Electrochemical Gradients, Membrane Pumps, and Ion Transport in Cells

What are the two combined forces that drive the diffusion of ions across a membrane?

Chemical and electrical forces

What is the term for the combined chemical and electrical forces that influence ion movement?

Electrochemical gradient

What happens when an ion diffuses down its electrochemical gradient?

It moves from an area of higher concentration to an area of lower concentration, influenced by both chemical and electrical forces.

What is an electrogenic pump?

A transport protein that generates a voltage across a membrane, storing energy for cellular work.

What is the main energy source used by electrogenic pumps?

ATP

Which type of pump do plants, fungi, and bacteria primarily use?

Proton pumps, which actively transport hydrogen ions out of the cell.

What type of pump do animals primarily use?

Sodium-potassium pumps.

how do plants use membrane potential?

plants use proton pumps to generate hydrogen gradients across cell membrane; cotransporter couples the movement of the hydrogen ions back down its concentration gradient to the active transport of sucrose into cell; this is how plants load sucrose into their veins for transport around plant body

when pumps are disrupted

normally, sodium in waste is reabsorbed in the colon to maintain constant level in body; when a person has diarrhea, waste is expelled too fast for reabsorption, causing sodium to drop; drinking a concentrated salt (NaCl) and glucose soln enables uptake through Na+/glucose transporters in intestine

receptor mediated endocytosis

vesicle formation is triggered by solute binding to receptors; receptor proteins bound to specific solutes from extracellular fluid are clustered in coated pits that form coated vesicles; emptied receptors are recycled to plasma membrane by the same vesicle

What is the function of low density lipoproteins (LDL) in the human body?

LDL carries cholesterol to human cells via receptor-mediated endocytosis.

What condition is associated with a missing or defective LDL protein?

Familial hypocholesterolemia.

What happens to cholesterol in individuals with familial hypocholesterolemia?

Cholesterol accumulates in the blood, building up lipids and narrowing blood vessels.

What are the potential health risks associated with the accumulation of cholesterol in the blood?

Potential heart damage or stroke.

metabolism and pathways

metabolism is the totality of an organism's chemical reactions; in a metabolic pathway, a specific molecule is altered in a series of steps to produce a product

What is kinetic energy?

Kinetic energy is energy associated with motion.

What is energy?

Energy is the capacity to do work.

What can energy be used to do?

Energy can be used to move matter against opposing forces, like friction and gravity.

What is thermal energy?

Thermal energy is kinetic energy associated with random movement of atoms and molecules.

potential energy

energy that matter possesses because of its location and structure; chemical energy is potential energy available for release in chemical reactions

the first law of thermodynamics

energy of the universe is constant; can be transferred and transformed but it cannot be created or destroyed; principle of conservation of energy

the second law of thermodynamics

during energy transfer, some energy is converted to thermal energy and lost as heat, becoming unavailable to do work; every energy transfer increases entropy of the universe

entropy

measure of molecular disorder, or randomness

spontaneity

processes that increase entropy of universe occur spontaneously; spontaneous processes occur without energy input, can happen quickly/slowly; processes that decrease entropy are nonspontaneous and require input of energy

What do cells create from less organized starting materials?

Ordered structures

What balances the increase in order within living systems?

Catabolic breakdown of organic forms of matter, releasing heat and small molecules

Can entropy decrease in a particular system like an organism?

Yes, as long as the total entropy of the system and surroundings increases

Free energy change, delta G

Gibbs free energy, G, can be simplified and referred to as free energy; free energy is a portion of a system's energy that can do work when temp and pressure are uniform throughout system; change in free energy during a reaction is related to temperature and changes in enthalpy and entropy

delta G=delta H-T*delta S

delta G = change in free energy

delta H = change in enthalpy (total energy)

delta S = change in entropy

T = temperature in kelvin

spontaneous processes

the change in G for a process can be used to determine whether it is spontaneous or not; delta G is negative for all spontaneous processes; delta G is 0 or positive for all nonspont processes

free energy, stability, and equilibrium

delta G represents the difference between free energy of the final state and free energy of initial state

delta G = G final - G initial

if a reaction has negative delta G, the system loses free energy and becomes more stable

spontaneity and equilibrium

equilibrium: the point at which forward and reverse reactions occur at same rate, describes rate of maximum stability; systems never spontaneously move away from equilibrium; a process is spontaneous and can perform work only when moving towards equilibrium

How can chemical reactions be classified based on free-energy changes?

Chemical reactions can be classified as exergonic or endergonic.

What are exergonic reactions?

Exergonic reactions are those that proceed with a net release of free energy to the surroundings.

What are endergonic reactions?

Endergonic reactions are those that absorb energy from the surroundings.

What characterizes exergonic reactions in terms of energy?

Products store less energy than reactants.

What is the sign of delta G in exergonic reactions?

Delta G is negative.

What does a negative delta G indicate about a reaction?

The reaction occurs spontaneously and is energetically favorable.

What does the magnitude of delta G determine?

The maximum amount of work a reaction can perform.

How much energy is available for work from one mole of glucose during cellular respiration?

686 kcal of energy.

Does breaking bonds during chemical reactions release or require energy?

It requires energy.

What happens to potential energy when products have lower free energy than reactants?

Potential energy is released when new bonds are formed after original bonds break.

What are endergonic reactions?

Reactions where products store more free energy than reactants.

What is the delta G of endergonic reactions?

Positive delta G.

Are endergonic reactions spontaneous?

No, they are nonspontaneous and require energy.

What does the magnitude of delta G indicate in endergonic reactions?

It determines the quantity of energy required to drive the reaction.

How much more free energy do the products of photosynthesis store compared to the reactants?

686 kcal more free energy per mole.

What do endergonic reactions convert light into?

Chemical energy.

What type of system eventually reaches equilibrium and can do no work?

Closed system

What type of system do living organisms represent?

Open system

Are the reactions of metabolism in living cells reversible?

Yes

Do living cells ever reach equilibrium?

No

What is a catabolic pathway?

A catabolic pathway is a series of reactions in a cell that releases free energy.

How do products behave in a catabolic pathway during cellular respiration?

In cellular respiration, products are pulled in one direction because they are reactants in the next step.

What ensures that equilibrium is never reached in a catabolic pathway?

A steady inflow of glucose and the release of waste products ensure that equilibrium is never reached.

cells doing work

transport work, chemical work, mechanical work

transport work

pumping substances across membrane against direction of spontaneous movement

mechanical work

beating cilia, contracting muscle cells

chemical work

pushing endergonic reactions

What molecule powers cellular work?

ATP

What is energy coupling in cells?

The use of an exergonic process to drive an endergonic one.

What mediates most energy coupling in cells?

ATP

structure of ATP

adenosis triphosphate is composed of ribose, adenine, and three phosphate groups; in addition to energy coupling, ATP functions as one of the nucleoside triphosphates for DNA

hydrolysis of ATP

energy is released from ATP when terminal phosphate bond is broken by hydrolysis (adding water molecules); energy does not directly come from phosphate bonds, but from chemical change to a state of lower free energy in products

how ATP provides energy that performs work

phosphorylation, transfer of phosphate group from ATP to another molecule, typically used to power endergonic reactions; the recipient molecule, a phosphorylated intermediate, is more reactive (less stable, w more free energy), than original molecule

how ATP drives transport and mechanical work

transport and mechanical work in cell also nearly always powered by ATP hydrolysis; ATP hydrolysis causes change in protein shape and binding ability

the regeneration of ATP

ATP is regenerated by addition of phosphate group to ADP; free energy needed to phosphorylate ADP comes from exergonic breakdown reactions (catabolism); the shuttling of inorganic phosphate and energy is called the ATP cycle, it couples energy yielding processes to energy consuming ones

How do enzymes speed up metabolic reactions?

By lowering energy barriers.

What are spontaneous reactions?

Reactions that do not need added energy but require activation energy, which can be gathered from the environment.

What is a catalyst?

A chemical agent that speeds up a reaction without being consumed by the reaction.

What is an enzyme?

A macromolecule (typically a protein) that acts as a catalyst to speed up a specific reaction.

What is activation energy (Ea)?

The initial energy needed to break bonds of a reaction.

How is activation energy often supplied?

Heat in the form of thermal energy absorbed from the surroundings.

What happens when enough energy is absorbed to break bonds?

Molecules become unstable and reach the transition state.

What occurs as atoms settle into new, more stable bonds?

Energy is released to the surroundings.

In an exergonic reaction, what is true about the energy released?

The formation of new bonds releases more energy than was invested in breaking the old bonds.

What does the activation energy barrier determine?

The rate of spontaneous reactions.

For some reactions, how can the activation energy (Ea) be overcome?

It can be low enough for thermal energy at room temperature to overcome the activation barrier.

What is generally true about the activation energy of most reactions?

Most reactions have a high activation energy (Ea).

how enzymes speed up reactions

adding heat is not a useful way to speed reactions in cells bc it can cause proteins to denature; heat is also impractical bc it would speed up reactions, not just those that are needed; instead, organisms carry out catalysis, the process by which a catalyst selectively speeds up a reaction w/o itself being consumed; an enzyme catalysis a reaction by lowering the Ea barrier enough for reactions to occur at moderate temps; an enzyme cannot change delta G, only speeds up reaction that would occur eventually

What is the term for the reactant that an enzyme acts on?

Substrate

What forms when an enzyme binds to its substrate?

Enzyme-substrate complex

What does each enzyme catalyze?

A specific reaction

Can an enzyme recognize its specific substrate among closely related molecules?

Yes

What is the active site of an enzyme?

The active site is the region on an enzyme, often a pocket or groove, that binds to the substrate.

What happens when a substrate enters the active site of an enzyme?

The enzyme changes shape slightly, tightening like a handshake.

What is induced fit in the context of enzymes?

Induced fit is the result of interactions between chemical groups on a substrate and the active site.

How is a substrate typically held in an enzyme's active site?

By weak bonds, such as hydrogen bonds.

What effect does increasing substrate concentration have on an enzyme-catalyzed reaction?

It can speed up the reaction rate.

What happens when all enzyme molecules have their active sites engaged?

The enzyme is saturated.

How can the reaction rate be increased if an enzyme is saturated?

By adding more enzymes.

What do enzymes do to activation energy (Ea) during a reaction?

They lower it by stretching or straining bonds for substrate bonding.

What type of amino acid residue is typically found in an enzyme's active site in neutral solutions?

An acidic amino acid residue.

What is the optimal temperature for an enzyme?

Each enzyme has an optimal temperature at which it catalyzes its reaction at the maximum possible rate.

How does temperature affect the reaction rate of enzymes up to the optimal temperature?

The reaction rate increases with the temperature increasing up to the optimal temperature.

What happens to the reaction rate of enzymes beyond the optimal temperature?

The rate of reaction begins to drop beyond the optimal temperature.

What is the optimal pH for pepsin?

2

What is the optimal pH for trypsin?

8

What determines the optimal pH of an enzyme?

The environment in which it is typically active.

What are cofactors?

Nonprotein helpers that assist enzymes.

How do cofactors bind to enzymes?

They can bind permanently or reversibly with the substrate.