Biology 4.2 Active Transport Active Transport in Cells

Biology 4.2 Active Transport Low concentration Movement Against a Concentration Gradient A concentration gradient is the difference in the amount of diffusion of a substance between two areas: an area of low concentration and an area of high concentration. Look at the illustration at right. The top part has no red in it (0% diffusion). The very bottom has all red (100% diffusion). In between we have a gradient as the amount of red diffuses from 100% to 0%. gradient High concentration

Biology 4.2 Active Transport Low concentration red Movement Against a Concentration Gradient One area may have a high concentration of a solution. Another area may have a low concentration of the same solution. The diffusion across the area where the two connect is the concentration gradient Gradient High concentration red

Biology 4.2 Active Transport Sometimes the concentration of a substance outside a cell moves through the cell’s membrane from an area of high concentration of the substance toward an area of lower concentration. This is called movement across a concentration gradient. Yellow dye diffuses into water

Active Transport In passive transport, no energy is required to move substances across a concentration gradient. Substances are also moved across cell membranes by a system called active transport. Active transport requires energy.

Active Transport Passive transport is like swimming downstream with a current pushing you. No energy is needed. Active transport is like swimming upstream against a current. You use energy to go against the current.

Active Transport Active transport allows substances to move from an area of low concentration to an area of high concentration, against the concentration gradient.

Active Transport Unlike passive transport, active transport requires the cell to use energy because the substance is being moved against it’s concentration gradient. Most often, the energy needed for active transport is supplied by ATP. low high

Active Transport Some active processes involve carrier proteins. Carrier proteins used in active transport bind to specific substances on one side of the cell membrane and release them on the other side of the cell membrane. Think of a carrier protein as a small motor boat with an engine that carries you up the river “against the current” using it’s energy to move you.

Active Transport But in active transport, substances bind to carrier proteins where they are low in concentration and are released where they are high in concentration. Carrier proteins in active transport function as “pumps” that move substances across the cell membrane against their concentration gradient.

Sodium-Potassium Pump One of the most important membrane pumps in animal cells is a carrier protein called the sodium-potassium pump. The energy needed to power sodium-potassium pumps is supplied by ATP. The sodium-potassium pump transports three sodium ions out of a cell and two potassium ions into the cell. Thus, both sodium ions and potassium ions move against their concentration gradients.

Movement in Vesicles Many substances, such as proteins and polysaccharides, are too large to be transported by carrier proteins. In these cases, vesicles move these substances into cells through the membrane. Vesicle forms around substance Vesicle pulls substance through membrane

Movement in Vesicles The movement of a substance into a cell by a vesicle is called endocytosis. During endocytosis, the cell membrane forms a pouch around a substance. The pouch than closes up and pinches off from the membrane to form a vesicle inside the cell. Vesicle forms around substance Vesicle pulls substance through membrane Into the cell

Movement in Vesicles The movement of a substance by a vesicle to the outside of a cell is called exocytosis. During exocytosis, vesicles in the cell fuse with the cell membrane, releasing their contents.

Membrane Receptor Proteins The cells of your body must communicate with each other. Some cells release signal molecules that carry information to nearby cells and throughout the body. Hormones are one familiar example of signal molecules. Cells can receive the messages carried by certain signal molecules because the cell membrane contains specialized proteins that bind to these signal proteins. These proteins are called receptor proteins.

Membrane Receptor Proteins A receptor protein is a protein that binds to a specific signal molecule, enabling the cell to respond to the signal molecule. For example, the muscles of a person exercising could not contract without receptor proteins and signal molecules that tell the muscles when to contract and when to relax.

Functions of Receptor Proteins A signal molecule is bound by a receptor protein that fits that molecule. Within the lipid bilayer, the part of the receptor protein that fits the signal molecule faces the outside of the cell. The receiving and binding of a signal molecule triggers a change in the receiving cell.

Functions of Receptor Proteins This change in the cell can happen in three ways: By causing changes in the permeability of the receiving cells. (what can move in/out) By triggering the formation of second messengers inside the cell. (passing on the message) By activating enzymes inside the cell. (trigger a response inside the cell)

Changes in Permeability The receptor protein may be coupled with an ion channel. The binding of a signal molecule to the receptor protein causes the ion channel to open. This type of receptor protein is very important to the nervous system.

Second Messengers The receptor proteins may also cause the formation of a second messenger inside the cell. When it is activated, a second messenger acts as a signal molecule in the cytoplasm. The second messenger amplifies the signal of the first messenger, passing the signal along.

Second Messengers Some second messengers activate enzymes, triggering changes in the cell. Others change how or what the cell allows in by opening ion channels in the cell’s membrane. The diagram at right shows a signal molecule triggering an ion channel to open. Once opened, ions flood in attracted by ions of the opposite charge.

Enzyme Action The receptor protein may act as an enzyme. This means that when a signal molecule binds to the receptor protein, the receptor protein may speed up chemical reactions within the cell. Receptor proteins may also activate other enzymes within the cell or in the cell membrane, triggering chemical reactions within the cell.

4.2 Review: Major Concepts Active Transport is the movement of a substance against the concentration gradient of the substance. Active transport requires cells to use energy. In animal cells, the sodium-potassium pump uses energy supplied by ATP to transport sodium ions out of the cell and potassium ions into the cell simultaneously.

4.2 Review: Major Concepts During endocytosis, substances are moved into the cell by a vesicle that pinches off from a cell membrane. During exocytosis, substances move out of a cell as materials inside a vesicle are released from a cell as the vesicles fuse with the cell membrane.

4.2 Review: Major Concepts Communication between cells often involves signal molecules that are bound to receptor proteins on cells. A signal molecule that is bound by a receptor protein on a cell can change the activity of the cell in three ways: by enabling specific ions to cross the cell membrane, by causing the formation of a second messenger, or by speeding up the chemical reactions of a cell