Osmosis and Active Transport
Osmosis: Review Examples
Free Water vs. Bound Water
This concept was previously introduced, referring to the distinction between water molecules readily available for diffusion and those associated with solutes.
U-Shaped Tube Example
Setup: A U-shaped tube is divided by a membrane permeable to sodium chloride (NaCl) but not to glucose.
Side A (Initial): Contains 0.5 M NaCl.
Side B (Initial): Contains 0.4 M NaCl and 0.4 M Glucose.
Salt Diffusion (Initial):
Net diffusion of salt occurs from Side A to Side B because 0.5 M > 0.4 M.
Salt Equilibrium: Salt diffuses until its concentration equalizes on both sides.
After salt reaches equilibrium, both Side A and Side B will have 0.45 M NaCl (assuming initial average between 0.5 M and 0.4 M).
Glucose: Glucose cannot pass through the membrane, so its concentration remains unchanged on Side B at 0.4 M, and absent on Side A.
Solute Concentration After Salt Equilibrium:
Side A: Total solutes = 0.45 M NaCl (since no glucose is present on Side A).
Side B: Total solutes = 0.45 M NaCl + 0.4 M Glucose = 0.85 M.
Conclusion: Side B (0.85 M) is concentrated, and Side A (0.45 M) is dilute.
Free Water Concentration and Osmosis:
Side A (dilute) has a higher concentration of free water molecules.
Side B (concentrated) has a lower concentration of free water molecules.
Net Movement of Water: According to the law of osmosis, water will move from an area of high concentration of free water (dilute solution) to an area of low concentration of free water (concentrated solution).
Therefore, the net movement of water will be from Side A to Side B.
Principle: Water always moves towards the more concentrated solution to equalize the concentrations.
Cell/Bag Osmosis Example
Setup: A cell (or bag) whose membrane is permeable to glucose and fructose, but not to sucrose.
Inside the bag: Contains 0.03 M sucrose.
Outside the bag: Contains 0.1 M sucrose.
Glucose and Fructose Diffusion: Glucose and fructose will diffuse across the membrane until their concentrations reach equilibrium (i.e., become equal on both sides).
Determining Concentrated Solution: After glucose and fructose concentrations equilibrate, the critical factor for net water movement, since sucrose is impermeable, is the overall solute concentration.
Conclusion (as stated): The solution inside the bag is concentrated, implying that the overall solute concentration inside is higher than outside.
Net Movement of Water: Since the inside of the bag is concentrated, water will move from the outside (dilute) to the inside (concentrated).
Therefore, the net movement of water will be into the bag.
Effect: The amount of solution inside the bag will increase.
Tonicity: Types of Solutions
Tonicity compares the concentration of solutes in a solution outside the cell to the concentration of solutes inside the cell.
Isotonic Solution ("Iso" = Same)
Definition: The concentration of solutes outside the cell is the same as the concentration of solutes inside the cell.
Example: Both inside and outside the cell have 0.9% salt concentration.
Free Water Concentration: Because solute concentrations are equal, the concentration of free water molecules is also the same inside and outside the cell.
Net Water Movement: The amount of water entering the cell equals the amount of water leaving the cell. There is no net movement of water.
Cellular Outcome:
Animal Cells (e.g., Red Blood Cells): Remain normal and healthy ("happy").
Plant Cells: Are typically flaccid (loss of turgor, but not plasmolyzed, as they usually require a slightly hypotonic environment to be turgid).
Hypertonic Solution ("Hyper" = High)
Definition: The concentration of solutes outside the cell is higher than the concentration of solutes inside the cell.
Example: Outside solution is 0.9% salt, inside is 0.5% salt.
Free Water Concentration:
Outside solution (concentrated) has a low concentration of free water.
Inside solution (dilute) has a high concentration of free water.
Net Water Movement: Water moves from the cell (high free water) out of the cell (low free water) to the concentrated external solution.
Cellular Outcome: The cell shrinks.
Animal Cells: Undergo crenation (shriveling).
Plant, Fungal, and Bacterial Cells (with cell walls): Undergo plasmolysis. The plasma membrane pulls away from the cell wall, but the cell wall itself remains intact.
Hypotonic Solution ("Hypo" = Low)
Definition: The concentration of solutes outside the cell is lower than the concentration of solutes inside the cell.
Example: Outside solution is 0.2% salt, inside is 0.9% salt.
Free Water Concentration:
Outside solution (dilute) has a high concentration of free water.
Inside solution (concentrated) has a low concentration of free water.
Net Water Movement: Water moves from the outside (high free water) into the cell (low free water) to the concentrated internal solution.
Cellular Outcome: The cell swells.
Animal Cells: The cell continues to swell and will burst (lyse) due to lack of a rigid cell wall.
Plant, Fungal, and Bacterial Cells (with cell walls): The cell swells and becomes turgid (hard or rigid). The central vacuole fills with water, and the cell wall prevents the cell from bursting, maintaining turgor pressure.
Summary of Osmotic Concepts
If one solution is hypotonic, the solution it is compared to is hypertonic.
Water will always move from a hypotonic solution to a hypertonic solution (i.e., towards the side with higher solute concentration).
Example: If Arm A is dilute (hypotonic) and Arm B is concentrated (hypertonic), water moves from A to B.
Real-World Applications of Tonicity
Carrot Sticks in Fresh Tap Water: Become stiff and hard (turgid).
This indicates tap water is hypotonic to the carrot cells, causing water to move into the cells.
Celery Stalks:
In fresh water: Become stiff and hard (turgid), similar to carrots. The fresh water is hypotonic to the celery cells.
In 0.15 M salt solution: Become limp and soft (flaccid/plasmolyzed). The salt solution is hypertonic to the celery cells, causing water to move out.
Conclusion: Cells of the celery stock are hypertonic to fresh water (water enters them) but hypotonic to the salt solution (water leaves them).
Red Blood Cells (RBCs) in 1.8% Sodium Chloride (NaCl) Solution:
Given internal RBC concentration is 0.9% NaCl.
The 1.8% NaCl solution is hypertonic to the RBCs (because 1.8% > 0.9%).
Water will move out of the RBCs, causing them to shrink.
Conversely, the RBC solution (0.9%) is hypotonic to the external solution (1.8%).
Glucose Solutions and RBCs:
If 5% glucose is isotonic to RBCs, then:
1% glucose is hypotonic to RBCs (as 1% < 5%), causing water to enter and potentially burst the cell.
10% glucose is hypertonic to RBCs (as 10% > 5%), causing water to leave and the cell to shrink.
Active Transport
Definition: Movement of substances against their concentration gradient (from an area of low concentration to an area of high concentration).
Energy Requirement: Requires energy, typically in the form of ATP.
Specificity: Carrier proteins involved in active transport are specific to the substances they transport.
Types of Carrier Proteins in Active Transport
Uniporters: Transport a single solute in one direction.
Symporters: Transport two different solutes simultaneously in the same direction.
Antiporters: Transport two different solutes simultaneously in opposite directions (one into the cell, one out of the cell).
Sodium-Potassium Pump (Na^+/K^+ Pump)
Example: A classic example of an antiporter active transport system.
Concentration Gradients Maintained:
Na^+ concentration is low inside the cell and high outside the cell.
K^+ concentration is high inside the cell and low outside the cell.
Mechanism: The pump uses ATP to move ions against their gradients:
3 molecules of Na^+ are pumped out of the cell (from low to high concentration).
2 molecules of K^+ are pumped into the cell (from low to high concentration).
This transport occurs in opposite directions.
Importance: Crucial for muscle contraction and nerve impulse transmission.
Osmoregulation: Salmon Life Cycle
Salmon are unique in their ability to survive in both freshwater and seawater, undergoing behavioral adaptation in estuary environments (where freshwater meets ocean water).
They lay eggs in freshwater, migrate to the ocean to grow into adults, and then return to freshwater to lay eggs and die.
Strict freshwater fish die in ocean water, and strict ocean fish die in lakes.
Salmon in Ocean Water (High Solute - Hypertonic Environment)
Solute Concentration: Ocean water has a high solute concentration (e.g., 3.5% solutes), making it hypertonic to the fish's cells (which have approximately 1% solutes).
Osmotic Challenge: Water tends to move out of the fish's cells into the hypertonic ocean water via osmosis.
Adaptations to Prevent Dehydration:
Drinks large volumes of water: The fish actively drinks liters of ocean water to compensate for water loss.
Excretes concentrated urine: To conserve water and minimize further water loss.
Actively transports salts out: Gills contain specialized cells that use active transport to excrete excess salts (from the ingested ocean water) back into the surrounding seawater, against their concentration gradient.
Salmon in Freshwater (Low Solute - Hypotonic Environment)
Solute Concentration: Freshwater has a very low solute concentration (e.g., less than 0.1% solutes), making it hypotonic to the fish's cells.
Osmotic Challenge: Water tends to move into the fish's cells from the hypotonic freshwater via osmosis, potentially causing them to swell and burst.
Adaptations to Prevent Swelling/Bursting:
Does not drink water: The fish does not need to drink water, as water is constantly entering its body.
Produces very dilute urine: To excrete the excess water that continually enters its body, minimizing internal volume.
(Implied) Actively absorbs salts: To prevent critical loss of salts from its body into the dilute environment (though not explicitly stated in the provided transcript for freshwater).
Bulk Transport
Mechanism: Substances are transported into or out of the cell without passing through the phospholipid bilayer. Instead, the cell or cell membrane engulfs or expels them.
Energy Requirement: This process requires cellular energy.
Endocytosis ("Endo" = Inside)
Definition: The process by which cells take substances into the cell.
Types of Endocytosis:
Phagocytosis ("Cell Eating"):
Involves the ingestion of solid particles or particulate matter (e.g., bacterial cells).
The cell membrane extends pseudopods to engulf the particle, forming a large vesicle called a food vacuole.
Pinocytosis ("Cell Drinking"):
Involves the ingestion of liquid droplets or dissolved solutes.
The cell membrane invaginates, forming small vesicles filled with extracellular fluid.
Receptor-Mediated Endocytosis:
Highly specific process where particular substances (ligands) bind to specific receptor proteins on the cell surface.
Once bound, both the receptors and the attached particles are selectively engulfed by the cell, forming a coated vesicle.
Exocytosis ("Exo" = Outside)
Definition: The process by which cells export substances out of the cell.
Mechanism: Also known as secretion or export.
Substances are packaged into secretory vesicles within the cell.
These vesicles then move to the plasma membrane, fuse with it, and release their contents into the extracellular space.
Examples: Export of hormones, neurotransmitters, and digestive enzymes.
Introduction to Thermodynamics
First Law of Thermodynamics: Conservation of Energy
Principle: Energy cannot be created or destroyed.
Transformation: Energy can only be transformed from one form to another.
Example: Solar energy is transformed (e.g., during photosynthesis).
Second Law of Thermodynamics: Entropy
Principle: While energy transformations occur, they are never 100% efficient.
Energy Loss: In every energy transformation, some energy is lost, typically in the form of heat.
Entropy Increase: This released heat increases the entropy (disorder or randomness) of the environment.
Example: Molecules of water move from a highly ordered solid state (ice) to a less ordered liquid state, and then to a highly disordered gaseous state (steam), demonstrating increasing entropy.
Definition of Energy
Energy: The capacity to do work or bring about change.
Units: Measured in calories (cal) or joules (J).
Forms of Energy
Kinetic Energy: Energy associated with motion.
Examples: Movement of molecules, speeding car, falling rock, a thrown or rolling ball, heat, light.
Potential Energy: Stored energy based on an object's position or structure.
Examples: Chemical energy (found in food, gas, wood) and gravitational energy (e.g., a person standing at the top of stairs or ready to slide).
Energy Transformation Examples
Girl on Stairs/Slide:
Going up the stairs: Kinetic energy.
Standing at the top ready to slide: Potential energy.
Sliding down: Potential energy converted to kinetic energy.
Burning Wood: Wood possesses potential (chemical) energy. When burned, this potential energy is converted to kinetic energy in the form of heat and light.
Solar Energy to Glucose (Photosynthesis):
Solar energy (light) is a form of kinetic energy.
Photosynthesis transforms this solar energy into chemical energy in ATP (potential energy).
This chemical energy in ATP is then further transformed into chemical energy in glucose (also potential energy).
Thus, there is a transformation: Kinetic (light)
ightarrow Potential (ATP)
ightarrow Potential (Glucose).
Glucose to ATP (Cellular Respiration):
Animals consume plants or other animals, metabolizing glucose (chemical potential energy).
Cellular respiration transforms the chemical energy stored in glucose into chemical energy in ATP (another form of potential energy).
ATP to Mechanical Work (Muscle Contraction):
The chemical energy in ATP (potential energy) is transformed into mechanical energy (kinetic energy) to perform cellular work, such as muscle contraction.
ATP hydrolysis releases free energy (approximately -7.3 ext{ kcal/mol}) for this process.
Entropy in Transformations: In all these energy transformations, heat is released, which increases the entropy of the environment, in accordance with the Second Law of Thermodynamics.
Oxidation and Reduction (Redox Reactions)
Definition: Chemical reactions that involve the transfer of electrons between reactants.
Atomic Particles Review: Protons (+, positive charge), Electrons (- , negative charge), Neutrons (neutral charge).
Oxidation
Definition: A substance loses electrons.
Mnemonic Devices:
LEO the lion says GER: Lose Electrons Oxidation.
OIL RIG: Oxidation Is Loss (of electrons).
Reduction
Definition: A substance gains electrons.
Charge Change: When a substance gains electrons, its overall charge becomes more negative or is "reduced" (e.g., from 0 to -1).
Mnemonic Devices:
LEO the lion says GER: Gain Electrons Reduction.
OIL RIG: Reduction Is Gain (of electrons).
Coupled Reactions
Oxidation and reduction reactions are always coupled; if one substance is reduced, another substance must be oxidized.
Example: Formation of Sodium Chloride (Na + Cl
ightarrow NaCl)
Reactants Initial State:
Sodium Atom (Na): 11 protons, 11 electrons = net charge of 0.
Chlorine Atom (Cl): 17 protons, 17 electrons = net charge of 0.
Electron Transfer: Chlorine is more electronegative than sodium, so it attracts and accepts an electron from sodium.
Products Final State:
Sodium Ion (Na^+):
Protons: 11
Electrons: 10 (original 11 minus 1 lost electron)
Net charge: +1
Outcome: Sodium lost an electron, so it was oxidized.
Chloride Ion (Cl^-):
Protons: 17
Electrons: 18 (original 17 plus 1 gained electron)
Net charge: -1
Outcome: Chlorine gained an electron, so it was reduced (its charge was reduced from 0 to -1).
Conclusion: In this reaction, chlorine is reduced, and sodium is oxidized.
Reducing and Oxidizing Agents
Reducing Agents: These are the substances that donate electrons to another substance, causing the other substance to be reduced. In donating electrons, the reducing agent itself becomes oxidized.
Oxidizing Agents: These are the substances that accept electrons from another substance, causing the other substance to be oxidized. In accepting electrons, the oxidizing agent itself becomes reduced.
(Further discussion on these agents to continue in the next meeting.)