Active Transport

Section 4-2

Some substances are transported against a concentration gradient.

Facilitated diffusion moves sugars and amino acids across the cell membrane DOWN their concentration gradient.

Cells must transport certain amino acids, sugars, and other substances into the cytoplasm where the concentration of that substance is higher in the cell than outside the cell. TRANSPORTING AGAINST THE CONCENTRATION GRADIENT

REMEMBER….

DOWN the concentration gradient → going from a high concentration to a low concentration

AGAINST the concentration gradient → going from a low concentration to a high concentration

Active Transport

The transport of a substance across the cell membrane AGAINST its concentration gradient

ACTIVE TRANSPORT REQUIRES ENERGY! Most often, the energy used is in the form of

ATP Some active transport processes involve carrier

proteins. These proteins bind to specific substances on one side of cell and releases it on the other/opposite side of cell.

Active Transport (con’t)

The difference in active transport is that substance is moving against its concentration gradient (low to high). In facilitated diffusion, the substance moves down its concentration gradient (high to low).

Carrier proteins in active transport act as pumps that move substances against concentration gradient. This is why they are often called membrane pumps.

Sodium-Potassium Pump

One of the most important membrane pumps in animal cells.

Transports 3 sodium ions (Na+) out of the cell and 2 potassium ions (K+) into the cell.

Sodium Ions – usually the concentration outside of cell is higher, so it is against its concentration gradient.

Potassium Ions – concentration is usually higher on the inside of the cell than on the outside of the cell.

Sodium-Potassium Pump (con’t)

The sodium-potassium pump actively transports Na+ and K+ against their concentration gradients through the use of ATP.

Reason for Importance of Pump

1. prevents Na+ from accumulating in the cell. If

accumulates in the cell, can cause cell to burst.

2. helps maintain the concentration gradients of Na+ and

K+ across the cell membrane. Can be used to help

transport other substances across cell membrane.

Vesicles move substances ACROSS membranes

Endocytosis – movement of a substance into a cell by a vesicle.

Exocytosis – movement of a substance by a vesicle to the outside of a cell.

Membrane Receptor Proteins Receive Information

Some cells release signal molecules to carry information to nearby cells and through the body. (Example: Hormones)

Cells must respond to important information and filter out unimportant information.

Receptor Proteins – protein that binds to a specific signal molecule, enabling the cell to respond to the signal molecule, which is located in the cell membrane

Functions of Receptor Proteins

When a signal molecule encounters a cell, the signal molecule binds to the receptor protein.

Receptor protein is specialized to fit the specific signal molecule.

Receptor proteins are embedded in the lipid bilayer of the cell membrane, and the part of the protein that fits the signal molecules faces the outside of the cell.

Functions of Receptor Proteins (con’t)

The binding of a signal molecule to its matching

receptor protein causes a change in the activity of the

receiving cell. This change can occur in 3 ways.

1. By causing changes in the permeability of the

receiving cell.

2. By triggering the formation of second messengers

inside the cell.

3. By activating enzymes inside the cell.

Changes in Permeability

Receptor protein may be coupled with an ion channel.

Binding of a signal molecule to the receptor protein causes the ion channel to open, allowing specific ions to cross the cell membrane.

Second Messengers

Receptor protein may cause the formation of a second messenger

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) signal molecule.

Second Messengers (con’t)

Second Messengers can change the

functioning of a cell in several ways

1. Some second messengers activate

enzymes, triggering a series of

biochemical reactions in the cell.

2. Other second messengers change the

permeability of the cell by opening ion

channels in the cell membrane.

Enzyme Action

Receptor protein may act as an enzyme When a signal molecule binds to the receptor

protein, the receptor protein speeds up chemical reactions inside the cell.

Receptor proteins may also trigger other enzymes inside the cell or cell membrane. This triggers chemical reactions in the cell.

The signal molecule causes changes in the functioning of the receiving cell.

Drugs can affect the binding of signal molecules to a receptor protein

Heroin

1. imitates signal molecules by binding to

receptor proteins on a receiving cell

2. alters function of cell

Beta Blockers

1. Bind receptor proteins

2. prevent heart rate from increasing too

rapidly by blocking the signal molecules.

Conclusion

Active Transport requires energy – ATP. Carrier Proteins in Active Transport – called

membrane pumps. Sodium-Potassium Pump – transports 3 Na+

out of cell and 2 K+ into cell

Conclusion (con’t)

Endocytosis – brings substance into cell Exocytosis – takes substance out of cell Receptor Protein – binds to a specific

substance and then enables cell to respond appropriately.

Second Messengers – amplifies the signal from the first messenger

Homework

Section 4-2 Review Questions

Page 86

Questions 1-5