Cell Membranes

Why are membranes so important ?

Chapters 6 and 7

Membranes can be...

• Fences• Gates• Glue• Identifiers• Energy Makers• Transporters• Communicators

Phospholipid Bilayer(turn to page 107)

• Phospholipid– 1 polar “hydrophilic” head--phosphate– 2 nonpolar “hydrophobic” tails—glycerol and fatty

acid• More fluid if there are “kinks” in their lipid

tails – oil is fluid and butter is solid at room temp

because of this structure– Cholesterol helps maintain proper membrane

fluidity –neither too soft nor too firm

• The steroid cholesterol has different effects on membrane fluidity at different temperatures

• At warm temperatures (such as 37°C), cholesterol restrains movement of phospholipids

• At cool temperatures, it maintains fluidity by preventing tight packing

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Polarhydro-philicheads

Nonpolarhydro-phobic

tails

Polarhydro-philicheads

Fluid Mosaic Model(page 108)

• What is meant by “Fluid Mosaic Model”?

• The phospholipid bilayer is a liquid structure.– The proteins, glycoproteins, and glycolipids are

like rafts, some floating, some anchored.

Extracellular fluid

CarbohydrateGlycolipid

Transmembraneproteins

Glycoprotein

Peripheralprotein

Cholesterol

Filaments ofcytoskeleton

Cytoplasm

Extracellularmatrix protein

The membrane’s importance will be seen thoughout Biology:

• Look for other pages which show the membrane illustration- (blue lollipops), especially see chapters 42-51 -Animal Form and Function.

• 996-997 Endocrine

• 1016 Immune

• 942 Nervous System

• Also: 174-175 Cell Respiration

• 187 Photosynthesis

Making a slide for electron microscope (page 110)

• Tissue is embedded in epoxy, then cut to very thin (1 m) slices and placed on a grid.

• Electrons are bounced off and image is made.

• Alternatively, the tissue is frozen, cracked, and a cast is made with platinum.

• Can see proteins, carbs, pits, pores, channels.• Pictured in the next slide.

Figure 7.4

Knife

Plasma membrane Cytoplasmic layer

Proteins

Extracellularlayer

Inside of extracellular layer Inside of cytoplasmic layer

TECHNIQUE

RESULTS

Types of Membrane Proteins(pg 111)

• Transporters –Sodium-Potassium Pump• Enzymes- Electron Transport • Cell Surface Receptors-ie: on neuromuscular

junction• ID markers—blood markers • Adhesion proteins (to glue cells together) • Attachment to cytoskeleton –to link to other

cells.

Structure of Membrane Proteins112-113

• Notice the helical structures of these proteins.

– This is a good example of how the secondary and tertiary protein structures (hydrogen bond interactions) come into play.

• When you see the purple “jelly-bean” proteins in your book, it is a symbol for this more accurate depiction

Transport through membrane

• Non-polar molecules (ex: O2 and N2) can diffuse thru membrane.

• Large charged molecules must pass thru channels (ex: glucose, amino acids).

• Water has special channels called aquaporins

Movement Into and Out of Cell

• Distinguish between simple diffusion, facilitated diffusion, and active transport

• Simple diffusion- Oxygen and CO2

• Facilitated diffusion-Glucose in RBC• Active Transport— Na/K pump

How substances traverse the membrane

CELL TRANSPORT

DIFFUSION ACTIVE TRANSPORTREQ'S ENERGY

SIMPLE DIFFUSION

FACTILIATEDDIFFUSION

REQ'SPROTEIN

OSMOSISdiffusion of water

through a membrane

PUMPSSODIUM POTASSIUM

PUMP

ENDOCYTOSISEXOCYTOSIS

Osmosis

• Osmosis-Diffusion of Water through a semi permeable membrane down its concentration gradient.

• Osmotic concentration- a measure of the amount (concentration) of a solute in a solution

Figure 7.14Lowerconcentrationof solute (sugar)

Higher concentrationof solute

Sugarmolecule

H2O

Same concentrationof solute

Selectivelypermeablemembrane

Osmosis

Concept Map Activity

Tonicity (Osomolarity)

Differentiate between hypotonic, hypertonic and isotonic.

What would happen to red blood cells in…pure water

blood 5% saltwater solution

Tonicity

These are all relative termsHypotonic— less concentrated with soluteHypertonic— more concentratedIsotonic— same concentration

What would happen to red blood cells in…pure water – a hyotonic solution – will burst (lysis)

blood- an isotonic solution—stay same 5% saltwater –a hypertonic solution-will shrink

Figure 7.15

Hypotonicsolution

Osmosis

Isotonicsolution

Hypertonicsolution

(a) Animal cell

(b) Plant cell

H2O H2O H2O H2O

H2O H2O H2O H2OCell wall

Lysed Normal Shriveled

Turgid (normal) Flaccid Plasmolyzed

Water Potential

• Water potential determines the direction that water will flow through a cell.

• The higher the water potential—the higher water’s tendency to leave.

• (Psi)

Water Potential

• p = Pressure Potential(Pressure built up in a water gun or a water

balloon).

• s = Solute Potentail(Potential of Solutes in Dialysis Bag or Potato)

• = p + s

– Water potential = pressure potential + solute potential

Sodium-Potassium Pump

• Active transport • 3 sodium out/ 2 potassium in • Seen in nerve cells

– Maintains membrane potential. Also important in muscle contraction and fertilization.

– Sodium Potassium Pump Animation

BULK TRANSPORT

• EXOCYTOSIS/ ENDOCYTOSIS• COOLEST ANIMATION EVER

WHAT CELL PARTS DO YOU NOTICE?– Cool Animation http://www.mhhe.com/

biosci/genbio/espv2/data/cells/003/index.html

Cell Signaling

• Cells know about their environment based on chemical signals from nearby cells.

• Cells can communicate information such as– “I’m infected”– “It’s getting crowded”– “This body is too hot, turn on sweat glands”

– And every other possible thing that a cell could need to “know”.

CELLS SIGNAL ONE ANOTHER WITH CHEMICALS(page 126)

• To study specific receptors, researchers can – 1) use immunochemistry to create antibodies that

are specific for the receptor proteins that are sought

– 2) use molecular genetics to create mutations that cause changes in the receptor, after which the structure and function of receptors can be studied

• See what functions are different or lost after mutation

Types of cell-signaling mechanisms

• Direct Contact- see table and page 127• Paracrine Signaling, message transmitted

to neighboring cells – Synaptic signaling —neurotransmitters carry

messages between nerve cells

• Endocrine signaling —hormones travel through circ system

Fig. 7.2 (TEArt)

Synaptic signaling

Nerve cell

Neurotransmitter

Synaptic gap

Targetcell

Endocrine signaling

Hormone secretion intoblood by endocrine gland

Blood vessel

Distant target cells

Gapjunction

Paracrine signaling

Adjacenttarget cells

Secretory cell

Direct contact

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Cell Communicating Mechanisms

• Table 7.1, page 128• Figure 7.13, page 137

Microvilli

Tight junction

Adherens junction(anchoring junction)

Intermediatefilament

Desmosome(anchoring junction)

Gap junction(communicatingjunction)

Hemidesmosome(anchoring junction)

Basal lamina

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Hormone(testosterone)

EXTRACELLULARFLUID

Receptorprotein

DNA

mRNA

NUCLEUS

CYTOPLASM

Plasmamembrane

Hormone-receptorcomplex

New protein

– Steroid hormones- testosterone, estrogen– Bind to intracellular receptors

1 The steroid hormone testosterone passes through the plasma membrane.

The bound proteinstimulates thetranscription ofthe gene into mRNA.

4

The mRNA istranslated into aspecific protein.

5

Testosterone bindsto a receptor proteinin the cytoplasm,activating it.

2

The hormone-receptor complexenters the nucleusand binds to specific genes.

3

Signal Transduction Pathways(Major Players) copy onto separate

paper)• Signal protein- Binds to a receptor, such as insulin,

epinephrine, or caffeine• Receptor ( receptor protein)-- Changes shape when

signaled.• GDP and GTP- guanine diphosphate and guanine

triphosphate- used for energy transfer (like ADP/ ATP)

• G protein- a membrane receptor that turns on an effector protein, which begins signal amplification.

Fig. 7.10 (TEArt)

Signal molecule

Receptorprotein

Activatedadenylyl cyclase

Amplification

Amplification

Amplification

Amplification

GTP G protein

2

1

3

4

5

6

7

Enzymatic product

Enzyme

Proteinkinase

cAMP

Not yetactivated

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Second Messenger SystemSecond Messenger System

Signal Transduction Pathways

• Read about Viagra, Levitra, and Cialis– See AP New Curriculum File

Signal Transduction Pathways

• In the previous example, if the hormone is epinephrine, and it acts on the liver, then the enzyme product will be lots of the enzyme glycogen phosphorylase.

• This enzyme breaks down glycogen into glucose.• What does glucose do?

Signal Transduction Pathways

• Molecule affects the cell without entering the cell• G-protein-linked receptors on membrane

– Hormone binds receptor-1st messenger– This triggers an intracellular response via a G protein– Cascade of events is regulated by a second messenger -

cAMP (cyclic AMP)

– Examples —peptide hormones (epinephrine)-- rod cells in eye detect light

--plant responses

Signal Transduction is Highly Regulated

• The amount of final product is regulated by

– Synthesis and breakdown of enzymes– Activation or inhibition of enzymes by other

moleculeles.– Environmental Signals

• Opening of ion channels will change balance of product, important in muscle and nerve cells.

• Gene expression- Genes may be turned on or off, so that more or fewer proteins are available for the cascade.

Bacterial Interaction

• TED TALKS– Bonnie Bassler- Bacterial Communication– V. fischeri Secretes hormone-like product

• As cell number increases, concentration increases• This chemical is used to for cell-cell communication

Signal Transduction Pathways

• Watch Chapter 7 animations: “Signal Amplification” and “Second Messengers”

• http://highered.mcgraw-hill.com/sites/0072437316/student_view0/chapter7/animations.html#

• See Learn Genetics Website http://learn.genetics.utah.edu/content/begin/cells/insidestory/

• The receptor has binding sites both outside of membrane and inside.

• In signal amplification, 1 cell surface receptor can activate many G-protein molecules

• The end result is high levels of the final product.

• Signal transduction is highly regulated

• This PPT is available on Wise’s Website

Surface area-to-volume ratios affect a biological system’s ability to obtain necessary resources or eliminate waste products. Evidence of student learning is a demonstrated understanding of each of the following:1. As cells increase in volume, the relative surface area decreases and demandfor material resources increases; more cellular structures are necessary toadequately exchange materials and energy with the environment. Theselimitations restrict cell size.To foster student understanding of this concept, instructors can choose anillustrative example such as:• Root hairs• Cells of the alveoli• Cells of the villi• Microvilli2. The surface area of the plasma membrane must be large enough to adequatelyexchange materials; smaller cells have a more favorable surface area-to-volumeratio for exchange of materials with the environment.