Chapter 7.2 and 7.4, The Cell's Plasma Membrane and Cellular Transport

Function of the Plasma Membrane In Chapter 1 we discussed one of the seven characteristics of life is the ability to maintain homeostasis, or a stable internal environment. One of the key roles of the plasma membrane is to maintain homeostasis within a cell. An important property of a plasma membrane is it's ability to be selectively permeable. Selective permeability is a property of cells that allows certain substances in and keeps other substances out.

Selective Permeability

Structure of the Plasma Membrane Remember from chapter 6 that one of the important lipids in biology is the phospholipid. A phospholipid is two fatty acid tails that are non-polar (repelled by water) and a phosphate head that is polar (attracted to water). The plasma membrane is made up of a sandwich of many phospholipids that separate the cell's interior from the exterior. This two-layered sandwich-like structure made of phospholipids is known as a phospholipid bilayer.

Phospholipid Bilayer

Other Structrural Components of the Plasma Membrane Other important substances present in the plasma membrane are cholesterol, proteins, and carbohydrates. One important type of protein that transports substances across the plasma membrane is called transport proteins. Cholesterol keeps the fatty acid tails in the plasma membrane from sticking together. Carbohydrates can identify chemical signals on the plasma membrane.

Transport Proteins

The Fluid Mosaic Model A good analogy that is sometimes used to describe how the substances in the plasma membrane behave is to think of the plasma membrane as a bath tub full of water with apples floating on top. The apples would represent the phospholipids, and the water would represent the cytoplasm This analogy describes what is known in biology as the fluid mosaic model This model is used to describe the way in which substances behave and interact on the plasma membrane.

Fluid Mosaic Model

Diffusion One of the ways in which cells receive the substances and nutrients they need is by simple diffusion. Diffusion is the movement of particles from areas of high concentration to areas of low concentration until the concentration is equal and equilibrium in achieved. Think of a pitcher of water and some food coloring. If I added a drop of food coloring to the water, the dye would spread out evenly until the color of the water was all the same.

Diffusion and Equilibrium Diffusion occurs until equilibrium is achieved and the concentration gradient is zero.

Rate of Diffusion Several factors influence the rate of diffusion. The three main factors affecting the rate of diffusion in cells are concentration gradient, temperature, and pressure. The higher the concentration, temperature and pressure, the faster the rate of diffusion.

Movement of larger particles across the plasma membrane Some substances are small enough to move across the plasma membrane by simple diffusion. Other particles that are much larger, require assistance from transport proteins. When objects require transport proteins in order to move across the plasma membrane, this movement is called facilitated diffusion. When energy is not required in order to move objects across the cell membrane, it is called passive transport. Facilitated diffusion is a form of passive transport.

Differences in Simple Diffusion and Facilitated Diffusion

Methods of Facilitated Diffusion There are several ways that transport proteins accomplish facilitated diffusion. One way is by channel proteins. Channel proteins allow only certain size particles to move by passive transport. Another type of transport protein that operates passively, is referred to as carrier proteins. Carrier proteins can change shape to move the particles across the plasma membrane In both cases, the substances are still moving from areas of high to low concentration, and no energy is required, so these are both examples of passive transport.

Channel and Carrier Proteins

The Movement of Water across the Plasma Membrane Remember that we just said some substances are too big to cross the plasma membrane. Water is not too big to move across the plasma membrane. It can move by simple diffusion. So, if equilibrium of a substance that is too big to move across the membrane can not occur, then water can move in the direction with the higher concentration to achieve equilibrium. Movement of solvent, rather than solute. This process is called osmosis.

Osmosis

How does osmosis affect cells? If a cell is in a solution (for instance, blood) that has the same concentration of a solute as the solution, the cell would be isotonic to the solution. In this case no osmosis will occur and the cell will not change in volume or size. If a cell is in a solution that has a lower concentration of a solute (for instance, salt) than the cell, then the solution is hypotonic to the cell. In this case, osmosis will move water inside the cell and the cell could swell and burst. If a cell is in a solution that has a higher concentration of solute than the cell, then the solution is hypertonic to the cell. In this case, osmosis will move water out of the cell and the cell will shrivel up.

How Osmosis Affects Cells

How does a cell move a substance from a low to a high concentration? So far, we have only discussed ways cells move substances by passive transport from areas of high to low concentration. So how does a cell move a substance from a lower concentration to a higher concentration? In order to do this, a cell must use energy. This is called active transport. To do this, special carrier proteins called pumps move substances against the concentration gradient from areas of low to high concentration.

Sodium Potassium Pump One common type of active transport is a pump called the sodium potassium ATPase pump. The function of the sodium potassium pump is to carry sodium out of the cell and bring potassium into the cell. In order to do this an enzyme called ATPase uses energy from ATP to pump 3 sodium ions out of the cell and pump 2 potassium ions into the cell.

How do solutes too large to move by diffusion or facilitated diffusion move across the plasma membrane? Some solutes are simply too big to move by simple diffusion or facilitated diffusion. These solutes require a different method to get across the plasma membrane. If a large object needs to leave the cell, the plasma membrane can engulf the solute and eject the substance by exocytosis. If a large object needs to enter the cell, the plasma membrane can engulf the solute and draw it into the cytoplasm by endocytosis.

Endocytosis and Exocytosis