3.3: Transport in plants

Question 1

What makes up a stem?

Answer 1

• Cortex
• Epidermis
• Sclerenchyma
• Collenchyma
• Phloem
• Xylem

Question 2

What makes up a root?

Answer 2

• Exodermis
• Epidermis
• Xylem
• Phloem
• Endodermis
• Cortex
• Pericycle in epidermis - layer of meristemic tissues

Question 3

What is cambium?

Answer 3

In between xylem and phloem
Layer of meristem cells that divide to produce new xylem and phloem

Question 4

What is a xylem and what are the features?

Answer 4

Tissue that transports minerals and ions up the plant (transpiration stream)

• Supports the plant
• Dead - no organelles makes it a hollow tube
• Lignin - to withstand pressure of water uptake
• Bordered pits - to allow water an mineral ions to leave xylem

Question 5

What is the xylem parenchyma?

Answer 5

Stores food
Contains tannin (bitter chemical)

• This is a defence against herbivores

Question 6

What is a phloem?

Answer 6

Transports assimilates (sugars) from source to sink

Question 7

Why is sucrose transported in the phloem?

Answer 7

It is less reactive so it is not used up in respiration

Question 8

What are features of the phloem?

Answer 8

Alive - can do active transport which is important for translocation

• Sieve tube elements - no organelles to allow mass flow of sap
• Sieve plate
• Companion cell - keeps phloem alive and is involved in translocation
• Plasmodesmata - gap that links cytoplasm of adjacent cells

Question 9

What is transpiration?

Answer 9

Loss of water vapour by evaporation through the upper leaf surface and the stomata

Question 10

What is the typical pathway of water during transpiration?

Answer 10

1. Water enters the leaf through the xylem and moves by osmosis into the cells of. the spongy mesophyll
2. Evaporates from the cells walls of the spongy meosphyll
3. Water vapour moves by diffusion out of the leaf through the open stomata due to water vapour potential gradient

Question 11

What is the capillary action in the transpiration stream?

Answer 11

1. Minerals actively transported into xylem from root which lowers the water potential and water enters by osmosis
2. Cohesion of water molecules enables water to move by mass flow, pulled upwards by tension from above
3. Movement of water out of xylem into leaves creates low hydrostatic pressure thus tension
4. Osmosis moves water across the leaf and water evporates from cell surface
5. Diffusion of water vapour out leaf

Question 12

What are advantages and disadvantages of transpiration?

Answer 12

Advantages:
Cools down plants - water vapour absorbs heat
Causes transpiration stream - delivers water and mineral ions

Disadvantages:
Loss of water - lose tugor pressure which causes wilting and reduces the SA of leaves and stem exposed to surrounding environment - reduces transmission

Question 13

Watrer vapour moves out of the cell becoming water vapour and will diffuse out of the stomata: Causes

Answer 13

1. WP to decrease in one cell
2. Other cell with higher WP will have its water be drawn to first cell
3. Original cell with higher WP will have lower compared to third one
4. Third cell will have its water be drawn to second cell
5. This continues until it reaches the xylem
6. The pull of water from the root up to the stem, through the xylem is the transpiration stream

Question 14

Where does cohesion and adhesion occur in transpiration?

Answer 14

Cohesion - HB between water molecules causes it to stick together
Adhesion - HB between water molecules and walls of xylem

Question 15

What are the factors that affect transpiration?

Answer 15

• Temperature
• Windy conditions
• Light intensity
• Humidity

Question 16

What does temperature do to transpiration?

Answer 16

Molecules including water have higher KE and move faster
Increases the water concentration gradient

Question 17

What does windy conditions do to transpiration?

Answer 17

The air movement blows the H2O molecules on the inside of the leaf away.
Therefore move away faster
Maintains a high water vapour potential gradient

Question 18

What does light intensity do to transpiration?

Answer 18

More stomata will open, for more CO2 to enter, for more photosynthesis to occur and H2O to leave

Question 19

What does humidity do to transpiration?

Answer 19

There are more water molecules in the air
Reduces the water concentration gradient between leaf and air

Question 20

What is the apoplast pathway?

Answer 20

Movement of water through spaces of cell wall by tension

Question 21

What is the symplast pathway?

Answer 21

Movement through cytoplasm (diffusion) and plasmodesmata (osmosis)

Question 22

What is the casparian strip?

Answer 22

In the endodermal cell and its made of waxy suberin
Waxy - apoplast pathway no longer works

• Water goes through symplast pathway

Question 23

What are the steps of water transport in plants?

Answer 23

1. H2O moves across root cells in apoplast or symplast pathway
2. H2O moves across endodermis - moves through apoplst pathway + meets casparian strip

• H2O forced from cell wall into cytoplasm and carry on in SP (Plasma membrane)

3. H2O moves in xylem

• Move ions into xylem by active transport therefore water potential decreases in xylem
• H2O moves into xylem by osmosis down a water potential gradient
• Root pressure is generated - pushes H20 up xylem

Question 24

Where does phloem loading occur?

Answer 24

Apoplast pathway

Question 25

What is translocation?

Answer 25

Transport of assimilates from source to sink

Question 26

What occurs in translocation?

Answer 26

1. H+ pumped out of companion cell through H+ pumps by active transport (increases H+ outside of companion cell) 2. H+ wants to diffuse back into CC so gets co-transportted with sucrose into CC through co-transporters 3. Sucrose diffuses from CC into sieve tube element through plasmodesmata (decreases WP inside the STE) 4. H2O moves into STE by osmosis (generates tugor pressure for mass flow)

Question 27

What is mass flow?

Answer 27

Assimilates flow from source to sinks down pressure gradient

Question 28

What happens in phloem unloading?

Answer 28

1. Sucrose diffuse from phloem to sinks because it has a low sugar concentration 2. Sucrose is moved to other cells or converted to other forms - maintains concentration gradient between cells and phloem 3. Loss of assimilates increases WP of STE * H2O moves out into cells by osmosis or enters xylem's transpiration stream

Question 29

What are adaptions of root hair cells?

Answer 29

Large surface area due to shape - increases water uptake for osmosis Permanent vacuole containing cell sap, which is more concentrated than soil water - ensures a high water potential gradient is maintained

Question 30

What are xerophytes?

Answer 30

Plants with adaptations to reduce water loss and are therefore found in locations with limited water, for example, the desert * Marram grass is found on sand dunes, which despite being next to the ocean, there is limited water due to sand beiing so porrous

Question 31

What are adatations of xerophytes?

Answer 31

* Curled leaves to trap moisture to increase local humidity * Hairs to trap moisture to increase local humidity * Sunken stomata to trap moisture to incease local humidity * Thicker cuticle to reduce evaporation * Large root network to reach more water

Question 32

What are hydrophytes?

Answer 32

Plants which live in or on water, so require adaptations to survive in an excess of water * Water lillies grow on the surface of water

Question 33

What are advantages of hydrophytes?

Answer 33

Ensures no additional water us retained in the plant and efficient water loss: * Short roots * Very thin to no waxy cuticle * Stomata permenantly open and in the top surface of lead Ensures light is still absorbed for photosynthesis: * Leaves being large, wide and on the surface of leaf

Question 34

What is root pressure?

Answer 34

* As water moves into the roots by osmosis it increases the volume of liquid inside the root and therefore the pressure inside the root increases. This is known as root pressure * This increase in pressure in the roots forces water above it upwards (positive pressure)

Question 35

What happens in mass flow?

Answer 35

1. The sucrose in the STE has decreased the water potential 2. Water moves to STE which creates high hydrostatic pressure 3. Water acts as as medium to transport he assimilates from an areas of high hydrostatic pressure to low 4. Once it reaches an area of low HSP the sucrose is used by cells and surroounding tissures 5. Sucrose diffuses from the STE to surrounding tissues which decreases its WP 6. Water moves from STE to surrounding tissues by osmosis down a WP gradient

Question 36

What is the source and an example?

Answer 36

Part of the plant that loads materials into the transport system E.g leaves photosynthesise and the sugaars made are moved to others part of the plant

Question 37

What is the sink?

Answer 37

Part of the plant where materials are removed from the transport system