mass transport in plants

Explain the 2 types of mesophyll tissue:

palisade mesophyll contains many chloroplastss for high rate of photosynthesis, and elongated cells increase diffusion

spongy mesophyll contains many air spaces for large internal surface area

describe and explain the role of stomata and guard cells (4)

1. stomata are pores mainly found in lower epidermis

2. guard cells control the opening and closing of stomata

3. open during the day to allow CO2 to diffue into leaf for photosynthesis, closed at night to reduce water loss

4. many stomata means all mesophyll cells are close to stoma

Explain the adaptations for large surface area in the leaf (2)

1. elongated cells of the palisade mesophyll

2. air spaces provide large internal surface area of mesophylls

explain the short diffusion pathways in leaves (3)

1. leaf is thin

2. many stomata so all mesophyll are close to a stoma

3. interconnecting air spaces so are mesophyll cells are in contact with air

explain the mechanisms to maintain steep concentration gradients in leaves (1)

• rate of photosynthesis maintains the concentration gradient

explain the structure and function of a xylem (6)

1. transports water and inorganic ions

2. developing xylem is composed of living cells

3. mature xylem is composed of dead cells that lose their end walls and cell contents

4. movement of water and ions is a passive process

5. lignin (polymer) provides strength and keeps xylem vessel open as a continuous tube

describe and explain transpiration (2)

• the evaporation od water molecules from the surface of the mesophyll cells

• water molecules diffuse out of leaf via the stomata by osmosis

explain cohesion-tension theory (6)

1. water is lost from the leaf because of transpiration

2. lowering water potential of mesophyll cells

3. this creates a water potential gradient, causing water to move across the leaf by osmosis

4. water molecules show cohesion due to hydrogen bonds joining them together and polar attraction

5. water moving into mesopyll from xylem causes tension on the water column in xylem

6. therefore water is pulled up the xylem in a continuous water column

explain adhesion (1)

• force of hydrogen bonds that form between water molecules to wall of xylem

explain the factors affecting rate of transpiration (4)

1. temperature increase leads to water molecules gaining more kinetic energy, leading to more water molecules lost so more transpiration

2. light intensity increases opening of stomata to allow CO2 to enter leaf for photosynthesis, so more water molecules lost so more transpiration

3. humidity increases water potential outside of leaf, so water potential / diffusion gradient /is less steep between leaf and air, so less transpiration

4. increased air movement leads to increased rate of transpiration

describe and explain 5 adaptations of xerophytes

1. plants are adapted to live in habitats that are low water availability

reduce water loss:

• thick waxy cuticles: reduces evaporation

• low stomatal density: reduces rate of transpiration

• hairs surrounding stomata: traps water molecules, maintaining humid air around stomata, reducing water potential gradient

• sunken stomata in pits: water vapour trapped, maintaing humid air around stomata, decreasing water potential gradient

• leaf rolling: creates humid environment surrounding stomata

Describe the structure and function of your phloem (2)

1. transport of organic solutes, mostly SUCROSE

2. composed of sieve tube elements and companion cells

Describe and explain the structures of sieve tubes and companion cells (2)

1. sieve tube elements are very narrow elongated cells and very few organelles, that have perforated ends (sieve plates) for mass transport

2. companion cells have lots of organelles and mitochondria to produce ATP for the loading and unloading of sucrose into and out of phloem

Explain translocation (6)

1. sucrose is actively transported into phloem sieve tubes

2. by co-transport of sucrose with H+ by companion cells

3. lowering water potential of phloem, causing water to enter sieve tubes from xylem by osmosis

4. increasing hydrostatic pressure in phloem

5. a pressure gradient is established in plhoem causing mass flow to sinks

6. sucrose is unloaded from phloem sieve tubes by active transport

Explain what sinks are

• respiring cells - meristems, roots

• storage tissues / organs

explain the co-transport of sucrose from leaf to sieve tube

1. H+ actively transported out of companion cell

2. H+ diffuses back into companion cell, through co-transport protein, along with sucrose

3. sucrose enters phloem from companion cell via facilitated diffusion

Name the two pieces of evidence for mass flow

1. ringing experiment

2. autoradiography

Explain the ringing experiment (4)

1. removal of phloem tissue from stem

2. stem immediately above ring / where phloem is removed swells due to accumulation of sugars, instead of going to roots / sinks

3. therefore phloem transports sugar

explain autoradiography (4)

1. plants are supplied with radioactively labelled 14C

2. during photosynthesis, 14C is is incorporated into sucrose, radioactively labelling the sucrose

3. radioactive sucrose is transported through phloem to sinks, where it can be detected

4. x-ray film turns black when radiactvity is detected