Cell Energetics: Photosynthesis and Chloroplasts

Cell Energetics

• Describes chemical-energetic processes in a cell, including photosynthesis in autotrophic plants and cellular respiration in most organisms.

Photosynthesis

• Photosynthesis is a biochemical process occurring in green plants and some bacteria.
• Plants produce sugar through photosynthesis, which fuels their growth.
• Enables autotrophy, or self-sufficiency.
• It is a metabolic process that produces sugar from carbon dioxide and water.
• Energy comes from sunlight.
• Photosynthesis occurs in plant chloroplasts.
• The overall reaction is separated into two reactions located in different areas:
  • 6CO2 + 6H2O => C6H12O6 + 6O2
• Light-dependent reactions form ATP and NADPH+H+.
• Light-independent reactions produce sugar.

Chloroplasts

• They are cell organelles.
• Photosynthesis occurs in chloroplasts.
• They are named after the pigment chlorophyll they contain.
• Chloroplasts are a starting point for all reactions.
• Photosynthesis takes place here.
• The carbohydrates produced store assimilation starch.
• Photosynthesis occurs in the thylakoid membrane.
• Thylakoid membranes contain photosystems and ATP synthase.
• Stacking increases the surface area, which supports structure and function.

Water and Nutrient Uptake

• Water and nutrients are absorbed through the symplast.
  • All cells in a tissue are connected through plasmodesmata.
• Entry in the symplast is facilitated via the cell membrane.
• The apoplast is the entirety of intercellular spaces.
• All substances must pass through the endodermis via a membrane.

Light-Dependent Reactions

• These reactions require light to proceed:
  • 12 H2O + 12NADP+ + 18ADP + 18Pi => 6O2 + 12NADPH + H+ + 18ATP
• Light-dependent reactions take place along the thylakoid membrane in the chloroplast.
• They generate ATP as an energy carrier, and NADPH+H+ as a reducing equivalent.

Light Excitation

• Light strikes a photosystem, which has a light-harvesting antenna complex.
  • This complex contains pigments that converge on a reaction center with two chlorophyll a molecules.
• Chlorophyll is green, so it absorbs all colors except green light, which it reflects.
• When light strikes, electrons in photosystem II are energized and raised to a higher energy level. The signal is relayed until chlorophyll a is excited.
• This excitation creates an electron gap that must be filled.

Photolysis

• The resulting electron gap is filled by photolysis.
• A water-splitting enzyme complex breaks down water, releasing two electrons.
• One complex fills two electron gaps.
• A cyclic electron transport happens when there is enough reducing equivalent and no NADP.
• Ferredoxin transfers electrons back to plastoquinone instead of NADP+ reductase.
• ATP continues to be formed independently of NADPH+H+.

Light-Independent Reactions

• Light-independent reactions:
  • 6CO2 + 12NADPH + H+ + 18ATP => C6H12O6 + 12NADP+ + 18ADP + Pi + 6H2O
• These reactions do not require light, occurring in the stroma of the chloroplast.
• They require products from light-dependent reactions.
• The end product of light-independent reactions is glucose, and they consume energy.

Calvin Cycle

• It includes four phases

1. Fixation phase: CO2 is bound to RDP by the enzyme "Rubisco".
2. Reduction Phase: PGS is reduced to GAP by NADPH+H+. Then two C3 molecules leave to form glucose.
3. Regeneration Phase: reactions regenerate GAP to Ribulose phosphate.
4. Activation Phase: Ribulose phosphate is phosphorylated to RDP.

Photosynthesis Balance

• Exactly 48 light excitations are needed to produce 1 glucose molecule
  • 18 ATP is involved.
  • 12 NADPH+H+ are involved.
• Stomata allow for gas exchange.
• Water is absorbed through the root.
• The sum equation derived is the addition of the partial equation of both steps:
  • 6CO2 + 18H2O => C6H12O6 + 6O2 + 12H2O
• The water absorbed during light-dependent reactions is balanced by water release after the Calvin cycle.
• ATP and NADPH+H+ are steadily consumed in the Calvin cycle.
• They are regenerated during the light-dependent reactions.

Transpiration

• Transpiration is the evaporation of water through stomata or the outer skin of plants.
• Transpiration creates a suction force, drawing new water from the soil up into the leaves.
• Transpiration cools the plant, protecting the plant, and especially the leaves, from overheating.
• Plants can partially control transpiration.
  • Stomata can be regulated for controllable stomatal transpiration.
  • Cuticular transpiration is not controllable.
• Total transpiration is the sum of both types of transpiration.

Stomatal Transpiration

• Water evaporates through the stomata and the pant can partially regulate this process.
• Gas exchange of both oxygen and carbon dioxide occurs in parallel, which cannot be managed independently of transpiration.
• Leaves have guard cells that make up the stomata.
• Plants can regulate the stomata via intracellular pressure, which relies on water content.
• Transpiration depends on the width of the stomal opening.

Cuticular Transpiration

• Water evaporates through the cuticula of the plant.
• Transpiration usually occurs to a lesser extent.
• Plants cannot control this.
• The hydrophobic cuticula is a waxy layer that discourages water. Water can still evaporate based on the temperature.

Transpiration Influences

• Temperature: Evaporation cools the plant, so transpiration increases in warmer temperatures.
• Humidity: High humidity retains water in the plant. Less transpiration occurs.
• Light Intensity: More light exposure enables greater photosynthesis, which requires more.
• Wind speed: In higher air flow, evaporated water quickly dissipates, leading to increased transpiration.
• Water Availability: If the plant can't absorb more water, it cannot transpire, leading to the plant drying out.

Plant Types

Hydrophytes

• They are plants that grow entirely or partially in water and are called aquatic plants.
  • Stomata bulge outwards at the top.
  • They lack transpiration protection.

Hygrophytes

• They are plants that grow in particularly moist areas, called. The name means moisture plants.

Xerophytes

• They are plants adapted to dry environments and called dry plants.
  • Transpiration protection is necessary.
  • They have many stomata and thin cuticula.

Photorespiration

• Rubisco can act as a carboxylase (carbon fixator) but also as an oxygenase.
  • In this case, it binds oxygen to ribulose biphosphate.
  • The product is toxic 2-phosphoglycolate.
• The recovery is called photorespiration.
• In photorespiration, carbon dioxide is lost without being absorbed.
  • More CO2 is consumed with less conversion of ATP and none of NADPH+H+.
• Photorespiration occurs when there is more oxygen than carbon dioxide.
• It is particularly problematic on hot days
  • In order to avoid transpiration, the plant must close its stomata.
    • No CO2 absorption causes a lower content.

Photosynthesis Rates and Light Compensation Point

• The gross photosynthesis rate refers to the photosynthesis rate without any deduction of glucose loss though cell respiration.
• The net photosynthesis rate refers to the photosynthesis rate after subtracting the glucose loss through cell respiration. Plant gain build biomass-can occur here.
• Energy in the form of carbohydrates remains available for further construction.
• The light compensation point is where the necessary illumination intensity allows CO2 absorption to compensate CO2 production occurring during respiration.
• The green curve is a shade-tolerant leaf.
  • Less cells
  • Less cell respiration
  • Less light incidence is needed for compensation
• The red curve is a sun-tolerant leaf.
  • Contains more cells
  • Has more cell respiration
  • Requires more light incidence to compensate
• Plants can only live if they have a positive net photosynthesis rate. Meaning, the photosynthesis rate needs to be above the light compensation point.

Cellular Respiration

• Cellular respiration is a metabolic that is responsible for generating energy in cells. It breaks down glucose into ATP (adenosine triphosphate).
• It describes the reverse metabolic process of photosynthesis that yields water, carbon dioxide, and energy from glucose and oxygen.
• ATP provides energy.
• Cell respiration takes place in the powerhouse of the cell, the mitochondrion.
  • C6H12O6 + 6O2 => 6CO2 + 6H2O
• The overall reaction occurs in four steps that are spatially separated to create compartmentation.
  • Glycolysis occurs in the cytoplasm.
  • Oxidative decarboxylation is in the mitochondrial matrix.
  • The citric acid cycle happens in the mitochondrial matrix
  • The respiratory chain is located within the inner mitochondrial membrane.
• They are enclosed in two membranes and contain a double compartment, matrix, and intermembrane space.
• The inner membrane forms invaginations in the matrix (cristae):
  • Cristae synthesize adenosine triphosphate and generate energy.
• Mitochondria are referred to as the "powerplant of the cell."
• They also contain DNA, ribosomes, etc.

Glycolysis

• Glycolysis is the first step in cell respiration, where glucose is converted into a C3 molecule (pyruvate).
  • C6H12O6 + 2NAD + 2ATP + P₁ => 2C3H3O2 + 2NADH + H+ + 2ATP + 2H+
• Occurs in the cell's cytoplasm.
• Regulated by the key enzyme phosphofructokinase between fructose-6-phosphate and fructose-1,6-biphosphate.

Oxidative Decarboxylation

• It is the second step in cell respiration, where pyruvate is transferred into the citrate cycle.
  • 2Pyruvat + 2NAD+ + 2H+ + Coa – SH => 2C3H„О – СpA – S + 2NADH + H+ + 2CO2
• Occurs inside the matrix of the mitochondrion.

Citric Acid Cycle

• The third step in cell respiration where most reducing agents (reductants) are resynthesized.
• It occurs after the oxidative Decarboxillierung.

Respiratory Chain

• Is the last cellular respiration step, which generates most of the ATP.
• It takes place along the inner mitochondrial and uses compartmentation of the intermembrane space to operate as a proton pump.

Overall Balance and Degree of Effectiveness

• Molecular process:
  • C6H12O6 + 6O2 + 38ADP + P; => 6CO2 + 6H2O + 38АТР
• Effectiveness of cell respiration:
  • One mol of glucose contains 2922 kJ.
  • One mol of ATP contains 28.9 kJ.
  • 38 mole of ATP are formed from one mol of glucose; This equals 1098.2 kJ.
  • This comes out to an effectiveness degree of 38.9% of the energy that is made available from glucose.
  • These 38 mol ATPs required 6 mol O2.
  • And 6 mol of CO2 are created.

Glycolysis Regulation with the Key Enzyme Phosphofructokinase

• ATP is an allosteric inhibitor of phosphofructokinase.
• ADP is an allosteric activator of phosphofructokinase.
• High ATP concentrations inhibit the enzyme
  • Low conversion of Fructose-6-Phosphate causes its concentration to rise.
• High ADP concentrations activate the enzyme
  • Increased conversion of Fructose-6-Phosphat causes its concentration to sink and forms Fructose-1,6-biphosphat.

C4 and CAM Plants

• C4 problem:
  • Very high water stress in dry region
    • Thicker cuticula against cuticular transpiration
  • Stomata closed against stomatal transpiration
  • The uptake of CO2 for the Calvin Cycle is necessary.
  • Stomata must be closed to prevent water loss.
  • Stomata must be open so that the Calvin Cycle can occur.
• These requirements are met with spatial separation of the light and dark-dependent reactions.

The C4 Cycle

• It functions through carbon dioxide fixation in a C4 molecule, which then gives this to the bundle sheath cells through intercellular material transport.
  • The generation happens at the mesophyll cells located at the stomata.
• The PEP-Carboxlase binds even the smallest amounts of CO2, remaining closed.
  • Begins pumping CO2 in the mesophyll cells by fixation of CO2.
  • Accumulation of CO2 completes in the sheath cell once malate is created.
• Allows for near-normal photosynthesis rate with minimal stomatal transpiration.
• Only NADPH+H+ and NADP+ is transported between cells.
• Renewal is made possible only through light-dependent reactions.
• The mesophyll cell and bundle sheath cells produce ATP during the cell respiration.

CAM-Cycle

• Solves the same problem through time division, in addition to space-division.
• The stomata open at night when temperatures are lower. Low water loss and CO2 is taken up and largely fixed as malate
  • Malate is stored as applesauce in the vacuole which leads to sinking of the pH-value.
• The stomata close during the day
  • Decreases water loss through limited stomatal transpiration Malate is decarboxylized allowing the CO2 content of the cells to increase, causing photorespiration to barely occur.
• Sunlight allows the pH-value to rise causing the applesauce stored malate value to rise to capacity.