Plant systems - Phloem Flashcards
1
Q
Phloem
A
Living tissue made up of several cell types including sieve tube elements and companion cells
2
Q
Sieve tube elements
A
- Cells without nuclei and other organelles that join end to end to form sieve tubes
- Their end walls are perforated with pores and are known as sieve plates
- Strands of cytoplasm containing phloem proteins extend from one sieve tube to the next through the sieve plates
3
Q
Companion cell
A
- Very metabolically active
- Large nucleus and dense cytoplasm with high numbers of RER and mitochondria
- Connected to neighbouring sieve tube elements via plasmodesmata
- ->Allowing exchange of materials to take place
4
Q
Translocation
A
The active movement of the soluble products of photosynthesis, like sucrose and amino acids, through the phloem from sources to sinks
5
Q
Source
A
Any region of a plant where sucrose is formed
- Usually leaves
- In deciduous plants it is the roots during winter as leaves fall off and stored starch in roots is hydrolysed to sucrose
6
Q
Sink
A
Any region of the plant where sucrose is stored as starch
- Growing regions of plants that need a source of sucrose
- Roots, flowers and buds etc
7
Q
Ringing experiment
A
- A ring of bark is removed from just below a section of a tree trunk or woody stem
- Phloem lies just beneath so it is removed in that section too
- After time the tissue above the ring swells, and below the ring shrinks/withers due to lack of nutrients
- Liquid in swollen tissue shown to be amino acids and sucrose
- Suggesting that removal of the phloem interrupts the downward movement of these substances
- Proves translocation of sucrose and amino acids occurs in the phloem
8
Q
Aphids experiment
A
It is difficult to extract sap and to measure the speed of translocation as when the phloem is cut phloem proteins clog pores in sieve plates to prevent loss of sap.
- Aphids feed on sap within phloem sieve tubes using a very narrow stylet mouthpiece
- The stylet cannot be blocked by phloem proteins, so if cut off from Aphid sap flows out freely
- The experiment in conjunction with radioactive source of CO2 have shown the rate of translocation is much faster than rate of diffusion alone
- Suggesting an additional mechanism is involved in the process of translocation
9
Q
Radioactive CO2
A
- If plants photosynthesise in the presence of radioactive CO2 its sucrose will incorporate it when it is formed
- Allows transport and location of sucrose to be visualised
- Transverse section (vertical) cut through the stem and placed on photographic film
- Positions of exposure and therefore radioactivity coincide with position of the phloem
- Indicating it is the phloem that translocates the sucrose made by photosynthesis
10
Q
Theories of translocation
A
- The Mass flow hypothesis
- Active processes
- Cytoplasmic streaming
11
Q
Mass flow hypothesis / Translocation as a passive process
A
Explains how sucrose synthesised at the source (e.g. leaves) is translocated via the phloem to the sink (e.g. roots)
LOADING SUCROSE AT SOURCE:
- Excess glucose synthesised during photosynthesis is converted to the disaccharide sucrose in mesophyll cells
- The sucrose enters companion cells via the plasmodesmata
- Sucrose is then loaded into the sieve tube elements by facilitated diffusion via plasmodesmata
- Water potential in sieve tubes is lowered
- Water from adjacent xylem vessels is drawn in by osmosis, increasing the hydrostatic pressure
- Water moves down a hydrostatic pressure gradient to sink
UNLOADING SUCROSE AT SINK:
- Sucrose moves down a concentration gradient into tissues of the sink
- This increases water potential in sieve tubes so water moves into xylem by osmosis
- This lowers the hydrostatic pressure in sieve tubes near the sink, creating a hydrostatic pressure gradient
- Sucrose is hydrolysed by enzymes inside sink cell into glucose and fructose to maintain a concentration gradient between sieve tube and sink
12
Q
The mass flow model
A
X represents source
Y represents sink
T represents phloem
S represents xylem
- X contains concentrated sucrose solution surrounded by partially permeable membrane
- Y contains dilute sucrose solution surrounded by partially permeable membrane
- Tendency for loss of water from both X and Y into S by osmosis. But tendency is greater for X
- Water moves down a water potential gradient into X, increasing hydrostatic pressure in X
- The solution is forced out of X and into Y via tube T
- Hydrostatic pressure in Y will increase, which forces water out into S, down a water potential gradient
13
Q
Limitations of mass flow hypothesis
A
- Rate of translocation is 10,000 times faster than if moving just by diffusion, suggesting translocation is an active process and not a passive one
- Companion cells are very biochemically active, but mass flow does not suggest a role for them
- Phloem have high oxygen consumption, suggesting translocation is an active process
- Does not take into account the presence of the sieve plates in the sieve tubes
14
Q
Translocation as an active process
A
- H+ ions are actively transported out of companion cells into the mesophyll cells
- Creates excess H+ ions in the mesophyll
- Hydrogen ions move back into the companion cells down their concentration gradient via co-transporter proteins
- Sucrose is carried at the same time into companion cells
- Sucrose then loaded into into the sieve tubes down a concentration gradient via apoplast or symplast pathway
15
Q
Cytoplasmic streaming theory
A
- Strands of cytoplasm containing tiny microtubules exist between phloem sieve tube elements, running through sieve pores in sieve plates
- The theory suggests solutes also move within this stream
- This accounts for movement of solutes in both upwards and downwards directions
