Chapter 3 Cell Structure and Function pt. 2

4. Diffusion and Osmosis

Materials move across membranes because of concentration differences

Diffusion and osmosis are types of passive transport.

• Passive transport: The movement of molecules across a cell membrane without energy input from the cell.
  • Saves cells a lot of energy.

Diffusion

• Diffusion: The movement of molecules in a fluid or gas from a region of higher concentration to a region of lower concentration.
  • When particles move and collide, they scatter to places where there are less particles.
  • Diffusion is passive transport
• Concentration gradient: The difference in concentration of a substance from one location to another.
• Diffusion is important for moving substances across the membrane.
  • Substances that aren’t highly concentrated in a cell are more likely to diffuse in a cell (Ex. oxygen).

Osmosis

• Osmosis: The diffusion of water molecules.
  • Like other molecules, water molecules will go to areas with lower water concentration
• There are different ways a solution can be identified as:
• Isotonic: When a solution has the same concentration of dissolved particles as the cell.
  • The cell’s size remains constant as the water comes and goes at an equal rate.
• Hypertonic: When a solution has a higher concentration of dissolved particles than a cell.
  • Water flows out of the cell at a faster rate which causes the cell to shrivel.
• Hypotonic: When a solution has a lower concentration of dissolved particles than a cell.
  • More water flows into the cell which expands or even bursts the cell.

Some molecules diffuse through transport proteins

• Certain molecules cross easier through the membrane by transporting proteins.
• Facilitated diffusion: The diffusion of molecules across a membrane through transport proteins.
• Still a form a passive transport (No energy is required).

5. Active Transport Endocytosis, and Exocytosis

Cells use energy to transport materials that cannot diffuse across a membrane.

Proteins can transport materials against a concentration gradient.

• Pumps are transport proteins that move materials AGAINST a concentration gradient.
• Active transport: Moves molecules across a membrane from an area of lower concentration to an area of higher concentration.
  • Active transport proteins move the molecules with chemical energy.

Endocytosis and exocytosis transport materials across the membrane in vesicles.

• Transport in vesicles lets substances enter or leave a cell without having to cross the membrane.

Endocytosis

• Endocytosis: The process of taking liquids or fairly large molecules into a cell by surrounding them in a membrane.
• Phagocytosis: A type of endocytosis where the cell membrane surrounds large particles.
• Think endo = in

Exocytosis

• Exocytosis: The release of substances out of a cell by sending materials out of the membrane through a vesicle.
  • The exact opposite of Endocytosis
  • Vesicles fuse with the cell membrane and releases substances outside of the cell.
• Think exo = out