Plant Systems: Xylem and Phloem Transport

Xylem vessel elements are a key component of the water-conducting system in plants.

Cells form tissues, tissues form organs, and organs form systems.
Xylem transports water, while phloem transports organic nutrients.
Autotrophs produce their own organic molecules; heterotrophs ingest them.
The digestive system breaks down food for absorption.
The circulatory system transports nutrients, oxygen, and wastes.
Substances are exchanged across cell surfaces through diffusion, osmosis, and active uptake.

Adhesion: Attraction between water molecules and other surfaces.
Cohesion: Attraction between water molecules.
Cuticle: A protective layer on the surface of plants.
Xylem: The tissue responsible for the transport of water and dissolved minerals from the roots to the rest of the plant.
Phloem: The tissue responsible for the transport of nutrients and sugar produced through photosynthesis from the leaves to other parts of the plant.
Lignin: A substance that strengthens plant cell walls.
Phloem: Tissue that transports sugars in plants.
Root Hair Cells: Cells on the surface of plant roots that absorb water and nutrients.
Root Pressure: Pressure in the roots that helps push water up the xylem.
Stomata: Pores on the surface of leaves that allow for gas exchange.
Tissue: A group of similar cells that perform a specific function.
Tracheid: A type of water-conducting cell in plants.
Translocation: Movement of substances in the phloem.
Transpiration: Evaporation of water from plants.
Transpiration Stream: The movement of water from the roots to the leaves.
Transpirational Pull: The force that pulls water up the xylem due to transpiration.
Vascular Bundle: A strand of vascular tissue in plants.
Vascular Plant: A plant that has vascular tissue.
Vascular Tissue: Tissue that transports water and nutrients in plants.
Villi: Finger-like projections in the small intestine that increase surface area for absorption.
Xylem: Tissue that transports water in plants.

Physiological challenge: Transporting water and minerals from roots to canopy against gravity.
Multicellular plants have specialized cells organized into tissues, organs, and systems.
Two main systems: shoot system and root system.

Above ground.
Functions: transportation of resources, gas exchange, photosynthesis, sexual reproduction.

Specialized transporting tissue.
Cells differentiated for obtaining energy, producing organic compounds, distributing materials, removing wastes, and exchanging gases.
Composed of:
- Phloem: transports sugars (sucrose) and other plant products, both upwards and downwards using thin-walled living cells.
- Xylem: transports water and minerals upwards using dead, thick-walled tubes.

Plants obtain water through roots (except algae and aquatic plants).
Terrestrial plants require a root system and vascular tissue.
Water moves against gravity from roots to leaves through the xylem.
Xylem is composed of:
- Tracheids: elongated, narrower dead cells with lignin.
- Xylem vessel elements: hollow, lignified tubes formed from dead cells.
Lignin: a strong organic polymer that thickens xylem cell walls.

Mineral ions enter root hairs by diffusion and active transport.
$CO_2$ diffuses out of root cells.
$O_2$ diffuses into root cells from air pockets in the soil.
Root hairs increase surface area for absorption and penetrate soil particles.
Water moves into epidermal cells by osmosis; dissolved minerals move in by diffusion and active transport.
Root tip region absorbs water six times faster than other regions.
Root hairs can provide a surface area 130 times greater than the shoot.
Parenchyma cells form the main root body beneath the epidermis.
Vascular tissue core in the middle of the root.
Water moves from root hair cells to parenchyma cells and then to xylem vessels.
Water movement creates root pressure.
Severed stems exude fluid due to root pressure.
Root pressure can raise water up to 6.4 m.

Two plant systems: shoot and root.
Vascular tissue: xylem and phloem.
Xylem: water-conducting tissue (tracheids and vessels).
Root hairs: increase surface area for water absorption.
Water and ions enter roots by osmosis (water) and diffusion/active transport (ions).
Phloem: transports sugars and other plant products.

Cuticle: waterproof wax layer on leaves, reducing water loss.
Stomata: pores on leaves for gas exchange and water vapor release.
Guard cells: specialized cells that open and close stomata.
Mesophyll cells: wet cell walls inside the leaf, air spaces contain water vapor.
Water vapor diffuses out of the leaf through open stomata.
Up to 98% of absorbed water is lost through transpiration.
Factors affecting transpiration rate:
- Temperature: higher temperature increases evaporation and water vapor loss.
- Humidity: higher humidity decreases water vapor loss.
- Air movement: increased air movement increases water vapor loss.

Objective: Examine water transport through a celery stalk.
Materials: celery stalk, red food dye/eosin, razor blade, microscope, slides, coverslips, needles.
Method:
1. Place celery in colored solution.
2. Examine dye distribution in stalk and leaves.
3. Cut thin transverse sections of stem and leaf.
4. Observe sections under the microscope.
5. Tease apart the dyed tissue with mounted needles.
6. Examine prepared slides of Helianthus (sunflower) stem sections.

Phloem tissue: sieve tube cells and companion cells.
Sieve tube cells: arranged in long tubes with perforated end walls (sieve plates).
Companion cells: support sieve tube cells lacking nuclei.
Phloem transports organic substances (sucrose, amino acids, mineral nutrients).
Translocation: movement in the phloem, transporting nutrients from synthesis sites to use/storage sites, both upwards and downwards.