3.1.3 Transport in Plants

Question 1

What cell types do the phloem tissue contain?

Answer 1

• phloem fibres
• phloem parenchyma
• sieve tube elements
• companion cells

Question 2

What is the movement of water from roots to leaves called?

Answer 2

• the transpiration stream

Question 3

Describe cohesion, tension and adhesion of water

Answer 3

cohesion and tension:

• water evaporates from the leaves at the top of the xylem (transpiration)
• this creates tension (suction) which pulls more water into the leaf
• water molecules are cohesive, so when some are pulled others follow
• this means the whole column of water in the xylem moves upwards
• water enters the stem through root cortex cells

adhesion:
- as well as being attracted to each other, water molecules are attracted to the walls of xylem vessels
- this helps water to rise up through the xylem vessels

Question 4

What is active loading?

Answer 4

• active loading is used to move substances into the companion cells from surrounding tissues, and from the companion cells into the sieve tubes, against a concentration gradient
• the concentration of sucrose is usually higher in the companion cells than the surrounding tissue cells and higher in the sieve tube cells than the companion cells
• so sucrose is moved using active transport and co-transporter proteins

Question 5

Why do plants need transport systems?

Answer 5

• plants need water, minerals, sugars
• direct diffusion is too slow
• small surface area : volume ration

Question 6

What is transpiration?

Answer 6

• The loss of excess water by diffusion through the stomata of leaves of a plant into the atmosphere is called transpiration.

Question 7

Describe characteristics of hydrophilic plants

Answer 7

• air spaces om the tissues help the plants to float and act as a store of oxygen for respiration
• e.g. water lilies have large air spaces in their leaves, allowing them to float, increases amount of light received. air spaces in the roots and stems allow oxygen to move from the leaves down to the underwater part
• stomata are usually only present on the upper surface pf floating leaves. maximises gas exchange
• often have flexible leaves and stems. they are supported by water around them. flexibility helps prevent damage by water current s

Question 8

Why is transpiration due to gas exchange

Answer 8

• a plant needs to open stomata to let in co2 so it can produce glucose by photosynthesis
• but this also lets water out as there is a higher conc. of water inside leaf than outside so water moves down the water potential gradient when stomata are open

Question 9

What are hydrophilic plants

Answer 9

• plants that are adapted to live in water

- they don’t need adaptation to reduce water loss

Question 10

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

Answer 10

• by changing the dissolved substances at the sink

- this makes sure theres always a lower conc at the sink

Question 11

Describe companion cells

Answer 11

• lack of a nucleus and other organelles in sieve tube elements mean they can’t survive on their own
• therefore, theres a companion cells for every sieve tube element
• carry out living functions for both themselves and their sieve cells
• provide energy for active transport of solutes

Question 12

Describe the symplast pathway

Answer 12

• goes through the living parts of cells, the cytoplasm
• cytoplasms of neighbouring cells connect through plasmodesmata
• water moves through the symplast pathway via osmosis

Question 13

Describe the transport of water through the xylem

Answer 13

• xylem vessels transport water all around the pant
• at the leaves, water leaves xylem and moves into cells mainly by apoplast pathway
• water evaporates from cell walls into the spaces between cell in the leaf
• when the stomata open, the water diffuse out of the leave into the surrounding air, down the water potential gradient
• this is called transpiration

Question 14

What is the source and sink

Answer 14

• translocation moves substances from source and sink
• source is where substance is made (high conc)
• sink is where it is used up ( low conc)

Question 15

What are xerophytic plants?

Answer 15

• they are adapted to reduce water loss by transpiration

- live in dry climates

Question 16

What are the four factors that affect transpiration rate?

Answer 16

• light: lighter it is the faster the transpiration rate. stomata open when it gets light, so Co2 can diffuse into the leaf. stomata is closed when dark, so little transpiration
• temperature: higher the temp, faster the transpiration rate. warmer water molecules have more energy so they evaporate from the cells inside the leaf faster. this increases the water potential gradient between the inside and outside the lead, making water diffuse out of the lead faster
• humidity: lower the humidity, faster the transpiration rate. if air around the plant is dry, the water potential gradient between the leaf and the air is increased, which increases transpiration
• wind: the windier it is, the faster the transpiration rate. lots of air movement blows away water molecules from around the stomata. this increases the water potential gradient, increases the rate of transportation

Question 17

What is the phloem tissue?

Answer 17

• mainly transports sugars both up and down the plant

Question 18

Describe the characteristics of xerophytic plants

Answer 18

• marram grass has stomata that are sunk in pits, so they’re sheltered from the wind. this slows transpiration
• also has a layer of hairs on epidermis, traps most air around the stomata, which reduces the water potential gradient between leaf and the air, slowing transpiration
• in hot orw tiny conditions, marram grass roll their leaves, trapping moist air, slowing transpiration. reduces the exposed surface area for losing water and protects stomata from wind
• both marram grass and cacti have a thick, waxy layer on the epidermis. this reduces water loss by evaporation because the layer is waterproof
• cacti have spines instead of leaves, reduces surface area for water loss
• cacti also close their stomata at the hottest times of the day

Question 19

Describe the movement of water in terms of water potential

Answer 19

• water always moves from areas of high water potential to areas of low water potential, going down a water potential gradient
• soil generally has high water potential, plant lower
• therefore, a water potential gradient that kee[s water moving through the plants from rights to leaves

Question 20

Describe the apoplast pathway

Answer 20

• goes through the non-living parts of the cells, the cell walls
• the walls are very absorbent and water can diffuse through them, as well as pass through spaces between them
• water can carry solutes and moves from areas of high hydrostatic pressures to low hydrostatic pressure
• an example of mass flow
• when water in the apopa-last pathway gets into the endodermis cells in the root, its path is blocked by a waxy strip in the cell walls, called the Casparian strip
• now water has to take the symplast pathway
• water has to pass through a cell membrane, which control whether substances in water can enter or not as it is partially permeable membrane
• once past this barrier, water moves into the xylem

Question 21

How does water reach the xylem?

Answer 21

• plants get water from soil, through the root and into the xylem to be transported
• they water through root hair cells, then passes through root cortex, including endodermis to reach xylem
• water is drawn into roots via osmosis
• it traves down a water potential gradient

Question 22

What are the two paths that water can travel through the roots (root cortex) into the xylem

Answer 22

• symplast pathway

- apoplast pathway

Question 23

What is the xylem tissue?

Answer 23

• xylem tissue transports water and mineral ions in solution
• move up the plant from the roots to the leaves

Question 24

What mechanism move water against gravity?

Answer 24

• cohesion, tension and adhesion

Question 25

Describe the theory of mass flow hypothesis

Answer 25

• active transport is used to actively load the solutes into the sieve tubes of the phloem at the source (e.g. leaves)
• this lowers the water potential inside the sieve tubes, so water enters the tubes by osmosis from the xylem and the companion cells
• this creates a high pressure inside the sieve tubes at the source end of the phloem
• at the sink end, solutes are removed from the phloem to be used up
• this increases the water potential inside the sieve tubes, so water also leaves the tubes by osmosis
• this lower the pressure inside the sieve tubes
• the result is a pressure gradient from the source end to the sink which pushes solutes along the sieve tubes when needed

Question 26

Describe the process of active loading

Answer 26

• in the companion cell, ATP is used to actively transport hydrogen ions (H+) out of the cell and into surrounding tissues cells
• this sets up a concentration gradient. there are more H+ ions in the surrounding tissue than in the companion cell
• an H+ ion binds to the co-transport protein in the companion cell membrane and re-enters the cell
• a sucrose molecule binds to the co-transport protein at the same time. the movement of the H+ ion is used to move the sucrose molecule into the cell, against its concentration gradient
• sucrose molecules are then transported out of the companion cells and into the sieve tubes by the same process
• the breakdown of ATP supplies the initial energy needed for the active transport of the H+ ions

Question 27

Which is the main pathway water travels?

Answer 27

• apoplast

- it has the least resistance

Question 28

How does the xylem support the plant in different parts?

Answer 28

root:
- xylem is in the centre surrounded by phloem to provide support for the root as it pushes through the soil

stem:
- xylem and phloem are near the outside to provide scaffolding to reduce bending (phloem in outside, xylem on inside)

leaf:
- xylem and phloem make up a network of veins which support think leaves

Question 29

can a plant be both a source and a sink?

Answer 29

• yes

Question 30

Describe sieve tube elements

Answer 30

• these are living cells that form the tube for transporting solutes through the plant
• they are joined end to end to form sieve tubes
• the ‘sieve’ parts are the end walls, which have lots of holes to allow solutes to pass
• unusually for cells, sieve tube elements have no nucleus, a very thin cytoplasm and few organelles
• cytoplasm of adjacent cells is connected through holes in sieve plates

Question 31

What is translocation?

Answer 31

• the movement of dissolved substance to where they’re needed in a plant
• dissolved substances are sometimes called assimilates
• an energy-requiring process the that happens in the phloem

Question 32

What makes up the plant’s vascular system?

Answer 32

• xylem and phloem

Question 33

How are xylem vessels adapted for their function?

Answer 33

• they are very long, tube-like structures formed from cells (vessel elements) joined end to end
• there are no end walls, making an uninterrupted tube that allows water to pass up through the middle easily
• cells are dead, so contains no cytoplasm
• walls are thickened with lignin, helping to support the xylem vessels and stops them collapsing inwards
• amount of lignin increases as cell gets older
• water and ions move into and out of vessels through small pits in walls where there’s no lignin