Lecture 1- Photosynthesis

Overview of Photosynthesis

• Photosynthesis Definition: Process by which solar energy is converted into chemical energy.

  • Equation: 6 CO2 + 6 H2O → C6H12O6 + 6 O2

• Importance of Photosynthesis:

  • Essential for life; forms the foundation of ecosystems.

  • Provides energy for consumers and decomposers.

Components of Photosynthesis

Ingredients Required

• Sunlight: Primary energy source.

• Carbon Dioxide (CO2): Gas absorbed from the atmosphere.

• Water (H2O): Taken from the soil, crucial for chemical reactions.

Pigments Involved

• Pigments: Molecules that capture light energy.

  • Chlorophyll a: Main pigment, absorbs blue and red light; reflects green.

  • Chlorophyll b: Accessory pigment, absorbs blue and orange light.

  • Carotenoids: Accessory pigments that absorb blue light and appear red, orange, or yellow.

Photosynthesis Process

Stages of Photosynthesis

1. Light-Dependent Reactions (LDR):

   • Occur in the thylakoid membranes of chloroplasts.

   • Convert solar energy into chemical energy (ATP and NADPH).

   • Water is split to provide electrons and produce O2.

2. Calvin Cycle (Light-Independent Reactions):

   • Occurs in the stroma of chloroplasts.

   • Uses ATP and NADPH from LDR to convert CO2 into glucose (sugar).

   • Main enzyme involved is Rubisco.

LDR Detailed Steps

• Photon Absorption: Photosystem II absorbs light, exciting electrons.

• Water Splitting: Electrons lost from chlorophyll molecules are replaced by splitting water.

• Electron Transport Chain (ETC): Electrons are transferred through proteins, pumping H+ ions into the thylakoid space, creating a gradient.

• ATP Production: H+ ions flow back into the stroma through ATP synthase, generating ATP.

• NADPH Formation: Electrons are transferred to NADP+ to form NADPH.

Calvin Cycle Detailed Steps

• Carbon Fixation: CO2 is fixed into organic molecules.

• Reduction Phase: ATP and NADPH from the LDR are used to convert 3-PGA into G3P.

• Regeneration: Some G3P is used to regenerate RuBP, enabling the cycle to continue.

Absorbance Spectrums

• Chlorophyll Absorption:*

  • Does not absorb green light; hence leaves appear green due to reflected light.

  • Different pigments absorb various wavelengths of light, increasing the efficiency of photosynthesis.

Relationship between Photosynthesis and Cellular Respiration

• Complementary Reactions: Photosynthesis stores energy in glucose, whereas cellular respiration converts glucose back into usable energy (ATP).

  • Photosynthesis Equation: 6 CO2 + 6 H2O → C6H12O6 + 6 O2.

  • Cellular Respiration Equation: C6H12O6 + 6 O2 → 6 CO2 + 6 H2O + ATP.

Clicker Questions Review

• Essential to understand how mutations, ratios of pigments, and conditions affect the processes of light-dependent reactions and the Calvin cycle.

Overview of Photosynthesis

Photosynthesis Definition

Photosynthesis is the biochemical process by which solar energy is harnessed by plants, algae, and some bacteria to convert carbon dioxide (CO2) and water (H2O) into glucose (C6H12O6) and oxygen (O2). This process not only provides energy for the organisms that perform it but also forms the base of the food chain for almost all life on Earth.

Equation

The overall chemical reaction of photosynthesis can be represented by the following equation:

[ 6 CO_2 + 6 H_2O \rightarrow C_6H_{12}O_6 + 6 O_2 ] This equation illustrates how six molecules of carbon dioxide combine with six molecules of water in the presence of sunlight to produce one molecule of glucose and six molecules of oxygen.

Importance of Photosynthesis:

• Essential for Life: Photosynthesis is vital for sustaining life on Earth by forming the foundation of ecosystems. It produces not only food but also oxygen, which is essential for aerobic organisms, including humans.

• Energy Provision: The glucose produced is utilized by plants for energy through cellular respiration and is also made available to other organisms through the food chain as herbivores consume plant material. Decomposers rely on the leftovers or dead organic material, further connecting the web of life.

Components of Photosynthesis

Ingredients Required

1. Sunlight: The primary energy source captured by chlorophyll and other pigments within the chloroplasts of plant cells.

2. Carbon Dioxide (CO2): This gas is absorbed from the atmosphere through small openings called stomata located mainly on the leaves.

3. Water (H2O): Taken from the soil through the plant roots, water is critical for the chemical reactions involved in photosynthesis.

Pigments Involved

• Pigments: These are molecular compounds that absorb specific wavelengths of light.

  • Chlorophyll a: The main pigment in photosynthesis, which absorbs light most efficiently in the blue-violet and red spectrums while reflecting green, giving plants their characteristic color.

  • Chlorophyll b: An accessory pigment that complements chlorophyll a by absorbing additional wavelengths of light, particularly blue and orange light, thereby enhancing the efficiency of photosynthesis.

  • Carotenoids: These accessory pigments absorb blue light and reflect red, orange, or yellow, and play a role in protecting chlorophyll from damage caused by excess light.

Photosynthesis Process

Stages of Photosynthesis

• Light-Dependent Reactions (LDR):

  • Occur in the thylakoid membranes of the chloroplasts.

  • Convert solar energy into chemical energy in the form of ATP and NADPH.

  • The process involves splitting water molecules to provide electrons, resulting in the release of oxygen as a byproduct.

• Calvin Cycle (Light-Independent Reactions):

  • Occurs in the stroma of chloroplasts where ATP and NADPH from LDR are used to convert CO2 into glucose.

  • The key enzyme involved in carbon fixation is Rubisco (Ribulose bisphosphate carboxylase/oxygenase).

LDR Detailed Steps

1. Photon Absorption: In photosystem II, chlorophyll absorbs photons of light, which excites electrons to a higher energy state.

2. Water Splitting: The excited electrons that leave chlorophyll are replenished by splitting water molecules into oxygen, protons, and electrons.

3. Electron Transport Chain (ETC): Excited electrons move through a series of proteins, creating a proton gradient by pumping H+ ions into the thylakoid space.

4. ATP Production: Protons flow back into the stroma through ATP synthase, a process known as chemiosmosis, leading to the production of ATP.

5. NADPH Formation: Electrons are transferred to NADP+ (nicotinamide adenine dinucleotide phosphate), generating NADPH for use in the Calvin cycle.

Calvin Cycle Detailed Steps

1. Carbon Fixation: CO2 is incorporated into a 5-carbon sugar, ribulose bisphosphate (RuBP), forming an unstable 6-carbon compound.

2. Reduction Phase: ATP and NADPH from the LDR are used to convert 3-phosphoglycerate (3-PGA) into glyceraldehyde-3-phosphate (G3P).

3. Regeneration: A portion of G3P is used to regenerate RuBP, allowing the cycle to continue, and the remaining G3P can be converted into glucose and other carbohydrates.

Absorbance Spectrums

• Chlorophyll Absorption: Chlorophyll does not absorb green light efficiently; hence, leaves appear green as this light is reflected. Different pigments capture various wavelengths of light, thus increasing the overall efficiency of photosynthesis by utilizing a broader spectrum of sunlight.

Relationship between Photosynthesis and Cellular Respiration

Photosynthesis and cellular respiration are complementary biochemical processes. Photosynthesis stores energy in glucose, while cellular respiration is the process by which that glucose is broken down to release energy in the form of ATP for cellular activities. The equations for these processes are:

• Photosynthesis: [ 6 CO_2 + 6 H_2O \rightarrow C_6H_{12}O_6 + 6 O_2 ]

• Cellular Respiration: [ C_6H_{12}O_6 + 6 O_2 \rightarrow 6 CO_2 + 6 H_2O + ATP ].

Clicker Questions Review

Understanding how factors such as mutations, ratios of pigments, and environmental conditions affect the processes of light-dependent reactions and the Calvin cycle is essential for comprehending the dynamics of photosynthesis.