Life at the Edge The plasma membrane is the boundary that separates the living cell from its...

Life at the Edge

• The plasma membrane is the boundary that separates the living cell from its nonliving surroundings

• The plasma membrane exhibits selective permeability, allowing some substances to cross it more easily than others

Figure 5.1

© 2014 Pearson Education, Inc.

Cellular membranes are fluid mosaics of lipids and proteins

• Phospholipids are the most abundant lipid in the plasma membrane

• Phospholipids are amphipathic molecules, containing hydrophobic and hydrophilic regions

• The fluid mosaic model states that a membrane is a fluid structure with a “mosaic” of various proteins embedded in it

Hydrophilichead

Hydrophobictail

WATER

WATER

Hydrophilic regionof protein

Hydrophobic region of protein

Phospholipidbilayer

The Fluidity of Membranes

• Phospholipids in the plasma membrane can move within the bilayer

• Most of the lipids, and some proteins, drift laterally

• Rarely does a molecule flip-flop transversely across the membrane

Lateral movement(~107 times per second)

Flip-flop(~ once per month)

Movement of phospholipids

• As temperatures cool, membranes switch from a fluid state to a solid state

• The temperature at which a membrane solidifies depends on the types of lipids

• Membranes rich in unsaturated fatty acids are more fluid than those rich in saturated fatty acids

• Membranes must be fluid to work properly; they are usually about as fluid as salad oil

Fluid

Unsaturated tails preventpacking.

Cholesterol

Viscous

Saturated tails packtogether.

(a) Unsaturated versus saturated hydrocarbon tails

(b) Cholesterol reducesmembrane fluidity atmoderate temperatures,but at low temperatureshinders solidification.

ViscousFluid

Unsaturated hydrocarbontails with kinks

Membrane fluidity

Saturated hydro-carbon tails

• The steroid 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

Cholesterol within the animal cell membrane

• Some proteins in the plasma membrane can drift within the bilayer

• Proteins are much larger than lipids and move more slowly

Membrane proteins

Mixedproteinsafter1 hourHybrid cell

Human cell

Mouse cell

Membrane Proteins and Their Functions

• A membrane is a collage of different proteins embedded in the fluid matrix of the lipid bilayer

• Proteins determine most of the membrane’s specific functions

• Peripheral proteins are not embedded

• Integral proteins penetrate the hydrophobic core and often span the membrane

Fibers ofextracellularmatrix (ECM)

Glycoprotein

Carbohydrate

Microfilamentsof cytoskeleton

Cholesterol

Integralprotein

Peripheralproteins

CYTOPLASMIC SIDEOF MEMBRANE

EXTRACELLULARSIDE OFMEMBRANE

Glycolipid

• 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

EXTRACELLULARSIDEN-terminus

C-terminusCYTOPLASMICSIDE

Helix

• Six major functions of membrane proteins:

– Transport

– Enzymatic activity

– Signal transduction

– Cell-cell recognition

– Intercellular joining

– Attachment to the cytoskeleton and extracellular matrix (ECM)

EnzymesSignal

ReceptorATP

Transport Enzymatic activity Signal transduction

Glyco-protein

Cell-cell recognition Intercellular joining Attachment to thecytoskeleton and extra-cellular matrix (ECM)

The Role of Membrane Carbohydrates in Cell-Cell Recognition

• Cells recognize each other by binding to surface molecules, often carbohydrates, on the plasma membrane

• Membrane carbohydrates may be covalently bonded to lipids (forming glycolipids) or more commonly to proteins (forming glycoproteins)

• Carbohydrates on the external side of the plasma membrane vary among species, individuals, and even cell types in an individual

Synthesis and Sidedness of Membranes

• Membranes have distinct inside and outside faces

• The asymmetrical distribution of proteins, lipids and associated carbohydrates in the plasma membrane is determined when the membrane is built by the ER and Golgi apparatus

Plasma membrane:

Cytoplasmic face

Extracellular faceTransmembraneglycoprotein

Plasma membrane:

Secretedprotein

Vesicle

Golgiapparatus

Glycolipid

Secretoryprotein

Transmembraneglycoproteins

ER

Membrane structure results in 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

The Permeability of the Lipid Bilayer

• Hydrophobic (nonpolar) molecules, such as hydrocarbons, can dissolve in the lipid bilayer and pass through the membrane rapidly

• Polar molecules, such as sugars, do not cross the membrane easily

Transport Proteins

• Transport proteins allow passage of hydrophilic substances across the membrane

• Some transport proteins, called channel proteins, have a hydrophilic channel that certain molecules or ions can use as a tunnel

• Channel proteins called aquaporins facilitate the passage of water

• Other transport proteins, called carrier proteins, bind to molecules and change shape to shuttle them across the membrane

• A transport protein is specific for the substance it moves

Passive transport is diffusion of a substance across a membrane with no energy investment

• Diffusion is the tendency for molecules to spread out evenly into the available space

• Although 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

Diffusion of one solute

• Substances diffuse down their 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

• The diffusion of a substance across a biological membrane is passive transport because it requires no energy from the cell to make it happen


Diffusion of two solutes


Effects of Osmosis on Water Balance

• Osmosis is the diffusion of water across a selectively permeable membrane

• The direction of osmosis is determined only 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

Lowerconcentrationof solute (sugar)

Higherconcentrationof sugar

Same concentrationof sugar

Selectivelypermeable mem-brane: sugar mole-cules cannot passthrough pores, butwater molecules can

H2O

Osmosis

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 that inside the cell; cell gains water

• Animals and other organisms without rigid cell walls have osmotic problems in either a hypertonic or hypotonic environment

• To maintain their internal environment, such organisms must have adaptations for 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 vacuole50 µm

50 µmContracting vacuole

Water Balance of Cells with Walls

• Cell walls help maintain water balance

• A plant cell in a hypotonic solution swells until the wall opposes uptake; the cell is now turgid (firm)

• If a plant cell and its surroundings are isotonic, there is no net movement of water into the cell; the cell becomes flaccid (limp), and the plant may wilt

• In a hypertonic environment, plant cells lose water; eventually, the membrane pulls away from the wall, a usually lethal effect called plasmolysis

Animalcell

Lysed

H2O H2O H2O

Normal

Hypotonic solution Isotonic solution Hypertonic solution

H2O

Shriveled

H2OH2OH2OH2OPlantcell

Turgid (normal) Flaccid Plasmolyzed

Facilitated Diffusion: Passive Transport Aided by Proteins

• In facilitated diffusion, transport proteins speed movement of molecules across the plasma membrane

• Channel proteins provide corridors that allow a specific molecule or ion to cross the membrane

• Carrier proteins undergo a subtle change in shape that translocates the solute-binding site across the membrane

EXTRACELLULARFLUID

Channel protein Solute

CYTOPLASM

Carrier protein Solute

Active transport uses energy to move solutes against their gradients

• Facilitated diffusion is still passive because the solute moves down its concentration gradient

• Some transport proteins, however, can move solutes against their concentration gradients

The Need for Energy in Active Transport

• Active transport moves substances against their concentration gradient

• Active transport requires energy, usually in the form of ATP

• Active transport is performed by specific proteins embedded in the membranes

• The sodium-potassium pump is one type of active transport system

Cytoplasmic Na+ bonds tothe sodium-potassium pump

CYTOPLASMNa+

[Na+] low[K+] high

Na+

Na+

EXTRACELLULARFLUID

[Na+] high[K+] low

Na+

Na+

Na+

ATP

ADP

P

Na+ binding stimulatesphosphorylation by ATP.

Na+

Na+

Na+

K+

Phosphorylation causesthe protein to change itsconformation, expelling Na+

to the outside.

P

Extracellular K+ bindsto the protein, triggeringrelease of the phosphategroup.

PP

Loss of the phosphaterestores the protein’soriginal conformation.

K+ is released and Na+

sites are receptive again;the cycle repeats.

K+

K+

K+

K+

K+

Diffusion Facilitated diffusion

Passive transport

ATP

Active transport

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:

– A chemical force (the ion’s concentration gradient)

– 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 proton pump

H+

ATP

CYTOPLASM

EXTRACELLULARFLUID

Proton pump

H+

H+

H+

H+

H+

+

+

+

+

+

–

–

–

–

–

Cotransport: Coupled Transport by a Membrane Protein

• Cotransport occurs when active transport of a solute indirectly drives transport of another solute

• Plants commonly 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

Diffusionof H+

Sucrose

H+

H+

H+

H+

H+

H+

+

+

+

+

+

+

–

–

–

–

–

–

Bulk transport across the plasma membrane occurs by exocytosis and endocytosis

• Small molecules and water enter or leave the cell through the lipid bilayer or by transport proteins

• Large molecules, such as polysaccharides and proteins, cross the membrane via vesicles

Exocytosis

• In exocytosis, transport vesicles migrate to the membrane, fuse with it, and release their contents

• Many secretory cells use exocytosis to export their products

Endocytosis

• In endocytosis, the cell takes in macromolecules by forming vesicles from the plasma membrane

• Endocytosis is a reversal of exocytosis, involving different proteins

• Three types of endocytosis:

– Phagocytosis (“cellular eating”): Cell engulfs particle in a vacuole

– Pinocytosis (“cellular drinking”): Cell creates vesicle around fluid

– Receptor-mediated endocytosis: Binding of ligands to receptors triggers vesicle formation

Phagocytosis

Foodvacuole

“Food”or otherparticle

CYTOPLASM

Pseudopodium

Solutes

EXTRACELLULARFLUID

Pseudopodiumof amoeba

An amoeba engulfing a bacteriumvia phagocytosis (TEM)

Bacterium

Food vacuole 1 m

Pinocytosis

Plasmamembrane

Coatprotein

Coatedpit

Coatedvesicle

Pinocytotic vesicles forming(TEMs)

0.25

m

Top: A coated pit Bottom: A coatedvesicle forming during receptor-mediated endocytosis (TEMs)

0.25

m

Receptor-MediatedEndocytosis

ReceptorPlasmamembrane

Coatprotein

Animations and Videos

• How Diffusion Works

• Diffusion

• Osmosis

• Bozeman - Osmosis Demo

• Bozeman - Diffusion Demo

• Plasmolysis

• Hemolysis and Crenation

• Contractile Vacuole

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

http://www.stolaf.edu/people/giannini/flashanimat/transport/diffusion.swf

Animations and Videos• Bozeman - Water Potential

• How Facilitated Diffusion Works

• Sodium-Potassium Pump Exchange

• Bozeman - Transport Across Cell Membrane

• Cotransport

• Proton Pump

• Amoeboid Movement

• Second Messengers (cAMP and Ca+2 Pathways)


• Chemical Synapse – 1

• Chemical Synapse – 2

• Voltage-Gated Channels and the Action Potential

• Clathrin-Coated Pits and Vesicles

• Receptors Linked to a Protein Channel

• Passive Transport

• Active Transport by Group Translocation


• Secondary Active Transport

• Organization of the Golgi

• Antiport

• Uniport...Carrier Protein

• Gated and Non-gated Channels

• Symport

• Cellulose Synthesis during Elongation

• Signal Transduction Pathway


• Signaling by Secreted Molecules

• Signal Transduction

• 2nd Messenger

• Signal Amplification – 1

• Signal Amplification – 2

• Cotranslational Targeting of Secretory Proteins to the ER

• Mechanism of Tyrosine Kinase


• Chapter Quiz Questions – 1

• Chapter Quiz Questions - 2

Which of the following best describes the structure of a biological membrane?

• two layers of phospholipids with proteins embedded between the two layers

• a mixture of covalently linked phospholipids and proteins that determines which solutes can cross the membrane and which cannot

• two layers of phospholipids with proteins either spanning the layers or on the surface of the layers

• a fluid structure in which phospholipids and proteins move freely between sides of the membrane

• two layers of phospholipids (with opposite orientations of the phospholipids in each layer) with each layer covered on the outside with proteins

Which of the following statements about osmosis is correct?

• If a cell is placed in an isotonic solution, more water will enter the cell than leaves the cell.

• Osmotic movement of water into a cell would likely occur if the cell accumulates water from its environment.

• The presence of aquaporins (proteins that form water channels in the membrane) should speed up the process of osmosis.

• If a solution outside the cell is hypertonic compared to the cytoplasm, water will move into the cell by osmosis.

• Osmosis is the diffusion of water from a region of lower water concentration to a region of higher water concentration.

Which of the following statements about osmosis is correct?

• If a cell is placed in an isotonic solution, more water will enter the cell than leaves the cell.

• Osmotic movement of water into a cell would likely occurif the cell accumulates water from its environment.

• The presence of aquaporins (proteins that form water channels in the membrane) should speed up the process of osmosis.

• If a solution outside the cell is hypertonic compared to the cytoplasm, water will move into the cell by osmosis.

• Osmosis is the diffusion of water from a region of lower water concentration to a region of higher water concentration.

Which of the following amino acids would most likely be present in the outer side of a transmembrane domain of an integral membrane protein?

• a charged amino acid like lysine

• a polar amino acid like serine

• a special amino acid like glycine or proline

• a hydrophobic amino acid like valine

• any of the above, with no preference

Which of the following molecules will diffuse most quickly across a lipid bilayer membrane? [Hint: Which one has a nonpolar covalent bond that leads to no partial charge on the atoms?]

• H2O

• O2

• H2PO4

• glucose

• Na

Cells (e.g., bacteria) are taken up by other cells (e.g., an immune cell) by which of the following?

• pinocytosis

• exocytosis

• receptor-mediated endocytosis

• phagocytosis

• facilitated diffusion

Consider the amino acids along the helices of a membrane spanning protein (see figure). Those facing the fatty acid chains would be expected to be _____, while those facing the inner pore would often be ______.

• acidic; basic

• hydrophilic; hydrophobic

• aromatic; acidic

• hydrophobic; hydrophilic

• cysteines; glycines

Based on the current model of the membrane, which statement is incorrect?

• Glycoproteins tend to have oligosaccharides on their outward facing side.

• Transmembrane proteins often bind with cytosolic proteins, but not with extracellular molecules.

• The combinations of phospholipids in the two faces of the membrane often differ.

• Phospholipids tend to move faster laterally along the membrane than do the proteins.

• Some transmembrane proteins function as active transport systems.

Assume that each of the following items experiences a similar magnitude of energy difference driving their diffusion across a pure lipid bilayer. If ranked in order from fastest to slowest, which of the following items would likely be second in terms of how much of it crosses the bilayer in a given time?• molecular oxygen

• sucrose

• insulin

• glucose

• water

As shown on the next slide, two solutions with similar solute concentration are separated by a membrane that allows only water to pass, and one solution is of greater volume than the other. Which choice best describes what will happen next?

• The energy in the pressure gradient will drive water to the left side, which will cause the solute concentration on the right side to increase.

• Water will pass to the right side and increase its volume.

• Any water that passes to the left side will create a tension on the right side, pulling the water back to the right side.

• The water will flow to the left side, down its pressure gradient, until the two sides achieve equal height.

• Since there is no concentration gradient present, there will be no net movement of water.

Two solutions with similar solute concentration are separated by a membrane that allows only water to pass, and one solution is of greater volume than the other. Which choice best describes what will happen next?

• The energy in the pressure gradient will drive water to the left side, which will cause the solute concentration on the right side to increase.

• Water will pass to the right side and increase its volume.

• Any water that passes to the left side will create a tension on the right side, pulling the water back to the right side.

• The water will flow to the left side, down its pressure gradient, until the two sides achieve equal height.

• Since there is no concentration gradient present, there will be no net movement of water.

One strategy used by many animal species to avoid the need for their body cells to have either a stiff external cell wall or active contractile vacuoles is to

• have membranes that are impermeable to water and keep it from entering their cells.

• use aquaporins to actively pump water out through a hydrophilic pore.

• keep the concentration of solutes in their cytosol the same as that found in pond water.

• maintain a high internal pressure to constantly push extra water out of cells.

• carefully regulate the solute concentration of the extracellular solution to which their cells are exposed.

An artificial liposome, whose membrane contains no proteins, is loaded with a 0.03 M sucrose solution and put into pure water (see figure on next slide). Which choice best describes what will quickly happen next?

• Since there are no membrane proteins, nothing will cross the membrane.

• Sucrose will diffuse out of the liposome.

• Water will diffuse down its concentration gradient into the liposome, causing it to burst.

• Water will enter the liposome, diluting the sucrose concentration there down to 0 M.

• The pressure of the surrounding solution will push water out of the liposome, causing it to shrink over time.

An artificial liposome, whose membrane contains no proteins, is loaded with a 0.03 M sucrose solution and put into pure water. Which choice best describes what will quickly happen next?

• Since there are no membrane proteins, nothing will cross the membrane.

• Sucrose will diffuse out of the liposome.

• Water will diffuse down its concentration gradient into the liposome, causing it to burst.

• Water will enter the liposome, diluting the sucrose concentration there down to 0 M.

• The pressure of the surrounding solution will push water out of the liposome, causing it to shrink over time.

A correct distinction between facilitated diffusion and active transport of a substance across a biological membrane is that• active transport requires conformational changes in the transport protein associated with the transport process, and facilitated diffusion does not.

• active transport requires an integral membrane protein to carry out the transport, and facilitated diffusion does not.

• facilitated diffusion requires a protein lined pore in the membrane, and active transport does not.

• facilitated diffusion depends on an existing energy gradient acting on the transported substance, while active transport makes such a gradient.

• facilitated diffusion requires cellular energy (often from ATP hydrolysis), but active transport does not.

Consider various transport systems in a hypothetical cell (see figure). Which one of these systems would both be a passive system and not alter the membrane potential through its operation?

• A

• B

• C

• D

• E

In the hypothetical cell with the membrane transport systems and conditions shown (see figure), which represents a sodium ion channel, and energy gradients in what form influence the direction of movement of sodium ions through this channel?

• A; concentration gradient

• B; concentration and electrical gradients

• C; concentration gradient

• D; electrical gradient

• E; concentration and electrical gradients

Receptor-mediated endocytosis produces vesicles that

• typically deliver the items they take up to the nucleus of the cell.

• carry macromolecules and cells for delivery to the lysosomal compartment.

• assist in the removal of certain items from the cytosol of the cell.

• when formed cause there to be more total surface area available in the plasma membrane.

• have receptors which can indicate at their cytosolic side if ligands are bound or not on their extracellular/ lumen side.

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