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Ch. 5 – The Cell Membrane and Signal Transduction

Originally prepared by Kim B. Foglia Revised and adapted by Nhan A. Pham

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Cell Membrane §  Cell membrane (typically 8 nm thick) -  separates living cell from surrounding aqueous

environment -  regulates what comes in and out of the cell

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Phospholipids

Fatty acid

Phosphate §  amphipathic molecules with

hydrophilic phosphate head and hydrophobic tails

§  arranged as a bilayer

“repelled by water”

“attracted to water”

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Phospholipid Bilayer

polar hydrophilic

heads

nonpolar hydrophobic

tails

polar hydrophilic

heads

§  serves as a cellular barrier

H2O sugar

lipids

salt

waste

impermeable to polar molecules

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§  membrane is fluid with a collection (“mosaic”) of proteins embedded in or attached to the bilayer

Fluid Mosaic Model

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

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Membrane Protein Structure

§  Integral proteins -  stretches of nonpolar, hydrophobic amino acids coiled

in α helices -  embedded in phospholipid bilayer

§  Peripheral proteins -  stretches of polar, hydrophilic amino acids facing the

aqueous environment -  not embedded, anchored to membrane

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Integral proteins

Peripheral proteins

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Classes of Amino Acids What do these amino acids have in common?

nonpolar and hydrophobic

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What do these amino acids have in common?

polar and hydrophilic

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Functions of Membrane Proteins Outside

Plasma membrane

Inside Transporter Cell surface

receptor Enzyme activity

Cell surface identity marker

Attachment to the cytoskeleton

Cell adhesion

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Membrane Carbohydrates §  Structure -  usually short, branched chain of

about 15 monomers -  can be glycolipids or

glycoproteins §  Function -  play a key role in cell-cell

recognition -  help cells to distinguish one cell

from another (in organ and tissue development)

-  basis for rejection of foreign cells by immune system (antigens)

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Diffusion through Phospholipid Bilayer §  What molecules can get through directly? -  fats and other lipids -  hydrophobic, nonpolar substances

inside cell

outside cell

lipid salt

aa H2O sugar

NH3 § What molecules can NOT

get through directly? -  polar molecules (H2O) -  ions (salts, ammonia -  large molecules

(starches, proteins)

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Simple Diffusion §  Movement of particles from HIGH to LOW concentration

(down concentration gradient) §  “passive transport” since no energy is needed §  Very slow!!!

diffusion

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Diffusion rate depends on:

1.  The diameter of the molecules or ions

2.  The temperature of the solution. How so?

3.  The concentration gradient in the system. The greater the concentration gradient, the more rapidly a substance diffuses.

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Facilitated Diffusion §  Diffusion through: -  channel proteins

(most often ligand- or voltage-gated channels)

-  carrier proteins §  specific molecules travel

across cell membrane down their concentration gradient

§  NO energy needed!

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How specific?

§  Glucose transport protein only transports… glucose – not fructose (its structural isomer)

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The Special Case of Water

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Osmosis is just diffusion of water §  Concentration of solutes in and out of cell determines

the direction of osmosis in animal cells. §  How about in plant cells?

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Hypotonic §  Hypotonic (in distilled water) -  high concentration of water around cell -  cell gains water, swells and can burst Ex: Paramecium cell

freshwater

ATP

1

No problem, here

KABOOM!

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Hypertonic §  Hypertonic (in salt water) -  low concentration of water

around cell -  animal cell loses water and can

die Ex: shellfish -  plant cells plasmolyze but can

recover

saltwater

2

I will survive!

I’m shrinking.

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Isotonic §  Isotonic (in mild salt solution) -  no difference in concentration

of water between cell and environment

-  no net movement of water Ex: blood cells in blood plasma

balanced

3

I could be better…

That’s perfect!

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To sum it up…

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Aquaporins §  Water molecules move rapidly into and out of cells

through protein channels

1991|2003

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Active Transport §  Cells may need to move

molecules against concentration gradient

§  Like what?

§  involves carrier proteins

§  conformational shape change transports solute from one side of membrane to other

§  Energy (ATP) is needed!

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Active Transport Animations http://bcs.whfreeman.com/hillis1e#667501__674130__

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Active transport §  various models and mechanisms…

ATP ATP

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Osmoregulation §  freshwater (hypotonic) - water flow into cells and salt

loss -  uptake of salt ions by gills -  excrete large amounts of urine

(water) §  saltwater (hypertonic) - water loss from cells -  excrete salt ions from gills -  excrete small amounts of urine

hypotonic

hypertonic

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simple diffusion

facilitated diffusion

active transport

ATP

Summary

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How about really large molecules? §  Large molecules into and out of cell through vesicles

and vacuoles §  Endocytosis

- phagocytosis = “cellular eating” - pinocytosis = “cellular drinking”

§  exocytosis

exocytosis

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Endocytosis

phagocytosis

pinocytosis

receptor-mediated endocytosis

fuse with lysosome for digestion

non-specific process

triggered by molecular signal

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Endocytosis Animations http://bcs.whfreeman.com/hillis1e/#667501__674131__

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Signal Transduction §  A signal transduction

pathway – a sequence of molecular events and chemical reactions that lead to a cellular response, following the receptor’s activation by a signal

§  Autocrine signals affect the same cells that release them.

§  Paracrine signals diffuse to and affect nearby cells.

§  Hormones travel to distant cells.

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What’s involved? §  a signal, a receptor, and

a response

-  signal molecule (ligand) binds to receptor on protein

-  receptor changes shape

-  shape change initiates a response

§  3 types: ion channel, protein kinase, and G protein-linked receptors

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Ion channel receptors

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Protein Kinase Receptors §  Ligand binds à new

conformation

§  activates a domain on the cytoplasmic side of the transmembrane protein that has catalytic (protein kinase) activity

§  ATP + protein → ADP + phosphorylated (activated) protein

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G Protein-linked Receptors §  Ligand binding exposes a site that can bind to a

membrane protein, a G protein (partially inserted in the lipid bilayer, and partially exposed on the cytoplasmic surface).

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G Protein-linked Receptors

http://www.youtube.com/watch?v=qOVkedxDqQo&feature=youtu.be

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§  A second messenger – carries the signal from the receptor to the interior of cell

§  In the fight-or-flight response, epinephrine (adrenaline) activates the liver enzyme glycogen phosphorylase.

§  Experimental evidence?

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Cyclic AMP §  The second messenger is

cyclic AMP (cAMP).

§  Second messengers allow the cell to respond to a single membrane event with many events inside the cell.

§  They amplify the signal by activating more than one enzyme target.

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Amplification

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Amplification

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To sum it up §  Cell functions change

in response to environmental signals:

-  opening of ion channels

-  alterations in gene expression (as transcription factor)

-  alteration of enzyme activities