Unit 3 - Energy of Life

Question 1

What is energy coupling and how does it work?

Answer 1

Definition: Using an exergonic process to drive an endergonic one.

Process: The primary source of energy for cells is ATP. One of the three phosphate groups is hydrolyzed and breaks off from ATP which turns it into ADP. This also releases energy that is used to drive reactions that require it.

Question 2

How do enzymes and substrates interact? Describe the steps of catalysis.

Answer 2

1. Substrate enters the active site of the enzyme and the enzyme changes shape slightly (induced fit)
2. Formation of the enzyme-substrate complex which is held together by weak interactions like hydrogen bonds and ionic bonds
3. Substrates are converted to products
4. Products are released
5. Active site is available for new substrate molecules

Question 3

Cofactors

Answer 3

Nonprotein helpers that are bound tightly to the enzyme and help it function properly for catalysis (EX: metal ions like zinc and iron)

If the cofactor is organic, it’s referred to as a coenzyme (EX: vitamins)

Question 4

Competitive inhibition

Answer 4

These inhibitors bind to the active site of the enzyme, thus competing with the substrate. They are often chemically similar to the normal substrate molecules and reduce the efficiency of enzymes by blocking the substrate from entering

Question 5

Non competitive inhibition

Answer 5

These inhibitors do not directly compete with the substrate molecule. Instead, they bind to another part of the enzyme which causes it to change shape so that the active site is now non-functional

Question 6

Feedback inhibition

Answer 6

A metabolic pathway is stopped by the product of the enzyme which acts as an inhibitor. This enables the cell to conserve its chemical resources

Question 7

Allosteric inhibition (activators, inhibitors, sites)

Answer 7

Many enzyme regulators bind to an allosteric site on the enzyme which is a specific binding site but not the active site. Once bound, the shape of the enzyme is changed, and this can either stimulate or inhibit enzyme activity.

Activators stabilize the active form and inhibitors stabilize the inactive form

Question 8

What is the equation for cellular respiration?

Answer 8

C6H12O6 + 6 O2 —> 6 H2O + 6 CO2 + ATP

Question 9

What are the reactants and products in glycolysis?

Answer 9

Reactants: glucose, 2 ATP (energy investment), 2 NAD+

Products are two pyruvate molecules (3C; store most of the potential energy of the original glucose molecule), 2 NADH (high energy electron carriers), 4 ATP

Net gain of 2 ATP

Question 10

Steps of pyruvate oxidation

Answer 10

Pyruvate (3C) loses a carbon through the formation of 1 CO2. (*remember this process occurs TWICE for every glucose molecule)

1 NADH is also created.

The acetyl group (2C) is attached to a molecule called coenzyme A to form acetyl coA

coA escorts acetyl coA into the Krebs cycle before leaving and being recycled

Acetyl group enters the first reaction

Question 11

Reactants and products of the citric acid cycle

Answer 11

Reactants: two acetyl (2C) and one 4C molecule (*remember this process occurs TWICE for every glucose molecule)

Products: 4 CO2, 6 NADH, 2 FADH2, and 2 ATP

Question 12

What are the reactants and products of pyruvate oxidation?

Answer 12

Reactants: 2 pyruvates, 2 coenzyme A

Products: 2 CO2, 2 NADH, 2 acetyl coA

Question 13

Steps of oxidative phosphorylation

Answer 13

1. NADH and FADH2 carry high energy electrons that they transfer to the complexes of the electron transport chain
2. The strong attraction of oxygen (due to its high electronegativity) pulls electrons down the chain
3. Oxygen is the final electron acceptor
4. At the same time, each oxygen atom picks up 2 H+ ions from the aqueous solution and 2 electrons to form an H2O molecule
5. The flow of electrons down the chain is an exergonic process that releases energy. This energy is then used to pump H+ ions across the inner mitochondrial membrane from the matrix into the inter membrane space (inside to outside). This forms a concentration gradient called the proton motive force which stores potential energy
6. The H+ ions concentrated in the inter membrane space diffuse through ATP synthase, converting the potential energy into kinetic energy that spins the enzyme and creates ATP

Question 14

What is chemiosmosis?

Answer 14

Energy stored in the form of an H+ gradient across a membrane is used to drive cellular work; form of energy coupling

EX: synthesis of ATP

Question 15

How do bears generate body heat but not ATP during hibernation?

Answer 15

A channel protein called the uncoupling protein is activated, allowing H+ ions to diffuse back down their concentration gradient into the mitochondrial matrix, which leads to a reduction in the proton gradient across the membrane. Heat energy is released as the protons move, but less ATP is made because of the decreased concentration of H+ ions available to move across ATP synthase

Question 16

Calculate how much ATP is produced by cellular respiration

Answer 16

Total products:
10 NADH
2 FADH2
6 CO2 
6 H2O
4 ATP (substrate level phosphorylation)

1 NADH —> 2.5 ATP
1 FADH2 —> 1.5 ATP

25 + 3 + 4
= 30-32 total ATP

Question 17

Anaerobic respiration vs fermentation

Answer 17

Anaerobic respiration still ATP without oxygen but still uses an electron transport chain. For example, some prokaryotes use sulfate ion as the final electron acceptor.

Fermentation also occurs without oxygen but it is only the stage of glycolysis. ATP is generated by substrate level phosphorylation (lactic acid and alcoholic fermentation).

Question 18

Alcoholic fermentation and lactic acid fermentation

Answer 18

In aerobic respiration, NADH transfers electrons to the ETC and can be recycled as NAD+. However, without oxygen, NADH must transfer its electrons to somewhere else so that there is a sufficient supply of NAD+ to accept electrons from glucose

In alcoholic fermentation, pyruvate is converted to ethanol which releases CO2 and oxidizes NADH, in the process creating more NAD+ (yeast cells)

In lactic acid fermentation, NADH drops off its electrons to pyruvate which reduces it and forms lactic acid. NAD+ is also created

Question 19

What is the equation for photosynthesis?

Answer 19

6 CO2 + 6 H2O + light energy —> 6 O2 + C6H12O6

Question 20

How do leaf and chloroplast anatomy relate to photosynthesis?

Answer 20

The stroma is the dense fluid filled interior of the chloroplast.

Within the stroma are membranous sacs called thylakoids that are stacked into grana.

Inside the thylakoids membrane is the thylakoids space

Chlorophyll is located in the thylakoids membranes and is the light absorbing pigment that drives photosynthesis and gives plants their green color

Leaves contain many pores called stomata through which CO2 enters and O2 + water vapor exit

Question 21

What are the steps of the light reactions?

Answer 21

1. A photon of light strikes one of the chlorophyll pigment molecules in a light-harvesting complex of photosystem II (PSII)
2. Energy is transferred from pigment molecule to pigment molecule until it reaches the reactor-center complex, which uses the energy from the light to boost one electron to a higher energy level
3. The primary electron acceptor traps the high energy electron and they then move down the electron transport chain to photosystem I (PSI)
4. During this whole process, H2O is breaking down into 2e-, 2 H+, and an oxygen atom. O2 flows through the stomata as a waste product. The electrons go to step 1 to restart the process
5. H+ ions are pumped from the stroma to the thylakoid space (outside to inside) using energy from the exergonic fall of electrons down the ETC. This creates a proton gradient that is then used in chemiosmosis
6. H+ ions diffuse back into the stroma and pass through ATP synthase which converts ADP and a phosphate group into ATP
7. In PSI, two excited electrons from the ETC, an H+ ion, and a molecule of NADP+ combine to form NADPH
8. Now the products can move onto the Calvin Cycle! ATP provides chemical energy and NADPH provides electrons that can reduce compounds

Question 22

What are the steps of the Calvin cycle?

Answer 22

Phase 1 - Carbon fixation
3 molecules of CO2 are incorporated into 3 molecules of a 5C sugar called RuBP. The enzyme that catalyzes this step is rubisco. This creates 6 molecules of a 3C compound.

Phase 2 - Reduction
6 ATP and 6 NADPH are put in, creating 6 ADP and 6 NADP+. This results in 6 molecules of G3P sugar. For every 3 molecules of CO2, 5 molecules of G3P return to the beginning of the cycle to regenerate RuBP and 1 exits to be used by the plant cell.

Phase 3 - Regeneration of RuBP
5 molecules of G3P which is a 3C sugar is rearranged into 3 molecules of the 5C sugar RuBP.

*remember that this 3 CO2 cycle occurs twice for every glucose molecule

Question 23

What do plant cells use G3P for?

Answer 23

Can be converted to glucose for short-term energy storage or cellulose as building material for the cell wall.

Question 24

List 3 similarities between cellular respiration and photosynthesis.

Answer 24

1. Both processes have an electron transport chain, with the flow of electrons down the chain being an exergonic process that releases energy that can be used to establish a proton motive force.
2. In both cases proton pumps move hydrogen ions against their electrochemical gradient in order to create potential energy across a membrane
3. ATP synthase then couples the diffusion of hydrogen ions across the membrane to the phosphorylation of ADP to form ATP

Question 25

List 3 differences between cellular respiration and photosynthesis.

Answer 25

1. Photosynthesis occurs in the chloroplast of plant cells while cellular respiration occurs first in the cytosol and mitochondria of both animal and plant cells.
2. In cellular respiration, the mitochondria use chemiosmosis to transfer chemical energy from food molecules such as glucose to ATP, while chloroplasts in photosynthesis transform light energy from the sun into the sources of energy like glucose.
3. In photosynthesis the electrons in the electron transport chain are supplied by the breakdown of water into two hydrogen ions, one oxygen atom, and two electrons. In cellular respiration, the electrons are supplied by the breakdown of organic molecules such as glucose.

Question 26

Where is pH the highest in photosynthesis and cellular respiration?

Answer 26

Photosynthesis: inside the stroma since H+ ions are pumped into the thylakoid space

Cellular respiration: inside the matrix since H+ ions are pumped into the intermembrane space

*remember H+ ions are pumped into the “space”

Question 27

Where do each of the stages of cellular respiration take place?

Answer 27

Glycolysis - cytosol
Pyruvate oxidation - mitochondrial matrix
Krebs/Citric Acid Cycle - mitochondrial matrix
Oxidative phosphorlyation - inner mitochondrial membrane (cristae folds)

Question 28

What helps maintain the fluidity of the plasma membrane?

Answer 28

Cholesterol

Question 29

List 1 similarity and 3 differences between substrate-level phosphorylation and oxidative phosphorylation.

Answer 29

Similarity
In both cases, a phosphate group is transferred to ADP in order to form ATP

Differences

1. Substrate level phosphorylation is more direct, as an enzyme transfers a phosphate group from a substrate to ADP in order to form ATP by using the energy obtained from a coupled reaction. Oxidative phosphorylation is indirect, as ATP is generated from the oxidation of NADH & FADH2. The transfer of electrons provides energy for pumping protons to form an electrochemical gradient which then powers and ATP synthase to form ATP.
2. Substrate level phosphorylation produces a smaller amount of ATP (4 ATP) than oxidative phosphorylation (30 - 32 ATP)
3. Substrate level phosphorylation occurs in the first two stages of cellular respiration, glycolysis and the Krebs cycle, while oxidative phosphorylation is the last stage of cellular respiration.

Question 30

How do you overcome a competitive inhibitor?

Answer 30

Add more substrate to increase the probability of a substrate binding to the active site rather than an inhibitor

Question 31

How do you stop an enzyme-catalyzed reaction?

Answer 31

Add acid/base to change the pH of the system or raise/lower the temperature significantly

This will denature the enzyme so it is no longer functioning

Question 32

How is free energy (delta G) affected by doubling the concentration of the enzyme?

Answer 32

No effect

Question 33

What is a catabolic reaction? Give two examples and describe why they fit the definition.

Answer 33

An exergonic process that releases energy

Breaks down large substances into smaller components [hydrolysis]

EX: cellular respiration (breaks down glucose into H2O and CO2); digestion (breaks down macromolecules into monomers)

Question 34

What is an anabolic reaction? Give two examples and describe why they fit the definition.

Answer 34

An endergonic process that absorbs energy

Creates large molecules from smaller components [dehydration]

EX: photosynthesis (makes glucose from H2O and CO2), translation (making proteins from amino acids)

Question 35

What is the function of the electron transport chain in cellular respiration? How much ATP does it produce?

Answer 35

To allow high energy electrons carried by NADH and FADH2 to slowly release their energy that is used to create a concentration gradient and store potential energy

The ETC does not produce ANY ATP

Question 36

Where is pH the lowest in photosynthesis and cellular respiration?

Answer 36

Photosynthesis: inside the thylakoid space since H+ ions are pumped into there

Cellular respiration: inside the intermembrane space since H+ ions are pumped into there

*remember H+ ions are pumped into the “space”

Question 37

What is the path of electrons in the light reactions?

Answer 37

Water –> PSII –> PSI –> NADPH

Question 38

What step in the light reactions is oxygen created?

Answer 38

In the water-splitting step in PSII (light reactions)

Water is split into H+ ions and oxygen while releasing electrons for the electron transport chain

Question 39

Where do the stages of photosynthesis occur?

Answer 39

Light reactions: thylakoid membrane

Calvin Cycle: stroma (cytosol in chloroplasts)

Question 40

How many turns of the Calvin Cycle make 1 G3P?

Answer 40

3

Question 41

What happens if a chemical is added that makes the thylakoid membrane permeable to H+ ions?

Answer 41

Reduced ATP synthesis because the cell cannot establish a proton gradient to store potential energy

Question 42

What is the result of adding more substrate to the reaction mixture?

Answer 42

Enzyme activity and the amount of product will increase until the reaction reaches its equilibrium point (active sites are all occupied and the enzyme is saturated with substrate) or until the substrate is used up by the reaction