Bio Exam 2 Study Guide

Signaling Pathways in Plants

• Signal Reception: Cell surface receptors, which are often proteins, bind to specific molecules like hormones or environmental stimuli, initiating the signaling process.

• Signal Transduction: Messengers spread the signal within the cell, leading to the activation of proteins and enzymes that relay the signal further.

• Response: Includes processes such as:

  • Phototropism: Growth towards light.

  • Stress Responses: Closing stomata during drought.

  • Immune Defense Activation: Combatting pathogens.

Perception of Light Stimulus

• Light is perceived by phytochromes, which absorb light and undergo conformational changes, activating the plant's response.

• De-etiolation (Greening): Light triggers seedlings to stop elongating, develop chlorophyll, and prepare for photosynthesis.

• Phototropism: When a plant is exposed to light from one direction, it bends towards the light as auxin accumulates on the shaded side, promoting cell elongation.

Discovery of Auxins

• Initial Observations: Darwin and Darwin (1880) discovered that plant shoots bend towards light, suggesting a signal sensed by the tip.

• Chemical Signal Confirmation: Boysen-Jensen (1913) showed that a chemical signal from the tip causes bending, confirmed by barriers made of agar or mica.

• Experimental Predictions: In a setup with light from one side, the shoot tip will bend towards light due to auxin accumulation. With light from all directions, the plant will not bend due to even auxin distribution.

Hormonal Sources and Effects

• Auxin:

  • Produced in: Apical meristem, young leaves, seeds.

  • Effects: Promotes elongation, root formation, phototropism, gravitropism, and maintains apical dominance.

• Cytokinins:

  • Produced in: Roots, seeds, fruits, growing tissues.

  • Effects: Stimulates cell division, promotes lateral bud growth, delays leaf aging.

• Gibberellin (GA):

  • Produced in: Seeds, young leaves, roots, flowering tissues.

  • Effects: Stimulates stem elongation, seed germination, flowering, and fruit development.

• Abscisic Acid (ABA):

  • Produced in: Leaves, roots, seeds (especially during stress).

  • Effects: Inhibits growth, promotes seed dormancy, induces stomatal closure during drought.

• Ethylene:

  • Produced in: Ripening fruit, wounded tissues, aging leaves.

  • Effects: Regulates fruit ripening, leaf abscission, and stress responses.

Types of Plant Responses to Light

• Phototropism: Plants grow towards light, controlled by auxin, promoting elongation.

• Seed Germination: Light can act as a signal to break dormancy and initiate germination, regulated by phytochromes.

• Shade Avoidance: Plants grow longer to reach sunlight, as shaded petioles and stems elongate to improve photosynthesis.

Types of Phototropism, Gravitropism, and Thigmotropism

• Positive Phototropism: Growth towards light maximizing photosynthesis.

• Negative Phototropism: Growth away from light, aiding in nutrient absorption in dark conditions.

• Positive Gravitropism: Growth towards gravity anchors the plant, seeking water and nutrients.

• Negative Gravitropism: Growth against gravity seeks light and air.

Nutrient Discovery and Soil Interaction

• Early agricultural practices indicated specific soil substances are vital for plant growth.

• Jan van Helmont (1600s) emphasized factors beyond water.

Cation Uptake and Soil Nutrients

• Cations are obtained through ion exchange; roots release hydrogen to access cations.

• Acidic soils have high proton concentrations, making nutrients less available, and can increase the solubility of toxic metals.

• Justus von Liebig identified key macronutrients essential for plant growth in the 19th century. Hydroponic experiments in the 1930s revealed the necessity of elements like iron and zinc.

Effects of Acid Rain

• Caused by atmospheric pollutants from coal combustion.

• Impacts include damage to plant tissues and human health risks, leading to nutrient deficiencies and leaching from soil.

• Solutions include shifting energy production from fossil fuels to renewable sources like solar and wind power.

Mobile vs. Immobile Nutrients

• Mobile Nutrients: Deficiencies appear in older leaves (e.g., nitrogen, phosphorus, potassium). Transferred to new growth as needed.

• Immobile Nutrients: Deficiencies appear in younger leaves (e.g., calcium, sulfur, iron). Cannot be moved around within the plant.

Water Potential Factors

• Water Potential: Potential energy of a unit of water.

• Determined by solute potential and pressure potential.

Osmosis in Plant Cells

• Water potential dictates the direction of osmosis, moving from high to low concentration (higher potential to lower potential).

Mineral and Water Absorption

• Water absorption occurs via root hairs, using apoplastic (outside wall) and symplastic (through cytoplasm) pathways.

• It crosses the endodermis via the casparian strip before entering the vascular cylinder.

• Mineral absorption occurs via active transport. Regulation includes selective permeability, ion pumps, and signaling based on nutrient availability.

Bulk Flow Mechanism

• Bulk Flow: Pressure-driven transport of water and solutes through xylem and phloem.

• Cohesion-Tension Hypothesis: Describes how transpiration creates tension to pull water upward, with cohesion between water molecules supporting continuous flow.

Xylem vs. Phloem Transport

• Xylem Transport:

  • Directionality: Unidirectional (upward only).

  • Substances: Water and minerals.

  • Driving Force: Transpiration.

  • Mechanism: Cohesion-tension hypothesis.

  • Energy Requirement: Passive process.

• Phloem Transport:

  • Directionality: Bidirectional (source to sink and vice versa).

  • Substances: Sugars, amino acids, hormones.

  • Driving Force: Pressure flow created by osmotic pressure differences.

  • Mechanism: Pressure flow hypothesis.

  • Energy Requirement: Active process.

Root Pressure and Water Movement

• Root Pressure: Caused by osmotic pressure pushing water upward, possibly causing guttation.

• Primary Driver: Transpiration is the main driver of water movement through xylem.

Transpiration Regulation

• Regulated through:

  • Stomatal control (guard cells open/close based on water availability).

  • Leaf structure adaptations (cuticle, leaf size, trichomes).

  • Root-to-shoot signaling.

Sugar Sources and Sinks

• Sugar Sources: Leaves (photosynthesis) and storage organs (e.g., tubers).

• Sugar Sinks: Roots, fruits, flowers. Movement occurs via phloem.

Pressure-Flow Model in Phloem Transport

• High sugar concentration lowers solute potential, drawing water from xylem into phloem.

• Increased pressure in phloem drives sugar from source to sink.

• Sugar unloading at the sink leads to water exiting phloem back to xylem.

Photosynthesis Overview

• General Equation: CO2 + H2O + light energy → C6H12O6 + O2

• Process:

  • CO2 from the atmosphere enters leaves.

  • Mesophyll cells fix CO2 into organic compounds.

  • Water serves as an electron donor and energy from sunlight is utilized.

  • Produces sugar and oxygen.

Chloroplast Structure

• General Structure: Double-membrane organelle with outer and inner membranes, stroma, and thylakoids.

• Light Reactions: Occur in thylakoid membranes.

• Calvin Cycle: Takes place in stroma.

Effects of Light on Photosynthetic Pigments

• Pigments absorb light energy, exciting electrons to initiate photosynthesis.

• Chlorophyll-a is most affected by red and blue light.

Accessory Pigments

• Expand photosynthetic capacity and protect from UV damage, aiding plant adaptation to varying environments.

Light Reactions and ATP/NADPH Production

• Photosystem II (PSII): Absorbs light energy, excites electrons, leading to water splitting (O2 released).

• Electrons pass through ETC, protons create a gradient for ATP generation via ATP synthase.

• Photosystem I (PSI): Absorbs light energy and contributes to NADPH production by reducing NADP+ in the stroma.

Cyclic vs. Non-Cyclic Electron Flow

• Non-Cyclic Flow: Involves both PSII and PSI, produces ATP and NADPH, and splits water.

• Cyclic Flow: Involves only PSI, produces ATP to balance ATP/NADPH ratio.

Calvin Cycle Stages

• Stage 1 - Carbon Fixation

  • CO2 combines with 5C sugar to form two 3C sugars.

• Stage 2 - Reduction

  • Sugars phosphorylated and reduced to form 2 G3P molecules.

• Stage 3 - Regeneration of RuBP

  • G3P used to regenerate RuBP.

• The Calvin Cycle must run 6 times to produce one glucose molecule.

Function of Rubisco

• Rubisco incorporates CO2 into organic molecules, required in high abundance due to its inefficiency, ensuring sufficient carbon fixation for photosynthesis.

Photorespiration

• Definition: A metabolic process where oxygen is consumed and carbon dioxide is released.

• Occurs under conditions of high oxygen and low carbon dioxide, causing rubisco to bind to oxygen instead of carbon dioxide.

C4 vs. CAM Photosynthesis

• C4 Photosynthesis:

  • Type of Plants: Found in grasses (corn, sugarcane, some dicots).

  • Cell Types: Involves mesophyll and bundle-sheath cells.

  • CO2 Fixation: CO2 fixed in mesophyll, transported to bundle sheath.

  • Role: Creates an environment low in oxygen and high in CO2 for rubisco.

• CAM Photosynthesis:

  • Type of Plants: Found in succulents (cacti, agave).

  • Cell Types: Involves mesophyll cells only.

  • CO2 Fixation: CO2 fixed at night and stored for use during the day.

  • Role: Reduces photorespiration and water loss.

Comparisons of Photosynthesis and Cellular Respiration

• Photosynthesis converts CO2 and water into sugars and oxygen using light energy, while respiration converts sugars and oxygen into CO2 and water, releasing energy. Both processes are essential to plant and animal life.