LIFE ON THE EDGE

INTRO TO THE PLASMA MEMBRANE

• 1915: membranes isolated from RBC’s were analyzed & found to be made of lipids & proteins

• 1925: scientists (Gorter & Grendel) suggested membrane was made of phospholipid bilayer. (hydrophobic parts are sheltered by the hydrophillic parts).

MEMBRANE MODELS (from past to present)

Where are the proteins?

• 1935: scientists Davidson & Danielli suggested the membrane was like a sandwich.– Phospholipid bilayer b/w layers of proteins.

• 1950’s: came the invention of the electron microscope.– Pictures seemed to support the sandwich model

• 1960’s: scientists had problems with model– Cells with different functions differed in structure and

chemical make-up.– Proteins in membranes were not very soluble (they were

amphipathic). If these proteins were the “bread” part of the sandwich the hydrophobic parts would be in an aqueous environment.

•1972: Singer and Nicolson proposed the membrane proteins were dispersed & individually inserted into the phospholipid bilayer.

•Only the hydrophillic portions are protruding

Fluid Mosaic Model

Freeze fracture (method of preparing cells for the electron microscope)- confirmed Singer and Nicolson’s model.

Fluid mosaic model

• Not static or rigid• Proteins & lipids drift back & forth

(laterally)• Movement is rapid

– Proteins move more slowly than lipids• Some move as directed (motor proteins

attached)• Some don’t move (anchored)

• The temperature at which the membrane solidifies is dependent on the types of lipids in the membrane

• Unsaturated lipids create a kink, preventing the fatty acids from packing together as tightly, thus decreasing the melting temperature (increasing the fluidity) of the membrane.

The fluidity of a lipid bilayer depends on both its composition and its temperature

Unsaturated vs Saturated Fatty Acids in the Membrane

• Plants, animals, & bacteria adapt to decreasing temperatures by increasing the proportion of unsaturated fatty acids in the membrane.

• They decrease the proportion of unsaturated fatty acids in the membrane & increase the saturated fatty acid content in the membrane when temps are high.

The ability of some organisms to regulate the fluidity of their cell membranes by altering lipid composition is called homeoviscous adaptation.

NOW LET’S TALK CHOLESTEROL

-Is amphipathic.

-The hydrophobic steroid ring which is near the hydrocarbon tails of the phospholipids tend to immobilize the tails.

-Restricts random movement so membrane doesn’t turn to mush.

-Keeps lipids more separated so they don’t crystalize.

(temperature buffer)

How would you expect the saturation levels of membrane fatty acids to differ in plants adapted

to cold environments & plants adapted to hot environments?

• Plants in cold environments would probably have more unsaturated fatty acids in their membrane, since those remain fluid at lower temps.

• Plants in hot environments would probably have more saturated fatty acids to so they would be closer to each other, causing the membrane to be less fluid. Being less fluid would help them stay intact at higher temperatures.

It’s the “mosiac” turn • Proteins - Over 50 different types• Different cells = different proteins• Proteins determine the function of the

membrane therefore the function of the cell.

2 Major Types of Proteins

• Integral:

• Peripheral

-many are transmembranes – amphimathic

-exposed to both aqueous solutions (inside & outside of the cell)

-not embedded in the lipid bilayer

-they are appendages to integral proteins & are attached to the cytoskeleton

Integral

Peripheral

Moves molecules in & out of the cell using passive or active passageways.

Catalyze reactions inside the cell

Convey signals from the outside of the cell to the inside; functions like a light switch.

Bind cells together to make tissues

Allow immune system to recognize what is self & not self

Allows cells to bind to each other or a substrate

ENZYAMTATIC

INTRACELLULAR JOINING

ATTACHMENT TO CYTOSKELETON & THE ECM

TRANSPORT

SIGNAL TRANSDUCTION

RECOGNITION

CARBOHYDRATES OF THE MEMBRANE

• Cell to Cell Recognition:– This comes in handy when in the embryonic stage– Basis for rejection of organ transplants by immune system.

• 2 types– Glycolipids- carbohydrate bonded to a lipid– Glycoprotein- carbohydrate bonded to a protein

• Function as markers- distinguish one cell from the other.– Ex: human blood types: each have variation in their

carbohydrate markers.

How does the endoplasmic

reticulum make the plasma membrane?

WHAT IS THE PLAMA MEMBRANE MADE OF?

• LIPIDS– PHOSPHOLIPIDS (amphipathic)– CHOLESTEROL

• PROTEINS– INTEGRAL PROTEINS (amphipathic)– PERIPHERAL PROTEINS

• CARBOHYDRATES– GLYCOLIPIDS (carbohydrate and lipid)– GLYCOPROTEINS (carbohydrate and protein)

CONSTRUCTING A MODEL OF THE CELL MEMBRANE

• Construct a model of the cell membrane using diagrams you have been provided in your text, during lecture, and in this lab.

• You may use construction material (assorted food & non-food materials: assorted pasta, assorted cereals, glue, pipe cleaners, yarn, and more).

• Make sure your model includes the following components:a. Phospholipid bilayer (heads & tails!)b. Integral proteinsc. Peripheral proteinsd. Glycolipidse. Carbohydratesf. Glycoproteinsg. Cholesterol

• Make a key to show what each item represents• Answer the Summary Questions on the next slide.

How do molecules pass in and out of the plasma membrane?

• If you (the molecule) are small and non-polar OR small & uncharged you can pass through the lipid bilayer.

• The hydrophobic tails of the lipid bilayer impedes ions and polar molecules.– This includes glucose and water!!

Transport Proteins• Hydrophylic substances (some ions and polar

molecules) pass through the membrane by way of the channel proteins – aquaporins facilitate the passage of water

Transport Proteins

• Carrier proteins: these proteins change shape as they pass substances through. They are highly selective.

What mechanisms drive molecules across the membrane?

• Passive Transport– Diffusion– Osmosis– Facilitated diffusion

• Active Transport– Sodium Potassium Pump/Electrogenic pump– Cotransport– Exocytosis– Endocytosis

Diffusion

• A substance will travel from where it is more concentrated to where it is less concentrated.– Substances travel down its concentration gradient.

• No work needs to be done– No need for energy

• It is a spontaneous process• Most travel of substances across is by diffusion

– EX: oxygen transfer; CO2 transfer

Osmosis

• diffusion of water molecules• across a selectively permeable membrane

• Movement is from areas of high potential (high free water concentration) and low solute concentration to areas of low potential (low free water concentration) & high solute concentration.

WATER MOLECULES CLUSTERED AROUND HYDROPHILIC MOLECULES ARE UNAVAILABLE TO TRAVEL THROUGH THE MEMBRANE.

Solutions of Solutions of Osmosis Osmosis

•HYPERTONIC:

•Has a higher solute concentration and a lower water potential compared to the solution on the other side of the membrane.

•HYPOTONIC:

•Has a lower solute concentration and a higher water potential than the solution on the other side of the membrane

•ISOTONIC:

•Have equal water potentials

Cells without cell walls

• In a hypotonic environment, water enters cell and it swells and may burst

• In a hypertonic environment, water leaves the cell and it shrinks

OSMOREGULATION:CONTROL OF WATER BALANCE

Turgor Pressure• most plant cells live in hypotonic environment• water moves into cells, pushing cell membrane

against cell wall• cell wall is strong enough to resist pressure• pressure from the water is called turgor pressure

Plasmolysis

• plant cells in hypertonic environment• water leaves cells• cell membrane moves away from cell wall• loss of turgor pressure (wilting in plants)

• In plant cells, the presence of a cell wall prevents the cells from bursting, but pressure does eventually build up inside the cell and affects the process of osmosis.

• When the pressure inside the cell becomes large enough, no additional water will accumulate in the cell even the though cell still has a higher solute concentration than does pure water.

• So movement of water through the plant tissue cannot be predicted simply through knowing the relative solute concentrations on either side of the plant cell wall.

• Instead, the concept of water potential is used to predict the direction in which water will diffuse through living plant tissues.

• In a general sense, the water potential is the tendency of water to diffuse from one area to another under a given set of parameters.

Osmosis and Water PotentialWhen water diffuses through a selectively permeable membrane from an area of high water potential to an area of low water potential this is called osmosis.

Water potential measures the tendency of water to diffuse from one compartment to another compartment in plant cells based on solute concentration and pressure.

ψ = ψP + ψS

Solute potential :AKA osmotic potential (ψs) is dependent on the solute concentration

Pressure potential (ψP) results from the exertion of pressure-either positive or negative

The water potential of pure water in an open beaker is zero because both solute and pressure potential is zero.

Increase in positive pressure raises the pressure potential & the water potential. Add solute; this lowers the solute potential thus decreasing the water potential

= -iCRTTHE SOLUTE POTENTIAL (ΨS)

i = ionization constant (sucrose = 1; salt = 2)

C = molar concentration

R = pressure constant = 0.0831 liter bars/mole-K

T is temperature in °K = 273 + 0C

A 0.15M solution of sucrose at atmospheric pressure (ψP=0) and 250C has an osmotic potential of -3.7bars and a water potential of -3.7bars.

A bar is measure of pressure & is the same as 1 atmosphere at sea level.

What if the solution was NaCl? What would the water potential be? -7.4bars

(osmotic potential)

Calculating Water Potential

Example: You have allowed slices of cucumber to sit overnight in several different sucrose solutions. You weighed the slices beforehand and find that the slice that you put in the 0.5M sucrose solution has not had any change in its mass.

Using this data: 1) determine the molar concentration of solutes within the cucumber cells, 2) calculate the solute potential (ψS) of the 0.5M sucrose solution, and 3) calculate the water potential within the cucumber slice. Assume temperature is 22 °C. 1bar = 0.1megapascal = 1kg/cm2

1) The concentration of solutes in the cucumber will be the same as the concentration of sucrose at which the cucumber slice has not gained mass, 0.5M.

2) ΨS = -iCRT

T in °K = 273 + 22 = 295

ΨS = -(1.0)(0.5mol/liter)(0.0831liter bars/mole °K)(295 °K) = -12.26 bars

3) Ψ = ΨS + ΨP At equilibrium the water potential of the cucumber will be

Ψ = ΨS + 0 equal to the water potential of the solution.

Ψ = -12.26

How can you measure water potential in plant cells?

• You can do this by measuring/calculating change in mass, change in length, or change in volume over time in plant sections from potatoes.

• DESIGN LAB:– TO IDENTIFY THE CONCENTRATIONS OF THE

SUCROSE SOLUTIONS AND THEN USE THE SOLUTIONS TO DETERMINE THE WATER POTENTIAL OF THE PLANT TISSUES.

FACILITATED DIFFUSION

CHANNEL PROTEIN

CARRIER PROTEIN

MOVE CHARGED POLAR MOLECULES

ACROSS MEMBRANE

Hydrophillic passageway

EX: aquaporins

EX: Cysteine transporter

ACTIVE TRANSPORT

• Where free energy (often provided by ATP) is used by proteins embedded in the membrane to “move” molecules &/or ions across the membrane & to establish or maintain concentration gradients.

• Membrane proteins are necessary

WHICH MEMBRANE PROTEINS ARE USED?

CARRIER PROTEINS

SODIUM-POTASSIUM PUMP

•Contributes to the membrane potential

•Pumps 3 Na+ out of cell for every 2 K+.

•Creates a positive charge from cytoplasm to extracellular fluid.

•Stores energy in the form of voltage

•Major electrogenic pump of animals

•Proton pump for plants, fungi, & bacteria.

AN EXAMPLE OF ACTIVE TRANSPORT

What is a nerve impulse?• Nerve impulse is misleading. We will call it an

action potential instead• Can be measured in the same way as

electricity is measured– Voltage

• Millivolts

• The conductor of a neuron is the axon– Is covered by a myelin sheath

• Increases the rate at which an action potential passes down an axon.

Resting potential

• Area of a neuron that is ready to send an action potential but is not currently sending one.

• This area is considered polarized– Characterized by the active transport of sodium ions

(Na+ ) out of the axon cell & potassium ions (K+) into the cytoplasm.

– There are negatively charged ions permanently located in the cytoplasm

– This collection of charged ions leads to a net positive charge outside the axon membrane & negative charge inside.

Action Potential• Described as a self-propagating wave of ion movements in and

out of the neuron membrane• This is the diffusion of the Na+ & the K+ .

– Sodium channels open & then potassium ones do to.• This is the “impulse” or action potential• It is a nearly instantaneous event occurring in one area of the

axon = depolarization– This area initiates the next area on the axon to open up the

channels.• This action continues down the axon.

• Once an impulse is started at the dendrite end that action potential will self-propagate itself to the far axon end of the cell.

Return to Resting Potential• Remember that one neuron may send dozens of

action potentials in a very short period of time.• Once an area of the axon sends an action

potential it cannot send another until the Na+ & K+ have been restored to their positions at the resting potential.

• Active transport is required to move the ions = repolarization– The time it takes for a neuron to send an action

potential & then repolarize is called: the refractory period of that neuron.

Maintenance of Membrane Potential by Ion Pumps

• Electrical potential across the membrane= voltage (separation of opposite charges).– Cytoplasm has a negative charge compared to the

extracellular fluid

• Voltage across membrane = membrane potential– Membrane potential favors passive transport of

cations (+) into cell & anions (-) out of the cell

So… what causes diffusion of ions?

• Electrochemical gradient– Electrical force– Concentration gradient

• EX: Na+ concentration inside a resting nerve is much lower than the concentration outside it.– When the cell is stimulated gated channels open & Na+

“fall” down their electrochemical gradient driven by the concentration gradient of the Na+ & the attraction of the cations to the negative side of the membrane.

COTRANSPORT

EXOCYTOSIS

PANCREAS SECRETING INSULIN TO THE BLOOD STREAM

ENDOCYTOSIS

CELLULAR DRINKING

CELLULAR EATING

What is the importance of active transport in biological systems?

• It is responsible for enabling us to absorb more food from our intestines.

• If there is no active transport, then most of the food that we eat will be wasted.

• If absorption only depends on diffusion, then once the concentration of food in and out of the intestinal cells becomes the same, then absorption will stop.

• But as we have experienced, we can have 2nd helpings or even 3rds of some of our favorite food. So we have active transport to thank (blame?)for that.

CHOLESTEROL

• Cholesterol is an important component in various systems throughout the body.

• Cholesterol is necessary for the creation of certain bile salts necessary for the digestion of saturated fats and sugars.

• It is also used in the manufacturing of certain hormones, like testosterone.

• Helps your body make vitamin D. • Important in the creation of cells, because it contributes

to cell structure by strengthening cell walls.

High Density LipoproteinAverage is around 40-60 mg/dL.

• HDL travels through the bloodstream after being produced in the liver. – As it travels it picks up excess cholesterol. – It then carries this excess cholesterol back to the

liver where it is used as a building block for bile.

• By using the cholesterol to create bile there is less in your arteries that can cause blockage.

• Therefore, HDL is considered good since it helps to reduce cholesterol in the bloodstream.

Low Density LipoproteinsYour goal is to have a number that is less than 100 mg/dL,

though 100-129 mg/dL is optimal.

• In general, LDL is not bad – it is having too much LDL is when the problems begin.

• LDL carries cholesterol through the blood and delivers it to cells that need it to build things such as hormones or fat.

• The problem is that LDL is soft enough to get into the walls of your arteries.

• This can lead to a process that builds up plaque in the arteries, eventually leading to a heart attack.

ATHEROSCLEROSISNormally cholesterol (LDL’s) binds to a receptor site on the cell membrane and then is brought into the cell by

way of endocytosis.

If LDL receptor sites are defective or just aren’t even there…..

HOW ARE CARRIER PROTEINS LIKE ENZYMES?

They have specificity, they have conformational changes, they are dependent on concentrations…