Relate the structure of the xylem to its function [6] #
Describe how the structure of the xylem vessels is adapted for their function of transporting water up the stem[6] ZIMSEC November 2008/2/12(a)]
- (Xylem vessels) are continuous this enables an unbroken column of water;
- (Xylem vessels )lack contents/hollow/ dead this enables an unrestricted flow of water;
- (Xylem vessels) walls are lignified for support and strength;
- Lignin makes vessels water proof/ increases adhesion of water molecules
- Vessels are narrow for capillarity;
- pits to allow lateral flow of water
- parenchyma cells have thin cell walls with lignin to assist in food storage
- Xylem vessels efficiently transport water and mineral ions from the roots to the rest of the plant.
- They are composed of hollow, dead cells that lack cytoplasm and organelles. This provides more room for water movement.
- The cells are stacked end-to-end, forming long, continuous tubes without cross walls between the vessel elements. Lignin, a tough woody substance, strengthens and waterproofs the cell walls, preventing collapse under the forces of water transport.
- The lignified walls also provide critical structural support to the plant.
- Small pores known as pits are scattered throughout the xylem vessel walls.
- These allow lateral transport of water and minerals between adjacent vessels and cells, enabling the xylem network to distribute water and nutrients throughout the plant body.
Sclerenchyma #
- Sclerenchyma fibers provide structural support in vascular bundles along with xylem and phloem.
- Composed of bundles of elongated, dead cells with hollow lumens.
- Cell walls are thickened with lignin and contain high levels of cellulose.
- Lignin increases strength and waterproofing.
- Extra cellulose also adds strength.
- The thickened, rigid cell walls give sclerenchyma fibers mechanical strength.
- Allows them to support plant structures without collapsing under pressure.
- The long dead cells are stacked together in bundles that run through vascular tissues.
- Act like cables reinforcing the plant body against bending and twisting.
With reference to the structure of a leaf, explain the difference between evaporation and transpiration. #
| evporation | transpiration |
|---|---|
| formation of water vapour from water / conversion of water from liquid (form) to gas(eous form) ; | idea of loss of water vapour, to external atmosphere / from the aerial parts of a plant ; |
| requires, energy / heat ; | ref. diffusion, down water potential gradient / from high to low water potential / from less negative to more negative water potential ; |
| (water loss) from, surface / cell walls, of (spongy) mesophyll (cells); | through stomata ; |
| ref. to, intercellular / air, spaces ; e.g. evaporation into air spaces, | diffusion from air spaces through stomata |
State and explain three ways in which the structure of xylem vessels is adapted to transport water. #
- cellulose, cell wall / lining;
- allows adhesion of water;
- thick (cellulose) cell wall;
- prevents collapse / idea of providing support (under tension);
- lignin
- waterproofing / prevents water loss;
- lignin ; A rings / spirals / thickening
- prevents collapse / idea of providing support (under tension);
- no cytoplasm / lack of contents / hollow / empty lumen;
- less resistance to / unimpeded / uninterrupted / unhindered
- lack of end walls / continuous tube;
- less resistance to / unimpeded / uninterrupted / unhindered
- pits / pores;
- lateral movement / movement around air bubbles / supplies (water) to (surrounding), cells / tissues;
- wide / large diameter / large lumen;
- so large volume of water can be transported;
- Phloem vessels consist of two specialized cell types: sieve tube elements and companion cells
- Sieve tube elements:
- Stacked end to end to form long sieve tubes for transport
- Living cells with perforated end plates called sieve plates
- Allow solutes to flow between cells
- Lack nuclei and most organelles
- Provides more room for solute transport
- Rely on adjacent companion cells
- Companion cells:
- Closely associated with sieve tube elements
- Contain nuclei and many mitochondria
- Provide ATP energy to power active transport
- Connected to sieve tube elements by plasmodesmata
- Allow sharing of solutes and communication
- Sucrose is actively loaded into the sieve tube elements by the companion cells
- This lowers water potential, pulling water in by osmosis
- Pressure flow transports sugars and other solutes through the phloem
State and explain two ways in which the structure of a phloem sieve tube is adapted for the transport of assimilates. #
- sieve pores ;
- allow easy flow (from sieve tube element to sieve tube element) ;
- sieve plate ;
- (may) prevent sieve tubes from bursting ;
- cell (surface) membrane / plasma membrane ;
- prevents loss, of sucrose / assimilates / phloem sap ;
- little cell contents
- little resistance
- plasmodesmata ;
- allows flow, to / from, companion cells ;
- thin walls ;
- for, rapid / easy, entry of water (at source, to build up pressure) ;
Differences Between Xylem and Phloem
| Feature | Xylem | Phloem |
|---|---|---|
| Components | Tracheids, vessels | Sieve tube elements, companion cells |
| Cell type | Non-living cells | Living cells |
| Cell walls | Lignified | Non-lignified |
| Transport direction | Unidirectional (upward) | Bidirectional |
| Main transport | Water and minerals | Sugars and organic compounds |
| Method of transport | Transpiration-cohesion-tension | Pressure flow |
| Energy source | Passive transport | Active transport |
| Function | Provides structural support, transports water and minerals | Transports products of photosynthesis |
Describe the pathways and mechanisms of water transport from the soil to the xylem in the root[6] #
[ZIMSEC November 2008/12(b)
Describe the pathway taken by water as it passes from the root hair cells into the cells which are specialised for transport of water and mineral ions.
- through cortex / via cortical cells ;
- apoplast pathway
- (by) via cell walls (of adjacent cells) ;
- symplast pathway
- via cytoplasm and plasmodesmata ;
- ref. vacuolar pathway ;
- ref. apoplast to symplast / pathway described, at endodermis ;
- (via) passage cells ;
- ref to, suberised / Casparian, strip ;
Describe how sucrose moves from the leaf to the sinks e.g. root[8] #
[ZIMSEC June 2006/2/11(a)]
Describe how the assimilate eg sucrose is transported from the source to the sink.
- H+ / protons, (move) out of companion cells by, active transport ;
- H+ / protons, diffuse (back) in with / cotransport sucrose, into companion cells ;
- via, cotransporter / cotransporter described ;
- sucrose, diffuses / AW, into (phloem) sieve, tube / element, via plasmodesmata ;
- (entry of sucrose into sieve tube so) water potential lowers ;
- water enters by osmosis ;
- (hydrostatic) pressure builds up ; A pressure difference created
- unloading at, sink / named sink, gives a difference in pressure (between source and sink) ;
- (so) mass flow ;
Multiple Choice Questions #
1. Which of the following adaptations is characteristic of xylem vessels?
A) Presence of companion cells
B) Living cells with a nucleus
C) Rigid lignified cell walls
D) Lack of sieve plates
2. What is the main function of the Casparian strip in plant roots?
A) Facilitating apoplastic transport
B) Enhancing cell-to-cell communication
C) Preventing uncontrolled water movement through the apoplast
D) Promoting symplastic transport
3. Which mechanism is responsible for the majority of long-distance water movement within plants?
A) Root pressure
B) Capillary effect
C) Osmosis
D) Transpiration pull
4. Which hypothesis explains the movement of sugars from source to sink in the phloem?
A) Cohesion-Adhesion Hypothesis
B) Pressure Flow Hypothesis
C) Active Transport Hypothesis
D) Transpiration-Cohesion Hypothesis
5. Which tissue is responsible for the transport of sugars and organic compounds within a plant?
A) Xylem
B) Epidermis
C) Cambium
D) Phloem
6. Which mechanism of water movement is most evident during the night when transpiration is low?
A) Root pressure
B) Osmosis
C) Capillary effect
D) Transpiration pull
7. Which pathway of water movement within roots involves crossing cell membranes via plasmodesmata?
A) Symplast pathway
B) Apoplast pathway
C) Casparian pathway
D) Vacuolar pathway
8. Which adaptation of xylem vessels contributes to their structural strength and prevention of collapse?
A) Lignification
B) Photosynthesis
C) Mitosis
D) Transpiration
9. What is the primary driving force for the movement of sugars in the phloem?
A) Root pressure
B) Active transport of sugars
C) Transpiration pull
D) Gravitational force
10. Which cells are closely associated with sieve-tube elements and provide metabolic support?
A) Xylem vessels
B) Tracheids
C) Parenchyma cells
D) Companion cells
