meaning the cell membrane has pulled away from the cell wall
• Plasmolysed

  When the cell's cell membrane has pulled away from the cell wall
• Active transport is the movement of particles through a cell membrane from a region of lower concentration to a region of higher concentration using energy from respiration