Chapter 7 Lecture.ppt

Overview: Life at the Edge

Concept 7.1: Cell membranes are fluid mosaics of lipids and proteins

• Phospholipids: amphipathic, both h-phob. and h-phil. regions, in bilayer

• Fluid Mosaic Model: membrane is fluid structure with “mosaic” of various

proteins embedded in it

Phospho -lipid

bilayer _Hydrophobic

regions of protein

The Fluidity of Membranes

• _{Phospholipids in plasma membr. can move}

within the bilayer

• _{Most lipids, some prot., drift laterally}

• _{Molec. rarely flip-flop transversely across the}

membrane

(a) Movement of phospholipids

Lateral movement

(107 times per second)

Flip-flop

Membrane Matters

• _{Temps cool, membranes switch from fluid state}

to solid state (depends on lipids)

• _{Membr. must be fluid to work properly; about}

as fluid as salad oil

• _{Membr. w/ unsat fatty acids more fluid}

Fluid

Unsaturated hydrocarbon tails with kinks

Viscous

Good Cholesterol

• _{Diff. effects on memb. fluidity at diff. temps}

• _{Warm temps (37°C), chol. restrains movement}

of p-lipids

• _{Cool temps, maintains fluidity by preventing}

tight packing

Cholesterol

Membrane Proteins and Their Functions

• Many diff. prot. embedded in the fluid matrix of the lipid bilayer

• _{Prot. determine most of PM’s specific fxn’s}

Varied Proteins

• Peripheral prot.: on surface of membrane

• _{Integral prot.: penetrate hydrophobic core,}

can be transmembrane, h-phob regions w/ nonpolar aa

N-terminus

C-terminus

 Helix CYTOPLASMIC_SIDE

(a) Transport

ATP

(b) Enzymatic activity Enzymes

(c) Signal transduction Signal transduction

Signaling molecule

Receptor

(d) Cell-cell recognition

Glyco-protein

The Role of Membrane Carbohydrates in Cell-Cell Recognition

• _{Cells recog. ea. other by binding to surface}

molec., often carbs, on the PM

The Permeability of the Lipid Bilayer

• _{H-phob molec., can dissolve in lipid bilayer,}

pass through membrane rapidly

• _{Polar molec. (sugars) do not cross membr.}

Transport Proteins

• _{Allow passage of h-phil. substances across}

membr.

• _{Channel prot. have h-phil. channel that certain}

molec. or ions can use as a tunnel

• _{Aquaporins facilitate the passage of water}

molec. shape deflects

Carrier Proteins

• _{Bind to molec., change shape to shuttle them}

across the membrane

• Protein is specific for the substance it moves

Concept 7.3: Passive transport: diffusion across a membrane w/ no E used

• _{Diffusion: substance move from [high] to [low]}

(net), down [gradient]; passive

• _{Dynamic equilibrium: substances move at}

Effects of Osmosis on Water Balance

• _{Osmosis: diffusion of water across memb.}

Lower

concentration of solute (sugar)

H₂O

Higher

concentration of sugar

Selectively permeable membrane

Same concentration of sugar

Water Balance of Cells Without Walls

• _{Tonicity: ability of solution to cause cell to gain or}

lose water

• Isotonic: [solute] same in/out cell; no net H₂O flow

• Hypertonic: [sol.] > outside cell; H₂O flows out

• Hypotonic: [sol.] < outside cell; H₂O flows in

Fig. 7-13

Hypotonic solution

(a) Animal

cell

(b) Plant

cell

H₂O

Lysed

H₂O

Turgid (normal)

H₂O

H₂O

H₂O

H₂O Normal

Isotonic solution

Flaccid

H₂O

H₂O Shriveled

Water Balance of Cells with Walls

• Cell walls give pressure against H₂O: turgid

• _{plant cell / env. isotonic, cell flaccid (limp),}

plant may wilt

Facilitated Diffusion: Passive Transport Aided by Proteins

• _{Facilitated Diff: transport prot. help molec.}

diffuse across PM

• _Specificity

• _{Ion channels: open/close response to stimulus}

(gated channels)

EXTRACELLULAR FLUID

Channel protein

(a) A channel protein

Solute

CYTOPLASM

Solute Carrier protein

Need Energy for Active Transport (AT)

• AT: moves substance against its [gradient]

• _{Requires E (ATP)}

• Carrier prot., solute binds, ATP donates P,

prot. changes shape, translocates solute which is released. Once released, prot. back to

original form, binds to another solute.

2

EXTRACELLULAR

FLUID

[Na+] high

[K+_{] low}

[Na+_{] low}

[K+_{] high}

Passive transport

Diffusion Facilitated diffusion

Active transport

How Ion Pumps Maintain Membrane Potential

• _{Membrane potential: voltage difference}

across membrane caused by differences in distribution of anions/cations

Like PB and J

• Electrochemical Gradient: drive diffusion of ions

• _{chem F (ion’s [gradient])}

Pump It Up!

• _{Electrogenic Pump: trans. prot. that}

generates voltage across a membrane

• _{Animals: Na}+ - K+ ion pump

• Cell-walled: H+ pump

Cotransport: Coupled Transport by a Membrane Protein

• _{Cotransport: AT of solute indirectly drives}

transport of another solute

• _{H+ pumps drive transport of nutrients into cell}

Proton pump – – – – – – + + + + + + ATP H+ H+ H+ H+ H+ H+ H+ H+ Diffusion of H+

Sucrose-H+

cotransporter

Sucrose

Concept 7.5: Bulk transport by exocytosis and endocytosis

• Move lg. molec.

• _{Requires E}

• _{Exocytosis: trans vesic. fuse w/ PM, dump}

Endocytosis

• Take in in macromol. forming vesicles from PM

• _{3 types:}

– _{Phagocytosis (“cell eating”)}

– Pinocytosis (“cell drinking”)

– Receptor-mediated (ligand binds to specific

PHAGOCYTOSIS EXTRACELLULAR

FLUID CYTOPLASM Pseudopodium “Food”or other particle Food vacuole PINOCYTOSIS 1 µm Pseudopodium of amoeba Bacterium Food vacuole

An amoeba engulfing a bacterium via phagocytosis (TEM)

Plasma membrane

Vesicle

0.5 µm

Pinocytosis vesicles forming (arrows) in a cell lining a small blood vessel (TEM)

RECEPTOR-MEDIATED ENDOCYTOSIS Receptor Coat protein Coated vesicle Coated pit Ligand Coat protein Plasma membrane