Cellular Respiration and Photosynthesis Quiz

Photosynthesis Overview

• Photosynthesis is a process that affects the entire planet.
• It involves the conversion of light energy into chemical energy (sugars).
• The process uses carbon dioxide and water, and produces oxygen and carbohydrates.

Photosynthesis Details (Campbell and Reece)

• Edition 7, 8, and 9 of Campbell and Reece's work contains information on photosynthesis, within Chapter 10.
• Chapter 10 includes an overview, and details on specific concepts (10.1, 10.2, 10.3, 10.4) excluding specific sections on 'cyclic electron flow', and 'photorespiration'.
• Chapter 9 also contains an overview, concentrating on the stages of cellular respiration within concept 9.1.

Photosynthesis and the Biosphere

• Photosynthesis significantly affects the biosphere, atmosphere, hydrosphere and lithosphere.

The Pyramid of Life

• The pyramid of life illustrates the flow of energy from the base (photosynthesizing organisms) to the top (carnivores).
• Photosynthesizing organisms capture energy from photons of sunlight, and energy flows up through herbivores, and carnivores.

Autotrophs and Heterotrophs

• Autotrophs: Utilize inorganic carbon (e.g., CO2), and require an outside energy source to produce organic compounds (e.g., sugars).
  • Plants, algae, and prokaryotes are autotrophs.
• Heterotrophs: Need an organic supply of carbon (e.g., sugars), and depend on dead or living biological material to obtain energy.
  • Consumers such as herbivores, carnivores, and decomposers are heterotrophs. Plants can sometimes be heterotrophs (for e.g. broomrape).
• Autotrophs and heterotrophs both oxidise organic carbon to produce CO2 via respiration.

The Carbon Cycle

• The carbon cycle demonstrates the movement of carbon throughout the environment (atmosphere, land plants, soil, ocean)
• Relevant factors include combustion, destruction of vegetation, photosynthesis, respiration, and decay of organic matter.
• Relevant quantities of carbon are in billion tonnes.

Leaf Structure

• Leaves have multiple parts, including the upper and lower epidermis, cuticle, palisade parenchyma, spongy parenchyma, guard cells, xylem, phloem, and stoma.
• Chlorophyll located in mesophyll (parenchyma) cells.
• Mesophyll cells contain up to 40 chloroplasts in each cell.
• Chlorophyll contributes to the leaves' green colour.

Chloroplast Structure

• Chloroplasts possess an outer membrane, inner membrane, stroma, and thylakoids that are stacked in structures called grana.

Endosymbiosis Theory

• The endosymbiosis theory explains how chloroplasts evolved from photosynthetic prokaryotes.
• Unicellular photosynthetic algal cells were absorbed by a heterotrophic host cell, and became incorporated into the host cell's digestive system.

Chloroplast Components

• Chloroplasts possess a double membrane, chloroplast DNA (ctDNA), thylakoid membranes.
• Thylakoids are stacked in grana; stroma, and chlorophyll is located on thylakoid membranes.
• Internal lumen is a compartment inside thylakoid membranes.

Historical Discoveries

• Jan Baptista van Helmont (early 1600s): investigated the source of plant biomass, questioning whether soil was a primary source.
• Joseph Priestley (1733-1804): studied air, bad air, and dephlogisticated air (oxygen).

Chlorophyll and Light

• Chlorophyll absorbs light energy; absorbing a photon.
• Excited electrons subsequently fall back to ground state, releasing energy as heat or fluorescence.
• Photons excite chlorophyll and trigger various reactions.

Photosynthesis: A Two-Step Process

• Light reactions: Convert light energy into chemical energy (ATP and NADPH).
• Dark reactions (Calvin cycle): Incorporate CO2 into sugars using energy from light reactions.

Light Reactions (Detail)

• Light reactions are a redox process, involving the transfer of electrons from a donor (water) to an acceptor molecule with a lower redox potential, driven by energy absorbed from light photons.
• The process involves two photosystems (PSII and PSI) operating sequentially (in order).

Light Reactions (Product 1)

• NADPH: a form of reducing energy (energy plus electrons)
• NADPH is essential for later steps.

Light Reactions (Product 2)

• ATP: a form of chemical energy for cellular processes.
• ATP is essential for later steps in the Calvin Cycle.

Water Splitting

• Water splitting is integral to light reactions.
• 2 H₂O is converted into 4 H⁺ + O₂ + 4 e⁻

Acidification of the Lumen

• H+ ions accumulate within the thylakoid space, creating a gradient used to produce ATP.
• This process is driven by the generation of a H+ ion gradient between the thylakoid space and the stroma.

ATP Synthesis

• Energy stored in the H⁺ gradient is used by ATP synthase to generate ATP, through a process known as chemiosmosis.
• Chemiosmosis involves protons moving through a selectively permeable membrane, from high to low concentration.
• Such movement creates energy used in phosphorylation.

End-products of Non-cyclic Electron Flow

• NADPH: a reducing agent for dark reactions
• ATP: energy in a usable form for dark reactions

Calvin Cycle Phase 1 (Carbon Fixation)

• CO2 is linked to Ribulose Bisphosphate (RuBP).
• The enzyme RuBP carboxylase (Rubisco) plays a key role in this step, combining the two, while forming an unstable 6-carbon molecule, which immediately splits into two molecules of 3-Phosphoglycerate.

Calvin Cycle Phase 2 (Reduction/Phosphorylation)

• 3-Phosphoglycerate is phosphorylated using ATP.
• Then, 3-Phosphoglycerate is reduced using NADPH to produce Glyceraldhyde 3-phosphate.

Calvin Cycle Phase 3 (Regeneration)

• 5 out of 6 molecules of glyceraldehyde 3-phosphate are used to regenerate ribulose bisphosphate (RuBP), the carbon acceptor.
• The regeneration process converts 5 molecules of Glyceraldehyde 3-phosphate into 3 molecules of Ribulose bisphosphate (RuBP).
• This allows the cycle to continue, incorporating more CO2.

C4 Metabolism

• Adaptations that increase leaf CO2 concentrations without increasing transpiration.
• Gather all available CO2 into a small group of cells, a Calvin cycle prefaced with C4-cycle.
• CO2 is initially fixed to phosphoenolpyruvate (PEP), which is converted to a four carbon compound.

C4 Mechanism

• The 4-carbon compound is transported to bundle sheath cells.
• There, it releases CO₂ to be incorporated into the Calvin cycle.
• C3 compounds return to the mesophyll cells to start the cycle again.

C4 Function Linked to Structure

• In C4 plants, bundle sheath cells are responsible for fixing carbon dioxide via the Calvin Cycle.
• The rest of the mesophyll cells collect CO₂, and carry it over as a 4-carbon compound to bundle sheath cells.

C4 Operation

• Some key enzymatic steps that enable C4 metabolism include the binding of CO2 to phosphoenolpyruvate (PEP) by phosphoenolpyruvate carboxylase (PEPC), formation of a four-carbon compound (C4), transportation of that compound to bundle sheath cells, CO2 release in bundle sheath cells, and returning C3 molecules back to mesophyll cells to start the Calvin Cycle.

C4 Advantages

• They operate at very low CO2 levels (high affinity for CO2).
• They have a large catchment area for CO2.
• Thus, they work as a concentrating mechanism.

C4 Plants and Climate Issues

• C4 plants are well-adapted to hot and dry environments, and may play an important role in adapting plants under climate change.

Mitochondrial Respiration

• The process of aerobic respiration converts chemical energy in complex molecules (such as sugars, for example) into ATP.

Differences from Photosynthesis

Mitochondrial respiration is distinct from photosynthesis. Respiration involves obtaining chemical energy from complex molecules like sugars to create ATP, and is a shared process between animals and plants.

Three Stages of Respiration

• Glycolysis: Breakdown of glucose into pyruvate.
• Citric Acid Cycle (Krebs Cycle): Breakdown of pyruvate to CO2.
• Oxidative Phosphorylation: Use of electron transport chain to oxidize NADH and FADH2 via electron transport, generating a high concentration gradient and ultimately ATP production.

Other Points

• Fermentation is a type of anaerobic respiration, distinct from the aerobic version in that occurs outside mitochondria and is less efficient in producing ATP.