TRANSPORT IN PLANTS

Explain how the properties of water are related to the transport role of water in a stem.

• cohesion holds water molecules tgt

• allows chain of molecules to be pulled up xylem

• adhesion allows water mol to stick to xylem vessel walls

• by capillary action

• solvent to dissolve sucrose/ transport in phloem

what do transport systems in plants transport

transport , water / ions , from roots to , cells / other parts of the plant ✓ 5 (to) transport , sucrose / products of photosynthesis / (named) assimilates , from , source to sink ✓ 6 (to) transport (named) hormones produced in one part to site of action

Sodium chloride in solution dissociates into Na+ and Cl−. Explain how the Casparian strip prevents these ions from reaching the xylem of the plant by the apoplast pathway.

• Casparian strip is impervious to water

• forces water/ solution to go through cell membrane

• phospholipid bilayer repels ions

Give similarity between xylem & phloem structure.

• xylem vessels & phloem sieve tube elements lack nuclei/ contents

• both made up of cells joined end to end

State differences between xylem & phloem structure.

Phloem has, unlike xylem:

• cytoplasm

• some organelles

• sieve tube elements

• companion cells

• smaller lumen

• sieve plates

Xylem has, unlike phloem:

• wider lumen

• lignified cell walls

• pits

• vessels

State the function of the pits in xylem tissue.

lateral movement of water

1) precaution for potometer and explain

2) give rest of method

1)

• assemble/ cut shoot under water→ prevent air entering

  → prevent airlock forming

  → prevent air entering & blocking xylem

• check apparatus is airtight & watertight

• keep abiotic factor (e.g. humidity) constant→ since wld affect transpiration rate

• keep screw clip closed→ prevent water entering while measuring

• open end of stem in water→ prevent new air bubbles entering

DURING EXPERIMENT

• keep shoot still→ measure distance accurately

• use syringe to move air bubble→ prevent entering xylem

2)

• dry leaves

• measure distance air bubble travels per (named) time interval

• calculate volume of water uptake

• ref. maintaining (named) constant conditions

Given 2 leaves w/ different surface areas. Describe how the student could ensure that a valid comparison could be made between the two leafy shoots.

• find/ account for leaf area

• find transpiration rate per unit area

reasons for anomalously high distance moved by air bubble.

• misread distance

• air movement/ light intensity/ temperature increased or humidity decreased

• timed for longer than given interval

• bubble not returned to starting position

terrarium can be sealed and the plants will be able to grow for months or even years despite not being in contact with the outside atmosphere. Suggest one reason why the plants in a sealed terrarium continue to grow

• respiration produces CO2 and H2O which is used in photosynthesis

• photosynthesis produces O2 which is used in respiration

• decomposition replaces nutrients

Explain how glucose produced in photosynthesis is translocated to parts of the plant where glucose is metabolised or stored.

PHLOEM LOADING

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• glucose converted into assimilate/ sucrose in photosynthesising cells

• apoplast route

• active process

• protein pump in companion cell membrane

• H+ ion concentration gradient

• co-transport of H+ and sucrose into companion cell

  • active transport of sucrose

• structural adaptations of companion cell e.g.

  • many mitochondria (to provide ATP for active transport)

  • large surface area of cell membrane

• passive loading into sieve tube elements via symplast route

  • diffusion through plasmodesmata

• sucrose being actively loaded into sieve tube element decreases its water potential

• → water enters phloem by osmosis from xylem/ surrounding cells

• → increasing hydrostatic pressure at source end

MASS FLOW

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• hydrostatic pressure gradient from source to sink

• → transport of sucrose from source to sink

PHLOEM UNLOADING

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• diffusion of sucrose from phloem to surrounding cells

  • wherever cells need sucrose/ glucose

• → increases water potential of phloem

• → water leaves phloem into xylem by osmosis

• → decreasing hydrostatic pressure

• sucrose → glucose → respiration

• glucose→ starch for storage

• concentration gradient maintained between phloem & cells at sink end

Suggest why it is beneficial to the plant for the carbohydrate to be transferred throughout the plant in the form of sucrose rather than as an alternative carbohydrate.

• sucrose is more soluble so can be transported in sap

  • DO NOT MENTION IF Q SPECIFIES WHY NOT GLUCOSE

• sucrose is metabolically inactive so doesn’t get used (e.g. in respiration) during transport

• loading / unloading controlled/ uses transport proteins

• less likely to exit (sieve tubes)

How is transport in the phloem similar to transport in the xylem?

• both carry mineral salts

• solutes carried in solution

• both use mass flow/ generated hydrostatic pressure

How is transport in the phloem different from transport in the xylem?

• phloem carries carbohydrates

• transport in phloem can take place in both directions whereas transport in xylem can only take place up plant

• phloem uses living cells

• xylem uses capillary action/ cohesion & adhesion

Explain, with a suitable example, how some parts of the plant can act as both a ‘source’ and a ‘sink’.

• some parts of plant can store carbohydrates and release when needed

• e.g. roots and leaves can be sources and sinks at different times of the year