2. Transport through the root

Question 1

Why is water needed in plants

Answer 1

• Mineral ions & sugars are transported in aqueous solution
• Water is a raw material of photosynthesis
• Cooling effect (by transpiration)
• Turgor pressure - hydrostatic skeleton

Question 2

Simple overview of water movement through plants

Answer 2

1. Water uptake near root tips (HIGH WATER POT)
2. Water enters xylem
3. Water moves up xylem
4. Water moves from xylem to leaf cells
5. Evaporation of water into leaf air spaces
6. Transpiration of water vapour through open stomata into air (LOW WATER POT)
   (see s7)

Question 3

What is water potential

Answer 3

The tendency for water molecules to move within & between cells
Water moves from region of HIGHER water pot to region of LOWER water pot

Question 4

plasmolyzed. flaccid. turgid

Question 5

What has the highest water potential

Answer 5

Pure water

Question 6

How is water potential of a solution LOWERED

Answer 6

Presence of solutes lowers the water pot of a solution

Question 7

What does having water potential of 0 mean

Answer 7

Maximum water potential
- Therefore all solutions have a water pot less than 0 (-ve number)

Question 8

Uptake of water process

Answer 8

A passive process
By osmosis

Question 9

Uptake of minerals

Answer 9

Can be passive or active
By diffusion or active transport (respectively)

Question 10

How does water potential inside the cell affect movement of water

Answer 10

• If water pot inside the cell is low (higher salt concent) water will move in by osmosis
• If water pot inside the cell is high (low salt concent) water will move out by osmosis

Question 11

What happens in low external water potential

Answer 11

Water moves out of the cell
- Plants can survive this for short periods, as they can shrink the cell membrane away from the cell water
- The cell is said to be plasmolysed

Question 12

s14 for dia of movement of water from root hair to xylem

Question 13

Adaptations of root hair cells

Answer 13

• Very thin surface layer (wall & plasma membrane) - so diffusion & osmosis can happen quickly
• Microscopic in size - meaning they can penetrate easily between soil particles
• Large SA:V ratio
• Concentration of solutes in the cytoplasm of root hair cells maintains a water potential gradient between the soil water & the cell

Question 14

see s18… for structure of a root dia

Question 15

How does water travel through the root (via the root cortex) into the xylem

Answer 15

Water travels through the roots (via the root cortex) into the xylem by 2 different paths:
- The symplast pathway
- The Apoplast pathway
- Vacuolar pathway

Question 16

Symplast pathway

Answer 16

Water moves through the living parts of the cell - the cytoplasm.
- The cytoplasms of neighbouring cells connect through plasmodesmata. Water enters cells through the plasmodesmata (small channels in the cell walls).
- Each cell further away from the roots has a lower water potential so water is drawn through the plant.
- Water moves through the symplast pathway via osmosis

Question 17

Apoplast pathway

Answer 17

Water moves through the non-living parts of the cell - the cell walls & intracellular spaces
- The walls are very absorbent & water can simply diffuse through them, as well as pass through the spaces between them.
- Water moves via diffusion as its not crossing a partially permeable membrane (simply cell to cell or through intracellular spaces)
- The water can carry solutes & move from areas of high hydrostatic pressure to areas of low hydrostatic pressure. This is an eg of mass flow
- This is the fastest movement of water

Question 18

Vacuolar pathway

Answer 18

The same as the symplast pathway when the water moves through the cells vacuoles in addition to the cytoplasm
- This is the slowest route for water

Question 19

What is the reason for the presence of the Casparian strip

Answer 19

Not fully understood but scientists think that:
- this may help the plant control which mineral ions reach the xylem
- it plays a part in increasing root pressure

Question 20

What happens to the Casparian strip as the plant ages

Answer 20

Thickens (as more suberin is deposited) except in cells called the passage cells, allowing for further control of the mineral ions

Question 21

see s32 for dia of apoplast, symplast, vacuolar pathways

Question 22

What is the Casparian strip

Answer 22

An impermeable layer of suberin - a waxy material
As a result, all water in apoplast pathway is forced into symplast pathway

Question 23

Where is the Casparian strip

Answer 23

The endodermis cells in the root contains the Casparian strip

Question 24

Explain the casparian strip (+following events)

Answer 24

1. When water in the apoplast pathway reaches the endodermis cells in the root, its path is blocked by a waxy strip in the cell walls - the Casparian strip
2. It is a waterproof, impermeable layer. This is bc of the waxy layer of suberin in walls of endodermal cells
3. In order to cross the endodermis, water that has been moving through cell walls in apoplast pathway, must now move through the cell surface membrane & into the cytoplasm - forced into symplast pathway
4. This is useful bc it means the water must go through a cell membrane, which are partially permeable & so are able to control whether or not substances in the water get through
5. As a result, the cell membrane can remove any toxic solutes from the soil, only allowing necessary water molecules/mineral ions to enter
6. Once past this barrier, the water moves into the xylem

Question 25

Evidence for active transport in root pressure

Answer 25

- Effect of Cyanide - Effect of Temperature - Reactant availability - Guttation

Question 26

Evidence for active transport in root pressure: **Effect of Cyanide**

Answer 26

Cyanide stops the mitochondria from working, therefore root pressure decreases

Question 27

Evidence for active transport in root pressure: **Effect of Temperature**

Answer 27

Root pressure increases as temp increases & decreases as temp decreases, suggesting an enzyme controlled chemical reaction

Question 28

Evidence for active transport in root pressure: **Reactant availability**

Answer 28

If oxygen levels or respiratory substrate levels drops, root pressure decreases

Question 29

Evidence for active transport in root pressure: **Guttation**

Answer 29

Sap & water will move out of cut stems, suggesting they are actively pumped out & not drawn up by transpiration

Question 30

How does water enter a plant

Answer 30

- Water must get from the soil, through the root & into the **xylem** to be transported around the plant - Water moves through **root hair cells** & then passes through **the root cortex**, including the **endodermis**, to reach the xylem - Water is drawn into the roots via **osmosis** - meaning it travels down a **water potential gradient**

Question 31

How does water potential affect movement of water into a plant

Answer 31

- Water always moves from areas of **higher water potential** to areas of **lower water potential** - it goes down a **water potential gradient** - The **soil** around roots generally has a **high water potential** (theres lots of water there) & **leaves** have a **lower water potential** (bc water constantly evaporates from them) - This creates a **water potential gradient** that keeps water moving through the plant in the right direction, **from roots (high)** to **leaves (low)**

Question 32

Which of the 2 is the most efficient pathway

Answer 32

Both are used, but the main one is the **apoplast pathway** bc it provides the **least resistance**

Question 33

Complete movement of water

Answer 33

Water moves through the **root** into the **xylem**, then up the xylem & out of the **leaves**

Question 34

What happens after the water moves through the root into the xylem

Answer 34

- **Xylem vessels** transport the water **all around** the plant - At the **leaves**, water **leaves** the xylem & moves into the cells mainly by the **apoplast pathway** - Water **evaporates** from the cell walls into the **spaces** between cells in the leaf - When the **stomata** (tiny pores in the surface of the leaf) open, the water **diffuses** out of the lead (down the water potential gradient) into the **surrounding air** - The loss of water from a plant's surface is called **transpiration**

Question 35

What is the movement of water from **roots to leaves** called

Answer 35

Transpiration stream

Question 36

What are the 3 mechanisms that move the water in the Transpiration stream

Answer 36

- Cohesion - Tension - Adhesion

Question 37

3 mechanisms that move water in Transpiration stream: **Cohesion & tension**

Answer 37

**Cohesion & tension** help water move up plants, from roots to leaves, **against** the force of gravity 1. Water **evaporates** from the **leaves** at the 'top' of the xylem (**transpiration**) 2. This creates a **tension (suction)**, which pulls more water into the leaf 3. Water molecules are **cohesive** (they stick tg) so when some are pulled into the leaf, others follow. This means the whole **column** of water in the **xylem**, from the leaves down to the roots, **moves upwards** 4. **Water** enters the stem through the **root cortex cells**

Question 38

3 mechanisms that move water in Transpiration stream: **Adhesion**

Answer 38

**Adhesion** is also partly responsible for the **movement of water**. 1. As well as being attracted to eachother, water molecules are **attracted** to the **walls** of the xylem vessels 2. This helps water to **rise up** through the xylem vessels