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Bio 141 Chapter 6

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What are lipids? Why are some lipids are liquid at room temperature whereas other lipids are solid at room temperature? Are all fats considered to be lipids? Are all lipids considered to be fats? If not, what else besides fats are considered lipids? What distinguishes lipids and phospholipids?

Lipids are carbon-containing compounds that are found in organisms and are largely nonpolar and hydrophobic; they do not readily dissolve in water

Saturated lipids are solid at room temperature because they have no double bonds, unsaturated lipids are liquid at room temperature because they have double bonds and the lack of hydrogen reduces opportunities for attraction between molecules and the melting point falls

All fats are considered to be lipids, but not all lipids are considered fats. Fats are nonpolar molecules composed of three fatty acids that are linked to a three carbon molecule called a glycerol

Lipid groups: fats, steroids (including cholesterol), and phospholipids

Lipids are different from phospholipids because phospholipids are amphipathic, containing a polar head and a nonpolar tail region; however, lipids are entirely nonpolar, containing a 3 Carbon glycerol at the head instead of a polar head.


Define the terms hydrophilic, hydrophobic, and amphipathic. Why do amphipathic lipids play such a central role in biology?

Hydrophilic: (“water loving”) having a tendency to mix with, dissolve in, or be wetted by water.

Hydrophobic: (“water fearing”)the physical property of a molecule that is seemingly repelled from a mass of water

Amphipathic: compounds that contain both hydrophilic and hydrophobic elements (ex: phospholipids and cholesterols)

Amphipathic lipids play a central role in biology because they can form phospholipid bilayers, which are related to the selective passage of nutrients and materials into cells and waste out of cells, which allows cells and organisms to survive.


In the absence of specific carrier proteins, which of the following can readily diffuse across the plasma membrane of a cell: ATP, glucose, amino acids, fatty acids, water, gases, proteins, ions, polysaccharides, glycerol, hemoglobin, ethanol?

Water, gases, and fatty acids???


Using artificial liposomes researchers showed that phospholipid bilayer membranes containing different amounts of cholesterol have different permeability to glycerol. What was the observed relationship between cholesterol content and glycerol permeability? How can the effect of cholesterol be explained? What other experimental variable used in these experiments affected membrane permeability (and membrane fluidity) in a consistent way? What was the effect observed in this case and what is a reasonable explanation for the observed effect?

  • Scientists observed that phospholipid bilayers that contain more cholesterol are less permeable to glycerol. Short, unsaturated layers are more permeable than long saturated ones.
  • The effect of cholesterol may be explained by the fact that steroid rings in cholesterol are bulky; therefore, adding cholesterol fills gaps that would otherwise be present in the hydrophobic section of the membrane
  • The other experimental variable used in these experiments that also impacted permeability in a consistent way was temperature. Phospholipid bilayer permeability and fluidity consistently increase with an increase in temperature but decreases with an increase in cholesterol.
  • This may be explained by the fact that as temperature drops, the molecules in a bilayer more more slowly, causing the tails to pack together more tightly, some even solidify. This makes the layer more impervious to molecules.

Cells of fish that live in the near-freezing waters off Alaska are likely to have bilayer membranes that contain phospholipids with what kinds of tails? How much cholesterol are their membranes likely to contain (only a little, a moderate amount, or a lot?)?

Their cells will most likely have bilayer membranes that consist of phospholipids with short, unsaturated tails. Due to the low temperature of the water surrounding the fish and its bodily fluids, the membranes will need to be easy to pass through since they will not be very fluid.

Short, unsaturated tails have a shorter distance for necessary substances to pass into and out of the cell and will have spaces that will make such diffusion more easy. Their membranes are likely to contain only a little cholesterol- cholesterol inhibits the passage of molecules into and out of the cell.


What is the difference between simple diffusion and osmosis? What type of membrane is always involved in osmosis? Is a similar type of membrane involved when one observes the diffusion of a solute across the membrane? If not, what is the difference, if any, between the two types of membranes?

  • Both are passive types of transport.
  • Osmosis:
    • Always occurs across a semipermeable membrane
    • Only solvent (water) passes through
    • Only in liquid
  • Diffusion:
  • Can occur in any mixture??? as well as one with a semipermeable membrane
  • Both solute and solvent molecules move freely
  • Can happen in gas, liquid, and solids

The red blood cells (RBCs) in your body contain mainly hemoglobin (Hb) which is a large protein that cannot pass through the RBC membrane. The RBC membrane is also impermeable to sucrose. If a red blood cell is placed into a sucrose solution in which the concentration of sucrose on the outside of the cell is less than the concentration of the Hb on the inside, will the RBC shrink, swell up or remain the same size? Explain your reasoning.

Cell will swell because concentration is only dependent on number and not type of molecule


How do ions and large polar molecules, such as glucose, get into cells? What are the various classes of proteins that affect passive membrane permeability? What, if any, is the difference between facilitated diffusion and passive transport?

  • Selective ion channels allow ions to move in response to an electrochemical gradient,
    • From areas of like charge to unlike charge
    • From regions of high concentration to low concentration
    • Many channels are gated and only open or close in response to a signal such as the binding of a particular molecule or a change in the electrical voltage across the membrane
    • Aquaporins for water molecules
  • Facilitated diffusion is the passive transport of substances that otherwise would not cross the membrane readily
  • Glucose binds to GLUT-1 and changes the shape in a way that moves the sugar through the hydrophobic region
  • GLUT-1 will bind to glucose molecule and change shape, allowing to enter cell.
  • Facilitated diffusion can also occur through carrier proteins, specialized membrane proteins that change shape during the transport process

How does active transport of material across membranes differ from the movement of material across membranes via diffusion and passive transport? How can dissolved materials be moved from the outside to the inside of a cell when the inside concentration of the material is already high?

The active transport of materials across membranes differ from facilitated diffusion and passive transport because active transport involves the use of energy from ATP to allow materials into the cell against their concentration gradient; passive transport and diffusion are normally down the concentration gradient.

Active transport is the movement of substances against their electrochemical gradient

  • ATP provides the energy by transferring a phosphate group to an active transport protein called a pump
  • Sodium potassium pump, both sodium and potassium are moving from high concentration to low concentration
  • ? sodium then used previously in active transpot can then be used as a cotransport and brought back in thru original pump

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