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A Level Biology > Translocation > Flashcards

Translocation Flashcards

1
Q

Where does translocation take place ?

A

In phloem vessels

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2
Q

What is translocation ?

A
  • Is the movement of assimilates within phloem sieve tubes ( eg. Sucrose/ amino acids , hormones etc. ) from where they are made ( source ) to where they are required ( sink )
  • it is an active process
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3
Q

What direction is the movement from source to sink ?

A

Bidirectional

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4
Q

What is phloem Sap ?

A

Liquid being transported

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5
Q

What is glucose transported as in the phloem Sap ?

A

Sucrose

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6
Q

What is a source ?

A

This is the site where sucrose/ assimilates are made and loaded into the phloem ( high concentration)

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7
Q

What are some examples of sources ?

A
  • green leaves
  • green stems
  • storage organs eg. Tubers and tap roots
  • food stores in seeds
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8
Q

What is a sink ?

A

The site where sucrose / assimilates are unloaded from the phloem for use or storage

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9
Q

What are some examples of sinks ?

A
  • meristems ( apical or lateral ) that are actively dividing
  • roots that are growing and / or actively absorbing mineral ions
  • any party of the plant where the assimilates are being stored ( eg. Developing seeds, fruits or storage organs )
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10
Q

How is sugar transported around a plant ?

A
  • a sink removes sugar from phloem increasing the water potential
  • water leaves the sieve tubes by osmosis keeping the hydrostatic pressure low
  • so that Sap continuously flows from source to sink
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11
Q

What are the 3 stages to the movement of sucrose and assimilates from source to sink ?

A
  • active loading = at the source into the phloem sieve tube
  • mass flow = of sucrose through the sieve tub and elements ( involves water from xylem )
  • Active unloading = of source at the sink
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12
Q

What is active phloem loading ?

A
  • organic compounds eg. Sucrose produced at the source are actively loaded into the phloem sieve tubes by companion cells
  • loading of sucrose in the phloem sieve tubes requires ATP and co-transporter proteins
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13
Q

Explain what happens in active loading at the source ?

A
  1. H+ ions are actively pumped out of the cytoplasm of companion cells via a proton pumps into their cell walls ( involves the hydrolysis of ATP - active process )
  2. This increases the H+ ion concentration in the cell walls of the companion cells compared to the inside —-> creating a concentration gradient
  3. H+ ions re-enter the cytoplasm of the companion cell down their conc gradient via a co- transporter protein
  4. While transporting the H+ ions this co- transporter protein also carries sucrose molecules at a different binding site into the companion cell against the conc gradient for sucrose by facilitated diffusion
  5. The sucrose molecules them diffuse into the phloem sieve tubes via the plasmodesmata from the companion cells
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14
Q

What are some adaptations for active loading ?

A
  • companion cells have infoldings in their cell surface membrane to increase the available surface area for the active transport of solutes
  • many mitochondria to provide energy for the proton pump
  • this mechanism permits some plants to build up the sucrose in the phloem to up to three times the concentration of that In the mesophyll
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15
Q

What is transported generally by mass flow and why are they transported ?

A
  1. Phloem sap moves by mass flow up and down the plant
  2. Carbohydrates are generally transported in plants in the form of sucrose because:
  • it allows for efficient energy transfer and increased energy storage ( sucrose is a disaccharide and thus contains more energy )
  • it it less reactive than glucose as it is a non reducing sugar and thus no intermediate reactions occur as it is being transported
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16
Q

What happens in stage 2 - the mass flow of sucrose through phloem sieve tube ?

A
  1. Sugars/ sucrose/ assimilates enter the sieve tube element at the source = this lowers the water potential in the sieve tube
  2. Water enters the sieve tube by osmosis from the xylem
  3. This raises the hydrostatic pressure at the source
  4. When assimilates leave the siege tube at the sink this increases the water potential inside the sieve tube
  5. Water leaves the sieve tube by osmosis down a water potential gradient and lowers the hydrostatic pressure at the sink
  6. Water moves down the hydrostatic pressure gradient from high to low towards the sink also moving sucrose and other assimilates along the phloem
17
Q

Describe what happens in stage 3 - active unloading of sucrose at the sink

A
  1. Sucrose needs to be actively unloaded into the sink where it is needed
  2. Sucrose is actively transported out of the companion cells and then move out of the phloem sieve tubes into the sinks via the apoplastic or symplastic pathways
  3. In the sink sucrose is converted into other molecules eg. Starch this helps to maintain s conc gradient
  4. When sucrose diffuses out of the sieve tubes = this increases the water potential of the tube
  5. Water thus moves out of the sieve tube back into the xylem vessels by osmosis
  6. This creates a low hydrostatic pressure at the sink compared to the higher hydrostatic pressure at the source
18
Q

What are some uses of glucose ?

A
  • raw material for growth,repair and replacement of damaged parts
  • to make fats & oils ( energy stored in seeds )
  • energy stored as sucrose in fruit
  • energy stored as starch in leaves,seeds, roots and tubers
  • used immediately to release energy in respiration
  • to make cellulose the main structural material in cell walls
  • energy used to turn sugars, nitrates and other nutrients into amino acids which build up proteins
19
Q

State the evidence for mass phloem

A
  • collecting and studying the sap from plants with clotting sap eg. Castor oil plants
  • using aphids to collect the sap - after the aphid inserts it’s stylet ( tubular mouthpart ) scientists remove the aphids head and collect the sap that continues to flow
  • using radioactively labelled metabolites eg. Carbon -14 labelled sugars which can be traced during translocation
  • advances in microscope enabling the adaptations of companion cells to be seen
  • observations about the importance of mitochondria to the process of translocation
20
Q

Describe how aphids can be used to demonstrate translocation including the effect light intensity on the process

A
  • aphids pierce plant tissues and push their stylet directly into phloem vessels
  • if the aphid is removed leaving the stylet in place pressure in the phloem vessel continues to force sap out of the stylet
  • this can be to show the presence and concentration of assimilates in the phloem and pressure in phloem
  • and how these things can change with manipulation of other factors such as light intensity

A Level Biology (54 decks)

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