9.2 Transport in phloem

Question 1

Define translocation

Answer 1

TRANSLOCATION: movement of organic compounds (e.g. sugars, amino acids) from sources to sinks

Question 2

Sources vs sinks

Answer 2

SOURCE: where the organic compounds are synthesised – photosynthetic tissues (leaves)

SINK: where compounds are delivered for use/storage (roots, fruits and seeds)

Question 3

Explain phloem transport

Answer 3

Nutrient rich fluid - plant sap (hypertonic) transports organic compounds in phloem from sources to sinks via mass flow

Sources: org. comp via active transport (against conc gradient) through companion cells into phloem

Water enters phloem from xylem in osmosis - increases hydrostatic pressure - creates mass flow which moves org comp to sinks

Sinks: org comp taken up by companion cells - into sinks (used in metabolism/stored) - plant sap becomes hypotonic - water drawn back to xylem - hydrostatic pressure at sinks is lower

Question 4

Phloem anatomy and functions

Answer 4

• sieve element cell: compose the sieve tube, thick/rigid walls to resist pressure,
• sieve plates: porous - enable flow, separate different sieve elements
• companion cell: metabolic support for sieve elements (many mitoch), load/unload org comp from sources to sinks via active transport (transport proteins)

Question 5

Xylem and phloem in roots

Answer 5

Monocotyledons:

Xylem - more internally

Phloem vessels - more externally

Dicotyledons:

Xylem - centrally cross-like shape (‘X’ for xylem)

Phloem surrounding xylem

Question 6

Xylem and phloem in stems

Answer 6

Monocotyledons:

Phloem - positioned externally (towards outside of stem) – remember: phloem = outside

Dicotyledons:

Phloem and xylem vessels will be separated by the cambium (xylem on inside ; phloem on outside)

Question 7

Companion cell transport

Answer 7

Active transport (ATP required):

Apoplastic path: through cell wall - pumped in membrane proteins - H+ gradient created

Symplastic path: through cytoplasm - pass through plasmodesmata

Question 8

Factors of translocation rate

Answer 8

• rate of photosynthesis (affected by light intensity, CO2 concentration, temperature)
• rate of cellular respiration
• rate of transpiration (how much water enters the phloem)
• diameter of the sieve tubes (affect the hydrostatic pressure - differ between plant species)

Question 9

Explain aphids

Answer 9

Aphids - group of insects which feed primarily on sap extracted from phloem

Have mouthpiece - stylet - pierces the plant’s sieve tube to allow sap to be extracted

Penetration of the stylet aided by digestive enzymes - soften the intervening tissue layers

When stylet removed, sap continues to flow - hydrostatic pressure within the sieve tube

Question 10

How to measure translocation experimentally

Answer 10

Aphids can be used:

• collect sap at various sites along a plant’s length and thus provide a measure of phloem transport rates
• plant is grown in C14 carbon dioxide
• leaves convertinto radioactively-labelled sugars - transported by the phloem
• aphids positioned along the plant’s length - feed on the phloem sap
• aphid stylet taken out - sap continues to flow from the plant at the selected positions
• sap analysed for the presence of radioactively-labelled sugars
• translocation rate calculated based on the time taken for the radioisotope to be detected at different positions

Question 11

Xylem vs phloem

Answer 11