3.3 - C - Transport In Plants

Question 1

What are dicotyledonous plants?

Answer 1

Plants with 2 seed leaves and a branching pattern of veins in the leaf

Question 2

What is a meristem?

Answer 2

A layer of dividing cells

Question 3

Define phloem

Answer 3

Transports dissolved assimilates, such as sugars, up and down a plant

Question 4

Define vascular tissue

Answer 4

Consists of cells specialised for transporting fluids by mass flow (xylem and phloem)

Question 5

Define xylem

Answer 5

Transports water and mineral ions upwards in a plant

Question 6

Explain the cross-section (incl. vascular bundles) of a root of a plant

Answer 6

Xylem in x/+ shape
Phloem in 4 quadrants
Epidermis surrounds them (vascular tissue)
Around that is the thick cortex with an epidermis at the edge
Inside endodermis is the pericycle

Question 7

Define pericycle

Answer 7

A ring of meristem cells - found inside the endodermis in roots

Question 8

Explain the cross-section (incl. vascular bundles) of a stem of a plant

Answer 8

Vascular bundles found near outer cells of the stem
The xylem is near the inside of the bundle (closer to centre of stem), phloem on outside
In between the xylem and phloem is cambrium

Question 9

What is cambium?

Answer 9

A layer of meristematic cells capable of differentiating into new xylem and phloem vessels

Question 10

Explain the formation of vascular bundles in leaves

Answer 10

They form the veins of the leaf

The xylem is above the phloem

Question 11

Define what companion cells are

Answer 11

The cells that help load sucrose into sieve tubes

Question 12

Define what sieve tube elements are

Answer 12

They make up the tubes in phloem tissue that carry sap up and down the plant. They are separated by sieve plates

Question 13

List 5 structural aspects of xylem vessels and explain their functions

Answer 13

Continuous, hollow tubes with no end walls or contents - less resistance to flow of water and more space
Walls impregnated with lignin - strengthens wall, waterproofs wall, improves adhesion of water molecules - increases capillarity
Lignification in spiral pattern - flexibility and stretching of stem
Narrow lumen - more effective capillary action
Bordered pits (pores) in walls - can move around and avoid blockages

Question 14

List 5 structural aspects of sieve tube elements and explain their functions

Answer 14

Little cytoplasm, lots of absent organelles - less resistance and more space for transport
Siege plates - connects S.T.E to allow sucrose (as sap) through
Joined end to end to for tube - allow continuous transport
Bi-directional flow - allows sucrose to go both up and down the plant
Living - allows active processes

Question 15

List 3 structural aspects of companion cells and explain their functions

Answer 15

Many mitochondria - a lot of respiration is needed to provide lots of ATP for active processes
Nucleus - controls the functions of both the companion cell and S.T.E
Plasmodesmata - allows continuation of cytoplasm between companion cell and sieve tube element

Question 16

What is plasmodesmata?

Answer 16

Gaps in the cell wall containing cytoplasm that connects 2 cells

Question 17

List the 3 pathways

Answer 17

The apoplast pathway
The symplast pathway
The vacuolar pathway

Question 18

Explain the apoplast pathway

Answer 18

Water passes through the spaces in the cell walls and between cells. Doesn’t pass through any plasma membranes into cells. It moves by mass flow, not osmosis. This means dissolved mineral ions and salts can be carried with water

Question 19

Explain the symplast pathway

Answer 19

Water enters the cell cytoplasm through the plasma membrane. It passes through the plasmodesmata from one cell to the next

Question 20

Explain the vacuolar pathway

Answer 20

This is similar to the symplast pathway, but the water is not confined to the cytoplasm of the cells. It is able to enter and pass through vacuoles as well

Question 21

Define water potential

Answer 21

A measure of the tendency of water molecules to move from one place to another

Question 22

Define turgid

Answer 22

When a cell is full/saturated with water

Question 23

Define transpiration

Answer 23

The loss of water by evaporation out of plant’s leaves via the stomata

Question 24

What is a potometer?

Answer 24

A device that can measure the rate of water uptake as a leafy stem transpires

Question 25

List 8 factors of transpiration

Answer 25

``` Number of leaves Number and size of stomata Presence of a waxy cuticle Light Temperature Humidity Wind Availability of water ```

Question 26

Explain how and why the number of leaves affects water loss in transpiration

Answer 26

More leaves = more water loss | It’s a larger SA for water to evaporate out of

Question 27

Explain how and why the number and size of stomata affects water loss in transpiration

Answer 27

More/bigger stomata = more water loss | A larger SA allows more water to evaporate out of them

Question 28

Explain how and why the presence of a waxy cuticle affects water loss in transpiration

Answer 28

If it’s present - less water loss | It reduces water evaporating from stomata as its hydrophobic

Question 29

Explain how and why light affects water loss in transpiration

Answer 29

The lighter it is = the more water lost The stomata open wider in light to allow gas exchange for photosynthesis. If they are open there is a larger SA for water to evaporate out of

Question 30

Explain how and why temperature affects water loss in transpiration

Answer 30

Higher the temp = more water loss | More kinetic energy, water evaporates faster and water vapour diffuses out of leaves faster

Question 31

Explain how and why humidity affects water loss in transpiration

Answer 31

The higher the humidity = the less water lost When humid, air is more saturated with water. Whilst saturation in the air spaces in the leaf is still higher, there is a shallower water potential gradient for diffusion if water vapour

Question 32

Explain how and why wind affects water loss in transpiration

Answer 32

More wind = more water loss It carries water vapour that has just diffused from the leaf away, making the air immediately surrounding the leaf less saturated and maintaining a steeper water potential gradient

Question 33

Explain how and why water availability affects water loss in transpiration

Answer 33

More water in the soil = more water loss | The more water available, the more water it can lose

Question 34

List 5 things to do when setting up a potometer as to ensure the results are valid

Answer 34

Set it up under water so no air bubbles are inside it Ensure the shoot is healthy Cut the stem underwater to prevent air entering the xylem Cut the stem at an angle so a large SA is in contact with the water Dry the leaves

Question 35

How do you calculate volume in a cylinder?

Answer 35

V=pi x r^2 x length

Question 36

How do you calculate the rate of transpiration?

Answer 36

Rate = volume / time

Question 37

Explain why using a potemeter does not give an exact measure for rate of transpiration

Answer 37

Transpiration is the loss of water by evaporation from leaves. A potometer measures water uptake to replace loss. Some water may be used e.g. in photosynthesis rather than all evaporating from the leaves. Also uptake by detached shoots may not be the same as that of the whole plant

Question 38

Define adhesion

Answer 38

The attraction between water molecules and the walls of the xylem vessel

Question 39

Define cohesion

Answer 39

The attraction between water molecules caused by hydrogen bonds

Question 40

Explain water uptake and movement across the root

Answer 40

Root hair cells absorb mineral ions and water from soil via osmosis. It moves across the root cortex down a water-potential gradient to the endodermis of the vascular bundle via the apoplast pathway blocked by caspian strip.

Question 41

What drives continual osmosis into root hair cells?

Answer 41

Minerals are actively transported into the root hair cell from the soil. This decreases the water potential of the root hair cell

Question 42

List 6 adaptations of root hair cells

Answer 42

Big SA - water and minerals can be exchanged Lots of mitochondria - release energy during respiration needed for active transport Tube-like protrusion - penetrate between soil particles, reducing distance water and mineral ions need to travel Large vacuoles - have salts to speed up water absorption No cuticles - would prevent water absorption Thin walls - thin exchange surface

Question 43

What is the purpose of the casparian strip?

Answer 43

It blocks the apoplast pathway, ensuring nutrients pass through the cells

Question 44

Give 3 ways water is helped to move up xylem vessels in the stem

Answer 44

Root pressure - the push from the water entering the xylem vessels in the roots (doesn't move water far) Capillary action ‐ adhesion of water molecules to lignin in narrow xylem vessels can pull the water up the sides of the vessel Transpiration pull ‐ most of the driving force - evaporation causes cohesion and pulls up water in chains. It creates tension and proves the purpose of lignin: to keep the xylem from collapsing.

Question 45

How does water move across and out of the leaf?

Answer 45

Osmosis

Question 46

Define water potential

Answer 46

The measure of the tendency of water molecules to move from one place to another

Question 47

Why is water loss via transpiration unavoidable?

Answer 47

Gas exchange of carbon dioxide and oxygen occur through open stomata. Stomata open during the day when it is lightest and warmest. More stomata open means a larger area for water vapour to diffuse through.

Question 48

What is a xerophyte?

Answer 48

A plant that is adapted to reduce water loss by transpiration so that it can survive in very dry (arid) conditions

Question 49

List 7 adaptations of xerophytes

Answer 49

``` Epidermis covered in hair Thicker waxy cuticle Small leaves/needles Sunken stomata (in pits) Curled leaves Small air spaces in mesophyll Stomata shut in day, open in night ```

Question 50

What is a hydrophyte?

Answer 50

A plant that is adapted to living in water or where the ground is very wet

Question 51

List 3 adaptations of hydrophytes

Answer 51

Large air spaces in leaf - keeps leaf afloat so they are in the air and can absorb sunlight Stomata on the upper epidermis - exposed to the air not the water for gas exchange Leaf stem has many large air spaces - helps with buoyancy, also allows oxygen to diffuse quickly to the roots for aerobic respiration

Question 52

Explain how having the epidermis covered in hairs reduces transpiration to help xerophytes survive

Answer 52

Hairs trap water which stops the wind removing water vapour. More humid air around the leaf reduces water potential gradient - less evaporation and therefore transpiration

Question 53

Explain how a thicker waxy cuticle reduces transpiration to help xerophytes survive

Answer 53

Waxy cuticles are hydrophobic This prevents even more water passing through the epidermis of a plant Less evaporation causes less transpiration

Question 54

Explain how small leaves/needles reduce transpiration to help xerophytes survive

Answer 54

Small surface area and/or fewer stomata causes less evaporation and transpiration

Question 55

Explain how sunken stomata reduce transpiration to help xerophytes survive

Answer 55

Pits trap water vapour which stops the wind removing it. More humid air around leaves reduce water potential gradient so less evaporation and transpiration occurs

Question 56

Explain how curled leaves reduce transpiration to help xerophytes survive

Answer 56

Lower epidermis is not exposes to the atmosphere This traps and stops wind removing water vapour More humid air around the lead reduces the water potential gradient causing less evaporation and transpiration

Question 57

Explain how small air spaces in mesophyll reduce transpiration to help xerophytes survive

Answer 57

Less water can evaporate into air spaces and so the leaf quickly becomes saturated This reduces area for loss of water

Question 58

Explain how having stomata shut during the day and open at night reduces transpiration to help xerophytes survive

Answer 58

Dry/hot climates cause more evaporation and transpiration during the day whereas it’s cooler during night

Question 59

Define translocation

Answer 59

The transport of assimilates between the sources and sinks of a plant in the phloem tissue. This requires energy

Question 60

What are assimilates

Answer 60

Carbon containing compounds produced by a plant using the carbon from carbon dioxide

Question 61

What is a source?

Answer 61

Where sucrose and other assimilates are loaded into the phloem e.g. leaf

Question 62

What is a sink?

Answer 62

Where sucrose and other assimilate are unloaded from the phloem e.g. flower

Question 63

Explain how sucrose enters the phloem from the source by active loading

Answer 63

H+ ions are actively transported (requires ATP) out of the companion cells This produces a diffusion gradient for the H+ ions They move back into the companion cell via facilitated diffusion through co‐transporter carrier proteins along with sucrose Sucrose has been actively loaded into the companion cell There is a high concentration of sucrose in the companion cell compared to the sieve tube element, so it diffuses into it down the concentration gradient through the plasmodesmata

Question 64

Explain how sucrose moves along the phloem at the source

Answer 64

Sucrose is actively loaded into the sieve tube elements at the source This reduces the water potential in the sieve tube element Water enters the sieve tube elements by osmosis This increases the hydrostatic pressure in the sieve tube element near the source

Question 65

Explain how sucrose moves along the phloem at the sink

Answer 65

Sucrose is unloaded at the sink by diffusion (or active transport) and used in respiration/stored This increases the water potential in the sieve tube element Water moves into the sink via osmosis down the water potential gradient This reduces the hydrostatic pressure in the sieve tube element near the sink Water in the sieve tube element at the source moves down the hydrostatic gradient from source to sink This creates a flow which carries the sucrose and other assimilates along the phloem via mass flow either up or down the plant

Question 66

How are sieve tube elements adapted to allow mass flow to occur? (3)

Answer 66

Elongated elements, joined end to end to form a column Sieve plates with pores in end walls allow sucrose through Little cytoplasm and no nucleus ‐ less resistance to transport

Question 67

Give 3 pieces of evidence which prove phloem is used in translocation

Answer 67

Radioactively labelled carbon dioxide supplied for photosynthesis appears in phloem Aphids feeding on plant stems insert mouthparts into phloem Sugars collect above ring when tree is ringed to remove phloem

Question 68

Give 3 pieces of evidence which prove ATP is needed in translocation

Answer 68

Companion cells have many mitochondria Translocation is stopped if a poison which stops ATP production is given Flow of sugars is very high that ATP must be used ‐ much faster than would be possible with diffusion

Question 69

Give 2 pieces of evidence that prove translocation takes place

Answer 69

pH of companion cells is higher than surrounding cells (H+ ions reduce pH) Concentration of sucrose is higher in source than sink

Question 70

Give 3 pieces of evidence against translocation

Answer 70

Not all solutes in phloem move at same rate Sucrose moved to all parts of plant at same rate and doesn't go to places with lowest concentration faster The role of sieve plates is unclear

Question 71

Give 4 reasons why transpiration is important

Answer 71

It transports usefulness mineral ions up the plant It maintains cell turgidity Supplies water for growth, cell elongation and photosynthesis Supplies water that, as it evaporates, can keep the plant cool on a hot day