Transport in Plants

Question 1

Describe the function and structure of the Endodermis

Answer 1

• -> A single layer of endodermal cells surrounding the pericycle
• -> Cell walls of endodermal cells contain suberin which forms a impermeable band called the Casparian strip

Question 2

Decribe root pressure and how water moves from the apoplast pathway to the xylem

Answer 2

• -> The water potential of the endodermal cells is raised by water being forced into them by the Casparian strip
• -> The water potential of the fluid in the xylem decreases due to the active transport of sodium ions
• -> so… water flows into the xylem via osmosis down a water potential gradient which pushes water up the xylem and water already there further up the xylem

Question 3

What is cohesion-tension theory?

Answer 3

–> Describes the water movement up the xylem as being a result of a combination of the adhesion of water molecules to the vessel wall and the tension in the xylem wall as a result of cohesion

Question 4

What 4 factors affect the rate of transpiration?

Answer 4

• -> Light intensity
• -> Temperature
• -> Humidity
• -> Wind speed / Air movement

Question 5

What are the names of the 2 types of cells found in the Xylem?

Answer 5

• -> vessels

- -> tracheids

Question 6

How are the xylem and Phloem arranged in the leaves?

Why is this arrangement advantageous?

Answer 6

• -> Arranged in the midrib

- -> Helps leaf resist tearing and gives it flexibility

Question 7

How are the xylem and Phloem arranged in the roots?

Why is this arrangement advantageous?

Answer 7

• -> Xylem arranged centrally in a star shape with phloem outside it
• -> Helps anchor the plant into the soil and resists pulling forces

Question 8

How are xylem and Phloem arranged in the stem?

Why is this arrangement advantageous?

Answer 8

• -> Arranged towards periphery in a ring

- -> provides support to the stem and resists bending

Question 9

What is the function of the endodermis?

Answer 9

Regulates the movement of water, ions and hormones moving in and out of the xylem

Question 10

What is adhesion in terms of water moving up the xylem?

Answer 10

The charges of water molecules are attracted to the hydrophilic lining of the vessels in the xylem which allows water to be pulled upwards

Question 11

What is cohesion in terms of water moving up the Xylem?

Answer 11

• -> water molecules are attracted to one another and form hydrogen bonds.
• -> as water molecules leave the xylem from the leaf cells, they pull more water molecules up behind them
• -> this continuous pull produces tension in the water column

Question 12

What is the Casparian Strip?

Answer 12

• -> an impermeable band made from suberin which is waxy, strong and waterproof and found in the cell wall of endodermal cells
• -> drives the movement of water from the apoplast pathway into the cytoplasm

Question 13

Define transpiration

Answer 13

The evaporation of water vapour from the leaves or other above-ground areas of the plant, out through the Stomata into the atmosphere

Question 14

What is capillarity

Answer 14

the movement of water up narrow tubes by capillary action, but only for a short distance

Question 15

What is lignin?

Answer 15

• -> Present in tracheid cell walls in the xylem

- -> hard, strong and waterproof

Question 16

How do vessel cells in the xylem become hollow to allow water to travel up ?

Answer 16

1. the lignin builds up in their cell wall
2. the contents of the cell die and leaves an empty space called the lumen
3. the tissue develops and the end walls break down leaving a hollow tube which water climbs straight up

Question 17

How are root hair cells adapted for water intake?

Answer 17

Large surface area as there are many of them

Question 18

How does water enter the root hair cells?

Answer 18

1. The soil solution surrounding the root hair cells has a higher water potential (hypertonic) than the vacuole of the root hair cell (as it contains ions and sugars)
2. Water moves into the root hair cells via osmosis down a water potential gradient

Question 19

What is the Apoplast pathway?

Answer 19

• -> the most significant route

- -> water moves between the spaces in the cellulose cell walls in the xlyem

Question 20

What is the Symplast pathway?

Answer 20

Water moves through the cytoplasm and plasmodesmata from cell to cell
plasmodemata= strands of cytoplasm through cell wall pits

Question 21

What is the Vacuolar Pathway?

Answer 21

• -> minor route

- -> when water passes from vacuole to vacuole of cells

Question 22

What is the function of the Phloem

Answer 22

Responsible for the translocation of organic solutes , e.g. sucrose and amino acids

Question 23

How does Humidity affect the rate of transpiration?

Answer 23

• -> increasing humidity of air outside leaf reduces the water potential gradient between the inside and outside of leaf
• -> diffusion gradient decreases
• -> diffusion rate and transpiration rate decreases

Question 24

How does light intensity affect the rate of transpiration?

Answer 24

• ->Increasing light intensity increases stomatal opening
• -> stomata open wider at higher intensities
• -> Stomata at their widest during day and fully closed at night

Question 25

How does wind speed affect the rate of transpiration?

Answer 25

• -> as wind speed increases, saturated air is removed from the leaf surface more quickly so the water potential gradient between the inside and outside of leaf increases
• -> water vapour diffuses out of stomata more quickly
• -> transpiration occurs quicker

Question 26

How does temperature affect the rate of transpiration?

Answer 26

• -> Increasing temperature increases the water molecule’s kinetic energy
• -> increased rate of diffusion out of stomata into atmosphere
• -> increased transpiration rate

Question 27

How have xerophytes adapted to conserve water? (4 things)

Answer 27

• -> Sunken stomata trapping humid air and reducing the water potential gradient between the air spaces inside and outside of the leaf
• -> Hairs surrounding the stomata which trap water vapour and reduces the water potential gradient between the outside and inside of the leaf
• -> Rolled leaves to reduce the surface area over which transpiration occurs
• -> thick waxy cuticle which reduces water loss from leaf’s surface

Question 28

How have Mesophytes adapted to conserve water? (4 things)

Overwinter means live through the winter

Answer 28

• -> Stomata close if water is scarce
• -> shed leaves and become dormant during winter
• -> overwinter beneath ground as bulbs or corms
• > annual plants produce seeds that can overwinter

Question 29

How have Hydrophytes (i.e the water Lily) adapted to live in these conditions and receive adequate amounts of CO2 and light? (5 things)

Answer 29

• -> stomata on upper leaf surface rather than lower, so are in contact with the air
• -> Their stems and leaves have large air spaces providing buoyancy and Oxygen/Carbon Dioxide reservoirs
• -> poorly developed xylem tissue as there’s no need for water transport
• ->leaves have little to no cuticle
• -> support tissue not needed as water is a supporting medium

Question 30

What is a Hydrophyte? i.e Water Lily

Answer 30

A plant growing partially or fully submerged in water, so water loss is not an issue and have instead adapted to receive adequate amounts of light and carbon dioxide for photosynthesis.

Question 31

What is a Xerophyte? i.e Cactus

Answer 31

A plant adapted to live in dry conditions with little water by reducing water loss

Question 32

What is a Mesophyte? i.e Daisy

Answer 32

A plant living in regions which has an adequate supply of water, but must survive in times of the year when water is scarce or unavailable.

Question 33

Define translocation

Answer 33

The transport of soluble organic materials in a plant in the phloem. They are transported away from the site of synthesis (source) to parts of the plants (sink) to be used for storage or growth

Question 34

Describe sieve tubes and sieve tube elements in the phloem

Answer 34

• -> sieve tubes comprise of end-to-end cells called sieve tube elements
• -> sieve tube elements contain a cytoplasm, no nucleus and organelles that disintegrate during development to allow space for the transport of materials

Question 35

Describe sieve/end plates in the phloem

Answer 35

Areas in the sieve tube element where the cell walls have perforated

Question 36

Describe companion cells in the phloem

Answer 36

–> biochemically active with a dense cytoplasm, a large nucleus, many mitochondria and many rough endoplasmic reticulum for ribosomes and are connected to each sieve tube by plasmodemata

Question 37

Describe ‘Ringing Experiments’ to provide evidence for phloem transportation

Answer 37

• ->cylinders of outer bark removed all the way round a woody stem in a ring which removes the phloem
• -> after some time, as the plant photosynthesises, the phloem contents below and above the ring were analysed
• -> above the ring, there was lots of accumulated sucrose as it was translocated in the phloem but could not go down any further
• -> below the ring, there was no sucrose as the plant tissue used it and could not been replaced

Question 38

Describe ‘Radioactive Tracers’ to provide evidence for phloem transportation

Answer 38

• ->Plant photosynthesised in the presence of a radioactive isotope
• ->the stem section was placed on a photographic film which is exposed if there is a radiation source and produces a autoradiograph
• -> the position of exposure of radioactivity coincides with the the position of the phloem , showing it had transported the sucrose made with the isotope

Question 39

Describe ‘Aphid Experiments’ to provide evidence for phloem transportation

Answer 39

• -> Aphids have a needle like, hollow stylet which is inserted into a sieve tube and the phloem contents (sap) is exuded under pressure into the stylet
• -> the aphid was anaesthetised and removed, leaving only the stylet in the phloem
• -> as the sap was under pressure, it exuded from the stylet, was collected , analysed and had a presence of sucrose showing the phloem is where this material is transported

Question 40

Describe the stages of the mass flow theory (5)

Answer 40

passive process

1. photosynthesising cells (source) produce glucose which is converted to sucrose which lowers the water potential of the cells so water flows in via osmosis down a water potential gradient and the hydro static pressure forces sucrose into the sieve tubes of the phloem
2. This increase in solutes lowers the water potential of the sieve tubes so water moves in via osmosis down a water potential gradient from adjacent cells and the xylem which raises the hydro static pressure in the phloem
3. Sucrose moved by mass flow from high to low hydro static pressure to the roots and growing areas (sink) where it’s either converted to starch (storage) or glucose (respiration).
4. this loss of glucose from the phloem rises it’s water potential above that of the xylem and other cells so water enters xylem via osmosis from the phloem
5. this reduces the hydro static pressure in the phloem and water moves back up the xylem by transpiration to the source cells

Question 41

What does the mass flow theory fail to explain?

Answer 41

• -> doesn’t take into account the sieve/ end plates
• ->sucrose and amino acids move at different speeds and directions in the same tissue
• -> phloem transports 10,000 x faster than diffusion
• -> translocation inhibited by respiratory poisons and slowed at low temperatures
• -> companion cells are biochemically active with mitochondria but the mass flow theory doesn’t include them

Question 42

What are the 3 other theories that help explain what mass flow cant?

Answer 42

• -> May be an active process as low temperatures and cyanide inhibits translocation indicating it may use energy from respiration instead of being a passive process
• -> protein filaments may be used to pass through sieve tube pores so different solutes can be carried by different routes in the same tube
• -> cytoplasmic streaming may be responsible for the movement of solutes in different directions in individual elements