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Text Sections
Section 5.3 Moving Smaller Substances In and Out, p. 87
Introduction
Molecules can be transported across the plasma membrane via passive or active transport. Active transport is characterized as requiring energy in the form of ATP, while passive transport does not require energy. During active transport, pumps are often used to assist in moving a molecule against a concentration gradient.
Learning Objectives
- Understand how proteins can help molecules move across cell membranes.
- Discuss how simple diffusion and facilitated diffusion differ.
- Know the difference between passive and active transport.
W E B T U T O R I A L 5. 2
Passive and Active Transport
Narration
Membrane Proteins
Cell membranes are composed of phospholipid bilayers with interspersed proteins. These proteins help to move larger molecules and ions across the membrane. The cell's phospholipid bilayer is itself a semipermeable membrane. In simple diffu- sion, water molecules and gases like oxygen and carbon dioxide need only move down their concentration gradient to cross the membrane and move into or out of cells. Simple diffusion is an example of passive transport, which requires no expenditure of energy.
Facilitated Diffusion and Ion Channels
Larger molecules and ions cannot cross the membrane by simple diffusion. Some membrane proteins, called ion channels, provide a pore (or “channel”) for a specif- ic type of ion to pass from one side of the cell membrane to the other. This passive transport, which requires no expenditure of energy, is an example of facilitated dif- fusion. In this example, sodium ions move spontaneously down their concentration gradient from an area of higher concentration to an area of lower concentration through the pore.
Facilitated Diffusion and Transport Proteins
Membrane proteins also facilitate the diffusion of large molecules, such as glucose. The glucose transport protein is one such membrane protein. The glucose transport protein is extremely specific, transporting only glucose.
The import of molecules into cells through transport proteins is still powered by diffusion. When glucose enters a cell by means of the glucose transport protein, it does so because it is following its concentration gradient. If the concentration of glucose is the same on both sides of the cell membrane, no net movement of glu- cose will occur.
Active Transport and Pumps
Some membrane proteins use energy to pump substances against their concentra- tion gradients—that is, from a region of low concentration on one side of the mem- brane to a region of high concentration on the other. When energy is required for substances to cross the membrane, the process is called active transport. The sodi- um-potassium pump is an important example of active transport.
The pump picks up three sodium ions inside the cell. ATP provides the energy for the pump to change shape and release the sodium ions outside the cell. This change in shape then allows the pump to pick up two potassium ions from the outside and bring them to the inside.
