NOTES FOR BIOLOGY 1201

Section 001

Spring 2005

DR. STEVEN POMARICO

Photosynthesis transforms light energy trapped by chloroplasts into chemical bond energy and stores that energy in sugar and other organic molecules

         -Synthesis of energy-rich organic molecules from energy-poor molecules

                   (CO₂ and H₂O)

         -Uses CO₂ as a carbon source and light-energy as the energy source.

         -Directly or indirectly supplies energy to most living organisms.

Major Modes of Nutrition Among Organisms - 3.7.1

>>>>>Plants and other autotrophs are the producer of the biosphere

Organisms acquire organic molecules used for energy and carbon skeletons (a.k.a. food) by one of two nutritional modes:

                   1. Autotrophic nutrition

                   2. Heterotrophic nutrition

---Autotrophic nutrition

---Photoautotrophic

---Heterotrophic nutrition

The Leaf: Adaptations for Photosynthesis - 5.2.1

>>>>>Chloroplasts are the site of photosynthesis in plants

Leaves are the major organs of photosynthesis

         -Chlorophyll is the green pigment that gives a leaf its color

chlorophyll is also responsible for the absorption of the light energy

that drives photosynthesis

         -Chloroplasts are primarily in cells of mesophyll (in the leaf interior)

         -CO₂ and H₂O enter the leaf through pores called stomata

         -Water is absorbed by the roots and transported to the leaves through the                    vascular bundles.

Isotopes: Unraveling Photosynthesis - 2.1.4

>>>>>Evidence that chloroplasts split water molecules let researchers track atoms through photosynthesis.

The summary equation for photosynthesis:

6 CO₂ + 12 H₂O + light energy =>

                                                  C₆H₁₂O₆ + 6 O₂ + 6 H₂O

While glucose is shown, other carbohydrates are the main products of photosynthesis.

Water is present on both sides of this equation because photosynthesis uses and produces H₂O.

The equation may be written as a net equation:

6 CO₂ + 6 H₂O + light energy => C₆H₁₂O₆ + 6 O₂

This looks very similar to the reverse of cellular respiration:

C₆H₁₂O₆ + 6 O₂ => 6 CO₂ + 6 H₂O + Energy                                                         (ATP + heat)

Plants use both photosynthesis and respiration, however they are NOT the reverse of a single pathway.

The splitting of water

Initially it was hypothesized that the oxygen generated by photosynthesis came from carbon dioxide

CO₂ + light energy => C + O₂

the freed carbon would then combine with water to from carbohydrates:

C + H₂O => CH₂O

However C.B van Niel found that some purple-sulfur bacteria use H₂S rather than water for photosynthesis.

These bacteria still produce carbohydrates, but instead of generating oxygen they make sulfur.

CO₂ + 2 H₂S + light energy => CH₂O + 2 S + H₂O

Using this reaction as a model van Neil proposed a general photosynthesis reaction:

CO₂ + 2 H₂X + light energy => CH₂O + H₂O + 2 X

From that model the reactions for photosynthesis in plants can be written

CO₂ + 2 H₂O^* + light energy => CH₂O + H₂O + O₂^*

Photosynthesis as a redox process (antithesis of cellular respiration)

Remember in cellular respiration:

         -the process is exergonic

         -carbon and hydrogen atoms of the sugar are oxidized (lose in their sharing                    of electrons)

         -oxygen atoms are reduced (gain in their sharing of electrons).

         -electrons lose energy as they travel down the electron transport chain

         -energy is used to produce ATP

         -water is formed from oxygen

In photosynthesis:

         -the process is endergonic (requires light energy)

         -carbon and hydrogen atoms from carbon dioxide and water are reduced                              (gain in their sharing of electrons)

         -oxygen atoms of water are oxidized (lose in their sharing of electrons).

         -electrons gain energy from light energy as they travel up the electron                              transport chain

         -energy is used to produce sugar and ATP

         -oxygen is formed from water

Photosynthesis is a REDOX reaction:

In cellular respiration the electrons from sugar’s carbon and hydrogen atoms lose potential energy (oxidation) as they are transferred toward oxygen

In photosynthesis the electrons from water’s hydrogen atoms and carbon dioxide’s carbon atoms gain potential energy (reduction) as they are transferred toward sugar.

The Nature of Light - 5.2.4

>>>>>Photosynthesis transforms light energy into ATP and NADPH.

>The nature of sunlight

         Light is electromagnetic energy

                   Light energy has both wavelike and particle-like properties

                   1. wavelike properties

                             -Electromagnetic energy travels in waves.

                             -Wavelength

                             -Electromagnetic spectrum (gamma rays) 10^-3 nm to 10³ m                                                  (radio waves)

                             -Visible light 380 nm to 750 nm

                   2. particle like properties

                             -Electromagnetic energy also travels in particles.

                             -discrete particles called photons

The Structure of a Chloroplast - 5.2.2

Photosynthetic Pigments - 5.2.3

>Photosynthetic pigments: The light receptors

         -Different pigments absorb different wavelengths of light

         -Absorption versus reflection (or transmittance)

         -The pattern of absorption is called the absorption spectrum

         -The absorption spectrum of a pigment can be measured using a                                   spectrophotometer

         -Chlorophyll a is the pigment that participates in the light                           reaction

         -Other pigments (chlorophyll b, and carotenoids) are called accessory                              pigments

Photoexcitation and Electron Transfer - 5.2.5

>Photoexcitation of chlorophyll

         An electron in a chlorophyll molecule is boosted to an excited state from its                ground state by the absorbed light energy (photons).

         The excited electron may:

                   -be transferred to an electron carrier molecule

         or 

                   -fall back to the ground state, releasing its energy as:

                             Heat or light (fluorescence)

The Light Reactions: An Introduction - 5.3.1

>Photosystems are the light-harvesting complexes of the thylakoid membranes.

Chlorophyll a and the accessory pigments are arranged into a photosystem

Photosystems are composed of a few hundred pigment molecules

Components of a photosystem:

                   1. Antenna complex

                   2. Reaction-center chlorophyll

                   3. Primary electron acceptor

There are two types of photosystems

                   1. Photosystem I (PS-I)

                   2. Photosystem II (PS-II)

Photosystem 1 - 5.3.2

Photosystem I => P700

         Reaction center has a specialized chlorophyll that absorbs light best at 700 nm.

Photosystem 2 - 5.3.3

Photosystem II => P680

         Reaction center has a specialized chlorophyll that absorbs light best at 680 nm

>>>>>The light reactions transform light energy to chemical energy

---Light reaction

         -Solar energy to chemical energy

         -Occurs in the thylakoid membranes

         -Reduces NADP⁺ to NADPH

         -H₂O split, O₂ byproduct

         -Generates ATP by photophosphorylation

                   NOTE: no sugar produced

---photophosphorylation

The Light Reactions: A Summary - 5.3.4

>Noncyclic electron flow

The electrons that are excited from each of the photosystems can be donated to a primary electron acceptor.

         -Both photosystems are active

         -Electrons flow in a path from one electron carrier to the next.

                   Light excites electrons in PS-II (P680)

                   Electrons transferred to the primary electron acceptor

                   Electrons flow down the electron transport chain

                   Electrons transferred to PS-I (P700)

                   Light excites electrons in PS-I (P700)

                   Electrons transferred to the primary electron acceptor

                   Electrons transferred to NADP⁺

                   Electrons (and reducing power) are stored in NADPH

         -Electrons from the splitting of H₂O, replace the electrons lost from PS-II and

                    generate O₂ as a byproduct.

As the electrons flow through this non-cyclic pathway they are gaining energy

The flow of electrons and the splitting of water also creates a proton gradient across the thylakoid membrane, which then is used to synthesize ATP by chemiosmosis.

This process is known as noncyclic photophosphorylation

Pseudo-summary equation for Non-cyclic electron flow:

       4 Photons    + H₂O + NADP⁺ + H⁺ + 4 ADP + 4 P_i

(2@680 & 2@700)

                                   =>

       2 O₂ + NADPH + 4 ATP + 4 H₂O

>Cyclic electron flow

         -Only photosystem I is active

         -Electrons flow in a path form one electron carrier to the next.

                   Light excites electrons in PS-I (P700)

                   Electrons transferred to the primary electron acceptor

                   Electrons flow down the electron transport chain

                   Electrons transferred to PS-I (P700)

         -Electrons from the electron transport chain replace electrons lost from PS-I.

         -No H₂O split and no O₂ generated

-The flow of electrons creates a proton gradient across the thylakoid membrane, which is used to synthesize ATP by chemiosmosis.

This process is known as cyclic photophosphorylation.

Pseudo-summary equation for Cyclic electron flow:

2 Photons (@700) + 2 ADP + 2 P_i => 2 ATP + 2 H₂O

A Review of Photosynthesis - 5.4.3

>Comparison of chemiosmosis in chloroplast and mitochondria

---Chemiosmosis

Similarities

         -Electron transport chains move protons across membrane as electron are                                        moving down the chain.

         -ATP synthase is in the same membrane as the electron transport chain

         -Diffusion of H⁺ is coupled to ATP production

         -Many components are very similar in structure.

Differences

         1. Electron transport chain

                   -Origin of electrons

                             -in respiration they come from food via NADH

                             -in photosynthesis they come from H₂O or PS-I

                   -Origin of energy

                             -in respiration energy is chemical energy from food

                             -in photosynthesis energy is light energy.

         2. Spatial organization

                   -in the mitochondria, electrons are pumped out across the inner                                                  mitochondrial membrane

                   -in chloroplast electrons are pumped into the thylakoid space

The Calvin Cycle - 5.4.1

The Calvin Cycle: RuBP Regeneration - 5.4.2

>>>>>>The Calvin-Benson cycle uses ATP and NADPH to convert CO₂ to sugar

-Occurs in the chloroplast stroma

         -Incorporates CO₂ into organic molecules by carbon fixation, and then reduces the                               fixed carbon to carbohydrate.

---Carbon fixation

         -No direct light energy required

         -ATP and NADPH from the light reaction

         -NADPH provides the reducing power and ATP provides the chemical energy.

Calvin Cycle produces a 3-carbon sugar (glyceraldehyde 3-phosphate)

The Calvin-Benson Cycle is divided into 3 phases

         1. Input phase

                   CO₂ is fixed by rubisco

         2. Reduction phase

                   ATP is used for phosphorylation

                   NADPH is used for reduction

         3. Regeneration of CO₂ Acceptor

                   Sugar carbons are shuffled around to make 3 5-carbon sugars                                                            from 5 3-carbon sugars

To produce one molecule of glyceraldehyde 3-phosphate, 3 CO₂ molecules are fixed.

Two molecules of glyceraldehyde 3-phosphate are required to make one molecule of glucose

The summary reaction for the Calvin-Benson cycle

6 CO₂ + 18 ATP + 12 NADPH + 12 H₂O + 12 H⁺ =>

                            1 C₆H₁₂O₆ + 18 ADP + 18 P_i + 12 NADP⁺

1 glucose molecule consumes 18 ATP's and 12 NADPH

The ADP and NADP⁺ return to the light reaction

A summary reaction for photosynthesis

         48 Photons   + 12 H₂O + 6 CO₂ + 30 ADP + 30 P_i

(24@680 & 24@700)

                                            |

                                           V

              6 O₂ + 30 ATP + 36 H₂O + C₆H₁₂O₆

Note: The sections on: Photorespiration - 5.5.1 and C4 Plants and CAM Plants - 5.5.2 will not be covered in lecture. You will be responsible for this material and should know the MAIN POINTS.