Ecology, Plants, and Photosynthesis

Question 1

Action and Absorption Spectrum

Answer 1

Action spectrum: Graph showing rate of photosynthesis for each wavelength of light.

1. Green-yellow light shows lowest rate of photosynthesis
2. Red-orange light shows good rate of photosynthesis
3. Violet-blue light shows best rate of photosynthesis.

Absorption spectrum: Graph showing % light absorbed by pigments within chloroplast (e.g. chlorophyll a + b), for each wavelength of light.

1. Green-yellow light shows least absorption/
   most reflection.
2. Red-orange light shows some absorption/
   little reflection
3. Violet-Blue light shows most absorption/
   least reflection.

Chlorophyll is most abundant pigment, so rate of photosynthesis greatest at violet-blue light.

However, carotene, another, less common, pigment able to absorb green-yellow light. So even in small amounts, allow some (little) photosynth to occur at wavelengths of light that chlorophyll can’t absorb.

Question 2

Chloroplast Structure

Answer 2

1. Double/inner and outer membrane/envelope – 2 concentric continuous lines close together;
2. Grana: Stack of several disc-shaped subunits (thylakoids);
3. 70S ribosomes
4. Starch granules
5. Stroma: Fluid containing enzymes, including rubisco, which are important for LIR’s.
6. Thylakoids:
   
   1. Have a large surface area for light absorption.
   2. Thylakoid space: Small, which causes faster accumulation of H⁺, which enhances [H⁺] gradient;

Question 3

Photosynthesis

Answer 3

LDR’s: (In thylakoid space largely)

1. Photosynthesis occurs inside chloroplasts, which contain chlorophyll inside thylakoid membranes, which are arranged in groups called photosystems (I and II).
2. Chlorophyll in photosystem II absorbs light; which excites a free e¯ (photoactivation)
3. Excited e¯ pass along ETC from carrier to carrier (within thylakoid membrane); from photosystem II to photosystem I (in stroma);
4. e¯ flow releases NRG, which is used to pump H⁺ across thylakoid membs & into thylakoid space; creating [H⁺] grad in thylakoid space;
5. Chemiosmosis couples e¯ transport to ATP synth. (photophosphorylation); meaning that:
   
   1. When H⁺ diffuses back across thylakoid memb (down conc. gradient)(chemiosmosis).
   2. They pass through ATP synthase; which uses KE from movement of H⁺ down their conc. grad to synthesise ATP.
      (by combining ADP + Pi).
   3. This form of ATP synthesis
      = Non-cyclic photophosphorylation.
6. Light excites e¯ in photosystem I (photoactivation); to reduce NADP⁺ to NADPH;
7. H₂O lyses to form O₂ + H⁺ + e¯ of which:
   
   1. O₂ is largely released as a waste product
   2. e¯ used to replace e¯ lost by photosystem II;
8. In cyclic photophosphorylation e¯ from photosystem I return to it via ETC between photosystem I and II; which causes H⁺ to be pumped across thylakoid membrane again.

LIR’s/Calvin Cycle: (In stroma)

1. LIRs take place in stroma of chloroplast; to produce carbs; using LDR products.
2. Rubisco catalyses; the fixation of CO₂ to RuBP; (C-fixation); to form an unstable 6-C compound; which splits into 2 glycerate 3-phosphate molecules;
3. LDR’s produce ATP + NADPH; of which:
   
   1. ATP provides nrg to reduce glycerate 3-phosphate;
   2. NADPH provides H for reduction of glycerate 3-phosphate; to triose phosphate;
4. Some triose phosphate used to regenerate RuBP;
5. Some triose phosphate used to synthesise glucose & (after several cycles) starch;

Question 4

Factors of Photosynthesis

Answer 4

Limiting Factor: Factor furthest from its optimum & serves to control a process.

1. Increasing limiting factor with other factors constant increases rate;
2. Increasing non-limiting factor with other factors constant has no effect on rate;

Light intensity: Limiting at low intensity;

1. ↑ intensity ↑ photosynthesis up to plateau;
2. Low light intensity → ↓ rate of LDR’s → ↓ ATP + NADPH (LDR products) → ↓ LIR’s → ↓ photosynth.
3. High light intensity → ↑ rate of LDRs; until chloroplasts become saturated with light (plateau)

Temperature: Limiting at low & high temps;

1. ↑ temp ↑ photosynthesis up to certain temp, then ↓ dramatically;
2. Low temps limit rate of LIRs reactions;
3. High temps (temps > opt. temp of rubisco) → rubisco denatures → CO2 not fixed (dec.);
4. ↑ temp ↑ rate of enzyme catalysis in LIR’s (e.g. rubisco) → ↑ rate of photosynthesis.

[CO₂]: Limiting in bright light & warm temps; at low [CO₂]

1. ↑ [CO₂] ↑ photosynthesis up to plateau;
2. Low [CO₂] ↓ rate of C-fixation → ↓ rate of photosynthesis.
3. High [CO₂] ↑ rate of C-fixation → ↑ rate of photosynthesis until plateau
   (when another factor becomes limiting).

Question 5

Chromatography and Chloroplasts

Answer 5

1. Leaf pieces grinded in pestle + mortar, then
   propanone added & repeated.
2. Cover & leave until liquid turns dark green. Then decant or filter liquid.
3. Use capillary tube to apply extract to chromatography strip or thin layer strip until small dark spot formed.
4. Place chromatography strip into narrow glass tube containing chromatography solvent & leave it until solvent front has moved to top & separated pigments.
5. Chlorophyll composed of various pigments:
   
   • Carotene (top layer)
   • Xanothophyll
   • Chlorophyll a
   • Chlorophyll b

Question 6

Species, Pops, Comms, Ecos

Answer 6

1. Species: Groups of organisms that can interbreed to produce fertile offspring
   • Cross-breeding: When 2 diff species
     produce hybrid, reproductively sterile offspring (e.g. liger, mule). Rare.
2. Population: Group of organisms of same species living in same area at same time
   
   • Organisms that live in different regions
     are reproductively isolated & unlikely to interbreed, but speciation only occurs if populations can no longer interbreed.
3. Communities: Pops of diff species living together & interacting with each other.
4. Habitat: Env in which species normally lives.
5. Ecosystems: Communities (biotic factors) interacting with abiotic env.

Question 7

Modes of Nutrition

Answer 7

1. Autotrophs: Obtain inorg. nutrients from abiotic env to synthesise org. compounds
   using energy from sunlight (photosynth.) or oxidation of inorg. nutrients (chemosynth.)
2. These nutrients (e.g. C, N, H, O & P) obtained from air, water & soil.
3. Heterotrophs: Obtain org. nutrients from feeding off other living org. or dead org. matter.
   
   1. Consumers: Heterotrophs that feed on living or recently killed org. via ingestion.
   2. Scavengers: Consumers that feed on dead/decaying carcasses rather than hunt living prey. (e.g. hyenas, vultures, etc.).
   3. Detritivores: Heterotrophs that feed on detritius or humus via int. digestion.
      
      • Detritus: Dead org. matter, e.g. decaying org. material & fecal matter
      • Humus: Decaying leaf litter intermixed within topsoil.
   4. Saprotrophs: Heterotrophs that feed on detritus by secreting dig. enzymes into it, & absorbing the digested products.
      
      • Decomp. release elements like N₂ into ecosystem, so that they could be used again by other organisms.

Question 8

Nutrient Cycling

Answer 8

1. Auto obtain inorg. nutrients from air, water & soil; convert them into org comp.
2. Hetero ingest org. comp. & use them for growth & respiration, releasing inorg. byproducts
3. Saprotrophs decompose the dead org. remains
   & free inorg. materials into soil.
4. Return of inorg. nutrients to soil ensures continual supply of raw materials for autotrophs.
5. Cycle begins again.

Question 9

Ecosystem Sustainability & Mesocosms

Answer 9

3 main components required for ecosystem sustain.

1. Energy availability: Light from sun provides the initial energy source for almost all communities
2. Nutrient availability: Saprotrophs ensure constant recycling of inorg. nutrients within env
3. Recycling of wastes: Certain bacteria can detoxify harmful waste byproducts.
   (e. g. denitrifying bacteria).

Mesocosms: Enclosed env’s that allow small part of natural env to be observed under controlled conditions.

• Terrarium: Small transparent container (e.g. glass or plastic) in which selected plants (or animals) are kept & observed

Making a Self-Sustaining Terrarium:

1. Terrarium created using glass/plastic bottle with lid.
2. 1^st layer: pebbles/gravel/sand for drainage purposes.
3. 2^nd layer: Thin layer of activated charcoal to prevent mold & help aerate soil.
4. 3^rd layer: Thin moss cover to create barrier between lower layers & soil.
5. 4^th layer: Pre-moistened growing medium
   (i.e. potting mix).
6. Choose plants that ideally grow slowly
   & thrive in some humidity & inspect plant for diseases/insects before placing in terrarium.
   (e.g. most ferns, club moss, etc.)
7. Ensure terrarium placed in location that provides continuous source of light AND
   doesn’t experience fluctuating temp. conditions (i.e. avoid direct sunlight)
8. Don’t initially over-water plants – once right humidity established, terrarium can go months without watering
9. Occasional pruning may be required, but nutrients in soil dec, so plant growth should slow down.

Question 10

Testing for association between 2 species using Chi-squared test with data obtained from quadrat sampling

Answer 10

1. 2 species existing within given env depends on potential interactions between them.
2. + Association: If 2 species typically found within same habitat.
   
   • (e.g. predator/preys or symbiotic relations)
3. - Association: If 2 species typically not found within same habitat.
   
   • Species typically show - association if competition exists for same resources
   • Some parts of habitat better for some species than others:
   • Competitive Exclusion: 1 species
     utilising resources more efficiently than another → dec. survival of other species
   • Resource Partitioning: Both species
     altering use of env to avoid direct comp.
   • If species do not interact, NO association & their distribution = indep of 1 another.
4. Presence of 2 species within given env
   determined using quadrat sampling:
   1. Quadrat: Rectangular frame of known dimensions used to establish pop density
   2. Quadrats placed inside defined area in either random arrangement or according to a belted transect.
   3. # of individuals of given species countedor estimated via % coverage
   4. Sampling process repeated many times in order to gain a representative data set
   5. Quadrat sampling used for counting plants & sessile animals, but not useful for motile organisms.
   6. In each quadrat, presence or absence of each species is identified
   7. Allows for # of quadrats where both species present to be compared against total # of quadrats.
5. Chi-Squared Test: Determines if there is a stat. sig. association between distr of 2 species
   
   1. Identify hypotheses (H₀ vs. H₁):
      • H₀: No sig diff between distr of 2
        species (i.e. NO ASSOCIATION)
      • H₁: Sig diff between distr of 2 species (i.e. ± ASSOCIATION)
   2. Construct freq. table (obs vs. exp)
      
      • Expected freq = (Row total × Column total) ÷ Grand total
   3. Apply chi-squared formula
      
      • Ʃ(O - E)² ÷ E
   4. Determine degree of freedom (df):
      
      • df = (m – 1) (n – 1)
      • m = # of rows; n = # of columns
      • df should always be 1 with 2 species
   5. Identify the p value.
      
      • p values indicate probability that relationship down to chance.
      • p value should be
      • if chi-squared > 3.8
        → reject H₀ ; accept H₁.
      • if chi-squared → reject H₀ ; accept H₁
   6. _​If H₁ accepted, determine which type of association the species have:
      1. If not usually in same habitat/quadrat
         = negative
      2. If usually in same habitat/quadrat
         = positive

Question 11

Energy Flow

Answer 11

1. Autotrophs gather energy from sun or chem
   processes to synth. org. comp. from inorg. comp.
2. Photosynthesis converts light energy into chem energy, which is stored in org. comp.
3. Heterotrophs ingest these organic compounds in order to derive their chemical energy (ATP) via feeding off other organisms.
   
   1. Food chain: shows linear feeding relationships between species in a comm.
   2. Trophic Lvls: Position org. occupies within a feeding sequence (e.g. food chain).
4. When org. comp. broken down via cell resp, ATP produced to fuel metabolic processes
   required for growth and homeostasis.
5. A by-product of these chem reactions is heat, which is released from organism
6. Not all energy stored in org. comp. transferred
   via heterotrophic feeding, some lost by:
   1. Being excreted in organism’s faeces.
   2. Remaining unconsumed as uneaten portions of food (e.g. bones, cellulose, etc)
7. Chem energy produced by organism
   converted into:
   1. KE: (e.g. during muscular contractions)
   2. Electric: (e.g. during transmission of nerve impulses)
   3. Light: (e.g. producing bioluminescence)
   4. All these reactions = exothermic & release heat as a by-product.
8. Living org’s can’t turn this heat into useful
   energy, so released from org. & lost from
   ecosystem (unlike nutrients, which are recycled
9. Hence ecosystems require continuous energy influx from an external source (i.e. Sun)

Question 12

Biomass

Answer 12

1. Energy transformations in living orgs never 100% efficient
2. Most nrg lost to org. either in respiration, (released as heat), excreted in faeces (e.g. cellulose), or unconsumed (e.g. bones).
3. 90% available nrg lost between trophic lvls & higher trophic lvls store less energy as C-compounds, so have less biomass
4. Biomass: Total mass of group of organisms, consists of cells, tissues and c-compounds.
5. Biomass + nrg ↓ along food chains with loss of CO₂, H₂O & waste products (e.g. urea) to env.
6. Explains limits to # of trophic lvls:
   
   1. As higher trophic lvls receive ↓ nrg/
      biomass from feeding, they need to eat more to obtain enough amounts
   2. By eating more, they expend more nrg
      (& biomass) hunting for food
   3. If nrg to hunt > nrg from food
      → trophic lvl unfeasible
7. Pyramid of Nrg: Graphical representation of amount of nrg at each trophic lvl of food chain.
   • Units: Nrg/area/time (e.g. kJ m^–2 yr^–1)
   • Pyramids of energy never appear inverted as some of energy stored in 1 source always lost upon transfer.
   • Each level roughly 1/10 of size of
     previous lvl.
   • Producers → 1º → 2º → 3º

Question 13

Carbon Cycle

Answer 13

1. Autotrophs convert inorg CO₂ into org
   comp’s (carbs, lips & prots) via photosynthesis
2. Autotrophs use CO₂ for photosynthesis
   → [CO₂] in org to atm (or water)
   → CO₂ passively diffuses into autotroph as required:
   • In aquatic autotrophs, CO₂ diffuses directly into autotroph.
   • In terrestrial plants, CO2 diffuses into autotroph through stomata.
3. Heterotrophs can’t synthesise their own org comp’s, so obtain org comp’s via feeding
4. All organisms produce nrg needed to power metabolic processes via cell respiration.
5. Build up of CO₂ in respiring tissues creates [grad] → removed by passive diffusion
6. Compensation Point: Net CO₂ assimilation = 0
   (Photosynth in auto’s = Respiration in hetero’s)
7. If net photosynthesis > cell respiration occuring in biosphere, atm [CO₂] should drop.
8. If net respiration > photosynthesis occurring in biosphere, atm [CO₂] should rise.

Question 14

CO₂ in H₂O

Answer 14

1. Oceans = major C-sink & absorb most of anthropomorphic CO₂ emissions.
2. CO₂ solubility = temp-dependent (+ soluble when cooler) → ↓CO₂ absorbed as temps ↑
3. Oceans absorb atm [CO₂]:
   
   • Some remains as dissolved (g)
   • Most combines with H₂O:
     CO₂ + H₂O ⇄ H₂CO₃
   • Which dissociates:
     H₂CO₃ ⇄ HCO₃¯ + H⁺
4. H⁺ produced lowers ocean pH & so dissolved [CO₂] ∝ H⁺ produced / acidity.
5. Since start of industrial revolution ocean pH↓.
6. Molluscs & coral reefs also absorb dissolved CO₂ & CO₃²¯ in rocks & combine them with Ca²⁺ to produce CaCO₃ used to form hard coral exoskeletons & mollusc shells.
7. H⁺ (from H₂CO₃) combines with CO₃²¯ to reform acid, which dec. free CO₃²¯ in H₂O, so molluscs + corals less able to produce CaCO₃.
8. Shells + coral exoskeletons that do form likely thinner & dissolve if ocean conditions + acidic.
9. Hence inc. [dissolved CO₂] threatens viability of coral reefs & molluscs.
10. Coral reef disapp → loss of shoreline protect.
    & habitat, altering coastal ecosystems.
11. ↓ revenue from tourism & food industries predicted to cost economies trillions.
12. ↑ dissolved [CO₂] in oceans → invasive algae species to flourish (more photosynthesis).

Question 15

Methanogenesis and Methane

Answer 15

1. Methanogens: Archaeans (prokaryotes) that produce methane (CH₄) as a metabolic by-product in anaerobic conditions including:
   
   1. Waterlogged soils
      (e. g. swamps and marshes)
   2. Marine sediments
      (e. g. in the mud of lake beds)
   3. Landfill sites (high pressure)
   4. Digestive tract of ruminant animals
      (e. g. cows, sheep, goats)
2. Methanogens produce CH₄ from via anaerobic digestion of CH₃COOH & CO₂:
   
   1. CH₃COOH → CH₄ + CO₂
   2. CO₂ + 4H₂ → CH₄ + 2H₂O
3. CH₄ builds underground or diffuses into atm.
4. Org matter buried in anoxic conditions (e.g. sea beds), forms natural gas (i.e. CH₄) underground
5. [CH₄] affected by:
   1. Rising # of domesticated cattle likely inc.
      [CH₄] being released into atm.
   2. CH₄ produced from organic waste in anaerobic digesters not allowed to escape, instead it is burned as a fuel.
   3. CH₄ naturally oxidised to form CO₂ + H₂O (CH₄ + 2O₂ → CO₂ + 2H₂O)
      → short life-cycle → atm [CH₄] low
      despite lots made.

Question 16

Peat and Fossils

Answer 16

1. Saprotrophs decompose detritus in soils &
   return nutrients to soil (recycles nutrients).
2. Decomp requires O₂ (cell resp. needed to fuel digestive reactions)
3. Waterlogged soils may lack oxygenated air spaces, thus possess anaerobic conditions
4. Anaerobic respiration by organisms in these regions produces organic acids (e.g. ethanoic), resulting in acidic conditions
5. Saprotrophs can’t function effectively in anaerobic/acidic conditions
   → prevents decomp.
6. Lots of compressed partially decomposed organic matter form C-rich acidic peat.
7. When peat deposits compressed under sediments, heat + press force out impurities & remove moisture, producing high [C] coal.
8. Burial + compaction of org. matter under deposited sediments (e.g. clay/mud) cause org matter to be heated & hydrocarbons to form,
   which form oil and gas, which are forced out of source rocks & accumulate in porous rocks.(e.g. sandstone)
9. FF formation takes place over long time, making them non-renewable nrg source.

Question 17

Carbon Fluxes

Answer 17

1. Global carbon fluxes are very large, measured in gigatonnes (billion tonnes)
2. Due to carbon fluxes being large & based on measurements from many diff sources, estimates have large uncertainties.
3. Main factors for C exchange (DOLCE FFIC):
   
   1. Deforestation will reduce removal of atm CO₂ via photosynthesis
   2. Oceanic temps also determine how much C stored as dissolved CO₂ or as HCO₃¯
   3. Inc. # of ruminant livestock (e.g. cows) will produce higher levels of CH₄.
   4. Climate events (e.g. El Niño, La Niña) change rate of C-flux ocean ⇔ atm
   5. Volcanic Eruptions release C-comps from Earth’s crust into atm.
   6. FF burning releases CO₂ into atm.
   7. Forest fires release high [CO₂] when plants burn (loss of trees also reduces photosynthetic C-uptake)
   8. Melting of polar ice caps → decomp of frozen detritus → releases CO₂ + CH₄.
4. Atmospheric [CO₂] measured at Mauna Loa Observatory (in Hawaii) since 1958 by Charles Keeling:
   
   1. [CO₂] fluctuate annually/seasonally.
   2. Global CO₂ trends conform to northern hemisphere patterns as it contains more of planet’s land mass (i.e. more trees)
   3. [CO₂] steadily inc. since industrial revolution (due to inc. burning of FF’s).
   4. Atm [CO₂] currently at highest recorded since measurements began.

Question 18

Greenhouse Gases

Answer 18

1. Greenhouse Gases(GG): Gases that absorb LW
   radiation, but let SW radiation pass through.

GG’s with largest warming effect within atm:

1. H₂O(g):
   
   • Created via evaporation of bodies (e.g. oceans) & transpiration
   • Removed via precipitation & CO₂
2. CO₂:
   
   1. Made by cell resp. & biomass (e.g. trees & FF) combustion.
   2. Removed via photosynth. & absorption by oceans
3. CH₄: Emitted from waterlogged soils, landfills,
   (g) product of ruminants, permafrost melting.
4. NO_x: released naturally by certain bacteria & artificially in exhaust by certain vehicles.

[CO₂]: [GG] inc. most rapidly in atm is CO₂ due to
combustion of FF.

1. When FFs combusted to release nrg, CO₂
   released as a by-product.
2. Inc. more amounts produced by human’s inc. reliance on FFs following industrial revolution.
3. Efforts to reduce reliance on FFs involve
   exploiting alt nrg sources (e.g. solar power).

Question 19

Greenhouse Effect

Answer 19

1. Green Eff: Natural process whereby atm traps & retains heat to prevent temp fluctuations & ensure Earth maintain moderate temps needed by organisms to maintain life processes.
   
   1. Insolation = SW radiation, which is absorbed by Earth’s surface, & re-emitted as LW radiation.
   2. GG absorb & re-radiate LW, hence retaining heat within atm.
2. Without green eff, Earth’s temps would drop sig. at night in absence of direct sunlight.
3. While GG’s occur naturally, man is inc. GG
   emission via (DAT):
   1. Deforestation: Tree removal → less CO₂ removed from atm via photosynth.
   2. Transport: More people = more cars
   3. Inc. Agriculture: Involves land clearing for cattle grazing & ruminant cattle producing CH_4​

Question 20

Global Warming

Answer 20

1. GGs retain heat & so main contributor to GW.
2. Inc. in [GG] should therefore correlate with an inc. in global temp.
3. Long-term weather patterns (climate) may also be influenced by [GG].
4. Scientists predict that inc. in [GG] will lead to Enhanced green eff (EGE), resulting in:
   
   1. More freq. extreme weather conditions (e.g. heat waves, cyclones, more powerful tropical storms, etc.)
   2. Some areas becoming + drought prone, whilst other areas becoming + prone to longer + heavier rainfall periods.
   3. Changes to circulating ocean currents →
      longer El Niño (warming) & La Niña
      (cooling) events.
   4. But effects = hypotheses, so uncertain.
5. Link between global temps & [CO₂] established by analysing data over long time period using ice cores taken from Antarctica.
   1. Ice cores provide evidence of the env
      conditions at time of freezing
   2. By analysing gas bubbles trapped in ice, historical [CO₂] & air temps (via oxygen isotopes) can be deduced
   3. Data collected from ice core show that:
      1. Strong + correlation between [CO₂]
         & temp (↑ [CO₂] ∝ ↑ temperature)
      2. Existence of fluctuating [CO₂] cycles which appear to correlate with global warm ages & ice ages.
      3. Current [CO₂] highest in recorded history.
6. Industrial revolution introduced new processes that sig. inc. man’s FF use.
7. FF burning releases CO₂ as by-product, leading to steady inc. in its atm [CO₂]
8. When fuel emissions, atmospheric CO2 concentrations and global temps
   compared, trends are revealed:
   1. Strong + correlation between inc. FF emissions & rising atm [CO₂].
   2. Atm [CO₂] inc. sig. since pre-industrial age
   3. Nearly half of CO₂ emissions remained in atm. (rest absorbed by sinks).
   4. Inc in atm [CO₂] correlates with inc. in average global temp.
   5. Whilst correlation ≠ causation, mounting evidence suggests CO₂ emissions linked to global temp changes, but other factors likely also contribute, such as:
      
      1. Milankovitch cycles in Earth’s orbit.
      2. Variation in sunspot activity.

Question 21

Water Transport in Plants

Answer 21

1. Water transported in xylem tissue.
2. Xylem walls are thick & impregnated with lignin, which is strong & allows xylem to withstand low pressures without collapsing.
3. When mature, xylem cells are nonliving, so flow of water among them → passive process.
4. Pressure inside xylem vessels usually < atm.
   pressure, but rigid structure prevents xylem vessels from collapsing.
5. Water pulled up from xylem in continuous stream because:
   
   1. Cohesion: H₂O molecules = polar; & delta - charge on O atom in 1 H₂O molecule attracts H in neighbouring H₂O molecule.
   2. Adhesion: H₂O molecules also attracted to hydrophilic parts of xylem cell walls.
6. Adhesive property of H₂O & evaporation generate tension forces in leaf cell walls.
7. When H₂O evaporates from wall surface in leaf, adhesion causes H₂O to be drawn through cell wall from xylem vessels in leaf veins.
8. Even if press. in xylem already low, force of adhesion between H₂O & cell walls in leaf strong enough to suck H₂O out of xylem, further reducing press.
9. Transpiration Pull: Low press. of xylem generates pulling force that’s transmitted through H₂O in xylem vessels down stem & to ends of xylem in roots.
10. Energy for this comes from heat that causes transpiration (no extra energy needed)
11. Pulling of water upwards in xylem depends on cohesion that exists between water molecules.
12. Cavitation: Liquids unable to resist low press.
    in xylem vessels & liquid column breaks (occurs with most liquids, unusual with water).
13. Water travels in xylem along 2 pathways:
    
    • Apoplast Pathway: Water travels xylem from cell wall to cell wall
    • Symplast Pathway: Water travels xylem through cytoplasm to cytoplasm.

Question 22

Water Absorption from Soils

Answer 22

1. AT of mineral ions in roots causes absorption of water by osmosis.
2. Mineral ions only absorbed by AT if they make contact with appropriate pump protein. This can occur by diffusion, or by mass flow when H₂O carrying ions drains though soil.
3. Some ions move through soil very slowly because ions bind to surface of soil particles.
4. To overcome this, fungi grows on surface of roots and sometimes even into cells of root.
5. Hyphae grow out into soil, absorb slow-moving ions like phosphate from soil, then supply ions to roots.
6. Most of these plants supply sugars & other nutrients to fungus, so both fungus & plant benefit.

Question 23

Xerophyte Adaptations

Answer 23

1. Xerophytes: Plants adapted to growing in deserts and other dry habitats. They do this by:
   
   • Inc. rate of water uptake from soil.
   • Dec. rate of water loss by transpiration.
2. Some xerophytes have short life cycles completed by brief period when H₂O available after rainfall. They then remain dormant as embryos inside seeds until next rains.
3. Others are perennial and rely on storage of water in specialised leaves, stems or roots.
4. Most cacti are xerophytes:
   
   • Spines instead of leaves prevent transpiration.
   • Stems contain H₂O storage tissue &
     become swollen after rainfall.
   • Pleats allow stem to expand & contract in volume rapidly.
   • Epidermis of cactus stems has thick waxy cuticle.
   • Stomata in plant stems (unusual), but spaced more widely than typical in leaves.
   • Stomata also open at night rather than at day, when it’s cooler & transpiration occurs more slowly.
   • Crassulacean Acid Metabolism (CAM): CO₂ absorbed at night & stored in form of malic acid, which releases CO₂ during day, allowing photosynthesis even when stomata is closed.
5. Marram Grass is also a xerophyte:
   
   • Rolled leaf, which creates localised env. of H₂O(g) that helps to prevent H₂O loss.
   • Stomata sit in small pits within curls of structure, which make them less likely to open & to lose water.
   • Folded leaves have hairs on inside to slow or stop air movement, much like many other xerophytes.
   • This slowing of air movement once again dec. amount of H₂O(g) being lost.

Question 24

Halophyte Adaptations

Answer 24

Halophytes: Plants adapted to saline soils.

• Leaves are reduced to small scaly structures or spines.
• Leaves shed when H₂O scarce & stem turns green & takes over function of photosynthesis when leaves absent.
• H₂O storage structures develop in leaves
• Thick cuticle & multiple layered epidermis.
• Sunken stomata
• Long roots, which search for water.
• Structures for removing salt build-up present.

Question 25

Phloem

Answer 25

1. Phloem composed of sieve tubes, which are composed of columns of sieve tube cells.
2. Individual sieve tube cells separated by perforated walls (sieve plates).
3. Sieve tube cells closely associated with companion cells.
4. Phloem has 3 functions:
   
   1. Loading of carbohydrates at sources
   2. Transport of carbohydrates
   3. Unloading carbohydrates at sinks

Question 26

Translocation

Answer 26

1. Translocation: Transport of organic solutes in a plant. Requires:
   
   1. Sources: Areas where sugars and amino acids are loaded into phloem.
   2. Sinks: Where sugars and amino acids are unloaded and used.
2. Sometimes sinks turn into sources, or vice versa. Phloem tubes must therefore transport in both directions. Also no valves or central pump like blood vessels.
3. However, fluid does flow inside tubes because of press. gradients & energy is needed to generate press. so both blood & phloem movement are active processes
4. Phloem Loading: AT used to load organic compounds like sucrose into phloem sieve tubes at source.
5. Sucrose not as readily available for plant tissues to metabolise directly in respiration, therefore makes good transport form of sugar as it won’t be metabolised during transport.
6. Plasmodesmata: Connections between plant cells located in cell walls.
7. Once sucrose reaches companion cell, it’s converted to an oligosaccharide to maintain [sucrose] gradient.
8. Sucrose build-up draws H₂O into companion cell through osmosis.
9. Rigid cell walls combined with incompressibility of H₂O result in build-up press., causing H₂O to flow from this area of high press. to area of low press. (following hydrostatic press. gradient).
10. At sink end, sucrose withdrawn from phloem & either utilised as energy source for growth or converted to starch.
11. In either case, loss of solute causes reduction in osmotic press. & H₂O carrying solute to sink then drawn back in to transpiration stream in xylem.

Question 27

Adaptations of Phloem sieve tubes

Answer 27

1. Sieve tubes composed of living sieve tube cell columns. “Living” as they depend on membrane to maintain [sucrose] made by AT.
2. Sieve tube cells closely associated with companion cells to share functions:
   
   1. Partly due to sieve tube cell & its companion cell sharing same parent cell.
   2. Companion cells perform most genetic & metabolic functions of sieve tube cell & maintain viability of sieve tube cell.
   3. Lots of mitochondria in companion cell to carry out AT of sucrose = specialised.
   4. Infolding of companion cell inc. phloem loading capacity using apoplastic route.
3. Plasmodesmata connect cytoplasm of companion cells with sieve tube cells & have larger diameter than plasmodesmata found in other parts of plant to allow movement of oligosaccharides and genetic elements between 2 cells.
4. Sucrose build-up in sieve tube cell-companion cell pair requires presence of AT proteins or enzyme activity in companion cells to produce oligosaccharides.
5. Rigid sieve tube cell walls allow for establishment of press. necessary to achieve flow of phloem in sieve tube cell.
6. Sieve tube cells separated by perforated walls (sieve plates) → Perforated walls + reduced cytoplasm = low resistance to flow phloem sap.

Question 28

Measuring Rate of Phloem Sap flow

Answer 28

Hemiptera is a group of insects that mainly eat phloem in plants due to their nutrient rich fluid.

Aphids (an example of a Hemiptera) penetrate plant tissue to reach phloem using mouth parts called stylets.

If aphid is anaesthetised and stylet severed, phloem will continue to flow out of stylet and both rate of flow and composition of sap can be analysed.

The closer the stylet is to the sink, the slower the rate at which phloem sap will come out.

Question 29

Meristems

Answer 29

1. Meristems: Regions made of undifferentiated cells undergoing active cell division.
2. Apical Meristems: Meristems found at tips of stems and roots.
   
   • Shoot Apical Meristem (SAM) responsible for stem growth.
   • Root Apical Meristem (RAM) responsible for root growth.
3. Lateral Meristems: Meristems found around established stems. Cause plant to grow laterally.
4. Cells in meristems small, so divide repeatedly to produce more cells, which absorb nutrients & H₂O & so inc. in volume & mass.
5. SAM throws off cells needed for growth of stem & produces cell groups that grow & develop into leaves and flowers.
6. With each division, 1 cell remains in meristem, whilst other inc. in size & differentiates as it’s pushed away from meristem region.

Question 30

Auxin in Phototropisms

Answer 30

1. Phototropism: Growing/turning of organism in response to a unidirectional light source
2. Auxins (e.g. IAA): Plant hormones produced by tip of a shoot & mediate phototropisms.
   
   • Stimulates growth at low [IAA].
   • Inhibits growth at high [IAA].
3. Auxin makes cells enlarge, but eradicated by light. So only builds up on shaded side of plant, so only shaded side grows, resulting in shoot bending towards light.
4. Auxin causes cell elongation by activating proton pumps that expel H⁺ ions from
   cytoplasm to cell wall
5. Resultant dec. in pH within cell wall causes cellulose fibres to loosen (by breaking bonds that hold them together)
6. Makes cell wall flexible & capable of stretching when H₂O influx promotes cell turgor.

Question 31

Auxins in Gene Expression

Answer 31

1. Light absorbed by phototropins (proteins) in photoreceptors.
2. When certain wavelength absorbed, their conformation changes.
3. They can bind to receptors within cell, which control transcription of specific genes.
4. Genes involved said to code for PIN3 proteins located in plasma membrane of cells in stem that transport auxin from cell to cell.
5. PIN3 effects for shoots:
   
   1. Position & type of PIN3 protein can vary to transport auxin to where growth needed.
   2. If phototropins in tip detect greater light intensity on q side of stem than other, auxin transported laterally from side with brighter light to more shaded side.
   3. Higher [auxins] on shadier side of stem cause greater growth on this side, so stem grows in curve towards source of brighter light.
   4. Leaves attached to stem will therefore receive more light and be able to photosynthesise at greater rate.
6. PIN3 effects for roots:
   
   • Upward shoot growth & downward root growth occur in response to gravity (geotropisms).
   • If root placed on its side, gravity causes statoliths to accumulate on lower side of cells, which leads to distribution of PIN3 transporter proteins that direct auxin transport to bottom of cells.
   • High [auxin] inhibit root elongation so top cells elongate at higher rate than bottom cells causing root to bed downward.

Question 32

Micropropagation and its uses.

Answer 32

1. Micropropagation: In-vitro process, produces
   many identical plants with desirable features,
   depends on totipotency of plant tissues.
2. Stock plant with wanted feature identified, & its meristem sterilised & cut into pieces (explants).
3. Explant placed in sterilised growth media with equal amounts of auxin + cytokinin that lead to undifferentiated mass (callus) formation.
4. Hormones added to make different media:
   
   • Rooting Media: Growth media contains ratio of auxin to cytokinin of 10:1;
     allows root dev.
   • Shoot Media: Growth media contains ratio of cytokinin to auxin of 10:1;
     allows shoot dev.
5. Once roots & shoots developed, cloned plant can be transferred to soil.

Micropropagation used for rapid bulking up:

1. Micropropagation techniques used to produce virus-free strains of plants (useful for exporting)
2. Viruses transported within plant from cell-cell through vascular tissue & via plasmodesmata.
3. Apical meristem therefore often free of viruses.
4. Micropropagation faster and takes up less space than traditional production methods.
5. Micropropagation is therefore also more cost-effective.
6. Used in preserving plant species like orchids, which are targets for collection in wild.
7. Used for wild replanting and for being a method of commercial production for orchids.
8. Orchids also produce few seeds and the few seeds produced also difficult to germinate.
9. Micropropagated plantlets stored in liquid nitrogen (Cryopreservation).

Question 33

Photoperiods and Flowering

Answer 33

1. Flowering controlled by phytochrome, which is affected by light (photoperiodicity).
2. 2 forms of phytochrome exist:
   
   • P_R absorbs red light of wavelength 660nm and is converted into P_FR.
   • P_FR absorbs far-red light, of wavelength 730nm and is converted into P_R.
3. P_FR = active form of phytochrome & receptors present in cytoplasm bind to P_FR but not P_R.
4. Sunlight contains more red light, so P_R converted rapidly to P_FR, causing P_FR to form predominantly during day, but less stable than P_R, so gradually reverts to P_R form during night
5. In LDP: Large amounts of P_FR remain at end of short night, which promotes transcription of flowering genes when they bind to receptor.
6. In SDP: Small amounts of P_FR remain at end of long night, so inhibition of transcription of flowering genes falls and plant flowers.

Question 34

Flower Forcing

Answer 34

1. Flower Forcing: Procedure designed to get flowers to bloom out of season (e.g. holiday time)
2. Siam tulip is a LDP sold as cut flowers all year, but it only produces flowers during rainy season.
3. Providing additional light in middle of night leads to flowering in off-season provided that enough humidity & nutrients provided.

Question 35

Draw and Label a Flower

Answer 35

Question 36

Mutualistic Relationships between flowers and pollinators

Answer 36

1. Sex reproduction involves pollen from 1 plant’s stamen being transferred to stigma of another plant.
2. Pollen transferred by wind, animals or H₂O.
3. Animal pollinators: bees, bats, birds & butterflies.
4. Mutualism: Close association between 2 organisms where both benefit from relationship.
5. Pollinators gain food (nectar) & plant gains means to transfer pollen to another plant.

Question 37

Fertilisation and Seed Dispersal

Answer 37

1. Fertilisation: Fusion of male gamete nuclei (in pollen grain) with female gamete (in ovule) to form zygote.
2. Each pollen grain on stigma grows a tube (pollen tube) from style to ovary carrying male gametes to fertilise ovary (located in ovule).
3. Fertilised ovule develops into seed and ovary develops into fruit.
4. Seed Dispersal: Moving away of seeds from parental plant (after fertilisation) before germination, reducing competition for resources and helping spread of species.
5. Type of seed dispersal depends structure of fruit:
   
   • Dry and explosive
   • Fleshy and attractive promotes animals to eat them, which then plant seeds in dung.
   • Feathery or winged promotes wind dispersal.
   • Hook catch onto animal coats.

Question 38

Structure of Seed

Answer 38

Question 39

Germination + Requirements

Answer 39

1. Germination: Early growth of seed.
2. Dormancy: Seeds may delay germination after planting to avoid germination at bad time. Time period also allows seed dispersal.
3. Seeds need water: As they’re usually dry & cells need H₂O activate metabolic processes + growth of embryo root and shoot.
4. Seeds need O₂: Cell needs ATP made by aerobic respiration to grow, which needs O₂.
5. Seeds need warmth: As germination involves enzyme-catalysed metabolic reactions that require optimum temps.
6. May also require: Fire, light/dark, freezing, prior animal digestion, or washing (removes inhibitors).
7. Gibberellin Synthesis: Needed for growth
   
   1. ​Gibberellin: Hormone that stimulates mitosis and cell division in embryo.
   2. In starchy seeds, also stimulates amylase production, needed to break down starch in food reserves into maltose.
   3. Starch = insoluble & immobile, so used as store, whereas sucrose and glucose =
      soluble & can be moved from food reserves to where needed.
   4. Embryo root & shoot need sugars for growth, together with AAs & other substances released from food stores.
   5. Glucose also required for aerobic respiration.

Question 40

Reasons why crops may be failing

Answer 40

1. Age of seed.
2. Soil temp too high/low
3. Soil too dry –> dehydration
4. Not enough light/darkness from being sown below soil surface/above soil surface (respect.)
5. Waterlogged and anaerobic soils, so seedlings die of ethanol poisoning.
6. Pests like slugs & snails ate seedlings or seeds.

Question 41

Calvin’s experiment to elucidate carboxylation of RuBP:

Answer 41

1. Melvin Calvin & his team replaced regular 12CO₂ with radioactively labelled 14CO₂.
2. They took algae samples at very short time intervals so that as few C-molecules formed as possible.
3. Photosynthetic products separated by 2-D paper chromatography, which separates molecules based on solubility in diff solvents.
4. Radioactive products identified by autoradiography,
5. Like this, reactants & products of LIR’s seq. in order of which C-molecule was produced 1^st.

Question 42

Modelling Xylem

Answer 42

1. Models allow one factor to be studied independently
2. Capillary tubes model adhesion between water and xylem vessel walls.
3. Porous pot models flow in xylem vessel due to transpiration from the leaf.