transport in plants

Question 1

Why do plants need transport systems?

Answer 1

• there are multicellular with a low surface area to volume ratio
• Diffusion is too slow to meet their metabolic needs
• substances may be moved over long distances

Question 2

What are the two main transport tissues?

Answer 2

Xylem and phloem tissue

Connected in a vascular bundle

Question 3

What does the xylem transport?

Answer 3

Transports water and mineral ions around plants

Question 4

What does xylem tissue do?

Answer 4

• Transport water and mineral ions
• Provides structural support

Question 5

What is the structure of the xylem tissue?

Answer 5

• Hollow tube (no cytoplasm)
• Thick lignified wall
• non-lignified pit
• elongated, tubes without end walls
• lack organelles
• walls are thickened with lignin for support
• non-lignified pits that allow movement of water and ions into and out of vessels

Question 6

What does the phloem tissue do?

Answer 6

Transport sugars and amino acids (assimilates)

Question 7

What is the phloem tissue mostly made out of?

Answer 7

Made up of mostly sieve tube elements and companion cells

Question 8

What is the structure of the phloem tissue?

Answer 8

Companion cells
- Nucleus
- Cytoplasm
- mitochondria

Sieve plate
Sieve tube element

Question 9

What are the adaptations of sieve tube element?

Answer 9

• Connected end to end to form sieve tubes
• sieve plates with pores at their ends to allow flow of sugars and amino acids
• lack nuclei and most organelles
• a thin layer of cytoplasm

Question 10

What are the adaptations of companion cells?

Answer 10

• connected to seive tube elements through pores (plasmodesmata)
  -Cytoplasm contains a large nucleus, many mitochondria to release energy for the active transport of substances through the seive tube elements, many ribosomes for protein synthesis

Question 11

What is the distribution of phloem tissues in roots, stem and leaf?

Answer 11

The vascular tissues are distribute to differently throughout various parts of the plant

In the root
- Xylem forms central cylinder surrounded by phloem
- This provides support as the root grows through soil

In the stem
- xylem and phloem are in the outer region
- This provides/forms ‘scaffolding’ to resist bending

In the leaves
- xylem and phloem form a network of veins
- This provides support for thin leaves

Question 12

How does water move through a plant?

Answer 12

1. Water enters the plants root hair cells via osmosis.
2. It moves through the cell cytoplasm or cell walls towards the xylem.
3. The xylem transports water from the roots up to the leaves.
4. Water is used for photosynthesis.
5. Some water evaporates from leaf cells by transpiration and diffuses out of the plant.

Question 13

What is the apoplast pathway through the plant?

Answer 13

• Water moves through spaces in the cell walls and between cells
• This occurs due to the water potential gradients

Question 14

What is the symplast pathway through a plant?

Answer 14

• water moves from cell to cell through the cytoplasm and plasmamodesmata
  This occurs due to water potential gradients

Question 15

What is the Casparian strip?

Answer 15

• blocks apoplast pathway in epidermis
• Bond of waterproof substance called Suberin
• Forces water out of the apoplast pathway into the symplast pathway

Question 16

What is the pathway of water through the leaf cells?

Answer 16

• after the xylem transports water up through a plant, water exits into leaf cells
• travels from the xylem to photosynthesising leaf cells mainly via the apoplast pathway
• water then evaporates from cell walls in the leaf into air spaces so it can exit the plant through its stomata

Question 17

What is the cohesion-tension theory?

Answer 17

Explains how water moves upwards through the xylem against gravity

Cohesion - hydrogen bonding causes water molecules to stick together and move as one continuous column
Adhesion - hydrogen bonding between polar water molecules and nonpolar cellulose in xylem vessels pulls water up through the xylem

Transpiration pull - evaporation of water leaves creates the transpiration pull, distension is transmitted down the whole water column due to cohesion

This causes water to be pulled up through xylem vessels

Question 18

How does transpiration relate to gas exchange?

Answer 18

• Water evaporates from the moist surfaces of mesophyll cells
• Stomata open so they can absorb carbon dioxide for photosynthesis
  -Provides a pathway for water vapour loss through the open stomata
• Water vapour moves down a water potential gradient from the air spaces in the leaf into the atmosphere

Question 19

What are the factors affecting the transpiration rate?

Answer 19

Light intensity, temperature, humidity, and windspeed

Question 20

How does light intensity affect transpiration rate?

Answer 20

As light intensity increases the stomata open for the maximum carbon dioxide absorption this increases transpiration rate

Question 21

How does temperature affect transpiration rate?

Answer 21

Evaporation of water molecules is faster due to the higher kinetic energy

Question 22

How does humidity affect the transpiration rate?

Answer 22

Low humidity increases the water vapour gradient between the leaf and atmosphere

Question 23

How does windspeed affect transpiration rate?

Answer 23

High windspeed increase water potential gradient between leaf and the atmosphere

Question 24

How do you use a potometer in the experiment?

Answer 24

1. Cut the shoot underwater at a slant to increase the surface area.
2. Put the shoot into potometer underwater.
3. Ensure equipment is airtight.
4. Dry the leaves, leave to acclimatise
5. Close tap, form a bubble, record the position of the bubble.
6. Measure the distance moved.

Question 25

How do you calculate the rate of transpiration?

Answer 25

1. Calculate the cross-section area of the capillary tube Pie x radius squared 2. Calculate the volume of water uptake. Pi x radius squared x distance 3. The rate of water uptake. Rate of transpiration = volume of water uptake/time taken

Question 26

What is the structure of the leaf from the surface to the underneath?

Answer 26

Waxy cuticle Upper epidermis Palisade mesophyll cell Spongy mesophyll cell Xylem and phloem Lower epidermis Guard cells and stomata

Question 27

What are the roles of the structures in the leaf?

Answer 27

Waxy cuticle - reduces water loss Air spaces – interconnecting spaces Palisade mesophyll cells – photosynthesis Spongy mesophyll cells – photosynthesis Stomata – small pores,open/close Vascular tissue – transports water and nutrients

Question 28

What are the adaptations in the leaf for gas exchange?

Answer 28

Air spaces – network for gases to quickly diffuse in/out of the leaf Mesophyll cells – large surface area Stomata – open and close

Question 29

How do plants limit water loss?

Answer 29

1. They have a waterproof waxy cuticle on their leaves. 2. They have guard cells that can close the stomata when needed.

Question 30

What are xerophytes?

Answer 30

They live in dry environments with limited water availability

Question 31

How are xerophytes adapted?

Answer 31

Waxy cuticle – reduces water loss through evaporation Rolling/folding of leaves – encloses the stomata on the lower surface to reduce airflow and the evaporation of water Hairs on leaves – traps moist air against leaf surface to reduce the diffusion gradient Sunken stomata in pits – reduce airflow and evaporation Small, needle light leaves – reduces surface area Water storage organs – conserve water for when there is a low supply

Question 32

What does mass flow propose?

Answer 32

It proposes translocation occurs due to pressure gradients

Question 33

What is the process mass flow?

Answer 33

1. At source, solutes (sucrose) are actively loaded into sieve tube elements from companion cells. 2. This decreases water potential in STE. 3. Water enters STE from xylem and cc by osmosis. 4. This increases hydrostatic pressure in the STE at the source. 5. At the sink, solutes are actively removed from STE 6. This increases the water potential in STE at the sink. 7. Water leaves the xylem, this decreases hydrostatic pressure at the sink 8. This creates a pressure gradient, pushing solutes from the source to areas of low pressure at the sink At the sink, solutes are actively unloaded from the STE into cc. They can then move into sink cells where the solutes can be used, for example in respiration or stored.

Question 34

How does active loading in the phloem at the source occur?

Answer 34

1. Hydrogen ions are actively transported out of cc into surrounding source cells. 2. Hydrogen ions are co-transported along its concentration gradient back into CC with sucrose 3. Sucrose can then diffuse along its concentration gradient through plasmodesmata from cc to STE

Question 35

What is translocation?

Answer 35

Mass flow of assimilates from one part of a plant, the source, to another part of the plant, the sink.

Question 36

What are assimilates?

Answer 36

Substances manufactured and modified in plants such as sucrose

Question 37

What are the features of translocation?

Answer 37

– It requires energy – It transport substances from sources (leaves) to sinks (roots) – Water provides the medium in which these substances dissolve for transport in the phloem – It maintains a concentration gradient using enzymes