3.1.3 - Transport in Plants

Question 1

Why do multicellular plants need transport systems?

Answer 1

Metabolic demands, size, and SA:V

Question 2

What are dicotyledonous plants?

Answer 2

Plants that make seeds that contain two cotyledons

Question 3

What is a cotyledon?

Answer 3

An organ that acts as a food store for the developing embryo plant

Question 4

What is the vascular system?

Answer 4

A series of transport vessels running through the stem, roots, and leaves

Question 5

What are the two main types of transport vessels?

Answer 5

Xylem and phloem

Question 6

How are the transport tissues arranged?

Answer 6

In vascular bundles

Question 7

Describe the structure of the xylem

Answer 7

Non-living tissue. Xylem vessels are the main structures, and they are long, hollow structures made by several columns of cells fusing together end to end. The walls of the xylem vessels are lignified in spirals or non-joined circles. There are also parenchyma packing around the xylem vessels, storing food. The unlignified parts of the xylem vessel are called bordered pits, and it is through these that the water leaves the xylem

Question 8

What does the xylem transport?

Answer 8

Water and mineral ions

Question 9

What else does the xylem tissue provide?

Answer 9

Support to the plant

Question 10

What is the phloem?

Answer 10

Living tissue that transports food in the form of organic solutes around the plant

Question 11

Describe the structure of the phloem

Answer 11

The main transporting vessels are sieve tube elements. Phloem sieve tubes are made of many cells joined end to end to form a long, hollow structure. In the areas between the cells, walls are perforated to form sieve plates. Companion cells are closely linked to sieve tube elements by plasmodesmata. Companion cells act as a life support system for the sieve tube elements.

Question 12

How does water enter the root?

Answer 12

Root hair cells on the root actively transport mineral ions from the soil into the root, decreasing the water potential inside the root. This causes water to enter the root via osmosis

Question 13

How are root hairs adapted to their function?

Answer 13

• They are small so they can penetrate between soil particles
• Large SA:V
• Lots of mitochondria

Question 14

What are the two ways water can move across the root?

Answer 14

The symplast and the apoplast pathways

Question 15

Describe the symplast pathway

Answer 15

Water moves through the continuous cytoplasm of living cells that is connected through the plasmodesmata.

Question 16

Describe the apoplast pathway

Answer 16

Water moves through the cell walls and the intercellular spaces. Water is pulled along by cohesive forces.

Question 17

What happens when the water reaches the endodermis?

Answer 17

Water from the apoplast pathway joins the symplast pathway, as the Casparian strip prevents water in the apoplast pathway from continuing through.

Question 18

Why is the diversion of water to the cytoplasm important?

Answer 18

In order to reach the cytoplasm, the water must pass through the plasma membrane, which prevents any potentially toxic solutes in the water from reaching living tissue

Question 19

What happens once the water is inside the vascular bundle?

Answer 19

Water returns to the apoplast pathway to enter the xylem itself and move up the plant.

Question 20

What is root pressure?

Answer 20

The active pumping of minerals into the xylem to produce movement of water by osmosis

Question 21

What is transpiration?

Answer 21

The loss of water vapour from leaves and stems of plants

Question 22

What is the transpiration stream?

Answer 22

The movement of water through the plant and eventually out

Question 23

How does the transpiration stream work?

Answer 23

• Water molecules evaporate from the surface of mesophyll cells into the air spaces in the leaf and exit via diffusion
• Loss of water lowers the water potential of the mesophyll cell, so water moves into the cell from an adjacent one via osmosis
• This repeats until the xylem is reached. Water moves out of the xylem by osmosis
• Water molecules form H bonds with the xylem vessels. This is adhesion. They also form H bonds with each other. This is cohesion. The combined effects result in water exhibiting capillary action

Question 24

What is capillary action?

Answer 24

The process by which water can rise up a narrow tube against gravity. Water is drawn up the xylem in a coninuous stream to replace the water lost. This is the transpiration pull

Question 25

What does the transpiration pull do?

Answer 25

Creates a tension in the xylem, results in water moving across the roots from the soil.

Question 26

What is the model of water moving up the plant in a continuous stream called?

Answer 26

The cohesion tension theory

Question 27

What factors affect transpiration?

Answer 27

Light, humidity, temperature, wind

Question 28

What is translocation?

Answer 28

The movement of organic compounds in the phloem from source to sink

Question 29

What is a source?

Answer 29

An area of the plant that produces more assimilates than it uses

Question 30

What is a sink?

Answer 30

An area of the plant that uses more assimilates than it produces

Question 31

What are assimilates?

Answer 31

The products of photosynthesis

Question 32

Describe the active loading of the phloem via the apoplast pathway

Answer 32

In the companion cells, sucrose is moved into the cytoplasm across the cell membrane in an active process. H+ ions are actively pumped out of the companion cell into the surrounding tissue using ATP. H+ ions return to the companion cell with sucrose via cotransport. This increases the sucrose concentration of the companion cells and so sucrose can then enter the sieve tube elements via the plasmodesmata.

Question 33

What does the increase in sucrose concentration in the sieve tube cause?

Answer 33

Water moves in by osmosis. This increases hydrostatic pressure. The sucrose then moves down the pressure gradient to the sink, in a process called mass flow

Question 34

Describe the unloading of the phloem

Answer 34

Sucrose is unloaded from the phloem at any point the cells need it. The sucrose diffuses from the phloem into the surrounding cells

Question 35

What happens after the sucrose is unloaded?

Answer 35

The water potential in the sieve tube increases, so water re-enters the xylem via osmosis.

Question 36

What are xerophytes?

Answer 36

Plants that are adapted to live and reproduce where water availability is very low.

Question 37

Describe ways xerophytes conserve water

Answer 37

Thick waxy cuticle, sunken stomata, reduced stomata, reduced leaves, hairy leaves, curled leaves, succulents, leaf loss, widespread shallow roots, deep roots

Question 38

What are hydrophytes?

Answer 38

Plants that are adapted to living in water

Question 39

Describe ways hydrophytes are adapted to their environment

Answer 39

Very thin or no waxy cuticle, many always-open stomata on the upper surfaces, reduced structure, wide and flat leaves, small roots, air sacs, aerenchyma

Question 40

What are aerenchyma and what do they do?

Answer 40

Specialised parenchyma tissue. They have many large air spaces, which makes the leaves and stems more buoyant, as well as form a low-resistance internal pathway for the movement of substances like oxygen to tissues below the water.