2.6: Membrane Transport

As previously discussed in topic 2.5, cell membranes don't let most things go through them. This has benefits to the cell, but also several downsides, the largest being that large molecules (like glucose) cannot enter the cell. Topic 2.6 is about how the cell circumvents these problems.

Vocab List

• Concentration gradient
  • Dynamic equilibrium
• Diffusion
• Passive transport
  • Simple diffusion
  • Facilitated diffusion
• Active transport
• Endocytosis
  • Phagocytosis
  • Pinocytosis
  • Receptor-mediated endocytosis/li>
• Exocytosis

Written Explanation

Cell Transport:

The cell wall is a powerful barrier between the interior and exterior of the cell. However, the cell still needs some substances to enter, and thus has several methods of cell transport (moving things into and out of the cell).

The first thing to know about cell transport is the concentration gradient. The concentration gradient describes the relative concentration of a substance (solute) across an area.

When the concentration gradient is even across an entire area (like in the second example shown above), it can be said the concentration is at a dynamic equilibrium. When the concentration gradient is not evenly distributed, diffusion occurs. Diffusion is the net movement of solutes from an area of high concentration to an area of low concentration.

Passive Transport:

The simplest type of cell transport is passive transport. Passive transport does not cost the cell energy. In simple diffusion (a type of passive transport), molecules are moved across the cell membrane by, simply, diffusion. As you learned in 2.5, not all molecules can freely diffuse across the membrane because of the cell membrane's selective permeability, which also means that not all molecules can participate in simple diffusion.

For the molecules that cannot cross the membrane by simple diffusion (due to being too large or being charged), there is facilitated diffusion. In facilitated diffusion (another type of passive transport), molecules move down the concentration gradient and across the membrane by going through channel proteins in the membrane. You could say that the movement of solute is "facilitated" by the channel proteins.

Active Transport:

Active transport, just like passive transport, aims to move helpful molecules into the cell. However, active transport goes against the concentration gradient, which means that it uses energy. Most active transport is carried out by transport proteins in the cell membrane which use energy (in the form of ATP) to bring in or take out solutes from the cell.

Endocytosis is a type of active transport that takes in materials to the cell. Endocytosis involves the cell membrane pinching off a part of itself to form a vesicle or vacuole filled with the contents of the outside of the cell. Phagocytosis is a type of endocytosis in which large solid particles are consumed by the cell. Pinocytosis is where several small particles are consumed by the cell. Lastly, receptor-mediated endocytosis involves receptor proteins on the outside of the cell membrane detecting and grabbing onto a desired solute before closing in to form a vesicle. Receptor-mediated endocytosis has coat proteins around its vesicles and receptor proteins to allow the cell to better identify the contents of a vesicle.

The last type of active transport is exocytosis. In exocytosis, vesicles rejoin the cell membrane, dumping their contents outside of the cell.