Chapter 7: Transport In Plants

Question 1

What is the name of the main transport system in plants?

Answer 1

The vascular system

Question 2

What are the two types of main vascular tissue in the vascular system?

Answer 2

The xylem and the phloem

Question 3

What are the cells in the xylem

Answer 3

• Xylem vessel element
• sclerenchyma (fibre)
• Parenchyma
• tracheid

Question 4

What are the cells in the phloem?

Answer 4

• Sieve tube element
• Companion cells
• Parenchyma
• sclereids (stone cells, sclerenchyma)

Question 5

How do xylem vessel elements join up to form the xylem vessel?

Answer 5

Xylem vessel element join end to end to form the xylem vessel

Question 6

How do the sieve tube elements form the sieve tube?

Answer 6

They join end to end

Question 7

What is the function of xylem tissues thickening?

Answer 7

• for transport of water and dissolved mineral ions

- for mechanical support

Question 8

What are the structural adaptations of the xylem vessel for transport?

Answer 8

• Hollow lumen (with no cytoplasm)
• No end walls
• lignified walls
• Arranged end to end
• Pitted

Question 9

What properties of the xylem elements aid in mechanical support of the plant structure?

Answer 9

• Lignin thickenings (5 types)
• primary cell wall is made up of cellulose
• secondary cell wall (during differentiation, lignin is formed as an additional layer beneath the primary cell wall)

Question 10

Define dicotyledonous

Answer 10

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

Question 11

Why do plants require a transport system?

Answer 11

● 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 12

What is the xylem?

Answer 12

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 13

What is the phloem?

Answer 13

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 14

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

plants.

Answer 14

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

Question 15

Relate the structure of the xylem to its function.

Answer 15

● Long, continuous columns made of dead tissue, allowing
transportation of water

● Contains pits which allow sideways movement of water
between vessels

● Thickened with a tough substance called lignin which
provides structural support

Question 16

Relate the structure of the phloem to its function.

Answer 16

● 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 17

Describe the arrangement of the vascular bundle in

dicotyledonous roots.

Answer 17

● Vascular bundle enable transport as well as structural support

● Xylem vessels arranged in 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 18

Describe the arrangement of the vascular bundle in

dicotyledonous stems.

Answer 18

● 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 is made
of meristem cells which are involved in production of new xylem
and phloem tissue

Question 19

Describe the arrangement of the vascular bundle in

dicotyledonous leaves.

Answer 19

● 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 20

Draw a labelled diagram of the
arrangement of the vascular bundle in
the root and stem of dicotyledonous
plants.

Answer 20

(see diagram in pmt notes)

Question 21

State the importance of water potential (ψ) in the

movement of water from roots to shoots.

Answer 21

● Water moves down a ψ gradient

● Roots have a high concentration of minerals. Water moves from the
soil into the root, down its ψ gradient

● ψ decreases as you move up the plant. This results in a ψ gradient,
facilitating the movement of water through the plant by osmosis

Question 22

Define transpiration.

Answer 22

● 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 23

Name the factors that affect the rate of transpiration.

Answer 23

● Increased light increases transpiration
● Increased temperature increases transpiration
● Increased humidity decreases transpiration
● Increased air movement increases transpiration
● Waxy cuticle prevents transpiration

Question 24

How can we measure transpiration rate?

Answer 24

● Using a potometer

● Plant cutting placed in a water-filled tube that
contains an air bubble. Rate of transpiration is
calculated by measuring the movement of the air
bubble over time

Question 25

Explain what is meant by the apoplastic pathway.

Answer 25

● One of 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 (impermeable layer made of suberin) and is forced through the symplastic pathway

Question 26

Explain what is meant by the symplastic pathway.

Answer 26

● 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 plasmodesmata ● To begin this pathway, water must be actively transported into cells

Question 27

Explain the cohesion-tension theory.

Answer 27

● Water molecules form hydrogen bonds with each other, causing them to ‘stick’ together (cohesion) ● 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 28

Define the term xerophyte.

Answer 28

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

Question 29

Give adaptations of xerophytes that allow them to | live in dry conditions.

Answer 29

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

Question 30

State what is meant by “source to sink”.

Answer 30

The movement of plant assimilates from a producing or storage region, the “source” (e.g. leaves) to a region of requirement, the “sink” (e.g. developing fruits).

Question 31

Define translocation.

Answer 31

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

Question 32

Summarise the mechanism of translocation.

Answer 32

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

Question 33

what is mass flow

Answer 33

the mass flow of water from the source to the sink down the hydrostatic pressure gradient is a means of supplying assimilates such as sucrose to where ​ ​ they are needed.