AP Biology Ch. 5 – The Cell Membrane and Signal Transduction Originally prepared by Kim B. Foglia Revised and adapted by Nhan A. Pham
AP Biology
Ch. 5 – The Cell Membrane and Signal Transduction
Originally prepared by Kim B. Foglia Revised and adapted by Nhan A. Pham
AP Biology
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