Another view:

Each NADH_{mitochondrial} is worth 2.5 ATP, each NADH_cytosol is worth 1.5 ATP, and each [FADH₂] is worth 1.5 ATP gives a yield of 30 ATP/glucose.

Pertinent factors:

1. Number of H⁺/(NADH ® ½ O₂)
2. Number of H⁺/(ADP + P_i ® ATP + H₂O)
3. ATP^4-_out ® ADP^3-_in "costs" a H⁺

Catabolism of the Various Food Molecules (Carbohydrates, Proteins, Fats) Converges to Pyruvate and Acetyl-CoA (web Figure).

Breakdown of food molecules generates pyruvate and acetyl-CoA for the citric acid cycle and electron transport/oxidative phosphorylation, culminating in ATP synthesis.

(Nucleic acids in foods are not important sources of energy: they provide energy only through catabolism of the pentoses they contain. Pentoses are broken down to form hexoses and trioses which can access the pathway of glycolysis.)

Regulation of Cellular Respiration (Figure 7.19)

Glycolysis:

3 enzymes are allosterically regulated:

1. Hexokinase (HK): feedback-inhibited by glucose-6-P.
2. PFK (phosphofructokinase)*: PFK is the ‘valve’ controlling the rate of glycolysis. PKF is feedback-inhibited by ATP and citrate; activated by AMP and ADP.
3. Pyruvate kinase (PK): feedback-inhibited by ATP; activated by fructose-1,6-bisphosphate.

Citric Acid Cycle:

1. Citrate synthase (CS): inhibited by ATP and NADH.
2. Isocitrate dehydrogenase (ICDH)*: allosterically activated by ADP and NAD⁺; inhibited by ATP and NADH.
3. Also, citric acid accumulation and transport into the cytosol leads to activation of fatty acid biosynthesis (storage of acetyl-CoA as fat).

The Pasteur Effect:

O₂ inhibits glucose consumption.

(Yeast cells growing aerobically consume much less glucose than yeast cells growing anaerobically).

Note: 2ATP/glucose under anaerobic conditions versus 30-some ATP/glucose under aerobic conditions.

How does it work? That is, how is glucose consumption inhibited under aerobic conditions?

ATP and citrate accumulation under aerobic conditions feedback-inhibits PFK, thereby inhibiting the rate of glucose entry into glycolysis, and the rest of the pathways of cellular respiration.

3.5 x 10²¹ kcal of solar energy reaches the earth each day.

1% of it is captured by photosynthetic organisms.

Photosynthesis: the transduction of light energy into chemical energy

Traditionally expressed in terms of CO₂ fixation:

6 CO₂ + 6 H₂O + energy ® C₆H₁₂O₆ + 6 O₂

essentially the reversal of cellular respiration.

More appropriately, since all of the O₂ evolved in photosynthesis comes from H₂O,

6 CO₂ + 12 H₂O + energy ® C₆H₁₂O₆ + 6 O₂ + 6 H₂O

The energy required can be expressed as nhu, where n = some number of photons of energy hu, where h = Planck’s constant and u = the frequency of light. (The energy, E = nhu = nhc/l, where c = speed of light and l = wavelength of light.)

Review chloroplast structure

The light reactions involve energy capture and O₂ evolution; the light reactions are associated with the thylakoid membranes.

The dark reactions involve CO₂ fixation; the dark reactions take place in the stroma.

Specifically, light (radiant electromagnetic energy) is transduced by a photochemical system (thylakoid membranes) into 2 forms of chemical energy:

1. ATP – phosphorylation energy
2. NADPH – a strong reducing agent

The source of electrons to reduce NADP⁺ a NADPH is water:

2H₂O + 2NADP⁺ + xADP + xP_I + nhu ®

O₂ + 2NADPH + 2H⁺ + xATP

The NADPH and ATP thus formed provide the chemical energy to drive CO₂ fixation in the dark.

Photosynthesis depends on the photoreactivity of Chlorophyll (Figure 8.9)

Chlorophyll (Chl) resembles heme, but it has Mg²⁺ instead of Fe²⁺.

Possible fates of the quantum of light energy absorbed by a photosynthetic pigment molecule (web figure).

Resonance energy transfer (exciton transfer) is an important mechanism in harvesting light energy.

Photo-excitation of Chl leads to e^- transfer to a primary electron acceptor (the photochemical event), creating an ‘electron hole’ in the Chl molecule (Chl⁺). Electron transfer from water fills this ‘hole’.

Photosynthetic units are localized within the thylakoid membranes. Photosynthetic units consist of a specialized pair of chlorophyll a molecules (the reaction center Chl) and several hundred molecules of Chl a and accessory light- harvesting pigment molecules serving as an antenna to collect light energy and channel it to the reaction center via exciton transfer.

Only reaction center Chl can perform the photochemical event: conversion of light energy to chemical energy by an e^- transfer from Chl*.

Eukaryotic phototrophs possess 2 kinds of photosystems, PS I and PSII, and a cytochrome-containing redox chain (Figures 8.11 & 8.12).