EEB162 Pink Slides Midterm
1. Why are plants important to the biosphere? What aspects of our biology depend on plant life?
Plants make up ~99.9% of terrestrial biomass and are the foundation of all ecosystem productivity.
They generate oxygen, remove CO₂, provide food, medicines, and materials essential to human survival.
Virtually all life—including ours—depends on the oxygen, food, and ecosystem balance provided by plants.
2. What are 5 characters that might define a good model plant for understanding genetics and physiology?
Small size.
Short life cycle.
Ease of propagation.
Small, well-understood genome.
Amenable to genetic manipulation.
3. Why are plants phylogenetically diverse?
Plants have diversified over 450 million years, especially among angiosperms (400,000+ species) and gymnosperms (~1,100 species), evolving a vast range of forms and adaptations across ecosystems.
4. What are 3 features of cells that are distinctive and typical of plants?
Cellulose-based cell walls.
Large vacuoles (80–90% of cell volume).
Chloroplasts for photosynthesis.
5. What is the difference between apoplast and symplast?
Apoplast: The space outside plasma membranes (cell walls); forms a continuous non-living pathway.
Symplast: The living continuum of cytoplasm connected by plasmodesmata.
5a. Which is a better system for conducting water?
Apoplast is better for water conduction due to its open, continuous space.
5b. Which is a better system to conduct large biomolecules?
Symplast is better for transporting larger biomolecules (up to ~2 nm in diameter).
6. What are the three major kinds of plant tissues and their functions?
Dermal tissue: Protection (epidermis, cuticle, guard cells).
Ground tissue: Photosynthesis and support (parenchyma, collenchyma, sclerenchyma).
Vascular tissue: Transport (xylem—water/nutrients; phloem—sugars/signals).
7. Explain why plants are phylogenetically unique.
Plants form a monophyletic group in the tree of life, characterized by traits like cellulose walls, chloroplasts, and life cycles with alternation of generations, making them a distinct evolutionary lineage.
8. What are 3 reasons that plants need water? Which of the three reasons accounts for the bulk of the water use?
Photosynthesis (reactant).
Cell expansion (growth).
Transpiration (bulk of water use)—to access CO₂.
9. How does transpirational cooling work in terms of the high latent heat of vaporization of water?
Evaporation of water at the leaf surface absorbs heat (due to high latent heat), cooling the plant passively.
10. Capillarity: How far can it push water up a tree?
Capillarity alone is insufficient to push water up tall trees; it plays a small role in seedlings.
10a. How high can water rise in cell wall pore capillaries (radius = 100 nm)?
About 150 meters.
10b. How high in xylem vessels (radius = 25 micrometers)?
Only about 0.6 meters.
11. Please explain and give the basis for the following four properties of water, and give an example of why the properties are important for plant life.
a. Water is a supersolvent:
Due to polarity and hydrogen bonding; dissolves nutrients and ions essential for uptake and transport.
b. Water has a high specific heat and latent heat of vaporization:
Stabilizes leaf temperature and cools plants during transpiration.
c. Water has high cohesion and high tensile strength:
Allows water to be pulled up tall trees via tension in xylem (cohesion-tension theory).
d. Water has high surface tension:
Supports capillary action and keeps cell walls wetted.
12. Explain these two forms of water transport:
a. Diffusion: Movement driven by concentration gradients (e.g., vapor diffusion out of leaves).
b. Bulk Flow: Mass movement driven by pressure gradients (e.g., water in xylem).
13. For a tree in the garden, there are five times as many xylem conduits in a branch than in a petiole (stem of the leaf, narrow) and the conduits in the branch are twice as wide as those in the petiole. How many times higher is the hydraulic conductance in the branch?
Conductance scales with the number of conduits × (radius)⁴.
(5 × (2)⁴) = 5 × 16 = 80× higher.
14. For the following types of water transport, please give the equation indicating the driving force. Also please state where in the plant you would expect each type to apply:
a. Diffusion:
Flow = -D × (Δc / Δx); applies to vapor diffusion out of stomata.
b. Bulk flow:
Flow = Kh × (ΔΨP / Δx); applies to xylem sap movement.
c. Osmosis:
Flow = Lp × ΔΨ; applies to water movement into cells.
15. A cell with S = -0.9 MPa and P = 0.4 MPa is placed in solution with Ψ = -0.4 MPa. The cell changes volume by 10%. Which direction does water flow and what are S and P for the cell at equilibrium?
Initial Ψ = -0.9 + 0.4 = -0.5 MPa < -0.4 MPa, so water flows into the cell.
At equilibrium: Ψ = -0.4 MPa; S and P adjust to sum to -0.4 MPa.
16. Cell A which has S = -0.9 MPa and P = 0.4 MPa is in contact with Cell B which has S = -0.5 MPa and P = 0.5 MPa. Which direction does water flow?
ΨA = -0.5 MPa; ΨB = 0 MPa → water flows from A to B.
17. What are two concepts/approaches to measure plant status? What is an advantage of each?
Relative Water Content (RWC): Simple, fast.
Water Potential (Ψ): Sensitive to drought and gives info on driving forces for water movement.
18. What are the leaf pressure volume curve parameters? Which is thought to be the most directly related to a species’ drought tolerance?
πo, πtlp (TLP), modulus of elasticity (ε), apoplastic fraction.
πtlp (TLP) is most predictive of drought tolerance.
19. Why is it difficult for plants to withdraw water from dry soil?
a. Driving force: Pressure gradient; Ψp becomes very negative in dry soils.
b. Soil moisture: As soil dries, air-water interfaces curve more → stronger tension.
c. Transport coefficient: Hydraulic conductivity drops steeply as soil dries.
20. What parts of the roots are most important for water uptake from soil?
a. Root hairs (over 60% surface area).
b. Pathways: Apoplast, symplast, transmembrane.
c. Tissue: Endodermis.
**d. Water crosses via Casparian strip, forcing entry into symplast—plants invest to control solute uptake.
21. How does water move from the soil to the root xylem?
Via root hairs → cortex (apoplast, symplast, transmembrane) → endodermis (forced symplast) → xylem.
22. Suppose that a pine tree has tracheids of 10 μm diameter and a sunflower has vessels of 50 μm diameter. Kh is higher for the sunflower by up to how many times?
(50 / 10)⁴ = 625× higher.
23. Please explain the structure and function of the xylem, including the following terms:
a. Tracheid: Dead, lignified tube-like cell in xylem.
b. Vessel: Wider dead tubes, connected end to end (mostly angiosperms).
c. Pits: Openings allowing water movement between conduits.
d. Poiseuille’s law: Flow scales with radius⁴.
e. Hydraulic conductance: Flow rate per pressure gradient.
f. Conduit radius: Strongly influences flow.
g. Cohesion-tension theory: Water pulled up by tension from transpiration.
h. Cavitation: Air bubbles block conduits, stopping flow.
24. How do the following factors influence transpiration rate?
a. Larger stomatal aperture: Decreases stomatal resistance → increases transpiration.
b. Higher temperature: Increases VPD → increases transpiration.
c. Lower relative humidity: Increases VPD → increases transpiration.
d. Slower wind speed: Increases boundary layer resistance → decreases transpiration.
e. A large leaf: Increases boundary layer resistance → decreases transpiration.
25. Please explain the structure and function of the stomata, including the following terms:
Guard cells: Swell/shrink to open/close stomata.
Plasmodesmata: Absent between guard cells and neighbors (isolation).
Turgor pressure: Drives guard cell swelling.
Cellulose microfibrils: Reinforce guard cell walls.
Stomatal resistance: Controls vapor diffusion.
26. What is the driving force for water movement out of the leaf? What is the driving force for water movement from root to leaf? How are the two linked? What is the main factor that determines the overall flow through the plant?
Out of leaf: VPD (concentration gradient).
Root to leaf: Pressure gradient (bulk flow).
Linked: Transpiration creates tension that pulls water up.
Main factor: VPD (external humidity/temperature).
27. UCLA replaced the plants in the UCLA court of sciences with native ones on the basis that these will require less water. Which traits would you measure at leaf or whole plant level to test whether this is indeed the case?
Leaf TLP, leaf area, stomatal density, root depth, xylem anatomy (vessel size), transpiration rate.
28. Name the 7 macronutrients and 9 micronutrients. What are their roles?
Macronutrients (7):
N (Nitrogen)
K (Potassium)
Ca (Calcium)
Mg (Magnesium)
P (Phosphorus)
S (Sulfur)
Si (Silicon)
Micronutrients (9):
Cl (Chlorine)
Fe (Iron)
B (Boron)
Mn (Manganese)
Na (Sodium)
Zn (Zinc)
Cu (Copper)
Ni (Nickel)
Mo (Molybdenum)
Roles:
Components of amino acids, nucleic acids, chlorophyll (N, P, S, Mg).
Components of cell walls (Si, B).
Ions for osmotic balancing (K, Na, Cl).
Enzyme cofactors (Ca, Mn).
Components of redox pigments (Fe, Zn, Cu, Ni, Mo).
29. Plant A has yellowing leaves, while Plant B has yellowing new leaves. What mineral deficiencies might they be suffering? Why do these different symptoms manifest?
Plant A (yellowing old leaves):
Likely deficiencies in mobile nutrients (N, P, K, Mg), which are relocated from old to young tissue.
Plant B (yellowing new leaves):
Likely deficiencies in immobile nutrients (S, Ca), which cannot be moved once deposited.
Reason:
Mobile nutrients are reallocated within the plant; immobile nutrients remain fixed, causing deficiencies to appear where new growth occurs.
30. What soil factors influence root nutrient uptake? How do plants release nutrients from soil? Give two reasons why plant roots need to keep growing.
Soil factors:
Soil texture (surface area for nutrient binding).
Soil pH (influences nutrient solubility).
How plants release nutrients:
Exude H+ ions to displace cations.
Release chelators to solubilize bound ions (e.g., Fe³⁺).
Why roots keep growing:
To explore new soil regions and access fresh nutrients.
To overcome depletion zones near existing root surfaces.
31. Explain the structure of mycorrhizae and their benefit for plant mineral nutrition:
Structure:
Ectomycorrhizae: fungal sheath around roots; hyphae between cortex cells.
Arbuscular mycorrhizae: hyphae penetrate cortex cells (but not protoplast), forming arbuscules inside cells, surrounded by a periarbuscular membrane.
Benefit:
Greatly expand the absorption area.
Improve phosphate uptake and other mineral access, especially from nutrient-poor soils.
32. Why is nodule formation by nitrogen fixing bacteria in legumes a case of “everyday endosymbiosis”?
The bacteria enter root hairs and form an infection thread that brings them inside root cells.
Bacteria are enclosed within plant-derived vesicles and work symbiotically to fix nitrogen.
This is true endosymbiosis: bacteria live inside plant cells, contributing N, while receiving sugars and controlled oxygen from the plant.
33. What is the driving force for solute transport?
The chemical potential difference across membranes, which includes contributions from:
Concentration gradient
Voltage gradient
(Minorly) Pressure gradient
34. How can the Nernst equation be used? To make a prediction of the ions inside the cell at equilibrium (based on voltage differences)
The Nernst equation calculates the equilibrium voltage (membrane potential) for a specific ion based on its inside vs. outside concentration.
It predicts the expected concentration gradient at equilibrium if the membrane potential is known.
35. What are the three kinds of solute transporters? What are the primary and secondary active transport? How do they differ?
Three transporters:
Channels: passive, rapid diffusion through pores.
Carriers: bind solute, undergo conformational change to transport; slower than channels.
Pumps: use energy to move solutes against gradients.
Primary active transport:
Directly uses energy (ATP hydrolysis) to transport ions (e.g., H+-ATPase).
Secondary active transport:
Couples the downhill movement of one ion (often H+) with the uphill transport of another solute (symport or antiport).
Difference:
Primary is directly energy-driven;
Secondary is indirectly driven by exploiting the gradient created by primary transport.
36. How do non-leguminous plants take up nitrogen from soil to the leaf mesophyll, and assimilate it into amino acids? Name key enzymes involved.
Uptake:
NO₃⁻ absorbed via nitrate-proton co-transporters into roots.
Transported to leaves via xylem.
Assimilation:
Nitrate reductase: converts NO₃⁻ → NO₂⁻ (cytosol).
Nitrite reductase: converts NO₂⁻ → NH₄⁺ (plastids).
Glutamine synthetase + glutamate synthase: incorporate NH₄⁺ into amino acids.
37. Why do organisms need nitrogen? Why do organisms compete strongly for nitrogen? Why is nitrogen metabolism so costly?
Need:
N is essential for amino acids, nucleic acids, chlorophyll, and enzymes.
Competition:
N is scarce in bioavailable forms despite its abundance in the atmosphere.
Once fixed, nitrogen is a highly sought resource among plants, microbes, and animals.
Cost:
Nitrogen metabolism requires breaking the stable N₂ triple bond, demanding large energy inputs (e.g., 12–16 ATP to assimilate nitrogen).
38. What are the important properties of light as a particle and as a wave that influence plants?
Wave: Wavelength determines energy (visible spectrum: 400–700 nm).
Particle: Photons carry energy (E = hν) needed to excite pigment electrons.
38a. What is the overall reaction of photosynthesis?
6 CO₂ + 6 H₂O + light → C₆H₁₂O₆ + 6 O₂.
38b. Why do plants harvest sunlight in the same range of wavelengths as the animal eye uses for vision? Include the important properties of light and of the pigment molecules in your answer.
Visible light (400–700 nm) has just enough energy to excite electrons in pigment molecules (chlorophylls with conjugated double bonds) without breaking bonds—perfect for light harvesting and safe for molecular stability.
39. What is a pigment molecule, and how does it function? Use the following terms in your answer: Light harvesting, absorption spectrum, conjugated double-bond.
A pigment molecule (like chlorophyll) absorbs light (specific absorption spectrum) via its conjugated double bonds, which stabilize excited states. It channels energy for light harvesting to power photosynthesis.
40. What is the light-harvesting antenna? Where is it located?
A protein-pigment complex (mostly chlorophylls + carotenoids) that absorbs light and transfers energy to the reaction center.
Located in the thylakoid membrane of chloroplasts.
41. What is meant by the Z-scheme of photosynthesis?
The Z-scheme describes the flow of electrons from water (via PSII → cytochrome b₆f → PSI) to NADP+, driven by light. It’s shaped like a Z when redox potential is plotted.
42. Chloroplasts are treated with a herbicide. Oxygen continues to be generated but CO₂ assimilation ceases. What is the herbicide?
Likely DCMU, which blocks electron flow after PSII. Oxygen evolves, but ATP/NADPH aren’t made to fuel CO₂ fixation.
43. What are the reactants and products of the light reactions? Where do the light reactions occur?
Reactants: Light, H₂O, NADP+, ADP + Pi.
Products: O₂, NADPH, ATP.
Occur in the thylakoid membranes.
44. Sketch out the electron transport reactions, and describe how ATP is produced.
Light excites PSII (P680) → splits water → electrons pass to plastoquinone → cytochrome b₆f → plastocyanin → PSI (P700) → ferredoxin → NADP+ → NADPH.
Proton gradient from water splitting + electron flow powers ATP synthase (photophosphorylation).
47. Name two ways that the carbon reactions are chemically coordinated with the light reactions, so that they can run at the same rates.
Rubisco activation by light (high stromal pH, Mg²⁺).
Ferredoxin-thioredoxin system activates Calvin cycle enzymes when light reactions are active.
48. Sketch out the three main components of the Calvin cycle.
1. Carboxylation: CO₂ + RuBP → 2× 3-PGA.
2. Reduction: 3-PGA → G3P (using ATP, NADPH).
3. Regeneration: G3P → RuBP (using ATP).
49. Name 5 ways that the Calvin cycle is regulated.
Rubisco activation by rubisco activase.
Rubisco light activation via pH/Mg²⁺ changes.
Ferredoxin-thioredoxin light activation of other enzymes.
Feedback inhibition by sugar buildup.
Gene expression regulating enzyme amounts.
50. What is the importance of compartmentalization, regulation of key steps, redundancy, and feedback in metabolic design? Please give two examples of each in photosynthesis reaction.
Compartmentalization:
Light reactions in thylakoid; Calvin cycle in stroma.
Sucrose synthesis (cytoplasm) vs. starch synthesis (chloroplast).
Regulation of key steps:
Rubisco carboxylation.
ATP synthase coupling to proton gradient.
Redundancy:
PSII and PSI both have their own light-harvesting antennae.
Cyclic electron flow as backup ATP generator.
Feedback:
Rubisco deactivated by sugar buildup.
Starch synthesis increases when sucrose builds up.
51. What is photorespiration? What are the reactants and products? Please name the toxic byproduct and the cost of its disposal. Why might this have evolved?
Rubisco fixes O₂ to RuBP → 3-PGA + phosphoglycolate.
Phosphoglycolate → 3-PGA + peroxide (toxic byproduct).
Disposal uses ATP + NADPH.
May have evolved due to ancient high CO₂ (less O₂ competition) or as a way to burn excess ATP/NADPH.
52. What are the different roles in the plant of starch and sucrose? What makes these two molecules suitable for their different functions? What makes them unsuitable for the opposite function?
Starch: Storage (insoluble polymer of glucose; compact, no osmotic effect).
Sucrose: Transport (soluble, disaccharide; mobile in phloem).
Starch is unsuitable for transport (insoluble); sucrose unsuitable for storage (osmotic pressure).
53. Sketch out the major steps of sucrose and starch synthesis. How do these synthesis differ in metabolites and reaction location?
Sucrose:
Triose phosphates → Fructose-6P + UDP-glucose → Sucrose.
Location: Cytoplasm.
Starch:
Triose phosphates → ADP-glucose → Starch.
Location: Chloroplast.