Cell Membrane and Cellular Transport

How Molecules Enter and Exit Cells

• The cell membrane determines what enters and exits the cell, and it is how the cell interacts with its environment.
• The cell membrane is a phospholipid bilayer structure that provides a semipermeable barrier between the inside and the outside of the cell.

Cell Survival

• The cell needs some substances to be brought into the cell and some to be moved out of the cell in order to survive.
• The cell membrane controls the transport of molecules between the outside environment and the interior of the cell to maintain homeostasis.

Homeostasis

• Homeostasis is the balance of conditions inside the cell for normal function.
• The cell membrane plays a crucial role in helping the cell maintain homeostasis.

Passive/Simple Diffusion

• Passive diffusion is the natural tendency of molecules to spread out to fill up the spaces available to them.
• In simple diffusion, molecules spontaneously move from areas of high concentration to areas of low concentration due to random movement.
• Diffusion happens when molecules move from a high concentration to a low concentration, moving down or with their concentration gradient.
• Examples of diffusion include:
  • The diffusion of a drop of ink placed in a beaker of water.
  • Smelling cookies baking the moment you walk into a kitchen.
  • Adding sugar to your coffee and the sugar molecules spreading out as they dissolve.
  • Leaving a can of soda open for a long time, and the carbon dioxide "fizz" diffusing into the air.
• Diffusion can also happen across a membrane, where molecules spread into or out of a cell, following their natural tendency to spread out, moving down their concentration gradient.