Download Handout: Photosynthesis: Light Reactions

Biology 20
Unit C: Photosynthesis and Cellular Respiration
6.1 Chloroplasts and Photosynthetic Pigments
Structure of the Leaf (Review)
3 µm to 8 µm in length and 2 µm to 3 µm in diameter
Epidermis: transparent covering over the top of the leaf that controls water loss
Palisade layer: thick section of chloroplasts
Spongy layer; storage of food and exchange of gases
Stomata: pores in the leaf for gas exchange, found mainly in the lower leaf
Vascular bundle: xylem: water and minerals, phloem: nutrients
Photosynthesis is the process that converts energy from the sun into chemical
energy that is used by living cells
 Solar energy is the ultimate source of energy for most living things
 5% of the light energy is converted into carbohydrate molecules such as
glucose (chemical energy)
Light
 is a type of electromagnetic (EM) radiation familiar forms include: Irays, microwaves and radio waves
 The short wavelengths have high energy and the long wavelengths
have low energy
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 Light is a mixture of photons
 Light of wavelengths 380nm and 750 nm form the visible spectrum
FIGURE 6 Page 183
Chloroplasts
Structure
 Leaves are the primary photosynthetic organs of most plants
 Chloroplast: a membrane bound organelle in plant and algal cells that
carries out photosynthesis
 Two limiting membranes, an outer and inner, these membranes enclose a
interior space filled with a protein rich semiliquid called the stroma
 Within the stroma is a system of membrane bound sacs called thylakoids
which stack on top of one another for form columns called grana, has
approximately 60 grana, each consisting of 30-5- thylakoids
 Adjacent grana are connected to one another by unstacked thylakoids celled
lamellae
 Photosynthesis takes place within the stroma and partly within the thylakoid
membrane
Chlorophyll
 Contain pigments: *chlorophyll A (blue-green) and B (yellow-green), and
the carotenoids carotene (orange), xanthophylls (yellow)
 Cholorophyll A and B are the primary pigments involved in photosynthesis
*is the only pigment that can transfer the energy of light to the carbon
fixation reactions of photosynthesis
 Each pigment absorbs different wavelengths of light
 Chlorophyll a and b absorb pigments with energies in the blue-violet and red
regions of the spectrum and reflect those with wavelengths between 500 nm
and 600 nm (green) that is why our eyes see as green light
 Carotenoids , hydrocarbons are built into the thylakoid membrane, absorbs
blue wavelengths but reflect orange and yellow
 Excessive light intensity can damage chlorophyll. Some carotenoids can
accept energy from chlorophyll, thus providing a function known as
photoprotection.
 With the onset of cooler autumn temperatures, plants stop producing
chlorophyll molecules and disassemble those already in the leaves. This
causes the yellow, red and brown colors of leaves to become visible
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Your Assignment: Page 185
6.2 The Reactions of Photosynthesis
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Means “light-building”
Takes place in the chloroplasts
stores light energy as chemical energy in chemical compounds
low energy reactants produce high energy compounds
Chlorophyll molecules MUST absorb light energy ( which is stored in the electrons of
the molecule)
6C02 + 6H20
light/chlorophyll
C6H1206 + 602
This light energy will be converted to glucose and then stored as chemical energy in
adenosine triphosphate (ATP)
Of the many energy rich molecules in living things, none is more significant than ATP
It is the molecule that is used in all living cells to perform cellular processes such as
the synthesis of needed chemicals, the active transport of materials across the cell
phosphate groups
high energy bonds
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membrane without its continual supply cell functions would come to a stop
 The phosphate bonds are extremely high energy bonds therefore when
broken a large amount of energy is released
ATP
ADP
Energy
AMP
Energy
 ATP can be made from ADP by a process called phosphorylation which is the
addition of a phosphate group
 Cells make ATP in a system called the electron transport chain
Photosynthesis is made up of 2 systems
1. light dependent reactions
2. dark reaction (carbon fixation)
1. Light Reactions (capturing the light)
 Occurs in the stroma and thylakoid membrane of the chloroplast
 Made up of two systems
1. Photosystem I
2. Photosystem II
 There are two types of chlorophyll
a) chlorophyll “a” (P680) – absorbs 680 nm light and is found in
Photosystem II
b) chlorophyll “b” (P700) – absorbs 700 nm light and is found in
Photosystem I
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Your Assignment: Page 187
Handout: Photosynthesis: Light Reactions
Photosystem II and I
 light enters Photosystem II and is trapped by chlorophyll “a” (P680)
 e- from P680 are boosted to a higher energy level and transferred to an eacceptor (this is the first oxidation-reduction reaction)
 boosting these electrons requires energy and each time an electron moves up
to a higher energy level it has a greater amount of energy
 e- are passed “downhill” through a series of electron acceptors within a
transport chain; during this time energy is released and is used to set up the
proton (H+) gradient
 proton gradient allows ADP to be phosphorylated to make ATP
 P680 needs to replace the e- it lost, so some of light energy absorbed by
p680 is used to split water by photolysis:
2 H20
4 H+ + 4e- + 02
** 4 e- go to P680 as replacement e5
** 4 H+ go to the proton gradient
** 02 goes to the atmosphere
 More light energy is trapped by chlorophyll “b” (P700) in Photosystem I
 e- from P700 are boosted to a higher energy level and transferred to an eacceptor molecule
 e- are transferred “downhill” through a series of electron acceptors until they
are picked up by the final acceptor NADP+ which is reduced (gains 2e- and 1
H+) and becomes NADPH
 e- removed from P700 are replaced with e- from P680
Proton Gradient
 Electrons from Photosystem II are transferred along an electron transport
chain and across the thylakoid membrane into the inner surface
 Some of their energy is used to pull H+ ions across the membrane, resulting
in a buildup of positive charge within the lumen
This process results in increasing concentration and electrical gradients across the
thylakoid membrane
** [H+] in stroma is low
** [H+] is thylakoid space is high
 The hydrogen ions are unable to escape the lumen except through
specialized protein complexes embedded into the membrane called ATP
synthetase complexes
 As the hydrogen ions rush through these complexes they releases energy
and some of the energy released is used to combine ADP and Pi to form ATP
 The process of making ATP using energy from an H+ ion gradient is called
chemiosmosis
END RESULT
Overall, the light reactions consume water and result in the formation of
 ATP
 NADPH *** used in the dark reactions
 oxygen
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FACTORS AFFECTING THE RATE OF PHOTOSYNTHESIS
a. wavelength of light: blue and red = increased rate
green = decreased rate
b. Intensity of light: dim light = decreased rate
bright light = increased rate to a point
c. Temperature: increased temperature = increased rate
lowered rate = decreased rate to a point
d. Concentration: increased carbon dioxide or water concentration = increased rate,
usually it is the water concentration that limits the rate
QUIZ
2. Dark Reactions
Also called the carbon-fixation reaction or the Calvin Cycle
A whole series of reactions that take place day and night
Occurs in the stroma
depends on the light reaction … it needs the ATP and NADPH
3 ATP and two NADPH are consumed for every CO2 that enters the cycle
To build one sugar molecule requires energy from 18 ATP and the electron
And protons by 12 NADPH
 C02 reaches photosynthetic cells through the stomata (openings in leaves
 and stems)
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ATP
2x3C
ADP + Pi
2 PGA
CO2
1C
unstable
molecule
NADPH
NADP+
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6C
Steps
1. CO2 enters the cycle and combines with ribulose biphosphate (RuBP)
(a 5 carbon sugar)
2. CO2 + RuBp forms an unstable 6 carbon sugar which breaks apart to form 2,
3 carbon molecules called phophoglyceric acid (PGA)
3. ATP is used by each PGA molecule to take the hydrogen away from
NADPH, forming 2 molecules of phosphoglyceraldehyde (PGAL)
4. PGAL has 2 possible fates:
1. can be joined in pairs to form glucose-3-phosphate
which is used to make glucose, starch or cellulose
2. can be converted into RuBP which keeps the CalvinBenson cycle going
END RESULT
 Glucose
The Fate of Glucose
 It depends on the needs of the animal or plant
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 If energy is needed in either plants or animals, glucose is used in
cellular respiration
 In plants, glucose is converted into starch and cellulose if it’s not
needed
 In animals, glucose is converted into glycogen and is stored in the
liver and muscles…excess is stored as fat
Pigment Lab
Your Assignment: Review page 200 #'s 1-13
7.1 Useable forms of energy
To meet the energy demands of cells, molecules must be able to
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Provide an immediate source of energy for cellular processes
Temporarily store and transport chemical energy
Store energy supplies over long term
Transfer energy within photosynthesis and respiration processes
 Oxidation occurs when elements/compounds lose electrons (or hydrogen)
LEO – (lose of electrons)
 Reduction occurs when elements/compounds gain electrons (or hydrogen)
GER – (gain of electrons)
 Each time electrons are transferred in oxidation-reduction reactions, energy is
made available for the production of ATP
Carbohydrates are the most useable sources of energy
 Plants use starch as an energy storage compound (excess as cellulose)
 Animals use glycogen as an energy storage compound (excess as fat)
Cellular Respiration releases energy that is stored in glucose bonds and this energy
is used to synthesize ATP molecules
Note: Active transport mechanisms, such as a sodium-potassium pump, allow
humans to efficiently absorb nutrient molecules
7.2 Glycolysis
 Glucose is carried in the bloodstream
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Glucose enters cells when insulin is present
Insulin is a hormone that is secreted by the pancreas into the bloodstream
Insulin increases the permeability of the cell membrane to glucose
As soon as glucose is inside the cytoplasm, a phosphate group is attached to
it by phosphorylation which makes it too bid to fit back through the cell
membrane
 The breakdown of glucose in the cell involves four stages
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(1) Glycolysis
(2) Pyruvate oxidation
(3) Krebs cycle
(4) Electron transport and chemiosmosis
1. Glycolysis
 Occurs in the cytoplasm
 Anaerobic …No oxygen needed!
 10 step process, 2.2 % efficiency
 Starting with glucose a 6 carbon sugar, Glycolysis provides TWO
3-carbon pyruvate (pyruvic acid) molecules
Steps
1. Glucose enters the cytoplasm
2. Glucose is phosphorylated and becomes glucose-6-phosphate…the Pi comes
from the conversion of ATP to ADP
3. Glucose-6-phosphate is phosphorylated again, then rearranged to become
fructose 1,6-diphosphate…the Pi comes from another ATP
4. Fructose 1,6-diphosphate breaks down into 2 PGAL molecules
5. Each PGAL is oxidized (loses a hydrogen) and becomes PGA by a molecule
called nicotinamide adenine dinucleotide (NAD, stray electron acceptor)…the
NAD is reduced (gains a hydrogen) and becomes NADH (***really high energy
molecule***)
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 Diagram
1 glucose
ATP
ADP
Pi
P
1 glucose-6-phosphate
P
1 fructose 1,6-diphosphate
ATP
ADP
Pi
P
P
NAD+
H
H
P
ADP + Pi
NADH
ATP
P
ADP + Pi
ATP
P
2 PGAL
ADP + Pi
NAD+
ATP
NADH
2 PGA
ADP + Pi
ATP
2 pyruvic acid
Note:
 During Glycolysis, oxidation-reduction reactions occur in which two
positively charged chemical compounds; NAD+ remove hydrogen
atoms from the pathway to form two NADH molecules and release two
H+ ions in to the cytoplasm
 In the later stage of Glycolysis enough energy is released to join four
ADP molecules with four Pi molecules to form four ATP molecules
 When Glycolysis is complete the cell has consumed a single glucose
molecule and produced two ATP molecules, two NADH molecules and
two pyruvate molecules
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 These ATP molecules are available to supply energy for cellular
functions
End results
 2 pyruvate molecules
 2 ATP are used as activation energy
 4 ATP are produced (net gain of 2 ATP)
 2 NADH
7.3 Aerobic Cellular Respiration
C6H1206 + 602 + 6H20
36 Pi/36 ADP
6C02 + 12H20 + 36 ATP
matrix – contains enzymes, H2O, Pi, CoA
cristae - folds
outer membrane – permeable to most
molecules
inner membrane – permeable to
pyruvic acid and ATP
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Mitochondria Structure
Parts of the mitochondria:
 Double membrane: outer protects the mitochondria and an inner that
performs many functions associated with cellular respiration
 Matrix: protein rich liquid that fills the innermost space
 Intermembrane Space: lies between inner and outer membrane (role in
aerobic respiration)
2. Pyruvate Oxidation
Recall, that by the end of Glycolysis stage 1, the cell had forms 2 ATP, 2 NADH and
2 pyruvate, all in the cytoplasm
These materials enter the two mitochondrial membranes in to the matrix
i. A CO2 is removed from each pyruvate and released as a waste product
(this step is the source of one-third of the carbon dioxide that you breathe out)
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ii. The remaining 2 carbon portions are oxidized by NAD+
iii. In the process, each NAD+ gains two hydrogen from the pyruvate and the
remaining 2 carbon compound becomes acetic acid (this reaction transfers
high energy hydrogen’s to NAD+)
iv. a compound called co-enzyme A attaches to acetic acid forming acetyl-CoA
and prepares the two carbon acetyl portion of this molecule for entering the
Kreb’s cycle
3. Kreb’s Cycle
 Kreb’s cycle takes place in the matrix of the mitochondria
 Anaerobic … no oxygen required but … it can’t work without the electron
transport chain which is aerobic and requires oxygen
 8-step process discovered by Sir Hans Krebs in 1937
 Each step is catalyzed by an enzyme
Steps
1. Pyruvic acid (3C) is oxidized when it joins with CoA (coenzyme A) to form acetyl
CoA (2C), releasing CO2 (-1C) and forming NADH
2. Acetyl CoA (2C) joins a 4C molecule (oxaloacetic acid), releasing CoA and
forming citric acid (6C)
3. Citric acid is oxidized twice releasing 2 CO2 (-2C) and making 2 NADH and 1 ATP
4. Two more reactions in the cycle produces 1 NADH and 1 FADH2 (flavin adenine
dinucleotide)
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 Diagram
CO2 (-1C)
pyruvic acid (3C)
NAD+
NADH
acetyl CoA (2C)
CoA
CO2 (-1C)
CoA
citric acid (6C)
NADH
-ketogluteric acid (5C)
CoA
oxaloacetic acid (4C)
ADP
ATP
CoA
NADH
fumeric acid (4C)
CO2 (-1C)
NADH
succinic acid (4C)
FADH2
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END RESULTS OF KREB’S CYCLE – for every 2 pyruvic acid
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2 ATP
8 NADH
2 FADH2
6 C02
4. Electron Transport Chain
 Occurs in the cristae
 Aerobic …needs oxygen
NADH and FADH2 transfer hydrogen atoms and electrons they carry to a series of
compounds associated with the inner mitochondrial membrane
 High energy electrons from NADH and FADH are passed “downhill” to
electron carriers called cytochromes (passed like a baton handed from runner
to runner in a relay race)
 Energy released by electrons are used to pump protons from the matrix of the
mitochondria to the outer membrane, setting up the proton gradient (just like
in the light reactions)
 Protons moves back across the inner membrane to the matrix, down the
gradient … this activates ATP synthase and allows ATP to be made from
ADP and Pi
 At the end of the electron transport chain is oxygen which is the strongest
electron acceptor
 Oxygen (strong electron acceptor) accepts the electrons along with hydrogen
ions forming water
***for every 2 electrons that pass from NADH to oxygen … 3ATP are made
***for every 2 electrons that pass from FADH to oxygen … 2 ATP are made
END RESULT OF ELECTRON TRANSPORT CHAIN
 32 ATP
 6 water (12(NADH + FADH) = 6 water)
TOTAL for CELLULAR RESPIRATION
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 36 ATP ( 2 from glycolysis, 2 from Kreb’s cycle and 32 from the electron
transport chain)
 6 carbon dioxide from Kreb’s Cycle
 6 water from electron transport chain
 Diagram
cristae
outer membrane
inner membrane
intermembrane
space
high [H+]
H+
matrix
low [H+]
2 epump
H+
-
2e
+
H
pump
2 e-
+
H
pump
+
2 e-
H
H+
H+
pump
+
H
ATP
2 e½ O2
H2O
H+
H+
ADP + Pi
phospholipid bilayer
ATP synthetase
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7.4 Anaerobic Respiration
 Without oxygen the ETC cannot operate and as a result, anaerobic organisms
have evolved several ways of recycling NAD+ and allowing glycolysis to
continue
 One method involves transferring the hydrogen atoms of NADH to certain
organic molecules instead of the ETC this process is called fermentation.
 There are a dozen different forms of fermentation
 There are two main types used by eukaryotes which have different end
products
 both take place in the cytoplasm
 In both methods glucose is not completely oxidized
 Both begin with glycolysis as their first step
 Two methods:
Note: glucose is not completely oxidized
(1) Alcohol Fermentation
C6H12O6 + 2 ADP +2Pi  C2H5OH + CO2 + 2ATP
(ethanol)
 NADP’s produced during glycolysis pass their hydrogen atoms to
acetaldehyde
 acetaldehyde is a compound formed when a carbon dioxide molecule is
removed from pyruvate by the enzyme pyruvate decarboxylase
 this forms ethanol, which is use in alcoholic beverages.
This is carried out by yeast to make products such as bread and pastries, wine, beer,
liquor and soy sauce
Example
Bread is made by mixing live yeast cells with starch and water. The yeast ferments
the glucose from the starch and releases carbon dioxide and ethanol. The small
bubbles of carbon dioxide gas cause the bread to rise and the ethanol evaporates
when the bread is mixed.
(2) Lactic Acid Fermentation
C6H12O6+2ADP +2Pi  2C3H6O3 +2 ATP
(lactic acid)
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 Under normal conditions, animals such as humans obtain energy from
glucose by aerobic respiration.
 Under strenuous exercise, muscles demand more ATP than can be supplied
by aerobic respiration.
 Additional ATPs are supplied by lactic acid fermentation
 NADH produced in glycolysis transfers its hydrogen atoms to pyruvate in the
cytoplasm of the cell
 This regenerates NAD+ and allows glycolysis to continue…… This changes
pyruvate into lactic acid.
Accumulation of lactic acid causes sore muscles, stiffness and fatigue, when the
exercise stops the lactic acid is converted back to pyruvate and aerobic
Notice that the first stage for both aerobic and anaerobic respiration is
glycolysis
Your Assignment: Lactic Acid Handout
Your Assignment: Page 228 #'s 1-8
Your Assignment: Review Page 232 #'s 1-14, 15, 16, 17,& 20
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HISTORY OF PHOTOSYNTHESIS AND RESPIRATION
Select two of the scientists from the following list. For each one, give a brief
biography of his life and his contribution to our knowledge of photosynthesis or
respiration. Your report may be in the form of a poster or essay. The essay would
be NO longer than one page per scientist.
LIST:
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Hans Krebs
Samuel Ruben
Martin Kamen
Jean Senebier
C.B. van Niel
Andre Jagendorf
Antoine Lavoisier
Linus Pauling
Daniel Arnon
Jan Ingenhousz
Nicolas de Saussure
Julius Meyer
Thomas Engelmann
Melvin Calvin
Peter Mitchell
Robert Hill
Albert Lehninger
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