Cells and Their Environment Chapter 4

The cell membrane controls what enters and leaves the cell to maintain a metabolic balance or homeostasis. This process is called transport.

Passive transport may occur spontaneously with no energy input from the cell. This type of transport is called passive transport.

The most simple type of passive transport is diffusion The most simple type of passive transport is diffusion. Diffusion is the movement from an area of high concentration to an area of low concentration. This difference in concentrations of the system is known as concentration gradient.

This transport is as simple as us being able to smell cookies baking in the teacher’s lounge. The odor molecules are in high concentration down there near the oven. The kinetic energy of the cookie smell drives it outward to lower concentration, down the hall, and to our noses.

When a concentration gradient becomes equalized, we say that a state of concentration equilibrium is reached. The particles keep moving, but no net movement occurs in either direction.

The ability for diffusion to take place in a cell depends on the size and polarity of the particle and the chemical nature of the cell membrane. The cell membrane allows free diffusion of small or nonpolar molecules.

Another type of passive transport is the movement of water from an area of high concentration to low concentration. This is called osmosis. Osmosis of water may occur simultaneously with the diffusion of particles.

When solute particles are in lower concentration and water molecules in higher concentration, the solution is considered hypotonic. When the solute particles are in higher concentration and water molecules in lower concentration, the solution is considered hypertonic. Hypotonic Hypertonic

Water moves from hypotonic solution to hypertonic solution until equilibrium is reached. Solutions at equilibrium are considered isotonic.

The continual build up of water pressure in a cell creates support for the cell. This is called turgor pressure and keeps plants from wilting.

Turgor pressure can cause a cell to explode in a process called cytolysis.

Continued loss of water by a cell can cause the cell to collapse in a process called plasmolysis.

Membrane proteins may become involved in transport Membrane proteins may become involved in transport. When they do, we often call them carrier proteins.

Since ions such as Na+ and Cl- are not soluble in the lipid cell membrane, carrier proteins form ion channels for them to pass through. The inside of the cell is typically charged negative causing each ion channel to be specific to a certain ion.

Ion channels are of two types: Open – Ions are free to flow in and out at all times. Gated – Ions only flow at a certain stimulus such as the stretching of the cell membrane

Ion channels not only depend on the type of carrier protein but also on the specific charge of the ion.

Some particles may have the wrong shape or size to become involved in transport. The particles may bind to a carrier protein, however, and pass freely through the membrane by diffusion. This type of transport is called facilitated diffusion. Diffusion of glucose into muscle cells is an example of this kind of transport.

Facillitated Diffusion Happens in Three Steps: A carrier protein binds to a specific molecule. This causes the carrier protein to change shape and move the molecule inside the cell membrane. The molecule is sheilded from the cell membrane and released into the cell as the carrier protein resumes its original shape.

Sometimes cells are forced to move substances against the concentration gradient. This requires the cell to expend energy. This process is known as active transport.

Active Transport

Carrier proteins that are involved in active transport are sometimes called pumps because they move materials against the concentration gradient. The carrier proteins are similar to other carrier proteins in that they bind to a specific molecule and then change shape to transport the particle and release it. Pumps, however, require ATP to operate.

One example of this process is the sodium-potassium pump One example of this process is the sodium-potassium pump. In nerve cells, a carrier may attach to three sodium ions inside of the cell. This causes its shape to change in a way as to attach to two potassium ions outside of the cell. When the ions are attached, the protein inverts itself depositing the potassium ions into the cell and removing the sodium from the cell. In this way, an electrical potential change moves along the nerve cell.

Sometimes particles are too large to pass through the cell membrane by ordinary transport. Cells may still transport the particles by cutting off pouches of the cell membrane by endocytosis or exocytosis. Both of these processes require energy so are considered active transport.

There are a number of enzymes and chemicals bathing the cell at all times. It does not need to respond to all of these signals much like we sort out the signals that we receive through the day. It is the job of receptor proteins in the cell membrane to bind to chemical messengers that are important to the cell.

Receptor proteins can affect the cell in three ways: They may change the permeability of the cell membrane. They may activate a second messenger causing a specific reaction inside the cell. They may cause the action of an enzyme inside the cell.