Transport in Plants

Question 1

what is the need for plant transport systems

Answer 1

• need substances from the environment to live
• need to excrete substances
• small surface area to volume ratio
• high metabolic demands
• cannot use direct diffusion

Question 2

what is xylem tissue

Answer 2

• non-living tissue
• transport water and mineral ions
• no end walls allowing water to pass through
• walls are thickened with lignin to support xylem vessels and prevent collapsing
• water and ions move in and out of the vessels through small pits where there are no lignin

Question 3

what is phloem tissue

Answer 3

• living tissue
• transports food in the form or organic solutes around the plant from the leaves
• sieve tube elements
• companion cells

Question 4

what are sieve tube elements

Answer 4

• living cells that form the tube
• joined to end to end to form sieve tubes
• end walls have holes allowing solutes to pass through

Question 5

what are companion cells

Answer 5

• carries out living function for themselves and sieve cells
• plasmodesmata
• large nucleus
• dense cytoplasm

Question 6

what is the vascular bundle

Answer 6

• plants that have a specialised transport system
• xylem and phloem tissue

Question 7

what is plasmodesmata

Answer 7

pores connecting cytoplasm of neighbouring cells across cell walls

Question 8

what is the importance of water in a cell

Answer 8

• turgor pressure provides hydrostatic skeleton to support stems and leaves
• loss of water by evaporation cools the plant
• minerals and products of photosynthesis are transported

Question 9

how is water moved into the root

Answer 9

• root hair cell
• microscopic size to penetrate easily between soil particles
• large SA:V of each hair - thin surface layer for diffusion and osmosis to take place easily

Question 10

how does water move across the root

Answer 10

• apoplast pathway
• symplast pathway

Question 11

what is the symplast pathway

Answer 11

• water moves through the symplast (cytoplasm) connected by plasmodesmata

Question 12

what is the apoplast pathway

Answer 12

• water moves through cell walls and intercellular spaces
• cohesive and tension forces against the cell wall pulls the water up the plant
• continuous flow of water

Question 13

how does water move into the xylem

Answer 13

• water moves until it reaches the casparian strip in the endodermis

Casparian strip
- band of waxy material forming a waterproof layer
- all water in the apoplast pathway is forced into the symplast pathway

Question 14

how is water moved up the stem

Answer 14

• root pressure
• transpiration
• capillary action

Question 15

what is the evidence for the role of active transport in root pressure

Answer 15

• affect of cyanide - stops mitochondria from working and the production of ATP - no root pressure
• root pressure increases when temperature increases
• when oxygen and respiratory substrates fall root pressure falls

Question 16

what is transpiration

Answer 16

movement and loss of water from plants

Question 17

what is the process of transpiration

Answer 17

• water molecules evaporate from the surface of mesophyll cells into air spaces in the leaf and move out of the stomata into the air by diffusion
• loss of water by evaporation lowers the water potential of the cell
• water moves from the xylem into the mesophyll cells via pathways
• water moves out of the xylem by osmosis
• water molecules form hydrogen bonds with carbohydrates in the xylem vessels by adhesion
• water molecules form hydrogen bonds with each other and stick by cohesion
• water is drawn up by the xylem in a continuous stream to replace the water lost by evaporation
• transpiration pull results in tension in the xylem which moves water across the roots from the soil - cohesion tension theory

Question 18

what is the evidence for cohesion tension theory

Answer 18

• changes in tree diameters - transpiration at highest in the day so is tension in xylem vessels and tree shrinks in diameter
• broken xylem vessels - cut flower stems in water air is drawn in rather than water leaking out

Question 19

how does the stomata control the transpiration rate

Answer 19

• low turgor pressure cell wall closes
• favourable conditions guard cells pump in solutes by active transport increasing turgor
• cellulose hoops prevent cells from swelling in width
• water becomes scarce and hormonal signals from the roots can trigger turgor loss from the guard cells which close the stomatal pore and conserve water

Question 20

what are the facts affecting transpiration

Answer 20

Light
- low light intensity stomata are closed and high they are open (increasing diffusion of water vapour and evaporating)
- increasing light intensity increases rate of transpiration

Humidity
- high humidity lowers transpiration rate as there is a reduced water potential vapour gradient

Temperature
- increase in temperature increases kinetic energy and rate of evaporation

Air movement
- increases rate of evaporation and transpiration

Question 21

what is translocation

Answer 21

• transport system in the phloem
• transport of organic compounds from sources to sinks
• active process that requires energy

Question 22

what are sources and where are they found

Answer 22

• provide assimilates to the plants
• green leaves and stems
• storage organs - tubers
• food stores in seeds

Question 23

what are sinks and where are they found

Answer 23

• use assimilates from the plant
• growing roots / actively absorbing mineral ions
• actively dividing meristems

Question 24

what is the process of translocation

Answer 24

Phloem loading
- soluble products of photosynthesis are moved into the phloem from sources

Symplast route - passive process
- sucrose moves through cytoplasm of mesophyll cells into seive tubes by diffusion through plasmodesmata
- sucrose in sieve elements and water flows by osmosis
- creates pressure of water that moves sucrose through the phloem by mass flow

Apoplast route - active process
- sucrose travels through the cell walls and inter cellular spaces to the companion cells and sieve elements by diffusion
- in companion cells sucrose is moved into the cytoplasm

Phloem unloading
- diffusion of sucrose from the phloem to the surrounding cells

Question 25

what are xerophytes

Answer 25

• live in a low water area

Question 26

what are the adaptations of xerophytes

Answer 26

Thick waxy cuticle
- minimise water loss

Sunken stomata
- located in pits to reduce air movement producing still humid air that reduces water vapour

Reduced number of stomata
- reduces water loss and gas exchange

Reduced leaves
- reduces SA:V to minimise the amount of water loss

Hairy leaves
- create a microclimate of still humid air reducing water vapour and loss of water by transpiration

Succulents
- stores water when it is in plentiful supply and used later on

Root adaptations
- long deep roots to access water below the surface

Question 27

what are hydrophytes

Answer 27

• live partially or completely submerged in water

Question 28

what are the adaptations of hydrophytes

Answer 28

Thin / No waxy cuticle
- don’t need to conserve water

Open stomata on upper surface
- maximises gaseous exchange
- no loss of turgor - always water avaliable
- inactive guard cells

Reduced structure
- water supports the leaves and flowers

Small roots
- water diffuses directly into stems

Air Sacs
- enables leaves and flowers to float to the surface