3.1.3 Transport in plants

Question 1

Why do plants need transport systems

Answer 1

• in order to meet metabolic demands
• effectively move substances up and down and to compensate for relatively small surface area to volume ratio

Question 2

What are the four types of xylem cells that make up xylem tissue

Answer 2

• tracheids - water conducting
• vessel elements - water conducting
• parenchyma - food storage
• sclerenchyma - strength and support

Question 3

What is the function of thick lignified walls in xylem

Answer 3

adds strength to withstand hydrostatic pressure so vessels dont collapse

Question 4

What is the function of non lignified pits in xylem

Answer 4

allows for lateral movement of water and continual flow in case of air bubbles

Question 5

What is the function of mature xylem cells having no end plates

Answer 5

allows for the mass flow of water and dissolved solutes as cohesive/adhesive forces aren’t impeded

Question 6

What is the function of xylem cells having no protoplasm

Answer 6

(cells are dead when mature)
- doesn’t impede mass flow

Question 7

What is the role of the phloem

Answer 7

to transport assimilates (organic compounds) from source to sink

Question 8

What are the three types of cells which make up phloem tissue

Answer 8

• sieve tube elements
• companion cells
• parenchyma cells

Question 9

What is the function of sieve plates having sieve pores

Answer 9

allows for continuous movement of organic compounds

Question 10

What is the function of cellulose cell walls in sieve tube elements

Answer 10

strengthens the walls to withstand hydrostatic pressure that moves with assimilates

Question 11

What is the function of sieve tube elements having no ribosomes, vacuole or nucleus

Answer 11

maximises space for translocation of assimilates

Question 12

What is the function of thin cytoplasm in sieve tube elements

Answer 12

reduces friction to facilitate movement of assimilates

Question 13

What is the role of companion cells

Answer 13

• controls metabolism of associated sieve tube member
• role in loading and unloading of sugars into phloem

Question 14

What is the function of the nucleus and other organelles being present in companion cells

Answer 14

provides metabolic support to sieve tube elements and helps unloading of assimilates
- active transport etc

Question 15

What is the function of transport proteins in the plasma membrane in companion cells

Answer 15

moves assimilates in and out of sieve tube elements

Question 16

What is the function of large numbers of mitochondria in companion cells

Answer 16

provides ATP for active transport of assimilates out and in of companion cells

Question 17

What is the function of the presence of plasmodesmata in companion cells

Answer 17

acts as a link to sieve tube elements which allows compounds to move between the two

Question 18

Give 5 comparisons between xylem tissue and phloem tissue

Answer 18

living/dead cells
substances transported
direction of flow
presence of end walls
cell wall material (lignin present in xylem)

Question 19

How does the phloem protect itself in the case of injury/infection

Answer 19

sieve tubes are blocked by callose which limits cell sap loss and limits pathogen entry

Question 20

Give three features of dicotyledonous (dicot) plants

Answer 20

• seeds that contain two cotyledons (seed leaves)
• network of veins
• leaves that typically have broad leaf surface and petioles (stems)

Question 21

Give four adaptations of root hair cells

Answer 21

large surface area - increased rate of osmosis
- thin cell walls - shorter diffusion difference
- lots of mitochondria for active transport
- large vacuole with lots of minerals and cell sap - lowers water potential which maintains concentration gradient

Question 22

Describe the symplastic route

Answer 22

water and minerals move across cell membrane by osmosis
water moves through root cortex cells and endodermis through plasmodesmata
minerals are actively transported into xylem, lowering the water potential in the xylem, so water moves into the xylem through osmosis

Question 23

How do water and minerals move into the xylem from the roots

Answer 23

minerals are actively transported into xylem, lowering the water potential in the xylem, so water moves into the xylem through osmosis

Question 24

Describe the apoplastic route

Answer 24

water and minerals diffuse into epidermis cell walls and diffuse through cortex cell walls
waters meets casparian strip at endodermis
- waxy impermeable barrier made of suberin
water is forces onto symplastic pathway and into xylem

Question 25

Describe how water moves up the xylem

Answer 25

cohesion-tension theory - cohesion between water molecule and adhesion between water molecules and xylem walls due to hydrogen bonding results in continuous moving column of water water moves up xylem by mass flow down a hydrostatic pressure gradient

Question 26

When are guard cells open

Answer 26

open at night closed at midday and in hot weather

Question 27

Give two features of guard cells

Answer 27

lots of vacuoles to control opening and closing thick inner walls and thin outer walls so can bend to open more easily

Question 28

Give six factors affecting transpiration

Answer 28

- temperature - air movement - humidity - light intensity - stomatal density - surface area

Question 29

How does temperature affect rate of transpiration

Answer 29

increased rate of evaporation water has higher kinetic energy eventually, stomata will close to limit water loss

Question 30

How does air movement affect rate of transpiration

Answer 30

rate of transpiration increases airflow removes water vapour surrounding leaf, maintaining concentration gradient

Question 31

How does humidity affect rate of transpiration

Answer 31

rate of transpiration decrease higher water potential outside the leaves in the air water potential gradient is weakened

Question 32

How does light intensity affect rate of transpiration

Answer 32

rate of transpiration increases more water is needed in leaves for photosynthesis rate plateaus when light intensity is no longer the limiting factor

Question 33

What are xerophytes

Answer 33

plants that are adapted to dry and arid conditions

Question 34

Give 5 adaptations of xerophytes that make them well adapted to dry conditions

Answer 34

fleshy succulent leaves - act as a water store leaves curl/fold - reduced transpiration due to reduced surface area stomata only found in upper epidermis - open into the humid space created by hairs reduced numbers of stomata - reduces water loss thick waxy cuticle

Question 35

What are hydrophytes

Answer 35

plants that are adapted to living in freshwater - main concern is carbon dioxide in the day and oxygen at night e.g. water lily

Question 36

Give four common adaptations found in hydrophytes

Answer 36

leaves are thin, flat and are buoyant - closer to surface so increased photosynthesis thin waxy cuticle - little need to prevent water loss stomata located on upper surface of leaves - allows for gas exchange to occur with air instead of water reduced veins in leaves - xylem is significantly reduced as no need for water transport throughout plant

Question 37

Describe how sieve tube element cells are loaded

Answer 37

H+ ions are pumped out of companion cells creating a concentration gradient H+ ions diffuse back into companion cells down concentration gradient carries sucrose through a co-transporter protein this creates a concentration gradient of sucrose between the companion cell and phloem sucrose diffuses into adjacent sieve tube element through plasmodesmata

Question 38

Describe the mass flow hypothesis

Answer 38

active loading of sucrose into sieve tube elements decreases the water potential in the sap in the phloem water moves down a water potential gradient from the xylem to the phloem this increases the hydrostatic pressure in the phloem at sources sucrose diffuses out of the phloem into the sink, increasing the water potential in the phloem water moves out of the phloem into the xylem, reducing the hydrostatic pressure phloem sap moves from source to sink down a hydrostatic pressure gradient

Question 39

Describe the method for investigating the rate of transpiration

Answer 39

attach shoot to capillary tube and connect tube to a beaker of water set up environmental factor (hairdryer for airflow etc) leave for a set period of time and record end location of air bubble

Question 40

Give some methods of preventing failure of rate of transpiration practical

Answer 40

cut shoot underwater use vaseline to ensure apparatus is airtight - prevents air from entering xylem or apparatus