Diffusion

• Definition: The spontaneous movements of substances from one location to another due to their random thermal motion.

  • Source: Vander’s Human Physiology, ©McGraw Hills.

Movement of Molecules

• Individual molecules move randomly, however, the net flux is directed from regions of higher concentration to regions of lower concentration.

  • Net Flux: The overall movement of particles.

  • Factors influencing net flux include:

  • Concentration gradient

  • Type of substance

  • Medium (air, liquid)

  • Temperature

  • Surface area

  • Note: Only two directions are typically indicated in diagrams.

Osmosis

• Definition: The diffusion of free water across a selectively permeable membrane.

• Resource: Download for free at https://openstax.org/details/books/biology-2e, https://cnx.org/contents/GFy_h8cu@11.2:6X9o-0n6@12/Passive-Transport

Tonicity

• Definition of Tonicity: A measure of the ability of a solution to add or remove water from cells due to osmosis.

• Types of Tonic Solutions:

  • Isotonic Solution:

  • Does not cause a change in cell volume.

  • Hypotonic Solution:

  • Causes cells to swell in volume due to a lower concentration of non-penetrating solutes than in the cell interior.

  • Hypertonic Solution:

  • Causes cells to shrink in volume due to a higher concentration of non-penetrating solutes than in the cell interior.

  • Source: Vander's Human Physiology, McGraw Hills.

Effects of Tonic Solutions on Cells

• Isotonic Solution:

  • For animal cells, isotonic environment is ideal unless the cells have adaptations to counteract osmotic changes.

  • Illustration: Animal Cell in Isotonic Solution.

• Hypotonic Solution:

  • For plant cells, they are healthiest in hypotonic conditions and become turgid (firm) as they uptake water. This is balanced by the cell wall pushing back against internal pressure.

  • Illustration:

  • Lysed cells in hypotonic solution (animal cells exposed to too much water, bursting).

  • Turgid plant cells in hypotonic solution.

• Hypertonic Solution:

  • Animal cells become shriveled due to water leaving the cell.

  • Illustration:

  • Shriveled animal cells in hypertonic solution.

  • For plant cells, they may become plasmolyzed in a hypertonic environment (loss of water leading to cell membrane detachment from cell wall).

Water Balance of Living Cells

• Microscopic Observation of Plant Cells:

  • Organism: Elodea

  • Visual representation indicating:

  • (a) cells in hypotonic solution (turgid)

  • (b) cells in hypertonic solution (plasmolyzed).

Water Potential

• Water Potential (Ψ):

  • Refers to water’s potential energy—the capacity for water to perform work when moving from areas of higher free water potential to lower free water potential.

  • Measured as pressure in megapascal (MPa), where 1 atm = 0.1 MPa.

  • Formula:

  • $ext{Ψ} = ext{Ψ}<em>S + ext{Ψ}</em>P$

  • where:

    • ΨS: Solute potential (also termed osmotic potential) - it accounts for the effect of solutes on water potential (0 or negative).

    • ΨP: Pressure potential - indicates the effect of physical pressure on water potential (positive or negative).

  • By definition, for pure water, ΨS = 0 MPa.

• Implications:

  • Knowing water potential values allows predictions regarding directions of water flow due to differences in water potential.

Estimating Water Potential

• Example with Potato Cells:

  • When potato cells are immersed in an isotonic sucrose solution, their mass remains unchanged, suggesting equal water potential between the cells and solution:

  • $ext{Ψ}<em>{ ext{cell}} = ext{Ψ}</em>{ ext{outside solution}}$

  • Through calculating the water potential of the sucrose solution known to be isotonic, the water potential of the potato cells can be derived.

Water Potential Calculation

• Calculating Solute Potential:

  • Assumption: Pressure potential of the sucrose solution is 0 (since it is open to the atmosphere)

  • Therefore, the water potential becomes equal to the solute potential.

  • $ext{Ψ} = ext{Ψ}<em>S + ext{Ψ}</em>P$

  • $ext{Ψ}<em>P = 0 ext{, hence, } ext{Ψ} = ext{Ψ}</em>S$

• Identifying Isotonic Solutions:

  • Procedure involves measuring the mass of potato pieces before and after soaking in varying molar concentrations of sucrose solutions.

  • Note: In the lab setup, the mass should be monitored for significant changes indicating isotonic conditions.

Data Collection for Osmolarity Experiment

• Table Structure (Example Data):

  • Sucrose Molarity (M): 0.0, 0.2, 0.3, 0.4, 0.5, 0.6

  • Final Weight (g): Recorded values after incubation

  • Initial Weight (g) measured pre-incubation

  • Weight Change (g): Final - Initial values

  • Percentage Change in Weight:

  • $ext{Percent Change} = rac{ ext{Weight Change}}{ ext{Initial Weight}} imes 100$

• Final Note: Use graphical representations to estimate sucrose concentrations that resulted in no change in weight for potato pieces (indicative of isotonic solution).

Experiment Setup for Diffusion of Molecules

• Experiment B: Diffusion of molecules through a selectively permeable membrane, utilizing dialysis tubing bags in solutions

• Objective: Determine which molecules pass through membrane by diffusion.

  • Initial Setup includes:

  • Tubing Configuration: Closed at one end, immersed in a beaker of water and iodine solution (IKI).

  • Target Molecules: H2O, I2KI, glucose, starch.

Testing for Starch Presence

• IKI Test:

  • Post-addition of IKI to an unknown solution, observe color change:

  • Purple or black indicates starch presence.

  • Pale yellow-amber color indicates absence of starch.

Testing for Reducing Sugars

• Benedict's Test:

  • When Benedict's reagent is added and heated:

  • Color changes from blue to green, orange, or orange-red indicate the presence of reducing sugars (with color indicating concentration).

  • Maintains blue color if reducing sugar is absent.

Review of Lab Experiments

• **Focus on the following exercises:

  1. Exercise 1.B – Diffusion of Molecules Through a Selectively Permeable Membrane.

  2. Exercise 3A – Estimating Water Potential by Change in Weight.

• Ensure to correct titles in lab manuals as needed.

• Additional study material available in Biol 108 lecture notes, pages 788 - 791.

• Lab manual text excerpts to be reviewed from pages 57-58.

Diffusion

What it is:
Imagine you spray air freshener in one corner of a room. Slowly but surely, the scent spreads throughout the entire room, even without a fan. That's diffusion! It's the natural scattering of tiny particles (like solid, liquid, or gas molecules) from an area where there's a lot of them (high concentration) to an area where there are fewer of them (low concentration). This happens because molecules are always jiggling around randomly due to their thermal energy.

How molecules move:
Individual molecules move in all directions chaotically. However, when you look at the overall movement (what we call net flux), more molecules will end up moving from the crowded area to the less crowded area than vice-versa, until they are evenly spread out.

Think of Net Flux like this:
If you have 100 people in one room and 10 in another, even if everyone is randomly walking, more people will eventually walk from the first room to the second simply because there are more of them in the first room to begin with.

Factors that influence how fast and far diffusion happens:

• Concentration gradient: The bigger the difference in concentration between two areas, the faster the net flux. (Imagine a very strong smell versus a faint one – the strong one spreads faster).

• Type of substance: Smaller, lighter molecules (like a gas) generally diffuse faster than larger, heavier ones (like a protein).

• Medium (air, liquid): Diffusion is usually faster in gases than in liquids, and even slower in solids, because molecules have more room to move around in gases.

• Temperature: Higher temperatures mean molecules have more energy, so they move faster and diffuse more quickly.

• Surface area: A larger surface area for diffusion (like the large surface area of your lungs) allows more molecules to cross at once, speeding up the process.

Osmosis

What it is:
Osmosis is a special type of diffusion, specifically the diffusion of water molecules across a selectively permeable membrane. Think of a screen door: it lets air through easily but keeps bugs out. A selectively permeable membrane is like that for water – it lets water pass through (usually very easily) but restricts the movement of other dissolved substances (solutes).

Analogy:
Imagine two swimming pools separated by a special divider. The divider lets water flow through, but it blocks sugar molecules. If one pool has a lot of sugar dissolved in it (and therefore less 'free' water), and the other has pure water, water will naturally flow from the pure water side to the sugary side to try and dilute the sugar, even without any pumping.

Tonicity

What it is:
Tonicity is a term we use to describe how a solution will affect the volume of a cell, primarily by causing water to move in or out through osmosis. It's essentially a measure of the effective solute concentration of a solution outside a cell, relative to the inside of the cell.

Types of Tonic Solutions:

1. Isotonic Solution (”Iso” means equal):

   • Definition: The concentration of non-penetrating solutes (solutes that can't cross the cell membrane) outside the cell is the same as inside the cell. Because there's no net movement of water, the cell's volume doesn't change.

   • Analogy: If you have an equal number of people on both sides of a swinging door, even if people are constantly going back and forth, the net number on each side stays the same.

   • Effect on Cells: For animal cells, an isotonic environment is ideal. Their cells are happy and maintain their shape.

2. Hypotonic Solution (”Hypo” means under/less than):

   • Definition: The concentration of non-penetrating solutes outside the cell is lower than inside the cell. This means there's a higher concentration of free water outside the cell.

   • Effect on Cells: Water will rush into the cell to try and dilute the higher solute concentration inside. This causes the cell to swell. If an animal cell takes in too much water, it can burst (we call this Lysis).

   • Analogy: Imagine a very thirsty sponge (the cell) placed in clean water (hypotonic solution). The sponge will soak up the water and expand.

   • Plant Cells in Hypotonic Solutions: Plant cells are actually healthiest in hypotonic conditions. As they take in water, their internal pressure (turgor pressure) builds up, pushing against their strong cell wall, making the cell firm or "turgid." This is why plants wilt when they don't get enough water – their cells become flaccid instead of turgid.

3. Hypertonic Solution (”Hyper” means over/more than):

   • Definition: The concentration of non-penetrating solutes outside the cell is higher than inside the cell. This means there's a lower concentration of free water outside the cell.

   • Effect on Cells: Water will rush out of the cell to try and dilute the higher solute concentration outside. This causes the cell to shrink or shrivel.

   • Analogy: If you place a plump grape inside a bowl of extremely sugary syrup (hypertonic solution), water will leave the grape and go into the syrup, causing the grape to shrivel into a raisin.

   • Plant Cells in Hypertonic Solutions: When plant cells are in a hypertonic environment, they lose water, and their cell membrane pulls away from the cell wall. This process is called plasmolysis, and the plant wilts severely and can die if the condition persists.

Water Potential (Ψ)

What it is:
Water potential is a way to precisely measure the potential energy of water. Think of it as water's "tendency" to move from one area to another. Water naturally moves from areas of higher water potential (more free water, less solutes, higher pressure) to areas of lower water potential (less free water, more solutes, lower pressure).

• Measurement: It's measured in units of pressure, like megapascal (MPa).

• Formula: The total water potential ($\Psi$) is made up of two main components:

  • $\Psi = \Psi<em>S + \Psi</em>P$

  • Where:

    • $\Psi_S$ (Solute Potential): This accounts for the effect of dissolved solutes. More solutes mean less free water, so solute potential is always zero or a negative number. Pure water has a solute potential of 0 MPa.

    • $\Psi_P$ (Pressure Potential): This accounts for any physical pressure exerted on the water. For example, the pressure of water in a plant cell pushing against its cell wall would be a positive pressure potential. If water is under tension (being pulled), it would be negative.

• Why it's useful: By knowing the water potential values for different parts of a system (like inside and outside a cell), scientists can predict exactly which way water will move.

Diffusion

• Definition: The spontaneous movements of substances from one location to another due to their random thermal motion.

  • Source: Vander

Vander’s Human Physiology, ©McGraw Hills.

Movement of Molecules

• Individual molecules move randomly, however, the net flux is directed from regions of higher concentration to regions of lower concentration.

  • Net Flux: The overall movement of particles.

  • Factors influencing net flux include:

  • Concentration gradient

  • Type of substance

  • Medium (air, liquid)

  • Temperature

  • Surface area

  • Note: Only two directions are typically indicated in diagrams.

Osmosis

• Definition: The diffusion of free water across a selectively permeable membrane.

• Resource: Download for free at https://openstax.org/details/books/biology-2e, https://cnx.org/contents/GFy_h8cu@11.2:6X9o-0n6@12/Passive-Transport

Tonicity

• Definition of Tonicity: A measure of the ability of a solution to add or remove water from cells due to osmosis.

• Types of Tonic Solutions:

  • Isotonic Solution:

  • Does not cause a change in cell volume.

  • Hypotonic Solution:

  • Causes cells to swell in volume due to a lower concentration of non-penetrating solutes than in the cell interior.

  • Hypertonic Solution:

  • Causes cells to shrink in volume due to a higher concentration of non-penetrating solutes than in the cell interior.

  • Source: Vander's Human Physiology, McGraw Hills.

Effects of Tonic Solutions on Cells

• Isotonic Solution:

  • For animal cells, isotonic environment is ideal unless the cells have adaptations to counteract osmotic changes.

  • Illustration: Animal Cell in Isotonic Solution.

• Hypotonic Solution:

  • For plant cells, they are healthiest in hypotonic conditions and become turgid (firm) as they uptake water. This is balanced by the cell wall pushing back against internal pressure.

  • Illustration:

  • Lysed cells in hypotonic solution (animal cells exposed to too much water, bursting).

  • Turgid plant cells in hypotonic solution.

• Hypertonic Solution:

  • Animal cells become shriveled due to water leaving the cell.

  • Illustration:

  • Shriveled animal cells in hypertonic solution.

  • For plant cells, they may become plasmolyzed in a hypertonic environment (loss of water leading to cell membrane detachment from cell wall).

Water Balance of Living Cells

• Microscopic Observation of Plant Cells:

  • Organism: Elodea

  • Visual representation indicating:

  • (a) cells in hypotonic solution (turgid)

  • (b) cells in hypertonic solution (plasmolyzed).

Water Potential

• Water Potential ($\Psi$):

  • Refers to water’s potential energy—the capacity for water to perform work when moving from areas of higher free water potential to lower free water potential.

  • Measured as pressure in megapascal (MPa), where 1 atm = 0.1 MPa.

  • Formula:

  • $\text{Ψ} = \text{Ψ}S + \text{Ψ}P$

  • where:

    • $\Psi S$: Solute potential (also termed osmotic potential) - it accounts for the effect of solutes on water potential (0 or negative).

    • $\Psi P$: Pressure potential - indicates the effect of physical pressure on water potential (positive or negative).

  • By definition, for pure water, $\Psi S$ = 0 MPa.

• Implications:

  • Knowing water potential values allows predictions regarding directions of water flow due to differences in water potential.

Estimating Water Potential

• Example with Potato Cells:

  • When potato cells are immersed in an isotonic sucrose solution, their mass remains unchanged, suggesting equal water potential between the cells and solution:

  • $\text{Ψ}_{\text{cell}} = \text{Ψ}_{\text{outside solution}}$

  • Through calculating the water potential of the sucrose solution known to be isotonic, the water potential of the potato cells can be derived.

Water Potential Calculation

• Calculating Solute Potential:

  • Assumption: Pressure potential of the sucrose solution is 0 (since it is open to the atmosphere)

  • Therefore, the water potential becomes equal to the solute potential.

  • $\text{Ψ} = \text{Ψ}S + \text{Ψ}P$

  • $\text{Ψ}P = 0 \text{, hence, } \text{Ψ} = \text{Ψ}S$

• Identifying Isotonic Solutions:

  • Procedure involves measuring the mass of potato pieces before and after soaking in varying molar concentrations of sucrose solutions.

  • Note: In the lab setup, the mass should be monitored for significant changes indicating isotonic conditions.

Data Collection for Osmolarity Experiment

• Table Structure (Example Data):

  • Sucrose Molarity (M): 0.0, 0.2, 0.3, 0.4, 0.5, 0.6

  • Final Weight (g): Recorded values after incubation

  • Initial Weight (g) measured pre-incubation

  • Weight Change (g): Final - Initial values

  • Percentage Change in Weight:

  • $\text{Percent Change} = \frac{\text{Weight Change}}{\text{Initial Weight}} \times 100$

• Final Note: Use graphical representations to estimate sucrose concentrations that resulted in no change in weight for potato pieces (indicative of isotonic solution).

Experiment Setup for Diffusion of Molecules

• Experiment B: Diffusion of molecules through a selectively permeable membrane, utilizing dialysis tubing bags in solutions

• Objective: Determine which molecules pass through membrane by diffusion.

  • Initial Setup includes:

  • Tubing Configuration: Closed at one end, immersed in a beaker of water and iodine solution (IKI).

  • Target Molecules: H2O, I2KI, glucose, starch.

Testing for Starch Presence

• IKI Test:

  • Post-addition of IKI to an unknown solution, observe color change:

  • Purple or black indicates starch presence.

  • Pale yellow-amber color indicates absence of starch.

Testing for Reducing Sugars

• Benedict's Test:

  • When Benedict's reagent is added and heated:

  • Color changes from blue to green, orange, or orange-red indicate the presence of reducing sugars (with color indicating concentration).

  • Maintains blue color if reducing sugar is absent.

Review of Lab Experiments

These laboratory exercises are crucial for a practical understanding of fundamental concepts in cell biology, specifically the movement of substances across membranes and the implications for cellular integrity. Students should focus on the following:

• Exercise 1.B – Diffusion of Molecules Through a Selectively Permeable Membrane: This experiment illustrates the principles of diffusion and selective permeability, demonstrating which molecules can pass through a membrane and how their movement is influenced by concentration gradients and membrane properties.

• Exercise 3A – Estimating Water Potential by Change in Weight: This exercise provides a hands-on approach to understanding water potential, osmosis, and tonicity. By observing changes in cell mass, students can experimentally determine the water potential of cells and identify isotonic environments.

• Ensure to correct titles in lab manuals as needed for accurate referencing.

• Additional study material is available in Biol 108 lecture notes, pages 788 - 791, for a deeper theoretical background.

• Lab manual text excerpts pertinent to these experiments are to be reviewed from pages 57-58 to reinforce practical procedures and expected outcomes.

Diffusion

What it is:

Imagine you spray air freshener in one corner of a room. Slowly but surely, the scent spreads throughout the entire room, even without a fan. That's diffusion! It's the natural scattering of tiny particles (like solid, liquid, or gas molecules) from an area where there's a lot of them (high concentration) to an area where there are fewer of them (low concentration). This happens because molecules are always jiggling around randomly due to their thermal energy.

How molecules move:

Individual molecules move in all directions chaotically. However, when you look at the overall movement (what we call net flux), more molecules will end up moving from the crowded area to the less crowded area than vice-versa, until they are evenly spread out.

Think of Net Flux like this:

If you have 100 people in one room and 10 in another, even if everyone is randomly walking, more people will eventually walk from the first room to the second simply because there are more of them in the first room to begin with.

Factors that influence how fast and far diffusion happens:

• Concentration gradient: The bigger the difference in concentration between two areas, the faster the net flux. (Imagine a very strong smell versus a faint one – the strong one spreads faster).

• Type of substance: Smaller, lighter molecules (like a gas) generally diffuse faster than larger, heavier ones (like a protein).

• Medium (air, liquid): Diffusion is usually faster in gases than in liquids, and even slower in solids, because molecules have more room to move around in gases.

• Temperature: Higher temperatures mean molecules have more energy, so they move faster and diffuse more quickly.

• Surface area: A larger surface area for diffusion (like the large surface area of your lungs) allows more molecules to cross at once, speeding up the process.

Osmosis

What it is:

Osmosis is a special type of diffusion, specifically the diffusion of water molecules across a selectively permeable membrane. Think of a screen door: it lets air through easily but keeps bugs out. A selectively permeable membrane is like that for water – it lets water pass through (usually very easily) but restricts the movement of other dissolved substances (solutes).

Analogy:

Imagine two swimming pools separated by a special divider. The divider lets water flow through, but it blocks sugar molecules. If one pool has a lot of sugar dissolved in it (and therefore less 'free' water), and the other has pure water, water will naturally flow from the pure water side to the sugary side to try and dilute the sugar, even without any pumping.

Tonicity

What it is:

Tonicity is a term we use to describe how a solution will affect the volume of a cell, primarily by causing water to move in or out through osmosis. It's essentially a measure of the effective solute concentration of a solution outside a cell, relative to the inside of the cell.

Types of Tonic Solutions:

1. Isotonic Solution (”Iso” means equal):

   • Definition: The concentration of non-penetrating solutes (solutes that can't cross the cell membrane) outside the cell is the same as inside the cell. Because there's no net movement of water, the cell's volume doesn't change.

   • Analogy: If you have an equal number of people on both sides of a swinging door, even if people are constantly going back and forth, the net number on each side stays the same.

   • Effect on Cells: For animal cells, an isotonic environment is ideal. Their cells are happy and maintain their shape.

2. Hypotonic Solution (”Hypo” means under/less than):

   • Definition: The concentration of non-penetrating solutes outside the cell is lower than inside the cell. This means there's a higher concentration of free water outside the cell.

   • Effect on Cells: Water will rush into the cell to try and dilute the higher solute concentration inside. This causes the cell to swell. If an animal cell takes in too much water, it can burst (we call this Lysis).

   • Analogy: Imagine a very thirsty sponge (the cell) placed in clean water (hypotonic solution). The sponge will soak up the water and expand.

   • Plant Cells in Hypotonic Solutions: Plant cells are actually healthiest in hypotonic conditions. As they take in water, their internal pressure (turgor pressure) builds up, pushing against their strong cell wall, making the cell firm or "turgid." This is why plants wilt when they don't get enough water – their cells become flaccid instead of turgid.

3. Hypertonic Solution (”Hyper” means over/more than):

   • Definition: The concentration of non-penetrating solutes outside the cell is higher than inside the cell. This means there's a lower concentration of free water outside the cell.

   • Effect on Cells: Water will rush out of the cell to try and dilute the higher solute concentration outside. This causes the cell to shrink or shrivel.

   • Analogy: If you place a plump grape inside a bowl of extremely sugary syrup (hypertonic solution), water will leave the grape and go into the syrup, causing the grape to shrivel into a raisin.

   • Plant Cells in Hypertonic Solutions: When plant cells are in a hypertonic environment, they lose water, and their cell membrane pulls away from the cell wall. This process is called plasmolysis, and the plant wilts severely and can die if the condition persists.

Water Potential ($\Psi$)

What it is:

Water potential is a way to precisely measure the potential energy of water. Think of it as water's "tendency" to move from one area to another. Water naturally moves from areas of higher water potential (more free water, less solutes, higher pressure) to areas of lower water potential (less free water, more solutes, lower pressure).

• Measurement: It's measured in units of pressure, like megapascal (MPa).

• Formula: The total water potential ($\Psi$) is made up of two main components:

  • $\Psi = \Psi S + \Psi P$

  • Where:

    • $\Psi_S$ (Solute Potential): This accounts for the effect of dissolved solutes. More solutes mean less free water, so solute potential is always zero or a negative number. Pure water has a solute potential of 0 MPa.

    • $\Psi_P$ (Pressure Potential): This accounts for any physical pressure exerted on the water. For example, the pressure of water in a plant cell pushing against its cell wall would be a positive pressure potential. If water is under tension (being pulled), it would be negative.

• Why it's useful: By knowing the water potential values for different parts of a system (like inside and outside a cell), scientists can predict exactly which way water will move.