Ap Bio Unit 3

Enzymes, Photosynthesis, Cellular Respiration

Catabolic Pathways

• release energy by breaking down complex molecules into simpler compounds

• Examples: cellular respiration and breakdown of glucose in the presence of O2

• “downhill” reactions

Anabolic pathways

• consume energy to build complex molecules from simpler ones

• example: the synthesis of protein from amino acids

• “uphill” reaction

What is an exergonic reaction?

• a reaction that releases free energy and has a negative ΔG

• “energy outward”

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

What is an endergonic reaction?

• a reaction that requires an input of free energy and has a positive ΔG

• “energy inward”

What 3 kinds of work does a cell do?

• Chemical work - pushing endergonic reaction

• Transport work - pumping substance across membranes against the direction of spontaneous movement

• Mechanical work - such a beating cilia or contracting muscle cells

What is ATP hydrolysis?

adding water to break ATP down into ADP and phosphate

What is a phosphorylation?

transferring a phosphate group from ATP to another molecule

What is a phosphorylated intermediate?

a molecule with an added phosphate that is more reactive and less stable

What is the ATP cycle?

ATP breakdown and regeneration couple energy-releasing and energy-consuming processes

What is an enzyme?

• a macromolecule, typically a protein, that speeds up a specific chemical reaction

• Acts as a catalyst

What is activation energy (EA)?

• the initial energy required to break reactant bonds and start a chemical reaction

• heat in the form of thermal energy absorbed from the surrounding often supplies activation energy

• provides a barrier that determines the rate of spontaneous reactions

What is the transition state?

• An unstable state that reactants pass through as a chemical reaction proceeds

• most reactions have high EA, and need additional energy (usually heat) to reach the transition state

Catalysis

• process by which a catalyst selectively speeds up a reaction without itself being consumed

• an enzyme catalyzes a reaction by lowering the EA barrier enough for the reaction to occur at moderate temperatures

What are the four ways enzymes can lower activation energy?

• orient substrates

• stretch bonds to make them easier to break

• provide a favorable micro environment

• use active-site amino acids

How does temperature affect enzyme activity?

activity increases toward the optimal temperature, then decreases as the enzyme begins to denature

How does pH affect enzyme activity?

Each enzyme has an optimal pH, and activity may decrease when pH moves away from that optimum

Competitive Inhibitors

• A molecule that binds to the enzyme’s active site and competes with the substrate

• enzyme productivity is reduced because the inhibitor blocks the substrate from entering the active site

What is a noncompetitive inhibitor?

• A molecule that binds away from the active site and changes the enzyme’s function or shape

• makes the active site less effective at catalyzing the reaction

What is allosteric regulation?

• a regulatory molecule binds to one site on a protein and affects its function at another site

• this type of regulation may either inhibit or stimulate enzyme activity

What does an allosteric activator do?

It stabilizes the enzyme’s active shape and stimulates activity

What does an allosteric inhibitor do?

Its stabilizes the enzmye’s inactive shape and reduces activity

What does the second law of thermodynamics state?

Every energy transfer increases the entropy of the universe

What is photosynthesis

The process that converts solar energy into chemical energy stored in sugars

What are autotrophs?

Organisms that produce organic molecules from Co2 and other inorganic materials

What are heterotrphs?

Organisms that obtain organic material by consuming other organisms or organic matter

What does photosynthesis occur?

in chloroplasts

What are the three main structures of a chloroplast?

Stroma, thylakoids, and grana (stacks of thylakoids)

What are the inputs of Photosynthesis?

• CO2

• H2O

• Light energy

What are the overall products of photosynthesis?

Sugars and O2

What is the main purpose of the light-dependent reactions?

Convert light energy into ATP and NADPH.

Where do light-dependent reactions occur?

In the thylakoid membranes of chloroplasts.

What are the main inputs of light-dependent reactions?

• Light

• H2O

• ADP + phosphate

• NADP+

What are the outputs of light-dependent reactions?

• ATP

• NADPH

• O2

What happens when chlorophyll absorbs light?

Its electrons become excited and move to a higher energy state.

What is the role of Photosystem II?

• Absorbs light

• excites electrons

• replaces lost electrons by splitting water

What is the role of Photosystem I?

Absorbs light to re-energize electrons used to reduce NADP+ into NADPH

What is the difference between P680 and P700?

• P680 is Photosystem II

• P700 is Photosystem I

• Their numbers indicate optimal light wavelengths

Why is water split during the light-dependent reactions?

To replace electrons lost by Photosystem II and release oxygen and hydrogen ions

How is a proton gradient created during photosynthesis?

H+ ions accumulate inside the thylakoid as electrons move through the electron transport chain

What does ATP synthase do in photosynthesis?

Uses the flow of H+ ions down their gradient to produce ATP

What is photophosphorylation?

The production of ATP using light energy

What is the main purpose of the Calvin cycle?

Uses CO2, ATP, and NADPH to produce G3P, a sugar molecule

Where does the Calvin cycle occur?

In the stroma of the chloroplast

What are the inputs of the light independent reaction cycle?

CO2, ATP, and NADPH.

38. Q: What are the outputs of the Light independent reaction cycle?

• G3P

• ADP

• NADP+

What are the three phases of the Light Independent Reaction cycle?

• Carbon fixation

• reduction

• regeneration of RuBP

How many CO2 molecules are needed to produce one net G3P?

Three CO2 molecules

How much ATP and NADPH are needed to produce one net G3P?

• 9 ATP

• 6 NADPH

How many G3P molecules are needed to make one glucose?

2 G3P molecules

What is cellular respiration?

The process of breaking down glucose to produce ATP

What is the overall equation for aerobic cellular respiration?

Glucose + oxygen → carbon dioxide + water + energy (ATP and heat)

Where does glycolysis occur?

In the cytoplasm (cytosol)

What are the inputs and outputs of glycolysis?

• Input: glucose, 2 ATP, and 2 NAD+

• Output: 2 pyruvate, 4 ATP (2 net), and 2 NADH

What is the net ATP gain from glycolysis?

2 ATP

Does glycolysis require oxygen?

No, glycolysis can occur without oxygen

What happens during pyruvate oxidation?

• Pyruvate is converted into acetyl-CoA

• releasing CO2 and producing NADH

Where does the Krebs cycle occur?

In the mitochondrial matrix.

What are the products of one turn of the Krebs cycle?

• 1 ATP

• 3 NADH

• 1 FADH2

• 2 CO2

How many times does the Krebs cycle turn per glucose molecule?

Twice, because one glucose produces two acetyl-CoA molecules

What is the purpose of the electron transport chain in cellular respiration?

Uses electrons from NADH and FADH2 to create a proton gradient that powers ATP production

Where is the electron transport chain located in mitochondria?

In the inner mitochondrial membrane

What is the final electron acceptor in aerobic cellular respiration?

Oxygen, which combines with electrons and H+ to form water

How does the proton gradient help produce ATP in mitochondria?

H+ flows through ATP synthase, providing energy to make ATP

What else can the proton gradient’s energy produce besides ATP?

Heat

What is oxidative phosphorylation?

ATP production using the electron transport chain and chemiosmosis

Why is fermentation important?

Regenerates NAD+ so glycolysis can continue producing ATP without oxygen

What is the difference between alcohol fermentation and lactic acid fermentation?

• Alcohol fermentation produces ethanol and CO2

• lactic acid fermentation produces lactate

How did photosynthesis change Earth's early atmosphere?

Cyanobacteria released oxygen as a byproduct of photosynthesis, increasing atmospheric oxygen.

How do pigments help with photosynthesis?

They capture light energy and help convert it into chemical energy

In what molecule is chemical energy temporarily stored after the light-dependent reactions?

NADPH

What is a photosystem?

A light-capturing unit made of pigments and proteins in the thylakoid membrane

Where are Photosystem I and Photosystem II located?

Embedded in the thylakoid membranes of chloroplasts

What happens to the hydrogen ions released when water is split?

They contribute to the proton gradient inside the thylakoid

What do high-energy electrons do as they pass through the photosynthetic electron transport chain?

Help establish a proton gradient across the thylakoid membrane

Where does the carbon dioxide used in the Calvin cycle come from?

The atmosphere

What does ATP synthase do?

Allows H+ ions to flow down their gradient and uses that energy to make ATP from ADP and phosphate

What did experiments with ATP synthase demonstrate about proton gradients?

ATP synthase activity depends on a proton gradient across the thylakoid membrane

What process do both cellular respiration and fermentation begin with?

Glycolysis in the cytoplasm

Is fermentation the same process as anaerobic respiration?

No. Fermentation does not use an electron transport chain, while anaerobic respiration does

What is a conserved process?

A process shared by many or all living organisms, such as glycolysis.

Where are electron transport chains located in eukaryotic cells?

In the inner mitochondrial membrane and the thylakoid membrane of chloroplasts

Where is the electron transport chain located in prokaryotes?

In the plasma membrane

What is the electron transport chain made of?

Membrane proteins that transfer electrons through a series of coupled reactions

Why can't hydrogen ions easily cross the membrane?

They are charged, polar ions that cannot pass easily through the hydrophobic lipid bilayer

What happens to NADH and FADH2 when they donate electrons to the electron transport chain?

They lose electrons and become oxidized

What is decoupling of oxidative phosphorylation, and how does it produce heat?

Protons bypass ATP synthase, so the energy in their gradient is released as heat instead of making ATP

What happens when ATP loses its third phosphate group and becomes ADP?

Energy is released for cellular work