Simple and Facilitated Diffusion

1. Describe why the phospholipid bilayer is selectively permeable
   The phospholipid bilayer is selectively permeable because of its hydrophobic region within the bilayer. Specifically, the hydrophilic heads that face outward and the hydrophobic tails that face inward, only allowing specific molecules (typically small and non-polar) to enter and exit the cell. Simultaneously blocking off bigger polar molecules via repulsion by the hydrophobic tails in the interior.

2. List the molecule types in order from permeable to impermeable

• Permeable: Small, nonpolar molecules

• Mostly permeable: Small, uncharged, polar molecules

• Mostly impermeable: Large, uncharged, polar molecules

• Impermeable: ions and polar molecules with a charge
  
  

3. Define transmembrane integral protein
   Transmembrane integral proteins span the width of the phospholipid bilayer and can aid in the transportation of materials across the membrane
   
   

4. State what property integral proteins have
   They are amphipathic, which means that integral proteins posses both hydrophilic and hydrophobic properties. Integral proteins can interact with both water and nonpolar molecules, making them crucial to perform various biological functions.

5. Define concentration gradient
   The concentration gradient is the difference in concentration that can be visualized by a hill. It follows the typical gradient rule that a bigger gradient means a steeper slope, which results in a greater rate of change over time.
   
   

6. State what is meant by “down the concentration gradient”
   “Down the concentration gradient” refers to the movement from higher concentration to lower concentration levels.
   
   

7. Distinguish between active and passive transport (using the definitions in your answer)

Active transport requires energy to move substances across a cell membrane against their concentration gradient (from low to high concentration), while passive transport does not require energy and substances move naturally down their concentration gradient (from high to low concentration). 

8. List the types of passive transport

Passive:

• Does not require an additional output of energy (ATP)

• Simple diffusion

• Facilitated Diffusion

  • Osmosis: The process of water molecules moving through a semipermeable membrane from an area of higher concentration to an area of lower concentration.
    
    

9. List the types of active transport

Active:

• Does require an addition input of energy (ATP)

• Pump proteins: also known as transporters, are transmembrane proteins that move ions and solutes across cell membranes against a concentration gradient.

• Bulk transport: The movement of macromolecules such as proteins or polysaccharides into or out of the cell.

  • Exocytosis: the process by which cells move materials from within the cell into the extracellular fluid

  • Endocytosis: process whereby cells internalize membrane proteins such as receptors and solutes, from the extracellular space by engulfing them within plasma membrane vesicles

    • Pinocytosis: The uptake of fluids or macromolecules in small vesicles

    • Phagocytosis: The ingestion of large particles, such as bacteria
      
      

10. Diffusion (general)

    1. Define
       Diffusion is the movement of a substance down a concentration gradient (from high to low) and requires no input of energy because movement isp powered by the energy that is already present (molecular movement)
       
       

    2. State the type of transport
       Diffusion is a passive transportation type and does not require ATP (ex. Air freshener).
       
       

11. Factors that affect affect diffusion rate

12. Simple Diffusion

    1. Define
       Simple diffusion refers to the movement of a substance down a concentration gradient (high to low concentration).
       
       

    2. State the type of transport
       Simple diffusion is a passive transport type that requires no energy and uses energy that is already present (molecules movement).
       
       

    3. Define dynamic equilibrium and state the net movement of molecules while at dynamic equilibrium
       Dynamic equilibrium refers to the state where opposite processes occur at equal rates, resulting in a stable, balanced internal environment despite continuous activity and fluctuations. Essentially, the system is constantly adjusting to maintain a relatively constant state—a key factor in homeostasis.

• Equilibrium: equal concentration of a substance on either side of a semi-permeable membrane

• Dynamic: costante movement, but in equal amounts and opposite directions

No Net Movement.

4. Identify the types of molecules that can do simple diffusion

• Small, nonpolar molecules

• Permeable and some mostly permeable molecules

  • Ex:

    • Oxygen (O2)

    • Carbon Dioxide (CO2)

    • Some water (H2O)

KEY CONCEPT: Mostly impermeable and impermeable molecules cross the phospholipid bilayer with a transmembrane integral protein. They cannot cross without.

• The size or the polar nature of most molecules prevents them from crossing the cell membrane without assistance.

Factors that impact diffusion rate for simply diffusion (in addition to the other 4 factors):

• Polarity: The more polar the molecules, the slower the diffusion rate through a bilayer

• Charged ions/molecules: Unable to simple diffuse
  
  

13. Facilitated Diffusion

    1. Define
       Facilitated Diffusion refers to the movement of the molecules down a concentration gradient that is assisted (facilitated) by transport proteins.
       
       

    2. State the type of transport
       Facilitated Diffusion is passive transport, meaning no external energy is needed to move the molecules in and out of the phospholipid bilayer, and it uses the energy already present.
       
       

    3. List the requirements for facilitated diffusion

For facilitated diffusion to occur, the molecules require the assistance of a transmembrane integral protein.

• Channel proteins

• Carrier proteins

Examples of molecules that use facilitated diffusion:

• Glucose

• Ions (Na+, Cl-, K+, etc.)

4. Name the types of transmembrane proteins that are used in facilitated diffusion

Channel proteins: Many channel proteins only allow 1 type of ion/molecules through based on hydrophilic/hydrophobic side chains lining the channel and the size the channel

• Can sometimes be always open, but often, their opening and closing is carefully controlled

  • Ex: Ligand-gates channels: open/close in response to a ligand (molecule) binding

  • Ex: Voltage-gated channels: open/close in response to changes in charge along the membrane

Carrier proteins: Binds with a molecule/ion to undergo a conformational change and transfers the molecules to the other side of the membrane.

Molecules/ions that are mostly impermeable and impermeable and permeable cannot pass through the cell membrane without:

1. A transmembrane integral protein specifically for them

2. Said protein being open

5. Why must it be a transmembrane integral protein and not just an integral protein?
   It must be a transmembrane integral protein and not just an integral protein because only transmembrane integral proteins can function on both slides of the bilayer or transport molecules across it.
   
   
   

6. Describe how facilitated diffusion is controlled/remains selective
   The membrane only allows selective molecules and ions to pass through it given their properties, presence of a transmembran integral protein, and opening/closing of a protein.