Transport in Plants

Question 1

Explain why transpiration is a consequence of gaseous exchange

Answer 1

Plants need to open their stomata to let in CO2 so it can produce glucose by photosynthesis. This lets water out as there’s a high conc of water inside the leaf than outside so water moves out of the leaf down its water potential gradient when the stomata is open

Question 2

What is the importance of transpiration?

Answer 2

It transports useful mineral ions up the plant. It maintains cell turgidity. It supplies water for growth, cell elongation and photosynthesis. It supplies water that can keep a plant cool on a hot day (evaporates)

Question 3

Describe why light intensity affects transpiration rate

Answer 3

In light, stomata opens to allow gaseous exchange for photosynthesis. Higher light intensity increases the transpiration rate.

Question 4

Describe the 3 processes of transpiration

Answer 4

• water enters the leaf through the xylem and moves by osmosis into the cells of the spongy mesophyll (moves along apoplast pathway)
• water evaporates from the cell wallsof the spongy mesophyll
• water moves by diffusion out of the leaf down a water vapour potential gradient. Must be higher water vapour potential inside the leaf than outside

Question 5

Describe why temperature affects transpiration rate

Answer 5

A higher temp increases the transpiration rate.

• increases rate of diffusion through the stomata because water molecules have more kinetic energy
• it decreases the relative water vapour potential in the air (increases it in the leaf), allowing more rapid diffusion of molecules out of the leaf

Question 6

Define transpiration

Answer 6

Transpiration is the loss of water vapour from the upper parts of the leaves through the stomata

Question 7

Describe how humdity affects rate of transpiration

Answer 7

Higher humidity in the air will decrease the rate of water loss. There will be a smaller water potential gradient between the air spaces in the leaf and the air outside

Question 8

Describe active loading

Answer 8

• ATP is used by the companion cells to actively transport hydrogen ions out of their cytoplasm and into the surrounding tissue
• this sets up a diffusion gradient as there is more H+ ions outside than inside so H+ diffue back into the companion cell
• diffusion happens through cotransporter proteins; allow H+ to bring sucrose molecules into the companion cell
• as the concentration of sucrose molecules builds up inside the companion cells they diffuse into the sieve tube element through the numerous plasodesmata

Question 9

Describe how wind affects transpiration

Answer 9

It will increase transpiration. Air moving outside the leaf will carry away water vapour that has just diffused out of the leaf. This will maintain a higher water vapour potential gradient

Question 10

Describe how water avalibility affects transpiration

Answer 10

If there is little water in the soil then the plant can’t replace the water that is lost. If there is insufficient water in the soil then the stomata close and the leaves wilt

Question 11

How is a potometer used to estimate transpiration rates?

Answer 11

It is used to estimate the rate of water loss. Not exact as it measures water uptake by a cut shoot. 95% of water is lost by transpiration. There must be no air bubbles inside the apparatus. Water lost by the leaf is replaced from the water in the capillary tube. The movement of the meniscus at the end of the water column can be measured

Question 12

What must be ensured when setting up a potometer?

Answer 12

1. cut the shoot under water to prevent air from entering the xylem
2. ensure the root is healthy
3. dry the leaves
4. allow time for equlilbrium and acclimatise
5. cut stem at an angle to provide a large surface area for water uptake
6. note where the meniscus is at the start and end of the time period

Question 13

Formula of a cylinder and how to calculate transpiration rate?

Answer 13

v = (3.14)(r*l)(l)

rate = volume/time

Question 14

Describe the transpiration stream

Answer 14

The movement of water from the soil through the plant to the air surrounding the leaves. The force is the water potential gradient between the soil and the air in the leaf air spaces

Question 15

Describe the pathway by which water is transported from the root hair cells to the xylem

Answer 15

• Water enters from the soil to the root hair by osmosis down a water potential gradient (due to the active pumping of mineral ions)
• water enters the cortex by the apoplast pathway between the cell walls through the symplast pathway (membrane and plasmodesmata) and vaculolar pathway
• water enters the endodermis which has a casparian strip which blocks the apoplst pathway so must enter by the symplast pathway (selective mineral)
• water enters the xylem and minerals are moved using active transport which reduces water potential in the xylem creating a gradient

Question 16

Describe the casparian strip

Answer 16

A waterproof strip found in the endodermis. Blocks the apoplast pathway so must be transported by symplast pathway. Responsible for selective mineral uptake and controls water potential. Lowering xylems WP so water moves up the xylem.

Question 17

Explain how water is transported from the root cortex to the air surrounding the leaves

Answer 17

• minerals are actively transported into the xylem vessels lowering the WP in the xylem and water follows by osmosis
• root pressure pushes some of the water upwards (pressure in the root medulla forces water up the xylem a few meters)
• water evaporates from the leaves by transpiration. The water must be replaced, creating a low hydrostatic pressure (tension)
• water molecules are attracted to each other by cohesion forming a continuous column of water so that they can move by mass flow pulled upwards by the tension above
• Xylem vessels are very narrow these forces of attraction can pull the water up the sides of the vessels (adhesion)

Question 18

Describe how translocation occurs

Answer 18

Translocation occurs through the sieve elements by mass flow. Sucrose is loaded into the phloem at sources. This mechanism is active. The addition of glucose lowers the water potential of the sieve element sap. This causes water to enter from surrounding tissues by osmosis which increases the pressure of the sap.

Question 19

Explain by which mechanisms water moves through the leaf

Answer 19

Water enters the root cells by osmosis. It moves across the root via the symplast pathway. It moves up the stem by mass flow and also across the leaf via the apoplast pathway. It comes out of the leaf via diffusion.

Question 20

Why do plants need a transport system?

Answer 20

To ensure that all the of cells recieve sufficient nutrients. Plants have a smaller SA:V. They need to move water and minerals from the roots up to the leaves. Sugars from the leaves to the rest of the plant. Size and metabolic activity

Question 21

Describe the xylem and phloem structure in a root of a dicotyledonous plant

Answer 21

The vascular bundle is found at the centre of the root, the central core of the xylem
Provides strength to withstand the pulling forces
Around the bundle is the endodermis, just inside is layer of meristem cells called pericycle

Question 22

Suggest why there was a positive correlation between distance from the river and mean leaf hair density

Answer 22

As you get further away from the river, less water is available from the soil so the tree would need a higher leaf hair density to reduce the rate of transpiration by trapping water

Question 23

Describe the xylem and phloem structure in a stem of a dicotyledonous

Answer 23

Xylem is found towards inside of vascular bundle and phloem outside
Inbetween is layer of cambium (meristem cells)
Provides strength and flexibility to withstnad bending forces

Question 24

Describe the xylem and phloem in the leaf

Answer 24

Vascular form midrib and veins of a leaf, branching network veins get smaller
Xylem located on top of the phloem

Question 25

How would you dissect plant material

Answer 25

Staining tissue
Thin sections cut and viewed at low power
Allow stem to take up water by transpiration
Stem cut longitudinally or transversely

Question 26

Describe the structure of the xylem vessel

Answer 26

Lignin makes walls waterproof, strength and prevents collapse but kills them so the ends decay leaving a tube
Form spiral, annular pattern or reticulate = allows flexibility
Bordered pits are gaps, in vessels that are close together allow water to leave one vessel and pass into next + enter living parts of plant

Question 27

Describe the adaptations of xylem vessels

Answer 27

Can carry water to top:
- dead cells aligned end to end to form continuous column
- narrow water column doesn’t break and capillary action
- bordered pits water move sideways
- Spiral, annular, reticulate allows stretch as plant grows and bend
Flow not impeded:
- no cross walls
- no cell contents
- lignin prevents walls from collapsing

Question 28

Describe the structure of the phloem

Answer 28

Transports assimilates (sucrose and amino acids) sucrose dissolved in water = sap 
Contains sieve tubes made sieve tube elements and companion cells

Question 29

Describe sieve tube elements

Answer 29

Elongated elements line up end to end to form sieve tubes
- no nucleus and very little cytoplasm for mass flow of sap
Ends of sieve tube elements perforated cross walls sieve plates
- allow movement of sap from one element to the next
- get blocked by callose
Very thin walls

Question 30

Describe companion cells

Answer 30

In between sieve tubes there are small cells with large nucleus and dense cytoplasm, numerous mitochondria produce ATP AT
Active loading

Question 31

How are terrestrial plants adapted to reduce and replace water loss (as a result of photosynthesis)

Answer 31

• waxy cuticle reduce loos due to evaporation
• stomata underside reduce direct heating from sun
• deciduous lose leaves in winter (ground frozen and temp too low for photosynthesis)

Question 32

Describe and explain the adaptations of marram grass

Answer 32

Water in sand drains away, salty, exposed to windy conditions

1. leaf rolled so air is trapped inside air becomes humid reduces water loss (roll more tightly in dry conditions)
2. thick waxy cuticle on outer side of rolled leaf reduce evaporation
3. stomata inner side protected by enclosed air space
4. in pits lower epidermis also folded and covered by hairs reduce air movement
5. spongy mesophyll very dense few air spaces less surface area for evaporation

Question 33

Define xerophyte and other features of adaptation

Answer 33

A plant adapted to living in arid conditions

Question 34

Define hydrophytes

Answer 34

Live in water