UNIT 3: Part 2 Cellular Respiration and photosynthesis Hillis Textbook Chapter 6 Cellular respiration is a major catabolic pathway. Glucose is oxidized: carbohydrate 6O2 6CO2 6H2O chemical energy
Photosynthesis is a major anabolic pathway. Light energy is converted to chemical energy:
6CO2 6H2O light energy 6O2 carbohydrate
Cellular Respiration is AEROBIC (uses oxygen) A lot of energy is released when reduced molecules with many C—C and C—H bonds are
fully oxidized to CO2.
Oxidation occurs in a series of small steps in three pathways, followed by generation of ATP:
1. Glycolysis 2. Pyruvate Oxidation 3. Citric Acid Cycle 4. Electron Transport Chain CELLULAR RESPIRATION: REACTANTS: Glucose and oxygen
LEARN THE NAMES OF THE STEPS AND WHERE IT TAKES PLACE!
ATP Oxygen
PRODUCTS: Carbon Dioxide, water and ATP STEP ONE: GLYCOLYSIS Glycolysis: ten total reactions. Takes place in the cytosol. Starts with glucose
Final products: 2 molecules of NADH 2 molecules of ATP 2 molecules of pyruvate (pyruvic acid)
These ATP molecules were produced, however they don’t count because we USED two molecules at the beginning STEP TWO: PYRUVATE OXIDATION Pyruvate Oxidation: Happens in the mitochondria Starts with TWO separate pyruvates from glycolysis
Products: CO2 and acetate; acetate is then bound to coenzyme A (CoA)
2 Results in: pyruvate in 2 CO2 and 2 Acetyl CoA total STEP THREE: CITRIC ACID CYCLE
Citric Acid Cycle: Takes place in the mitochondrial matrix 8 reactions Starts with the two Acetyl CoA produced by pyruvate oxidation So, the cycle operates twice for every ONE glucose molecule that enters glycolysis
Each acetyl group is oxidized to two CO2. Oxaloacetate is regenerated in the last step to be re-used again when another acetyl CoA comes along. Energy carriers are produced: 6 NADH, 2 FADH2, 2 GTP STEP THREE: CITRIC ACID CYCLE Every glucose forms TWO acetyl CoA from pyruvate…
End result = 4 CO2, 6 NADH, 2 FADH2 and 2 GTP ALERT!!! The Citric Acid Cycle is also known as The Kreb’s Cycle and the TCA cycle STEP FOUR: ELECTRON TRANSPORT CHAIN
Electron transport/ATP Synthesis: + NADH is reoxidized to NAD and O2 is reduced to H2O in a series of steps. Respiratory chain—series of redox carrier proteins embedded in the inner mitochondrial membrane. Electron transport—electrons from the oxidation of NADH and
FADH2 pass from one carrier to the next in the chain.
STEP FOUR: ELECTRON TRANSPORT CHAIN
The oxidation reactions are exergonic; the energy is used to actively transport H+ ions out of the mitochondrial matrix, setting up a proton gradient. ATP synthase in the membrane uses the H+ gradient to synthesize ATP by chemiosmosis. About 32 molecules of ATP are produced for each fully oxidized glucose.
The role of O2: most of the ATP produced is formed by oxidative phosphorylation, which is due to the reoxidation of NADH.
Under anaerobic conditions (NO OXYGEN IS AVAILABLE), NADH is reoxidized by fermentation. The overall yield of ATP is only two—the ATP made in glycolysis.
Lactic acid fermentation: Alcoholic fermentation: End product is lactic acid (lactate). End product is ethyl alcohol (ethanol). Metabolic pathways are linked. Carbon skeletons (molecules with covalently linked carbon atoms) can enter catabolic or anabolic pathways. How do you think prokaryotes perform respiration? Photosynthesis involves two pathways: 1. Light reactions convert light energy into chemical energy (in ATP and the reduced electron carrier NADPH). 2. Carbon-fixation reactions use the ATP and NADPH, along with CO2, to produce carbohydrates. LIGHT REACTIONS:
Light is a form of electromagnetic radiation, which travels as a wave but also behaves as particles (photons). Photons can be absorbed by a molecule, adding energy to the molecule—it moves to an excited state.
In plants, two chlorophylls absorb light energy chlorophyll a and chlorophyll b.
LIGHT REACTIONS:
The light reactions use CHLOROPHYLL to trap energy from the sun! That is why they are considered “light” reactions. Chlorophyll molecule LIGHT REACTIONS:
When chlorophyll (Chl) absorbs light, it enters an excited state (Chl*), then rapidly returns to ground state, releasing an excited electron. Chl* gives the excited electron to an acceptor and becomes oxidized to Chl+. The acceptor molecule is reduced. Chl * acceptor Chl acceptor The electron acceptor is first in an electron transport system in the thylakoid membrane. Final electron acceptor is NADP+, which gets reduced:
NADP H 2e NADPH
ATP is produced chemiosmotically during electron transport (photophosphorylation).
LIGHT REACTIONS:
Two photosystems: •Photosystem I absorbs light energy at 700 nm, passes an excited electron to NADP+, reducing it to NADPH. • Photosystem II absorbs light energy at 680 nm, produces ATP, and oxidizes water molecules. CALVIN CYCLE:
The Calvin
cycle: CO2 fixation. It occurs in the stroma of the chloroplast. Each reaction is catalyzed by a specific enzyme.
1. Fixation of CO2:
CO2 is added to ribulose 1,5-bisphosphate (RuBP). Ribulose bisphosphate carboxylase/oxygenase (rubisco) catalyzes the reaction. A 6-carbon molecule results, which quickly breaks into two 3-carbon molecules: 3-phosphoglycerate (3PG). 2. 3PG is reduced to form glyceraldehyde 3- phosphate (G3P).
3. The CO2 acceptor, RuBP, is regenerated from G3P. When glucose accumulates, it is linked to form starch, a storage carbohydrate.
The C—H bonds generated by the Calvin cycle provide almost all the energy for life on Earth! Photosynthetic organisms (autotrophs) use most of this energy to support their own growth and reproduction. Heterotrophs cannot photosynthesize and depend on autotrophs for chemical energy.
Do we rely on plants?
Do plants rely on us? ALCOHOL PRODUCTION USING RESPIRATION: Catabolism of the beet sugar is a cellular process, so living yeast cells must be present.
With air (O2) yeasts used aerobic metabolism to fully oxidize glucose to CO2. Without air, yeasts used alcoholic fermentation,
producing ethanol, less CO2, and less energy (slower growth). PATHWAYS THAT HARVEST ENERGY RESPIRATION OVERVIEW: RESPIRATION OVERVIEW: GLYCOLYSIS RESPIRATION OVERVIEW: PYRUVATE OXIDATION FOR EACH PYRUVATE
KREB’S CYCLE FOR EACH ACETYL CoA
ELECTRON TRANSPORT CHAIN USES ALL THE ENERGY CARRIERS AND OXYGEN TO MAKE ATP Photosynthesis Overview: Photosynthesis Overview:
Light Reactions in the thylakoid membranes
Calvin Cycle in the stroma