2.Membranes

Chapter 7

Cell Membranes

A. Phospholipid bilayer

B. Fluid mosaic

– _{Phospholipids, cholesterol, proteins}

C. Selectively permeable

Hydrophilic region of protein

Hydrophobic region of protein Phospholipid

A. Phospholipid bilayer

Hydrophilic head Hydrophobic

tail

WATER

WATER • _{Membrane is made primarily of}

phospholipids

– _{Phospholipids are amphipathic: containing}

B. Fluid Mosaic Model

• _{The fluid mosaic model states that a membrane is a}

fluid structure with a “mosaic” of various proteins embedded in it

• _{Phospholipids can move within the bilayer}

– _{Most of the lipids, and some proteins, drift laterally}

– http://www.wisc-online.com/objects/index_tj.asp?objID=AP1101

Lateral movement

(~107 _{times per second)}

Fluid Mosaic Model

• _{Membranes must be fluid to work properly; they are}

usually about as fluid as salad oil (!)

– Membranes rich in unsaturated fatty acids are more fluid that those rich in saturated fatty acids

Viscous Fluid

Unsaturated hydrocarbon tails with kinks Membrane fluidity

Fluid Mosaic Model

• _{Some proteins in the plasma membrane can drift}

within the bilayer

• _{Proteins are much larger than lipids and move more}

slowly

Membrane proteins

Mixed proteins after 1 hour Hybrid cell

Fluid mosaic: Cholesterol in the membrane

• _{Cholesterol has different effects on membrane}

fluidity at different temperatures

– At warm temperatures (such as 37°C), cholesterol restrains movement of phospholipids

– At cool temperatures, it maintains fluidity by preventing tight packing

Cholesterol

Fluid Mosaic: Membrane Proteins

• _{Proteins determine most of the membrane’s specific}

functions

• _{Peripheral proteins are not embedded- sometimes}

bound to integral proteins

• _{Integral proteins penetrate the hydrophobic core and}

often span the membrane

Membrane Proteins

• _{Integral proteins that span the membrane are called}

transmembrane proteins

– _{The hydrophobic regions of an integral protein consist of}

one or more stretches of nonpolar amino acids, often coiled into alpha helices

EXTRACELLULAR SIDE N-terminus

C-terminus

CYTOPLASMIC SIDE

Functions of Membrane Proteins

• _{Six major functions of membrane proteins:}

1. Transport

2. Enzymatic activity 3. Signal transduction 4. Cell-cell recognition 5. Intercellular joining

6. Attachment to the cytoskeleton and extracellular matrix (ECM)

LE 7-9a

Enzymes Signal

Receptor ATP

LE 7-9b

Glyco-protein

Fluid Mosaic: Glycoproteins and glycolipids

• Cells recognize each other by binding to surface

molecules, often carbohydrates, on the plasma membrane

• Membrane carbs may be covalently bonded to lipids (glycolipids) or proteins (glycoproteins)

• Carbs on the external side of the plasma membrane vary among species, individuals, and cell types

• _{Chill techno cell dance}

Sidedness of Membranes

• _{Membranes have distinct}

inside and outside faces

• _{Asymmetrical distribution of}

proteins, lipids and

associated carbohydrates in the plasma membrane

– _{determined when the}

membrane is built by the ER and Golgi apparatus

Build a Membrane

•

Use: bobby pins, marshmallows, ziti, other

candy pieces

•

What are the THREE main macromolecules you

need?

– _{What are some of their functions?}

– _{How can you reflect the fact that form dictates}

function?

•

What TWO types of molecules are

C. Selective permeability

• _{A cell must exchange materials with its surroundings, a}

process controlled by the plasma membrane

• _{Plasma membranes are selectively permeable,}

regulating the cell’s molecular traffic

•

http://www.wiley.com/college/pratt/047139387

8/student/animations/membrane_transport/ind

ex.html

The Permeability of the Lipid Bilayer

(nonpolar) molecules can dissolve in the lipid bilayer and pass

through the membrane

• _{Polar molecules, such}

as sugars, do not cross the membrane easily

Transport Proteins

• _{Transport proteins allow passage of SPECIFIC}

hydrophilic substances across the membrane

– _{channel proteins: transport proteins with a}

hydrophilic channel that certain substances can use as a tunnel

oChannel proteins called aquaporins facilitate the passage of water

• carrier proteins: transport proteins that bind to

molecules and change shape to shuttle them across the membrane

The cell is so DRAMATIC

•

http://www.youtube.com/watch?v=GW0lqf4F

Passive transport: Diffusion

• _{Diffusion: tendency for molecules to spread out evenly}

into the available space

– _{Each molecule moves randomly,}

– _{Diffusion of a population of molecules may exhibit a net}

movement in one direction

• _{At dynamic equilibrium, as many molecules cross one}

way as cross in the other direction

Molecules of dye Membrane (cross section)

WATER

Net diffusion Net diffusion Equilibrium

Concentration Gradients

• _{Concentration gradient: the difference in concentration of a}

substance from one area to another

– No work must be done to move substances down the concentration gradient

• Diffusion is passive transport because it requires no input of energy

Net diffusion Net diffusion Equilibrium

Diffusion of two solutes

Net diffusion Net diffusion Equilibrium

Passive Transport: Osmosis

• _{Osmosis: the diffusion of water across a selectively}

permeable membrane

• The direction of osmosis is determined by a difference in total solute concentration

– Water diffuses across a membrane from the region of lower solute concentration to the region of higher solute

concentration

Water Balance of Cells Without Walls

• _{Tonicity is the ability of a solution to cause a cell to gain}

or lose water

• _{Isotonic solution: solute concentration is the same as}

that inside the cell; no net water movement across the plasma membrane

• _{Hypertonic solution: solute concentration is greater}

than that inside the cell; cell loses water

• _{Hypotonic solution: solute concentration is less than}

http://www.cat.cc.md.us/courses/bio141/lecguide/unit4/metabolism/growth/ hypertonicanim.html

www.cat.cc.md.us/.../ growth/hypotonicanim.html

www.cat.cc.md.us/.../ growth/isotonicanim.html

http://www.youtube.com/watch?v=sdiJtDRJQEc

http://www.connect.ab.ca/~lburns/osmosis.gif

http://www.youtube.com/watch?v=crpeX8nBgJE&NR=1

Osmoregulation

• Osmoregulation, the

control of water balance

– The protist Paramecium,

which is hypertonic to its pond water environment, has a contractile vacuole that acts as a pump

Filling vacuole 50 µm

Water Balance of Cells with Walls

• _{A plant cell in a hypotonic solution swells until the wall}

opposes uptake; the cell is turgid (firm)

• A plant cell in an isotonic solution : no net movement of water into the cell; the cell becomes flaccid (limp)

• _{In a hypertonic environment, plant cells lose water; the}

membrane pulls away from the wall, a usually lethal effect called plasmolysis

Plasmolysis

LE 7-13

Animal cell

Lysed

H₂O H₂O H₂O

Normal

Hypotonic solution Isotonic solution Hypertonic solution

H₂O

Shriveled

H₂O H₂O

H₂O H₂O

Plant cell

Calculating Water Potential

•

http://www.phschool.com/science/biology_pl

ace/labbench/lab1/watpot.html

•

http://www.phschool.com/science/biology_pl

Passive Transport: Facilitated Diffusion

• _{Facilitated diffusion:}

transport proteins speed movement of molecules across the plasma

membrane

– _{Channel proteins :}

corridors that allow a

specific molecule or ion to cross the membrane

– _{Carrier proteins: undergo a}

change in shape that

moves the solute-binding site across the membrane

Active transport

• _{Some transport proteins move solutes against their}

concentration gradients

• _{Active transport requires energy, usually in the form of}

ATP

LE 7-16

Cytoplasmic Na+ bonds to

the sodium-potassium pump

CYTOPLASMNa+

[Na+_{] low}

[K+_{] high}

Na+

Na+

EXTRACELLULAR FLUID[Na

+_{] high}

[K+_{] low}

Na+ Na+ Na+ ATP ADP P

Na+ _{binding stimulates}

phosphorylation by ATP.

Na+

Na+

Na+

K+

Phosphorylation causes the protein to change its conformation, expelling Na+

to the outside.

P

Extracellular K+ _binds

to the protein, triggering release of the phosphate group.

P

P

Loss of the phosphate restores the protein’s original conformation.

K+ _{is released and Na}+

sites are receptive again; the cycle repeats.

K+

K+ K+

K+

LE 7-17

Diffusion Facilitated diffusion Passive transport

ATP

Maintenance of Membrane Potential by Ion

Pumps

• _{Membrane potential is the voltage difference across a}

membrane

• _{Two combined forces, collectively called the}

electrochemical gradient, drive the diffusion of ions across a membrane:

1. A chemical force (the ion’s concentration gradient) 2. An electrical force (the effect of the membrane

potential on the ion’s movement)

• An electrogenic pump is a transport protein that

generates the voltage across a membrane

– _{The main electrogenic pump of plants, fungi, and bacteria is a}

Cotransport

• _{Cotransport: active}

transport of a solute

indirectly drives transport of another solute

– _{Plants: use the gradient of}

hydrogen ions generated by proton pumps to drive

active transport of nutrients into the cell

H+ ATP Proton pump Sucrose-H+ cotransporter Diffusion of H+

Bulk transport across the plasma membrane: exocytosis and endocytosis

•

Large molecules, such as polysaccharides and

proteins, cross the membrane via vesicles

•

Exocytosis: transport vesicles migrate to the

membrane, fuse with it, and release their contents

•

Endocytosis: the cell takes in macromolecules by

forming vesicles from the plasma membrane

– _{Phagocytosis (“cellular eating”): Cell engulfs particle in a}

vacuole

http://highered.mcgraw-hill.com/sites/0072495855/student_ view0/chapter2/animation__phagocytosis.html

– _{Pinocytosis (“cellular drinking”): Cell creates vesicle around}

fluid

– _{Receptor-mediated endocytosis: Binding of ligands to}

receptors triggers vesicle formation

Exocytosis

Phagocytosis:

a form of endocytosis

Pinocytosis (a form of Endocytosis)

LE 7-20c Receptor RECEPTOR-MEDIATED ENDOCYTOSIS Ligand Coated pit Coated vesicle Coat protein Coat protein Plasma membrane 0.25 µm