plant metabolism
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Plant Metabolism
Chapter 10
Outline
Introduction Enzymes and Energy Transfer Photosynthesis Respiration Additional Metabolic Pathways Assimilation and Digestion
Introduction
Photosynthesis - converts light energy to usable form
Respiration - releases stored energy• Facilitates growth, development and reproduction
Metabolism - sum of all interrelated biochemical processes in living organisms
Animals rely on green plants for O2, food, shelter and other products
Enzymes and Energy Transfer Enzymes regulate metabolic activities• Anabolism - forming chemical bonds to build
molecules−Photosynthesis
• Catabolism - breaking chemical bonds−Cellular respiration
Photosynthesis-respiration Cycle involves transfer of energy via oxidation-reduction reactions
Enzymes and Energy Transfer Oxidation-Reduction Reactions• Oxidation - loss of electron(s)• Reduction - gain of electron(s)• Oxidation of one compound usually coupled with
reduction of another• H atom lost during oxidation and gained during
reduction• O usually final acceptor of electron
Photosynthesis Energy for most cellular activity = adenosine
triphosphate (ATP)• Plants make ATP using light as energy source
−Takes place in chloroplasts and other green parts of organisms
6CO2 + 12H2O + light C6H12O6 + 6O2 + 6H2O
−Many intermediate steps to process, and glucose not immediate 1st product
Photosynthesis• CO2 reaches chloroplasts in mesophyll cells by
diffusion (stomata -> leaf interior)• Use of fossil fuels, deforestation, and other human
activities add more CO2 to atmosphere than is removed−Has potential to cause global increases in
temperature
−May enhance photosynthesis
Photosynthesis Less than 1% of all H2O absorbed by plants used
in photosynthesis• Most transpired or incorporated into plant materials
H2O source of e- in photosynthesis and O2 released as by-product
If H2O in short supply or light intensities too high, stomata close and reduce supply of CO2 available for photosynthesis
Photosynthesis ~40% of radiant energy received on earth visible
light• Violet to blue and red-orange to red wavelengths
absorbed• Green light reflected• Leaves absorb ~80% of visible
light reaching them
• Light intensity varies with time of day, season, altitude, latitude, and atmospheric composition
Visible light passed through prism
Photosynthesis Plants vary considerably in light intensities needed
for optimal photosynthetic rates
Temperature and amount of CO2 can be limiting
Photosynthesis If light and temps too high: ratio of CO2 to O2 inside leaves
may change
• Accelerates photorespiration - uses O2 and releases CO2
−May help some plants survive under adverse conditions
If light intensity too high: photooxidation - results in destruction of chlorophyll
If H2O in short supply or light intensities too high: stomata close and reduce supply of CO2 available for photosynthesis
Photosynthesis Several types of chlorophyll molecules• Mg end captures light• Lipid tail anchors into thylakoid membrane• Most plants contain chlorophyll a (blue-green color)
and chlorophyll b (yellow-green color)−Chlorophyll b transfers energy from light to
chlorophyll a
Chlorophyll a molecule
Photosynthesis Other photosynthetic pigments include carotenoids
(yellow and orange), phycobilins (blue or red, in cyanobacteria and red algae), and several other types of chlorophyll
Ca. 250-400 pigment molecules grouped in light-harvesting complex = photosynthetic unit• Two types of photosynthetic units work together in
light-dependent reactions
Two phases of photosynthesis:• Light-dependent reactions• Light-independent reactions
Photosynthesis Major Steps of Photosynthesis
Light-Dependent Reactions:• Thylakoid membranes of chloroplasts• H2O split apart, releasing e- and H+; O2 released• e- pass along e- transport system• ATP produced• NADP reduced to NADPH (used in light-independent
reactions)
PhotosynthesisMajor Steps of Photosynthesis
Light-Independent Reactions:• Stroma of chloroplasts• Utilize ATP and NADPH to form sugars• Calvin Cycle
−CO2 combines with RuBP (ribulose bisphosphate) and combined molecules converted to sugars (glucose)
−Uses ATP and NADPH produced during light-dependent reactions
Photosynthesis A Closer Look: Light-Dependent Reactions
Each pigment has own distinctive pattern of light absorption = absorption spectrum
When pigments absorb light, energy levels of e- raised• Energy from excited e-
released when drops back to ground state
• In photosynthesis, energy stored in chemical bonds
Photosynthesis A Closer Look: Light-Dependent Reactions
Two types of photosynthetic units: photosystem I and photosystem II• Photosystem II before photosystem I
• Both produce ATP
• Both photosystem I and photosystem II needed to produce NADPH and O2 as result of e- flow
Photosynthesis A Closer Look: Light-Dependent Reactions
Photosystem I = chlorophyll a, small amount of chlorophyll b, carotenoid pigment, and P700
• P700 = reaction-center molecule which uses light energy
• Remaining pigments = antenna pigments−Gather and pass light energy to reaction center
• Fe-S proteins - primary e- acceptors, first to receive e-
from P700
Photosystem II = chlorophyll a, B-carotene, small amounts of chlorophyll b, and P680
• Pheophytin (Pheo) - primary e- acceptor
Photosynthesis A Closer Look: Light-Dependent Reactions
Photosynthesis A Closer Look: Light-Dependent Reactions
• Photolysis - H2O-splitting, Photosystem II
– Light photons absorbed by P680, boosting e- to higher energy level
– e- passed to acceptor molecule, pheophytin, then to PQ (plastoquinone), then along e- transport system to photosystem I
– e- extracted from H2O replace e- lost by P680
– 1 O2, 4 H+ and 4 e- produced from 2 H2O
Photosynthesis A Closer Look: Light-Dependent Reactions
e- Flow and Photophosphorylation• e- transport system consists of e- transfer molecules• Photons move across thylakoid membrane by
chemiosmosis• Phosphorylation - ATP formed from ADP
Photosynthesis A Closer Look: Light-Dependent Reactions
Photosystem I• Light absorbed by P700, boosting e- to higher energy
level• e- passed to Fe-S acceptor molecule, Fd (ferredoxin),
then to FAD (flavin adenine dinucleotide).• NADP reduced to NADPH • e- removed from P700 replaced by e- from photosystem
II.
Photosynthesis A Closer Look: Light-Dependent Reactions
Chemiosmosis• Net accumulation of H+ in
thylakoid lumen occurs from splitting of H2O molecules and e- transport
• H+ gradient gives ATPase in thylakoid membrane potential to move H+ from lumen to stroma
• Movement of H+ across membrane = source of energy for ATP synthesis
Photosynthesis A Closer Look: Light-Independent Reactions
Calvin Cycle• 6 CO2 combine with 6 RuBP (ribulose 1,5-
bisphosphate) with aid of rubisco• Results in 12 3-C molecules of 3PGA (3-
phosphoglyceric acid)• NADPH and ATP supply energy and e- reducing 3PGA
to GA3P (glyceraldehyde 3-phosphate)• 10 of 12 GA3P restructured, using 6 ATP, into 6 5-C
RuBP• Net gain of 2 GA3P -> converted to carbohydrates or
used to make lipids and amino acids
The Calvin Cycle
Photosynthesis A Closer Look: Light-Independent Reactions
Photorespiration - competes with C-fixing role of photosynthesis• Rubisco fixes O2 instead of CO2
• Allows C3 plants to survive under hot dry conditions Dissipates ATP and accumulated e-, prevents
photooxidation• When stomata closed, O2 accumulates and
photorespiration more likely• Produces 2-C phosphoglycolic acid (processed in
perioxisomes)−Forms CO2 and PGA -> reenter Calvin cycle
−No ATP formed
Photosynthesis A Closer Look: Light-Independent Reactions
C4 Pathway - produces 4-C compound instead of 3-C PGA during initial steps of light-independent reactions• C4 plants - tropical grasses and plants of arid regions• Kranz anatomy
−Mesophyll cells with smaller chloroplasts with well-developed grana
−Bundle sheath cells with large chloroplasts with numerous starch grains
Photosynthesis A Closer Look: Light-Independent Reactions
C4 Pathway
• CO2 converted to organic acids in mesophyll cells
• PEP (phosphoenolpyruvate) and CO2 combine, with aid of PEP carboxylase
• Form 4-C oxaloacetic acid instead of PGA
• PEP carboxylase converts CO2 to carbohydrate at lower CO2 concentrations than does rubisco−No photorespiration
Photosynthesis A Closer Look: Light-Independent Reactions
C4 Pathway
• CO2 transported as organic acids to bundle sheath cells, released and enters Calvin cycle
• CO2 concentration high in bundle sheath = little photorespiration
• C4 plants photosynthesize at higher temps than C3 plants −Costs 2 ATP for C4
photosynthesis
Photosynthesis A Closer Look: Light-Independent Reactions
CAM Photosynthesis - similar to C4 photosynthesis as 4-C compounds produced during light-independent reactions, however:• Organic acids accumulate
at night (stomata open)• Converted back to CO2
during day for use in Calvin cycle (stomata closed)– Adaptation to limited
H2O supply and high light intensity habitat
Respiration Respiration - release of energy from glucose
molecules broken down to individual CO2 molecules• Initiated in cytoplasm and completed in
mitochondria• Aerobic respiration needs O2
C6H12O6 + 6O2 6CO2 + 6H2O + energy
Respiration
Anaerobic respiration and fermentation - carried on in absence of O2
• Release less energy than aerobic respiration• Fermentation equations:
−C6H12O6 2C2H5OH + 2CO2 + 2ATP
−C6H12O6 2C3H6O3 + 2ATP
RespirationMajor Steps of Respiration
Glycolysis - 1st phase• In cytoplasm
• No O2 required
• Glucose converted to GA3P (glyceraldehyde 3-phosphate)
• 2 ATP molecules gained
RespirationMajor Steps of Respiration
Citric Acid (Krebs) Cycle - 2nd stage• In fluid matrix of cristae in mitochondria• High energy e- and H+ removed• NADH, FADH2 , and small amount of ATP produced
• CO2 produced as by-product
Electron transport - 3rd stage• In inner membrane of mitochondria• NADH and FADH2 donate e- to e- transport system
• Produces ATP, CO2 and H2O
RespirationA Closer Look
Glycolysis• 3 Steps:– Phosphorylation - glucose becomes fructose 1,6-
bisphosphate– Sugar cleavage - fructose 1,6-bisphosphate split into 2 3-C
GA3P (glyceraldehyde 3-phosphate) molecules– Pyruvic Acid Formation - H+, energy and H2O removed
leaving pyruvic acid
• Before citric acid cycle, pyruvic acid loses CO2 and converted to acetyl CoA
• No O2 = anaerobic respiration and fermentation−H+ released during glycolysis transferred back to
pyruvic acid, creating ethyl alcohol or lactic acid
RespirationA Closer Look
Citric Acid (Krebs) Cycle• Acetyl CoA combines with oxaloacetic acid (O.A.),
producing citric acid• Each cycle uses 2 acetyl CoA, releases 3 CO2 and
regenerates O.A.
O.A. + acetyl CoA + ADP + P + 3NAD + FAD O.A. + CoA + ATP + 3NADH + H+ + FADH2 + 2CO2
• High energy e- and H+ removed, producing NADH, FADH2
and ATP.
RespirationA Closer Look
e- Transport and Oxidative Phosphorylation• Energy from NADH and FADH2 released as H+ and e-
passed along e- transport system• H+ build up outside mitochondrial matrix =
electrochemical gradient• Chemiosmosis couples transport of H+ into matrix with
oxidative phosphorylation = formation of ATP• O2 = ultimate e- acceptor, producing H2O as it combines
with H+
• Produces net gain of 36 ATP and 6 CO2 and H2O
Respiration
Factors Affecting the Rate of Respiration
Temperature• Increase from 20o C to 30o C, respiration rates double
H2O• Medium in which enzymatic reactions take place• Low H2O content - respiration rate reduced
O2
• Reduction in O2 - respiration and growth rates decline
Additional Metabolic Pathways Other processes contribute to growth development,
reproduction and survival• Includes production of sugar phosphates, nucleotides,
nucleic acids, amino acids, proteins, chlorophylls, cytochromes, carotenoids, fatty acids, oils, and waxes
Secondary Metabolism - metabolic processes not required for normal growth and development• Enable plants to survive and persist under special
conditions−Colors, aromas, poisons - give competitive edge
Codeine, Nicotine, Lignin, Salicin, Camphor, Menthol, Rubber
Assimilation and Digestion Assimilation - conversion of organic matter
produced in photosynthesis to build protoplasm and cell walls• Sugars transformed into lipids, proteins, or other
carbohydrates, such as sucrose, starch and cellulose
Digestion - conversion of starch and other insoluble carbohydrates to soluble forms• Nearly always hydrolysis process
Review
Introduction Enzymes and Energy Transfer Photosynthesis Respiration Additional Metabolic Pathways Assimilation and Digestion
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