CHAPTER 5Cell Structure & Function

OVERVIEW

Membrane

Models

Structure / Function

Permeability

Modification of Cell Surface

MODELS

MEMBRANE MODELS • Sandwich Model

– Concept: phospholipids form a double bilayer that is a filling between two layers of protein.– Evidence: electron microscopy images.– Model was revised and added to…

• Unit Membrane Model– Concept: all membranes are the same, just like all DNA is the same, and all amino acids are the

same. – Evidence: electron microscopy– Problem: some kinds of membranes looked different under the electron microscope– Not contested strongly until early 1970s.

• Fluid-Mosaic Model– Concept: the cell surface is a viscous fluid, with proteins embedded in it that can freely diffuse,

producing a mosaic pattern.– Evidence: mouse-human hybrid membranes– Problem: none, holds true to this day

Surface area of a sphere: 4*pi*r2

1925 There are enough phospholipids to make a bilayer.Gorder & Grendel proposed hydrophobic tails on the inside and polar heads on the outside.

1940s Proteins are part of the membrane.Danielli and Davson proposed the sandwhich model.

1950s electron microscopes were good enough to look directly at the membrane, confirming the sandwhich model.

Robertson suggests the membrane is standardized for all cells, and proposes the “unit membrane”

1972 Singer & Nicolson propose fluid-mosaic model, which fits observations of cells that the unit membrane model did not.

So who is right? Do the experiment to find out.

Hypothesis 1: Unit MembraneHypothesis 2: Fluid-Mosaic.

Experiment: cut open a plasma membrane and compare the inside and outside of a phospholipid layer.

Expected Results:

If the Unit Membrane is correctly, only the outside of the membrane will show proteins.

If the Fluid-Mosaic model is correct, the inside and outside will show proteins.

Figure 5.1d

Experiment 2

Hypothesis: The lipid membrane is fluid in nature.

Prediction: If the membrane is fluid, then embedded proteins should move freely in the fluid.

Test: merge cells of 2 species (having different proteins in their membrane), and determine whether the proteins mix with each other after fusion.

Results:

ORIGINAL ABSTRACT OF FLUID MOSAIC MODEL OF MEMBRANE STRUCTURE

• A fluid mosaic model is presented for the gross organization and structure of the proteins and lipids of biological membranes. The model is consistent with the restrictions imposed by thermodynamics. In this model, the proteins that are integral to the membrane are a heterogeneous set of globular molecules, each arranged in an amphipathic structure, that is, with the ionic and highly polar groups protruding from the membrane into the aqueous phase, and the nonpolar groups largely buried in the hydrophobic interior of the membrane. These globular molecules are partially embedded in a matrix of phospholipid. The bulk of the phospholipid is organized as a discontinuous, fluid bilayer, although a small fraction of the lipid may interact specifically with the membrane proteins. The fluid mosaic structure is therefore formally analogous to a two-dimensional oriented solution of integral proteins (or lipoproteins) in the viscous phospholipid bilayer solvent. Recent experiments with a wide variety of techniqes and several different membrane systems are described, all of which abet consistent with, and add much detail to, the fluid mosaic model. It therefore seems appropriate to suggest possible mechanisms for various membrane functions and membrane-mediated phenomena in the light of the model. As examples, experimentally testable mechanisms are suggested for cell surface changes in malignant transformation, and for cooperative effects exhibited in the interactions of membranes with some specific ligands. Note added in proof: Since this article was written, we have obtained electron microscopic evidence (69) that the concanavalin A binding sites on the membranes of SV40 virus-transformed mouse fibroblasts (3T3 cells) are more clustered than the sites on the membranes of normal cells, as predicted by the hypothesis represented in Fig. 7B. T-here has also appeared a study by Taylor et al. (70) showing the remarkable effects produced on lymphocytes by the addition of antibodies directed to their surface immunoglobulin molecules. The antibodies induce a redistribution and pinocytosis of these surface immunoglobulins, so that within about 30 minutes at 37 degrees C the surface immunoglobulins are completely swept out of the membrane. These effects do not occur, however, if the bivalent antibodies are replaced by their univalent Fab fragments or if the antibody experiments are carried out at 0 degrees C instead of 37 degrees C. These and related results strongly indicate that the bivalent antibodies produce an aggregation of the surface immunoglobulin molecules in the plane of the membrane, which can occur only if the immunoglobulin molecules are free to diffuse in the membrane. This aggregation then appears to trigger off the pinocytosis of the membrane components by some unknown mechanism. Such membrane transformations may be of crucial importance in the induction of an antibody response to an antigen, as well as iv other processes of cell differentiation.

Science. 1972 Feb 18;175(23):720-31

STRUCTURE & FUNCTION

PERMEABILITY

• Brownian Motion• Diffusion• Osmosis

Figure 5.5

MOVIES

• Diffusion– Watch some videos on diffusion to get the idea.– Simulation of a lateral diffusion in a lipid bilayer (1)– Simulation of lateral diffusion in a lipid bilayer (2)– Same thing, with “obstacles”– Trippy diffusion– Example of diffusion of a hydrogen ion through water

• Brownian Motion• Cellular Movements• Treadmilling Macrophage

OSMOSIS

(my original teaching demonstration)

When we think about diffusion, we don’t concern ourselves about the liquid (the solvent, e.g. water) in which something is dissolved.

A solute particle moving randomly in a solution of water (Brownian motion)

(Check out www.dhmo.org)

OHH

A water molecule

Brownian Motion of liquid water Molecules

But “water” is just a molecule,

bouncing around like any other.

To distinguish between diffusion of dissolved particles (the “solute”), and the surrounding liquid

(the “solvent”), we use the term “osmosis”

Why do we care about osmosis in biology?

$.02 Answer: Because it is essential for bringing water in and out of cells.

HOW OSMOSIS WORKS

Fill a divided chamber with water.

The constantly moving water molecules can pass freely through the barrier.

Add a solute to the Red side (such as salt)

The solute molecules act as a sort of molecular “sponge”, strongly binding up the water molecules, thereby removing them from the surrounding volume.

This results in an effective decrease in water concentration in the Red chamber.

The solute molecules are too big to pass through the barrier

The constantly moving water molecules, just like any other molecule, will diffuse from the area of HIGH concentration to the area of LOW concentration

(REDBLUE)

There will be some movement of water the other way, too, but the average will be a net increase in the red side.

The result is a net movement of water from the Blue side, where there are more “free” water molecules to the Red side.

Movement will continue until a Dynamic Equilibrium is achieved.

Net Movement of Water

Who Pulls, Who Pushes?

The solution with the higher concentration of solute is said to be “hypertonic” with respect to the solution of lower concentration.

The solution with lower concentration, in turn, is referred to as “hypotonic”

Hypertonic

More Concentrated

Hypotonic

Less Concentrated

Key Concept: Water always towards the hypertonic (more concentrated) solution (assuming there’s a semi-permeable membrane)

Hypertonic

More Concentrated

Hypotonic

Less Concentrated

Isotonic

Equal Concentration

When two solutions have the exact same concentration, they are said to be “isotonic”. (iso- ‘the same’)

Hypertonic, hypotonic, and isotonic, describe relationships between solutions. They are relative terms.

If you ever see the word describing a solution itself, you can usually figure out the context.

An isotonic saline solution for eyes, would be referring to its concentration relative to the liquid of your eye.

It answers the question, “which solution will pull the water away, and which will have water taken from it?”

A solution is never just “hypertonic”. It is always hypertonic with respect to another solution.

Hyper: “more” or “higher”

(hyperactive)

Hypo: “less” or “lower”

(hypothermia)

Osmotic Pressure Revealed

Lets give the solutions some breathing room.

What happens to the volume of each side?

Since we’re losing water on the blue side, and gaining it on the red, the blue side will decrease in volume – while the red increases.

On this side, water is pushing against gravity.

It’s doing work.

How much work?

That depends on the concentration of the solute – (or the differences between them, if both solutions have solutes) and the amount of pressure acting on each.

Question: Does the amount of work depend on the type of

dissolved substance?

NO. This is an example of a “colligative property” of a solution – all that matters is how much stuff is there, not what it is.

If you needed to weigh down the back of a pickup truck, all you really care about is how heavy (and handy) the stuff is – cement blocks, gravel, sand, or even filling it with water and letting it freeze - they all do the same thing.

The original science here was based on the theory that molecules in solution acted in the same way as noble gases – they didn’t react with anything else (including each other), but they bumped around and exerted pressures on their constraints.

How could we measure this pressure? Push back!

With the right amount of pressure, we can return both

sides to the same volumeThis value is the “Osmotic Pressure” of the solution.

Another word for this pressure is “Turgor”

The value of osmotic pressure is always negative.Why?

Like any other physical force, it’s a vector, and therefore has both value and direction. It’s negative by tradition, probably because it was a measure of water being pulled away from its starting point – like a vacuum cleaner.

And since we usually ascribe the force asserted by a vacuum to be negative – well, there you have it.

Going back to this earlier image: what if we filled the blue side

with more water? Would the red keep rising?

To a point. Eventually, the pressure of the water being pulled down by gravity would be stronger than the Turgor pressure.

This is hydrostatic pressure (static, because the water isn’t moving anywhere).

G

Keeping it AllStraight

Diffusion: Moving from more concentrated areas to less concentrated areas.

Osmosis: Moving from less concentrated areas to more concentrated areas.

Just keep in mind that the liquid in which something is dissolved acts just like any other molecule bumping around. It will diffuse from regions of low concentration to high.

Key Concept: Solutions with the higher solvent concentration will have the lower solute concentration. Solutions with the lower solvent concentration will have the higher solute concentration.

Because we’re generally more interested in what’s dissolved in a liquid, the language that’s normally used revolves around the solute. (When you think about salt water, you are thinking about how ‘salty’ it is – not how ‘watery’ it is!)

In fact, when we dive into the underlying chemistry and mathematics of osmosis, the concentration of water isn’t even used. Instead of talking in terms of concentrations, we talk in terms of how much the water will potentially move against the concentration gradient of the solute. This is called the “water potential” of the solution.

SUMMARYOsmosis…

…requires a semi-permeable membrane to occur

…is the net movement of solvent through a semi-permeable membrane, from regions of low solute (high solvent) concentration to high solute (low solvent) concentration.

...is a process that occurs between two solutions of different concentrations

…depends upon the concentration (but not type) of solute.

SUMMARYOsmotic Pressure…

… is the force exerted by a hypertonic solution on a hypotonic solution

…of a solution is always in relation to another solution.

hypertonic solutions draw water away from a hypotonic solution; Isotonic solutions have the same concentration – and therefore an osmotic pressure of 0.

… is always a negative number

Osmosis “IRL”

• When cells are placed in a hypotonic solution, what will happen?

– Red blood cells will swell, and lyse, or rupture. This is called osmotic lysis

– Plant cells, with sturdy cell walls, do not lyse in hypotonic solution. Instead of increasing volume, they become more rigid.

• This rigidity is a result of the osmotic (turogr) pressure discussed earlier – the water molecules trapped in a small space push against the inside of the cell wall.

A Practical Example of Osmosis:

Or, How Understanding Osmosis Helped Save the Children

Oral Rehydration Therapy (ORT)

Measuring spoons for oral rehydration therapy, 1981. Made by the company Teaching Aids at Low Cost, each spoon has instructions in a different language.

CHOLERA: EASILY FATAL, EASILY TREATABLE

• Diarrheal illness caused by a bacterial infection in the intestines (Vibrio cholerae)

• Left untreated, Cholera will cause massive, fatal dehydration from diarrhea and vomiting.– Rapid loss in blood volume (extreme cases: as short as an hour to a

hypotensive state, with death being 2-3 hours later if left untreated).

• Without treatment the death rate is as high as 50%; with treatment the death rate can be well below 1%

• It is still a major cause of death amongst third world children.

Blood Epithelial Cells Gut

High Low High

Low High

Na+

K+

WaterLow HighHigh

Sugar and salt are co-transported from the gut into the epithelial cells.

This causes a net increase in solute concentration inside the cells, causing the

cells to become hypertonic

Water from the hypotonic gut flows into the epithelial cells

Salt is also secreted into the blood, causing it to be hypertonic with respect

to the epithelial cells

Water from the hyptonic epithelial cells flows into the blood, increasing blood

volume.

Basis of Cholera Toxcity

Water Water

Blood Epithelial Cells Gut

Low Low HighNa+

Low LowK+

Water LowerHigh

Cholera uses a protein (the eponymous Cholera Toxin), which activates cellular proteins that, in the process of their normal activities, causes the excretion of salts into the gut.

Due to the new osmotic pressure of the now-hypertonic gut, water moves from the blood, through the epithelial cells, and into the gut, causing massive diarrhea.

The massive drop in blood loss results in a sharp increase in blood pressure, which can result in death if not treated.

HighHigh

Drinking water alone won’t help – it will just pass right through the hypertonic gut. Salt water won’t help, either – it will simply increase the hypertonicity of the gut.

But, we can take advantage of the fact that glucose and Na+ are co-transported into the epithelium,If we mix sugar into the salt water, this will drive the salt into the epithelium – and with it, the life-saving water.

Water Water

Even in an isotonic environment, cells are continually struggling to maintain their volume.

Cells experience a lot of leakage – ions such as Na+ and Cl- diffuse into the cell (down their concentration gradients).

Without an active mechanism to counter this, the cell would increase in solutes, become hypertonic, and therefore exert an

osmotic pressure on the hypotonic environment, drawing in water and eventually lysing.

So why don’t cells lyse?

Think of a rowboat with a leak. It’s not going to sink immediately, and as long as you can bale the water over the side quicker than the boat fills, you can prevent the boat from sinking. But if you stop putting in the effort, over

time, the boat will sink.

This leads into the next section of Active Transport; where diffusion and osmosis are both spontaneous processes, Active Transport requires the cell to expend energy, pumping molecules against their concentration gradient.

INTERNET LINKS

Fun• The dangers of dihydrogen monoxide can be found at

www.dhmo.org. • Try this fun simulation here.

Technical• Lab Excerise Link• Webster’s

various definitions and underlying mathematics.

TRANSPORT BY CARRIER PROTEINS

Facilitated Transport

Sometimes referred to as “passive” transportAllows molecules that are otherwise impermeable to the membrane to pass freely.No ATP is required.Molecules diffuse down their concentration gradient.Analogy: A doggie door. Small enough for dogs, but not (most) people.

Active Transport

Uses energy to move molecules against their concentration gradient.

Generally called “pumps”

Facilitated Transport

Active Transport

58Facilitated ActiveTransport:The Sodium-Potassium

PumpError in book’s slides!

VESICLES

VESICLES

Exocytosis

Endocytosis

Phagocytosis

Pinocytosis

Also includes Receptor-Mediated Endocytosis

MODIFICATION OF CELL SURFACES

Junctions Between Cells

Anchoring junctions

Desmosome

Tight junction

Gap Junction

Anchoring JunctionsAdhesion junctionsKey: plasma membranes have space between them; does not prevent movement of moleculesHeart, stomach, etc. Good for stretching

Desmosomes (not shown)A single point of contact. In skin cells.

Tight Junctionsproteins attach to each other directlyKey: plasma membranes are in contact, forming a water-impermeable barrier.

Gap JunctionsAct as pores. Allows cells to communicate with each other.

ADHESION JUNCTION

Desmosomes

Tight Junction

GAP JUNCTION

Extracellular MatrixWikipedia Link

polysaccarides + protein associating with the cells that produced them (and secreted them)

Fibrous proteinsCollagen (strength) Elastin (resilience)

Also includes proteoglycans

Proteoglycan

Very little “protein” portion!

Question: how do proteoglycans and glycoproteins differ?

• Another great link for the ECM

PLANT CELL WALLS.• Cell walls have pectin. This gives them flexibility.• Cells are held together by a “glue” called the middle

lamella.

• Woody plants have a second cell wall. – Constructed from cellulose, but more sturdy. Also, they

contain Lignin, which gives them strength.• Plasmodesmata are small, and allow plants to

exchange small particles like water, but can also remain different from each other because the bigger stuff can’t get through (and therefore, the cytoplasm of each cell will be different)

PLANT CELL WALLS.