Transport Across Cell Membrane

• Passive transport


  the movement of substances across the cell membrane without energy

  
• 3 types of passive transport


  diffusion, osmosis, facilitated diffusion

  
• Diffusion

  movement of substances from an area of a higher concentration to a lower concentration

  
• Concentration gradient

  difference in concentration between two sides of the cell membrane

  
• Osmosis

  diffusion of water with the concentration gradient across a selectively permeable membrane

  
• How does osmosis work?

  if the membrane is impermeable to the solute and solute concentrations are different on either side of the membrane, water will move across the membrane until both sides are equally concentrated
• Osmosis in blood cells

  when blood cells are in a solution, water diffuses in and out of the cell depending on the concentration of salt in the solution

  
• Hypertonic


  when a solution has a higher concentration of solute compared to the cell, causing more water to diffuse out of the cell making it shrivel up

  
• Isotonic

  when the concentration of a solution is equal to the cell's concentration of solute, so an equal amount of water is diffusing in and out of the cell (no net movement of water)

  
• Hypotonic


  when the concentration of a solution is lower than the cell's concentration of solute, so water diffuses into the cell, eventually causing it to burst

  
• Plasmolyzed

  vacuole shrinks inside a hypertonic solution as water diffuses out of the cell, causing cytoplasm to shrink

  
• Flaccid

  plant cell in an isotonic solution, vacuole and cytoplasm stay the same size (no net movement of water)

  
• Turgid

  plant cell in a hypotonic solution, cell swells, vacuole increases in size, and turgor pressure increases

  
• Facilitated diffusion


  diffusion with concentration gradients through specific channel or carrier proteins

  
• Channel proteins

  hydrophilic pathways through the membrane for a molecule to travel, the size of the opening determines which molecules can enter or leave the cell

  
• What are some properties of channel proteins?


  some remain open all the time while others have gates that open in response to a signal and they permit ions or polar molecules to move across the membrane
• Carrier proteins


  protein that binds to specific molecules and transports them across the membrane to the lower concentration side

  
• Active transport


  transport of solute against concentration gradient using ATP
• ATP

  Adenosine triphosphate, energy source required to move molecules or ions across the cell membrane

  
• What does ATP consist of?

  ribose, adenine, and 3 phosphate groups (triphosphate)
• ATP Hydrolysis


  water is added to ATP which produces useful energy of the cell, and breaks it down into ADP (Adenosine diposphate) by breaking apart one of the phosphate groups

  
• ADP dehydration


  energy for ATP synthesis comes from food or body (glycogen or fat) and water is lost in the process, producing ATP
• Active transport process in neurons

  1. Na+ ions bind from the cytosol (low concentration of Na+), and ATP is hydrolyzed, forming ADP and a phosphate that binds to the transport pump to switch to the E2 conformation

  2. Na+ is released outside of the cell (high concentration)

  3. K+ ions bind to the pump from outside the cell (low concentration of K+)

  4. Phosphate is released, and the pump switched back to the E1 conformation, and K+ is released into the cytosol (high concentration)
• Membrane-assisted transport

  to use energy to transport macromolecules that are too large to cross the membrane, and the cell forms vesicles to surround incoming or outgoing particles

  
• Types of membrane-assisted transport

  endocytosis and exocytosis

  
• Endocytosis


  process in which a cell engulfs extracellular material into the cell

  
• Types of endocytosis

  phagocytosis, pinocytosis, receptor-mediated endocytosis

  
• Phagocytosis


  engulfs large and solid particles, membrane folds inward and pinches off to form a vesicle

  
• Pinocytosis

  engulfs liquid particles, membrane folds inward and pinches off to form a vesicle

  
• Receptor-mediated endocytosis


  particles bind to receptor proteins and pinches off to form a coated vesicle inside the cell

  
• Exocytosis


  the vacuole fuses with the cell membrane, forming a vesicle which fuses with the cell membrane and releases contents out of the cell

  
• What happened when the dialysis tube with glucose solution was put into iodine solution?

  iodine entered the dialysis tube and water diffused into the the dialysis tube

  dialysis tube turned blue-black due to starch being unable to diffuse out, while the beaker solution is glucose-positive so glucose diffused into the beaker