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Membrane Structure and Function. Chapter 8. Practice problems – use the terms hypertonic, hypotonic, isotonic as appropriate. If you soak your hands in dishwater, your hands absorb the water and swell into wrinkles. This is because your skin cells are to the dishwater.

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practice problems use the terms hypertonic hypotonic isotonic as appropriate
Practice problems – use the terms hypertonic, hypotonic, isotonic as appropriate
  • If you soak your hands in dishwater, your hands absorb the water and swell into wrinkles. This is because your skin cells are to the dishwater.
  • Why does lettuce wilt in a vinaigrette salad dressing?
  • Why is seawater dangerous to drink?
  • Why will a marine fish die in freshwater?
plasma membrane the cell boundary
Plasma Membrane - the Cell Boundary
  • Controls what goes in and out of the cell
  • Semipermeable - allows some things to cross more easily than others
membrane structure
Membrane Structure

Phospholipids – main lipid in membranes

Amphipathic molecule – has a hydrophilic region & a hydrophobic region

membranes are fluid
Membranes are fluid
  • Held together by hydrophobic interactions
  • Most lipids and some proteins can move laterally
evidence for drifting of membrane proteins
Evidence for drifting of membrane proteins


Membrane proteins

Mixed proteinsafter 1 hour

Mouse cell

Human cell

Hybrid cell




Unsaturated hydrocarbontails

Saturated hydrocarbon tails

(a) Unsaturated versus saturated hydrocarbon tails

(b) Cholesterol within the animal cell membrane

what s part of the membrane
What’s part of the membrane
  • Phopholipids
  • Cholesterol
  • Proteins:
    • Integral proteins – embedded in lipid bilayer
    • Peripheral proteins – not embedded
  • Carbohydrate chains on proteins or lipids – found on outside face of membrane
    • Oligosaccharides (15 or fewer sugars)
integral protein
Integral protein

Hydrophobic region is in the membrane

Hydrophilic regions are exposed on either side of membrane


Signaling molecule

  • Functions of Proteins in Membrane




Signal transduction

(a) Transport

(c) Signal transduction

(b) Enzymatic activity


(f) Attachment to the cytoskeleton and extracellular matrix (ECM)

(e) Intercellular joining

(d) Cell-cell recognition

traffic across membranes
Traffic across membranes
  • What needs to go in and out of a cell?
membrane structure results in selective permeability
Membrane structure results in selective permeability
  • Small hydrophobic molecules – easily go through plasma membrane
  • Ions, polar molecules do not
    • (water, glucose)
  • Transport proteins aid in moving these molecules across membrane
passive transport
Passive Transport

-Energy is not required

-Substance moves down

concentration gradient

  • Passive transport of water
  • Hypertonic solution – more solute
  • Hypotonic solution – less solute
  • Isotonic solution – same amt of solute
facilitated diffusion
Facilitated Diffusion
  • Transport proteins help move polar molecules or ions across membrane
  • Channels – corridor for specific molecules or ions
  • Gated channels – open with a stimulus

- physical, electrical, chemical

  • Many change shape (conformation) in process
  • Water channel proteins
  • (2003 Nobel prize – co winner Pete Agre for work on aquaporins)

Move massive amounts of water molecules across the membrane – 3 billion per second

Water molecules move through in a single file

active transport
Active Transport
  • Requires energy (in form of ATP)
  • Moves substances against the concentration gradient
  • From low concentration to high concentration
sodium potassium pump
Sodium Potassium Pump
voltage across membranes
Voltage across membranes
  • Some ion pumps generate voltage across membranes – difference in charge across membrane
  • An ion diffuses down electrochemical gradient
  • Electrochemical gradient-
    • Chemical force – ion gradient
    • Electrical force- voltage across membrane
endocytosis exocytosis
Endocytosis, Exocytosis
  • Transport of large molecules
receptor mediated endocytosis
Receptor mediated endocytosis
  • Coated pits have receptors for specific molecule
  • Allow cells to get large amts of specific materials that may be in lower concentrations in the environment
  • Humans – cholesterol travels in LDLs – attach to receptors in pits
  • LDLs act as ligands – bind to LDL receptors in cell membrane
  • If LDL receptors are defective, leads to hypercholesterolemia (build up of LDL and cholesterol in the blood)