Cell Membrane

• Phospholipid bilayers are the foundationof cell membranes
• Amphipathic lipid molecules:In water, hydrophobic tails will orient towards each other (avoiding water) while polar heads happily orient towards outsides (interacting with water)
• Polar and charged (hydrophilic) molecules will not readily pass across the hydrophobic interior of the bilayer
• This bilayer forms a barrier between the cell (or interior cell compartments) and the aqueous (watery) surrounding environment
• All cells are enclosed by a phospholipid bilayer membrane that serves as a boundary defining the space of the cell
• The membrane separates the internal contents from the surrounding environment, thereby controlling cellular conditions within narrow, optimal parameters
• The maintenance of this optimal environment is known as homeostasis
• The cell membrane is selectively permeable
• To get across the cell membrane,something must either...• be able to pass the hydrophobic layer of the membrane• pass through a protein channel• be transported across by a carrier protein
• Diffusion is the net movement of molecules (solutes) from an area of HIGH concentration to an area of low concentration

  • Goal: equilibrium
• Permeable membrane: has holes that can let solute pass through
• Diffusing: move from higher concentration to lower:

  So its said they move down or with their concentration gradient
• Diffusion:

  High to Low = “with” or “down” the gradient
• Diffusion:

  Low to High = “against” or “up” the gradient
• Osmosis can be treated like a special case of diffusion

  • Goal: equilibrium
• Osmosis: happens when there are solutes that cannot quickly pass across the cell membrane and there is a higher solute concentration on one side of the membrane than the other
• When osmosis happens, the net (total) movement of water will be to the area of higher solute concentration (lower “water concentration”)
• Tonicity of the environment describes the concentration of solutes outside of a compartment (like a cell) compared to inside
• Tonicity:

  • Hypertonic [solute] higher outside
  • Isotonic [solute] equal to inside
  • Hypotonic [solute] lower outside
• The tonicity of a cell’s external environment determines osmotic behavior (how water moves into and out of the cell)
• Osmosis can thus potentially pose a challenge for cells..

  • If a red blood cell finds itself in a hypertonic environment (one with a higher solute concentration than that inside the cell), water will leave the cell by osmosis and the cell will shrink
• Osmosis can thus potentially pose a challenge for cells..

  • If a red blood cell finds itself in a hypotonic environment (one with a lower solute concentration than that inside the cell), water will move into the cell by osmosis and the cell will burst
• Osmosis can thus potentially pose a challenge for cells..

  • Cells solve the problem of water movement by using active transport to keep the intracellular fluid isotonic (at the same solute concentration as exists outside the cell)
• Passive transport of molecules across the cell membrane does not require any input of energy (what makes it “passive”)
• Passive transport

  • relies on the principles of diffusion
  • Molecules are moving down their concentration gradient
• Passive transport can occur via either• Simple diffusion• Facilitated diffusion
• Molecules that are small, uncharged, or nonpolar (e.g. O 2, CO 2) cross the plasma membrane by this process of simple diffusion, without any help
• The rate of simple diffusion is directly affected by the factors controlling membrane fluidity/permeability we discussed last class
• The cell membrane’s phospholipid bilayer is embedded with all kinds of membrane proteins that help molecules enter or leave the cell
• If a molecule uses the help of a protein to cross, it’s facilitated diffusion
• Molecules that cannot move across the lipid bilayer directly can diffuse passively down their concentration gradient through a membrane protein channel or carrier
• Facilitated diffusion

  • Channels have “gating” mechanisms that open or close them
  • Used by smaller molecules (like ions)
• Facilitated Diffusion

  • Carriers exist in 2 shapes, one open to the outside, one open to the inside
  • They’re uniquely shaped to fit a specific molecule (“lock and key mechanism”)
• • Binding of the transported molecule on one side induces conformational (shape) change, allowing transport to other side
  • Used by larger molecules
• passive transport works to the cell’s advantage only when the concentration gradient is in the right direction
• Active transport

  • Often the cell needs to move substances against the concentration gradient – that is, from areas of lower concentration to areas of higher concentration
• The movement of substances against their concentration gradient requires energy and is known as active transport
• Active transport can occur via either• Primary active transport• Secondary active transport
• Primary active transport

  • Uses the energy stored in ATP to actively pump molecules against / up their concentration gradient
• Primary active transport

  • Classic example is the Na + /K + pump:Cells like to keep Na + concentration low inside and K+ concentration high inside.
  • To maintain this optimum, cells use a membrane protein called an Na + /K + pump