Topic 4.4 Transport in Plants

Question 1

Xylem (GCSE)

Answer 1

Xylem transports water and minerals from the root to the leaves (dead cells).

Question 2

Phloem (GCSE)

Answer 2

Phloem transports sucrose and amino acids up and down the plant (source to sink).

Question 3

Cambium

Answer 3

-Unspecialised cells
-Divide by mitosis
-Differentiate into specialised cells
-Lead to formation of xylem and phloem

Question 4

Structure and functions of xylem

Answer 4

-Movement of water
-Support
-Contain pits, allowing water to move sideways between vessels
-Thickened with a tough substance, providing structural support

Question 5

Development of xylem: Start

Answer 5

-It starts off as living tissue
-The first xylem to for is protaoxylem
-Capable of stretching and growing as walls aren’t fully lignified

Question 6

Development of xylem: Strengthening

Answer 6

-Stem strengthens, cellulose fibrils are laid more or less vertically
-Stem can withstand the weight of the plant
-As the stem ages, cells become impermeable and die
-Tissue becomes stronger and more supportive
-Lignin amounts increase (metaxylem - lignified tissue)

Question 7

Development of xylem: Ageing

Answer 7

-As the stem ages, cells become impermeable and die
-Tissue becomes stronger and more supportive
-Lignin amounts increase (metaxylem - lignified tissue)
-End walls between cells break down so xylem forms hollow tubes running from the roots to the tip of the stems and leaves

Question 8

Evidence for xylem: Eosin dye

Answer 8

-A cut end of a shoot is placed in eosin dye
-Eosin dye is a transported with water in the xylem
-When sections of tissue are examined under a light microscope, eosin is clearly seen in the xylem only

Question 9

Evidence for xylem: Ringing experiments

Answer 9

-Ringing experiments involve killing a ring with a steam jet.
-This destroys living phloem cells but not the xylem cells.
-Eosin dye placed in the water shows that the upward movement of water through the plant is unaffected.

Question 10

Evidence for xylem: autoradiography

Answer 10

-If the plant is provided with water containing radioactive isotopes, these can be traced by autoradiography as they move through the plant
-The movement of minerals in the xylem can be followed in the same way

Question 11

Structure of the phloem

Answer 11

-Mature phloem is living tissue
-Transports food in the form of organic solutes and around the plant from the leaves where they are made by photosynthesis to where they are needed
-Can be transported both up and down the stems
-Sieve tube elements transports sugars around the plant
-Companion cells deigned for active transport of sugars into tubes
-Cytoplasms linked by plasmodesmata, allowing flow of substances between cells.

Question 12

Transpiration

Answer 12

Movement of water and minerals up the xylem

Question 13

Roots

Answer 13

-Water enters root hair cells by osmosis from the soil
-Very large SA:Vol ratio
-Water moves from root hair cell through more root cells to the xylem by: symplast pathway, apoplast pathway

Question 14

Symplast pathway

Answer 14

-Water moves by diffusion down the concentration gradient
-From root hair cells to the xylem
-Through the interconnected cytoplasm (symplast) of the cells in the root system
-It moves through gaps in the cellulose cell walls that allow strands of cytoplasms to pass through them
-The contents of the two cells are in contact

Question 15

Apoplast pathway

Answer 15

-Water is pulled across adjacent cell walls (apoplast) from the root hair cell to the xylem
-Up to half of the volume of cell wall can be filled with water
-Water entering the root hair from the soil has mineral ions dissolved in it and they are drawn up the apoplast pathway too
-The water moves across the cells of the roots in the cell walls until it reaches the epidermis which contains a waterproof layer called the casparian strip

Question 16

Mass transport in xylem

Answer 16

1. When he humidity of the atmosphere is less than that of the air spaces next to the stomata, the water vapour molecules diffuse out of the air spaces into the surrounding air.
2. Water lost is replaced by water evaporating from the cell walls of the surrounding mesophyll cells.
3. This is replaced by water reaching the mesophyll cells from the xylem.
4. A column of water is pulled up the xylem because water molecules form hydrogen bonds between one another and hence tend to stick together.

Question 17

Translocation in plants

Answer 17

-The substances transported in the phloem are called assimilates.
-Sucrose from sources is loaded into the sieve tube elements and transported to the sinks.
(Scientists are still not entirely clear about the process of translocation; research continues).

Question 18

Describe how the structure of the xylem vessels allows them to transport water

Answer 18

-Hallow/no cytoplasm
-Vessels have no end walls/are open at ends
-Vessels have pits/are strong so don’t collapse
-Lignin makes the walls waterproof

Question 19

How does lignin add strength to xylem tissue?

Answer 19

-Lignin holds the microfibrils
-Lignin keeps microfibrils parallel

Question 20

The cohesion-tension theory

Answer 20

Water molecules form hydrogen bonds with each other, causing them to ‘stick’ together (cohesion). The surfaces tension of water also creates this sticking effect. Therefore as water os lost through transpiration, more can be drawn up the stem from the roots.

Question 21

How does root pressure affect water movement?

Answer 21

High mineral content gives the root a low water potential, meaning there is strong osmotic flow into the roots. This creates a weak push effect, moving water from the roots into the stem.

Question 22

How does temperature affect rate of transpiration?

Answer 22

A higher temperature increases evaporation, increases amount of water vapour before saturation (increases concentration gradient) therefore increasing rate of transpiration.
(Eventualy there will be a limiting factor).

Question 23

How does light affect rate of transpiration?

Answer 23

A higher light intensity increases rate of photosynthesis, causing more stomata to open for gas exchange, therefore increasing rate of transpiration.

Question 24

How does humidity affect rate of transpiration?

Answer 24

High humidity means the water content of the air next to the leaf is high. This reduces the concentration gradient, therefore decreasing rate of transpiration.

Question 25

How does air movement affect rate of transpiration?

Answer 25

Large amounts of air movement blow moistures air away from the leaves, creating a steep concentration gradient. Therefore increases rate of transpiration.

Question 26

Mass-flow hypothesis of translocation

Answer 26

-Sugar loaded into sieve tubes via active transport
-Lowers water potential, causing water to move in from the xylem
-Hydrostatic pressure causes sugars to move

Question 27

Evidence for mass-flow hypothesis

Answer 27

-Sap is released when a stem is cut, therefore there must be pressure in the phloem
-There is a higher sucrose concentration in the leaves than the roots

Question 28

phloem loading

Answer 28

1.sympast pathway - sucrose moves by diffusion down a conc gradient into companion cells through cytoplasm and into seive cells then water moves by osmosis into seive tube causing positive hydro static pressure - passive
2.apoplast pathway - moves by diffusion down a conc gradient through cell wall to companion cell across membranes by active transport producing a high conc in the cytoplasm of companion cells , passes into seive tubes through plasmodesmata. water then moves due to os producing positive hydrostatic presssure

Question 29

parenchyma

Answer 29

thin walls of cellulose

Question 30

collenchyma

Answer 30

cell walls with thickened areas of additional cellulose

Question 31

sclerencyma

Answer 31

have lignified cell walls
2 types - sclereids and fiber (long and thin) cells
support

Question 32

guttation

Answer 32

when transpiration rates are low and drops of water are forced out of the leaves

Question 33

sources

Answer 33

site of sugar production

Question 34

sinks

Answer 34

where sugar is used or stored

Question 35

aerenchyma

Answer 35

when parenchyma is modified to create tissues with air spaces for buoyancy or aeration of tissues,

Question 36

How are sugars stored after being transported around the plant?

Answer 36

• Glucose is converted to sucrose for transporting around the plant in the phloem
• Then converted back to glucose or starch for storage

Question 37

The differences between the structures of xylem and phloem

Answer 37

• Xylem cell walls contain cellulose and lignin; phloem walls only contain cellulose.
• Xylem is hallow/no end walls; phloem ha sieve plates.
• Xylem has pits; phloem doesn’t.
• Xylem is dead; phloem has companion cells.