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12 Biology > Module 3: Transport In Plants > Flashcards

Module 3: Transport In Plants Flashcards

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1
Q

Xylem function

A

Transport water and dissolved minerals up the plant.

Xylem and phloem are found together in vascular bundles in the plant.

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2
Q

Phloem function

A

Transport sucrose and other assimilates up and down the plant.

Xylem and phloem are found together in vascular bundles in the plant.

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3
Q

Vascular bundles in the root

A

Found in the centre of the root.

Xylem in X shape in the middle.

Phloem in between arms of xylem X.

Around the vascular bundle is a ring of endodermis.

In the endodermis is a ring of meristem cells called pericycle.

Meristem cells are stem cells- undifferentiated cells that can still divide.

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4
Q

Vascular bundle in stem

A

Found near outer edge of stem.

Xylem closer to centre of stem.

Phloem near outside.

In between phloem and xylem is cambium- layer of meristematic cells that can differentiate into xylem and phloem.

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5
Q

Vascular bundle in leaves

A

Form the veins of leaves.

Xylem are above phloem.

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6
Q

Xylem tissue

A

Transport water and dissolved minerals up a plant (from roots to leaves).

Consist of: xylem vessels, fibres, parenchyma cells.

Long cells in column. They become lignified and cells die, contents and end walls decay forming xylem vessels.

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7
Q

Structure of xylem vessels

A

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8
Q

Phloem tissue

A

Transport sucrose and other assimilates up and down the plant.

Consist of sieve tube elements and companion cells.

Sucrose is transported as sap (dissolved in water).

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9
Q

Sieve tube elements

A

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10
Q

Companion cells

A

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11
Q

How does water enter a plant?

A

Through root hair cells via osmosis

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12
Q

How are root hair cells adapted?

A
  • Root hair cells are found on the epidermal layer of plant roots.
  • Hair like projection into the soil provides a large surface area for osmosis and mineral uptake into the roots.
  • Thin wall for a short diffusion path
  • Many mitochondria to provide energy for the active transport of minerals
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13
Q

How and why does water enter the root hair cells?

A

• Minerals are actively transported into the root hair cell from the soil
• This decreases the water potential of the root hair cell (below that of the
soil)
• Water moves into the root hair cell via osmosis across the cell surface membrane, down the water potential gradient

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14
Q

Water pathways

A 
Water needs to cross the cortex and endodermis to reach the xylem. This happens via osmosis between cells.
This is done in 3 ways:
- Apoplast pathway
- Symplast pathway
- Vacuolar pathway
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15
Q

Apoplast pathway

A

• Water travels through the cell walls in gaps
between the cellulose fibres
• Water does not cross membranes and so can
carry dissolved minerals with it

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16
Q

Symplast pathway

A

• Water crosses the cell surface membranes
via osmosis (through aquaporins) and
enters the cytoplasm
• It then can move through plasmodesmata
which links the cytoplasm from one cell to
the next

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17
Q

Vacuolar pathway

A

• Similar to symplast but water also moves through the vacuoles, not just the cytoplasm

18
Q

The casparian strip

A

• On the cell walls of the cells of the endodermis, there is
a strip of waterproof material called suberin known as
the Casparian strip
• The Casparian strip blocks the apoplast pathway
between the cortex and the xylem ‐ water now must
take the symplast pathway
• Minerals must be actively transported from the
cytoplasm into the xylem through carrier proteins in the
cell membranes
• This lowers the water potential in the xylem, so water
now crosses the cell surface membranes from the cortex
into the xylem via osmosis
• Water potential is lowest in the xylem than anywhere
else in the root ‐ this causes the osmosis of water from
the root hair cell from the cortex to the endodermis

19
Q

Summarise how water moves from the soil to the xylem

A

• Minerals actively transported into root hair cell (through carrier proteins)
• Water moves via osmosis from soil into root hair cells across cell surface
membrane (through aquaporins) down the water potential gradient
• Water can move via cell walls in the apoplast pathway
• Water can move via the cytoplasm in the symplast pathway, through plasmodesmata, linking the cytoplasm in neighbouring cells
• At the endodermis, the Casparian strip (made of suberin) blocks the apoplast pathway
• This makes the water enter the symplast pathway
• Water potential is most negative in the xylem due to the active transport of minerals into it
• This causes water to move into the xylem from the cells of the endodermis and cortex

20
Q

How does water move up the stem into the leaves?

A

Water moves up the xylem vessels from the root.

This is helped in 3 ways:
‐ root pressure ‐ the push from the water entering the xylem vessels in the roots (doesn’t move water far)
‐ capillary action ‐ adhesion (forces of attraction) of water molecules to lignin in narrow xylem vessels can pull the water up the sides of the vessel
‐ transpiration pull ‐ most of the driving force

21
Q

What is transpiration?

A

Transpiration is the loss of water by evaporation out of a plant’s leaves via the stomata.

22
Q

How does transpiration result in the movement of water up a stem?

A

1) Water evaporates from the surface of the mesophyll cells in the leaf and forms water vapour
2) Water vapour diffuses from a high water potential to a lower water potential out of the leaf, through the stomata
3) More water is drawn from the mesophyll cells via the symplast/apoplast pathways in the leaf replacing the water that has evaporated
4) This occurs via osmosis down the water potential gradient
5) This water is replaced by water from the xylem vessels (moving out via osmosis)
6) The loss of water from the xylem causes a low hydrostatic pressure at the top of the xylem
7) Water moves from a higher pressure (roots) to a lower pressure (down the pressure gradient) under tension
8) Water is therefore pulled up the xylem by mass flow
9) The cohesion of water molecules due to the hydrogen bonds between them causes them to stay as a long unbroken column of water during this process ‐ the transpiration stream

23
Q

Factors affecting transpiration

A

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24
Q

What is a potometer?

A

A piece of equipment that measures water uptake in a cut shoot.

25
Q

How do you set up a potometer?

A
  1. Cut a healthy shoot under water (to stop air entering xylem vessels)
  2. Cut shoot at a slant (to increase surface area)
  3. Check potometer is air bubble free
  4. Insert shoot into potometer under water
  5. Remove potometer from water and ensure, airtight joints around shoot dry leaves
  6. Keep, conditions constant
  7. Allow time for shoot to acclimatise
  8. Shut screw clip
  9. Keep ruler fixed and record position of air bubble on scale
  10. Start timing and measure distance moved per minute
26
Q

Potometer: Why is it not an exact measure of transpiration?

A
  • Transpiration is the loss of water by evaporation from leaves
  • A potometer measures water uptake to replace loss
  • Some water may be used e.g. in photosynthesis
  • Rather than all evaporating from the leaves
  • Also uptake by detached shoot may not be same as that of the whole plant
27
Q

What is a xerophyte?

A

A plant that is adapted to reduce water loss by transpiration so that it can survive in very dry (arid) conditions.

28
Q

How are xerophytes adapted to reduce water loss by transpiration?

A

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29
Q

Water loss via transpiration is unavoidable.

Why?

A
  • Gas exchange of carbon dioxide (in for photsynthesis) AND oxygen (out ‐ by product) occur through the open stomata
  • Stomata open during the day when it is lightest and hottest
  • More stomata open means a larger area for water vapour to diffuse through
30
Q

What is a hydrophyte?

A

A plant that is adapted to living in water or where the ground is very wet.

31
Q

How is a water lily leaf stem adapted to living in water?

A
  • Many large air spaces in the leaf. This keeps the leaves afloat so that they are in the air and can absorb sunlight.
  • The stomata are on the upper epidermis, so that they are exposed to the air to allow gaseous exchange.
  • The leaf stem has many large air spaces. This helps with buoyancy, but also allows oxygen to diffuse quickly to the roots for aerobic respiration.
32
Q

What is translocation?

A

The transport of assimilates between the sources and sinks of a plant in the phloem tissue. This requires energy.

33
Q

What is a source?

A

Leaves when photosynthesising make glucose which is converted to sucrose and then loaded into the phloem.

In the winter when little photosynthesis can take place, starch (or other carbohydrates) that has been stored e.g. in the roots is converted into sucrose and loaded into the phloem.

34
Q

What is a sink?

A

Anything that stores sucrose as starch (carbohydrates) e.g. roots.

Anything that uses the starch (carbohydrates) in respiration or growth.

35
Q

Describe the mechanism of transport in phloem involving active loading at the source and removal at the sink

A

• H+ ions are actively transported (requires ATP) out of the companion cells
• This produces a diffusion gradient for the H + ions
• They move back into the companion cell via facilitated diffusion through
co‐transporter carrier proteins along with sucrose
• Sucrose has been actively loaded into the companion cell
• There is a high concentration of sucrose in the companion cell compared to
the sieve tube element, so it diffuses into it down the concentration gradient through the plasmodesmata

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36
Q

How does sucrose move along the phloem

A

At the source:
• Sucrose is actively loaded into the sieve tube elements at the source.
• This reduces the water potential in the sieve tube element.
• Water enters the sieve tube elements by osmosis.
• This increases the hydrostatic pressure in the sieve tube element near the source.

At the sink:
• Sucrose is unloaded at the sink by diffusion (or active transport) and used in respiration/stored.
• This increases the water potential in the sieve tube element.
• Water moves into the sink via osmosis down the water potential gradient.
• This reduces the hydrostatic pressure in the sieve tube element near the sink.
• Water in the sieve tube element at the source moves down the hydrostatic gradient from source to sink.
• This creates a flow which carries the sucrose and other assimilates along the phloem via mass flow either up or down the plant.

37
Q

How are the sieve tube elements adapted to allow mass flow to occur?

A
  • elongated elements, joined end to end to form a column
  • sieve plates with pores in end walls allow sucrose through
  • little cytoplasm and no nucleus ‐ less resistance to transport

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38
Q

If a ring is cut around the bark of a tree, a swelling can occur above the ring. Why?

A

• phloem is in the bark and so sucrose cannot pass the cut area
• above cut acts as a sink so water moves into cells
• damage triggers increased cell
division to produce cells to store sugars
• cut causes infection

39
Q

Explain evidence for this mechanism of translocation

A
  • pH of companion cells is higher than surrounding cells (H+ ions reduce pH)
  • Concentration of sucrose is higher in source than sink

How we know phloem used:
• Radioactively labelled CO2 supplied for photosynthesis appears in phloem
• Aphids feeding on plant stems insert mouthparts
into phloem
• Sugars collect above ring when tree is ringed to
remove phloem

It needs ATP:
• Companion cells have many mitochondria
• Translocation is stopped if a poison which stops ATP
production is given
• Flow of sugars is very high that ATP must be used ‐
much faster than would be possible with diffusion

40
Q

Evidence against this mechanism of translocation

A
  • not all solutes in phloem move at same rate
  • sucrose moved to all parts of plant at same rate and doesn’t go to places with lowest concentration faster
  • (role of sieve plates is unclear)
41
Q

How are companion cells adapted?

A
  • Many mitochondria to produce ATP for active loading

12 Biology (12 decks)

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