Transport in plants

Question 1

The absorption of water by the root

Answer 1

Water can be absorbed passively into the cell wall which acts as a sponge
Water can move into the cell cytoplasm through osmosis as it has a lower water potential so water passes into the root air cell by osmosis down a water potential gradient

Question 2

Adaptations of root hair cells

Answer 2

Thin cellulose cell wall - short diffusion path
Large number of root hair each with a long extension into soil - increased surface area for absorption

Question 3

Symplast route

Answer 3

Water moves from the cytoplasm of one cell to the cytoplasm of adjacent cell by moving through the plasmodesmata linking the cells
Slower as obstructed by organelles

Question 4

Apoplast pathway

Answer 4

Water moves in the cell walls. Cellulose fibres in the cell wall are separated by spaces through which the water moves.
This pathway is the fastest due to less resistance because there’s no membrane

Question 5

The vaculoar pathway

Answer 5

Water moves from vacuo to vacuole
Slowest as there is resistance to move across membranes

Question 6

How is water moved into the xylem by osmosis?

Answer 6

There is a water potential gradient across the root cortex
It is highest in the root hair cells and lowest in the xylem so water moves down the water of potential gradient across the root

Question 7

Casperian strip

Answer 7

The endodermis cell walls are impregnated with Suberin forming a distinctive band called the Casperian strip
Suberin is hydrophobic so the Casperian strip prevents water moving further in the apoplast.
Water and the dissolved minerals it contains leave the apoplast and enter the cytoplasm before they move further across the route

Question 8

Why does water move from the root endodermis into the xylem across the endodermal cell membranes?

Answer 8

Increased hydrostatic pressure in root endodermal cells pushes water into the xylem
Decreased water potential in the xylem below that of the endodermal cells draws water in by osmosis across endodermal cell membranes

Question 9

Uptake of minerals

Answer 9

Mineral ions can also move along the apoplast pathway in solution
When they reach the endodermis, the Casperian strip prevents further movement so the mineral ions enter the cytoplasm by active transport and then diffuse or actively transported into the xylem

Question 10

The movement of water from roots to the leaves

Answer 10

Water always moves down a water potential gradient
The air has a very low water potential and the soil water has a very high water potential.
So water moves from the soil through the plant into the air

Question 11

The three main mechanisms for the movement of water from the roots to the leaf

Answer 11

Cohesion tension
Capillarity
Root pressure

Question 12

Cohesion

Answer 12

As water molecules leave the xylem cells in the leaf by transpiration they pull up other water molecules behind them in his xylem
The water molecules all move because they show cohesion
Cohesion is the attraction of water molecules for each other seen as hydrogen bonds resulting from the dipole structure of the water molecule

Question 13

Adhesion

Answer 13

Attraction between water molecules and hydrophilic molecules in the cell walls of the xylem

Question 14

Cohesion tension theory

Answer 14

Describes water movement up the xylem by this combination of adhesion of water molecules and tension in the water columns resulting from their cohesion

Question 15

Capillarity

Answer 15

The movement of water up narrow tubes in this case the xylem by capillary action

Question 16

Root pressure

Answer 16

The upward force on water in roots derived from osmotic movement of water into the root xylem

Question 17

Transpiration

Answer 17

The evaporation of water vapour from the leaves and other above ground parts of the plant out through the stomata into the atmosphere

Question 18

Transpiration stream

Answer 18

The continual flow of water in at the roots up the stem to the leaves and out to the atmosphere
Caused by cohesion and adhesion

Question 19

Loss of water by transpiration

Answer 19

The stomata must be open during the day to allow gas exchange between the leaf tissues and the atmosphere.
But this means that the plant loses valuable water
This gives rise to the transpiration stream

Question 20

Factors affecting the rate of transpiration

Answer 20

Temperature
Humidity
Wind speed
Light intensity

Question 21

Temperature

Answer 21

As temperature increases the kinetic energy of molecules, including water vapour increase
As a result, molecules will diffuse and evaporate faster and osmosis will take place at a higher rate

Question 22

Humidity

Answer 22

If the air contains small water vapour (is more humid) this decreases the water potential gradient and fewer water molecules can diffuse out of the leaves

Question 23

Wind speed

Answer 23

As wind increases water vapour on the surface of the leaf is blown away
The water potential gradient between the inside and outside of the leaf increases so there is a higher rate of diffusion of water molecules

Question 24

Light intensity

Answer 24

If higher the rate of photosynthesis in the guard cells of stomata increase and are more likely to open
The more stomata that open the higher the rate of diffusion of water molecules from the leaf

Question 25

Three angiosperms

Answer 25

Mesophytes Xerophytes Hydrophytes

Question 26

Mesophytes

Answer 26

E.g. rose Plants that live in habitat where there is sufficient water available for their survival They flourish with adequate water supply but need to survive unfavourable times of the year by shedding leaves surviving underground or as dormant seeds

Question 27

Hydrophytes

Answer 27

Plants that live in or on water and have a plentiful supply of water at all times e.g. Waterlily. They have little need for support and transport tissues have little to no cuticles and the stomata are only present on the surface of their leaves

Question 28

Xerophytes

Answer 28

Plants that live in areas of low water availability for example a desert or sand dunes e.g. marram grass Rolled leaves - this reduces the leaf area exposed to air and so reduces transpiration Sunken stomata - this reduces the water potential gradient between the inside of the leaf and the outside and so reduces the rate of diffusion of water out through the stomata Hairs - Steph interlocking has trapped water vapour and reduces the water potential gradient between the inside of the leaf and the outside Thick cuticle - wax is waterproof and so reduces water loss. The thicker this cuticle the lower the rate of transpiration through the cuticle.

Question 29

The two main types of cells in this xylem

Answer 29

Vessels and tracheids

Question 30

Tracheids

Answer 30

Occur in flowering plants but not in mosses Spindle shaped water conducting cells in the xylem

Question 31

Vessels

Answer 31

Occur only in angiosperms As lignin builds up in their cell walls, the contents die leaving an empty space, the lumen As the tissue develops the end walls of the cells breakdown leaving a long hollow tube like a drain pipe through which water climb straight up the plant The lignin is laid down in a spiral pattern

Question 32

What are the two functions of xylem?

Answer 32

Transport of water and dissolved minerals Providing mechanical strength and support

Question 33

Structure of the phloem

Answer 33

A living tissue and consists of several types of cells, including sieve tubes and companion cells

Question 34

Sieve tubes

Answer 34

Sieve tubes are the only components of flow obviously adapted for the flow of material. They compromise end to end cells called sieve tube elements. The end walls do not break down.

Question 35

Sieve tube elements

Answer 35

Sieve tube elements lose their nucleus and most of their other organelles during their development, allowing space for transporting materials

Question 36

Cytoplasm filaments

Answer 36

Cytoplasm filaments containing phloem protein extend from one sieve tube element to the next through the pores in the sieve plate

Question 37

Companion cells

Answer 37

Companion cells are biochemically very active as indicated by the large nucleus, dense cytoplasm containing much rough endoplasmic rectum and many mitochondria. They are connected to the sieve tube elements by plasmodesmata

Question 38

Translocation

Answer 38

The movement of the soluble products of photosynthesis such as sucrose and amino acids These products of photosynthesis are synthesised in the leaves (the source). They are translocated in the phloem to the other parts of the plants, the sinks, where they are used for growth and storage

Question 39

Mass flow hypothesis

Answer 39

The sucrose makes the water potential very negative and water passes into the cells by osmosis As water enters the leaf, hydrostatic pressure builds up forcing sucrose in solution into the phloem joining the source to the sink The pressure pushes the sucrose solution down the phloem and this movement is called mass flow It is pushed into the sink where sucrose is removed by: - Respiration - Stored as starch - Converted to cellulose The increased pressure forces water out of the sink into the xylem, bringing water back to the source

Question 40

Mass flow summary

Answer 40

Passive mass flow of sugar sugars from the flow of the leaf where there is the highest concentration (source), to other areas such as growing tissues where there is a lower concentration (the sink)

Question 41

Evidence against mass flow

Answer 41

- The rate of translation is 10,000 times faster than it would be if the substances were moving by diffusion - sieve plates with tiny pores act as a barrier impending flow - Sucrose and amino acids move at different rates and in different directions in the same phloem tissue.

Question 42

Four alternative theories

Answer 42

Puncture experiments Ringing experiments Aphid stylet experiment Radioactive tracer experiments

Question 43

Puncture experiments

Answer 43

If the phloem is punctured, the sap oozes out so there is a high-pressure inside and if the xylem is punctured, then air is sucked in showing that there is a low pressure in this xylem

Question 44

Ringing experiments

Answer 44

If a ring is cut around the outside of a stem, this holds transport in the phloem that allows transport in this xylem to continue Outcome - demonstrates that the phloem is responsible for the translocation of sugars in plants.

Question 45

Aphid stylet experiments

Answer 45

If the stylet is surgically separated from the aphid, the phloem will continue to flow out of the stylet

Question 46

Radioactive tracer experiments

Answer 46

Shows that organic compounds are transported downwards from the leaf to the roots This is done by radioactive isotopes which can be used to trace precisely where different compounds are being transported from two as well as measuring the rate of transport Outcome - radioactive compounds only detected in phloem not xylem

Question 47

Potometer

Answer 47

A simple potometer is a piece of capillary tubing to which a plant has been connected. The water uptake is measured by recording the time taken for a bubble in the tube to move a set distance.

Question 48

Difference between the volume of water taken up by the shoot and the volume lost on transpiration

Answer 48

The volume of water taken up by the shoot is very slightly higher than the volume lost on transpiration because a small volume of water: is used in photosynthesis is used to keep cells turgid Evaporates through the cuticle