TRANSPORT IN PLANTS

Question 1

Why do plants need a transport system

Answer 1

• low SA:V ratio, so direct diffusion would be too slow
• relatively high metabolic rate
• system allows areas of plants to obtain things they require
  E.g. roots cannot obtain all sugars, whilst leaves cannot obtain necessary water

Question 2

Describe key features of the xylem

Answer 2

• hollow tube
• no end walls, continuous
• pits, small holes allowing movement of water to neighbouring cells
• strengthened by dead lignin
• spiral, allowing flexibility

Question 3

Outline purpose of the xylem

Answer 3

Moves water and minerals up the plant to the leaves

Question 4

Outline purpose of phloem

Answer 4

Moves solutes (sucrose) around the plant

Question 5

What makes up the vascular bundle

Answer 5

• xylem
• phloem

Question 6

Describe key features of the phloem

Answer 6

• few organelles, due to thin cytoplasm
• sieve plate at either end for movement of sugar molecules
• companion cells containing organelles, supporting the phloem
  Ex. Have mitochondria to provide energy for phloem

Question 7

• describe and explain the distribution of the vascular bundle in the roots
• how are the xylem and phloem positioned

Answer 7

• in the middle of the root
• supports root when pushing through the soil
• xylem in “cross”, phloem as “background”

Question 8

• describe and explain the distribution of the vascular bundle in the stem
• describe the position of the helm and phloem

Answer 8

• around the edge
• provide scaffolding, prevent growing plants from bending
• xylem towards centre, phloem towards outside

Question 9

• describe and explain the distribution of the vascular bundle in the leaves
• describe positioning of the xylem and phloem

Answer 9

• all around, creating a network in the leaf
• supports leaf
• xylem towards top, phloem towards bottom

Question 10

• what is the cambium
• where is it positioned relative to the vascular bundle

Answer 10

• meristem cells
• produce new xylem and phloem cells
• found between xylem and phloem

Question 11

Define osmosis

Answer 11

• net movement of water from an area of high water potential to an area of low water potential
• across a partially permeable membrane

Question 12

• what are the 2 ways that water can move from root hair cells to the xylem
• which parts of the cell does water pass through in each

Answer 12

• symplast pathway (living parts/cytoplasm)
• apoplast pathway (non-living parts/cell wall)

Question 13

Describe the symplast pathway

Answer 13

• water enters root hair cells through osmosis
• enters through plasma membrane
• travels through cytoplasm of the cells in the cortex
• diffuses through plasmodesmata down a concentration gradient

Question 14

Describe the apoplast pathway

Answer 14

• water enter root hair cell by osmosis
• water moves through walls of cells and spaces between them in the cortex
• travels by mass flow, from an area of high hydrostatic pressure to low
• reaches waterproof casparian strip at the endodermis
• water enters plasma membrane (osmosis)
• travels via symplast pathway to the xylem

Question 15

• what is the casparian strip
• where is it
• what is its purpose

Answer 15

• waxy waterproof layer
• at the endodermis layer
• cannot be penetrated by water, a controls what enters the vascular bundle

Question 16

Describe how water moves in the xylem

Answer 16

• H2O evaporating at leaves creates tension
• pulls more water up in a continuous column due to cohesion
• supported by adhesion between water and sides of the xylem

Question 17

• define adhesion
• how does this relate to the transpiration stream

Answer 17

• formation of hydrogen bonds between H2O and the walls of the xylem
• aids process of water column moving up the xylem

Question 18

• define cohesion
• how does this relate to the transpiration stream

Answer 18

• formation of hydrogen bonds between H2O molecules
• cohesion between water molecules causes H2O to be moved up the xylem in a continuous column

Question 19

Outline the movement of water in leaves
- how does this affects transpiration

Answer 19

• H2O moves from xylem to mesophyll cells via symplast or apoplast pathway
• H2O evaporates
• diffuses out of leaf air spaces into surroundings
• down a concentration gradient
• creates tension whilst leaving, pulling more H2O molecules up the xylem

Question 20

• what is translocation
• what is the substance moved

Answer 20

• movement of substances/assimilates from the source to the sink via mass flow (high to low conc)
• sucrose is moved

Question 21

Why is sucrose moved during translocation instead of glucose

Answer 21

• sucrose is metabolically inactive
• soluble
• glucose would get used up as it moves around (metabolically active)
• is a disaccharide, so contains more energy

Question 22

What monosaccharides make up the disaccharide
- sucrose

Answer 22

• (alpha) glucose
• fructose

Question 23

How do enzymes help maintain a concentration gradient from the source to the sink

Answer 23

• change dissolved substances at the sink
• lower concentration at the sink
• maintains gradient

Question 24

• what is meant by the “source”
• what is meant by the “sink”

Answer 24

• where assimilates are sourced e.g. green leaves are the source of glucose
• where assimilates are required e.g. growing meristems

Question 25

State the term for how assimilates travel in the phloem

Answer 25

Via mass flow

Question 26

Describe how assimilates are loaded at the sink during translocation

Answer 26

1. Solutes enter sieve tubes via active transport - sieve tubes water potential lowered 2. H2O enters from xylem and companion cells via osmosis - increases hydrostatic pressure at the source

Question 27

Describe how assimilates are unloaded at the sink

Answer 27

1. Solutes diffuse out of phloem (high➡️low conc) - increases water potential in sieve tubes 2. H2O leaves sieve tubes via osmosis - hydrostatic pressure in sieve tubes is lowered

Question 28

- how do assimilates move from the source end to sink end in the phloem during translocation - what happens once they reach the sink

Answer 28

- via mass flow - travel down the pressure gradient - solutes pushed towards sink - are either used e.g. respiration or stored e.g. as starch once at the sink

Question 29

- what is a co-transport protein - how do they work (with concentration gradients)

Answer 29

- carrier protein that binds to 2 molecules at a time - the concentration gradient of one molecule moves the other against its concentration gradient

Question 30

- describe how H+ ions are used to move sucrose from surrounding tissue to the phloem sieve tubes

Answer 30

1. H+ actively transported out of companion cells to tissue cells 2. H+ and sucrose bind to co-transport protein in companion cell membrane at same time 3. - H+ diffuses back into companion cell - sucrose moves into companion cell against its conc. gradient 4. Same process to move sucrose from companion cells into phloem sieve tubes

Question 31

Define transpiration

Answer 31

- evaporation of water from a plants surface e.g. leaves - occurs due to gas exchange for photosynthesis

Question 32

State 4 factors that affect rate of transpiration

Answer 32

1. Light intensity 2. Temperature 3. Humidity 4. Wind speed

Question 33

- what happens to rate of transpiration with higher light intensity - why

Answer 33

- faster rate of transpiration - more light = more photosynthesis - stomata open to allow CO2 to diffuse into leaf - due to this H2O vapour diffuses out

Question 35

State 2 similarities between the xylem and phloem vessels

Answer 35

- both are complex tissues/made of more than one cell type - xylem vessels and phloem sieve tube elements both lack nuclei

Question 36

State 4 differences between xylem and phloem vessels

Answer 36

1. XYLEM = lignified PHLOEM = not lignified 2. XYLEM = wide lumen PHLOEM = small lumen 3. XYLEM = continuous tube PHLOEM = has sieve plates, so isn’t continuous 4. XYLEM = no cytoplasm PHLOEM = has cytoplasm