plants

Question 1

Why do plants transport water?

Answer 1

Maintain turgidity.
Transport nutrients across plant.
Create aqueous environments for reactions to occur. 
To cool plants by evaporation.
Photosynthesis.

Question 2

What is being transported around the plant in what direction ?

Answer 2

Water is taken up from the soil, through the roots by osmosis.
Water and ions are transported UP the plant in xylem vessels.
Water is lost in the leaves through transpiration.

CO2 enters the leaves through diffusion
CO2 used to make sucrose
Sucrose transported in phloem vessels up and down the plant .

Question 3

Why do plants need a specialised control system?

Answer 3

To move the products of photosynthesis (H20 and O2) around from their sit of origin.
Most plants are large and therefore require substances to be transported over large differences.
The plant has a small overall SA:Vol ratio.

Question 4

How is water taken in?

Answer 4

Water enters the root by osmosis . For this to occur, the water potential of the soil must be higher. In order for this to be achieved:

• Ions pumped INTO root hair cell by active transport.
• This makes the water potential of the soli greater than of the roots.
• Water enters root hair cell by osmosis.

Question 5

What is the pathway of the water once entered the root?

Answer 5

Root hair –> Epidermis –> Cortex –> Endothermis –> Pericycle

Question 6

What are the apoplast and symplast pathways?

Answer 6

Apoplast pathway - water travels through cellulose wall. It moves into the endodermis once it reaches the CASPARIAN STRIP.
The symplast pathway - water travels in the cytoplasm, moving from cell to cell by the plasmodesmata.

Question 7

What is the Casparian strip?

Answer 7

A ring of SUBERIN that is IMPERMEABLE to water.

Question 8

How does water move from the endodermal cells?

Answer 8

Salts are actively pumped into they xylem which means it has a lower water potential so water enters by osmosis. The pressure can also push water into xylem vessels and a few metres up the stem.

Question 9

What are the three processes involved in movement of water up the stem and through the leaf?

Answer 9

• Root pressure
• Capillarity
• Cohesion tensions theory.

Question 10

What is root pressure?

Answer 10

the active pumping of minerals into the xylem by root cells that produces a movement of water into the xylem by osmosis.

Question 11

What is the evidence for root pressure?

Answer 11

Cyanide is a METABOLIC POISON and inhibits AEROBIC RESPIRATION and therefore the release of ATP.
Root pressure disappears when water no longer moves into the xylem by osmosis.
Also, cutting a stem low down, it will exude sap.
For it to be forced out there must be root pressure as water from the xylem is leaving

Question 12

What are the problems with root pressure?

Answer 12

Root pressure not great enough to push water all the way to the top of the plant.
Water will still move up the plant if roots are removed.

Question 13

What is capillarity?

Answer 13

Cohesive forces between water molecules and adhesive forces between water molecules and walls of xylem vessels. The adhesion causes the water to be pulled up the vessels and the cohesion keeps the water molecules together.

Question 14

What evidence is there for capillarity?

Answer 14

• It explains why xylem consists of bundles of very narrow vessels.
• A greater height of liquid is achieved in a thinner tube. Greater surface areas of vessel walls mean greater adhesive pressure.

Question 15

What is a limitation of capillarity?

Answer 15

The maximum height achievable in a xylem vessel is one metre.

Question 16

What is cohesion-tension theory?

Answer 16

CO2 enters the leaf through the stomata to be used for photosynthesis. As a consequence, water vapour is lost due to TRANSPIRATION.
Water molecules evaporate from the surface of mesophyll cells into air spaces in the leaf and move out by diffusion, down a water potential gradient.
This process is repeated across the leaf, as water moves by osmosis to adjacent cells until it reaches the xylem. Water leaves the xylem by osmosis and water is pulled up in a continuous stream due to cohesion and adhesive forces.
This is TRANSPIRATION PULL.
It results in tension in the xylem which helps move water across the root from the soil.

Question 17

What is the evidence of cohesion-tension theory.

Answer 17

The relationship between water flow in the xylem and the diameter of the tree - diameter of the tree decreases as transpiration rate increases.
The tree is wider at night as less transpiration is taking place.
This is because there is more heat and wind during the day which makes water evaporate off of the mesophyll cells than at night.
Stomata are also open during the day but closed at night to prevent water escaping.
When a xylem vessel is broken:
- Air is drawn into the xylem vessels, rather than leaking out.
- Air breaks the column of water so adhesive forces stop. The plant can no longer move water up the xylem vessels and the cohesive forces have been broken.

Question 18

What is translocation?

Answer 18

Transport of assimilated in the phloem.

Question 19

How does sucrose enter the phloem?

Answer 19

• Companion cells actively pump H+ ions into surrounding mesophyll cells.
• H+ ions diffuse actively diffuse back into companion cells, co-transporting sucrose.
• Sucrose diffuses into sieve tube elements, down a concentration gradient.

Question 20

How is sucrose moved in the phloem?

Answer 20

• When sucrose enters the phloem, the water potential of the sieve tube elements is lowered.
• Water enters from surrounding cells and the xylem, down a water potential gradient by osmosis.
• This forces the contents of the phloem to flow. This is MASS FLOW.

Question 21

How does sucrose leave the phloem?

Answer 21

• Sucrose and other assimilates leave the phloem by diffusion into cells.
• The cells maintain a concentration gradient

Question 22

Summary equation for photosynthesis:

Answer 22

6CO2 + 6H20 –> C6H12O6 + 6O2

Question 23

What do plant cells obtain from H20 and CO2?

Answer 23

• Lipids
• Proteins
• Carbohydrates
• nucleic acid

Question 24

What additional elements are taken from the soil and what are they used to produce?

Answer 24

Phosphorus and nitrogen make nucleic acids and proteins.

Question 25

How do stomata open?

Answer 25

Cells surrounding the guard cells actively pump K+ ions into the guard cells. This lowers their water potential.
This causes water to enter by osmosis.
The guard cells swell but because their inner wall is thicker than the outer wall so a pore opens.

Question 26

How to stomata close?

Answer 26

K+ ions diffuse out of the guard cells and back to the epidermal cells.
Lower water potential in epidermal cells so water moves out by osmosis.
Guard cells become flaccid and the pore closes.

Question 27

How would a plant conserve water?

Answer 27

Waxy cuticle = water impermeable so water cannot be lost from upper palisade mesophyll cells.
Stomata on underside of leaf = less exposed to sunlight and wind which causes water loss.
Closable stomata = stomata are closed in low light as photosynthesis cannot take place. Water cannot leave through stomata.
Roots grow down to water in soil = gain access to more water, replaces water lost constantly.

Question 28

What is a xerophyte and where would on grow?

Answer 28

A plant which needs very little water to survive, such as cacti. They grow in hot, dry, breezy conditions such as dessert.

Question 29

What adaptions do xerophytes have?

Answer 29

Thick waxy cuticle= prevent water loss cos water impermeable
Sunken stomata= reduces air movement, produces micro-climate of still air that reduces the loss of water vapour by reducing water potential gradient and reducing transpiration.
Reduced number of stomata= reduces water loss by transpiration.
Reduced leaves= less water loss by evaporation – lowers SA:V ratio.
Hairy leaves= Create micro-climate of still, humid air (see above).
Curled leaves= Confines stomata within a micro-climate.
Succulents = Store water in specialized parenchyma tissues in the stem and root and is used in times of draught.
Root adaptions
Long tap roots = grow several metres into ground to access water
Mass of widespread, shallow roots = large surface area, absorbs all water before it evaporates.
Leaf loss = loses leaves in times of draughts to decrease loss of water through evaporation.
Avoid problems by becoming dormant (or die completely leaving seeds to germinate) or surviving as storage organs.

Question 30

What is a hydrophyte?

Answer 30

Plants that live in water such as water lilies. They face problems with water-logging as air spaces in leaves cannot be filled with water.

Question 31

What adaptions do hydrophytes have?

Answer 31

Very think or no waxy cuticle = water loss by transpiration not an issue.
Many stomata that are always open and on upper surface = maximise gaseous exchange.
Reduced structural support = Water supports leaves and flowers.
Wide, flat leaves = they spread out across the surface and catch as much air as possible.
Small roots = Water diffuses directly into stem and leaf
Large surface area of stems and roots under water = maximises area for photosynthesis and for oxygen to diffuse into submerged plants.
Air sacs = enable leaves and/or flowers to float.
Aerenchyma = this is specialized parenchyma tissue. Makes leaves and stems buoyant and forms a low-resistant internal pathway for movement of oxygen to tissues below water. Helps to cope anoxic conditions (no dissolved oxygen).

Question 32

What factors effect transpiration?

Answer 32

Light intensity – increase in light intensity increases the number of open stomata and this increases the rate in which water vapour diffuses out as is more frequently evaporates as it moves down a concentration gradient.
Humidity – this is a measure of water vapour in the air compared to the total concentration of water vapour that the air can hold.
Temperature – increase kinetic energy of water molecules and therefore increased in evaporation, so more water is lost. Also increases concentration of water vapour the air can hold. Relative humidity is therefore increased, as is the water potential gradient.
Air movements – Increases rate of evaporation as wind maintains the water potential gradient.

Question 33

What are important conditions of setting up a potometer?

Answer 33

Stem must be woody, must fit the tubing, must be assembled underwater, stem must be cut underwater, water on the underside of leaves must be blotted off.

Question 34

What is further evidence for phloem being used for translocation?

Answer 34

Removing a ring of bark which only removed phloem, not xylem, severely interrupted the flow of organic solutes down to the roots. Shows how the loss of the phloem is also the loss of the means to transport organic solutes.
When mature leaf is exposed to radioactive CO2 (14CO2), it can produce soluble carbohydrates containing 14C. When an autoradiograph of the cross section of the stem was examined, 14C was large restricted to the phloem. This shows how organic solutes are transported in the phloem.
A combination of electron microscopy and autoradiography has demonstrated that sucrose and amino acids are transported in the sieve tube elements rather than in any other cells of the phloem.
A feeding aphid may injects it’s mouth parts into the phloem. When the body is removed and the moth parts remain, the fluid forced out of the tubes from the mouth parts contain carbohydrates and amino acids. The solute concentrations are higher when in conditions that favour photosynthesis.