Xylem And Phloem

Question 1

Two main types of xylem tissue

Answer 1

Xylem vessel and xylem fibres

Question 2

Xylem vessels

Answer 2

Xylem vessels start as a series of plant cells running up the stem from the roots to the leaves- at a certain point the carbohydrate lignin forms within the cell walls- lignin is impermeable- the living contents of the cells die and end walls between cells break down.

-regions of cell wall remain free of longing- pits- allow water and dissolved substances to pass between vessels.

Question 3

What happens if a xylem vessel is blocked or damaged

Answer 3

Water can move through pits to different vessels- pits also allow water to move out of the xylem, like to cells in leaves

Question 4

Lignin

Answer 4

Arranged in spirals or rings, continuous apart from pits

-supports structure of xylem vessel, water is pulled up xylem vessels, this causes pressure in vessels to fall slightly, lignin prevents collapse.

Question 5

Xylem fibres

Answer 5

Formed from long narrow cells, very large amounts of lignin form in these cells- interior contents of cells die

-xylem fibres are not used to transport water- provide mechanical support for the plant.

Question 6

What does the xylem also contain

Answer 6

Parenchyma cells- can act as store of starch- parenchyma cells can also contain tannins which are bitter compounds which deter herbivores from eating the plant.

Question 7

What is an assimilate

Answer 7

-Glucose used to form other compounds like sugars and amino acids-assimilates

Question 8

Job of phloem

Answer 8

Transport assimilates from leaves to other parts of the plant like roots or flowers- up or down

Question 9

What’s the fluid moving in the phloem called

Answer 9

Phloem sap

Question 10

Is phloem living or dead

Answer 10

Living

Question 11

Two types of tissue in phloem

Answer 11

Sieve tube element cell- long line of cells arranged end to end, almost all organelles have been lost- interior mostly free to transport sap- end walls of cells have been modified to contain large pores- sieve plate

Companion cells- contain nucleus and lots of mitochondria, microscopic channels link the companion cells to the sieve tube element cells-plasmodesmata

Question 12

What’s a sieve plate

Answer 12

Modified end wall of sieve tube element cell- allow phloem sap to move between cells

Question 13

What are plasmodesmata

Answer 13

Microscopic channels which link the companion cells to the sieve tube element cells. Molecules such as ATP and proteins can move through the plasmodesmata into the sieve tube element cells

Question 14

Role of the companion cell

Answer 14

To provide essential molecules to the sieve tube element cells

Question 15

Do phloem tubes contain lignin in their cell walls

Answer 15

No

Question 16

Two types of phloem tissue which provide support

Answer 16

• Fibres and sclereids- both have thickened cell walls
• fibres are long and narrow whereas sclereids have a variety of shapes.

Question 17

Where do root hairs grow from

Answer 17

Cells in the epidermis of the root- water move in to the root hair by osmosis

Question 18

How are root hair cells adapted

Answer 18

-densely packed root hairs massively increase the SA:V ratio of the root

-The surface of the root hair consists only of the cell wall and cell membrane, makes surface extremely thin increasing rate of osmosis.

-water potential inside the root hair cell is lower than in the soil- mineral ions in root.

Question 19

How does water from root hair cells get to the xylem

Answer 19

Through the root cortex

Question 20

What’s the symplast pathway in the cortex

Answer 20

-water moves from the cytoplasm of one cell to the cytoplasm of an adjacent cell, to do this water moves through the plasmodesmata linking the cells

-The symplast pathway is driven by the water potential gradient between the root hair cells and the xylem, water continually moves into the root hair cells by osmosis from the soil

-symplast pathway is relatively low-pathway for water in the cytoplasm is obstructed by the organelles

Question 21

What’s the apoplast pathway in the cortex

Answer 21

-water moves within the cell walls and the spaces between the cells

-the cellulose cell walls have a relatively open structure allowing water to move easily between the cellulose fibres.

-apoplast pathway offers much less resistance to water flow than the symplast pathway

Question 22

What’s cohesion

Answer 22

Water molecules are attracted to each other- water molecules can form hydrogen bonds to each other- as water moves into the xylem and is carried away, more water moves along the apoplast pathway due to cohesion.

Question 23

What does water have to pass through before entering xylem

Answer 23

-Layer of cells called the endodermis- a band of waterproof material called Suberin runs around the cell wall-casparian strip

-because of casparian strip water can no longer move through the apoplast pathway, instead water passes through the cell membrane and into the cytoplasm- becoming part of the symplast pathway

-by forcing all water into cytoplasm, this allows the cell membrane to control which substances can enter the xylem

Question 24

What’s the casparian strip

Answer 24

Where cells contain a band of waterproof material called Suberin which runs around the cell wall- in endodermis

Question 25

Where is active transport used in the endodermis - root pressure

Answer 25

Cells in endodermis use active transport to pump mineral ions into the xylem- lowers water potential of xylem, triggering water to move into the xylem vessels by osmosis- root pressure

-active process requiring respiration

Question 26

What’s the surface of a leaf covered with

Answer 26

Waterproof layer called the waxy cuticle- reduces water loss from the surface of the leaf by evaporation

Question 27

What are the tiny pores on the lower surface of the leaf called

Answer 27

Stomata- open when the plant photosynthesises- CO2 in and O out.

Question 28

What are the surface of the cells in the leaf covered with

Answer 28

A thin layer of water-evaporates from surface of cells- internal leaf spaces contain a high concentration of water vapour.

-generally the level of water vapour in external air is relatively low - water vapour travels out of leaf

-this evaporation of water followed by diffusion of water out of the leaf is transpiration.

-conc of water vapour in cells starts to decrease, causes water to move by osmosis from adjacent cells, lowers water potential of these cells causing water to move into them, at some point this reaches the xylem, with water passing out of the xylem to adjacent cells.

-so when transpiration is taking place water is continually being pulled out the xylem vessels

Question 29

What is tension

Answer 29

-when transpiration is taking place water is continually being pulled out the xylem vessels- this pulling effect is tension.

Question 30

What’s the transpiration stream

Answer 30

The movement of water from the roots, up the xylem and out of the leaf

Question 31

What can water also form hydrogen bonds to- adhesion

Answer 31

Molecules in the xylem vessel walls like carbohydrates- adhesion.

Question 32

Effect of cohesion and adhesion-capillary action

Answer 32

Water can move up very thin tubes against the force of gravity - water moves up xylem vessels by capillary action to replace water lost by the xylem vessels.

Question 33

What is transpiration pull

Answer 33

water moves up xylem vessels by capillary action to replace water lost by the xylem vessels.

Question 34

What’s the whole process of transpiration called

Answer 34

Cohesion-tension theory

Question 35

Two pieces of evidence which support the cohesion tension theory

Answer 35

• If a plant stem is cut, then air is sucked into the xylem, suggesting xylem vessels are under tension. However the air prevents cohesion between the water molecules, so water movement stops.

-If we measure the diameter of a tree trunk we can see that this reduces when transpiration is at its maximum, this supports the idea that transpiration pull is generating a negative pressure or tension in the xylem.