Cell Permeability & Transport

🔹 1. Types of Transport

Type of Transport	Description	Requires Energy (ATP)?	Driving Force	Example
Diffusion	Movement of solutes from high to low concentration	❌ No	Concentration gradient	Oxygen into cells
Osmosis	Movement of water across a semi-permeable membrane from low solute to high solute concentration	❌ No	Water potential / solute gradient	Water into plant roots
Filtration	Movement of water and solutes through a membrane by hydrostatic pressure	❌ No	Pressure gradient	Kidney glomerulus
Facilitated Diffusion	Passive movement via transport proteins	❌ No	Concentration gradient	Glucose via GLUT
Active Transport	Movement against gradient using ATP	✅ Yes	ATP energy	Na⁺/K⁺ pump

2. Cell Models and Permeability

Artificial cell models (e.g., dialysis tubing) simulate selective permeability.

Example Testing Model:

Dialysis tubing filled with starch + glucose, placed in iodine solution.

Substance	Result	Explanation
Iodine	Enters tubing (turns starch blue-black)	Small molecule → permeable
Starch	Stays inside	Too large → impermeable
Glucose	Leaves tubing (can be detected in water)	Small enough → permeable

🧪 Testing Reagent:

Benedict’s solution: Tests for glucose (blue → orange when heated)
Iodine (IKI): Tests for starch (yellow-brown → blue-black)

3. Osmosis and Tonicity

Tonicity = Effect of solution on cell water balance

Solution Type	Solute Concentration	Water Movement	Result
Isotonic	Equal inside and outside	No net movement	Cell stays same
Hypotonic	Lower outside	Water enters cell	Cell swells (may burst)
Hypertonic	Higher outside	Water leaves cell	Cell shrinks (crenates)

Testing Osmosis with Cells:

Red blood cells or potato slices in different salt solutions can show tonicity effects:
- Hypotonic: Cells swell, turgid
- Hypertonic: Cells shrink, plasmolysis
- Isotonic: No visible change

4. Filtration in Cell Models

Often modeled using pressure-driven systems, such as:
- Filter paper
- Membranes in lab models (e.g., capillary filters)

Key Concepts:

Size of particles determines what is filtered out.
Driving force: Hydrostatic pressure (e.g., blood pressure in kidneys).

🔹 5. Summary of Testing Reagents & Outcomes

Transport Type	Reagent/Test	Positive Result	Indicates
Diffusion	Time-based observation	Dye or substance spreads	Movement along gradient
Osmosis	Weight change or volume	Cell/tissue gains or loses water	Water movement
Filtration	Particle detection in filtrate	Substances found in filter output	Size-based movement
Starch Test	Iodine	Blue-black color	Presence of starch
Glucose Test	Benedict's + heat	Orange/red color	Presence of glucose

| Term | Definition |

| -------------------------- | ------------------------------------------------------------------- |

| Permeability | Ability of a membrane to allow substances to pass through |

| Selective Permeability | Some molecules can pass, others cannot |

| Tonicity | Relative solute concentration of two solutions |

| Driving Force | The factor causing movement (concentration gradient, pressure, ATP) |

1. Types of Transport

Type of Transport	Energy Requirement	Movement Direction	Mechanism	Examples in Cells
Passive Transport	No ATP required	Down concentration gradient	Movement due to kinetic energy of molecules	Diffusion, Facilitated Diffusion, Osmosis
Simple Diffusion	No ATP	High to low concentration	Direct movement across lipid bilayer	O2 $,$ CO2, small nonpolar molecules
Facilitated Diffusion	No ATP	High to low concentration	Via integral membrane proteins (channels/carriers)	Glucose, ions (e.g., $Na^+$ ), amino acids
Filtration	No ATP (hydrostatic pressure)	Bulk flow based on pressure	Movement of water and solutes across a membrane due to pressure	Kidney glomeruli
Osmosis	No ATP	High to low water potential	Movement of water across a selectively permeable membrane	Water movement in/out of cells
Active Transport	ATP required	Against concentration gradient	Movement requires energy input, often pumps specific substances	Primary/Secondary Active Transport, Bulk Transport
Primary Active Transport	Direct ATP	Low to high concentration	Uses ATP directly to move solutes (e.g., $Na^+/K^+$ pump)	$Na^+/K^+$ pump, proton pump
Secondary Active Transport	Indirect ATP (ion gradient)	Low to high concentration	Uses energy from an established ion gradient (co-transport)	Glucose co-transport with $Na^+$
Bulk Transport	ATP required	In/Out of cell	Movement of large molecules or numerous molecules via vesicles	Endocytosis (Phagocytosis, Pinocytosis), Exocytosis

2. Cell Models and Permeability

Artificial cell models (e.g., dialysis tubing) simulate selective permeability due to their microscopic pores that allow smaller molecules to pass while retaining larger ones. This mimics the lipid bilayer of a cell membrane. The size of these pores is crucial for observable results.

Example Testing Model:

Dialysis tubing, which is a selectively permeable membrane, is typically filled with an

Membrane Transport

Why do cells have a membrane?

The cell membrane (also called the plasma membrane) protects the cell, holds its contents together, and controls what enters and exits.
It helps maintain homeostasis by regulating the internal environment.

How does the structure of the membrane make it semipermeable?

The cell membrane is made of a phospholipid bilayer:
- The heads are hydrophilic (water-attracting)
- The tails are hydrophobic (water-repelling)
This arrangement allows some molecules (like small nonpolar ones) to pass freely, while others (like large or charged molecules) need help, making the membrane semipermeable.

What are the roles of proteins in the membrane?

Transport proteins: Help substances cross (channels and carriers)
Receptor proteins: Receive signals from outside the cell
Enzymes: Speed up chemical reactions
Anchor proteins: Help maintain cell shape or attach to other cells

How can substances cross the cell membrane?

Passive transport (no energy):
- Simple diffusion
- Facilitated diffusion
- Osmosis
Active transport (requires energy):
- Uses ATP to move substances against a concentration gradient

3. Definitions

Diffusion

Movement of particles from high to low concentration until evenly spread.

Osmosis

The diffusion of water across a semipermeable membrane from high to low water concentration (or from low solute to high solute concentration).

4. Tonicity and Cell Behavior

Hypertonic solution

More solutes outside the cell → water leaves the cell → cell shrinks (crenates)

Isotonic solution

Equal concentration inside and outside → no net movement of water

Hypotonic solution

More water (less solute) outside → water enters the cell → cell swells or bursts (lysis)

5. Compare Osmosis and Diffusion

Feature	Diffusion	Osmosis
Substance moving	Solutes (e.g., gases, ions)	Water
Membrane required?	No (can happen in air/liquid)	Yes (semipermeable membrane)
Direction	High to low concentration	High water concentration to low water concentration

6. What determines the direction of water movement?

Water moves toward higher solute concentration (to dilute it).
This is driven by concentration gradients.

7. Osmotic Pressure

The pressure required to prevent water from moving across a membrane by osmosis.
It increases with higher solute concentration.

8. Osmolarity vs. Tonicity

Osmolarity: Total concentration of solute particles in a solution.
Tonicity: The effect of a solution on a cell’s shape/volume.
- Tonicity depends on both osmolarity and membrane permeability to solutes.

9. Active vs. Passive Transport

Feature	Passive Transport	Active Transport
Energy required?	No	Yes (ATP)
Movement direction	High → low concentration	Low → high concentration
Examples	Diffusion, osmosis, facilitated diffusion	Pumps, endocytosis, exocytosis

10. Beaker Scenario

A beaker is split by a semipermeable membrane. Both sides start with water. You add salt to the right side, not the left.

Which way does the salt move?

Salt (solute) doesn’t move if the membrane is impermeable to it.

Which way does the water move?

Water moves from left to right (low salt → high salt) by osmosis.

If the solutes cannot move, what happens to the osmotic pressure?

Osmotic pressure increases on the right side (with salt), drawing in more water.

11. Facilitated vs. Simple Diffusion

Feature	Simple Diffusion	Facilitated Diffusion
Uses proteins?	No	Yes (channel or carrier proteins)
Type of substances	Small, nonpolar (e.g., O₂, CO₂)	Larger or charged (e.g., glucose, ions)
Energy required?	No	No
Direction	High → low concentration	High → low concentration