Cell Bio Exam 2

characteristics of the membrane

• selectively permeable

• responsive to the environment

• capacity to change shape

cell membrane structure

Membranes are composed of lipid molecules

• amphipathic

• phospholipids are the most abundant type

Phospholipids spontaneously assemble into bilayers

Proteins are inserted into the membrane after the bilayer has formed

Bilayer formation

Hydrophilic (polar) molecules readily form H-bonds with aqueous surroundings

Hydrophobic (nonpolar) molecules cannot form H-bonds with water

Bilayer fluidity

the lipids can change places within the bilayer

not all membranes are created equal

membrane is not a homogenous molecule - many different types of lipids exist in a single bilayer

• different head groups, differences in the hydrocarbon tails

Factors that influence fluidity:

• length of the tail

• number of double bonds in the tail

• presence of additional lipids

• temperature

Fluidity depends on composition

the more tightly packed the lipids are the less fluid the membrane

the less tightly packed the lipids are the more fluid the membrane (unsaturated tails)

high temperatures increase fluidity and low temps decrease it because the phospholipids pack tighter together

additional lipids can be included in the membrane

animal cells modulate fluidity by adding cholesterol - a sterol molecule

the rigid steriod ring structure fills the spaces in the bilayer and gives structure

acts as a fluidity buffer

• high temps - helps stiffen the bilayer

• low temps - creates space between the lipids

phospholipids are assembled in the ER

• phospholipids are synthesized by enzymes associated with the ER

• synthesized as a single layer

  • new lipids are deposited only on the cytosolic side

  • creates an uneven number of lipids on each side of the bilayer

• Enzymes called scramblases redistribute lipids to make things even

  • selects lipids randomly

most membranes are asymmetrical

often an uneven distribution of different types of lipids between each monolayer

• glycolipids (lipids with sugar groups on one end) are located mainly on the extracellular side of the PM

• Glycolipids interact with extracellular matrix (either carbohydrate coat in animals called the glycocalyx or cell wall in plants) that protects the cell

membrane polarity is maintained between transfers

as vesicles move between the ER, golgi, and plasma membrane the two sides are maintained

• anything that is facing the cytosol will stay facing the cytosol whether its in the ER, golgi, or plasma membrane

Enzymes are required for asymmetry

membranes leave the ER move to the golgi where they encounter an enzyme flippase

• most flippases function at the golgi, but they can be found at any membrane

Flippases take specific lipids from one side of the membrane and flip it to the other

• each lipid has its own flippase

• some go cytosol to lumen side, or vise versa

Membranes are then moved to the plasma membrane, or other organelles

Scramblases are important for apoptosis

phosphatidylserine is always found on the cytosolic side

one of the first triggers of apoptosis is to activate a PM scramblase that removes asymmetry

phosphatidylserine on the extracellular serves as a signal for phagocytosis

membrane protein functions

proteins constitute up to 50% plasma membrane in animals

proteins have many different functions

receptor proteins

receive and transmit signals

anchor proteins

limit cytoskeleton to the membrane, or the extracellular matrix to the cell

transporters and channel proteins

move water, ions, nutrients, food, and waste

transmembrane proteins

extend all the way through the bilayer, with a portion extended out to the other side

made up of single or multiple alpha helices, or a beta sheet barrel

monolayer proteins

only anchored to one half of the PM

attached by a single alpha helix

integral proteins

are all secured to the membrane using hydrophobic interactions

lipid linked proteins

are attached to the membrane by lipid groups that are covalently attached to the protein

peripheral proteins

kept at the membrane via protein-protein interactions (hydrogen and ionic bonds)

what can cross the membrane by simple diffusion

hydrophobic molecules (O₂, CO₂, N₂)

Small uncharged polar molecules (H₂O, glycerol)

What crosses the membrane through facilitated diffusion?

large uncharged polar molecules (glucose and sucrose)

ions (Cl^-,K⁺)

hypertonic

more solute outside the cell vs inside (water will move out the cell)

hypotonic

more solute inside the cell vs outside (water moves into the cell)

glycogen

storage form of glucose that is not a solute

hypertonic - breaks down glucose

hypotonic - form glucose

passive transport

Solute moves down a concentration gradient and does not require energy

• simple diffusion: directly through the membrane

• facilitated diffusion: uses a protein

(carrier mediated, channel mediated, free)

electrochemical gradient

determines which way an ion will move based on:

• concentration (high to low)

• charge (opposites attract)

active transport

Solute moves against the concentration gradient, requires input of energy

Always requires a protein

(pump-mediated, carrier-mediated)

classes of transport proteins

channels and transporters

channel protein

passive only

moves ions

solute specifity determined by size and charge

transporter

passive or active

moves ions and uncharged molecules (lots of noncovalent interactions)

solute specifity determined by affinity for binding site (like an enzyme)

cell respiration

where cells harvest energy stored in food to make ATP

requires a membrane

mitochondria relation to bacteria

contain their own DNA (circular chromosomes, can translate/transcribe their own genes)

Divide by fission

Some genes from mitochondria have moved to the nucleus while other nuclear genes have moved to mitochondria

Mitochondria adaptability

Can change their shape, location, and number depending on what the cell needs

can fuse into long tubular networks

can rapidly increase division to meet high energy demands

mitochrondria organization

1. outer membrane

2. inner membrane

3. intermembrane space

4. matrix

mitochondria outer membrane

contains specialized porin channel proteins that allow large molecules (sugars and proteins) into the mitochondria

mitochondria inner membrane

Impermeable to ions and small molecules

highly convoluted forming a set of infoldings called cristae

contains ETC complex proteins

intermembrane space

region between the two membranes. because the outer membrane is so porous, it is very similar chemically to the cytosol

Matrix

Large internal space created by the inner membrane

Site of the citric acid cycle (Krebs)

Contains all the enzymes required for the breakdown of sugars

Mitochrondria also:

main Ca²⁺store

regulate immunity/immune cells

regulate apoptosis

regulate cell proliferation

serve as a location for biosynthetic pathways

help regulate membrane potential in neurons and detox ammonia in liver cells

Cellular respiration equation

C₆H₁₂O₆ + O₂ —> CO₂ +H₂O + ATP

Cellular Respiration

electrons derived from the oxidation of food molecules are transferred to a series of electron carrier proteins embedded in the membrane

NADH is an activated carrier that transfers electrons from one step of respiration to another

produces about 32 ATP

Electrochemical gradients provide the energy required to make ATP

Chemiosmotic coupling uses energy in a H+ gradient to power the formation of ATP

gradient often formed when electron flow powers the pumping of protons

Stages of cellular respiration

glycolysis (e^-)

citric acid cycle (e^-)

electron transport chain (H⁺)

Glycolysis

glucose is broken down into pyruvate, producing 2 pyruvates, 2 ATP, and 2 NADH

occurs in the cytosol

Krebs cycle

pyruvate is converted to acetyl CoA in the matrix

acetyl coA directly enters into the citric acid cycle

functions to oxidize acetyl-coA generating NADH and FADH₂

NA/K chart with polarizing and depolarizing stuff

3 main types of active transport pumps

ATP-pumps

coupled pumps

light driven pumps

ATP pumps

co-transporter/coupled transporter moves down its gradient using ATP

Coupled pumps (co-transport)

Uses the energy of a strong concentration gradient of one solute to power the movement of a second molecule against the gradient

Na/K pump

Both molecules move against their gradient

K+ is high inside the cell, and Na+ is high outside the cell

pumps 3 Na⁺ ions out and 2 K⁺ ions in the cell

cytosolic Ca²⁺

needs to be kept low to prevent it from activating signaling pathways in the cytosol

types of coupled transporters

symport and antiport

symport

two molecules move in the same direction

antiport (exchanger)

two molecules move in opposite directions

Na/glucose symport

glucose and Na are both moved from outside the cell in

Na⁺ moves down its gradient and glucose moves up

sodium/H⁺ antiport

used to manage pH in cells

H⁺ moves down its concentration gradient and Na⁺ moves up

plants, fungi, and bacteria use H⁺ gradients instead of Na⁺ to drive coupled pumps

don’t have Na⁺ pumps in their plasma membrane

instead uses H⁺ pumps to pump out protons

• makes the extracellular space acidic

• symports of sugars, amino acids, and nucleic acids is coupled with excess H⁺ ions

neurons

specialized cells that recieve, integrate, and transmit signals

have polarity on one end

membrane depolarization

used to facilitate rapid, long distance communication along axons

facilitated by the opening and closing of voltage gated ion channels, first Na⁺ then K⁺

voltage gated channels

can be:

• closed: but capable of opening when criteria are met

• open: free flow of ions down their electrochemical gradient

• inactivated: gate is in place, cannot be opened again until RMP resets

action potential

traveling wave of depolarization/repolarization

Na⁺ ions will spread to the next region to set off the next action potential

selectivity filter

very small region in the center of a channel that allows specific ions to move across the membrane

Determined by the amino acids present in the region

types of gated ion channels

voltage gated, ligand gated, and mechanically gated

voltage gated channels

have voltage sensing domains that can tell when there is a change in membrane potential

above a certain threshold, the channel will switch from closed to open

ligand gated channels

can be on either side of the membrane

only open when ligand is bound

why is the ligand not the ion the channel is specific for?

because the ion is always present so the channel would always be open

activated carriers

shuttle electrons between citric acid cycle and electron transport chain

oxidative phsosphorylation

Electrons from food are ultimately transferred to O_2, providing the energy to make ATP

Includes the ETC and ATP synthase, creating ATP

Products are CO₂ and H₂O

In the inner mitochondrial membrane

enzyme complexes help pump protons across the inner membrane

each complex in the chain is designed that the affinity for electrons increases with each complex

• NADH dehydrogenase complex contains many Fe-S centers, which have relatively low affinity for electrons

• cytochromes have heme groups (fe) that have high affinity for electrons

order is NADH dehydrogenase, cytochrome c reductase, then cytochrome c oxidase

specialized electron carriers

Q- ubiquinone and C- cytochrome C

Q- ubiquinone

small coenzyme that transfers electrons between NADH dehydrogenase and cytochrome c reductase

very hydrophobic because it acts inside the membrane

C- cytochrome c

small protein that contains heme (Fe rich) group that shuttles electrons between the cytochrome c reductase and cytochrome c oxidase

hydrophobic because it interacts with the membrane

succinate dehydrogenase

alternate pathway

activated carrier

accepts electrons from FADH₂ and passes them to ubiquinone (Q)

does not pump protons into the intermembrane space

ATP synthase

enzyme head always sticks into the matrix while the protein body(hydrophobic subunits) is embedded into the inner membrane

Last step of oxidative phosphorylation

electron flow in cellular respiration

powers the pumping of H⁺ protons across the membrane

protons are pumped from the matrix to the intermembrane space

Creates a concentration gradient across the inner membrane (high pH in the matrix (acidic) and low in the intermembrane space (basic))

Also creates a membrane potential - large amount of H⁺ in the intermembrane side makes that side of the membrane positively charged and the matrix side becomes negative

ATP synthase function

transmembrane carrier receives H^+, which causes the carrier to spin like a motor

• once the H⁺ has made a circle its released into the matrix

spinning of the carrier causes the stalk that connects the two components to rub against the proteins in the head, altering the conformation so it creates ATP

proton gradient in cellular respiration

Import of pyruvate and inorganic phosphate (P_i) are coupled to the proton concentration gradient (symport)

Import of ADP into the matrix and export of ATP out is performed by an antiport protein that uses the voltage gradient

fermentation

occurs when O₂ is low

glycolysis → pyruvate is broken down into waste products (lactic acid, ethanol, CO₂) when NADH is oxidized and NAD+ is replenished to be reused in glycolysis

photosynthesis reaction

CO₂ + H₂O + (light)energy → C₆H₁₂O₆ + O₂

3 membranes of chloroplasts

porous outer membrane

non-permeable inner membrane

thylakoid membrane - where chlorophyll is located

granum (grana plural)

stacks of thylakoid

stroma

choroplasts version of the matrix

light reactions

chloroplasts capture energy of sunlight and use it to make ATP and NADPH

in the thylakoid membrane

dark reactions

“synthesis” step

calvin cycle

captured energy converts CO₂ into glucose precursors

does not require light

chlorophyll

pigment that absorbs light adn helps transfer energy from the sun

photosystem

antenna complexes and the reaction center together

light reactions physiology

Arranged in an antenna complex, an array of proteins that contain thousands of chlorophyll and pigment molecules (allowing for energy transfer)

antenna complex funnels energy to the special pair located in the reaction center (protein complex), where the energy from the sun is used to energize electrons

light reactions steps

when the special pair has been excited enough, an electron will be transferred to an electron carrier

• creates a charge separation that starts electron flow between the reaction center and an electron transport chain

Photosystem II feeds electrons to an electron transport chain that powers the pumping of protons into the thylakoid space

• electrons are transferred to the electron carrier plastoquinone, which it takes to the cytochrome complex

• electron flow fuels H⁺ pumping and ultimately synthesis of ATP

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electrons from the electron transport chain are transferred to photosystem I and are used to replenish the special pair in PSI

• electrons are carried by the carrier protein plastocyanin (pC)

electrons from special pair in PSI reduce the Fe-S rich protein ferredoxin

• ferredoxin will transfer electrons to ferredoxin NADP reductase which will reduce the activated carrier NADPH

how does the lost electron get replaced?

a protein complex contains a water splitting enzyme that holds two water molecules and extracts electrons one at a time

• oxygen evolving complex

once 4 electrons have been removed the left over H⁺ is released

Calvin cycle

happens in the stroma

ATP and NADPH are used to power the production of sugars from atoms derived from CO₂ and H₂O

• carbon fixation

calvin cycle steps

CO₂ is combined with ribulose-1,5-bisphosphate (RuBP) by the enzyme Rubisco

• This reaction is energetically favorable

The intermediate is phosphorylated by ATP and reduced by NADPH to generate 6 of the 3-C sugar G3P

5 remaining G3Ps are used to regenerate ribulose-1,5-bisphosphate (back to step 1)

• requires ATP

product is 1 G3P

carbon fixation

Rubisco: The enzyme that attaches CO₂ to RuBP, a 5-carbon molecule, during the Calvin cycle.