Catalyst (12/13)

Catalyst (12/13)

1. Where in the body are more microvilli found? Why?

Kidney, intestines, nose, tastebuds—so the cell can absorb more of whatever substance (organs all involved in absorption)

2. Where in the body are more cillia found? Why?

Nose, ears, throat, fallopian tubes—clear things, move the egg past, ears—help feel sound waves (sound into a chemical signal)

THE PLASMA MEMBRANE

AP Biology-Chapter 7

Plasma Membrane Functions

A. Be protectiveB. Regulate transport in and out of cell or subcellular domainC. Allow selective receptivity and signal transduction by

providing transmembrane receptor that binds signaling molecules

D. Allow cell recognitionE. Provide anchoring sites for cytoskeletal filaments or

components of the extracellular matrix1. This allows the cell to maintain its shape and perhaps move to distant sites

F. Help compartmentalize subcellular domainsG. Regulate the fusion of the membrane with other membranes in

the cell via specialized junctionsH. Provide passageway across the membrane for certain

molecules

II. Membrane Model

A. Current Model: Fluid Mosaic Model1. Brain child of Singer and Nicholson (1972)

a) Derived from a model proposed in the 1930s by Daffson and Danielli

i. Two layers of lipids that were coated with proteinsii. Fit the electron properties of membranes, but not all

the details fitiii. Unable to account for all the things going into and out

of the cell• Biggest problem was how their model would get polar

molecules through—in their theory they would get stuck—but this is not what happens—especially with water (a polar molecule)

II. Membrane Model

1. Four components to the Fluid Mosaic Modela) Phospholipid bilayer

i. Direct from Daffson and Danielliii. 3 types of lipids

• Phospholipids• Glycolipids• Cholesterol “attracted to water”

“repelled by water”

Fatty acid

Phosphate

II. Membrane Model

b) Chunks of proteins that float IN the bilayeri. New proposal

polarhydrophilicheads

nonpolarhydrophobictails

polarhydrophilicheads

H2Osugar

lipids

salt

waste

impermeable to polar molecules

II. Membrane Model

c) Molecules can move aroundi. Both lipids and proteinsii. Lipid bilayer acts like a liquid crystal

Array of molecules arranged in some relation to each other, but they are not fixed in position

Hydrocarbon chains in constant motion, so molecules are free to rotate and can move laterally

iii. Not a lot of movement from one layer to another and if they do there are enzymes called flipases and flopases that put them back to where they started.

iv. Fluidity has 2 meaningsMovement of molecules within the liquid crystal

arrangementMotion of the fatty acid side chains of phospholipid.

II. Membrane Model

d) Lipid distribution on the two sides of the bilayer is asymmetric

i. Outside layer and inside layer are composed of different types of lipids

III. Membrane Structure

A. Phospholipid bilayer in which a variety of proteins are embedded.

1. Fluidity of fatty acidsa) Function of type of fatty acid—saturated vs. unsaturated.

i. If saturated, there are one or more double bonds. This makes the fatty acid rigid and bent (kinked)Kinks require a lot of space between the phospholipids

and this would make the bilayer unstableb) Nature’s solution—usually have one of each, unsaturated

and saturated on each phospholipidi. This is sufficient to maintain the lipid bilayer without it

being too packed to move around or too much room that it falls apart


1. Role of cholesterol in animal membranesa) Provides support helping to maintain membraneb) Reduces the permeability of membrane to many molecules

i. (e.g. some hormones—estrogens and testosterone will not cross membranes loaded with cholesterol)

c) Sits in the “crook” of the unsaturated fatty acid side chains

d) No or little cholesterol in the inner membrane


3. Two types of membrane proteinsa) Integral (or transmembrane)

i. Stick all or partway through the membraneii. To stick in the interior of the membrane the protein

must have a hydrophobic surface• This goes against what we learned about protein folding

—hydrophillic exterior and hydrophobic interior!iii. Tightly associated with the lipid bilayer. Cell

biologists can release them only by disrupting the cell membrane with detergents

iv. Different forms• Extend all the way through—transmembrane proteins• Partially embedded—either sticking out the interior or

exterior

Polar areasof protein

Nonpolar areas of protein


a) Peripherali. Sit on the cytoplamsic face of a membrane, with no

part sticking into the membraneii. Easy to dislodge

Catalyst (12/17)

Sketch a drawing of the plasma membrane. Be sure to include and label all components.

ANNOUNCEMENTS:--cell analogy poster due TOMORROW--cell quiz (chpt 6) THURSDAY


1. Structure and function of membrane proteinsa) Channel proteins: allows a particular molecule or ion

to cross the plasma membrane freely.


b) Carrier proteins: selectively interacts with a specific molecule or ion so that it can cross the membrane.


c) Cell recognition proteins: glycoproteins—a protein with a carbohydrate chain attached sticking above the membrane surface.

i. The carbohydrate chains of the glycoproteins combined with the carbohydrate chains of the glycolipids forms the glycocalyx.

ii. The glycocalyx serves as the cell’s “fingerprint”• Mark the cell as belonging to a particular individual and

tissue• Causes difficulties in organ transplants—patient’s cells do

not recognize the glycocalyx fingerprint of the donor organ and therefore reject it.


d) Receptor protein: is shaped in such a way that a specific molecule can bind to it (signal transduction).


d) Enzymatic protein: catalyzes a specific reaction.

Many Functions of Membrane Proteins

Outside

Plasmamembrane

InsideTransporter Cell surface

receptorEnzymeactivity

Cell surface identity marker

Attachment to thecytoskeleton

Cell adhesion

“Antigen”

“Channel”

Membrane is a collage of proteins & other molecules embedded in the fluid matrix of the

lipid bilayer

Extracellular fluid

Cholesterol

Cytoplasm

Glycolipid

Transmembraneproteins

Filaments ofcytoskeleton

Peripheralprotein

Glycoprotein

Phospholipids

1972, S.J. Singer & G. Nicolson proposed Fluid Mosaic Model

SELECTIVE PERMEABILITY

AP Biology-Chapter 7

I. The plasma membrane is semipermeable

A. The structure of the plasma membrane affects which types of molecules can freely pass through it.

1. Because the structure of the membrane is predominantly lipid, how lipid soluble a molecule is predominantly defines its ability to cross the membrane.

a) Small hydrophobic (nonpolar or non charged) molecules freely cross the membrane

b) Ions don’t freely cross the membrane. Charged=polar


2. The size of the molecule also influences its diffusibility, but the influence of size is secondary

a) Large uncharged or charged molecules do not freely cross the membrane

b) H20, which is polar but very small, does cross the membrane freely. Able to pass quickly through when fatty acid chains move momentarily out of the way.

Why H2O?•Is a dipole•Not high charge (like ions)—only slightly charged•Osmotic pressure

1st –charge2nd– size


B. The permeability of membranes to ions and large molecules is due mainly to the activity of specialized membrane proteins.

II. Molecules cross the plasma membrane in both passive and active ways

A. Passive transport1. Two types

a) Diffusion: molecules move from higher to lower concentration (that is down their gradient) until they are distributed equally

i. Osmosis: the diffusion of H2O across a semipermeable membrane.

(no energy needed)

osmosis

2nd Law of Thermodynamics—universe

tends toward disorder (entropy)

Molecules move from HIGH to LOW concentration


b) Facilitated diffusion: a carrier (or channel) protein speed the rate at which a solute crosses the membrane in the direction of decreased concentration. The carrier protein undergoes a change in shape as I moves a solute across the membrane.

Facillitated diffusion


A. Active transport1. Three types

a) Carrier-mediated active transport: small molecules and ions move against their concentration gradient. Carrier protein AND energy needed.

i. Sodium-potassium pump: sodium out, potassium in (example of carrier mediated active transport)

• 3 Na+ ions carried out for each 2 K+ ions carried in, therefore, the inside of the cell is negatively charged compared to the outside.

(requires the use of energy—usually ATP)

Na+/K+ pump


b) Exocytosis: vesicles, often formed by the ______________________ & carrying a specific molecule, fuse w/ the plasma membrane as secretion occurs.

i. Example: insulin is secreted into the blood stream by insulin-secreting cells through the process of exocytosis.

golgi apparatus

Where in the body does a lot of exocytosis happen?

“cell”

“out”

Catalyst (12/17)

Sketch a drawing of the plasma membrane. Be sure to include and label all components.

ANNOUNCEMENTS:--cell analogy poster due TOMORROW--cell quiz (chpt 6) THURSDAY


c) Endocytosis: cells take in substances by vesicle formation. A portion of the plasma membrane invaginates to surround the substance and then pinches off to form an intracellular vesicle

i. Two types• Phagocytosis: when the material to be taken in is large,

such as a food particle or another cell (eg. Macrophages engulf bacteria or other pathogens and worn out cells)

• Pinocytosis: occurs when vesicles form around a liquid or very small particle. (Occurs in blood cells, cells that line the kidney tubules, or intestinal wall and plant root cells)

• Phagocytosis can be seen with the light microscope, but an electron microscope is needed to see the vesicles formed in pinocytosis.

“cell eating”

“cell drinking”

“in” “cell”

Endocytosis and digestion of pathogen (bacteria)

Hydrolytic enzymes

Endocytosis and digestion of food particle


ii. Receptor-mediate endocytosis: is a form of pinocytosis that allows cells to take up specific kinds of molecules and then sort them within the cell.

• Ligand: a macromolecule that bind to a plasma membrane receptor

• Coated pit: when ligands bind the their receptors this causes the receptors to gather in the “coated pit” that has a layer of a fibrous protein called clathrin.

• Clathrin is a fibrous protein that coasts many vesicles that leave the golgi apparatus. It appears to aid in the formation of vesicles

• The fate of the vesicle and ligand depends on the ligand.e.g. after hormones enter and act on the cell, the hormone (ligand), the receptors, and the vesicle fuse with a lysosome and are digested.e.g. when cholesterol enters the vesicle and receptors are recycled back to the plasma membrane

Hormone/vitamin

Controlled by receptor proteins

III. Diffusion across cell membrane

A. Cell membrane is the boundary between inside & outside…

separates cell from its environment

INfoodcarbohydratessugars, proteinsamino acidslipidssalts, O2, H2O

OUTwasteammoniasaltsCO2H2O products

cell needs materials in & products or waste out

IN

OUT

Can it be an impenetrable boundary? NO!

Catalyst (1/9)

What do you already know about osmosis?Diffusion of water across a semipermeable

membrane

I. Osmosis: the diffusion of water across a semipermeable membrane

A. Passive transportB. Depends on the concentration of solutes on each

side of the membraneC. Tonicity:

1. Hypertonic: higher concentration of solute molecules

2. Hypotonic: lower concentration of solute molecules

3. Isotonic: same concentration of solute molecules

4. NOTE: water moves from hypotonic to hypertonic solution (FIX THIS IN YOUR NOTES!!!!!!!)

No energy needed (down gradient)

“Above normal or over”

“Below normal”

“Same” or “equal”

D. Water Potential: a concept that helps to describe the tendency of water to move from one area to another, particularly into or out of cells. (ψ)

1. Water potential determines the rate and direction of osmosis

2. Is measured in units of pressure—kPa, MPa, bar

TWO FACTORS affect Water Potential

Solute Concentration Pure water has a water potential of zero (ψ=0) A solution will have a lower concentration of water

molecules, so it will have a NEGATIVE water potential Addition of solutes lowers the water potential

Pressure In a plant cell, pressure exerted by the rigid cell wall

that limits further water uptake (osmosis into the cell).

When a plant cell is fully inflated with water it is called turgid.

Pressure potential is important in plant cells because they are surrounded by a cell wall which, is strong and rigid. When water enters a plant cell, its volume increases and the living part of the cell presses on the cell wall. The cell wall gives very little and so pressure starts to build up inside the cell. This has the tendency to stop more water entering the cell and also stops the cell from bursting. When a plant cell is fully inflated with water, it is called turgid. Pressure potential is called turgor pressure in plants)

ψp

ψp

ψp

ψp

Water potential (ψ) =

pressure potential (ψp )

+ solute potential (ψs)(osmotic)

This is an open container, so the ψp = 0This makes the ψ = ψsThe ψs =-0.23, so ψ is -0.23MPa, and water moves into the solution.

Water moves from a place of high water potential to a place of low water potential.

Equation given on AP TestWater potential (Ψ)= ψp+ψs

hypotonichypertonic

Low water potential

High water potential

hypertonichypertonic

Pressure (ΨP)

(ΨS)

Turgid!

1

2

3

IsotonicA=B

sameNo net movement

A hyperB hypo

A is lower

B is lower

A hypoB hyper AB

BA

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Catalyst (12/13)