Topic 3: Exchange + Transport: Transport in Plants p2

Question 1

Function of the bordered pits in xylem vessels

Answer 1

Bordered pits in xylem vessels allows for the lateral movement of water and dissolved ions out of the xylem and into specific tissue

Question 2

Why do large multicellular plants need a transport system?

Answer 2

Large multicellular plants need a transport system:
-Small surface area to volume ratio > diffusion not fast enough -> for substances eg sucrose
-Long distance from external surface to cells

Question 3

Adaptations of plants with no vascular bundles containing xylem

Answer 3

Adaptations of plants with no vascular bundles containing xylem:
-No/thin waxy cuticle -> wax production is a waste
-Large surface area to increase photosynthesis as transpiration is not an issue
-Many stomata to increase gas exchange
-Stomata on the top surface, as gas concentration is higher in air than water

Question 4

Why is nile blue stain used for observing plants under a microscope?

Answer 4

Nile blue stain - increases contrast/to make nuclei visible/ to show no nuclei in sieve tubes

Question 5

How can water still be lost from cut stem when all leaves have been covered by petroleum jelly?

Answer 5

Water can still be lost from cut stems due to evaporation from upper leaf surfaces

Question 6

Possible sources of error in transpiration practical

Answer 6

Sources of error:
-Not all lower leaf surface covered
-Leaks in apparatus
-Shoot not cut under water
-Error in reading position of meniscus

Question 7

Movement of water into root cells and into symplast pathways

Answer 7

Water moves into root cells and into the symplast pathway by osmosis

Question 8

Movement of water up the stem in the xylem and in the apoplast pathway

Answer 8

Water moves up the stem in the xylem and in the apoplast pathway by mass flow

Question 9

Movement of water out leaf via stomata

Answer 9

Water diffuses out of the leaf via stomata

Question 10

Why is it beneficial for carbohydrates to be in the form of sucrose when transported within plants?

Answer 10

-Sucrose is soluble and so can be transported in sap
-Metabolically inactive so not used/removed during transport

Question 11

Similiarities in the transport within the phloem and the transport within the xylem

Answer 11

Similarities in the transport within the phloem and the transport within the xylem:
-Solutes carried in solution in both
-Both carry mineral salts
-Both use mass flow/generated hydrostatic pressure

Question 12

Differences in transport within the phloem and transport within the xylem

Answer 12

Differences in transport within the phloem and transport within the xylem:
-Transport in phloem can take place in different directions but transport in the xylem only takes place up in the plant
-Phloem carries carbohydrates and xylem does not
-Phloem transports using living cells and xylem does not
-Xylem uses capillary action/cohesion and adhesion, whereas phloem does not

Question 13

How does the casparian strip prevent ions reaching the xylem in the apoplast pathway?

Answer 13

-Casparian strip does not allow solutions and water through
-Forces the solutions and water to pass through a plasma membrane
-Phospholipid bilayer repels ions/charged particles

Question 14

How do plants that live in soil with low water potential eg in salt marsh communities (halophytes)?

Answer 14

Halophytes (plants that live in soil with low water potential):
-Lower water potential inside root hair cells
-Actively transport ions into root hair cells

Question 15

How does mass flow of the phloem sap occur in plants with a vascular system?

Answer 15

How mass flow of the phloem sap occurs in plants with a vascular system:
-Sucrose/assimilates in sieve tube elements
-Assimilates enters sieve tube/phloem (at source) and lowers water potential in sieve tube
-Water enters sieve tube by osmosis
-Assimilates leave sieve tube and increases water potential in sieve tube

Question 16

How does increased air movement increase the loss of water vapour from leaves?

Answer 16

Increased air movement:
-Water vapour around stomata blow away
-Reduces water vapour potential around the stomata
-Increases water vapour potential gradient between air space in leaf and outside

Question 17

How does not having vascular tissue affect the size to which plants can grow?

Answer 17

Effect on not having vascular tissue on plant growth:
-Cannot grow large
-No support from xylem
-Uses only diffusion - no mass flow/rapid transport
-Diffusion too slow (short diffusion pathway, large surface area to volume ratio)

Question 18

Parts of the leaves where water may be lost, other than the leaf/stomata

Answer 18

Parts of the leaves where water may be lost, other than the leaf/stomata:
-Epidermis/Cuticle

Question 19

What increases the cohesion-tension theory (capillary action) in plant?

Answer 19

Evaporation at top of plant creates tension in the xylem

Question 20

Types of cells that can be found in the phloem tissue

Answer 20

Types of cells that can be found in the phloem tissue:
-Parenchyma, companion cells, sieve tube elements

Question 21

Benefits to plants of internal transport systems

Answer 21

Benefits to plants of internal transport systems:
-Surface area to volume ratio originally too small -> insufficient diffusion
-Transport system ensures molecules reaches all tissues
-Allows for a high metabolic rate

Question 22

Guard cell location

Answer 22

Guard cells are located at the leaves and contain chloroplast

Question 23

Substances transported by companion cells

Answer 23

Companion cells transport sucrose

Question 24

Substance example transported by root hair cells

Answer 24

Nitrate ions are transported by root hair cells

Question 25

Translocation definition

Answer 25

Translocation = the movement of nutrients around a plant - sugars, amino acids and other organic molecules

Question 26

Sources in phloems

Answer 26

Sources - these provide assimilated to the plant from reactions or storage for transport

Question 27

Examples of sources in plants

Answer 27

Sources:
-Green leaves and stems
-Storage organs eg tubers and root taps
-Food stores in seeds

Question 28

Sinks in plants

Answer 28

Sinks - these use assimilates from the plant in various processes

Question 29

Examples of sinks in plants

Answer 29

Sinks:
-Growing roots
-Active process in the roots and stem
-Meristem cells activity - dividing
-Developing stores eg seeds, fruits or storage organs

Question 30

Two routes by which assimilates are moved into the phloem

Answer 30

Two routes by which assimilates are moved into the phloem:
-Symplast route
-Apoplast route

Question 31

The symplast route by which assimilates move in the phloem

Answer 31

The symplast route by which assimilates move in the phloem:
-Assimilates are moved through the cytoplasm of mesophyll cells into the sieve tubes across connecting plasmodesmata
-Passive provess
-Assimilates are moved by changes in water potential of cells

Question 32

The apoplast route by which assimilates move in the phloem

Answer 32

The apoplast route by which assimilates move in the phloem:
-Assimilates diffuse through the cell wall and intermembrane spaces
-When they reach the companion cells, they are actively transported across the membrane into sieve cells cytoplasm
-Hydrogen ions act as co-transporters and actively move assimilates across the membrane

Question 33

Process of active loading in plants

Answer 33

Active loading in plants:
-Hydrogen ions are actively pumped out of companion cells
-Flows back in down a concentration gradient, through a co-transporter protein + brings sucrose in with it

Question 34

Mass flow in the phloem

Answer 34

Mass flow in the phloem:
1). Sucrose actively loaded into sieve tube element and reduces the water potential
2). Water flows in by osmosis and increases the hydrostatic pressure in the sieve tube element
3). Water moves down sieve tube from higher hydrostatic pressure at source to lower hydrostatic pressure at sink
4). Sucrose is removed from the sieve tube by the surrounding cells and increases the water potential in the sieve tube
5). Water moves out of the sieve tube and reduces the hydrostatic pressure

Question 35

Evidence of mass flow in the phloem

Answer 35

Evidence for mass flow in the phloem:
-Advances in microscopy - allows us to see adaptation of companion cells for active transport
-Poisoning mitochondria - translocation stops
-Flow of sugars in phloem much faster than it would be by diffusion alone - shows active process is involved
-Aphids

Question 36

How can aphids be used as evidence for mass flow in the phloem?

Answer 36

Aphids - shows there is a positive pressure in the phloem that forces the sap out through a stylet - the pressure and flow rate in phloem is lower closer to sink than near the source - concentration of sucrose in phloem sap is also higher near source than sink

Question 37

Argument against the use of mass flow of substances in the phloem

Answer 37

Not all solutes in the phloem move at the same rate, and no one is sure yet of the use of sieve plates in the process

Question 38

Sieve tube elements in phloems

Answer 38

Sieve tube elements:
-No organelles to form hollow tube that transports substances
-Each has a companion cell - companion cells have the organelles and carries out the metabolic reactions

Question 39

Sieve plates in the phloem

Answer 39

Sieve plates in the phloem:
-Sieve plates have large pores that allow sap to move through the sieve tube elements.
-Sieve plates allow sugars to be transported through the phloem.

Question 40

Why is sucrose concentration low at sinks?

Answer 40

Sucrose concentration is low at sinks because it is being used up as it moves along the plant from source to sink