Transport in plants

Question 1

Why do plants require a transport system?

Answer 1

• multicellular with a low SA:V
• diffusion too slow to meet metabolic needs
• substances must be moved over long distances

Question 2

What is the function of xylem?

Answer 2

Transports H2Oand mineral ions around plants

Also provides structural support mostly made up of xylem vessels

Question 3

What are the adaptions of xylem vessels?

Answer 3

• elongated hollow tubes without end walls
  -lack organelles
  -walls are thickened with lignin for support
• have non-lignified pits allowing movement of water and ions into and out of vessels

Question 4

What is the function of phloem?

Answer 4

Transports sugars and amino acids (assimilates) around plants mostly made up of sieve tube elements and companion cells

Question 5

What are the adaptions of phloem vessels?

Answer 5

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

Question 6

What are the adaptions of companion cells?

Answer 6

• connected to sieve tube elements through pores (plasmodesmata)
• many ribosomes for protein synthesis
• cytoplasm contains large nucleus many mitochondria to release energy for the active transport of substances through the sieve tube elements

Question 7

In roots how are xylem and phloem tissues distributed?

Answer 7

Xylem forms central cylinder surrounded by phloem providing support as root grows through soil

Question 8

In stems how are xylem and phloem tissues distributed?

Answer 8

Xylem and phloem are in the outer region forming ‘scaffolding’ to resist bending

Question 9

In leaves how are xylem and phloem tissues distributed?

Answer 9

Xylem and phloem form a network of veins providing support for thin leaves

Question 10

After water enters the roots how does it move through the plant?

Answer 10

1) water enters plant’s root hair cells via osmosis
2) moves through cell cytoplasm or cell walls towards xylem
3) xylem transports water from roots up to leaves
4) water is used for photosynthesis
5) some water evaporates from leaf cells by transpiration and diffuses out of plant

Question 11

What is the apoplast pathway?

Answer 11

Water moves through spaces in cell walls and between cells due to the cohesive and adhesive properties of water

Question 12

What is the symplast pathways?

Answer 12

Water moves from cell to cell through the cytoplasm and plasmodesmata due to water potential gradients

Question 13

What is the casparian strip?

Answer 13

Band of waterproof substance (suberin) that surrounds the endodermis cells

forces water out of the apoplast pathway and into the symplast pathway

Question 14

What happens to water in the leaves?

Answer 14

After the xylem transports water up through a plant, water exits the xylem into leaf cells it travels from the xylem to photosynthesising leaf cells mainly via apoplast pathway

water then evaporates from cell walls in leaf into air spaces so it can exit the plant through its stomata

Question 15

What does the cohesion tension theory explain?

Answer 15

How water moves upwards through the xylem against gravity due to pulling

Question 16

What causes the cohesion tension theory?

Answer 16

Cohesion = H bonds causes water molecules to stick together and move as one continuous column

Adhesion = H bonds between polar water molecules and non-polar cellulose in xylem vessel walls pulls water upwards through xylem

Transpiration pull = water evaporation at leaves creates the transpiration pull and tension is transmitted down whole water column due to cohesion

Question 17

What is transpiration?

Answer 17

Evaporation of water from aerial parts of plants especially leaves

Question 18

Why does transpiration occur?

Answer 18

1) H2O evaporates from moist surfaces of mesophyll cells
2) stomata open so can absorb CO2 for photosynthesis
3) provides pathway for H2O vapour loss through open stomata
4) H2O vapour moves down a ψ gradient from air spaces in leaf into atmosphere

Question 19

What factors affect transpiration rate and how?

Answer 19

Light intensity = at high light intensities stomata open for max CO2 absorption for photosynthesis increasing transpiration rate

Temp = at high temps evaporation of H2O molecules is faster due to higher ke increasing transpiration rate

Humidity = low humidity increases H20 vapour gradient between leaf and atmosphere, increasing transpiration rate

Wind speed =high wind speeds increase H2O vapour gradient between leaf and atmosphere increasing transpiration rate

Question 20

What are the steps in using a potometer?

Answer 20

1) cut shoot underwater at slant to increase SA for H2O uptake
2) assemble potometer with shoot submerged in water
3) keep capillary tube end of potometer submerged throughout experiment
4) check apparatus is airtight
5) dry leaves and give shoot time to acclimatise
6) shut tap form an air bubble and record position
7) measure distance air bubble moves and time taken
8) change one variable at a time and keep everything else constant

Question 21

How do we calculate transpiration?

Answer 21

Rate of transpiration = volume of H2O uptake ÷ time taken

Question 22

What are the key structures in a typical leaf?

Answer 22

• upper epidermis
• air spaces.
• palisade mesophyll cells
• spongy mesophyll cells
• stomata
• lower epidermis
• vascular tissue

Question 23

What is the structure and function of the upper epidermis?

Answer 23

Waxy cuticle = reduces water loss from leaf surface

Question 23

What is the structure of air spaces in leaves?

Answer 23

Interconnecting spaces that run throughout mesophyll layer

Question 24

What is the structure and function of palisade mesophyll cells?

Answer 24

Cells located beneath upper epidermis to carry out photosynthesis

Question 25

What is the structure and function of spongy mesophyll cells?

Answer 25

Dispersed cells located beneath palisade mesophyll layer to carry out photosynthesis

Question 26

What is the structure and function of stomata?

Answer 26

Small pores surrounded by guard cells on underside of leaves that can open and close

Question 27

What is the structure of the lower epidermis of leaves?

Answer 27

Bottom layer of cells in a leaf that contains stomata and guard cells

Question 28

What are the adaptions of the leaf for efficient gas exchange?

Answer 28

Air spaces = provide network for gases to quickly diffuse in and out leaf and access photosynthesising cells Mesophyll cells = dispersed throughout leaf providing large SA across which gases can diffuse Stomata = open when conditions are suitable for photosynthesis allowing inward diffusion of CO2 and outward diffusion of O2 and close to minimise H2O loss

Question 29

How can plants limit water loss?

Answer 29

- waterproof waxy cuticle on leaves - guard cells that can close stomata when needed

Question 30

What are Xerophytes?

Answer 30

Plants adapted to living in dry environments with limited water availability without adaptations they would become desiccated (dry out) and die

Question 31

What are the adaptions of xerophytes to reduce water loss?

Answer 31

Thick waxy cuticle = reduces H2O loss through evaporation Leaf rolling/folding = encloses stomata on lower surface to reduce air flow and H2O evaporation Leaf hairs = trap moist air against leaf surface to reduce diffusion gradient of water vapour Sunken stomata in pits = reduce air flow and H2O evaporation Small needle-like leaves = reduce SA across which water can be lost Water storage organs = conserve water for when in low supply

Question 32

What is the mass flow hypothesis?

Answer 32

Proposes translocation occurs due to pressure gradients

Question 33

Explain the mass flow hypothesis?

Answer 33

1) at source solutes (e.g sucrose) are actively loaded into sieve tube elements from companion cells 2) decreases the Ψ in sieve tube elements 3) H2O enters sieve tube elements from xylem and companion cells by osmosis 4) increases hydrostatic pressure in sieve tube elements at source 5) at sink solutes are actively removed from sieve tube elements 6) increases the Ψ in sieve tube elements at sink 7) H2O leaves phloem by osmosis decreasing hydrostatic pressure at sink 8) creates pressure gradient pushing solutes from source to areas of lower pressure at sink

Question 34

What happens at the sink to solutes?

Answer 34

Solutes are actively unloaded from sieve tube element into companion cells can then move into sink cells where solutes can be used for example in respiration or stored

Question 35

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

Answer 35

1) H+ ions are actively transported out of companion cells into surrounding source cells 2) H+ is co-transported along its concentration gradient back into companion cells with sucrose 3) sucrose can then diffuse along its conc gradient through plasmodesmata from companion cells to sieve tube element

Question 36

What is translocation?

Answer 36

Mass flow of assimilates from one part of a plant (source) to another part of same plant (sink)

Question 37

What are assimilates?

Answer 37

Substances that have been manufactured or modified in the plant (e.g sucrose and AA) that are transported in phloem sieve tubes during translocation

Question 38

What are features of translocation?

Answer 38

- requires energy - transports substances from sources (like leaves) to sinks like roots) - H2O provides medium in which substance dissolve for transport in phloem - maintains a conc gradient using enzymes