3.3.4.2 Mass Transport in Plants

evidence for and against mass flow

For:

1. Ringing: phloem removal causes bulging of sugar above the ring. This sonfirms downward sugar flow

2. Aphid feeding: aphids pierce the phloem, then their bodies are removed leaving their mouth pieces behind, and sap can flow out. this showed us that sap flow is faster near leaves, confirming the existnace of a pressure gradient

3. Tracer use: radioactive tracers can trace organic substance movement

4. Metabolic inhibitors: stopping ATP production halts translocation, proving that active transport is involved

Against:

1. sugars travel to many different sinks, not just the one with the highest water potential

2. sieve plates act as significant physical barriers, overcoming resistance requires an excessive pressure for a reasonable mass flow rate

mass flow hypothesis (translocation)

1. Source (Leaf)

 -Photosynthesis converts glucose into sucrose in leaves. 

 -Sucrose is actively loaded from companion cells into phloem sieve tubes using ATP from mitochondria. 

-This lowers the water potential inside the sieve tubes (in the phloem)

- Water enters the phloem via osmosis from the xylem, creating high hydrostatic pressure. 

2. Flow

 -High pressure at the source and low pressure at the sink create a pressure gradient. 

 -Mass flow pushes solutes along the sieve tubes towards the sink. 

 ⭐️Higher sucrose concentration at the source increases the rate of translocation. 

3. Sink (Root)

 -Sucrose is actively removed from the phloem into the root (sink tissues). 

-It is used in respiration or stored as starch (insoluble and compact energy store). 

- Removing solutes increases water potential in the sieve tubes, causing water to leave by osmosis back to the xylem. 

- This results in low hydrostatic pressure at the sink end, maintaining the pressure gradient for sucrose to move down.

what ensures there's always a LOWER concentration at the SINK than at the source

enzymes. they maintain a concentration gradient by changing the solutes at the sink.

what's translocation

the movement of solutes/assimilates (amino acids and sugars) to where they're needed by the plant. It's an energy requiring process that happens in the phloem.

what are companion cells and where are they located

-they are cells located to either sieve tube element that carry out functions for sieve cells

-they help with loading and unloading of solutes and supply ATP to provide energy for active transport against a concentration gradient

what are the structure of sieve tubes and what do they do

-long, thin walls joined together to form tubes with sieve plates

-very few organelles, no nucleus, very thin cytoplasm

-transport organic solutes such as amino acids and sugars

how to measure the rate of uptake of water in different conditions leafy shoot, driven by transpiration (transpiration is the removal of water however the transpirational pull draws columns of water up the xylem)

1. cut a shoot underwater the prevent air entering the xylem (cut at a slant to increase surface area for water uptake)

2. assemble potometer underwater and insert the shoot with the apparatus still under the water

3. remove the apparatus from the water but keep the end of the capillary tube submerged in a beaker of water

4. check that the apparatus is watertight and airtight 

5. dry the leaves and allow time for the shoot to acclimatise, then shut the tap

6. remove end of capillary tube from the beaker of water until one air bubble has formed. then put it back into the water

7. record the starting position of the air bubble

8. start a stopwatch and record the distance moved by the bubble per unit of time. rate of air bubble movement= transpirational rate

⭐️only change one variable at a time e.g. temperature, all other conditions must be kept constant 

factors affecting the rate of transpiration

• Light intensity: stomata open when its light to let in carbon dioxide for photosynthesis = more transporation

• Temperature: at higher temperatures water molecules have more Ek, so they evaporate from the spongy mesophyll cells faster. This increases the water potential gradient between the inside and outside of the leaf, so water diffuses out faster.

• Humidity: the lower the humidity, the faster the transpiration rate. At high humidity there's more water in the air so the water potential gradient is reduced, decreasing the rate of transpiration.

• Wind: the wind removes moist air from around the leaf, increasing transpiration by increasing the water potential gradient.

3 pieces of evidence to support the cohesion tension theory

• transpirational pull puts the xylem under tension, causing a tree trunk to shrink in diameter. most evaporation from leaves happens during the day, causing an inward pull of xylem. due to the adhesion of water molecules to the walls of xylem vessels, the tree trunk is smaller during the day

• if a xylem vessel is broken, air enters, and the continuous column of water is broken to they cannot cohere and the tree cannot draw up water

• when a xylem vessel is broken air is drawn in rather than water leaking out, which proves that it's under tension

what is transpiration and what's the process of it, and how do they control the rate of it (diagram included)

transpiration is the evaporation of water from the leaves through the stomata

1. water diffuses through the air spaces between mesophyll cells, and out through the stomata down a water potential gradient

2. water lost from air spaces is replaced by water evaporating from cell walls surrounding mesophyll cells

3. water lost from mesophyll cells is replaced by water from the xylem by either symplast or apoplast pathway

4. this causes water to be pulled from the xylem, creating tension

⭐️plants control their rate of transpiration by altering the size of their stomatal pores (opening and closing guard cells)

what's the cohesion tension theory 

• water evaporates out of the stomata in the leaves

• this reduced water potential in cells, water is drawn out of the xylem creating a tension, making xylem vessels narrower (lignin in the walls stops them from collapsing)

• water is pulled up the stem by cohesion. (water molecules are cohesive due to hydrogen bonding, so they move through the xylem as a continuous column)

• more water enters the stem from the roots

• water moves from the soil into the root hairs via osmosis

how does the uptake of mineral ions happen in a plants roots

-they are absorbed by active transport (using ATP from respiration) against a concentration gradient

-theyre also absorbed by diffusion 

evidence for the function of the xylem (3 experiments)

1. Using dyes:          It you place a plant in a dye solution, the dye can be seen moving up the stem

2. Ringing experiments:          Removing a strip of tissue, removes the phloem but keeps the xylem in place. After ringing the plant dosent wilt, proving that it still has plenty of water

3. Radioactive tracers:           If a plants placed in a solution containing a radioactive mineral ion, the movement of them through a plant can be traced using autoradiography

⭐️this shows that the movement of ions in a plant happens in the xylem

definition of root pressure and how it works

-also what are some inhibiting factors

-what are 2 examples of what it causes

It's an upward force generated in the roots that pushes water up the xylem

 

How it works: 

-active transport moves ions into xylem

-this lowers water potential inside xylem

-water follows down a water potential gradient into the xylem via osmosis, creating an outward pressure

 

Inhibiting factors:

Low temperatures and respiratory inhibitors stop root pressure as they slow down respiration and active transport.

 

Causes: 

-it results in guttation (drops of water forced out the end of leaves)

-causes sap exuding from a cut, rooted stem

how does water enter the xylem

Water travelling via the apoplast pathway is blocked by the suberin-containing Casparian strip, forcing it into the cytoplasm of endodermal cells (symplast pathway)

 -Endodermal cells use ATP and carrier proteins to actively transport mineral ions into the xylem vessels. 

-this lowers the water potential inside the xylem 

 -Water moves from the endodermal cytoplasm into the xylem by osmosis

 - Because active transport requires ATP, this entry mechanism can only occur in living tissue.

The apoplast pathway 

• water passes through the non-living parts of the root (CELL WALLS)

• water diffuses through cell walls and passes through spaces external to the cell membrane

• this pathway avoids the contents of the living cell, meaning it needs a little resistance

• movement of water is driven by transpirational pull and cohesion between water molecules (hydrogen bonding)

• water moves across the root until it meets the endodermis. here the casparian strip (waterproof suberin in the endodermal cells) blocks water but minerals can continue along the apoplast pathway

• water is forced to cross into symplast pathway via osmosis before it can move into the xylem

The symplast pathway

• water moves continuously through the LIVING part of plant (CYTOPLASM)

• water enters root hair from soil across the cell wall and cell membrane via osmosis (high to low water potential)

• water passes from one cytoplasm to the next via plasmodesmata 

• as water leaves a cell, its water potential drops, which causes water from neighbouring cells to enter by osmosis, maintaining a contrast flow across the root and into the xylem.                          ⭐️ flow through the cytoplasm experiences resistance caused by organelles and cell membrane

How does water travel across the root?

1. symplast pathway- through cell surface membrane 

2. apoplast pathway- through cell walls, intercellular spaces

⭐️water in the apoplastic pathway moves into the symplast pathway . water in the symplast pathway stays there

What's each layer of the root structure?

Epidermis- single layer of cells with root hairs

Cortex- thick layer of packing cells containing stored starch 

Endodermis- single layer of endodermal cells containing a waterproof layer called the casparian strip. This is made of suberin, which prevents the movement of water between cells.

Pericycle- layer of undifferentiated meristematic (growing) cells

Vascular tissue- contains xylem and phloem cells. 

why is active transport necessary for the absorption of mineral ions?

why does this help the uptake of water?

Conc of mineral ions lower in soil than inside root hairs cell, they therefore cannot move via simple diffusion due to the diffusion gradient. ATP moves mineral ions against their concentration gradient, using energy.

 

Active transport of mineral ions lowers the water potential of root cells, establishing a water potential gradient that drives the uptake of water from the soil via osmosis.

How are the roots adapted to absorb water?

specialised root hairs to increase surface area for osmosis

What are the 2 functions of the roots?

• to anchor the plant into the ground

• the absorb water and minerals

Cohesion tension mechanism

---

• water evaporates from leaves at top of xylem (transpiration)

• this creates tension, pulling more water into the leaf

• water molecules are cohesive, meaning when some are pulled into the leaf others follow. This means the whole column of water in the xylem moves upwards

• water then enters the stem through roots

Explain how the xylem tissue is adapted for its function

tubes with no end walls = allows for continuous water columns

-no cytoplasm or organelles = there's no obstruction 

-thickening/lignin = withstand tension 

-pits in walls = allows lateral movement 

How is the xylem and phloem different in the root compared to in the stem?

In the root:
Both located at the centre of the root, forming a structure called stele
In the stem:
Vascular tissue is organised into vascular bundles, xylem is on the inside, phloem is on the inside.

What do cambium cells do?

Differentiate to form new xylem and phloem.

Role of the Xylem and Phloem

Xylem- transports water and dissolved mineral ions, up the plant from the roots to the leaves.
Phloem- transports sucrose (in solution) up and down the cells.

What’s the role of the stem, flower, roots and leaves?

Stem- SUPPORT/TRANSPORT (holds leaves in the light and carries water and minerals to the leaves and carries sugar from the leaves to the rest of the plant)

Flower- POLLINATION/REPRODUCTION (brightly coloured to attract insects for pollination. contain the male and female sex organs. make seeds which grow into new plants)

Roots- WATER UPTAKE/ANCHORAGE (roots anchor the plant in soil. they are covered in root hairs which increase surface area for absorption of water and minerals)

Leaves- PHOTOSYNTHESIS (plant makes its own food by the process of photosynthesis)
carbon dioxide + water —-> glucose + oxygen

What’s water conserved for during a shortage?

It’s conserved for stomatal opening to enable gas exchange and uptake of carbon dioxide for photosynthesis, as well as for the reaction of photosynthesis itself.

Why do plants use water?

-solvent for transport
-photosynthesis
-stomatal opening
-hydrolysis reactions
-Medium for chemical reactions
-component of cells/cytoplasm
-cell expansion
-turgor pressure 
-cell wall synthesis