Photosynthesis: Light & Calvin Cycle

• Photosynthesis is how plants, algae, and some bacteria convert light energy into chemical energy, producing glucose and oxygen.

Photosynthesis Stages

• Light-Dependent Reactions: Capture solar energy and convert it into ATP and NADPH.
• Calvin Cycle (Light-Independent Reactions): Utilizes ATP and NADPH to fix CO2 into organic molecules like glucose.

Overall Equation of Photosynthesis

• 6CO2+6H2O+light→C6H12O6+6O2

Light Reactions (Light-Dependent Reactions)

• Location: Thylakoid membranes of the chloroplast
• Chloroplasts are solar-powered chemical factories
• Thylakoids transform light energy into ATP and NADPH
• The Thylakoid Membrane has chlorophyll, absorbs Red and Blue wavelengths, and reflects green.
• Purpose: Convert light energy into chemical energy (ATP and NADPH) while releasing O2 as a byproduct.

Step 1: Light Absorption & Electron Excitation

• Photosystem II (PSII) absorbs light energy, exciting electrons in chlorophyll a molecules.
• Excited electrons are transferred to the primary electron acceptor.
• Photolysis of water occurs to replenish lost electrons: 2H2O→4H+ +4e-+02
• Enters the electron transport chain (ETC).

Step 2: Electron Transport Chain & ATP Synthesis

• Electrons travel through the ETC (plastoquinone (PQ), cytochrome complex, plastocyanin (PC)).
• The energy released pumps H+ ions into the thylakoid lumen, creating a proton gradient.
• ATP Synthase utilizes this gradient to convert phosphorylate ADP + Pi → ATP (chemiosmosis).

Step 3: Photosystem I (PSI) & NADPH Formation

• Electrons reach Photosystem I (PSI) and are re-excited by light.
• High-energy electrons are transferred to ferredoxin (Fd), then to NADP+ reductase, which forms NADPH: NADP++2e-+H+→NADPH

Calvin Cycle

• The Calvin cycle is anabolic
• Location: Stroma of the chloroplast.
• Purpose: Use ATP and NADPH to incorporate CO2 into organic molecules.
• Three Phases: Carbon Fixation, Reduction, and Regeneration of RuBP.

Step 1: Carbon Fixation

• Enzyme Involved: RuBisCO (Ribulose-1,5-bisphosphate carboxylase/oxygenase).
• CO2 combines with RuBP (5-carbon molecule) → forming an unstable 6-carbon compound.
• The compound immediately splits into two 3-phosphoglycerate (3-PGA) molecules.

Step 2: Reduction

• 6 ATP molecules phosphorylate 3-PGA into 1,3-bisphosphoglycerate (1,3-BPG).
• 6 NADPH molecules donate electrons to convert 1,3-BPG into glyceraldehyde-3-phosphate (G3P).
• Net Product: 1 G3P exits the cycle; the remaining 5 regenerate RuBP.

Step 3: Regeneration of RuBP

• 5 G3P molecules (3-carbons each) rearrange into 3 RuBP (5-carbons each).
• 3 ATP molecules are consumed in the process.

Glucose Formation & Cycle Turns

• 1 Turn of the Cycle produce 1 G3P.
• 2 Turns produce 2 G3P, which combine to form 1 glucose (C6H12O6).

Photosynthesis as a Redox Process

• Electron Flow: Photosynthesis reverses the electron flow of cellular respiration.
• H2O is oxidized( Lose Electrons) to O2, and CO2 is reduced (Gain Electrons) to glucose.
• Light energy drives this endergonic reaction.

Water Splitting Experiment

• Oxygen-18 isotope tracing was used to confirm that O2 comes from H2O, not CO2.
• Thylakoids use light to generate ATP and NADPH, while the Calvin Cycle assembles sugars.
• Excess sugar is stored as starch in chloroplasts or in the cells of roots, tubers, seeds, and fruits.
• Photosynthesis produces the O2 in the atmosphere and transports sugar to nonphotosynthetic cells throughout the plant as sucrose.

Key Vocabulary

• Aerobic respiration: A process by which cells use oxygen to break down glucose and produce energy (ATP), carbon dioxide, and water.
• Bromothymol blue (BTB): A pH indicator that changes color based on the presence of carbon dioxide. It turns yellow in acidic conditions (high CO2) and blue in basic conditions (high O2).
• Carbon dioxide-oxygen cycle: animals and plants exchange carbon dioxide and oxygen. Animals release carbon dioxide, and plants use it for photosynthesis, releasing oxygen in the process.
• Indicator: A substance that changes color to show the presence or amount of a particular substance.
• Interdependence: The mutual dependence between two organisms or systems, such as plants and animals relying on each other for gases like oxygen and carbon dioxide.
• Photosynthesis: The process by which plants use sunlight, carbon dioxide, and water to produce glucose (a sugar) and oxygen.

Objective

• Understanding how BTB (Bromothymol blue) indicates the presence of different gases (O2 and CO2) in water and how it changes color.

Experiment Setup:

• Snail test tube: The tube containing the snail turned yellow after 24 hours, indicating the presence of high levels of carbon dioxide (CO2).
• Plant test tube: The tube with the plant stayed blue, indicating a high concentration of oxygen (O2).

Analysis of Gas Release

• Plants in light: Release oxygen (O2).
• Plants in dark: Release carbon dioxide (CO2).
• Animals (snails) in light and dark: Release carbon dioxide (CO2) continuously.

Carbon Cycle:

• Animals breathe in oxygen (O2) and exhale carbon dioxide (CO2).
• Plants take in carbon dioxide (CO2) and release oxygen (O2) in sunlight.

Observations with Elodea and Elodea with snail:

• In light conditions, the levels of O2 and CO2 in the test tube remained stable (close to 6.0 ppm).
• Without light, the CO2 level increased, and the O2 level decreased due to the absence of photosynthesis by the plant.
• Animals need oxygen (O2) and produce carbon dioxide (CO2).
• Plants need carbon dioxide (CO2) for photosynthesis and produce oxygen (O2).
• The survival of each is dependent on the other: animals rely on oxygen produced by plants, and plants rely on the carbon dioxide produced by animals.

Activity C: The Carbon-Oxygen Balance

• Objective: To understand how oxygen (O2) and carbon dioxide (CO2) levels are related during photosynthesis and aerobic respiration.
• In light conditions, the oxygen level increases, and the carbon dioxide level decreases due to photosynthesis.
• In the dark, the oxygen level decreases, and the carbon dioxide level increases due to aerobic respiration.
• In both light and dark conditions, the total amount of oxygen and carbon dioxide remains constant (12 ppm) due to photosynthesis and respiration.
• The processes are complementary, the oxygen produced by plants is used by animals for respiration and the carbon dioxide released by animals is used by plants for photosynthesis.
• The balance between oxygen and carbon dioxide in the environment is maintained through the interconnected processes of photosynthesis in plants and aerobic respiration in animals. Both systems rely on each other, creating a stable cycle essential for life.

Cell Division

• Cell division is a fundamental process required for growth, development, repair, and reproduction in living organisms.
• Cell division allows for the continuity of life by ensuring that genetic material is faithfully replicated and passed on to daughter cells.

Cell Division Functions

• Asexual and Sexual Reproduction
  • In unicellular organisms, a single cell divides to form a new organism (e.g., bacteria, amoeba).
  • In multicellular organisms, cell division produces gametes (sperm & egg) for sexual reproduction.
• Growth & Development
  • Multicellular organisms start as a single fertilized egg (zygote), which divides to form the entire organism.
  • Embryonic development relies on highly regulated cell division.
• Tissue Repair & Renewal
  • Damaged or old cells are replaced through mitosis.
• Cell Cycle Regulation & Homeostasis
  • Ensures that cells only divide when needed and that division stops when no longer required.
  • Dysfunction in regulation can lead to cancer.

The Genome

• The genome is a cell's entire set of genetic material, consisting of DNA.
  • Prokaryotic genome: One circular DNA molecule.
  • Eukaryotic genome: Multiple linear DNA molecules, organized into chromosomes.

Chromatin

• Complex of DNA and proteins (histones) that condenses into chromosomes.
• Histones: Proteins around which DNA wraps to form nucleosomes.
• Chromosomes: Highly organized structures that ensure accurate DNA segregation during division.

Types of Cells

• Somatic Cells (Diploid, 2n): Contain two sets of chromosomes (one from each parent), such as human cells with 46 chromosomes (23 pairs).
• Gametes (Haploid, n): Contain one set of chromosomes, like human sperm and egg cells with 23 chromosomes.

Chromosome Duplication

• Each chromosome is copied, forming two identical sister chromatids in the S phase of interphase.
• Chromatids are joined at a centromere, the region where they remain attached.
• During Mitosis: The two sister chromatids separate and move into two nuclei. Once separated, each chromatid becomes an individual chromosome.

Cell Cycle

• The cell cycle describes the life of a cell from its formation to its division into two daughter cells.

Interphase (90% of the cell cycle)

• Period of growth and DNA replication.
  • G1 phase (First Gap): Cell grows, produces organelles & proteins.
  • S phase (Synthesis): DNA is replicated.
  • G2 phase (Second Gap): Preparation for mitosis.

Mitotic (M) Phase

• The phase where the cell divides, consisting of:
  • Mitosis: Division of the nucleus.
  • Cytokinesis: Division of the cytoplasm.

Mitosis

• Prophase: Chromosomes condense, the mitotic spindle (made of microtubules) begins forming, and centrosomes migrate to opposite poles.
• Prometaphase: The nuclear envelope breaks down, allowing spindle fibers to attach, kinetochores (protein structures at centromeres) form, and spindle fibers attach to kinetochores.
• Metaphase: Chromosomes align at the metaphase plate (equatorial plane) and the spindle checkpoint ensures all chromosomes are attached before proceeding.
• Anaphase: Sister chromatids separate and move to opposite poles, kinetochore microtubules shorten (pulling chromatids apart), and non-kinetochore microtubules lengthen (stretching the cell).
• Telophase: Nuclear envelopes reform around separated chromosomes, chromosomes decondense back into chromatin, and spindle fibers disassemble.
• Cytokinesis (Final Step): Animal cells form a cleavage furrow, and plant cells form a cell plate, leading to a new cell wall.

Cell Cycle Control System

• The cell cycle is regulated by a molecular control system that includes:
  • Cyclins: Proteins that regulate cell cycle timing.
  • Cyclin-Dependent Kinases (CDKs): Enzymes that drive the cycle forward.

Checkpoints in the Cell Cycle

• G1 Checkpoint ("Restriction Point"): Determines if the cell enters the cycle or exits to Go (quiescent state
• G2 Checkpoint: Ensures DNA replication is complete before mitosis. If errors are detected, the cell attempts to repair DNA or undergoes apoptosis.
• M Checkpoint ("Spindle Checkpoint"): Ensures that all chromosomes are properly attached to spindle fibers before anaphase prevents errors in chromosome separation.

Cancer Cells

• Cancer cells bypass regulatory signals and divide uncontrollably as well as key cancer features and ignore growth factor signals.
• Divide indefinitely (immortalized cells).
• Invade other tissues (malignant tumors).
• Spread via metastasis.

Cancer Treatment

• Radiation: Damages DNA in cancer cells.
• Chemotherapy: Disrupts the cell cycle. Targeted Therapy: Blocks specific molecules involved in cancer progression.

Binary Fission

• Prokaryotic cell division is simpler than mitosis.
  • DNA replication begins at the origin of replication.
  • Two copies move to opposite sides of the cell. The plasma membrane pinches inward and splits the cell.