Transport in Plants P1

Define mass flow

• the bulk transport of materials from one point to another due to a pressure gradient between 2 points

Characteristics of Mass flow

1. fluid in which the molecules are to be transported in

2. system of tubes - vascular system

3. mechanism by which a pressure difference between two points can be generated

What does xylem transport

• water

• mineral salts

• organic nitrogen

• hormones

What does phloem transport

• organic and inorganic solutes

Xylem - 2 important roles + what is it made up of

• mechanical support due to lignification

• transport of organic nitrogen, water, hormones and mineral salts

• made up of tracheids, vessels, parenchyma and sclerenchyma

Phloem what is it composed of

• living tissue → nuclei and cytoplasm

• sieve tube elements, companion cells, parenchyma and sclerenchyma

Tracheids - structure and function

• STRUCTURE

  • boarded pit

  • mainly found in gymnosperms

  • dead

  • primitive water conducting cells

  • overlapping tapering ends

  • lignified wall hollow lumen

• FUNCTION

  • conduction of water through hollow lumens

  • mechanical support

Vessel Elements

• STRUCTURE

  • mainly in angiosperms

  • lignified wall

  • simple pits

  • open at both ends

  • wide hollow lumen

• FUNCTION

  • rapid conduction of water

  • mechanical support

Parenchyma

• STRUCTURE

  • living

  • large - elongated

• FUNCTION

  • mechanical support

  • gas exchange

  • storage

Sclerenchyma

• STRUCTURE

  • dead

  • overlapping tapering ends

  • lignified wall

  • simple pits

  • narrow lumen

• FUNCTION

  • mechanical support

Sclerieds

• STRUCTURE

  • irregular

  • dead

  • lignified walls

  • simple pits

• STRUCTURE

  • mechanical support

Sieve tube elements

• STRUCTURE

  • perforation at the end walls

  • long tube like structure

  • cytoplasm - at perifery

  • lack nucleus

• FUNCTION

  • transports organic solutes

Companion Cells

• STRUCTURE

  • dense cytoplasm with numerous mitochondria

• FUNCTION

  • live support

Parenchyma, sclerenchyma and sclerides

• same as xylem

Water Potential

• the tendency of water molecules to move from one place to another Ψ/Ψw

• pure water has the max water potential - 0kPa

helps to understand if you’re stuck <3

Solute Potential

• measure of the change in water potential of a system due to the presence of solute molecules - Ψs

• negative value

Pressure Potential

• measure of change in water potential of a system due to the application of pressure

• Ψp

• positive value

Osmosis

movement of water molecules from a region of higher water potential to a region of lower water potential across a partially permeable membrane

Equation

Ψ = Ψs + Ψp

When a cell is placed in a HYPOTONIC SOLUTION

• Ψexterior is greater than interior

• water moves into cell

• TURGID

When placed in an ISOTONIC SOLUTION

Ψe = Ψi

When placed in a hypertonic solution

• Ψexterior < Ψinterior so water moves out of cell

• incipient plasmolysis - flaccid

• some of cell membrane has begun to separate from cell wall

When placed in a HYPERTONIC solution

• water moved out

• Ψe < Ψi

• cell is plasmolyzed all cell membrane has separated from cell wall.

Pathways through which water enters the root

• Apoplast pathway → through cell walls

• Symplast pathway → through cytoplasm

• Vacuolar pathway → through vacuoles

Describe the water pathways

• through the apoplast pathway water moves relatively quickly

• reaches casparian strip made of suberin waterproof - must pass through symplast or vacuolar pathways

• these pathways - water must pass through cell membrane which is differentially permeable → pathogens and minerals not needed by plant will not reach xylem

Transpiration

• the evaporation of water from the surface of a plant which occurs through leaves stomata and lenticels

Why is transpiration vital to a plant

• mineral ions needed by the plant enter with water

• water flow is needed for photosynthesis and mechanical support

• loss of water = cooling effect

External factors effecting transpiration

• Temperature → kinetic energy increases and hence evaporation increases

• Humidity → more humid = higher water potential in air, less transpiration

• Wind → less windy, water leaking forms a water boundary around stomata = higher w.p in that region less transpiration

• Light → stomata open during day = transpiration, transpiration at night is minimal only thru cuticle

Internal Factors affecting transpiration

1. Area of leaf = larger area, more stomata = more transpiration

2. Cuticle = more cuticle less transpiration, larger distance

3. Density of Stomata = rate increases, more stomata per unit area

Mechanism of stomatal aperture and closure

• guard cells have thicker and less elastic inner walls

• are attached at tips

• when they take up water and cell becomes turgid the inner walls stretches less creating a pore

• when cells become flaccid it closes again