Introduction to Metabolism



Cellular Energetics

Chapters 8 (Photosynthesis) & 9 (Cellular Respiration)

I. Overview of Photosynthesis and Cellular Respiration:

PHOTOSYNTHESIS (AUTOTROPHS)

CO2 + H2O C6H12O6 + O2

CARBON DIOXIDE WATER GLUCOSE OXYGEN

RESPIRATION (HETEROTROPHS and AUTOTROPHS)

II. Energy and Metabolism

METABOLISM – sum of all chemical reactions in an organism; reactions are organized into pathways:

• CATABOLIC – Releases energy – break down complex molecules into simpler molecules

o Ex. Hydrolysis (adding water to break),

o Ex. CELLULAR RESPIRATION (breaking down glucose for ATP)

• ANABOLIC – Requires energy to combine simpler molecules into more complicated one

o Ex. Condensation Reaction/Dehydration Synthesis (removing water to make),

o Ex. PHOTOSYNTHESIS (production of sugar “food” for plant)

EXERGONIC Reaction – spontaneous reaction, RELEASES energy “EXITS”

• Ex. Respiration

ENDERGONIC Reaction – non-spontaneous reaction, REQUIRES energy “ENTERS”

• Ex. Photosynthesis

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III. Energy in Cells – ATP

ADENOSINE TRIPHOSPHATE – Energy Currency of the Cell – molecule with unstable phosphate bonds that the cell breaks off to drive endergonic reactions

• Cell traps energy stored in ATP by transferring the last PHOSPHATE group from ATP to other compounds

• ATP that lost P group = ADP (adenosine diphosphate)

o ATP - Drives most cellular work

o Components:

▪ Adenine

▪ Ribose

▪ 3 phosphate groups

IV. CHAPTER 8 – PHOTOSYNTHESIS

PHOTO = LIGHT SYNTHESIS = TO MAKE : Anabolic process which converts solar E ( “food” (glucose) for survival. Occurs in the CHLOROPLASTS of AUTOTROPHS(producers).

Solar E is converted by the use of PIGMENTS, molecules that absorb certain wavelengths of light and reflect (what we see) others.

Primary photosynthetic pigments:

❑ CHLOROPHYLL A: main photosynthetic pigment, absorbs RED & BLUE light, reflects GREEN

Chlorophyll b & c absorb different wavelengths of blue & red

Accessory pigments:

❑ CAROTENES: absorb BLUE & GREEN reflect yellow and orange.

In fall, chlorophyll is broken down so we see the colors of the accessory pigments.

OVERVIEW OF PHOTOSYNTHESIS: USES ENERGY FROM THE SUN TO CONVERT WATER AND CARBON DIOXIDE INTO HIGH ENERGY SUGAR AND OXYGEN

(light)

Overall Equation: 6CO2 + 6 H2O ( C6H12O6 + 6O2

(chlorophyll)

PHASE I- LIGHT DEPENDENT REACTION energy captured from the SUN

❑ Occur in the THYLAKOID membranes – and involve PHOTOSYSTEMS - clusters of chlorophyll and proteins that act like antennae to absorb sunlight.

❑ Light energy is used to produce the energy rich compound ATP and an electron carrier NADPH. Electrons are given to NADP+ by breaking apart H2O molecules, and stripping the Hydrogen atoms of their ELECTRONS. Also released as a waste product by this breakdown is OXYGEN gas. (good for us!)

PHASE II- LIGHT INDEPENDENT REACTION: Energy from light reactions used to “fix” CO2 into sugars (AKA Calvin cycle, or Carbon Fixation)

❑ Occurs in the STROMA

❑ Use the ATP and the NADPH from the Light Dependent reactions to produce high energy SUGARS (like glucose – C6H12O6) from CARBON DIOXIDE.

❑ Does not require light, but will not occur if products from light reactions are not being made.

PHASE I: LIGHT REACTIONS (AKA Photophosphorylation)

Occur in the thylakoid membranes of chloroplasts

Two Stages:

1) Light Absorption: Light energy is used to break apart H2O into three things:

❑ Oxygen atoms: diffuse out as waste

❑ H ions: used to make NADPH (a coenzyme: H carrier, remember NADH from respiration?) and eventually ATP’s

❑ Electrons: used in photosystem I and II below

What is a photosystem? A combination of chlorophyll molecules plus accessory pigments where light absorption occurs. Photosystems act like antennas, capturing light energy and transporting electrons (an ETC) through the thylakoid membrane

2) Chemiosmosis: H+ diffuse across membrane fueling ATP synthesis – remember this from respiration too?

[pic]

Photosystem II ( Electron Transport Chain ( Photosystem I ( short ETC ( Chemiosmosis



PHASE II: CARBON FIXATION (AKA - CALVIN cycle, LIGHT INDEPENDENT reactions, C3cycle, DARK REACTIONS)

Occurs in the STROMA OF CHLOROPLASTS

CO2 enters the stroma with the help of RUBISCO (most plentiful enzyme on earth) which binds the CO2 to RIBULSOSE BIPHOSPHATE (RuBP) a 5-Carbon sugar, creating an unstable 6-Carbon compound.

Energy from ATP and NADPH (products of light RXN)

Combine six carbon dioxide molecules with six 5-carbon compounds and then…

Produce twelve 3-carbon compounds

Midcycle

Two 3-carbon molecules are removed (called G3P)

These become the building blocks of sugars, lipids, amino acids, etc.

Finally

Ten 3-carbon molecules are converted back into six 5-carbon molecules – go on to repeat the cycle

FACTORS AFFECTING RATES OF PHOTOSYNTHESIS

LIGHT: Increases then level off bc light rxns are proceeding as fast as possible and Calvin cannot keep up

CO2: Same as light… will level off bc light rxns cannot keep up

Low High

TEMPERATURE: Increase in temp makes molecules move faster but if too hot ( stomata close, decreasing rate of photo.

O2:

x

Too much = photorespiration

x – atmospheric oxygen

Photorespiration interferes with successful performance of Calvin Cycle, which is undesirable in plants.

WHY? B/C Rubisco (enzyme that binds CO2 to RuBP) can bind to either CO2 or O2.

In photosynthesis: Rubisco binds with CO2

CO2

RuBP glucose (sugars)

rubisco

In photorespiration: Rubisco binds with O2

O2

RuBP CO2 released – does not produce as much glucose

rubisco

Therefore, photorespiration reduces/decreases the amount of glucose made by plants.

❑ In high concentrations of carbon dioxide, photosynthesis is dominant process.

❑ In high concentration of oxygen, photorespiration is dominant process.

V. CHAPTER 9 CELLULAR RESPIRATION & FERMENTATION

• A series of chemical reactions that release ENERGY by breaking down GLUCOSE and making CO2 and H2O.

Overall Reaction: C6H12O6 + 6O2 ( 6CO2 + 6H2O

• GLUCOSE is the primary source of ENERGY at the cellular level. The stored energy in its bonds will be converted to cellular energy in the form of ATP.

• Most of the process takes place in the MITOCHONDRIA of all cells. (Not just animal cells – remember plant cells have mitochondria too!)

Anatomy of a Mitochondrion

❑ Mitochondria have two membranes, an outer membrane covering the organelle, and an inner, folded membrane.

❑ Folds are called CRISTAE and they increase surface area for the reactions of cellular respiration to occur.

❑ The inner membrane creates a fluid filled cavity called the MATRIX, where some of the reactions of cellular respiration take place.

[pic]

Reactions of Glycolysis and Respiration

Equation: C6H12O6 + 6O2 ( 6CO2 + 6 H2O

4 main stages

AEROBIC

Glycolysis

Prep for Citric Acid (Krebs) Cycle/Formation of Acetyl CoA

Citric Acid/Krebs Cycle

Electron Transport Chain and Chemiosmosis (Oxidative Phosphorylation)

GLYCOLYSIS – occurs in cytoplasm, breakdown of glucose into 2 molecules of pyruvate aka pyruvic acid (does not require oxygen)

(2NAD+ ( 2 NADH)

GLUCOSE 2PGAL (G3P) 2 pyruvate

(C6H12O6) (3C) (3C)

2ATP 4 ATP

END PRODUCTS:

2 pyruvate (each containing 3 C – Glucose has 6)

2 NADH (electron carriers)

NET gain of 2 ATP

If Oxygen is PRESENT:

2. PREPARATION for CITRIC ACID CYCLE – pyruvate enters mitochondrion - matrix

(2NAD ( 2 NADH)

2 pyruvates 2 acetate 2 Acetly CoA

(3C) (2C) (2C)

2CO2 2 CoA

END PRODUCTS:

2 NADH

2 Acetyl CoA

2 CO2 - waste (what do you exhale???)

3. Citric Acid Cycle/Krebs occurs in the mitochondrial matrix, complete breakdown of glucose and release of carbon dioxide. Remember there were 2 molecules of pyruvate, therefore the Citric Acid Cycle must make TWO turns before you get the products from the breakdown of the two pyruvate molecules

END Products NET End Products

o 3 NADH 6 NADH

o FADH2 2 FADH2

o ATP 2 ATP

o 2 CO2 4 CO2

End Products so far

|Process |ATP |NADH |FADH2 |

|Glycolysis |2 (Net) |2 |0 |

|Prep for Krebs |0 |2 |0 |

|Krebs Cycle |2 |6 |2 |

|TOTAL |4 (substrate level |10 |2 |

| |phosphorylation) | | |

4. Electron Transport Chain – produces energy that drives the synthesis of ATP in oxidative phosphorylation, ETC consists of proteins embedded in the inner membrane of the mitochondrion, oxygen is the final electron acceptor

FADH2 and NADH are “cashed in” – oxidized (lose H ions), NAD+ and FAD are free to pick up more Hydrogen Ions

ETC does not make ATP, it provides a source of energy for the production of ATP by chemiosmosis.

Chemiosmosis – embedded in the mitochondrial membrane (along with ETC) are protein complexes called ATP synthases. These phosphorylate ATP from

ADP + Pi, ATP synthases use energy from a proton gradient – built up by the ETC

End Products: Approximately 32-34 ATPs are generated for every glucose

| |Glycolysis |Formation of Acetyl CoA|Citric Acid |ETC & |TOTAL |

| | | |Cycle |Chem.(NADH and FADH2 are | |

| | | | |cashed in) | |

|ATP |2 |0 |2 |See below |4 |

|NADH |2 |2 |6 |3 ATP/ NADH |30 |

|FADH2 |0 |0 |2 |2 ATP/FADH2 |4 |

|CO2 |0 |2 |4 | | |

| | | | | |38 ATP |

If No Oxygen is Present – Anaerobic Respiration (still begins with glycolysis, therefore start with pyruvate)

DOES not produce ATP (ATP is made during glycolysis), regenerates NAD+ so glycolysis can continue

Fermentation- enables some cells to produce ATP without the use of oxygen

Alcoholic (plants & fungi)

CO2 NADH NAD+

Acetylaldehyde ethyl alcohol

Pyruvate

(3C) (2C)

Lactic Acid (animals & some bacteria)

NADH NAD+

Pyruvate lactic acid

Occurs in some bacteria and muscle cells (that is why you feel the BURN!)

PATHWAYS OF GLYCOLYSIS & RESPIRATION

GLUCOSE

C6H12O6

IF O2 PRESENT IF O2 ABSENT

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NADP+ is an electron carrier – it can accept a pair of high-energy electrons and transfer them, along with most of their energy to another molecule (When carrying electrons is called NADPH)

ATP synthases are built into inner membrane of the mitochondria – act like mini-turbines, when H ions pass through them they “spin” which generates energy to create ATP

ALCOHOLIC FERMENTATION

(plants & fungi)

FERMENTATION

CITRIC ACID CYCLE (Kreb’s Cycle)

PREP FOR CITRIC ACID CYCLE

GLYCOLYSIS (“glucose breaking”)

LACTIC ACID FERMENTATION

(animals & some bacteria)

ELECTRON TRANSPORT & ATP SYNTHESIS

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