Unit 1: Biochemistry - # 10 Passive and Active Transport (copy)

Passive Transport

• The movement of a substance across a membrane without the need to expend chemical energy (ATP)

• Universe tends towards disorder (entropy)

• If molecules are more concentrated on one side of a membrane, they will become equally distributed on both sides until equilibrium is reached

What are the three types of passive transport?

1. Simple Diffusion

2. Facilitated Diffusion

3. Osmosis

Simple Diffusion

• The ability of a substance to move across a membrane unassisted 

• Movement of a substance from high to low concentration (no energy needed) 

• Rate of diffusion depends on the concentration gradient between two sides of a membrane 

• Dynamic equilibrium - even after the concentration of molecules is the same on both sides, they continue to move from one side to the other 

What can Diffuse Through The Phospholipid Bilayer?

• Very small non-polar molecules can get through directly (eg. oxygen gas and carbon dioxide) 

• Small, uncharged polar molecules (eg. water and glycerol can also cross easily) 

What Molecules Cannot Can’t Through Directly?

• Ions (positively charged Cl, negatively charged K and positively charged Na) 

• Large uncharged polar molecules (polysaccharides and proteins)

Facilitated Diffusion

• Diffusion through transport protein channels 

• Channels help move specific molecules across cell membrane 

• Still driven by concentration gradient (high to low concentration) 

In what two ways does the membrane become semi-permeable?

1. Channel Proteins

2. Carrier Proteins

Channel Proteins

• A hydrophilic pathway in a membrane that enables water and ions to pass through

• Open tunnel

• Specific channels allow specific material across cell membrane

Carrier Proteins

• Protein changes shape and allows the solute to enter/exit the cell

• Form passageways through the lipid bilayer 

• Each binds to a specific solute and transports it across the bilayer

Osmosis

• Diffusion of water from high concentration of water (low amount of solute) to low concentration of water (high amount of solute).

• Water will move to the more concentrated side (more solute) to balance it out.

• Across a semipermeable membrane 

• Water will always chase the hypertonic side (high amount of solute) 

Concentration of Water

• Direction of osmosis is determined by comparing total solute concentrations 

• Hypertonic: More solute, less water 

• Hypotonic: Less solute, more water 

• Isotonic: Equal solute, equal water

Active Transport

• Cells may need to move molecules against concentration gradient 

• From low concentration to high concentration 

• Use of protein “pumps” 

• The term “active” refers to the fact that the cell has to expend energy = ATP

What are the two types of active transport?

1. Primary Active Transport

2. Secondary Active Transport Pumps

Primary Active Transport

• Pumps (carrier proteins) that move positively charged ions across the membrane 

• Electrochemical Gradient: 

  • Voltage across a membrane is a difference in electrical charge on either side of a membrane 

  • Forms as a result of many positive cations on one side of a membrane compared to the other 

  • Both the voltage difference and difference in ion concentration creates an electrochemical gradient 

  • It is a form of stored potential energy which is used in nerve impulse transmission or to make ATP 

Example of Primary Active Transport

• Sodium-potassium pump (Na+/K+ pump)

• Transports sodium ions (Na+) out of the cell and potassium ions (K+) into the cell, both against their concentration gradients.

• This process is essential for maintaining cell membrane potential and regulating cell volume and ion balance.

Secondary Active Transport Pumps

• Uses the concentration gradient of an ion set up by a primary active transport pump as its energy source 

• As the ion flows back along its concentration gradient, it brings a second molecule/ion along with it

Symport

A solute that moves through the membrane channel in the same direction as the driving ion

Antiport

The driving ion moves through the membrane in one direction providing energy for the transport of another molecule in the opposite direction