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Photosynthesis Overview

• Thylakoid Lumen: Site of the electron transport chain in chloroplasts, crucial for ATP synthesis.

• ATP Synthase: Enzyme that synthesizes ATP from ADP and inorganic phosphate (Pi) during photosynthesis.

• NADP+ / NADPH: Electron carrier involved in redox reactions, facilitating the conversion of solar energy into chemical energy.

• CO2: Carbon dioxide utilized in the Calvin cycle to produce glucose.

• Stroma: Fluid-filled space in chloroplasts where the Calvin cycle occurs.

• H+ (protons): Contributes to the proton gradient that drives ATP synthesis.

Photorespiration Issues

• Spatial & Temporal Separation: Mechanism to mitigate photorespiration.

  • RuBisCO Enzyme: Has a poor specificity for CO2 over O2, leading to potential carbon loss.

  • Photorespiration: Occurs mainly in light and can result in a carbon loss of up to 50% of fixed carbon.

The Carbon Cycle

• Photosynthesis Reaction: 6CO2 + 6H2O → C6H12O6 + 6O2

  • Role of Sunlight: Provides energy required for cellular tasks.

  • Cellular Respiration: Converts glucose back into CO2 and H2O, releasing energy for cellular functions.

History of Plants and Oxygen

• Evolution Timeline:

  • Photosynthesis evolved before the first plants appeared around 2000 million years ago.

  • Early forms of life: Prokaryotes responsible for creating oxygen-rich atmosphere, allowing for colonization of land (earliest animals, plants, fungi).

Atmospheric Changes Due to Photosynthesis

• Formation of an ozone layer due to increased oxygen levels.

• Enabled terrestrial life and enhanced ATP formation through oxidative phosphorylation.

Cell Signaling

Concept of Receptors

• Receptors: Proteins in cells that have specific binding sites for ligands (signals) and undergo shape changes to initiate a signal.

• Learning Outcomes: Classify types of chemical signals (polar vs. nonpolar) and recognize different receptor types.

Functions of Cell Membrane

• Structure: Fluid mosaic model containing proteins, lipids (phospholipid bilayer), and carbohydrates.

• Main Functions:

  • Acts as a barrier, allowing selective entry and exit of substances.

  • Recognizes and responds to external signals.

Types of Receptors

• Transmembrane vs Intracellular: Not all receptors are found on the membrane; some are inside the cell for nonpolar signals.

• Gap Junctions and Plasmodesmata: Facilitate rapid communication between adjacent cells.

Local and Distant Signaling

• Autocrine & Paracrine: Local signaling mechanisms without physical contact.

• Endocrine System: Uses circulatory system for long-distance signaling, where signals diffuse from vessels to target cells.

Receptor-Ligand Interaction

• Shape of receptors is critical for function; they bind ligands like a lock and key.

• Binding Mechanism: Reversible; receptor returns to inactive state after signal transduction.

Signal Transduction Models

• G-Protein Coupled Receptors: Activation involves GDP/GTP exchange, which influences effector proteins, leading to cellular responses.

• Second Messengers: Relay and amplify signals within the cell, enabling more complex responses.

• Protein Kinase Receptors: Utilizes phosphorylation to regulate signal transduction, involved in updating receptor states through active and inactive forms.

Specific Receptor Examples

• Example of Ligand: Caffeine binds to adenosine receptors impacting dopamine release. -Action Mechanism: Binding alters cell responses through various signaling pathways, demonstrating the relevance of receptor functionality in biology.