BOTANY LEC - Unit 3: Part 1 - Plant Physiology and Development (Photosynthesis and Light-Dependent Reactions)

Photosynthesis

Light driven conversion of water and carbon dioxide to form carbohydrates

Water (H2O), Carbon Dioxide (CO2)

Raw Materials in Photosynthesis

Photosynthesis

Light driven conversion of water and carbon dioxide to form carbohydrates

Water (H2O), Carbon Dioxide (CO2)

Raw Materials in Photosynthesis

Carbohydrates (C6H12O6)

Final Product in Photosynthesis

Chlorophyll

The primary pigment responsible for absorbing light energy in photosynthesis

Chlorophyll-a, Chlorophyll-b

Two Types of Chlorophyll

Chlorophyll-a

• The most important pigment (primary pigment) for photosynthesis

• Found in all photosynthetic organisms (except photosynthetic bacteria)

Chlorophyll-b

Absorbs light that Chlorophyll-a can’t

Stomata, Cuticle, Thylakoids

Structures Involved in Photosynthesis

Stomata

Allows entry of carbon dioxide and exit of oxygen

Cuticle

• Prevents water loss from the leaf surface

• Covers epidermal cell

Thylakoids

Contain chlorophyll and enzymes for light-dependent reactions

Grana

• Stacks found in thylakoids

• Contains the Stroma

Stroma

Contains starch, chloroplast, DNA, ribosomes, and enzymes

Light-Dependent Reactions

• The first stage of Photosynthesis

• Light energy is converted into biochemical energy

• This stage involves the transfer of energy from light to ATP and NADPH

1. Photoactivation of Chlorophyll-a

2. Transfer of energy to ATP and NADPH

3. Release of Oxygen

Key Events in Light-Dependent Reactions

Photoactivation of Chlorophyll-a

Chlorophyll-a absorbs light energy, resulting in the splitting of water molecules

Transfer of energy to ATP and NADPH

Energy is transferred to ATP and NADPH, producing energy-storing molecules

Release of Oxygen

Oxygen is release as a byproduct of the light-dependent reactions

Photosystems

Responsible for absorbing light energy and transferring it to ATP and NADPH

• Photosystem I

• Photosystem II

Two Types of Photosystems

Photosystem I

• First discovered

• Absorbs light energy; generates ATP

• Each photosynthetic unit consists of:

  • 200 or more molecules of chlorophyll-a

  • small amounts of chlorophyll-b

  • carotenoid pigment with protein attached

  • reaction-center molecule (P700)

• Absorbs light with wavelengths shorter than 700 nm

P700

Reaction-center of Chlorophyll-a

Photosystem II

• Acts Firsts

• Absorbs light energy; generates ATP and NADPH

• Each photosynthetic unit consists of:

  • chlorophyll-a

  • β-carotene (precursor of Vitamin A) attached to protein

  • small amounts of chlorophyll-b

  • reaction-center molecule (P680)

• Absorbs light with wavelengths shorter than 680 nm

P680

Reaction-center of Chlorophyll-b

Photolysis

• Water Splitting

• The process of splitting water into oxygen, protons, and electrons

• Light energy is used to split water molecules into oxygen, protons, and electrons

Photolysis Process

• The reaction-center of Photosystem 2 (PS2), P680, absorbs light energy and transfers electrons to a primary acceptor called pheophytin.

• The electrons then travel down the Electron Transport Chain (ETC) of Photosystem 2 (PS2), ultimately resulting in the formation of ATP and NADPH

Pheophytin

Primary acceptor

Electron Transport Chain (ETC)

Series of electron carriers that generate ATP from ADP and PI

Electron Transfer Process in PS2

• Involves the transfer of electrons from water to the reaction center, which is then reoxidized by the Electron Transport Chain.

• This process is coupled with the synthesis of ATP from ADP and PI

Electron Transfer

Electron Donor: H2O

Electron Acceptor: Reaction Center PS2 (P680)

Result: ?

Reoxidation of Reaction Center

Electron Donor: Reaction Center PS2 (P680)

Electron Acceptor: ETC

Result: ?

ATP Synthesis

Electron Donor: ETC

Electron Acceptor: ADP + PI

Result: ?

Photophosphorylation

• H2O (electron donor)

• NADPH (end product)

• ADP —> ATP

• splitting of H2O to create O2 and H+

Phosphorylation

• NADH (electron donor)

• Oxygen (electron acceptor)

• H2O (end product)

Plastoquinone (PQ)

• Releases and electron to an electron transport system

• Moves electron from H2O to a storage molecule NADPH (electron acceptor for PS I)

Electron Transport System

Consists of Fe-containing pigments (Cytochromes)

Cytochromes

Fe-containing pigments

Chemiosmosis

• As electrons pass along the electron transport system and protons move across thylakoid membrane by __________.

• Formation of ATP molecules (ADP + Phosphate)

• Cyclic

• Non-Cyclic

Types of Photophosphorylation

Cyclic Photophosphorylation

• Involves only Photosystem I

• Does not produce NADPH

• It generates ATP from ADP and PI

Non-Cyclic Photophosphorylation

• Involves both PS1 and PS2

• Produces both ATP and NADPH

Summary of Light Dependent Reactions

1. Sunlight

   • PS II

   • gain of 2 electrons and splitting of water molecules and releases oxygen as by-product

2. Electrons

   • move towards the pigment molecules and single bond goes to Chlorophyll-a

   • boosted to the ETC of PS II and gains energy

3. Electrons received by a primary acceptor and releases energy.

4. Energy released

   • charges photon and empowers ATP synthase

5. During the previous step

   • P700 of PS I absorbs light energy and also gets electrons (from the chloroplast ETC)

6. Electrons

   • move towards the pigment molecules towards Chlorophyll-a

   • electrons are boosted and receives energy

7. Primary acceptor receives excited electrons and travel down the ETC of PS I.

8. NADP+ picks up electrons at the end of ETC of PS I

   • picks up the H+ and comes out the ATP synthase creating NADPH2, ATP, and NADPH2

   • go to Calvin-Benson Cycle