Membrane Structure and Function

Chapter 5, pt 1

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

Phospholipids – main lipid in membranes

Amphipathic molecule – has a hydrophilic region & a hydrophobic region

Membranes are fluid

• Held together by hydrophobic interactions• Most lipids and some proteins can move

laterally

• http://www.dnatube.com/video/360/Fluid-Mosaic-Model

Evidence for drifting of membrane proteins

Mouse cell

Membrane proteins

Human cell Hybrid cell

Mixed proteinsafter 1 hour

RESULTS

Fluid

Unsaturated hydrocarbontails

Viscous

Saturated hydrocarbon tails

(a) Unsaturated versus saturated hydrocarbon tails

(b) Cholesterol within the animal cell membrane

Fluid Mosaic model of membrane structure

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

Hydrophobic region is in the membrane

Hydrophilic regions are exposed on either side of membrane

Enzymes

Signaling molecule

Receptor

Signal transduction

Glyco-protein

ATP

(a) Transport (b) Enzymatic activity (c) Signal transduction

(d) Cell-cell recognition (e) Intercellular joining (f) Attachment to the cytoskeleton and extracellular matrix (ECM)

•Functions of Proteins in M

embrane

Traffic across membranes

• What needs to go in and out of a cell?

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-Energy is not required-Substance moves downconcentration gradient

Osmosis• Passive transport of water• Hypertonic solution – more solute• Hypotonic solution – less solute• Isotonic solution – same amt of solute

Osmosis animation

• http://highered.mcgraw-hill.com/sites/9834092339/student_view0/chapter38/animation_-_osmosis.html

• - What is plasmolysis?• - What is turgor pressure?

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

Facilitated diffusion

• http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_facilitated_diffusion_works.html

Aquaporins

• 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

• Requires energy (in form of ATP)

• Moves substances against the concentration gradient

• From low concentration to high concentration

Sodium Potassium Pump

• http://highered.mcgraw-hill.com/sites/9834092339/student_view0/chapter38/sodium-potassium_exchange_pump.html

Voltage across membranes

• Some ion pumps generate voltage across membranes – difference in charge across membrane

• An ion diffuses down electrochemical gradient

•

Cotransport

Endocytosis, Exocytosis

• Transport of large moleculesAnimation:• http://www.youtube.com/watch?v=

4gLtk8Yc1Zc• Amoeba eating paramecium:• http://www.youtube.com/watch?v=

pvOz4V699gk

Types of 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)

Receptor mediated endocytosis – LDL animation

• http://www.dnatube.com/video/5092/Receptor-Mediated-Endocytosis-of-LDL-Cholesterol

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?