2.4 Active Transport, Simple Diffusion & Osmosis, Facilitated Diffusion, Endocytosis, Exocytosis, and Receptor Mediated Transport

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What is passive transport?

The transport of molecules through the use of the concentration gradient (from high conc. → low)
DOES NOT NEED ENERGY

What is active transport?

The transport of molecules against the concentration gradient (now from low conc. → high) by USING ENERGY (usually ATP)

Primary/Direct Active Transport

Move positive ions across the cell membrane, using energy to go against the concentration gradient

Primary Transport: Sodium-Potassium Pump

Pump Na⁺ ions OUT of the cell, K⁺ ions INTO the cell

1. Pump faces inside cell, has high affinity for Na⁺ ions → binds to 3

2. ATP also binds to pump, phosphate group breaks off and attaches separately, leaves ADP as byproduct

3. Binding to phosphate group tiggers pump to face outwards, loses affinity for Na⁺ ions → dumps 3 outside of cell

4. High affinity for K⁺ ions → binds to 2, no longer attracted to phosphate group → breaks off

5. Losing phosphate group triggers pump to face inwards, loses affinity for K⁺ ions → dumps 2 inside cytoplasm

   • Back to step 1

Secondary Active Transport

Energy source derives from primary pump’s concentration gradient
Symport: Transported solute moves in same direction as driving solute

• Eg. Sodium (driving) & Glucose

• Glucose goes against its gradient to follow sodium

Antiport: Transported solute moves in opposite direction as driving solute

• Eg. Sodium (driving) & Calcium

• Calcium goes against its gradient to go in opposite direction as sodium

Active Transport Example: Soil → Root Hairs

Low solute conc. in soil, high ion conc. in root hairs
Stored energy moves minerals into root hairs (against conc. gradient) for the plant to absorb

3 Things That Happen if Active Transport Malfunctions

1. Accumulate Waste Products

2. Disrupt Fluid Balance

   • Typically, active transport maintains fluid between cells + body compartments

   • Without = dehydration

3. Less/More Nutrient Intake

   • Without active transport bringing nutrients into the cell, less energy = impaired function

What is simple diffusion?

Moving molecules across the cell membrane without energy — following the concentration gradient

• Hydrophobic: enters lipid bilayr

• Hydrophilic: bounces back

What is osmosis?

The passive diffusion of water through the cell membrane

Solute Concentration vs Water Potential

HIGH SOLUTE CONC. = LOW WATER POTENTIAL

LOW SOLUTE CONC. = HIGH WATER POTENTIAL

water will go where there’s a higher solute conc.

What happens when a cell is placed in a hypotonic solution?

LOW SOLUTE CONC. OUTSIDE vs inside the cell
WATER MOVES IN → causes cell to swell, potentially burst

What happens when a cell is placed in a hypertonic solution?

HIGH SOLUTE CONC. OUTSIDE vs inside the cell
WATER MOVES OUT → causes cell to shrivel

What happens when a cell is placed in an isotonic solution?

SAME SOLUTE CONC. IN & OUT
Nothing happens

What does hyposmotic mean?

The cell OR solution has a LOWER solute concentration

What does hyperosmotic mean?

The cell OR solution has a HIGHER solute concentration

What does isosmotic mean?

The cell and solution have the same solute concentration

What is facilitated diffusion?

The transport of ions and polar molecules across the cell membrane via proteins

• Still follows concentration gradient

• Also dependent on transport protein efficiency

• No energy required!

Rate of Diffusion: Simple Diffusion vs. Facilitated Diffusion

Simple diffusion: As conc. diff. increases, rate of diffusion increases at a constant rate

Facilitated diffusion: As conc. diff. increases, rate of diffusion increase a lot UNTIL a max. amount (when there’s not enough proteins per molecule)

Facilitated Diffusion: Channel Proteins

• Channels solutes across membrane

• Much faster than carrier proteins!

• Eg. Malfunction = Cystic Fibrosis

  • CFTR channel can’t open

  • Water remains inside cells

  • Sticky mucus lining

Facilitated Diffusion: Carrier Proteins

• Binds to molecule to be carried across membrane (that’s why slower than channel proteins)

• Specific to solute (similar to enzymes)

1. Carrier proteins open towards high solute conc.

2. Solute binds to protein!

3. Triggers protein to open towards low solute conc.

4. Solute released, protein reverts to step 1

What is exocytosis and endocytosis?

Process that allow eukaryotes to transport large molecules

• Both REQUIRE ENERGY

Exocytosis: Export materials from cytosol → cell exterior

Endocytosis: Import materials from cell exterior → engulf inside cytosol

Cytosol: Fluid portion of cytoplasm

How does exocytosis work? Examples?

• Vesicles within cytoplasm release materials into plasma

• Eg. Enzymes + mucus secreted by cells lining digestive tracts

How does endocytosis work?

• Cell membrane surrounds substance outside membrane

• Pocket pinched off to form: endocytic vesicle

What are the steps for receptor mediated endocytosis?

1. Molecules outside of cell bind to specific receptor proteins on cell’s surface

2. Gather in a pit coated in clathrin proteins

3. Coated pit forms vesicle, separates from membrane

4. Vesicle loses protein coating → fuses with lysosome

5. Digested by enzymes

Receptor Mediated Endocytosis Malfunction: Cholesterol Example

• RME removes “bad cholesterol” (low-density lipoproteins) from blood

• Hypercholesterolemia: Missing LDL receptors = failure to remove LDL = dangerous levels in blood