Unit 7 - Transport in Plants

Question 1

Define Dicotyledonous

Answer 1

Plants that produce seeds that contain two cotyledons. They have two primary leaves

Question 2

Why do plants require a transport system? [2]

Answer 2

• To ensure all cells receive a sufficient supply of the nutrients they require
• Especially important as a plant must be able to transport substances up their stem, against gravity

Question 3

What is the xylem?

Answer 3

A non living, heavily lignified plant transport vessel responsible for the transfer of water and minerals from the roots to the shoots and leaves.

Question 4

What is the phloem?

Answer 4

A living plant transport vessel responsible for the transfer of assimilates to all parts of the plant. The phloem consists of sieve tube elements and companion cells.

Question 5

State what is meant by the term “vascular bundle” in plants

Answer 5

The vascular system in dicotyledonous plants. It consists of two transport vessel, the xylem and the phloem.

Question 6

Relate the structure of the xylem to its function [3]

Answer 6

• Long, continuous columns made of dead tissue, allowing the transport of water.
• Contains pits which allow sideways movement of water between vessels.
• Thickened with a tough substance called lignin which provides structural support

Question 7

Relate the function of the phloem to its function [3]

Answer 7

• Sieve tube elements transport sugars around the plant
• Companion cells designed for active transport of sugars into tubes
• Plasmodesmata allow flow of substances between cytoplasm of different cells

Question 8

Describe the arrangement of the vascular bundle in dicotyledonous roots [4]

Answer 8

• Vascular bundle enable transport as well as structural support
• Xylem vessels arranged in the an X shape in centre of vascular bundle. This enables plant to withstand various mechanical forces such as pulling.
• X shape arrangement of xylem vessels is surrounded by endodermis, an outer layer of cells which supply xylem vessels with water
• Inner layer of meristem cells known as the pericycle

Question 9

Describe the arrangement of the vascular bundle in dicotyledonous stems [3]

Answer 9

• Xylem located on inside in non wooded plants to provide support and flexibility to stem
• Phloem found on outside of vascular bundle
• Layer of cambium in between xylem and phloem. This made of meristem cells which are involved in productions of new xylem and phloem tissue

Question 10

Describe the arrangement of the vascular bundle in dicotyledonous leaves [2]

Answer 10

• Vascular bundles form the midrib and veins of a leaf.
• Dicotyledonous leaves have a network of veins, starting at midrib and spreading outwards which are involved in transport and support

Question 11

State the importance of water potential in the movement of water from roots to shoots [3]

Answer 11

• Water moves down a water potential gradient
• Roots have a high concentration of minerals. Water moves from the soil into the root, down the water potential gradient.
• Water potential decreases as you move up the plant. This results in a water potential gradient, facilitating the movement of water through the plant by osmosis

Question 12

Define transpiration [2]

Answer 12

• The evaporation of water from the leaves of a plant via open stomata
• Consequence of gaseous exchange. Occurs when the plant opens the stomata to exchange oxygen and carbon dioxide

Question 13

Name the factors that affect the rate of transpiration [5]

Answer 13

• Increased light increases
• Increased temp increases
• Increased humidity decreases
• Increased air movement increases
• Waxy cuticle prevents transpiration

Question 14

Explain what is meant by the apoplastic pathway [3]

Answer 14

• One of the three pathways by which water and minerals move across the root
• Water moves through intercellular spaces between cellulose molecules in the cell wall
• Water reaches the casparian strip and is forced through the symplastic pathway

Question 15

Explain what is meant by the symplastic pathway [3]

Answer 15

• One of three pathways by which water and minerals move across the root
• Water enters the cytoplasm through the plasma membrane and moves between adjacent cells via the plasmodesmata
• To begin this pathway, water must be actively transported into cells.

Question 16

Explain the cohesion-tension theory [2]

Answer 16

• Water molecules form hydrogen bonds with each other, causing them to stick together
• The surface tension of the water also creates this sticking effect, preventing water slipping down the xylem. As water is lost through transpiration, more can be drawn up the stem from the roots.

Question 17

Define the term zerophyte

Answer 17

Plants that are adapted to live and reproduce in dry habitats where water availability is low.

Question 18

Give adaptations of xerophytes that allow them to live in dry conditions [5]

Answer 18

• Small/rolled leaves
• Densely packed mesophyll
• Thick waxy cuticle
• Stomata often closed
• Hairs to trap moist air

Question 19

State what is meant by source to sink

Answer 19

The movement of plant assimilates from a producing or storage region, the “source” to a region of requirement, the “sink”

Question 20

Define translocation

Answer 20

The bulk movement of organic compounds in plants from sources to sinks via the phloem

Question 21

Summarise the mechanism of translocation [5]

Answer 21

• Companion cells actively transport H+ to surrounding tissue, establishing a concentration gradient
• H+ flows down their concentration gradient via a cotransporter along with sucrose
• Sucrose builds up in companion cells and diffuses into sieve tube elements via plasmodesmata
• This lowers water potential, causing water to move in from xylem
• Assimilates move along sieve tube towards areas of lower hydrostatic pressure. Sucrose diffuses into surrounding cells where it is needed.

Question 22

Whats the empty space in a stem called?

Answer 22

Parenchyma