Plants: Water Transport (Part 3) Flashcards
What are the xylem and phloem vessels?
The xylem and phloem make up the vascular tissue of plants, transporting water, minerals, and sugars through a series of interconnected tubes through the leaves, stems, and roots.
What is the xylem?
Transport water/dissolved minerals from the soil to the leaves.
- In mature plants, xylem cells are dead, with no minerals and only the cell wall remains, forming hollow tubes called xylem vessels.
- Consists of long hollow cells called tracheids, or vessel elements, which are joined by small pits, allowing water to flow through
What is the phloem?
Transports sugars produced along photosynthesis from leaves to roots/other places down
- Cylindrical cells joined end to end form phloem vessels which are living cells with porous cell walls. Sugary sap flows down phloem vessels and passes through such pores.
- Consists of sieve tubes, cylindrical cells joined by a sieve plate. A companion cell lies alongside each sieve tube cell, offering direction because the phloem cells lack organelles.
* sieve tube cells are alive but have no nucleus
Living: active transport
How is water uptaken in roots?
Roots are covered with epidermal tissue which is permeable to water only at the root tip. Water enters the root tips by osmosis until it reaches the xylem.
The xylem carries the xylem sap up to stems and leaves (branching into leaf surface veins) eventually being absorbed by all the cells of the plant.
FYI: The root system is around 1.5x the size of the top.
What do root hairs do?
They increase surface area for absorbing water and dissolved minerals. They are each an outgrowth of a single epidermal cell. Minerals enter the root by facilitated diffusion or active transport.
What is the xylem sap?
The solution of water and minerals in the root xylem.
(From the roots) (It’s hypertonic, which is why it requires A. Transport)
How do the properties of water assist water uptake?
Two properties of water allow xylem sap to rise great distances against gravity, which are cohesion and adhesion.
What is cohesion?
The attraction of water molecules to other water molecules due to their polar nature (this is what causes water to form droplets) (Move one, others follow).
The column of xylem sap can be broken in the vessel or by a bubble in the sap. All transport stops.
How could cohesion be affected like this? Give examples.
Break: snapped
Cut: animals
Weather:
- when water is frozen, it expands
- when water contracts, it goes back to liquid
= GAPS
What is adhesion?
The attraction of water molecules to other surfaces. (Obvious when water sticks to glass in a graduated cylinder, forming a meniscus—the curve).
Water also clings to the cellulose (polar) wall of a xylem vessel, preventing the sap from falling back towards the roots, and helping to fight the force of gravity.
What are the two main mechanisms that aid the upward movement of water in plants?
Root pressure and transpiration.
Push and pull, respectively.
Explain root pressure.
Root Pressure Pushes.
- Root cells are permeable to water only (nothing else can move passively)
- Root cells actively transport minerals into the xylem
- Causes water to diffuse (osmosis) into this hypertonic area, building root pressure in the xylem which forces fluid up the xylem vessels.
- A pressure gradient is formed, as plant cells are rigid (don’t expand)—bottom increasing pressure
- High pressure @ roots, low pressure @ leaves. [Pressure difference] pushes water up.
Will all transport cease due to a cut halfway up the stem?
No, root pressure will continue to push up beneath the cut. Fluid seeping from a cut stem of a plant occurs due to root pressure.
For example, cut a dandelion—
Explain Transpiration Pulls.
Transpiration of water from leaf stomata generates a pulling force, aiding the upward transport of water.
- The air is dry, inside of the leaf is comparatively more water, and the energy for xylem comes from heat of sun
- Water evaporates, and inside the leaf is drier, less water, so, slightly hypertonic to rest of plant
3a. Water then diffuses out of leaf cells into intracellular fluid where solutes are more concentrated.
4a. As evaporation continues, cohesion of water molecules draws the water up the xylem vessels, replacing evaporated water. Adhesion of water molecules to walls aids.
3b. Water molecules move to hypertonic leaf.
4b. Water molecules get pulled up plant because of adhesion
Transpiration increases when temperature rises, increasing water movement through xylem. It can be up to 50m/h.
Explain SUGAR transport in the phloem.
Sugars produced by the plaisade and spongy tissue cells of the leaf are transported to the stems and roots by phloem vessels.
1. Sugars, minerals, and other nutrients enter the phloem by acitve transport (as it’s already abundant)
2. Now, more hypertionic, water flows by osmosis, causing phloem cells to increase with turgor pressure (∴ phloem pressure)
3. Phloem sap flows down the [c gradient] and the fluid perssure (+gravity) forces the sap through pores in phloem cell walls and into surrounding cells
4 (simple). [Gradient] and gravity brings sap to roots.
4 (full). The nutrients are continually used up by tissues of the stem and roots presulting in a pressure gradient that causes a continual flow from leaf (high) to root (low).
Phloem transport ranges from 20cm/h to 100cm/hr, due to viscosity.