Plant Cell Biology: Membranes, Metabolism, and Transport
Cell Membranes: Composition and Structure
Cell membranes are lipid bilayers composed of lipids, proteins, and fatty acids. Their mosaic and fluid structure enables component sharing.
Plant Cell Organelles and Their Functions
Nucleus
Controls genetic material replication and transcription.
Plastids
Conduct photosynthesis.
Mitochondria
Carry out cellular respiration.
Endoplasmic Reticulum
Facilitates protein transport and processing.
Golgi Apparatus
Synthesizes non-cellulosic structural polysaccharides.
Vacuoles
Maintain cell turgor.
Simplast and Apoplast
Apoplast
The space outside the plasma membrane.
Symplast
The continuous space enclosed by plasma membranes connected by plasmodesmata.
Advantages of Membranes with Higher Unsaturated Fatty Acid Content
Promotes membrane fluidity, allowing survival in extreme environments.
Amphipathic Molecules
Possess both polar (hydrophilic head) and nonpolar (hydrophobic tail) regions. They form lipid bilayers with hydrophobic tails interacting and hydrophilic heads facing the aqueous phase.
Chemical Composition of Primary and Secondary Cell Walls
The primary cell wall (100-200nm thick) forms after cell division and consists of cellulose microfibrils (9-25%), hemicellulose, pectin, proteins, and phenols in a slightly acidic matrix. The secondary cell wall is thicker and contains a higher proportion of lignin (up to 35%).
Differences Between Hemicellulose, Pectin, and Cellulose
Pectin
Composed of acidic and neutral sugars, providing porosity, electrical charge, and cohesion.
Cellulose
A linear chain of D-glucose, forming strong microfibrils.
Xyloglucan
Links to cellulose microfibrils via hydrogen bonds.
Primary Cell Wall Structure
A network of cellulose microfibrils embedded in a matrix of hemicellulose and pectin.
Cellulose Synthesis and Microfibril Orientation
Synthesized at the plasma membrane surface by a multienzyme complex. Microfibril orientation influences cell wall strength.
Lignin and Lignification
Lignin, a complex polymer of aromatic alcohols, provides rigidity and resistance to the cell wall.
Plasmodesmata Formation
Plasmodesmata are channels connecting adjacent cell cytoplasms, enabling molecular exchange.
Matrix Polysaccharide Synthesis
Synthesized in the Golgi apparatus, packaged into vesicles, and exported to the cell surface.
Expansins and Cell Growth
Expansins disrupt hydrogen bonds between cell wall components, allowing cell expansion.
Plant Cell Survival Without Cell Walls
Possible only in isotonic environments.
Chlorophyll and Carotenoid Location in Cell Membranes
Their nonpolar nature facilitates association with membrane structures.
Photosystem Involvement in Light Energy Conversion
Photosystems are protein complexes that absorb light energy and convert it into electrochemical energy.
Light Energy Absorption and Dissipation by Pigments
Antenna complexes absorb light energy, which is then transferred to reaction centers or dissipated as heat.
Cyclic vs. Noncyclic Electron Transport
Cyclic transport involves only photosystem I, while noncyclic transport involves both photosystems I and II.
Water Photolysis and NADP+ Reduction Locations and Implications
Water photolysis occurs in the thylakoid lumen, while NADP+ reduction occurs in the stroma, creating a proton gradient for ATP synthesis.
LHCII Function and Abundance in Shade vs. Sun Leaves
LHCII is the light-harvesting complex of photosystem II. Shade leaves have more LHCII to capture more light in low-light conditions.
Relationship Between Pigment Absorption Spectrum and Photosynthetic Action Spectrum
The absorption spectrum indicates the wavelengths of light absorbed by pigments, while the action spectrum shows the effectiveness of different wavelengths in driving photosynthesis.
ATP and NADPH Use in the Carbon Reduction Cycle
ATP and NADPH provide energy and reducing power for carbon fixation.
Light Regulation of the Photosynthetic Carbon Reduction Cycle
Light activates Rubisco and other enzymes involved in the cycle.
Factors Determining Starch vs. Sucrose Synthesis
Enzyme regulation and metabolite concentrations influence the partitioning of carbon between starch and sucrose.
Photon Requirement for Fructose Synthesis in C3 Plants
At least 60 photons are required to synthesize one fructose molecule.
Photorespiration and Its Relationship with Photosynthesis
Photorespiration occurs when Rubisco reacts with O2 instead of CO2. It can protect photosystems from excess light energy.
Carbon Pathway in C4 Leaves
CO2 is initially fixed in mesophyll cells and then transported to bundle sheath cells for the Calvin cycle.
Advantages of High Chloroplast Density in C4 Bundle Sheath Cells
Facilitates CO2 concentration and reduces photorespiration.
Environmental Conditions Favoring C4 over C3 Plants
High temperatures and low CO2 concentrations.
CAM vs. C4 Metabolism
CAM plants separate carbon fixation and the Calvin cycle temporally, while C4 plants separate them spatially.
Maximum and Minimum ATP Synthesis from Glucose Oxidation
Maximum: 38 ATP. Minimum: 8 ATP.
Why Plants Need Respiration Despite Photosynthesis
Respiration provides energy for cellular processes when photosynthesis is not active.
Nitrate and Sulfate Assimilation
Plants reduce nitrate to ammonium and sulfate to sulfide for incorporation into organic molecules.
Main Source of Ammonium in Plant Tissues
Photorespiration.
Source of Reducing Power for Nitrate Reduction
Leaves: Photosynthetic reducing power. Roots: Tricarboxylic acid cycle.
Role of Glutamine Synthetase and Glutamate Synthase
Catalyze ammonium assimilation into glutamine and glutamate.
Nitrogen Export Form from Roots
Nitrate or amino acids.
Carbon Skeletons for Glutamine and Asparagine Synthesis
Derived from the tricarboxylic acid cycle.
Similarities Between Nitrate and Sulfate Uptake
Both are active processes involving proton cotransport.
Glutathione and Its Roles
Glutathione is a sulfur-containing tripeptide involved in sulfur storage, enzyme protection, and detoxification.
Source vs. Sink Determination
Sources produce sugars, while sinks consume or store them.
Energy for Phloem Transport
Generated by the accumulation of sugars, creating a pressure gradient.
Münch Pressure Flow Hypothesis
Explains phloem transport based on pressure differences between source and sink tissues.
Sugar Loading and Unloading in Phloem
Loading occurs at the source, and unloading occurs at the sink.
Raffinose Family Sugar Formation in Phloem
Provides energy for sugar loading and maintains the concentration gradient.
Zero Value in a Fully Turgid Cell
Water potential.
Turgor Pressure Function
Maintains cell shape and prevents excessive water uptake.
Soil-Plant-Atmosphere Continuum (SPAC)
A concept describing water movement through the plant and its environment.
Aquaporins
Water channels in cell membranes that regulate water flow.
Pressure Chamber Method for Water Potential Measurement
Applies pressure to a leaf to balance the negative pressure of water within the xylem.
Stomatal Opening Mechanisms at Dawn
Proton pumping, K+ influx, and malate synthesis.
Essential Properties for Stomatal Function
K+ pumping, malate synthesis, and proton pumping.
Leaf Resistance to Water Vapor Transport
Stomatal resistance.
Relationship Between Carbon Isotope Discrimination and Transpiration Efficiency
Negative linear relationship.
Light and CO2 Control of Stomatal Opening
Light provides energy for proton pumping, while low CO2 concentrations stimulate stomatal opening.
Transpiration Action Spectrum
Blue and red light.
Factors Increasing Transpiration Rate
Low wind speed and low relative humidity.
Abscisic Acid and Stomatal Closure
ABA induces stomatal closure under water deficit conditions.
Useful Functions of Transpiration
Cooling and nutrient transport.
Driving Force for Passive Water Absorption
Transpiration.
Flow Rate Differences in Tracheids and Vessels
Vessels have a much higher flow rate due to their larger diameter.
Reasons for Wilting in Specific Conditions
Cold water shock, sudden exposure to sun, and wind.
Water Movement to the Top of Tall Trees
Cohesion and adhesion of water molecules.
Causes of Cavitation in Xylem
Excessive tension, freezing, embolism, and pathogens.
Function of Bordered Pits in Xylem Vessels
Strengthen vessel walls and prevent embolism spread.
Counterproductive Fertilization During Drought
Can damage crops due to increased osmotic stress.
Essentiality of a Plant Nutrient
Required for completing the life cycle, cannot be replaced, and directly involved in metabolism.
Essential vs. Beneficial Elements
Beneficial elements are not required by all plants.
17 Essential Plant Elements and Their Roles
Macronutrients and micronutrients.
Chlorosis Due to Nutrient Deficiencies
Iron deficiency causes interveinal chlorosis in young leaves, while magnesium and nitrogen deficiencies affect older leaves.
Chelating Agent
An organic molecule that binds to metal ions.
Toxic Elements for Plants
Selenium.
Simple Diffusion, Facilitated Diffusion, and Active Transport
Different mechanisms of membrane transport.
Channel Proteins and Their Role
Selective pores for ion transport.
Differences Between Transporters, Channels, and Ion Pumps
Different types of membrane transport proteins.
Mechanisms Responsible for Specific Solute Transport
Simple diffusion, active transport, and facilitated diffusion.
Nernst Equation
Used to determine if ion transport is passive or active.
Radial Ion Movement Path in Roots
Symplastic and apoplastic pathways.
Differences Between Plant and Animal Hormones
Hormone synthesis, transport, and response mechanisms.
Tissue Sensitivity and Plant Hormone Activity
Hormonal response depends on both hormone concentration and tissue sensitivity.
Coleoptile Apex and Phototropism
Auxin transport from the apex causes phototropic curvature.
Bioassays for Distinguishing Plant Hormones
Plant-based tests to determine hormone activity.
Common Structural Features of Auxins
Aromatic ring and carboxylic acid group.
Auxin Growth Inhibition at High Concentrations
Can disrupt hormone signaling and metabolism.
Apical Dominance and Involved Hormones
Auxin inhibits lateral bud growth.
Effect of Rooting Hormone on Cuttings
Promotes root formation at the distal end.
Acid Growth Theory
Auxin stimulates proton pumping, acidifying the cell wall and promoting cell expansion.
Phytohormones Synthesized from Amino Acids
Auxins and cytokinins.
GA1 and GA4 vs. Other Gibberellins
GA1 and GA4 are bioactive forms with 19 carbon atoms.
Chemical Requirements for Cytokinin Activity
N6 side chain.
Skoog-Miller Model
Auxin/cytokinin ratio determines organogenesis.
Effects of Ethylene Insensitivity
Reduced apical hook, delayed senescence, and no triple response.
Effects of Abscisic Acid Deficiency
Reduced stress tolerance, early germination, and wilting.
Five Major Plant Hormone Classes and Their Primary Actions
Auxins, cytokinins, gibberellins, ethylene, and abscisic acid.