Transport In Plants

Question 1

Why do multicellular plants need transport systems

Answer 1

• they have a high surface area to volume ratio so they cannot rely on diffusion alone
• metabolic demands, oxygen and glucose needs to be transported to cells that don’t photosynthesise. Hormones and mineral ions will also need to be transported
• size, plants need effective transport systems to move substance up and down from the tip of the roots to the topmost leaves and stems

Question 2

What is a herbaceous dicotyledonous plant

Answer 2

Are plants that contain two organs that act as food stores, with soft tissue and a short life cycle.

Question 3

What is the vascular system is in dicotyledonous plant

Answer 3

Is a series of transport vessels running through the stem, roots and leaves. In herbaceous dicotyledonous plants it is made up of the xylem and phloem.

Question 4

What does the xylem do

Answer 4

Transports mineral ions and water from the roots to the shoots and leaves. They also provide structure and support.

Question 5

What does the phloem do

Answer 5

Transports food in the form of organic salutes around the plant. For example it supplies amino acids and sugars for respiration and photosynthesis

Question 6

4 types of cells that make up the xylem tissue

Answer 6

• xylem vessels
• tracheids
• parenchyma
• fibres

Question 7

Xylem structure

Answer 7

• made of hollow tubes joined end to end with lignified walls
• spirals of lignin run around the lumen of the xylem, reinforcing the wall
• xylem has bordered Pits

Question 8

4 types of cells that make up the phloem

Answer 8

• sieve tube element
• companion cell
• parenchyma
• fibres

Question 9

Structure of phloem

Answer 9

• companion cell they maintain their nucleus and all their organelles. They act as a life support for the sieve tube cells.
• sieve plates
• sieve tube element are living cells that form the tube for transporting food through the plant. Each sieve tube has a companion cells and defined by two sieve plates.

Question 10

Define transpiration

Answer 10

Is the loss of water vapour from the leaves and stems of a plant, mainly through the stomata of the leaves. The movement of water from roots to shoots to leaves is called the transpiration stream.

Question 11

How is water transported up the stem to the leaves

Answer 11

• water evaporates from the spongy mesophyll into the air spaces of the leaf.
• water vapour then diffuses out of the leaf via the stomata
• water moves from adjacent cells into the spongy mesophyll by osmosis down a water potential gradient
• this creates tension in the xylem this is know as transpiration pull
• within the xylem vessels columns of water are held together by cohesion and adhesion
• the column of water is pulled up the xylem by transpiration pull, capillary action and root pressure

Question 12

Evidence for cohesion and tension theory

Answer 12

Tree rings and broken xylem vessel

Question 13

Tree rings

Answer 13

When transpiration is highest during the day, tension in the xylem is at its highest too. As a result the tree shrinks in diameter. At night when transpiration is lowest, tension is lowest and diameter of the tree increases.

Question 14

Broken xylem vessel

Answer 14

In most circumstances air is drawn into the xylem rather than water leaking out. Once air is pulled in the plant can no longer move water up the stem as the continuous stream of water molecules held together by a cohesive force is broken.

Question 15

Factors affecting the rate of transpiration

Answer 15

• light intensity
• relative humidity
• temperature
• soil water availability
• air movement

Question 16

How does light intensity affect transpiration

Answer 16

The lighter it is the more transpiration that occurs because stomata’s open up when it gets light and close when dark.

Question 17

How does relative humidity affect transpiration

Answer 17

A high humidity results in a lower rate of transpiration as there is a shallower water potential gradient between the leaf and outside air.

Question 18

How does temperature affect transpiration

Answer 18

Warmer molecules have more energy, so water cells evaporate faster out of the spongy mesophyll into the air spaces of the leaf.

Question 19

How Soil water availability affects transpiration

Answer 19

The amount of water available in the soil affects transpiration rate. When there is less water, the plant will be under more water stress and rate of transpiration decreases.

Question 20

How does air movement affect transpiration

Answer 20

Increasing air movement increases transpiration as a steeper diffusion gradient of water is created. This is because there is less water vapour around the stomata when there is increased air movement.

Question 21

How do stomata control the rate of transpiration

Answer 21

When turgor pressure is low guard cells close the pore, so transpiration decreases. However when guard cells are turgid cell walls swell and open up the pore. This allows water to exit and the plant to transpire.

Question 22

Root hair cell adaptations

Answer 22

• Microscopic in size means they can penetrate easily between soil particles
• Large surface area to volume ratio, lots of them
• Thin surface layer, short diffusion distance, only cell wall and cell membrane
• Concentration of solutes in root hair cells maintain a water potential gradient between the root and soil.

Question 23

3 ways for water to travel across the root into the xylem

Answer 23

Apoplast pathway
Symplast pathway
Vacuolar pathway

Question 24

Apoplast pathway

Answer 24

• movement of water through the cell wall and intercellular spaces
• the walls of young roots and corhical cells are made from porous cellulose
• cohesive forces between water molecules create a tension which ensures a continuous flow of water
• water moves osmotically from high wp to low wp, this is the fastest route

Question 25

Symplast pathway

Answer 25

- water moves through the cytoplasm from one cell to the next - cytoplasms are connect by plasmodesmata - water moves osmotically from a cell with high wp to a cell of low wp - process continues till water reaches the xylem.

Question 26

Vacuolar pathway

Answer 26

- water enters the vacuole of a root hair cell - this raises it water potential - water passes to the next vacuole by cell wall, cell surface membrane, cytoplasm, vacuole - this is the slowest route

Question 27

What is the Casparian strip

Answer 27

- blocks the pathway of the apoplast - this is because each endodermal cell has a layer of Suberin a fatty waterproof substance in cell walls - so water from the apoplast is forced into they cytoplasm, joining the symplast pathway. But first they have to travel through the cell surface membrane . - the cell surface membrane can remove toxic salutes from the soil and only necessary water and minerals can enter

Question 28

Xerophytes

Answer 28

Are plants adapted to live in dry environment and are adapted to conserve water by reducing water loss and increasing there ability to absorb water. An example would be cacti and marrem grass.

Question 29

Xerophytes adaptations

Answer 29

- Thick waxy cuticle - Reduced number of stomata - Reduced leaves - Extensive shallow root system - Hairy leaves to trap humid air and reduce the diffusion gradient - Curled leaves to create a microclimate of humid air

Question 30

Hydrophytes

Answer 30

Are plants that live in aquatic environments and need special adaptations to cope with growing in water or in permanently saturated soils. They need adaptations to cope with low oxygen levels. An example of a hydrophyte is lilies.

Question 31

How are hydrophytes adapted

Answer 31

- many open stomata on the upper surface - reduced structure of plant as the water supports it - wide flat leaves, a high surface area to capture as much light - very thin or no waxy cuticle as they don't need to conserve water

Question 32

Define translocation

Answer 32

The transportation of assimilates from the source to sink. It is an active process where ATP is needed to create a pressure difference.

Question 33

What are the two types of phloem cells that are important in transporting organic substances

Answer 33

Sieve tube element and companion cell

Question 34

How are companion cells adapted to carry out its function

Answer 34

- Has many infoldings in the cell membrane to increase surface area for active transport of sucrose into the cytoplasm - Has many mitochondria to provide large amounts of ATP

Question 35

Examples of sources in terms of translocation

Answer 35

Green leaves and stems Storage organs such as tubers and tap roots Food stores in seeds

Question 36

Examples of sinks in terms of translocation

Answer 36

Roots and meristems as they are both active. | Meristems are actively dividing while roots require active transport.

Question 37

What is the mass flow hypothesis - how does sucrose get into the sieve tube element

Answer 37

- Hydrogen ions are pumped out of the companion cell by proton pump. This creates a high concentration outside the cell. - H+ Ions and sucrose bind to co-transporter proteins but at a different sites - H+ ions diffuse back into the companion cell bringing sucrose via co-transporter proteins - The sucrose diffuses into the sieve tube element from a high concentration to low concentration

Question 38

Mass flow hypothesis - how does sucrose travel down the sieve tube element and into sinks

Answer 38

- With there being a higher concentration of sucrose, water potential decreases, so water enters the sieve tube element by osmosis from the xylem. - This creates a high hydrostatic pressure and water moves down a hydrostatic pressure gradient. - Sucrose is the unloaded at the sink by diffusion and this increases water potential. - Therefore water moves out by osmosis, lowing hydrostatic pressure and maintaining a hydrostatic pressure

Question 39

Evidence for mass flow theory

Answer 39

- Advances in microscopy: allow us to see adaptations of companion cells for active transport - When mitochondria in the companion cells are poisoned, translocation stops - Rate of flow of sugar: rate will be much slower if it relied on diffusion alone, this suggests active transport - Aphids: aphids penetrate plant tissue to reach the phloem. A positive pressure forces sap out of the stylet. Concentration of sucrose is high closes to the source than the sink.

Question 40

How are guard cells adapted

Answer 40

They have unevenly thickened cell wall so that they can change shape