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