Phloem, Translocation, Mass Flow Hypothesis Flashcards
what is the function of the phloem?
transports organic substances (mainly sucrose) in plants
What are sieve tube elements?
Living cells that make up the phloem. They have perforated end walls called sieve plates to allow continuous flow of organic substances. They have no nucleus, very little cytoplasm and few organelles
what are companion cells?
Living cells that carry out the living functions for the sieve tube elements (one for each sieve tube element). e.g. provide ATP for active transport of solutes.
What is translocation?
the movement of organic substances (e.g. sucrose and amino acids) in the plant from source to sink. It is an energy requiring process. It can move in both directions up and down the phloem.
what is a source?
produces carbohydrates like sucrose during photosynthesis e.g. leaves
what is a sink?
areas which need energy such as growing regions (roots) or storage organs (potatoes)
What is mass flow?
movement down a pressure gradient
explain the process of mass flow hypothesis
-at the source (e.g. leaves), sucrose is actively transported by companion cells into the sieve tube elements in the phloem
-this lowers the water potential inside the phloem, causing water to move from the xylem into the phloem by osmosis.
-this creates high hydrostatic pressure at the source compared to the sink which causes mass flow towards the sink
-at the sink, sucrose is used in respiration or stored as starch creating a concentration gradient between the sink cells and phloem, so sucrose is removed from the phloem into sink cells (facilitated diffusion)
-the removal of sucrose, increases the water potential in the phloem so water moves back into the xylem via osmosis. this creates a lower hydrostatic pressure at the sink forming a pressure gradient.
suggest what would happen to the rate of translocation if the plant was treated with heat?
heat denatures respiratory enzymes so no ATP produced so no active transport. Also, heat denatures carrier proteins so no active transport so no movement of sucrose in the phloem
EVIDENCE FOR MASS FLOW: If a ring of bark (including phloem but not xylem) is removed from a woody stem, then a bulge forms above it. The fluid from the bulge has a higher concentration of sugars than the fluid below the ring. What does this show?
-shows that movement of sucrose can occur downwards as sugars can’t move past the area where the bark has been removed as there is no phloem
EVIDENCE FOR MASS FLOW: an aphid’s mouth can pierce the phloem, allowing it to feed. Scientists can snap off the aphid’s body leaving their mouth parts in the plant. If this is done near the leaves, the phloem sap flows out quicker than if it was further down the stem. What does this show?
It shows that there is a pressure gradient ( higher pressure at leaves/ source and lower pressure at sink) due to movement of water into and out of the phloem by osmosis.
EVIDENCE FOR MASS FLOW: Carbon-14 can be used as a radioactive tracer. Radioactive CO2 can be supplied to a plant which is used in photosynthesis to form sucrose, which will then be moved around the plant in translocation. The radioactivity can be traced using autoradiography , where the plant is placed onto photographic film. radioactivity is observed more at the leaves at first, then further away from the leaves over time. what does this show?
-The leaves photosynthesise, forming radioactive sucrose at the source
-The radioactive sucrose is then actively transported into the phloem and translocated towards the sink via mass flow.
Why does the addition of metabolic inhibitors into the phloem stop translocation?
-metabolic inhibitors stop respiration and ATP production
-so active transport of sucrose out of the companion cells into phloem stops.
-translocation/mass flow stops as no pressure gradient can be formed.
The rate of translocation is highest at midday. Why? (worth about 5 marks)
-At midday, light intensity and temperature is at its highest, so there will be the highest rate of photosynthesis.
-This means more glucose is produced so more glucose is converted into sucrose.
-This means more sucrose is actively transported by companion cells into sieve tube elements in the phloem, so more water enters the phloem by osmosis due to lowered water potential
-This creates high hydrostatic pressure at the source compared to the sink, increasing the pressure gradient down the phloem
-More sucrose moves by mass flow down the phloem.
