CHAPTER 7Membrane Structure and Function

Cell Membrane

Plasma Membrane Boundary that separates living cell from

surroundings May have been 1 of first evolutionary

steps Exhibits selective permeability Fluid mosaic model

What is selective permeability? Allows some substances to cross it

more easily than others Encloses a solution different from the

surrounding solution Permits the uptake of nutrients and

elimination of wastes

What makes a membrane? Lipids and Proteins and carbohydrates Proteins Phospholipids are most abundant

What makes phospholipids unique? They are amphipathic

Have both hydrophobic and hydrophilic regions

AMPHIPATHICPHOSPHOLIPIDS

Hydrophilichead

Hydrophobictail

WATER

WATER

AMPHIPATHICPROTEINS

Hydrophilic regionof protein

Hydrophobic region of protein

Phospholipidbilayer

What is the fluid mosaic model? Membrane is “fluid” structure “Mosaic” because of various proteins

embedded in or attached to the membrane

Bilayer – double layer due to phospholipids

How is the membrane fluid? Not static sheets Held together by hydrophobic

interactions Can shift laterally

rapid

How is the membrane fluid? Can shift transversely across

membrane, but it’s rare

FLUID MOSAIC MODEL AND PHOSPHOLIPIDS

Lateral movement(~107 times per second)

Flip-flop(~ once per month)

Movement of phospholipids

How does temp affect fluidity? As temp decreases the phospholipids

settle into a closely pack arrangement and the membrane solidifies

What role do unsaturated hydrocarbon tails play?

How does temp affect fluidity? As temp decreases the phospholipids

settle into a closely pack arrangement and the membrane solidifies

What role do unsaturated hydrocarbon tails play?Membrane remains fluid to a lower temp if

rich in phospholipids with unsaturated hydrocarbon tails

Why?

How does temp affect fluidity? As temp decreases the phospholipids

settle into a closely pack arrangement and the membrane solidifies

What role do unsaturated hydrocarbon tails play?Membrane remains fluid to a lower temp if

rich in phospholipids with unsaturated hydrocarbon tails

Why?○ Because of the kinks n tails

How does temp affect fluidity? As temp decreases the phospholipids

settle into a closely pack arrangement and the membrane solidifies

Steroid cholesterols effect the fluidity at different temperatures

What are the membrane proteins? Adds to the “mosaic” part of the model TONS OF THEM!!

More than 50 in plasma of RBC Proteins determine most of the

membrane’s function

What are the membrane proteins? Two types

IntegralPeripheral

What are the membrane proteins? Two types

Integral○ Penetrate the hydrophobic core of lipid bilayer○ Some are transmembrane

Span the entire membrane

What are the membrane proteins? Two types

Integral○ Penetrate the hydrophobic core of lipid bilayer○ Some are transmembrane

Span the entire membranePeripheral

○ Not embedded in lipid bilayer

6 Major Functions of the Proteins of the Plasma Membrane Transport Enzymatic Activity Signal Transduction Cell-cell recognition Intercellular Joining Attachment to the cytoskeleton and

Extracellular Matrix (ECM)

MEMBRANE PROTEIN FUNCTIONS

EnzymesSignal

ReceptorATP

Transport Enzymatic activity Signal transduction

MEMBRANE PROTEIN

FUNCTIONS

Glyco-protein

Cell-cell recognition Intercellular joining Attachment to thecytoskeleton and extra-cellular matrix (ECM)

6 Major Functions of the Proteins of the Plasma Membrane Transport

Provides a hydrophilic channel across the membrane that is selective for a particular solute

Shuttle substance from one side to the other by changing shape

6 Major Functions of the Proteins of the Plasma Membrane Transport Enzymatic Activity

Protein may be an enzyme with its active site exposed to substances in adjacent solution

Can work as a team to carry out sequential steps

6 Major Functions of the Proteins of the Plasma Membrane Transport Enzymatic Activity Signal Transduction

Receptor protein may have binding site with specific shape that fits the shape of a chemical messenger ○ Example: hormone

External messenger may cause shape change

6 Major Functions of the Proteins of the Plasma Membrane Transport Enzymatic Activity Signal Transduction Cell-cell recognition

Glycoproteins serve as identification tags that specifically recognized by membrane proteins of other cells

6 Major Functions of the Proteins of the Plasma Membrane Transport Enzymatic Activity Signal Transduction Cell-cell recognition Intercellular Joining

Membrane proteins of adjacent cells may hook together in various kinds of junctions○ Example:

Gap junctionsTight junctions

6 Major Functions of the Proteins of the Plasma Membrane Transport Enzymatic Activity Signal Transduction Cell-cell recognition Intercellular Joining Attachment to the cytoskeleton and

Extracellular Matrix (ECM)Microfilaments may be noncovalently bound to

membrane proteinsThis helps maintain cell shape and stabilize

location of certain membrane proteins

When is this important? Sorting tissues into organ as embryo For rejection of foreign objects by

immune system

How is this recognition accomplished? Cells recognize other cells by binding to

surface moleculesOften carbohydrates

These carbohydrates are usually short, branched chains 15 or few sugars

How is this recognition accomplished? Carbohydrates on Extracellular side of

plasma membrane vary: from species to speciesamong individuals of same specieEven from cell to cell

What is a glycolipid? Carbohydrate covalently bonded to lipid

What is a glycoprotein? Carbohydrate covalently bonded to

protein

Synthesis of Membrane Proteins and Lipids

Synthesis of Membrane Proteins and Lipids1. Synthesis of proteins and lipids in ER

Carbohydrates are added to make them glycoproteins Carbohydrates are then modified

2. Inside Golgi Complex, glycoproteins undergo further carbohydrate modification. Lipids also acquire carbohydrates (glycolipids)

3. Transmembrane proteins, membrane glycolipids, and secretory proteins are transported in vesicle to plasma membrane

4. Vesicles fuse with membrane, releasing secretory proteins from the cell

Synthesis of Membrane Proteins and Lipids

Plasma Membrane is Supramolecular Structure What is a Supramolecular Structure?

Many molecules ordered into a higher level of organization

Has emergent properties

Movement across the membrane Steady movement of small molecules

and ions in both directions Sugars, amino acids, and nutrients enter

cell Waste leave cell Regulation of inorganic ions Movement occurs at different rates

Movement across the membrane Nonpolar molecules are hydrophobic

and can dissolve in the lipid bilayer of the membranes and cross it easily without membrane proteins

Hydrophobic core of membrane impedes direct passage of ions and polar molecules (hydrophilic molecules)

Membrane Permeability Transport Proteins

Channel Proteins- provide a channel for hydrophilic molecules to move through.○ Aquaporins- allow water to pass through the

cell membrane quickly.Carrier Proteins- bind to molecules and shuttle

them across the membrane.

Diffusion

Diffusion- movement of molecules of any substance until they spread out evenly in the available space. (equilibrium).Diffusion is a spontaneous process, needing no energy

input. Rule of Diffusion: in the absence of a force, a

substance will diffuse from high concentration to low concentration.

Diffusion

A substance diffuses down its own concentration gradient, unaffected by the concentration of other substances.

Diffusion is a form of passive transport- movement that does not require the cell to use energy.

Osmosis• Osmosis- the diffusion of water. Water diffuses

from the region of lower solute concentration (higher free water concentration) to the area of higher solute concentration (lower free water concentration)- until equilibrium is reached. • Osmosis is a method of passive transport

Osmosis

Osmosis For dilute solution (like that found in

most biological fluids), solutes don’t affect water concentration

Instead, tight clustering of water molecules around the hydrophilic solute molecules makes some of water unavailable to cross membraneThis is NOT FREE WATER

Osmosis It’s FREE WATER that moves Water moves from areas of low Solute

concentration to high solute concentration

Osmosis

Tonicity- the ability of a surrounding solution to cause a cell to gain or lose water.Hypertonic- concentration of solution is more than the cell. Cell

will lose water, shrivel, and probably die. Hyper = “more” (when talking about nonpenetrating solutes)

Hypotonic- concentration of solution is less than the cell. Water will enter the cell and the cell will swell and lyse (burst).

Isotonic- concentration of solutions is the same on both sides of the membrane. No net movement of water = stable volume.

Osmosis Osmoregulation- the control of solute

concentrations and water balance. Less permeable membrane, contractile vacuole, etc.

Facilitated Diffusion Facilitated Diffusion- passive transport

aided by proteins. Frequently involves polar molecules.

Facilitated Diffusion Reminder:

Channel proteins are a type of transport protein that provide corridors that allow a specific molecule to cross the membrane

Aquaporins – type of channel protein Ion Channels- channel proteins that transport

ions down the concentration gradient. No energy required.Gated Channels- open or close in response to a stimulus.

Warm Up Exercise Explain the difference between osmosis

and diffusion. What is facilitated diffusion? What is the rule of diffusion regarding

concentration gradient?

Active Transport Active Transport- moves solute from

low to high concentration. Requires energy (usually ATP). Uses carrier proteins.Active transport allows a cell to have an internal

concentration different from its surroundings. Sodium-Potassium Pump- an example

of active transport that exchanges Na+ for K+ across the plasma membrane.

Active Transport Membrane Potential – the difference in

voltage across the cell membrane. (ranges from -50 to -200 mV) The inside of the cell is negative relative to the outside. This favors transport of cations into the cell and anions out

of the cell. Electrochemical Gradient- the combination

of the membrane potential (electrical force) and concentration gradient (chemical force). Ions diffuse not only down their concentration gradient, but

down its electrochemical gradient.

Active Transport Electrogenic Pump- a transport protein

that generates voltages across a cell membrane by maintaining a membrane potential.Ex. Sodium-potassium pump in animals and proton

pump in plants, fungi and bacteria

Cotransport Cotransport- active transport driven by

a concentration gradient.

Endocytosis/Exocytosis Exocytosis- the secretion

of large molecules by the fusion of vesicles with the plasma membrane. Requires energy.

Endocytosis- cell takes in molecules by forming new vesicles from the plasma membrane. Phagocytosis- cell eating Pinocytosis- cell drinking Receptor-Mediated

Endocytosis

Endocytosis/Exocytosis

Warm Up Exercise Define phagocytosis and pinocytosis. What does it mean for a cell to have a

concentration gradient?

Cell Signaling

Signal Transduction Pathway- a specific cellular response as a result of a received cellular signal.

Local Signaling

Local Regulators- influence cells in the nearby vicinity.Paracrine Signaling- broad range- can communicate with

many cells.Synaptic Signaling- occurs in the nervous system (more

specific)

Long Distance Signaling Hormones- chemicals

that aide in long distance signaling- released by specialized cells and travel in the blood stream.

Three Stages of Cell Signaling Reception- the target cell’s detection of

a chemical signaling molecule. (when the molecule binds to the receptor protein).

Transduction- the change that occurs on the protein due to the receptor binding.

Response- transduced signal triggers a specific cellular response.

Three Stages of Cell Signaling

Cell Surface Transmembrane Receptors G Protein Coupled Receptors

(GPCRs)- signaling molecule binds to GPCR which activates it and changes its shape. GPCR then binds an inactive G protein causing GDP to convert to GTP, activating the G protein. Protein binds to enzyme, activating it- triggering the next step in a cellular response.

Cell Surface Transmembrane Receptors

Cell Surface Transmembrane Receptors

Receptor Tyrosine Kinases (RTKs)- transfer phosphates from ATP to the amino acid tyrosine.

Cell Surface Transmembrane Receptors Ion Channel Receptors- includes a

region that acts as a fate when the receptor changes shape. Gate can open or close to allow flow of specific ions.

Cell Surface Transmembrane Receptors

Intracellular Receptors Intracellular

Receptors- found in the cytoplasm or nucleus of target cells.

Warm Up Exercise What are the three stages of cell

signaling? What are the two types of local

signaling? Which is the strongest? Hint: When a signal is transmitted to numerous

molecules, it is more amplified because it activates more than one molecule at the end of a pathway.

How are long-distance signals sent? What are the three main types of

transmembrane receptors?

Signal Transduction Protein Kinase- enzyme that transfers

phosphate groups from ATP to a protein.Phosphorylation- adding a phosphate

(which many times activates the protein)

Phosphorylation Cascade

Signal Transduction Protein Phosphatase- enzyme that

rapidly remove phosphate groups from proteins, a process called dephosphorylation. Usually inactivates protein kinases and help

turn off signal transduction pathway. Makes protein kinases available for reuse.

Second Messengers Second Messengers- small, water-

soluble, non-protein molecules/ions involved in signaling pathways. Cyclic AMP (cAMP)- (cyclic adenosine

monophosphate)- ATP is converted to cAMP by an enzyme (adenylyl cyclase) in the plasma membrane in response to an extracellular signal (usually a hormone).○ Phosphodiesterase- reduces cAMP to AMP

Second Messengers

Second Messengers cAMP usually

activates protein kinase A which phosphorylates other molecules in the signal transduction pathway.

Second Messengers Calcium (Ca2+) Ion- calcium pumps

actively transport calcium ions from the cytoplasm out of the cell or into the ER.

Second Messengers

Warm Up Exercise Explain the terms phosphorylation and

dephosphorylation. What enzymes are in charge of

phosphorylating and dephosphorylating? To what are the phosphates usually

added or removed? Why is this important in signal transduction?

Signal Response Signaling pathway

may regulate protein activity or synthesis.

Growth Factors- signaling molecules that initiate cell division pathways.

Cellular Response What transmembrane

receptor is involved here? What are the other ones?

What major secondary messengers do you see in this pathway?

What is the final response of this pathway?

Fine Tuning a Cellular Response Signal Amplification

Each relay molecule activates more molecules in the following step. This allows a small signal to elicit a huge response.

Specificity of Cell Signaling and Coordination of Response

Fine Tuning a Cellular Response

Fine Tuning a Cellular Response Signaling Efficiency

Scaffolding Proteins- large relay proteins to which several other smaller relay proteins are simultaneously attached.

Termination of the Signal

Apoptosis Apoptosis- programmed

cell death, or cell suicide.

Warm Up How does the length of the signal

transduction pathway affect the response?

What is the job of phosphodiesterase? When might a cell commit apoptosis?

Cell Division Cell Division Cell Cycle- the life of a cell from the time it

is first formed from a dividing parent cell, until its own division into 2 daughter cells.

Somatic Cells- produced by mitosis- genetically identical- contain 46 chromosomes (in humans)

Gametes- produced by meiosis- genetically unique- contain 23 chromosomes (in humans)

Cell Division Sister

Chromatids- joined copies of the original chromosome- joined at the centromere.

Cell Cycle Interphase

G1- cell growsS- Synthesis (DNA)G2- cell grows

M PhaseMitosis

○ Prophase○ Prometaphase○ Metaphase○ Anaphase○ Telophase

Cytokinesis

Cell Division

Cell Division

Cell Division Cleavage Furrow- in animal cells only Cell Plate- in plant cells only, forms cell wall.

Binary Fission Binary Fission-

cells grow to double their size and divide to form two cells. Occurs in

prokaryotes and single celled eukaryotes.

Single celled eukaryotes must still go through mitosis prior to division.

Exit Slip How many chromatids are in a duplicated

chromosome? A chicken has 78 chromosomes in its

somatic cells. How many chromosomes did the chicken inherit

from each parent? How many chromosomes are in each of the

chicken’s gametes? How many chromosomes will be in each

somatic cell of the chicken’s offspring?

Warm Up Exercise Compare cytokinesis in plant and animal

cells. During which stages of the cell cycle,

does a chromosome consist of two identical chromatids.

Cell Cycle Control System Three major cellular checkpoints occur

in G1, G2, and M phase. G0 Phase- a nondividing cellular state.

Regulatory Proteins Protein Kinases- provide go ahead signals

at G1 and G2 checkpoints.More specifically, cyclin-dependent kinase (Cdks)

Cyclins- proteins that bind to and activate protein kinase.

Regulatory Molecule Cyclins + Cdk = MPF (maturation

promoting factor) MPF activity is directly proportional to the

concentration of cyclins. Concentration/Activity rises during S and G2 phases and falls during M phase.

MPF phosphorylates many proteins, initiating mitosis. Growth Factors- protein released by certain cells that stimulates other cells to divide.

Cancer and Loss of Cell Cycle Controls Density Dependent Inhibition-

crowded cells stop dividing. Anchorage Dependence- to divide,

cells must be attached to a substrate.

Cancer and Loss of Cell Cycle Controls Transformation- when a normal cell

becomes a cancerous cell. Benign Tumors- do not cause serious

problems. Malignant Tumors- spread to new

tissues and impair the functions of one or more organs.

Metastasis- spread of cancer cells to new locations.

Cancer and Loss of Cell Cycle Controls

TUNNEL TRANSPORT PROTEINS

EXTRACELLULARFLUID

Channel protein SoluteCYTOPLASM

“GRABBER” TRANSPORT PROTEINS

Carrier protein Solute

Diffusion

Osmosis

.

Animalcell

Lysed

H2O H2O H2O

Normal

Hypotonic solution Isotonic solution Hypertonic solution

H2O

Shriveled

H2OH2OH2OH2OPlantcell

Turgid (normal) Flaccid Plasmolyzed

Positive Pressure Potential

Negative Pressure Potential

Solute Potential

ΨS = -iCRT

-i (ionization constant) C (molar concentration) R (pressure constant) T (temperature in Kelvin)

Total Water Potential problems

FACILITATED DIFFUSION

EXTRACELLULARFLUID

Channel protein SoluteCYTOPLASM

ACTIVE TRANSPORT

Cytoplasmic Na+ bonds tothe sodium-potassium pump

CYTOPLASM Na+[Na+] low[K+] high

Na+

Na+

EXTRACELLULARFLUID

[Na+] high[K+] low

Na+

Na+

Na+

ATP

ADPP

Na+ binding stimulatesphosphorylation by ATP.

Na+

Na+

Na+

K+

Phosphorylation causesthe protein to change itsconformation, expelling Na+

to the outside.

P

Extracellular K+ bindsto the protein, triggeringrelease of the phosphategroup.

PP

Loss of the phosphaterestores the protein’soriginal conformation.

K+ is released and Na+

sites are receptive again;the cycle repeats.

K+

K+

K+

K+

K+

Cell Voltage Gradient

Proton pumps

Proton pumps and co-transport

Phagocytosis & Pinocytosis

.

Receptor

RECEPTOR-MEDIATED ENDOCYTOSIS

Ligand

Coatedpit

Coatedvesicle

Coat protein

Coat protein

Plasmamembrane

0.25 µm

A coated pitand a coatedvesicle formedduringreceptor-mediatedendocytosis(TEMs).