Transport Across Membranes

• What is diffusion?


  Diffusion is the net movement of molecules from a high concentration to a low concentration until the molecules are equally distributed.
• What kind of process is diffusion?


  A passive process- it doesn’t require energy fro, respiration.
• How does gaseous exchange occur?


  Via diffusion.
• What is it called when molecules diffuse directly through a cell membrane?


  Simple diffusion.
• Why can oxygen and carbon dioxide diffuse easily through cell membranes?


  Because they are small molecules, so they can pass through the spaces between the phospholipid. They’re also non polar, which makes them soluble in lipids, so they can dissolve in the hydrophobic bilayer.
• What is the rate of diffusion increased by?


  Temperature, concentration gradient, surface area.
• What is facilitated diffusion?


  It allows the transport of larger polar molecules such as glucose and amino acids across membranes. It involves the use of channel proteins and carrier proteins, which the large or charged particles diffuse through.
• What kind of process is facilitated diffusion?


  A passive process- doesn’t require energy
• Is facilitated diffusion inhibited by respiratory inhibitors?


  Facilitated diffusion is not inhibited by respiratory inhibitors such as cyanide.
• How do carrier proteins move large molecules across membranes during facilitated diffusion?


  First, the large molecule attaches to the carrier protein in the membrane.

  Then, the carrier protein changes shape.

  Finally, the molecule diffuses out of the carrier protein to the opposite side of the membrane.
• How do channel proteins move charged particles across the membrane during facilitated diffusion?


  Channel proteins form pores in the membrane for the charged particles to diffuse through. Different channel proteins facilitate the diffusion of different charged particles.
• What does the rate of facilitated diffusion depend on?


  The concentration gradient and the number of channel or carrier proteins. Having more carrier or channel proteins increases the rate of diffusion.
• What is active transport?


  The movement of molecules or ions through a partially permeable membrane by carrier proteins against a concentration gradient.
• What does active transport require?


  Energy in the form of ATP which is produced during respiration.
• How do carrier proteins aid the transport of polar molecules during facilitated diffusion and active transport?


  Carrier proteins possess a specific tertiary structure complementary to polar molecules.
• What is the difference between active transport and facilitated diffusion?


  Active transport requires energy, facilitated diffusion does not.

  Active transport moves solutes from a low to a high concentration, in facilitated diffusion they move from a high to a low concentration.
• Which factors decrease the rate of active transport?

  Lowering the temperature, lack of oxygen, metabolic and respiratory inhibitors like cyanide. These lower the rate of respiration therefore decreasing active transport.
• What do cells involved in active transport contain?


  A large number of mitochondria to provide the ATP required via respiration.
• How does active transport take place?


  The phosphate group atpatches to the carrier protein. The energy from ATP activates the protein to accept the particle to be transported. The phosphate group is released and the energy attaching the phosphate group to the carrier protein is used to change the shape of the carrier protein, transporting the particle across the membrane.
• What are co transporters?


  A type of carrier protein.
• What do co transporters do?


  They bind two molecules at a time. The concentration gradient of one of the molecules is used to move the other molecule against its own concentration gradient.
• What is osmosis?


  The net movement of water molecules from a dilute solution to a concentrated solution across a partially permeable membrane.
• What is osmosis in terms of solute concentration?


  The movement of water from a low concentration of solute molecules to a high concentration of solute molecules.
• What is osmosis in terms of water concentration?


  The movement of water from a high concentration of water molecules to a low concentration of water molecules.
• What is water potential?


  The potential (tendency) of water molecules to leave a solution by osmosis.
• What water potential does pure water have?


  Pure water has the highest water potential. All solutions have a lower water potential than pure water.
• What does isotonic mean?


  When two solutions have the same water potential.
• What does the rate of osmosis depend on?

  The water potential gradient- the higher the water potential gradient, the faster the rate.

  The thickness of the exchange surface- the thinner it is the faster the rate.

  The surface area- the larger it is the faster the rate
• What is the value of pure water?

  Zero- has the highest water potential
• What does an increase in solute concentration do to the water potential?


  Causes the water potential to decrease, it becomes more negative.
• What does hypertonic mean?


  Lots of solute dissolved in it/ higher solute concentration compared to the cell.
• What does hypotonic mean?


  Having a lower solute concentration compared to another solution/ the cell.
• Net water movement during osmosis occurs until..


  Until both of the solutions were isotonic (of equal concentration)
• What happens to the net water movement when the solution becomes isotonic?


  The water movement still occurs but it is now equal in both directions so there would be no NET movement of water.
• How does water movement occur?

  Water movement occurs down a water potential gradient.
• What does the presence of ions in a cell do to the water potential?


  Lowers the water potential of the cell and water can enter it via osmosis.
• What happens as water enters the cell vacuole?


  The cell vacuole enlarges and the vacuole, cytoplasm and cell membrane exert an outward turgor pressure on the cell wall.
• Where is cell turgor essential?


  It is essential in providing support in plants
• What happens when a plant cell is placed in distilled water?


  The cell becomes turgid
• What happens when a plant cell is placed in a concentrated solution?

  The cell becomes plasmolysled.