Diffusion and Osmosis Concepts

• Water is vital for plant life, serving as the most abundant yet often the most limiting resource for growth and function
• Water content in plant tissues depends on tissue type, metabolic activity, and soil water status
• Fresh, actively growing tissues like fruits and flowers can be 90% water by weight
• Dry, non-actively tissues like seeds or wood can be 10% water or less by weight

Fresh Weight (FW) and Dry Weight (DW)

• Fresh weight (FW) is the weight of freshly harvested plant tissues
• Dry weight (DW) is the remaining weight after tissues are left in air or incubated in an oven at 70 °C for a few days to evaporate all water
• The difference between FW and DW indicates the water content in the tissue

Example of Water Content Calculation

• Start with a freshly harvested flower weighing 4g (FW = 4g)
• Incubate the flower at 70 °C for 3 days until all water evaporates, resulting in a dry weight of 0.5g (DW = 0.5g)
• Calculate the water content: Water content = FW – DW = 4 – 0.5 = 3.5g
• Water content percentage = (water content / FW) * 100 = (3.5/4) * 100 = 87.5%
• The high water content in metabolizing tissues reflects water's importance

Functions of Water in Plant Life

• Acts as a solvent for cellular molecules and a medium for enzymatic reactions
• Maintains the shape of plant organs and the plant body
• Transports solutes and nutrients up from the soil and throughout the plant body
• Cools the plant body
• Used for germination and growth of cells and the overall plant body
• Facilitates movement, such as the opening/closing of flowers and the movement of leaves in carnivorous plants (e.g., Venus flytrap)

Diffusion

• Molecules and ions are in constant, random motion
• Brownian movement, described by Robert Brown in 1827, explains this motion as bombardment of visible particles by invisible water molecules
• Diffusion: the movement of molecules or ions from high to low concentration regions
• Movement from high to low concentration is along a diffusion gradient
• Movement from low to high concentration is against a diffusion gradient
• Dynamic equilibrium is reached when molecules distribute evenly throughout available space
• Diffusion is spontaneous and doesn't need energy
• Diffusion rate is proportional to temperature and pressure
• Diffusion rate is inversely proportional to the density/viscosity of the medium and the size of diffusing molecules

Diffusion Example with Glucose

• It would take ~31 years for a glucose molecule to diffuse 1 meter long
• A glucose molecule can diffuse from one end of a cell to the other (10 micrometers) in about 0.88 seconds

Osmosis

• Solution: a mixture of substances where one is dissolved in another
• Osmosis: a special case of diffusion where water molecules move through a semipermeable membrane from high to low water concentration
• Semipermeable membranes have specific pore sizes, restricting larger molecules
• Plant cells have semipermeable membranes like the cell membrane and tonoplast

Osmosis Demonstration

• Tie a cellophane membrane over a thistle funnel filled with 10% sugar solution
• Immerse it in a beaker of distilled water, setting up demonstration of osmosis
• Water diffuses through the membrane from the distilled water to the sugar solution, causing the solution to rise inside the funnel

Osmotic Pressure and Solutions

• Osmotic pressure: the force driving water into the funnel through the semipermeable membrane
• Osmotic pressure is proportional to the concentration difference across the membrane
• Hypotonic solution: has a lower concentration of salt (higher concentration of water)
• Hypertonic solution: has a higher concentration of salt (lower concentration of water)
• Osmotic system: a hypotonic solution separated from a hypertonic solution by a semipermeable membrane

Water Potential

• Water potential (Ψ) measures free energy of water under temperature and pressure, relative to pure water
• Measured in pressure units (bar or pascal (Pa))
• 1 bar = 100000 Pa = 0.1 MPa
• Water potential (Ψw) of pure water at atmospheric pressure is zero
• Any solution (salt in water) has a water potential below zero (negative value)
• Water moves from high to low water potential
• Water in an osmotic system diffuses from pure water (Ψw = 0 MPa) to 0.1 M sucrose (Ψw = -0.24 MPa) demonstrating movement from high to low

Osmosis, Water Potential, and Salt Concentration

• During osmosis, water moves through a semipermeable membrane from a hypotonic solution with higher water potential or lower salt concentration
• Occurs to a hypertonic solution with lower water potential or higher salt concentration

Components of Water Potential

• The driving force of water movement example: ΔΨw = ΨA – ΨB
  • ΔΨw= the driving force
  • ΨA= water potential of solution A
  • ΨB= water potential of solution B
• Water potential (Ψw) is comprised of osmotic potential (Ψs) and pressure potential (Ψp)
• Ψs (osmotic potential/solute potential): determined by solute concentration
• Pure water has a solute potential is zero
• Adding solutes lowers water potential, making it negative
• Osmotic potential of any solution is always negative
• The higher the salt concentration, the lower (more negative) the osmotic potential
• Osmotic potential is sometimes symbolized by Ψπ
• Ψp (pressure potential): physical pressure exerted on water in a system:
  • Ψp at atmospheric pressure is zero
  • Ψp above atmospheric pressure is positive
  • Ψp below atmospheric pressure is negative

Plant Cell as an Osmotic System

• Plant cells have cellulose cell walls which provide rigidity and some elasticity
• Protoplast = cytoplasm with its components, surrounded by a plasma membrane
• Tonoplast: membrane surrounding a large vacuole inside the cytoplasm
• Cell membrane and tonoplast = semipermeable membranes that separate cytoplasmic solution from the outer environment
• Plant cell functions as an osmotic system where water moves from high to low water potential

Plasmolysis and Turgor Pressure

• Plasmolysis: the protoplast loses water and shrinks
• Occurs when the plant cell is surrounded by a high solute solution
• Deplasmolysis: the reverse of plasmolysis, the plasmolized cell returns to a turgid state when water diffuses back inside the protoplast
• Permanent plasmolysis: when plasmolysis is severe or prolonged, the cell dies and cannot deplasmolyze
• Turgor pressure: the turgid protoplast pushes against the cell wall, causing it to expand
• Wall pressure: the cell wall squeezes back on the protoplast, preventing further expansion
• Turgor and wall pressure are equal and opposite
• Turgid plant cells have their protoplast under pressure greater than atmospheric pressure

Additional Factors

• The crispness of lettuce is due to turgor pressure; cells explode when cell walls are ruptured during chewing
• Potassium ions regulate plant cell osmolarity
• Osmosis enables water movement from cell to cell by going from high to low water potential

Imbibition of Water by Seeds

• Seed imbibition: the initial stage of seed germination
• Seed imbibition enables seeds to transition from dormancy to active growth
• Involves absorption of water by dry seeds, leading to swelling and the activation of metabolic processes necessary for germination
• Imbibition: a physical adsorption process where water molecules adhere to seed food (starch/protein) and hydrophilic cell walls (cellulose, hemicellulose, pectins)
• Capillary action draws water into the microscopic pores and spaces of the seed coat and tissues
• The difference in water potential underlies the driving force behind imbibition
• Water causes the seed coat to hydrate, swell, and become permeable
• Water penetrates the embryo disrupting hydrogen bonds, causing cell walls to expand, which increases the seed volume
• Seed coat ruptures, facilitating water uptake and gaseous exchange

Factors Affecting Imbibition

• Seed Coat Structure: thickness, permeability, and composition of the seed coat
• Thick, impermeable coats may need mechanical scarification or pre-treatment
• Water Availability: Seeds in high water potential take up more water than seeds in low water potential
• Temperature: influences the rate of imbibition and metabolic activities. Higher the temperature, the faster the imbibition
• Texture/type of imbibants: seeds with loose structures imbibe easier than seeds that are in compact structures
• Seeds with starch/protein imbibe more water than seeds with oil.
• Wood is an excellent imbibant.

Explore the differences between diffusion and osmosis, factors affecting semipermeable membrane passage, and osmotic pressure. Understand how water concentration impacts plant cells and the role of semipermeable membranes. Investigate the effects of wind and water on molecule distribution and water content in plant tissues.