Chapter 1–7 Review: Basic Transport, Osmosis, Diffusion, Tonicity, Filtration, pH and Buffers

Brownian motion

Brownian motion: random zigzag movement of particles due to constant collisions with other molecules.
Role: drives diffusion by continually jostling particles, increasing the chance that solute particles encounter and disperse through the solvent.
When we say “random zigzag patterns,” we’re describing Brownian motion, which helps surface area and mixing of solute in solvent.

Solute and Solvent (key terminology)

Solute: the substance being dissolved; the particles or “stuff” that are dispersed in the solvent.
Solvent: the medium doing the dissolving; the substance in which the solute is dispersed; typically water, but can be air in some dispersal contexts.
Common pairing: solute in solvent (e.g., salt (solute) in water (solvent)).
Quick tip: write the words next to each other to anchor understanding (solite/solute, solvent).

Temperature and motion

Temperature effect: the rate of molecular motion increases with temperature.
Higher temperature → faster movement → faster diffusion.
Lower temperature → slower movement → slower diffusion.
Everyday example: hot water dissolves substances faster (e.g., tea dissolving sugar) than cold water, regardless of the solute.
Conceptual takeaway: heat up the system to speed up mixing; cool down to slow it.

Passive transport (no cellular energy required)

Definition: movement along a concentration gradient from high to low concentration.
Key idea: no cellular energy expenditure is required; particles simply move downhill along their gradient.
Related analogy: sledding down a hill from high to low position.
Forms of passive transport covered:
- Simple diffusion
- Facilitated diffusion
- Osmosis (special case involving water as solvent)

Simple diffusion

Definition: movement of solute particles from regions of higher concentration to regions of lower concentration.
Core concept: the solute (the particles) diffuse through the solvent.
Visual example: drop of dye in a glass of water gradually disperses until uniform color.
Three main factors affecting diffusion rate:
1) Temperature: higher temperature speeds up particle movement and diffusion.
2) Particle size: smaller particles diffuse faster; larger particles diffuse more slowly.
3) Concentration gradient: greater differences in concentration lead to faster diffusion.
Summary: simple diffusion is diffusion without a helper protein and does not require energy.

Facilitated diffusion

Definition: transport that uses membrane proteins to assist the movement of substances across the cell membrane.
It is still diffusion (no energy required), but it requires a helper gatekeeper protein to move the solute in or out.
Key distinction from simple diffusion: there is a membrane protein involved (gatekeeper).
Conceptual takeaway: diffusion can be either simple (no gatekeeper) or facilitated (needs a gatekeeper).

Osmosis

Osmosis: diffusion of water (the solvent) across a selectively permeable membrane.
Selectively permeable membrane: a membrane that allows water (and some solutes) to move but restricts certain solutes.
Core rule: when the membrane is selectively permeable, the solvent moves to balance concentrations when solute is confined to one side.
Important phrase: “selectively permeable membrane” should cue osmosis in your mind.
Direction of movement: water moves from higher water potential (lower solute concentration) to lower water potential (higher solute concentration).
Conceptual note: in osmosis, the solute does not move (in this context); instead, the solvent moves to balance solute concentrations.

Osmotic scenarios: hypertonic, hypotonic, isotonic

Important terms (in relation to the solution surrounding the cell):
- Hypertonic: the solution outside has a higher solute concentration than the inside (relative to the cell).
- Hypotonic: the solution outside has a lower solute concentration than the inside.
- Isotonic: the solute concentrations are equal inside and outside.
Classic diffusion vs osmosis framing:
- For diffusion of solutes, particles move from high to low concentration.
- For osmosis (water movement), water moves from high water concentration (low solute) to low water concentration (high solute).
Example scenario (cell and solution):
- Scenario 1: Inside the cell 40% solute, outside solution 0% solute.
- The cell is hypertonic relative to the outside solution.
- Water tends to move into the cell (to balance solute concentrations). The cell weight increases.
- Scenario 2: Inside the cell 0% solute, outside solution 40% solute.
- The cell is hypotonic relative to the outside solution.
- Water tends to move out of the cell toward the higher solute outside; the cell weight decreases.
- Scenario 3: Inside and outside both 0% solute (isotonic).
- No net water movement; the cell weight remains the same.
Isotonic balance point: the goal is a middle ground where solute and water concentrations balance to minimize net movement, around a 55/45 type equilibrium in the example context.
Visuals: red blood cell shapes change with tonicity (hypotonic makes a cell swell, hypertonic makes it shrink; isotonic keeps it like a donut/cheerio).
Important note: in osmosis, when water moves, the direction depends on relative water potential; the terminology is always relative to the two sides being compared.
Practical exam hint: expect questions asking you to identify hypertonic, hypotonic, or isotonic conditions and predict the direction of water movement and whether the cell will swell or shrink.

Quantitative practice and box-dasing examples (conceptual, not always numeric)

Boxed setup (dialysis bag analogy): compare solute concentrations inside the cell/box to the surrounding solution.
- Example A: inside 40% solute, outside 0% solute → cell is hypertonic; water moves into the cell; weight increases.
- Example B: inside 0% solute, outside 40% solute → cell is hypotonic; water moves out of the cell; weight decreases.
- Example C: inside 0% solute, outside 0% solute → isotonic; no net movement; weight stays the same.
Conceptual takeaway: always compare solute concentration on both sides to decide direction of movement; water moves to balance solute concentrations across the membrane.

Filtration

Filtration defined: movement driven by hydrostatic pressure (water pressure) from an area of higher pressure to lower pressure.
Analogy: coffee filter, water pressure pushes small particles through the filter; those particles move along with the liquid.
Key point: this is a pressure-driven process, not diffusion of solutes across a membrane via concentration gradients.
Summary: filtration = movement by hydrostatic pressure (↑ pressure → more filtration).

Acids, bases, pH, and buffers

pH basics: pH measures the hydrogen ion concentration; pH = -
log_{10}([H^+]).
pH scale: 0 (strong acid) to 14 (strong base); 7 is neutral.
Acids and bases:
- Acids: higher hydrogen ion concentration; lower pH.
- Bases: lower hydrogen ion concentration; higher pH.
Important counterintuitive note from this lecture: high [H^+] corresponds to a low pH (not high pH).
Blood pH range for a healthy human: approximately
7.35 \leq pH \leq 7.45.
Typical value around 7.40, which is slightly alkaline.
Buffers: substances that resist changes in pH by either accepting or donating hydrogen ions as needed.
- Mechanism: buffers balance H+ by binding excess protons when the environment becomes too acidic; they release protons when the environment becomes too basic.
- Common intracellular buffer: bicarbonate (HCO3−).
- Conceptual Henderson–Hasselbalch for buffers (for a weak acid HA and its conjugate base A−):
  pH = pK_a + \log \left(\frac{[A^-]}{[HA]}\right).
- In the bicarbonate buffering system, CO₂ hydration and equilibrium with carbonic acid drive the H⁺ binding/release, maintaining blood pH around the normal range.
Practical takeaway: buffers help keep pH from swinging too far in either direction, preserving physiological conditions.

Macromolecules: macro- vs micro-scale terminology

Macro- means large; micro- means small.
Terminology is helpful for quick comprehension as you study biochemistry and cell biology.
The lecturer emphasizes understanding testing agents and what each test detects; this is often explored in lab contexts.
Note: this section is more about terminology and testing context; the exact tests and agents are class-specific and can vary by course.

Key connections to foundational principles

Diffusion and osmosis both arise from Brownian motion and concentration gradients; energy is not required for diffusion but the process is driven by random motion.
Temperature, particle size, and concentration gradient are fundamental factors that modulate diffusion rates across all contexts.
Facilitated diffusion combines passive transport with membrane proteins, illustrating how barriers (cell membranes) alter diffusion efficiency.
Osmosis highlights the central importance of the solvent (water) and how selective permeability governs movement to balance solute distributions.
The tonicity framework (hypertonic, hypotonic, isotonic) is a practical tool for predicting cell volume changes and is essential for understanding physiological processes and medical scenarios.
pH, acids/bases, and buffers tie chemistry to biology; maintaining stable pH is critical for enzyme activity, protein structure, and metabolic function.
The bicarbonate buffering system links respiratory physiology (CO₂ levels) to acid-base balance, illustrating the integrative nature of biology.

Quick recap of common formulas and concepts (LaTeX)

Diffusion flux (general form):
J = -D \frac{dC}{dx}
Diffusion rate is proportional to the concentration gradient:
\text{Rate} \propto \Delta C
Water movement via osmosis is driven by water potential differences (conceptual): water moves from higher water potential to lower water potential, balancing solute concentrations.
pH definition:
\text{pH} = -\log_{10}([H^+]).
Henderson–Hasselbalch (buffer context):
\text{pH} = pK_a + \log\left(\frac{[A^-]}{[HA]}\right).
Isotonic/hypertonic/hypotonic definitions (relative comparison):
- Isotonic: inside concentration = outside concentration (Cin = Cout)
- Hypertonic: Cin > Cout (solution outside is relatively more concentrated)
- Hypotonic: Cin < Cout (cell interior is relatively more concentrated)

Exam-oriented reminders

Expect questions asking you to identify whether a solution is hypertonic, hypotonic, or isotonic relative to a cell and predict the direction of water movement and changes in cell size.
Be able to distinguish diffusion vs facilitated diffusion by whether a membrane protein (gatekeeper) is involved.
Be comfortable with basic buffering concepts and the bicarbonate buffer as a representative system in physiology.
Understand how to interpret simple dialysis or cell-possession scenarios in terms of solute vs solvent movement and tonicity.

Final note

The content emphasizes building a solid grasp of basic terms (solute, solvent, diffusion, osmosis, tonicity, facilitated diffusion, filtration), then applying them to practical examples and exam-style questions.
If any part feels murky, revisit the specific definitions and walk through the provided scenarios step by step to see which side is hypertonic/hypotonic and where the water or solute is moving.