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Lipids, Membranes, and Membrane Transport in General Biology

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  • What is the primary function of the plasma membrane?
    The plasma membrane serves as a selective barrier that separates life from nonlife, allowing entry of needed materials, keeping damaging materials out, and facilitating chemical reactions necessary for life.
  • What defines lipids chemically and why are they insoluble in water?
    Lipids are carbon-containing compounds that are insoluble in water due to a high proportion of nonpolar C-H bonds, making them hydrophobic.

  • What is the difference between saturated and unsaturated fatty acids? Which is healthier?

    • Saturated fatty acids have only single bonds between carbons and maximum hydrogen atoms, while unsaturated fatty acids have one or more double bonds causing kinks in the chain.

    • Unsaturated fats b/c double bonds

  • How does bond saturation affect the physical state of lipids?
    Highly saturated lipids are solid at room temperature, especially with long hydrocarbon tails, while highly unsaturated lipids are liquid at room temperature.

  • Name the three most important types of lipids found in cells. Does lipid structure vary?

    • Steroids, fats (triacylglycerols), and phospholipids

    • Yes, very. Hydrocarbon skeleton can be put together in many different ways

  • What structural feature distinguishes steroids from other lipids?
    Steroids have a bulky, four-ring structure and differ by functional groups attached to the rings.
  • What is the primary role of fats in cells?
    Fats store energy, containing many high-energy bonds in fatty acid chains, allowing storage of twice as much chemical energy as carbohydrates.
  • How are fats formed chemically?
    Fats form by dehydration reactions between the hydroxyl group of glycerol and the carboxyl group of fatty acids, joined by ester linkages.
  • What are phospholipids composed of and what is their primary role?
    Phospholipids consist of glycerol linked to a phosphate group and two hydrocarbon chains; their primary role is to form cell membranes.
  • What does it mean that phospholipids are amphipathic?

    Phospholipids have both

    • Hydrophilic (polar, head) region with a glycerol and a phosphate group.

    • Hydrophobic (nonpolar, tail) region made of a hydrocarbon chain.

    Allows them to form plasma membranes

  • What structures do amphipathic lipids form in water? Are they insoluble or soluble?

    • Amphipathic lipids form micelles (spherical aggregates - circle - vesicles) or lipid bilayers (paired sheets) spontaneously in water.

    • Insoluble: the hydrophilic head interacts with water; the hydrophobic tail does not

  • Why do phospholipid bilayers form spontaneously in water despite decreased lipid entropy?
    Although lipid molecules become less disordered, the overall entropy increases because water molecules form cages around nonpolar tails, increasing water entropy.

  • What is selective permeability in lipid bilayers? Examples?

    Selective permeability means small or nonpolar molecules cross phospholipid bilayers quickly, while charged or large polar substances cross slowly or not at all.

    • Oxygen - small & nonpolar; moves across quickly

    • Glucose - large & polar; moves slower across

  • How do hydrocarbon tail length and saturation affect membrane permeability?

    • Longer and saturated hydrocarbon tails increase hydrophobic interactions, making membranes denser and less permeable (longer & denser tails = less permeable)

    • Unsaturated tails with kinks weaken the barrier and increase permeability.

  • How does cholesterol affect membrane permeability?
    Cholesterol increases the density of the hydrophobic section, reduces membrane permeability, and forces phospholipid tails closer together.
  • How does temperature influence membrane fluidity and permeability?
    Lower temperatures decrease membrane fluidity by slowing molecular movement and tightening hydrophobic tail packing, which decreases permeability.
  • What is diffusion and what drives it across membranes?

    Diffusion is the spontaneous movement of molecules from high to low concentration along a concentration gradient, increasing entropy.

    • Concentration gradient created by a difference in solute concentrations

  • What is osmosis and how does it differ from diffusion?

    Osmosis is the diffusion of water across a selectively permeable membrane from low to high solute concentration to equalize solute concentrations.

  • Define hypertonic, hypotonic, and isotonic solutions relative to a cell.

    Hypertonic solutions have higher solute concentration outside the cell, causing water to exit; hypotonic solutions have lower solute concentration inside the cell, so water enters; isotonic solutions have equal solute concentrations.

    • Hypo - bursts

    • Isotonic - equilibrium

    • Hyper -shrinks

  • What is the fluid-mosaic model of membrane structure?

    The cell membrane is flexible, with proteins scattered throughout it that can move around.

  • What are integral and peripheral membrane proteins?

    Proteins embedded in cell mem. that help transport substances across it, receive signals, or perform other cell functions.

    • Integral = embedded in membrane.

    • Peripheral proteins = attached to surface of the membrane (outside or inside cell) that help w/ cell signaling, support, & other functions.

  • How do membrane proteins move within the membrane?

    Most lipids and some proteins move sideways within the membrane, but flip-flopping across layers is rare.

  • What roles do channel proteins play in membranes? Give an example

    Channel proteins form pores allowing selective ions or small molecules to diffuse passively across membranes

    • Aquaporins ("water pores") - allow water to cross plasma membrane

  • What are gated channels and how are they regulated? Example?

    Gated channels open or close in response to signals like molecule binding or voltage changes, regulating ion and molecule flow.

    • Aquaporin

  • How do carrier proteins facilitate diffusion?
    Carrier proteins selectively bind solutes on one side of the membrane, change shape, and release them on the other side, assisting passive transport.
  • What distinguishes active transport from passive transport?
    Active transport moves substances against their concentration gradient using energy input, often from ATP, unlike passive transport which moves substances down gradients without energy.
  • Describe the sodium–potassium pump function.
    The sodium–potassium pump uses ATP to actively transport Na+ out and K+ into the cell against their concentration gradients.
  • What is secondary active transport (co-transport)?

    Secondary active transport uses energy stored in electrochemical gradients created by pumps to move other molecules against their gradients without direct ATP use.

    • Gradients represent potential energy

  • Why is selective permeability important for cells?

    Selective permeability allows cells to maintain an internal environment different from the outside, essential for life processes and favored by natural selection.

  • What are hydrocarbons? What are isoprenoids?

    • Non-polar molecules that contain only carbon and hydrogen; C-H bonds; hydrophobic

    • Polymer of hydrocarbons--hydrocarbon chain; building blocks for complex lipids

  • What is a fatty acid? Structure?

    • Hydrocarbon chain bonded to a carboxly (-COOH) group; contains 14-20 carbon atoms; can be saturated or unsaturated

  • Structure of steroids? Examples?

    • Lipid

    • Bulky, four-ring structure

    • Differ from each other by attached functional groups

    • Estrogen, testosterone, cholesterol

  • Structure of fats? Primary role? Are they polymers?

    • 3 fatty acids linked to a glycerol (aka triacylglycerols, triglycerides)

    • Energy storage bc large number of high-energy bonds

    • No

  • Functions of lipids?

    Store chemical energy, act as pigments for photosynthesis, signal between cells, form waterproof coatings on skin & cells, act as vitamins in cellular processes.

  • What are vesicles? Liposomes?

    • Vesicles: small membrane-bound sacs (a lipid bilayer enclosing an aqueous interior).

    • Liposomes: artificial (lab-made) membrane-bound vesicles formed from phospholipid bilayers.

      • Liposomes are a type of vesicle!

  • What is the difference between concentration and solute concentration? Give an example of a process for each

    Concentration = amount of a substance in a solution. Solute concentration = amount of dissolved substance.

    • Osmosis - water moves from low to high solute concentration

    • Diffusion -particles move from high to low concentration.

  • What is used to measure permeability of lipid bilayers?

    • Planar bilayers - lipid bilayers constructed across a hole in a glass or plastic wall, separating two aqueous solutions

  • How do phospholipids in plasma membranes move in the bilayer?

    Laterally/sideways

  • Why are dissolved solutes in constant random motion?

    Due to their thermal energy, which causes particles to move randomly. The higher the temperature, the faster they generally move.

  • What is equilibrium? What is passive transport?

    • Occurs when molecules or ions are randomly distributed throughout a solution; molecules are still moving, but no net movement.

    • Occurs when substances diffuse across a membrane in the absence of an outside energy source

  • What are the main types of cell membrane transport?

    • Passive transport - occurs without energy

      • Simple diffusion

      • Facilitated diffusion

      • Osmosis

    • Active transport - requires energy

  • Facilitated diffusion vs. simple diffusion

    • Simple diffusion: Small molecules move from high to low concentration directly through the membrane (ex: oxygen).

    • Facilitated diffusion: Molecules move from high to low concentration through a channel or carrier protein (ex: glucose).

  • How do plant cells behave in a hypotonic solution and a hypertonic solution? How do red blood cells act in hypertonic solutions?

    • Plant cells become turgid (swollen), but do not lyse (burst) in hypotonic solutions; in hypertonic solutions, the plasma membrane and cytoplasm pull away (shrink away) from the rigid cell wall (aka - plasmolysis)

    • Red blood cells will undergo crenation (shrink and develop spiked or wrinkled margins)

  • What is the role of phospholipids and proteins in the membrane?

    Phospholipids form the structure; proteins help substances cross.

  • What does amphipathic mean, and how do proteins form passageways across the lipid bilayer?

    Amphipathic means having hydrophilic and hydrophobic regions. Protein folding creates shapes that can form passageways.

  • What are protocells?

    Simple vesicle-like structures harboring nucleic acids, possible intermediates in cell evolution. Their permeable fatty-acid bilayers let nucleotides enter, allowing RNA to be synthesized inside.

  • What detergents? What proteins effect permability?

    • Small amphipathic molecules that can form micelles; can separate membrane proteins from the membrane (often used to isolate & study those proteins); water-soluble

      • Allowed identification of different membrane proteins

    • Channels, carriers, pumps (push substances)

  • What are ion channels? What are electrochemical gradients?

    • Specialized membrane proteins; form pores, or openings, in membrane; allow ions to cross membrane

    • Occurs when ions build up on one side of plasma membrane; Ions diffuse down their electrochemical gradient

  • What is cystic fibrosis & what is it caused by?

    Secretions of cells that produce mucus, sweat, and digestive juices become abnormally concentrated & block pathways

    Caused by defects in the transmembrane protein CFTR (cystic fibrosis conductance

    transmembrane regulator) which allows passage of chloride ions (Cl-)

  • How does glucose diffuse across a membrane?

    • Carrier protein named GLUT-1 increases membrane permeability to glucose; it changes shape when it binds to glucose and allows its diffusion across the membrane