Exam 3

amt of membranes in prok v.s. euk cells

prok → 1 plasma membrane to separate cell from outside

euk → add’l internal membranes enclose indiv organelles

what inhibits ions (ex. Cl-) from passing thru plasma membrane?

the hydrophobic interior of plasma membrane

3 important functions of plasma membrane

1. receptor proteins in it act as sensors that enable cell to receive info abt changes in its environment & respond to them

2. passes nutrients inwards & waste products outwards

3. expands for cell growth & mvmt

the most common phospholipid in cell membranes is ____. describe its parts.

• Phosphatidylcholine

• its parts:

  • Choline: the hydrophilic head

  • Phosphate: links choline to glycerol

  • Glycerol: links phosphate to 2 hydrocarbon chains

  • 2 hydrocarbon chains: which originate as fatty acids

why are amphipathic phospholipids useful?

• form a lipid bilayer

• separate aqueous environment inside cell from aqueous environment outside cell

  • hydrophilic heads = attracted to water

  • hydrophobic tails = don’t like water

describe the self-sealing qualities of cell membrane

• created by same forces which make it amphipathic

• this closed structure = stable bc it avoids exposure of hydrophobic hydrocarbon tails to water, which would otherwise be energetically unfavorable

what are liposomes?

a pure phospholipid in a close, spherical formation

what influences the fluidity of the phospholipid bilayer?

• Temperature

• Types of hydrocarbon tails (phospholipid composition)

• Hydrocarbon tails that do not pack closely together inc the fluidity of the membrane (saturated vs unsaturated tails)

  • Unsaturated fatty acid tails increase the fluidity

• Short tails inc fluidity of membrane

how are lipid bilayers synthesized?

• Newly synthesized phospholipid molecules are added to cytosolic side of the ER membrane

• Scramblases then transfer some of these molecules to the opposite monolayer so that the entire bilayer expands

  • phospholipids randomly distributed

• Flippases (enzyme found in Golgi) help maintain asymmetry of plasma membrane in animal cells

  • fixes random distribution and makes sure phospholipid distribution is asymmetric

purpose of cholesterol in cell membrane

• cholesterol fits into gaps btwn phospholipid molecules in lipid bilayer → stiffens it → makes it more rigid & less permeable

• can also prevent hydrocarbon chains from coming tg & crystalizing

how are phospholipids & glycolipids distributed in cell membrane in relation to each other?

• they’re distributed asymmetrically

the 2 sides of the lipid bilayer are…

1. cytosolic: faces cytosol

2. noncytosolic: faces exterior of cell OR interior of internal organelle

Membranes ____ (do/don’t) retain their orientation even after transfer between cell compartments

• DO

• original cytosolic surface remains facing the cytosol; Noncytosolic surface continues to face away from the cytosol

describe glycolipids

• they’re located mainly in plasma membrane

• found only in the Noncytosolic half of the bilayer

• sugar groups facing exterior of cell

• they acquire their sugar groups in Golgi (since enzymes that add sugars are confined inside Golgi)

what are transmembrane proteins?

• protein that goes all the way through membrane, and is sticking out from both sides

describe a transmembrane hydrophilic pore

• Five transmembrane α helices forma water-filled channel across the lipid bilayer

• The hydrophobic amino acid side chains on one side of each helix contact the hydrophobic hydrocarbon tails, while the hydrophilic side chains on the opposite side of the helices form a water filled pore

what are monolayer-associated membrane proteins?

protein only on one side of the membrane

what are lipid-linked membrane proteins?

lipid connects protein to membrane

what are protein-attached (peripheral membrane) proteins?

sticking out from membrane

if you want to break up a lipid bilayer, you have to…

• use a detergent

• Small amphipathic lipid-like molecules which only have one hydrophobic tail

• Spontaneously aggregate into clusters, or micelles, in water

• Able to squeeze in btwn lipids & proteins in plasma membrane, separate them, & keep them in water soluble complexes

what is a cell cortex?

• meshwork of fibrous proteins that are attached to the cytosolic surface of the membrane

  • cell membrane lays on top of it

• determines shape of cell AND its mechanical properties

what’s spectrin?

• main component of cell cortex

• a long, thin, flexible rode and is attached to cytosolic surface of membrane

• Spectrin dimers, tg w/ a smaller # of actin molecules, are linked tg in a netlike meshwork that is attached into plasma membrane by binding of at least 2 types of attachment proteins to 2 kinds of transmembrane proteins

  

how is the lateral mobility of the plasma membrane proteins restricted?

Proteins can be:

• Tethered to the cell cortex inside the cell

• Tethered to the extracellular matrix molecules outside the cell

• Tethered to proteins on the surface of another cell

• Diffusion barriers which can restrict proteins to a particular membrane domain

The eukaryotic cell surface is coated with ____. describe them/their functions

• sugars

• Attached to both lipids and proteins in the extracellular face only

• Functions:

  • Absorb water

  • give the cell a slimy surface

  • prevent blood cells from sticking to one another or to the walls of blood vessels

  • Play an important role in cell-cell recognition and adhesion

describe neutrophils, oligosaccharides, & their roles in battling infection

• cell-surface carbohydrate on neutrophils (aka WBC) is recognized by the cells lining the blood vessels at sites of infection

• Oligosaccharides (short sugar chains) carried by glycolipids and glycoproteins on the extracellular faces of neutrophils

• Oligo. recognized by lectins on the endothelial cells lining the blood vessels @ site of infection

• Provide “ID” tags for cells, which are recognized by immune system, and unfortunately, also pathogens

• neutrophil now able to migrate to infected tissue

  

describe bacteriorhodopsin

• transporter protein that crosses the lipid bilayer as 7 α helices

• acts as a proton pump during the light-activated pumping cycle

• When retinal (chromophore) absorbs a photon of light, it changes shape, & causes the protein to undergo a series of small conformational changes

• The changes result in the transfer of 1 H⁺ moving across the bilayer along the pump channel

• In presence of sunlight, thousands of bacteriorhodopsin pump H⁺ out of cell, generating a concentration gradient of H⁺ across the plasma membrane, which can be converted into ATP

how can photobleaching techniques measure the rate of lateral diffusion of a membrane protein?

• FRAP: Fluorescence Recovery After Photobleaching

• GFP: Green Fluorescent Protein derived from a jellyfish

  • glows when exposed to correct wavelength of light

• The membrane protein of interest is tagged with GFP

• The cell expressing the GFP fusion protein is irradiated with a laser beam that irreversibly bleaches the fluorescent group in a spot

• rate of “fluorescence recovery” = diffusion rate of tagged membrane bound proteins

what is Single-particle tracking (SPT) microscopy, and why is it useful?

• Labeling proteins w/ antibodies coated w/ gold particles

• The gold spheres appear as tiny black dots under the microscope

• The movement of individually tagged proteins can be tracked using video microscopy

• The patterns of movement range from random diffusion to complete immobility

• Some proteins switch between these different kinds of motion

  

what kind of molecules can pass rapidly across the bilayer?

• small hydrophobic molecules

• small uncharged polar molecules

Na⁺ is the most plentiful positively charged ion (cation) ____ the cell, while K⁺ is the most plentiful ____

outside; inside

transporter v.s. channel proteins

• transporter: undergoes conformational changes to transfer specific molecules across the membrane

• channel: Has a hydrophilic pore, through which specific molecules can diffuse

describe passive transport

• No energy required

• If a molecule moves from an area of high concentration to an area of low concentration, then the molecule is moving down its concentration gradient

• Simple diffusion: no transport protein required

• Facilitated diffusion: transport proteins required

describe active transport

• Requires energy

• If a molecule moves from an area of low concentration to an area of high concentration, then the molecule is moving up its concentration gradient

describe the 2 conformational states of a transport protein

• The binding sites for the solute are exposed on the outside of the membrane

• The same sites are exposed on the other side of the membrane

Both _______ and _______ drive passive transport

concentration gradients; electrical forces

Electrochemical gradient is determined by what 2 factors?

• Concentration gradient of an ion

  • it overrides every other factor that can influence

• Distribution of positive and negative charges on either side of the membrane called membrane potential

describe the 3 diff types of active transport

• Coupled Transport: mvmt of one molecule down its concentration gradient is coupled to the movement of another molecule up its concentration gradient

• ATP-driven pump: mvmt of one molecule up its concentration gradient is coupled to the hydrolysis of ATP to ADP

• Light-driven pump (most are in bacteria): mvmt of one molecule up its concentration gradient is coupled to the light energy

Why does a cell need to control the concentration of Na⁺ and K⁺ across its membrane?

• cell must maintain high concentration of Na⁺ on the extracellular side of the membrane, as the movement of Na⁺ into the cell is coupled to the movement of other molecules across the membrane (coupled transport)

• cell must maintain high concentration of K⁺ on the cytoplasmic side of the membrane for proper functioning of the nervous system

purpose of ATPase

• enzyme that converts ATP to ADP

• component of Na⁺-K⁺ pump

6 steps of sodium-potassium pump

1. Na+ binds to protein transporter

2. pump phosphorylates itself, hydrolyzing ATP'

   1. phosphate is in high-energy linkage w/ transport protein

3. phosphorylation triggers conformational change & Na+ is ejected

4. K+ binds

5. pump dephosphorylates itself

   1. phosphate removes itself from transport protein

6. pump returns to OG confirmation & K+ is ejected inside cell

describe the Ca²⁺ pump & why it’s important

• Intracellular signaling often involves an increase of intracellular (cytosolic) Ca²⁺, therefore, the intracellular concentration of Ca²⁺ must be kept low

• Similar to Na⁺-K⁺ pump: requires ATP

• Contains ATPase within pump

describe the 2 types of couple transport

• Symport: 2 molecules are transported in the same direction across the membrane

• Antiport: 2 molecules are transported in the opposite directions across the membrane

describe uniport

only one molecule is transferred across the membrane

The glucose-Na⁺ symport protein uses the ______ to drive the import of glucose via _____

• electrochemical Na⁺ gradient

• symport

H⁺ is pumped into ______ in animal cells and _____ in plant cells to control the _______ & _______ of _________

• lysosomes

• vacuoles

• pH & function of organelles

If the concentration of solutes inside a cell is higher than outside, then…

water will move in by osmosis, causing the cell to swell (cell lysis)

If a cell is placed in a high-salt solution, then…

water will rush out (crenate)

describe aquaporins

• allow the selective passage of water molecules across a membrane (osmosis)

The majority of channels in animals and plants are _____ (describe them)

• ion channels

• Depends on the diameter/shape of an ion and the amino acids in the interior of the pump (the ion will squeeze through these)

• Opposite charges work best

• Because ions are surrounded by water, the channels can get “jammed”, forcing the flow of ions to slow down

• Most ion channels are also gated (switch between open and closed confirmation)

describe voltage-gated channels

• Controlled by membrane potential

• major player in transmission of signals between nerve cells

• Controlled by extremely sensitive voltage sensors

  • (Specialized charged protein domains on ion channel proteins)

• Changes in the membrane potential above certain threshold → exerts sufficient electrical force on voltage sensors → channel to opens/closes

describe ligand-gated channels

Controlled by binding of a ligand (such as neurotransmitters) to the channel protein (receptor)

describe stress-activated channels

• Controlled by mechanical force

• Ex: sounds trigger the opening of ion channels in ear hair cells

how do stress-gated ion channels allow us to hear?

• basilar membrane below and the tectorial membrane above are not lipid bilayer membranes, but sheets of extracellular matrix

• Sound vibration causes the basilar membrane to vibrate up and down, causing the stereocilia to tilt

• Each stereocilium on each hair cell is attached to the next shorter stereocilium by a fine filament

• The tilting stretches the filaments, which pull open stress-gated ion channels in the stereocilium membrane, allowing positively charged ions to enter from the surrounding fluid

• The influx of ions activates the hair cells, which stimulate underlying endings of the auditory nerve fibers that convey the auditory signal to the brain.

Ions on each side of the membrane decide the _____. define it

• membrane potential = Voltage diff across membrane due to a slight excess of positive ions on one side and of negative ions on the other

A typical potential for an animal cell plasma membrane is about ______, measured relative to the surrounding fluid

-60 mV (inside negative

why are K⁺ leak channels important for the membrane potential?

• The high intracellular concentration of K⁺ is in part generated by the Na⁺-K⁺ pump, which actively pumps K⁺ into the cells

• In a resting cell, K⁺ leak channels are the main ion channels open in the plasma membrane

what kinds of channels are used for action potentials, and how do they work?

• ligand-gated channels

• they open temporarily & cause depolarization of the plasma membrane (change in membrane potential) at that particular site

  • Membrane potential becomes less negative

• Depolarization stimulates opening of voltage-gated Na⁺ channels

  • Leads to further depolarization

how do rapid changes in membrane potential trigger an action potential?

• resting membrane potential: -60 mV

• stimulus depolarizes the plasma membrane by about 20 mV to make the membrane potential -40 mV

• -40 mV is the threshold value for initiating an action potential

• After an action potential is triggered, the membrane rapidly depolarizes and reaches +40 mV before it returns to its resting state

describe the 3 conformations a voltage-gated Na+ channel can adopt

• open (Na+ can flood into cell)

• inactive (unpolarized membrane; closed & CANNOT reopen)

  • this state allows us to ensure that action potential only travels in one direction

• closed (polarized membrane & membrane @ rest; ready to go)

In order for the plasma membrane to return to its resting membrane potential after experiencing an action potential, 2 events occur:

• Na⁺ channels automatically switch to an inactive state

• Voltage-gated K⁺ channels open

  • K⁺ moves out of the cell (down its electrochemical gradient in the depolarized cell)

synaptic vesicles contain…

neuorotransmitters

at the nerve terminal, an ____ signal is converted into a ____ signal. how?

• electrical; chemical

• When an action potential reaches a nerve terminal, it opens voltage-gated Ca²⁺ channels in the plasma membrane, allowing Ca²⁺ to flow into the terminal

• The increased Ca²⁺ in the nerve terminal stimulates the synaptic vesicles to fuse with the plasma membrane, releasing their neurotransmitter into the synaptic cleft

A ____ signal is converted into an ____ signal by transmitter-gated ion channels at a synapse. how?

• chemical; electrical

• The released neurotransmitter binds to and opens the transmitter-gated ion channels in the plasma membrane of the postsynaptic cell

• The resting ion flows alter the membrane potential of the postsynaptic cell, thereby converting the chemical signal back into an electrical one

excitatory vs inhibitory neurotrans

• Excitatory neurotransmitters activate ion channels that allow the passage of Na⁺

• Inhibitory neurotransmitters activate ion channels that allow the passage of Cl^-

Three stages of cellular metabolism lead from food to waste

1. breakdown of large food molecules to simple subunits

   1. mostly occurs outside cells, although lysosomes can digest large molecules in the cell interior

2. breakdown of simple subunits to acetyl CoA; limited amts of ATP & NADH produced

   1. occurs mainly in cytosol: except for the final step of conversion of pyruvate to acetyl groups on acetyl CoA, which occurs in the mitochondria

3. complete oxidation of acetyl grp in acetyl CoA to H2O & CO2; large amts of ATP produced on inner mitochondrial membrane

   1. occurs entirely in the mitochondria

3 general steps of glycolysis

1. energy investment to be recouped later

   1. Glucose is converted to fructose 1,6-bisphosphate

2. cleavage of 6-carbon sugar to two 3-carbon sugars

3. energy generation

explain step 1 of glycolysis

1. one molecule of glucose experiences catalysis of hexokinase, which phosphorylates the glucose

   1. irreversible reaction

2. rearrange molecule into fructose

3. fructose 1,6-bisphosphate produced by another phosphorylation w/ phosphofructokinase

   1. irreversible reaction

net results of glycolysis

• 2 pyruvate, 2 ATP, 2 NADH

• synthesis of ATP in glycolysis is known as substrate-level phosphorylation

  • Involves the transfer of a phosphate group directly from a substrate molecule

when animal muscle cells & certain bacteria are lacking oxygen, pyruvate is converted to _____. what’s this process called? how does it occur?

• lactate

• this process is anaerobic; it’s called fermentation

• glycolysis → pyruvate + NADH → lactate + NAD+

when yeast is lacking oxygen, pyruvate is converted to _____. what’s this process called? how does it occur?

• ethanol & CO2

• this process is anaerobic; it’s called fermentation

• glycolysis → Pyruvate --> acetaldehyde + CO2 --> ethanol (NADH is oxidized to NAD⁺)

how does the oxidation of pyruvate result in acetyl CoA?

• Pyruvate is oxidized to acetyl CoA & CO2 by pyruvate dehydrogenase

• pyruvate (3C) + NAD+ + CoA → acetyl CoA + CO2 NADH

• occurs in mitochondrial matrix

• one acetyl CoA per pyruvate

how does the oxidation of fatty acids result in acetyl CoA?

• long fatty acid chain → acetyl CoA + NADH + FADH2

• chops off 2C at a time from carbon chain → multiple acetyl CoA per pyruvate

• occurs in mitochondria

where does the citric acid cycle/Kreb’s cycle occur? why?

• in the mitochondria

• bc pyruvate & fatty acids are directed there for acetyl CoA production

the citric acid cycle catalyzes the complete ____ of ___ atoms in _____ _____

• oxidation

• carbon

• acetyl CoA

what are the net products of the Kreb’s cycle?

• 3 NADH

• 1 FADH2

• 1 GTP

• 2 CO2

per 1 acetyl CoA

products of each step of the Kreb’s cycle

Citrate Is Kreb’s Starting Substrate For Making Oxaloacetate

1. C: citrate

2. I: isocitrate

3. K: ketoglurate

4. S: succinyl CoA

5. S: succinate

6. F: fumarate

7. M: malate

8. O: oxaloacetate

Glycolysis and the TCA cycle provide the precursors needed to …. & give example

• synthesize many important biological molecules

• many of the intermediates formed are used for biosynthetic (anabolic) pathways, where they’re converted into amino acids, nucleotides, lipids, & other small organic molecules

To maintain order with cells, all organisms need to ____ through _____

• constantly replenish their ATP pools

• sugar or fat oxidation

Why do cells synthesize food reserves?

To store energy when food is plentiful for times of scarcity.

What is a key role of metabolic regulation?

Directing metabolites into anabolic or catabolic pathways.

pyruvate is a very common substrate; name some reactions it’s involved in

• Pyruvate dehydrogenase converts pyruvate to acetyl CoA

• Lactate dehydrogenase converts it to lactate

• Another enzyme can convert it to oxaloacetate

• Another can change it to alanine

_____ = “reverse” reactions from glycolysis; describe it

• Gluconeogenesis

• makes blood glucose from small non-carbohydrate organic molecules like lactate, pyruvate, or amino acids

• mainly in the liver

• requires energy

• bypasses glycolysis’s irreversible steps 1, 3, and 10 using 4 bypass reactions

• Both glycolysis & glycogenesis pathways are tightly regulated

Animal cells store ____ to provide energy in times of fasting; describe it

• glycogen

• its structure is very similar to that of starch in plants

  • branched polymer of glucose

  • starch has many few branch pts than glycogen

What regulates glycogen synthesis and breakdown?

• Low energy: Cells break down glycogen for energy.

• Glycogen phosphorylase: Breaks glycogen → glucose-1-phosphate → glucose-6-phosphate (enters glycolysis).

• Glucose-6-phosphate (G6P):

  • Activates glycogen synthase (glycogen synthesis).

  • Inhibits glycogen phosphorylase (glycogen breakdown).

• ATP also inhibits glycogen phosphorylase.

Why is fat a better energy store than glycogen?

• ~2× more energy per gram than glycogen.

• Glycogen binds water, making its stored mass ~6× greater for equal energy.

• Energy reserves: Glycogen lasts ~1 day; fat lasts nearly 1 month.

auditory hair cells are ____ gated

stress-mechnically

_____ leak channels are important for maintaing the resting membrane potential of -60 to -80 mV in animal cells

potassium

how is glucose absorbed from gut lumen into intestinal epithelial cells?

• Na+-glucose symporter on apical membrane uses Na+ moving down its electrochemical gradient to drive glucose uptake against its gradient (active transport)

• Na+/K+ pump uses ATP to move Na+ out & K+ in, maintaining this gradient

how does glucose leave intestinal epithelial cells, & where does it go?

• GLUT uniporters on basolateral membrane allow glucose to exit via facilitated diffusion

• moves down its concentration gradient into extracellular fluid for use by other tissues

what are key membrane regions & structures in intestinal glucose absorption?

• apical membrane faces gut lumen

• basolateral membrane faces extra cellular fluid

• tight junctions seal neighboring cells & maintain distinct membrane domains

• Na+ enters apically w/ glucose & is pumped out basolaterally using ATP

the high concentration of H+ ions can be found in…

intermembrane space (space btwn outer & inner membranes)

which part of ATP synthase is responsible for linking the diff moving parts?

stator, specifically its peripheral stalk

How does the electron transport chain (ETC) generate ATP?

• NADH and FADH₂ donate electrons to ETC in inner mitochondrial membrane

• e- pass through carriers to progressively lower energy states → released energy pumps H⁺ across the membrane → proton gradient created → ATP synthesis from ADP

• Finally, O₂ accepts the electrons and combines with H⁺ to form H₂O

  

The essential requirements for harnessing energy in the form of ATP are…

• A membrane containing a series of electron carriers (ETC), a pump protein, and an ATP synthase

• Sources of high-energy electrons derived from the oxidation of food and of protons

describe the stages of chemiosmosis during oxidative phosphorylation

Stage 1: Electron transport and proton pumping

• High-energy electrons release energy as they move through the electron transport chain.

• This energy powers proton pumps that move (H+) across the membrane, creating a high concentration of protons on one side.

Stage 2: ATP synthesis

• Protons flow back across the membrane through ATP synthase, moving down their electrochemical gradient.

• ATP synthase uses this energy to combine ADP and inorganic phosphate to produce ATP.

  

describe chemiosmotic coupling

• Uses energy from e- transport to pump H⁺ across inner mitochondrial membrane, creating a proton gradient

• H⁺ then flows through ATP synthase, driving ATP production from ADP + phosphate

describe some basic characteristics of mitochondria

• Contain own DNA and RNA

• Constantly change shape and position in the cell

• Position varies between cell types depending on where the majority of ATP is needed

a mitochondrion is organized into 4 separate compartments; describe them

• matrix: Innermost compartment; contains enzymes for pyruvate oxidation, fatty acid oxidation, and the citric acid cycle.

• inner membrane: Folded into cristae; contains the electron transport chain and ATP synthase for oxidative phosphorylation.

• outer membrane: Surrounds the mitochondrion; contains porins that allow small molecules (≤ 5,000 Da) to pass through.

• intermembrane space: Space between the inner and outer membranes; accumulates H⁺ during electron transport to create the gradient used for ATP synthesis.

how do NAD⁺ and NADH carry electrons and hydrogen during cellular respiration?

(A) Reduction: NAD⁺ → NADH

1. NAD⁺ accepts a high-energy electron and becomes a negatively charged intermediate.

2. It then accepts an H+ from the surrounding water.

3. The carrier becomes reduced NADH, carrying high-energy electrons.

(B) Oxidation: NADH → NAD⁺

1. NADH donates its high-energy electron to another molecule.

2. This creates a positively charged intermediate.

3. The intermediate releases H+ into the surrounding water, regenerating oxidized NAD⁺.

How does the ETC create a proton gradient in cellular respiration?

1. NADH and FADH₂ donate electrons to the ETC.

2. Their energy powers H⁺ pumps across the inner mitochondrial membrane, creating a gradient that drives ATP synthase.

3. O₂ accepts electrons and H⁺ to form H₂O.