Transport in Plants

Question 1

What are cross-walls?

Answer 1

The equivalent of sieve plates in phloem which are pores at the end of sieve tube elements. Sieve plates are a type of plasmodesmata

Question 2

What are sieve plates?

Answer 2

Plasmodesmata between sieve tube elements. They are the pores at the end of each sieve tube element

Question 3

What are assimilates?

Answer 3

Substances that have become part of the plant eg. sucrose and amino acids that are contained in sap

Question 4

Why does water flow into the phloem at the source end?

Answer 4

Because sucrose enters the sieve tube elements at the source end meaning that the water potential at the source end is decreased therefore water moves into the phloem at the source end from the surrounding tissues, by osmosis, which increases the hydrostatic pressure at the source end

Question 5

What is the source?

Answer 5

Any part of the plant which loads sucrose into the sieve tube

Question 6

When would the roots be the source?

Answer 6

Early spring when sucrose is required for growth. Energy that is stored as starch and is converted to sucrose and moved to other parts of the plant to enable them to grow

Question 7

When would the leaves be the source?

Answer 7

Late spring, summer and early autumn (whilst the leaves are green). Sugars are produced in photosynthesis and converted int sucrose and loaded into the sieve tubes. The sucrose is transported to other areas of the plant that may be growing (meristems - found in shoots) or to the roots for storage

Question 8

What is the sink?

Answer 8

Any part of the plant that removes sucrose form the sieve tubes

Question 9

Why does water leave the phloem at the sink end?

Answer 9

When sucrose is required in other areas of the plant sucrose can leave the phloem by either diffusion or active transport. When the sucrose leaves the phloem the water potential increases in the phloem and therefore it moves out of the phloem and into the surrounding tissues. This reduces hydrostatic pressure at the sink end meaning that there is a high pressure at the source end and a low pressure at the sink end so a pressure gradient is created.

Question 10

Describe the process of mass flow

Answer 10

• Sucrose is actively loaded into the phloem from the companion cells, this decreases the water potential at the source end, this causes water to flow move into the source end from surrounding tissues by osmosis which increases the hydrostatic pressure at the source end
• Sucrose is removed from the sink end by either diffusion or active transport which results in an increase in the water potential at the sink end. This means that water flows from the phloem and into the surrounding tissues so the hydrostatic pressure is lower at the sink end
• This means there is a hydrostatic pressure gradient with a high hydrostatic pressure at the source end and a low hydrostatic pressure at the sink end
• The pressure gradient causes sap to flow from the high hydrostatic pressure at the source to the low hydrostatic pressure at the sink

Question 11

What is xylem tissue made up of?

Answer 11

• Vessels to carry the water and dissolved mineral ions
• Fibres to support the xylem
• Living parenchyma cells that act as packing tissue to separate and support the cells

Question 12

How is the xylem vessel formed?

Answer 12

• As the vessels develop lignin enters the cell walls which waterproofs them but at the same time also kills the xylem vessels
• The contents of the cell decay leaving a long column of dead cell with no contents, this long column is called the xylem vessel

Question 13

What does the lignin in the cell walls of the xylem vessels do?

Answer 13

• Makes the cell walls of the vessels waterproof
• Keeps the vessels open even when water is in short supply
• When the lignin thickens it forms patterns in the cell walls
• This prevents the vessel from being too rigid and allow the stem or branch to bend

Question 14

What are bordered pits and how are they formed?

Answer 14

• Bordered pits are small gaps in the cell walls of the xylem vessels
• They are formed when lignification isn’t completed in small parts of the cell wall leaving small gaps

Question 15

What is the function of bordered pits?

Answer 15

• The bordered pits from each vessel are aligned so that water can travel from one xylem vessel to another
• The bordered pits also allow water to leave the xylem and enter other living parts of the plant

Question 16

What adaptations does xylem have to its function ?

Answer 16

• Xylem vessels can carry water from the roots to the top of the plant because:
• They’re made of dead cells aligned end to end to from a continuous column
• The tubes are narrow so the column of water isn’t easily broken and capillary action can be effective
• Bordered pits in the lignified walls allow water to pass sideways from one vessel to another
• Lignin patterns allow xylem to stretch as plant grows and allows the stem to branch or bend

Question 17

What is capillary action?

Answer 17

Movement along the surface of a solid due to the adhesion between the water and the surface

Question 18

Why sis the flow of water through xylem not impeded?

Answer 18

• There are no cross-walls between cells
• No cell contents or cytoplasm
• Lignin thickening prevents cell walls fro collapsing

Question 19

What is phloem tissue?

Answer 19

• Phloem is a tissue that is used to transport assimilates around the plant (mainly sucrose and amino acids, sucrose is dissolved in water to from sap)
• Phloem consists of sieve tubes that are made up of sieve tube elements and companion cells

Question 20

Sieve tube elements

Answer 20

• Elongated sieve tube elements are lined up end to end to form sieve tube tubes
• They contain no nucleus and very little cytoplasm
• This leaves space for the mass flow of sap to occur
• At the ends of sieve tube elements are perforated cross-walls called sieve plates, the perforations in the cross-walls allow the movement of sap from one element to the next

Question 21

What are the functions of sieve plates?

Answer 21

• Keep the lumen open
• Can carry out a mechanism to block the sieve tube after injury or infection by depositing callose which blocks the sieve tube to prevent the flow of pathogens and loss of sap

Question 22

What are companion cells?

Answer 22

• In between sieve tubes are small cells each with a large nucleus and a dense cytoplasm (a cytoplasm with contents that are closely packed together)
• These are companion cells
• Companion cells have many mitochondria to provide the energy required for active processes such as loading sucrose into the sieve tube
• Companion cells carry out the metabolic processes needed to actively load assimilates into the sieve tube

Question 23

Describe the process of active loading

Answer 23

• Sucrose is loaded into the sieve tube by an active process
• It involves use of the energy from ATP produced by mitochondria in the companion cells
• This energy is used to pump H+ ions out of the companion cells using a proton pump
• This increases their concentration outside of the companion cells and decreases their concentration inside the companion cells
• As a result a concentration gradient is created
• Hydrogen ions diffuse back into the companion cells through special cotransporter proteins that only allow the movement of H+ ions if they’re accompanied by sucrose molecules
• This is known as cotransport or secondary active transport
• As sucrose concentration increases in the companion cell, it can diffuse into through the plasmodesmata and into the sieve tube

Question 24

How does ATP release energy?

Answer 24

When it is broken down into ADP and an inorganic phosphate, energy is released