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Rabu, 03 Juni 2009

Photosynthesis
All Materials © Cmassengale

I. Capturing the Energy of Life
A.All organisms require energy
B.Some organisms (autotrophs) obtain energy directly from the sun and store it in organic compounds (glucose) during a process called photosynthesis
6CO2 + 6H2O + energy -->  6O2 + C6H12O6
II. Energy for Life Processes
A.Energy is the ability to do work
B.Work for a cell includes growth & repair, active transport across cell membranes, reproduction, synthesis of cellular products, etc.
C.Work is the ability to change or move matter against other forces (W = F x D)
D.Autotrophs or producers convert sunlight, CO2, and H2O into glucose (their food)
E.Plants, algae, and blue-green bacteria, some prokaryotes, are producers or autotrophs
F.Only 10% of the Earth’s 40 million species are autotrophs
G.Other autotrophs use inorganic compounds instead of sunlight to make food; process known as chemosynthesis
H.Producers make food for themselves and heterotrophs or consumers that cannot make food for themselves
I.Heterotrophs include animals, fungi, & some bacteria, & protists

  III.      Biochemical Pathways
A.Photosynthesis and cellular respiration are biochemical pathways
B.Biochemical pathways are a series of reactions where the product of one reaction is the reactant of the next
C.Only autotrophs are capable of photosynthesis
D.Both autotrophs & heterotrophs perform cellular respiration to release energy to do work
E.In photosynthesis, CO2(carbon dioxide) and H2O (water) are combined to form C6H12O6 (glucose) & O2 (oxygen)
 6CO2 + 6H2O + energy -->  6O2 + C6H12O6
F.In cellular respiration, O2 (oxygen) is used to burn C6H12O6 (glucose) & release CO2(carbon dioxide), H2O (water), and energy 
G.Usable energy released in cellular respiration is called adenosine triphosphate or ATP
 
IV. Light Absorption in Chloroplasts
A.Chloroplasts in plant & algal cells absorb light energy from the sun during the light dependent reactions
B.Photosynthetic cells may have thousands of chloroplasts
C.Chloroplasts are double membrane organelles with the an inner membrane folded into disc-shaped sacs called thylakoids
D.Thylakoids, containing chlorophyll and other accessory pigments, are in stacks called granum (grana, plural)
E.Grana are connected to each other & surrounded by a gel-like material called stroma
F.Light-capturing pigments in the grana are organized into photosystems

 V. Pigments
A.Light travels as waves & packets called photons
B.Wavelength of light is the distance between 2 consecutive peaks or troughs

C.Sunlight or white light is made of different wavelengths or colors carrying different amounts of energy
D.A prism separates white light into 7 colors (red, orange, yellow, green, blue, indigo, & violet) ROY G. BIV
E.These colors are called the visible spectrum

F.When light strikes an object, it is absorbed, transmitted, or reflected
G.When all colors are absorbed, the object appears black
H.When all colors are reflected, the object appears white
I.If only one color is reflected (green), the object appears that color (e.g. Chlorophyll)
VI. Pigments in the Chloroplasts
 

A.Thylakoids contain a variety of pigments ( green red, orange, yellow...)
B.Chlorophyll  (C55H70MgN4O6) is the most common pigment in plants & algae
C.Chlorophyll a & chlorophyll b are the 2 most common types of chlorophyll in autotrophs
D.Chlorophyll absorbs only red, blue, & violet light
E.Chlorophyll b absorbs colors or light energy NOT absorbed by chlorophyll a
F.The light energy absorbed by chlorophyll b is transferred to chlorophyll a in the light reactions

G.Carotenoids are accessory pigments in the thylakoids & include yellow, orange, & red

VII. Overview of Photosynthesis        6CO2 + 6H2O C6H12O6 + 6O2
A.Photosynthesis is not a simple one step reaction but a biochemical pathway involving many steps
B.This complex reaction can be broken down into  two reaction systems --- light dependent & light independent or dark reactions
Light Reaction:         H2O O2 + ATP + NADPH2
Water is split, giving off oxygen.
This system depends on sunlight for activation energy.
Light is absorbed by chlorophyll a which "excites" the electrons in the chlorophyll molecule.
Electrons are passed through a series of carriers and adenosine triphosphate or ATP (energy) is produced.
Takes place in the thylakoids.
Dark Reaction:         ATP + NADPH2 + CO2 C6H12O6
Carbon dioxide is split, providing carbon to make sugars.
The ultimate product is glucose.
While this system depends on the products from the light reactions, it does not directly require light energy.
Includes the Calvin Cycle.
Takes place in the stroma.



VIII. Calvin Cycle
A.Carbon atoms from CO2 are bonded or "fixed" into organic compounds during a process called carbon fixation
B.The energy stored in ATP and NADPH during the Light Reactions is used in the Calvin cycle
C.The Calvin cycle has 3 main steps occurring within the stroma of the Chloroplast
        STEP 1
CO2 diffuses into the stroma from surrounding cytosol
An enzyme combines a CO2 molecule with a five-carbon carbohydrate called RuBP
The six-carbon molecule produced then splits immediately into a pair of three-carbon molecules known as PGA
      STEP 2
Each PGA molecule receives a phosphate group from a molecule of ATP
This compound then receives a proton from NADPH and releases a phosphate group producing PGAL
These reactions produce ADP, NADP+, and phosphate which are used again in the Light Reactions.
      STEP 3
Most PGAL is converted back to RuBP to keep the Calvin cycle going
Some PGAL leaves the Calvin Cycle and is used to make other organic compounds including amino acids, lipids, and carbohydrates
PGAL serves as the starting material for the synthesis of glucose and fructose
Glucose and fructose make the disaccharide sucrose, which travels in solution to other parts of the plant (e.g., fruit, roots)

Glucose is also the monomer used in the synthesis of the polysaccharides starch and cellulose

D.Each turn of the Calvin cycle fixes One CO2 molecule so it takes six turns to make one molecule of glucose
IX. Photosystems & Electron Transport Chain
A.Only 1 in 250 chlorophyll molecules (chlorophyll a) actually converts light energy into usable energy
B.These molecules are called reaction-center chlorophyll
C.The other molecules (chlorophyll b, c, & d and carotenoids) absorb light energy and deliver it to the reaction-center molecule
D.These chlorophyll molecules are known as antenna pigments
E.A unit of several hundred antenna pigment molecules plus a reaction center is called a photosynthetic unit or photosystem
F.There are 2 types of photosystems --- Photosystem I & Photosystem II
G.Light is absorbed by the antenna pigments of photosystems II and I
H.The absorbed energy is transferred to the reaction center pigment, P680 in photosystem II, P700 in photosystem I
I.P680 in Photosystem II loses an electron and becomes positively charged so it can now split water & release electrons  (2H2O   4H+   +   4e-   +  O2)   
J.Electrons from water are transferred to the cytochrome complex of Photosystem I
K.These excited electrons activate P700 in photosystem I which helps reduce NADP+ to NADPH
L.NADPH is used in the Calvin cycle
M.Electrons from Photosystem II replace the electrons that leave chlorophyll molecules in Photosystem I

X. Chemiosmosis (KEM-ee-ahz-MOH-suhs)
A.Synthesis or making of ATP (energy)
B.Depends on the concentration gradient of protons ( H+) across the thylakoid membrane
C.Protons (H+) are produced from the splitting of water in Photosystem II
D.Concentration of Protons is HIGHER in the thylakoid than in the stroma
E.Enzyme, ATP synthetase in the thylakoid membrane, makes ATP by adding a phosphate group to ADP

XI. Alternate Pathways
A.The Calvin cycle is the most common pathway used by autotrophs called C3 Plants
B.Plants in hot, dry climates use alternate pathways to fix carbon & then transfer it to the Calvin cycle
C.Stomata are small openings on the underside of leaves for gas exchange (O2 & CO2)
D.Guard cells on each side of the stoma help open & close the stomata
E.Plants also lose H2O through stoma so they are closed during the hottest part of the day

F.C4 plants  fix CO2 into 4-Carbon Compounds during the hottest part of the day when  their stomata are partially closed
G.C4 plants include corn, sugar cane and crabgrass
H.CAM plants include cactus & pineapples
I.CAM plants open their stomata at night and close during the day so CO2 is fixed at night
J.During the day, the CO2 is released from these compounds and enters the Calvin Cycle
XII. Factors Determining the Rate of Photosynthesis
A.Light intensity - As light intensity increases, the rate of photosynthesis initially increases and then levels off to a plateau
B.Temperature - Only the dark, not the light reactions are temperature dependent because of the enzymes they use (25 oC to 37oC)
C.Length of day
D.Increasing the amount of carbon dioxide available improves the photosynthesis rate
E.Level of air pollution

 

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Cellular Respiration
All Materials © Cmassengale
C6H12O6 + 6O2 -----> 6CO2 + 6H20 + energy (heat and ATP)

Energy
Capacity to move or change matter
Forms of energy are important to life include Chemical, radiant (heat & light), mechanical, and electrical
Energy can be transformed from one form to another
Chemical energy is the energy contained in the chemical bonds of molecules
Radiant energy travels in waves and is sometimes called electromagnetic energy. An example is visible light
Photosynthesis converts light energy to chemical energy
Energy that is stored is called potential energy
Laws of Thermodynamics
1st law- Energy cannot be created or destroyed.
Energy can be converted from one form to another. The sum of the energy before the conversion is equal to the sum of the energy after the conversion.
2nd law- Some usable energy is lost during transformations.
During changes from one form of energy to another, some usable energy is lost, usually as heat. The amount of usable energy therefore decreases.
Adenosine triphosphate (ATP)
Energy carrying molecule used by cells to fuel their cellular processes
ATP is composed of an adenine base, ribose sugar, & 3 phosphate (PO4) groups

 
The PO4 bonds are high-energy bonds that require energy to be made & release energy when broken

ATP is made & used continuously by cells
Every minute all of an organism's ATP is recycled
Phosphorylation refers to the chemical reactions that make ATP by adding Pi to ADP
ADP + Pi + energy «  ATP + H2O
 
Enzymes  (ATP synthetase& ATPase) help break & reform these high energy PO4 bonds in a process called substrate-level phosphorylation
When the high-energy phosphate bond is broken, it releases energy, a free phosphate group, & adenosine diphosphate (ADP)

 
Enzymes in Metabolic Pathways:
Biological catalysts
Speeds up chemical reactions
Lowers the amount of activation energy needed by weakening existing bonds in substrates

Highly specific protein molecules
Have an area called the active site where substrates temporarily join

 
Form an enzyme-substrate complex to stress bonds
Enzyme usable
enzyme substrate complex
 
 
Energy Carriers During Respiration:
NADH: A second energy carrying molecule in the mitochondria; produces 3 ATP


FADH2: A third energy carrying molecule in the mitochondria; produces 2 ATP

Mitochondria:
Has outer smooth, outer membrane & folded inner membrane
Folds are called cristae
Space inside cristae is called the matrix & contains DNA & ribosomes
Site of aerobic respiration
Krebs cycle takes place in matrix
Electron Transport Chain takes place in cristae 

Cellular Respiration Overview:
C6H12O6 + 6O2 -----> 6CO2 + 6H20 + energy (heat and ATP)
Controlled release of energy from organic molecules (most often glucose)
Glucose is oxidized (loses e-) & oxygen is reduced (gains e-)
The carbon atoms of glucose (C6H12O6) are released as CO2
Generates ATP (adenosine triphosphate)

The energy in one glucose molecule may be used to produce 36 ATP
Involves a series of 3 reactions --- Glycolysis, Kreb's Cycle, & Electron Transport Chain
Glycolysis:
Occurs in the cytoplasm
Summary of the steps of Glycolysis:
a. 2 ATP added to glucose (6C) to energize it.
b. Glucose split to 2 PGAL (3C). (PGAL = phosphoglyceraldehyde)
c. H+ and e- (e- = electron) taken from each PGAL & given to make 2 NADH.
d. NADH is energy and e- carrier.
e. Each PGAL rearranged into pyruvate (3C), with energy transferred to make 4 ATP (substrate phosphorylation).
f. Although glycolysis makes 4 ATP, the net ATP production by this step is 2 ATP (because 2 ATP were used to start glycolysis). The 2 net ATP are available for cell use.
g. If oxygen is available to the cell, the pyruvate will move into the mitochondria & aerobic respiration will begin.


 
Net Yield from Glycolysis
4 NADH2
2 CO2
4 ATP ( 2 used to start reaction)
h. If no oxygen is available to the cell (anaerobic), the pyruvate will be fermented by addition of 2 H from the NADH (to alcohol + CO2 in yeast or lactic acid in muscle cells). This changes NADH back to NAD+ so it is available for step c above. This keeps glycolysis going!
Alcoholic Fermentation



Lactic Acid Fermentation

Aerobic Respiration:
Occurs in the mitochondria
Includes the Krebs Cycle & the Electron Transport Chain
Pyruvic acid from glycolysis diffuses into matrix of mitochondria & reacts with coenzyme A to for acetyl-CoA (2-carbon compound)
CO2 and NADH are also produced

Kreb's Cycle:
Named for biochemist Hans Krebs
Metabolic pathway that indirectly requires O2 
Kreb's Cycle is also known as the Citric acid Cycle
Requires 2 cycles to metabolize glucose
Acetyl Co-A (2C) enters the Kreb's Cycle & joins with Oxaloacetic Acid (4C) to make Citric Acid (6C)
Citric acid is oxidized releasing CO2 , free H+, & e- and forming ketoglutaric acid (5C)
Free e- reduce the energy carriers NAD+ to NADH2 and FAD+ to FADH2
Ketoglutaric acid is also oxidized releasing more CO2 , free H+, & e-
The cycle continues oxidizing the carbon compounds formed (succinic acid, fumaric acid, malic acid, etc.) producing more CO2, NADH2, FADH2, & ATP
H2O is added to supply more H+
CO2 is a waste product that diffuses out of cells
Oxaloacetic acid is regenerated to start the cycle again
NADH2 and FADH2 produced migrate to the Electron Transport Chain (ETC)

Net Yield from Kreb's Cycle (2 turns)
6 NADH2
2 FADH2
4 CO2
2 ATP
Electron Transport Chain:
Found in the inner mitochondrial membrane or cristae
Contains 4 protein-based complexes that work in sequence moving H+ from the matrix across the inner membrane (proton pumps)
A concentration gradient of H+ between the inner & outer mitochondrial membrane occurs
H+ concentration gradient causes the synthesis of ATP by chemiosmosis
Energized e- & H+ from the 10 NADH2 and 2 FADH2 (produced during glycolysis & Krebs cycle) are transferred to O2 to produce H2O (redox reaction)
O2  +  4e-  +  4H+  2H2O

Energy Yield from Aerobic Respiration
Glycolysis
Kreb's Cycle
Total
4 NADH2
6 NADH2
10 NADH2 x 3 = 30 ATP
0 FADH2
2 FADH2
2 FADH2 x 2 = 4 ATP
2 ATP
2 ATP
                         4 ATP
 
 
38 ATP
Most cells produce 36- 38 molecules of ATP per glucose (66% efficient)
Actual number of ATP's produced by aerobic respiration varies among cells

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