Lecture 5

Membranes

Why does osmosis matter?http://www.livescience.com/37227-man-overdoses-on-soy-sauce.html?cmpid=514645

Yesterday’s Exit Ticket

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Prokaryotes Animals Plants

No nucleus True nucleus True nucleus

Cell wall (featuring peptidoglycan)

No cell wall Cell wall(featuring cellulose)

No membrane-bound organelles

Membrane-bound organelles (including mitochondria, but NOT chloroplasts or vacuole)

Membrane-bound organelles (mitochondria, chloroplasts, vacuole)

DNA DNA DNA

Ribosomes Ribosomes Ribosomes

Cytoplasm Cytoplasm Cytoplasm

Cell Membrane Cell Membrane Cell Membrane

Sim

ilaritie

sD

iffere

nce

s

Key Themes

(2) “Think Like a Biologist”: Understand What Life Is. “Unity” of life: What are the common features of all life?

• Structure and function of biological membranes

• Maintenance of a suitable internal environment at the cost of energy input

Today’s agenda:

• Fun with membranes

• Review of the key concepts for the exam

Key Functions of Membranes

Membrane Structure and Function

1) Provide a barrier around cells & sub-cellular spaces

2) Provide controlled passageways for wanted & unwanted substances

Which macromolecules do which?

Phospholipid bilayer provides ±impenetrable barrier

Proteins provide selective & controllable passageways(“selective permeability”)

Phospholipid bilayer - as the basic membrane structure

Phospholipids have hydrophilic & hydrophobic regions.

Fig. 7.2

1. Be able to relate the basic structure of biological membranes to their principal functions

Fluid-Mosaic Membrane

• Membranes: typically “fluid” with consistency of salad oil (fluidity level varies with temperature!)

• Membranes: mosaic of phospholipids & proteins

Phospholipidbilayer

Hydrophobic regionsof protein

Hydrophilicregions of proteinFig. 7.3

The effect of unsaturated versus saturated phospholipids on membrane fluidity

In organisms that do not regulate body temperature (microorganisms, plants, & non-regulating animals)

Fluid Viscous

Unsaturated hydrocarbontails with kinks

Saturated hydro-carbon tails

Fig. 7.5 (b)

2. Be able to identify factors affecting membrane fluidity in various organisms

3. Be able to relate saturated and unsaturated fatty acids to the ecology of organisms

http://www.ecoworld.com/maps/world-ecoregions.html

Macademia nutAustralia & Hawaii

WalnutNortheast US & N Europe

Canola

TemperateTemperateMediterranean

&Olive oil

Tropical

versusPalm & coconut oil

Role of cholesterol in animal membranesActs as a “temperature buffer”

CholesterolFig. 7.5 (c)

Fig. 5.15

• Prevents hydrophobic chains from packing too closely together: increases fluidity at low temperatures• Limits lateral phospholipid movement & stabilizesmembranes at high temperatures

Passage of Molecules across the Plasma Membrane

Hydrophobic, non-polar molecules cross membranes with ease.

http://www.colorado.edu/ebio/genbio/07_11_MembraneSelectivity_A.html

4. Be able to predict the passage of hydrophilic (polar) and hydrophobic (nonpolar) molecules through biological membranes

Hydrophilic molecules cannot slip through hydrophobic core of membrane: Require help of proteins that span the entire membrane.

Passage of Molecules across MembranesHydrophilic, polar molecules cannot slip through membrane; their transport requires help of proteins that span entire membrane.

Fig. 7.15 (a)Cytoplasm

Extracellularfluid

Solute

Predict which portions of a membrane-spanning protein (allowing passage of polar or charged ions/molecules) are hydrophilic:

Fig. 7.15 (a)Hydrophilic regions (R groups!) of protein

Hydrophobic regions(R groups!) of protein

Predict which portions are hydrophobic:

5. Structure and function of membrane channels: Be able to predict where amino acids with hydrophilic versus

hydrophobic rest groups are found in transport proteins

Nonpolar R groups: hydrophobic

Glycine Alanine Valine Leucine Isoleucine

Methionine Phenylalanine Tryptophan Proline

Fig. 5.17(a)Amino acid R (rest) groups

Arginine HistidineAspartic acid Glutamic acid Lysine

ElectricallyCharged

R groups:hydrophilic

Fig. 5.17(b&c)

Polar R groups: hydrophilic

Asparagine GlutamineSerine Threonine Cysteine Tyrosine

http://www-als.lbl.gov/als/science/sci_archive/54aquaporin.html

Aquaporins: Membrane-spanning

protein channels allowing (polar) water to move

across (hydrophobic) lipid

membranes

5. Structure and function of membrane channels (example aquaporins)

Two aspects of movement across membranes:

• Predict when a protein is needed for movement:

For small non-polar, hydrophobic substances?

For polar, hydrophilic substances?

• Predict when ATP energy is needed for movement:

When substances move from high to low concentration, i.e. along their concentration gradient?

When substances are moved from low to high concentration, i.e. “uphill” against the concentration gradient?

No

Yes

No

Yes

6. Be able to predict when when ATP energy is needed to fuel active transport

Overview of the two possibilitiesPassive transport

Diffusion Facilitated diffusion

Active transport

ATP

Fig. 7.17

“Downhill”“Uphill”

ex. fructose, H2O

Predict how glucose moves from the gut into intestinal cells when the glucose concentration in the gut is higher than in the intestinal cells after a meal:

A) by passive transport

B) by active transport

Passive transport

Facilitated diffusion

Think-Pair-Share

Molecules of dye

Fig. 7-11a

Membrane (cross section)

WATER

Net diffusion Net diffusion

(a) Diffusion of one solute

Equilibrium

Water crosses membranes by

OSMOSISdown its

concentration gradient

7. Be able to predict the direction of water movement via osmosis

http://isite.lps.org/sputnam/Biology/U3Cell/Unit3Notes_cell.htm

Salt (Na+) retention & high blood pressure

Lowerconcentrationof solute (sugar)

Fig. 7-12

H2O

Higher concentrationof sugar

Selectivelypermeablemembrane

Same concentrationof sugar

Osmosis

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

Plasmolyzed

Hypertonic solution

Osmosis = passive (net!) movement of water across membranes along/down the concentration gradient

Net water movement follows only the water gradient (regardless of what kinds of dissolved

compounds are involved)

Intravenous saline solution (1) with similar concentration of all dissolved compounds, like salts & sugars, combined as the blood plasma

Net water movement into or out of red blood cells?

(2) Intravenous “solution” of pure water

Net water movement into or out of red blood cells?

(3) Intravenous solution more concentrated in salt & sugars

Net water movement into or out of red blood cells?

• The Crash Course for Membranes is particularly good!!

http://www.youtube.com/watch?v=dPKvHrD1eS4&list=PL3EED4C1D684D3ADF

3:07-3:43

5 minute break

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Overview of the two possibilitiesPassive transport

Diffusion Facilitated diffusion

Active transport

ATP

Fig. 7.17

“Downhill”“Uphill”

K+/Na+ pump

K+

Na+

Na+/K+ Pump

• Cells want to pump Na+ out

• Cells want to pump K+ in

K+

Na+

ATP

Active transport and the sodium-potassium pump

Both Na+ and K+ are moved AGAINST their concentration gradient

http://www.colorado.edu/ebio/genbio/07_16ActiveTransport_A.html

See Fig. 7.16 for a six panel, blow-by-blowdescription of the sodium-potassium pump.

8. Be able to apply the principal features and functions of an ATP-fueled ion pump to the Na+/K+ pump

Fig.8.7

http://onlinephys.com/circuit1.html

ATP fuels the Na+/K+ pumpNa+ accumulates “on top of the hill” (against its concentration gradient)

Na+ flows downhill again

Releasing useful energy

Cotransport: Using potential energy

ATP

Na+

Cotransport: Using potential energy

This potential energy can be used… To transport other molecules

AGAINST their concentration gradient

The Na+ gradient built up by the Na+/K+ pump also fuels the secondary active transport

of glucose (& other substances) AGAINST their concentration gradient

Via Na+/glucose cotransport, whereNa+ flows back downhill & drags

glucose uphill AGAINST its concentration gradient

https://www.youtube.com/watch?v=LyvmM1lKtWs

https://www.youtube.com/watch?v=svAAiKsJa-Y

Predict how glucose moves into intestinal cells when glucose concentration is lower in the gut than in the intestinal cells:

A) through a glucose channel

B) directly through the lipid bilayer

C) via Na+-glucose cotransport fueled by the

Na+/K+ pump

D) directly through the ATP-fueled Na+/K+ pump

Think-Pair-Share

Predict how glucose moves into intestinal cells when glucose concentration is higher in the gut than in the intestinal cells after a meal: A) through a glucose channelB) directly through the lipid bilayerC) via Na+-glucose cotransport fueled by the Na+/K+ pumpD) through the ATP-fueled Na+/K+ pump

Passive transport

Facilitated diffusion

Think-Pair-Share

Membrane Bioflix Exo- and Endocytosis

Fig. 7.9Overview of functions of membrane proteins

(a) Transport

ATP

(c) Signal transduction

Signal transduction

Signaling molecule

Receptor

Let’s look at the two major classes of hormones: Protein hormones and steroid hormones

Predict which hormones can pass directly through the lipid bilayer of membranes:

A) Protein hormonesB) Steroid hormones

Think-Pair-Share

10. Be able to predict the principal differences in signal transduction of a protein hormone versus a steroid hormone

(a) Water-soluble protein hormones relay message via signal transduction pathway to a gene regulatory protein.

Eighth ed. = Fig. 45.5

(a) Water-soluble protein hormones relay message via signal transduction pathway to a gene regulatory protein.

Eighth ed. = Fig. 45.5

(b) Lipid-soluble (e.g. steroid) hormones move into nucleus & bind directly to gene regulatory protein.

See also Fig. 11.8

Open Forum

Questions leading up to Monday’s exam?

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