carbon assimilation and phloem

Question 1

carbon cycle

Answer 1

-most carbon released by plants is released back into the atmosphere via respiration (short term, by plants and animals consuming plants)
- only small percentage trapped, entering longer term geological cycle
- formation of sedimentary rock, fossil fuels
- carbon is trapped for a long time but not permanently (erosion, burning of fossil fuels)

Question 2

main 2 types of leaves within angiosperms

Answer 2

• monocot leaves, parallel veins, grasses and cereals e.g. maize
• dicot leaves, netted veins, more complex vasculature e.g. sunflower

Question 3

cross section of dicot leaf

Answer 3

• epidermis
• palisade mesophyll
• spongy mesophyll
• lower epidermis (thinner)

Question 4

epidermis

Answer 4

• lack chloroplasts
• very large cells
• cells have a convex surface, focuses light
• amplifies light so mesophyll receives 2-3x more light than leaf surface

Question 5

palisade mesophyll

Answer 5

• columnar cells with multiple chloroplasts
• columnar cells channel light so it passes through multiple chloroplasts/ chlorophyll molecules

Question 6

spongy mesophyll

Answer 6

• large air spaces scatter light back into columnar cells
• increases time each photon stays in leaf
• photon density is higher inside leaf than out

Question 7

stomata

Answer 7

• CO2 in
• O2 and H2O out
• thickened guard cell wall controls whether stomatal pore opens or closes
• closed when flaccid
• open when turgid
• subsidiary cell pumps ions in and out of guard cell (active transport), controls water content of guard cell through osmosis and therefore whether stomatal pore is open or not, controlled by hormones
• amount of stomata depends on environment
• stomata open in daytime when there is enough water
• stomata close in dark or drought conditions

Question 8

movement of fixed carbon

Answer 8

• exported from mesophyll into vasculature
• diffusion from high to low concentration
• apoplastic route, through channels in cell walls
• symplastic route through plasmodesmata between cells (smooth ER)

Question 9

transport from source to sink

Answer 9

• direction is different depending on life stage
• source = photosynthate exporting organ (leaf) or storage organ (root)
• sink = photosynthate-importing organ (growth, leaves, seeds, storage, roots)

Question 10

phloem sap

Answer 10

• mainly sugars (carbohydrate)
• amino acids
• organic acids
• proteins
• macro and micronutrients
• transports hormones/ signalling molecules

Question 11

phloem structure

Answer 11

• single phloem cells = sieve tube elements
• connected by sieve plates
• lack nucleus and mitochondria
• each has companion cell connected by branched plasmodesmata , provides sieve tube element with proteins and energy for transport

Question 12

stem structure

Answer 12

• epidermal layer
• cortex, mechanical strength, storage, some photosynthesis
• lignified fibres
• phloem
• cambium layer, meristem layer, source of future divisions (tree rings), regulates position of phloem
• xylem
• pith, involved in storage

Question 13

sugars transported by phloem

Answer 13

• tend to be non-reducing complex sugars
• mainly sucrose (disaccharide)
• sometimes sucrose bound to galactose(s) and non-reducing sugar alcohols
• reducing sugars (e.g. glucose, fructose, galactose) too reactive (can be oxidised)

Question 14

transport of sugars into phloem, pressure-flow model

Answer 14

• sucrose accumulated against concentration gradient
• sugars actively loaded
• H+ actively pumped out of phloem into cell wall space
• creates concentration gradient, sucrose driven across plasma membranes by cotransport with H+ ions through specialised pores
• water follows by osmosis, from source cells, companion cells, xylem, increasing pressure
• pressure forces sucrose down into sink tissue (pressure flow, faster flow rate than sucrose diffusion)
• unloaded at sink, water follows through osmosis

Question 15

phloem signal molecules, FT

Answer 15

• FT produced in leaves
• triggered by circadian rhythms in Rhabydopsis (day-length sensor in leaves), some plants need period of cold to trigger it
• moves up through phloem to shoot apical meristem (forming leaves through mitosis)
• conversion into inflorescent meristem, triggers transcription factors that intitiate flowering

Question 16

phloem-mediated signals, venus flytrap

Answer 16

• carniverous
• rapid electrical signalling to trigger closure of modified leaves when prey touches sensory hairs (more than one)
• action potential generated through ion fluxes
• action potential travels through phloem down to specialised motor cells at hinge
• triggers change in turgor pressure through flow of ions, modified leaves close rapidly

Question 17

photosynthesis

Answer 17

• CO2 + 6H2O = C6H12O6 + 6O2
• light-dependent stage (ATP, NADPH)
• calvin cycle (G3P simple sugar, Rubisco)

Question 18

rubisco

Answer 18

• calvin cycle
• CO2 fixation enzyme
• most abundant protein on earth
• evolved in prokaryotes ~2.45bya