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UA / Biology / BSC 300 / What is the function of a biological membrane?

What is the function of a biological membrane?

What is the function of a biological membrane?

Description

School: University of Alabama - Tuscaloosa
Department: Biology
Course: Cell Biology
Professor: John yoder
Term: Spring 2019
Tags: cellular biology and MCAT Biology
Cost: 50
Name: cell bio test 2 study guide
Description: this study guide covers the last 5 lectures that will be on exam 2
Uploaded: 02/02/2019
17 Pages 49 Views 3 Unlocks
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Test 2


What is the function of a biological membrane?



Biomembrane Structure 

1. Plasma membrane: all cells have these; outer boundary of cell separating from world

a. Phospholipid bilayer with polar head groups and hydrophobic tails

b. Fluid mosaic model of biomembranes

i. Noncovalent interactions between phospholipids and

between phospholipids and proteins provide membrane

integrity and resilience

ii. Individual phospholipids spin and diffuse laterally within plane of membrane

iii. Enables organelles to assume their typical shapes

iv. Enables membrane budding and fusion

c. Proteins: large component of model

i. Integral membrane

ii. Lipid-anchored


What is the composition and function of the cell membrane?



iii. Peripheral

2. Bilayer structure of biomembranes

a. Composed of amphipathic phosphoglycerides, sphingolipids, and cholesterol If you want to learn more check out What are the 3 types of intermolecular forces?
If you want to learn more check out What does a data management platform do?

b. Formation of bilayer is energetically favorable

i. Forms spontaneously

ii. Van der waals interactions between hydrocarbon tails iii. Polar head groups interact through ionic and H-bonding to stabilize each other

3. Formation of Lipid Bilayer

a. Increased concentration of fatty acids in water form micelles i. Hydrophobic interior composed entirely of fatty acyl chains b. High concentration of phospholipids in water form bilayers 4. The faces of cellular membranes

a. Plasma membrane

b. Vesicles and some organelles


What are the molecules responsible for membrane transport?



i. Bounded by single membranes

c. Nucleus, mitochondrion, and chloroplast organelles

i. Enclosed by two membranes

d. Different cell types have different combos of lipids-regulated by cell intentionally 

5. The chemical composition of membranes

a. Composition

i. Lipid and protein components bound by noncovalent bonds ii. Contain carbs

b. lipids=amphipathic

i. Phosphoglycerides: most abundant

ii. Sphingolipids

iii. Sterols

c. Phosphoglycerides

i. Glycerol backbone; tails; head-a polar group esterified to a phosphate; highly amphipathic

ii. 4 major head groups If you want to learn more check out What we usually think of when we think about personality?

1. Phosphatidylcholine (PC)

2. Phosphatidylethanolamine (PE)

3. Phosphatidylserine (PS)

4. Phosphatidylinositol (PI)

iii. Plasmalogens: one fatty acyl chain attached to glycerol and one attached by an ether linkage and same head group as others

d. Sphingolipids: fatty acid and sphingosine and 2 long tails i. Sphingomyelins

ii. Glycolipids

e. Sterols: membrane components (in animals=cholesterol) i. Can be very prevalent in membranes

ii. Weakly amphipathic

6. Membrane lipids and fluidity

a. Exist in gel or fluid-like consistency depending on temp

b. Fluidity required for optimal performance 

c. Transition temp of phosphoglycerides

i. Temp where lipids change from liquid to gel

ii. Depends on length and degree of unsaturation in fatty acyl chains

iii. Must adjust fluidity for different environmental

temperatures

1. Saturases remove C double C bond If you want to learn more check out What are the different chemical processes?

2. Desaturases add C double C bond

3. Reshuffle chains to get different packing

d. Gel: favored by longer, more saturated fatty acyl chains; strong van der Waals interactions

e. Fluid: short fatty acyl chains form fewer van der Waals interactions; unsaturated

f. Role of cholesterol-prevents regular packing of saturated fatty acyl chains; tends to abolish sharp transition temperature; without it, membranes would crystallize We also discuss several other topics like Why does heart failure cause pink sputum?

i. Enhances rigidity while preserving fluidity

ii. Insert themselves into membrane

7. Asymmetry of Membrane Lipids

a. Membrane lipids move easily within a leaflet but only rarely flip-flop

b. Exoplasmic leaflet on outer layer while cytosolic leaflets on inner layer

8. Effect of Lipid Composition on Bilayer Thickness and Curvature a. Pure sphingomyelins (SM) bilayer: thicker than phosphoglyceride bilayer; increase thickness

b. Phospholipids: PC and PE form a natural curvature

9. Lipid Rafts: in outer leaflet of plasma membrane

a. Sphingomyelin and cholesterol organize into “rafts” that float within the more fluid environment 

b. Provide a favorable environment for cell surface receptors and GPI-anchored proteins

10. Membrane Proteins

a. Integral proteins/membrane spanning proteins We also discuss several other topics like How to read polls?

i. Penetrate and pass thru lipid bilayer

ii. Are amphipathic

iii. Parts exist on either side of membrane 

iv. Often hydrophobic ɑ helices

v. Can assume beta barrel configuration

vi. Quaternary structure of these possible

b. Peripheral Proteins 

i. Attached to membrane by weak noncovalent bonds and easily solubilized

ii. Located entirely outside bilayer

c. Lipid-anchored proteins: covalent attachment 

i. On either face of bilayer

11. Membrane Carbs

a. Covalently linked to lipids and proteins on extracellular surface of bilayer

b. Glycoproteins have short, branched carbs for interactions with other cells

12. Mechanisms of Transport of cholesterol and phospholipids between membranes

a. Vesicles bud off and fuse with target membrane to transfer lipids b. Lipids transferred directly by membrane embedded proteins between contacting membranes

c. Small, soluble lipid transfer proteins mediate transfer

Transmembrane Transport of Ions and Small Molecules 1. Movement Across Cell Membranes

a. Selective permeability

i. Net flux: difference between influx and efflux of materials ii. Flux can occur by passive diffusion or active transport 2. Energetics of Solute Movement

a. Diffusion: spontaneous movement from high concentration to low concentration

i. Can reach equilibrium

ii. Free energy change depends on concentration gradient for nonelectrolytes

iii. For electrolytes free energy change depends on

electrochemical gradient (can only let through positive or negative charges)

b. Passive transport: high to low concentration, exergonic, spontaneous

c. Active transport: nonspontaneous, endergonic, low to high concentration, needs an external energy source to go against its gradient

3. Relative Permeability

a. Pure phospholipid bilayer

i. Permeable: many gases and small, uncharged, water-soluble molecules (polar)

ii. Semipermeable: water, ethanol, urea

iii. Impermeable: anything w a charge (ions), large polar molecules

iv. Large nonpolar molecules can go through 

v. The more nonpolar, the easier it moves through the

membrane

4. Membrane Transport Proteins

a. Transport proteins: form a protein-lined pathway across membrane that enables hydrophilic substances to move thru membrane without contacting hydrophobic interior

b. 3 main classes:

i. Channels (passive)

1. Facilitate movement of specific ions or water down

electrochem gradient

2. Nongated open all the time

3. Gated open only in response to signals from chemicals or electrical

ii. Transporters (active)

1. 3 groups that facilitate movement of specific small

molecules or ions

2. Uniporters, symporters, and antiporters

iii. ATP-powered pumps (active)

1. Energy released by ATP hydrolysis drives movement

against electrochemical gradient

2. Directly phosphorylate pump=covalent

3. Binding and hydrolysis of ATP=noncovalent

c. Multiple membrane transport proteins function together in plasma membrane

d. Facilitated (uniport) transport is faster and more specific than simple diffusion

e. GLUT Uniporters: in pmembrane of most mammalian cells; transport glucose

i. Rate of substrate movement far higher than simple diffusion ii. Transported molecule partition coefficient k is irrelevant iii. Transport is reversible

iv. Vmax depends on # of uniporters

v. Transport is specific

5. The Diffusion of Water Through Membrane

a. Osmosis: spontaneous diffusion of water thru a semipermeable membrane that goes from low conc to high conc

b. Cells swell in hypotonic

c. Cells shrink in hypertonic

d. Remain unchanged in isotonic

e. Aquaporins: specialized protein channels that only transport water

6. 4 Classes of ATP-powered transport proteins

a. P-class pumps: named for transfer of phosphate to transporter

b. V-class pumps: complex; acidifies by pumping in H+; use ATP hydrolysis

c. F-class pumps: similar to V-class pumps; synthesize ATP in mitochondria and bacteria

d. ABC Superfamily: ATP-binding cassette; xenobiotic transporters; primarily efflux pumps responsible for drug resistance

7. Operational model of pmembrane sodium/potassium pump a. Phosphorylation state produces conformational change for numbers of Na+ or K+ it wants

b. Used to establish an electrochemical gradient

8. Generation of a transmembrane electric potential (voltage) depends on the selective movement of ions across a semipermeable membrane a. Na+ and Cl- pumped out, K+ pumped in

9. Resting vs Action Potential

a. Resting Potential: membrane potential of a nerve or muscle cell, subject to changes when activated (-70)

b. Action Potential: when cells stimulated, Na+ channels open, causing membrane depolarization

c. Refractory period: Na/K p-type pump re-establishing membrane potential

d. 90% of energy in cell comes from electrochemical gradients 10. Movement of Ions Across Cell Membranes

a. Diffusion

b. Membranes impermeable to ions EXCEPT thru ion channels i. Integral membrane proteins

ii. Ion channels let ions pass thru

iii. Selective and bidirectional in direction of electrochemical gradient

11. Types of Gated Channels

a. Voltage gated channel: responds to charge potential

b. Ligand gated channel: ligand binds to channel (allosteric regulation) ligand not transported thru channel

c. Mechano-gated channel: dependent on stretch or pressure

12. Co-Transport: type of active transport; energetically favorable a. Coupling active transport to existing ion gradients

b. 2 different molecules going together

c. Symports: transport in same direction

d. Antiports: transport in opposite directions

13. Sodium/Glucose Symporter

a. Secondary transport: the Na+ gradient helps to transport glucose by a Na+/glucose co-transporter

b. Symporter

c. Na+ (exergonic); glucose (endergonic)

Cellular Energetics Part 1:Glycolysis 

1. Metabolism: collection of biochemical reactions that occur within a cell a. Metabolic pathways: sequences of chem reactions

i. Catabolic pathways: larger to smaller molecules; release E ii. Anabolic pathways: invested E makes complex molecules from smaller ones; uses ATP and NADH

2. Redox Reactions: Leo ger-losing electrons oxidizing, gaining electrons reducing

a. Gain H+: hydrogenation reaction

b. Lose H+: dehydrogenation reaction

c. Strong reducing agents+weak oxidizing agents

d. Strong oxidizing agents +weak reducing agents

e. Sequential Redox Reactions: Electron Transport Chain (ETC) i. Ae-+B couple then A+Be

ii. Essentially power/energy generators

f. Redox of organic compounds

i. Energy released when carbon oxidized, rich in energy

because of numerous hydrocarbon bonds

1. Enough energy released from one glucose to generate

36-38 ATPs

ii. Catabolism of glucose occurs in series of small steps in order to capture energy 

3. Adenosine + ribose=adenosine

a. 1 phosphate is AMP; 2 phosphates is ADP; 3 phosphates is ATP b. Connected with phosphoanhydride bonds

c. ATP is ideal link between pathways

4. The Glycolytic Pathway

a. Glycolysis occurs in cytoplasm 

b. Harvests only a fraction of energy from glucose

c. Anaerobic pathway

d. Series of 10 reactions degrades one 6-carbon glucose to two 3-carbon pyruvates

i. 7 energetically unfavorable steps

ii. 1,3, and 10 are energetically favorable; drive entire pathway forward 

5. Energy Investment to get Reactions Going

a. Hydrolysis of ATP in 2 of first 3 reactions

i. 1st step: activates glucose w addition of phosphate

(hexokinase)

ii. 3rd step: adds second phosphate to fructose-1,6

bisphosphate (phosphofructokinase-1)

iii. Steps 4 and 5: fructose 1,6-bisphosphate split into 2 molecules of G3P

iv. 1,6-carbon sugars to 2-3 C phosphate 

b. Remaining 5 reactions generate ATP and NADH

i. Step 6: 2 NADH from Pi 

ii. Step 7: 2 ATP from 2ADP 

iii. Step 10: pyruvate kinase 

1. NADH, FADH2,NADPH 

iv. Substrate level phosphorylation for all 3 above 

6. Mechanisms by which Cells Phosphorylate ADP->ATP a. Substrate level phosphorylation

b. Oxidative phosphorylation: mitochondria

c. Photophosphorylation: chloroplasts

7. Net Equation

a. Glucose+2 ADP+2 Pi+2NAD+ yields 2 pyruvate +2 ATP +2 NADH +2 H+ +2 H2O

8. Regulation of Metabolic Pathways

a. Regulation of glucose uptake

i. Most cells Km= 1 mM

b. Concentration of rate-limiting enzymes in pathway

i. hexokinase , phosphofructokinase, and pyruvate kinase are irreversible steps

ii. Insulin stimulates transcription

iii. Glucagon deregulates transcription

c. Allosteric regulation of enzymes: noncovalent

i. Hexokinase can be allosterically inhibited by

glucose-6-phosphate

ii. Phosphofructokinase inhibited by ATP, activated by AMP iii. Pyruvate kinase inhibited by ATP, activated by

fructose-1,6-bisphosphate

d. Covalent modification of enzymes

i. Phosphorylation of pyruvate kinase slows it down;

dephosphorylated pyruvate kinase more active

9. Anaerobic vs Aerobic Metabolism of Glucose

a. Absence of oxygen: fermentation (inefficient)

b. Presence of oxygen: pyruvate in mitochondria citric acid cycle 10. Mitochondrial Structure and Function

a. Bean-shaped organelles that can change shape

b. Size and number differ from cell to cell

i. Low energy needs less

c. Outer boundary/outer membrane

i. Enclose intermembrane space

ii. Inner membrane has 2 connected domains

1. Inner boundary membrane

2. Cristae: folds where ATP machinery is at

d. Matrix: contains circular DNA, ribosomes, and enzymes

e. Mitochondrial membranes have peptidoglycan from eukaryotes i. Inner membrane: projects into interior of mitochondrion 1. invaginations=cristae

ii. Cristae increases surface area for ATP production 

f. Mitochondria undergo binary fission to increase number or can fuse with one another to regulate morphology

g. No multicellular organism can exist solely by anaerobic

metabolism has ever been identified

Cellular Energetics Part 2: TCA and Fatty Acid Oxidation 1. Glycolysis and Aerobic Respiration

a. Anaerobes populated early earth; O2 accumulated in atm after cyanobacteria appeared

b. Aerobes use oxygen to extract more energy from org molecules 2. Citric Acid Cycle (TCA): nine sequential reactions oxidize acetyl CoA to CO2 capturing high E e-’s in NADH and FADH2

a. Reaction 1: 2 acetyl CoA condenses with 4-C oxaloacetate to create 6-C citrate

b. Many combine steps 2 and 3

c. ATP formed in glycolysis and TCA cycle forms 6 ATP but the net is 4 ATP

3. Pyruvate Dehydrogenase Complex

a. Oxidizes pyruvate into acetyl CoA

b. Precedes TCA

c. CO2 released

d. Occurs in mitochondrial matrix from cytosol 

e. In bacteria and archaea, process occurs in cytosol 

4. GTP is equivalent to ATP: Steps 5 and 6

a. Substrate level phosphorylation: succinyl-CoA-its potential E is used to drive formation of an E rich phosphate bond. GTP formed

and used to synthesize ATP by actions of another enzyme-swaps nitrogenous bases

5. By end of TCA cycle- glucose fully oxidized

a. 4 NADH, 1 FADH2, and 1 GTP/ATP for each pyruvate (2

pyruvates)

b. Purpose of TCA cycle is NOT to yield ATP, it is to produce high energy electron carriers (NADH and FADH2) that’re used to produce ATP through oxidative phosphorylation 

6. NADH shuttles

a. Glycerol-3-phosphate shuttle(36 ATP): NADH enters as FADH2 in complex

b. Malate-aspartate shuttle(38 ATP): NADH from glycolysis enters as NADH in complex 1

7. Energy for ATP Generation

a. About 3 ATPs from NADH, 2 ATPs from FADH2, 2 ATPs per pair shuttled from G3P shuttle

b. 3 ATPs per pair shuttled from malate aspartate shuttle

8. Oxidation of Fatty Acids in Mitochondria

a. Generates ATP (purpose of it) 

b. Acetyl CoA

c. NADH and FADH2 electron carriers

9. Fats

a. Significant source of energy

b. Stored as triglycerides

c. Fatty acids removed from glycerol and CoA attached to carboxyl end d. Fatty acyl CoA transported into mitochondria

i. Undergoes beta oxidation in mitochondria

ii. Produces 1 acetyl CoA molecule w each turn of TCA cycle 10. The ETC and Generation of Proton-Motive Force 

a. Stage III flow of e- from NADH/FADH2 thru ETC provides E to drive proton transport across inner mitochondrial membrane generating a proton-motive force

b. Composed of five types of e- carriers

i. Prosthetic groups in 4 of 5 types

ii. Many part of large complexes with more than one e- carrier 1. Flavoproteins: FAD or FMN, FMN in complex 1=NADH, FAD in complex II=FADH2

2. Cytochromes: contain heme prosthetic groups w iron

3. Three copper atoms: alternate btwn Cu2+ and Cu3+

4. Iron-Sulfur Proteins: iron linked to non-heme sulfur centers 5. Ubiquinone (CoQ): no prosthetic group, lipid soluble,

embedded inside mitochondrial membrane

11. E- carriers part of 4 membrane-spanning complexes

a. Only goes I to III to IV or II to III to IV

b. Carriers arranged in order of increasing redox potential form i. I, III, II, IV

c. E-’s passed from one to the next, last e- acceptor is O2

d. Complex II does not transport protons 

e. 3 places during transport w substantial release of free energy i. Occurs as e-’s transferred to next carrier

ii. Protons picked up by molecules when reduced

iii. Oxidation tends to release proton

f. NADH= 10 H+ pumped across 

g. FADH2= 6 H+ pumped across 

12. Oxidative Phosphorylation: indirect E input into ATP synthesis; direct E input into rotational catalysis but no transfer of a high E phosphate bond a. Spontaneous and energetically favorable

14. ATP Synthase: both synthesizes and hydrolyzes ATP

a. Fo base and an F1 sphere

i. F-type ATPase

15. The Machinery for ATP Formation: binding change mechanism a. Binding change mechanism states: movement of protons thru ATP synthase alters binding affinity of active site

i. Each active site goes thru distinct conformations

ii. 360 degree rotation of sphere=3 ATPs formed

iii. Binding sites can be open or closed

b. ATP Production: ᇫG= +7.2 kcal/mol

i. ADP +Pi =ATP

ii. Very tight conformation=influences Keq 

1. Keq= [1 ATP]/[1 ADP][1 Pi] =1 

2. ᇫG=0 

iii. Rotation prevents reverse reaction from occurring, so no eq stopping ATP production and no hydrolysis of ATP=surplus ATP Cellular Energetics Pt 3 

1. Heterotrophs: depend on external energy source of organic compounds 2. Autotrophs: capable of living on CO2 as principal carbon source a. Chemoautotrophs: use chem energy stored in inorganic molecules b. Photoautotrophs: use radiant energy of sun to convert CO2 into organic compounds; carry out photosynthesis

3. Photosynthesis: principal end products are O2 and polymers of 6 carbon sugars; light capturing and ATP generating photosynthesis reactions occur in chloroplast thylakoid membranes 

a. Oxygen generated in light dependent reactions

b. Oxidizes H2O to O2; respiration reduces O2

c. Reverse of mitochondrial respiration

d. Requires energy from sunlight

e. Plants also use mitochondrial respiration

f. Two Reactions 

i. Light dependent: ATP and NADPH (steps 1-3); occurs in thylakoid membrane

ii. Light independent: need ATP and NADPH from light

dependent (step 4); occurs in stroma

g. Structure of Leaf and Chloroplast

i. Chloroplast photosynthesis: produces energy rich sugars broken down for energy by mitochondria

ii. Stroma: enzymes to oxidize CO2

iii. Thylakoid membrane: contain chlorophylls

iv. Granum: stack of adjacent thylakoids

4. Absorption of Light

a. Low energy e- taken from water during photosynthesis 5. Stages of Photosynthesis:

a. Absorption of light energy

b. E- transport

c. ATP synthesis

d. Light independent reaction

6. Light absorbed by thylakoid membrane pigments

a. chlorophyll-A: absorbs violet-blue and red light most strongly b. Chlorophyll A and B in plants

c. Carotenoids: absorb light in blue-green region

i. Allow chloroplasts to gain energy from more spectra of light ii. Accessory pigments

7. Primary event in photosynthesis: photoelectron transport a. Transport from energized reaction center chlorophyll-A pair i. Located close to thylakoid membrane lumen face

b. Multiprotein light-harvesting complex: in cyanobacterium i. Six reaction center chlorophyll molecules

ii. Two special pair chlorophylls initiate transport

iii. Two bridging chlorophylls transfer energy to special-pair chlorophylls

iv. Photosynthetic units: each has several hundred chlorophylls 8. 2 Plant Photosystems: PSI(P700) and PSII(P680)

a. PSI- boosts e- to a level above NADP+

b. PSII- converts H2O to O2; gets e- from H2O (photolysis) c. PSII->PSI

i. Linear phosphorylation/carb synthesis

9. Cytochrome Bf Complex: accepts e- from QH2

a. Generates proton motive force

b. Bf complex Q cycle translocates additional protons across membrane into thylakoid membrane

10. PSI reaction center (P700)

a. Each e- released moves to stromal surface

b. Reduces NADP to NADPH

i. Making an e-carrier for light independent reactions 

c. PSII gives an e- used to reactive PSI complex

11. Q cycle: flow of e- from PSII to PSI

a. Plastocyanin carries e- to PSI from PSII

12. Contributions to Protein Gradient

a. Splitting of H2O in lumen releases H+

b. Translocation of plastoquinol from stromal side of membrane to lumen side and subsequent oxidation

c. Oxidation of NADP+ in stroma

13. Cyclic Photophosphorylation: ONLY produces ATP and DOES NOT produce NADPH 

a. Carried out by PSI independently of PSII

b. E- constantly cycle thru PSI and cytochrome Bf complex c. Thought to provide addl ATP; common under stress conditions in plants

d. No carb synthesis

e. LHCII moved from front PSII to back PSI

14. CO2 metabolism during photosynthesis

a. Calvin Cycle fixes CO2 into organic molecules in reactions in chloroplast stroma 

b. Occurs in C3 plants

c. Primary way atmospheric carbon is fixed

d. Calvin Cycle

i. Carboxylation of RuBP to form PGA

ii. Reduction of PGA to GAP using NADPH and ATP from light reactions

iii. Step 1: carboxylation

1. Rubisco: large multisubunit complex; 3

molecules/sec; most abundant protein on Earth

iv. Step 2: reduction (2 reactions)

1. Consumes ATP and NADPH 

2. 1 G3P for every 3 turns of the Calvin Cycle

v. Step 3: regeneration (multiple reactions)

1. GATP->ribose 1,5 bisphosphate

vi. Equation: 6 CO2 +12 NADPH + 18 ATP yields C6H12O6+18 ADP+ 18 Pi+ 12 NADP

15. Photorespiration: uptake of O2 and release CO2 

a. Rubisco also catalyzes attachment of O2 to RuBP to produce 2-phosphoglycolate(toxic to cell) 

b. Happens if temp high and/or ratio of oxygen to CO2 esp high c. Produces no ATP and leads to net loss of C and N which slows plant growth

d. Uses chloroplast, peroxisome, mitochondrion

e. Highly energy intensive process (consumes ATP)

16. Alternate Pathways for Carbon Fixation

a. C4 Plants: used in hotter climates

i. CO2 taken up thru stomata on leaf surface

ii. H2O lost thru stomata when open; so they close during day iii. Developed to increase conc of CO2 delivered to rubisco to be more efficient (in bundle-sheath cells)

iv. Produces PEP that combines w CO2 to produce oxaloacetate or malate

b. CAM Plants: carry out light reactions and CO2 fixation at diff times of day using the enzyme PEP carboxylase

i. Stomata open at night, CO2 collected at night

ii. Carboxylation of PEP

iii. Malic acid stored in vacuole to be used in daylight

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