PharmacognosyPhotosynthesis

Prof. Suleiman OLimat

THE BASICS OF PHOTOSYNTHESIS

Plant Metabolism

Light Energy Harvested by Plants & Other Photosynthetic Autotrophs

6 CO2 + 6 H2O + light energy → C6H12O6 + 6 O2

• Photosynthesis is the process by which autotrophic organisms use light energy to make sugar and oxygen gas from carbon dioxide and water

AN OVERVIEW OF PHOTOSYNTHESIS

Carbondioxide

Water Glucose Oxygengas

PHOTOSYNTHESIS

The Most Important Equation in Biology

• The Calvin cycle makes sugar from carbon dioxide– ATP generated by the light

reactions provides the energy for sugar synthesis

– The NADPH produced by the light reactions provides the electrons for the reduction of carbon dioxide to glucose

LightChloroplast

Lightreactions

Calvincycle

NADP

ADP+ P

• The light reactions convert solar energy to chemical energy– Produce ATP & NADPH

AN OVERVIEW OF PHOTOSYNTHESIS

Steps of Photosynthesis

• Light hits reaction centers of chlorophyll, found in chloroplasts

• Chlorophyll vibrates and causes water to break apart.

• Oxygen is released into air• Hydrogen remains in chloroplast

attached to NADPH• “THE LIGHT REACTION”

Steps of Photosynthesis

• The DARK Reactions= Calvin Cycle• CO2 from atmosphere is joined to H

from water molecules (NADPH) to form glucose

• In most plants, photosynthesis occurs primarily in the leaves, in the chloroplasts

• A chloroplast contains: – stroma, a fluid – grana, stacks of thylakoids

• The thylakoids contain chlorophyll– Chlorophyll is the green pigment that captures

light for photosynthesis

Photosynthesis occurs in chloroplasts

• Chloroplasts contain several pigments

Chloroplast Pigments

– Chlorophyll a – Chlorophyll b – Carotenoids– Xanthophyll

Figure 7.7

Chlorophyll a & b•Chl a has a methyl group •Chl b has a carbonyl group

Porphyrin ring delocalized e-

Phytol tail

• Two connected photosystems collect photons of light and transfer the energy to chlorophyll electrons

• The excited electrons are passed from the primary electron acceptor to electron transport chains– Their energy ends up in ATP and NADPH

In the light reactions, electron transport In the light reactions, electron transport chains generate ATP, NADPH, & Ochains generate ATP, NADPH, & O22

• The electron transport chains are arranged with the photosystems in the thylakoid membranes and pump H+ through that membrane– The flow of H+ back through the membrane is

harnessed by ATP synthase to make ATP– In the stroma, the H+ ions combine with NADP+ to

form NADPH

Chemiosmosis powers ATP synthesis in the light reactions

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Introduction Metabolism: (Gr. metabole = change) the totality of the chemical changes in living cells which involves the buildup and breakdown of chemical compounds. Primary metabolism: biosynthesis, utilization and breakdown of the essential compounds and structural elements of the living organism, such as: sugars and polysaccharides; amino acids, peptides and proteins (including enzymes); fatty acids; and nucleotides. The starting materials are CO2, H2O and NH3. All organisms possess similar primary metabolic pathways and use similar primary metabolites.

Primary Metabolites

• Primary metabolites are compounds that are commonly produced by all plants and that are directly used in plant growth and development.

• The main primary metabolites are carbohydrates, proteins, nucleic acids, and lipids.

Carbohydrates• Carbohydrates are the sugars made up of

glucose and its isomers• Carbohydrates come in many different sizes:• Monosaccharides made up of one sugar unit

(glucose or fructose)• Disaccharides made up of two sugar units

(sucrose is a glucose and a fructose)• Polysaccharides are polymers made up of more

than two sugar units

Structural Polysaccharides

• The most common structural polysaccharide in plants is cellulose. It makes up 40 to 60% of the cell wall. It is also the most common polymer on earth

• Cellulose is extremely strong due to its chemical organization. It is made of a long chain of beta-glucose molecules – 100 to 15,000 glucose molecules

Proteins

• Proteins make up most of the remaining biomass of living plant cells.

• A protein consists of one or more polypeptides made up of amino acids. Plants make amino acids from the products of photosynthesis through a very complex process involving the acquisition of N, usually in the form of NH4, and involving the use of large amounts of energy, in the form of ATP and NADPH.

Enzymes

• Enzymes catalyze biochemical reactions. Most proteins in living cells are enzymes.

• Pure enzymes that maintain their activity when removed from plants are commercially important to us.

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Introduction Secondary metabolism: refers to the biosynthesis, utilization and breakdown of smaller organic compounds found in the cell. These compounds, called secondary metabolites, arise from a set of intermediate building blocks : acetyl coenzyme A (acetyl-CoA), mevalonic acid (MVA) and methyl erythritol phosphate (MEP), shikimic acid, and the amino acids phenylalanine/tyrosine, tryptophan, ornithine and lysine.

SCoA

O CO2H

CH3HO

OH

CO2H

OH

OH

HO

NH2R

CO2H

NNH2

CO2H

H

H2N CO2H

NH2

H2NCO2H

NH2

HO

CH3HO

OP

OH

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Introduction Relationship between primary and secondary metabolism: • The processes and products of primary metabolism are similar in most organisms, while those of secondary metabolism are more specific.

• In plants, primary metabolism is made up of photosynthesis, respiration, etc., using CO2, H2O, and NH3 as starting materials, and forming products such as glucose, amino acids, nucleic acids. These are similar among different species.

• In secondary metabolism, the biosynthetic steps, substrates and products are characteristic of families and species. Species which are taxonomically close display greater similarities (and metabolites); those which are distant have greater differences.

3. Secondary metabolites and Biosynthesis (Dayrit)

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PRIMARY METABOLITES INTERMEDIATE METABOLITES SECONDARY METABOLITES

CO2 + H2O Glucose

Polysaccharides

Pentose phosphateErythrose-4-phosphate

Phosphoenol pyruvate

Shikimate

Aromatic compounds(C6

-C1; C6-C2)

Phenylpropanoids (C 6-C3)

Lignans

PyruvateCitric acidcycle

Aromaticamino acids

Aliphaticamino acids

Aromatic alkaloids

Mixed alkaloids

Aliphatic alkaloids

Acetyl-CoA Polyketides Polyphenols

PhenylpropanoidsFlavonoids

Fatty acidsPolyacetylenesProstaglandins

Mevalonic acid TerpenesSteroidsCarotenoids

+NH3

Iridoids

Aliphaticamino acids

Alkaloids

Overview of Secondary

Metabolism

* Metabolites found in higher organisms only

*

*

*SCoA

O

CO2H

CH3HO

OH

CO2H

OH

OH

HO

NH2R

CO2H

NNH2

CO2H

H

H2N CO2H

NH2

Plant Secondary Metabolites• Plants make a variety of less widely distributed

compounds such as morphine, caffeine, nicotine, menthol, and rubber. These compounds are the products of secondary metabolism, which is the metabolism of chemicals that occurs irregularly or rarely among plants, and that have no known general metabolic role in plants.

• Secondary metabolites or secondary compounds are compounds that are not required for normal growth and development, and are not made through metabolic pathways common to all plants.

• Most plants have not been examined for secondary compounds and new compounds are discovered almost daily.

Plant Secondary Metabolites• Secondary compounds are grouped into classes

based on similar structures, biosynthetic pathways, or the kinds of plants that make them. The largest such classes are the alkaloids, terpenoids, and phenolics.

• Secondary compounds often occur in combination with one or more sugars. These combination molecules are known as glycosides. Usually the sugar is a glucose, galactose or rhamnose. But some plants have unique sugars. Apiose sugar is unique to parsley and its close relatives.

Functions of Secondary Compounds

• The most common roles for secondary compounds in plants are ecological roles that govern interactions between plants and other organisms.

• Many secondary compounds are brightly colored pigments like anthocyanin that color flowers red and blue. These attract pollinators and fruit and seed dispersers.

• Nicotine and other toxic compounds may protect the plant from herbivores and microbes.

• Other secondary compounds like rubber and tetrahydrocannabinil (THC) from cannabis plants have no known function in plants.