Transport Of Organic substances in the phloem Flashcards
What is translocation?
-the process by which organic molecules and some mineral ions are transported from one part of the plant to another
How are biological molecules transported in flowering plants?
-phloem
What is the structure of the phloem?
- it’s made up of sieve tube elements
- long thin structures arranged end to end
- their end walls are perforated to form sieve plates which are called companion cells
What is transported through the phloem?
- organic molecules like sucrose and amino acids
- inorganic ions like potassium chloride phosphate and magnesium ions
What are sources?
-sites that produce sugars during photosynthesis
What are sinks?
- sugars produced at sources are transported to sinks where they are used directly or stored for future use
- they can be found anywhere in a plant
Why can translocation in the phloem occur in any direction up or down?
-the sinks can be listed above or below the source
How is translocation proved?
-the mass flow theory
What are the phases of mass flow theory?
- transfer of sucrose into sieve elements from photosynthesising tissue
- mass flow of sucrose through sieve tube elements
- transfer of sucrose from the sieve tube elements into storage or other sink cells
How is sucrose transferred into sieve elements from photosynthesising tissue?
- sucrose is manufactured from the products of photosynthesis in cells with chloroplasts
- the sucrose diffuses down a concentration gradient by facilitated diffusion from the photosynthesising cells into companion cells
- hydrogen ions are actively transported from companion cells into the spaces within cell walls using ATP
- these hydrogen ions then diffuse down a concentration gradient through carrier proteins into the sieve tube elements
- sucrose molecules are transported along with the hydrogen ions in a process known as co transport
- the protein carriers are known as co transport proteins
How does the mass flow of sucrose through sieve tube elements occur?
- the sucrose produced by photosynthesising cells is actively transported into the sieve tubes
- this causes the sieve tubes to have a lower water potential
- the xylem has a much higher water potential so water moves from the xylem into the sieve tubes by osmosis creating a high hydrostatic pressure within them
- at the respiring cells sucrose is either used up during respiration or converted to starch for storage
- these cells therefore have low sucrose content and so sucrose is actively transported into them from the sieve tubes lowering their water potential
- due to a lowered water potential water also moves into these respiring cells from the sieve tubes by osmosis
- the hydrostatic pressure of the sieve tubes in this region are lowered because of water entering the sieve tube elements at the source and leaving at the sink so there is a high hydrostatic pressure at the source and low at the sink meaning there’s mass flow of sucrose solution down this hydrostatic gradient in the sieve tubes
What is mass flow?
- the bulk movement of a substance through a given channel or area in a specified time
- it’s a passive process that occurs as a result of the active transport of sugars so the process as a whole is active and can be affected by temperature and metabolic poisons
Which evidence supports the mass flow hypothesis?
- there is a pressure within the sieve tubes as shown by the sap being released when they are cut
- the concentration of sucrose is higher in leaves than in roots
- the downward flow in the phloem occurs in daylight but ceases when leaves are shaded or at night
- increases in sucrose levels in the leaf are followed by similar increases in sucrose levels in the phloem later
- metabolic poisons and lack of oxygen inhibit translocation of sucrose in the phloem a little later
- companion cells possess many mitochondria and readily produce ATP
Which evidence questions the mass flow hypothesis?
- the function of the sieve plants is unclear as they would seem to hinder mass flow
- not all solutes move at the same speed as they would is movement occurred by mass flow
- sucrose is delivered at more or less the same rate to all regions rather than going more quickly to the ones with the lowest sucrose concentration which the mass flow theory would suggest
How is sucrose transferred from the sieve tube elements into storage and other sink cells?
-sucrose is actively transported by companion cells out of the sieve tubes and into the sink cells
