Today is Tuesday, October 21st, 2014

Today is Tuesday, October 21st, 2014
In This Lesson: Cell Membranes and Transport (Lesson 4 of 5) Today is Tuesday, October 21st, 2014 Stuff You Need: Guided Reading On Your Desk Pre-Class: Have a seat. When your partner arrives, say hi, ask how they’re doing, and then tell them all about the cell membrane. Anything you recall. This works even better if you don’t tell them why you’re doing it.

Today’s Agenda The Cell Membrane Where is this in my book? Structure
Function You know…osmosis? Diffusion? It’s gonna get “insane in the membrane…” Where is this in my book? Chapter 7

By the end of this lesson…
You should be able to describe in detail the structure of the cell membrane and link it to its functions. You should be able to predict the outcome of an osmotic process. You should be able to use the water potential equation.

So now then… Okay, you giant piles of cells you…
Where do we go from here? Well, let’s think about our last two units: Evolution turned into speciation when we considered the “interactive” perspective. Ecology turned into community ecology when we considered the “interactive” perspective. Cells are going to be turning into cell membranes and transport – it’s what allows multicellular organisms to be so darn interesting. And yes, it’s how cells interact with their environments.

Cell Membrane Structure Overview
The cell membrane is around 8 nm thick. For perspective, the thickness of human hair is around 99,000 nm. It’s composed of: Lipids Mainly phospholipids and some cholesterol. Carbohydrates Signal molecules attached to… Proteins Embedded in the membrane. How was it discovered? TED: Ethan Perlstein – Insights into Cell Membranes Via Dish Detergent

The Phospholipid Bilayer
The cell membrane is primarily composed of phospholipids. The cell is sitting in a water-based environment, therefore: Each phospholipid has a polar (hydrophilic) head… It’s a phosphate group. …and a non-polar (hydrophobic) pair of tails. They’re fatty acids. Because it has hydrophilic and hydrophobic parts, it’s amphipathic. Note: Amphoteric = acid/base. Different. Polar Non- polar

These phospholipids are then arranged in a bilayer. Key: Don’t confuse a bilayer for a “double membrane.” A bilayer is one membrane with two layers. Polar Non- polar

Aside: Soap Ever wonder how soap works?
Soap, like the cell membrane, is amphipathic. Unlike the membrane, soap molecules form a micelle (basically a phospholipid monolayer). Because it’s got a lot of non-polar regions, soap can dissolve cell membranes of other cells. Like pathogens’.

The bilayer acts as a semi-permeable barrier. Polar molecules can’t get in or out. H2O Salt Sugar Polar Heads Non-Polar Tails Polar Heads Lipids Waste

Quick Note: Permeability
Some things are “impermeable:” Raincoats, balloons, brick walls. Some things are “permeable:” Air, water. Some things are “semi-permeable:” Nets, gates, cell membranes. Semi-permeability is sometimes called selective permeability. Can you guys think of more examples for these?

Back to the Phospholipid Bilayer
Importantly, the composition of the bilayer is not constant. A certain percentage is composed of phospholipids with unsaturated fatty acid tails; the rest with saturated tails. Unsaturated hydrocarbons lead to increased fluidity. The lower the temperature, the more unsaturated the membrane needs to be to prevent freezing. Cholesterol is also in the membrane and acts to increase viscosity except at low temperatures.

And why the fluidity? To allow for movement of embedded membrane proteins. This view of the membrane is the Fluid Mosaic Model:

Another View

Membrane Proteins Membrane proteins provide the bulk of the cell-specific (or organelle-specific) functions. There are two main types: Peripheral Proteins They’re stuck to the outside of the cell. Example: Antigens (cell markers) Integral Proteins They’re stuck within and usually span the membrane. Example: Transmembrane Proteins or Transport Proteins

Non-polar areas of protein
So why proteins? Polar areas of protein What do you see in the picture to the right? What are the blue things with two tails? Phospholipids What’s the yellow thing wedged in there? Cholesterol What are the red squiggly lines? -Helices Non-polar areas of protein

Non-polar areas of protein
So why proteins? Polar areas of protein Remember how amino acids can be polar or non-polar? That makes proteins (also amphipathic) a great candidate for transmembrane proteins. The hydrophobic regions act as anchors to the membrane. Non-polar areas of protein

Fluid Mosaic Model Those anchors are needed because the phospholipids move frequently. It’s the Fluid Mosaic Model, remember? Studies of hybrid cell membranes made of a combination of human and mouse cells confirmed this:

What do they do? Example: Channel Protein Signal Transduction Protein
Transport Enzymes Cell Surface Receptor Example: Antigen Cell-Cell Recognition Cell Cohesion Attachment to cytoskeleton

Cell Surface Proteins Cell surface proteins play a key role in recognition between cells. This aids in development of organs and tissues. Antigens are proteins on the cell surface that cause a response from the immune system. They’re how the body “rejects” cells that are foreign.

Cell Surface Proteins Take a look at the image to the right. See those two orangey things? They’re carbohydrate chains. One’s coming from a lipid, making it a glycolipid. The other is coming from a protein, making it a glycoprotein. These carbohydrate chains make the cell identifiable to other cells.

Cell Membrane Function Overview
Cells must take in and release substances: Food in, products and waste out. They can do it with one of two general modes: Passive Transport (does not require energy) Diffusion Facilitated Diffusion Osmosis Active Transport (requires energy) Endocytosis Exocytosis Molecular Transport To fully understand these, we need to understand concentration gradient.

Concentration Gradient
Concentration refers to the amount of a substance in a certain area. Particles diffuse down their concentration gradient. What does that mean? In passive transport, particles always go from an area of high concentration to an area of low concentration. Fun Fact: Passive transport occurs in part to satisfy the second law of thermodynamics, AKA entropy.

Warning: Steep Grade High Low

High Concentration In Passive Transport, particles move from areas of high concentration to areas of low concentration. Substance Concentration Gradient Low Concentration

Diffusion Demo Diffusion in Air

What can diffuse? Can diffuse: Can’t diffuse: Lipids CO2 O2
H2O and other polar molecules Ions and other charged particles Large molecules (like starches and proteins)

Facilitated Diffusion
HIGH LOW Simply put, it’s diffusion with help. Those particles that can’t diffuse can get through channel proteins. No energy needed. This leads to semi-permeability for molecules that can’t otherwise diffuse. There are specific channels for specific molecules, too. inside cell outside cell sugar aa H2O salt NH3

Summary of Passive Transport

Osmosis Osmosis is basically the same thing as diffusion, only with water molecules and some form of a barrier. Osmosis is another form of passive transport. Just like in diffusion, in osmosis, water moves from areas of high water concentration to low water concentration. Or, water moves from areas of low solute concentration to areas of high solute concentration.

Osmosis Which drink has more liquid in it? Drink A Drink B ICE ICE ICE

Which side has more water on it?
Osmosis in a U-Tube Side A Side B Here’s a way to look at it all put together. Notice how the water level will even change - that’s how strong this force is. Which side has more water on it?

Tonicity Hypertonic solution Hypotonic solution Isotonic solution
Relatively more solute than surroundings. Relatively less free water than surroundings. Free water is water not busy hydrating a dissolved solute. Hypotonic solution Relatively less solute than surroundings. Relatively more free water than surroundings. Isotonic solution The same amount of solute as the surroundings. No net water change. A hypertonic solution is one that has relatively more solute. Think of hyper = “more!” Water will flow TOWARD a hypertonic solution. A hypotonic solution is the opposite. An isotonic solution -guesses? It’s one with no relative difference in solution levels. It is already at equilibrium.

Substance Isotonic Solutions
Water does not experience a net movement in isotonic solutions. There is no concentration gradient. Substance No concentration gradient No net movement of water

And now, I present to you…
…the key to EVERYTHING!!!!!!* *osmosis-related. Draw this in your notebook. Make it BIG. Hypotonic Hypertonic H2O Flow

Osmosis in Plant Cells As we have learned, plant cells are good at holding water. If they’re placed in a hypertonic solution, however, they lose water and wilt. Their cells undergo plasmolysis. Place them in a hypotonic solution and they will swell slightly, like a garden hose with water. Their cells become turgid. In animal cells, without a cell wall, the cell may burst in a process called cytolysis.

Osmosis – The Big Idea

Osmosis – The Big Idea Blood hypertonic, surroundings hypotonic
Imagine an animal cell. More “stuff” is dissolved in the animal’s cytoplasm than is dissolved outside in the ECM. What happens to the cell? What happens If it has a LOT more dissolved in it than the outside? What if the outside has more? [slide picture] What might be a solution to having a cell burst from being hypotonic? Which kinds of organisms have solved this? [Fungi/Plant Cells with Cell Walls] So how do animal cells without cell walls deal? That’s for Thursday. For now, here’s a little bit of a preview as we learn about the next kind of passive transport. Blood hypertonic, surroundings hypotonic Blood hypotonic, surroundings hypertonic Isotonic solutions

Managing Water Balance
Animals: Kidneys. Methods to either remove salt or pump in water. Unicellular Organisms: Contractile Vacuoles Pump water out at a cost of ATP (energy). Maintaining water balance is just another aspect of homeostasis.

Osmosis in Kidneys

Osmosis in Kidneys The proximal Loop of Henle is the part of the nephron (kidney component) responsible for re-absorbing water from urine. With this in mind, would you guess that desert animals have larger or smaller Loops of Henle than other animals?

Osmosis in Kidneys

Osmosis in Merriam’s Kangaroo Rats

Osmosis Supplements Egg Osmosis Gummi Bear Osmosis
Woman Dies After Water Drinking Contest CrashCourse – In Da Club – Membranes and Transport

Stop everything, you liar!
“You said at the beginning of this PowerPoint that polar substances like H2O can’t diffuse into the cell through the membrane, yet now you’re talking about osmosis being like water diffusion. How could that be?” said the student. “For a while scientists noticed the same thing. Water clearly efficiently enters a cell, but how?” replied the teacher, quietly appreciative of his student’s skepticism.

Aquaporins Aquaporins are channel proteins that move water rapidly into the cell through facilitated diffusion. They were discovered by these two in 1991. They shared the 2003 Nobel Prize in Chemistry. Roderick MacKinnon Rockefeller Peter Agre Johns Hopkins

Equilibrium For things like diffusion and osmosis, eventually the solutes reach a point where there is no net change in molecule movement. This is equilibrium. We call it “dynamic equilibrium” because the molecules are still moving, but there is no net change in concentration or movement.

Equilibrium When dynamic equilibrium is reached, diffusion and osmosis stop. Molecular motion continues, though. 1.0% Sugar Net Water Flow Inward 0.75% Sugar No Net Water Flow WATER WATER 0.50% Sugar WATER WATER

Osmosis Practice Problems
We’re going to talk about water potential soon, but for now, let’s get the concept of osmotic movement under our belts. Following this slide are four osmosis practice problems, all multiple choice. Get them all correct on the whiteboards and, uh… …um… 

Osmosis Practice Problem SAMPLE
Suppose a human blood cell (saline concentration 0.9%) is sitting in a beaker of 2% NaCl. Will it shrink, expand, or remain unchanged? Make a sketch! The blood cell will shrink. Hyper Hypo 0.9% 2%

Osmosis Practice Problem #1
If you soak your hands in dishwater, you may notice that your skin absorbs water and swells into wrinkles. This is because your skin cells are _______________ to the _______________ dishwater. hypotonic…hypertonic hypertonic…hypotonic hypotonic…hypotonic isotonic…hypotonic hypertonic…isotonic

You decide to buy a new fish for your freshwater aquarium. When you introduce the fish into its new tank, the fish swells up and dies. You later learn that it was a fish from the ocean.

Based on what you know of tonicity, the most likely explanation is that the unfortunate fish went from a(n) _______________ solution into a(n) _______________ solution. isotonic…hypotonic hypertonic…isotonic hypotonic…hypertonic hypotonic…isotonic isotonic…hypertonic

In osmosis, water always moves toward the ____ solution: that is, toward the solution with the ____ solute concentration. isotonic…greater hypertonic…greater hypertonic…lesser hypotonic…greater hypotonic…lesser

The concentration of solutes in a red blood cell is about 2%. Sucrose cannot pass through the membrane, but water and urea can. Osmosis would cause red blood cells to shrink the most when immersed in which of the following solutions? a hypertonic sucrose solution a hypotonic sucrose solution a hypertonic urea solution a hypotonic urea solution pure water

Water Potential Last thing before the lab: Water potential.
Yes, it has to do with potential energy. Water flows toward the hypertonic solution. Or… Water flows toward the lower water potential value (). Let’s discuss this further. Heads-up: In your book, water potential is discussed in Chapter 36, p782–785.

Water Potential Conceptual View
The nearby water molecules create a hydration shell around the Na+, decreasing the amount of free water in that location. Suppose you have a semi-permeable membrane separating some water. The membrane is impermeable to everything except water. So the right side, in this case, has a lower water potential () than the left side, making it hypertonic, and making water flow toward it. You dissolve some NaCl, which dissociates into Na+ and Cl- (just the Na+ shown). H O H O H O H O Na+ H O H O H O H O left >  right

Water Potential Units Water potential is measured in units called megapascals (MPa) or (more commonly) bars. Remember kPa from chemistry? Kilopascals? Out of curiosity: 1 MPa ≈ 1000 kPa 1 MPa = 10 bars 1 bar ≈ standard atmospheric pressure For perspective, the pressure of a plant cell is 0.5 MPa, or about twice as much as a car tire’s air pressure.

Water Potential Equation
Water potential has two components: Solute Concentration (S – Solute Potential) Physical Pressure (P – Pressure Potential) Together, they make up the water potential equation:  = S + P

Water Potential Forces
Think of water as being able to be pushed away from an area or to be pulled toward an area. For example, dissolved solutes in an area pull the water toward that area. Since water always moves toward the lowest water potential area, solutes make  go down. S, which is solute potential, is thus familiar: The more solute in an area, the greater the pull to bring water near, the lower the value of S. You can almost think of S as a “pull magnitude.”

Solute Potential: S Pure water has a S of 0.
Makes sense: no dissolves solutes in the water, therefore no pull. Key: Solute potential (S) is always either 0 or negative. More negative = more pull. It can’t be positive. There’s no way to dissolve particles and have water go away from them.

Solute Potential: S Solute potential is formally calculated this way:
S = -iCRT C is concentration in molarity (M). R is the ideal gas constant: liter  bars / mol  K (it’s given). T is the temperature in Kelvin. K = °C + 273 i is the ionization constant. This one needs some more explanation, starting with a diagram.

Dissociation Bound ions in… …component ions out. Ca Cl Cl- Ca2+ Cl-

S = -iCRT Ionic compounds break down into their components in solution. Let’s say you put a mole of NaCl into water. It’ll dissociate into one mole of Na+ and one mole of Cl-, or two moles total. The ionization constant (i) is 2. Let’s say you put a mole of C12H22O11 (sucrose) into water. It won’t dissociate. It stays one mole of C12H22O11. The ionization constant (i) is 1. Key: i = how many particles into which a solute breaks.

Water Potential Forces
P is the pressure potential and it’s a little different – there’s no direct formula for it. In the same way S = 0 for pure water, P = 0 for normal atmospheric pressure in an open container. However, P can be positive or negative. Positive would be like squeezing a water balloon – you’re pushing water away. Negative would be like sucking on a straw – you’re pulling water closer.

Pressure Potential: P
When would P be negative? Perhaps in plants, when transpiration at the leaf draws water up in a column through the plant stem. When would P be positive? Perhaps in plants, when the cell becomes so turgid and swollen that the cell wall begins to push back down.

Water Potential Summary
Water moves from high water potential areas to low water potential areas. So: Relatively high water potential = hypotonic. Relatively low water potential = hypertonic. S is the solute potential and gets lower as solute concentration rises. It’s never positive. P is the pressure potential and is (+) for pushing events and (-) for pulling events. (0) for neutral, isotonic events. Key: Most problems will require you to find  outside and  inside, then compare.

Water Potential Practice Problem
A cell in an open beaker is in equilibrium with its environment. Its P = 2.3 bars, the temperature is 24 °C, and the beaker contains a 0.25 M NaCl solution. What is the cell’s concentration of NaCl? Equilibrium means cell = beaker and 24 °C = 297 K. beaker = P + S beaker = 0 + (-iCRT) beaker = 0 + -(2) (0.25 M) (0.0831) (297 K) beaker = bars

Water Potential Practice Problem
A cell in an open beaker is in equilibrium with its environment. Its P = 2.3 bars, the temperature is 24 °C, and the beaker contains a 0.25 M NaCl solution. What is the cell’s concentration of NaCl? beaker = bars = cell cell = P + S bars = 0 + (-iCRT) bars = 0 + -(2) (C) (0.0831) (297 K) bars = (C) C = 0.25 M

For more practice… Water Potential Practice Questions worksheet
Need help with water potential? Visit my website and head to the Fact Sheets section for a video review of the concepts of water potential courtesy a different AP teacher. Note: Chapter numbers referenced therein are now outdated.

Ade: Poseidon and Water Potential
Who’s the Greek/Roman mythological god of the sea/water? Poseidon/Neptune And what does Poseidon/Neptune carry? A trident And what does a trident look like?  Oooooh! Podon!

Okay then…I think you’re ready.
Lab 4!

Active Transport What happens when a cell gets greedy?
What I mean is, what happens when a cell has within it a higher concentration of a certain molecule than is present outside the cell, yet still wants more? This is where active transport comes in – we’re going to need to expend a little energy to get what we want.

High Concentration Active Transport Substance Concentration Gradient ENERGY NEEDED! Low Concentration

Quick Note: Transport Proteins
I’ve been mentioning transport proteins quite loosely this whole lesson. Here’s something concrete about them: Channel proteins are basically just tunnels for polar stuff to diffuse in/out. They’re simple. Carrier proteins are a bit slower, but they allow for active transport and the movement of nonpolar stuff. They also typically undergo shape changes to do their work. They’re usually glycoproteins.

Channel vs. Carrier Channel Protein Carrier Protein

Transport Proteins

Back to Active Transport
Active transport costs ATP to move molecules against their concentration gradient. Proteins in the membrane that do this undergo a conformational change in the process:

Active Transport Classic Example
The Proton Pump (chloroplasts)

Cotransport Proton Pump (moves in two molecules)

The Sodium-Potassium Pump (neurons)

Sodium-Potassium Pump
We’re going to look at this one in greater detail as it’s very important to know. Fun fact: The mere fact that you’re reading these words means many of your cells are using this pump right now. It’s such a good example of multiple forms of cell transport that you should expect related questions from both the AP Exam and me. Anon!

Sodium-Potassium Pump
Three sodium ions inside the cell bind to a carrier protein. ATP causes a conformational change in the protein, releasing the Na+ ions to the ECM. Two potassium ions then bind to the carrier, causing another conformational change that releases the K+ ions to the cytoplasm. Repeat. Key: Both ions are moving against their gradients. Key: Not only are these concentration gradients, we can also call them electrochemical gradients because the particles are ions and thus electrically charged.

Na-K Pump Na-K Pump animation

Active Transport: Three Forms #toomanynotes
Exocytosis – Removing stuff from the cell. Endocytosis – Bringing stuff into the cell. Phagocytosis – “Cell Eating” – when a cell engulfs a large particle/other cell. The vesicle fuses with the lysosome. Amoeba Eats Two Paramecia video Pinocytosis – “Cell Drinking” – a continuous intake of small dissolved particles in the nearby solution. Receptor-Mediated Endocytosis – Pinocytosis except the cell is bringing in particles that have bonded to receptors on the outside of the membrane. Molecular Transport – a general term for using protein pump-like structures embedded in the membrane.

Exocytosis Easy one:

Endocytosis Will fuse with lysosome for digestion. Phagocytosis
Non-specific process. Pinocytosis Triggered by receptors outside the cell. Helps for low-concentration “targets.” Receptor-Mediated Endocytosis

Cell Contact Points One last thing – it’s a straggler cell membrane structure detail. When two cells neighbor one another, you are bound to get one of several kinds of connections: Tight Junctions Desmosomes Gap Junctions Plasmodesmata

Tight Junctions Tight junctions are the tightest connections between cells in nature. They are only found in epithelial cells (cells that form the skin and other coverings). Makes sense – they need to form a barrier, so being really closely associated is logical. They form a tight seal and prevent passage of fluids.

Aside: Tight Junction Regulation
Tight junctions can be “adjusted” to allow certain molecules through. There are a rather large number of proteins that regulate tight junctions:

Desmosomes Desmosomes form an anchoring connection between cells.
While tight junctions prevent passage of molecules, desmosomes help cells gain structure and strength. Intermediate filaments of one cell link with intermediate filaments of another cell. Adherens junctions are similar (but different) junctions to desmosomes.

Gap Junctions Gap junctions have pores allowing for signaling and coordination between cells. Where tight junctions served to prevent movement of molecules, gap junctions promote it.

Cell Junctions

Plasmodesmata Plasmodesmata are small openings between plant cells.
Allow for communication through cell walls.

Cell Junctions Summary
Tight Junctions Barriers that prevent flow of materials. Desmosomes Anchors to make tissue stronger. Gap Junctions Spaces that promote communication between cells. Plasmodesmata Pores for communication between plant cells.

Cell Transport Summary

Closure So what’s the point of the cell membrane?
By now you should have lots of answers. Perhaps one we haven’t discussed enough is homeostasis. All these membrane functions, all these pumps and structures…they all serve to help regulate the cell’s “chemical balance.” As you know, homeostasis is “maintaining internal balance,” and you should now be able to see how well the cell membrane can do that.

Closure Turn to your partner (say hello), and think of an real-world example for each of the following: Diffusion Facilitated Diffusion Osmosis Active Transport (any form)

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