How do materials enter and leave cells

There are 3 main types of passive processes. Diffusion relies on kinetic energy and a concentration gradient. Kinetic energy is affected by temperature, size of molecules, steepness of the gradient, and the medium the molecules are in. Anything that increases the kinetic energy of the molecules will increase the rate of diffusion. Diffusion is the movement of molecules from an area of high concentration to an area of lower concentration.

There are different types of diffusion: simple in which the molecule passes directly through the phospholipid bilayer and facilitated which uses the integral membrane proteins as channels. Simple diffusion occurs when the molecules are either very small or lipid soluble and pass through the phospholipid bilayer of the cell membrane. Some examples of substances that use this process are oxygen O 2 , carbon dioxide CO 2 , and lipids.

The molecules will move from an area of high concentration down its diffusion or concentration gradient to the lower concentration until equilibrium is reached. Once the concentration is the same on both sides of the membrane, the molecules continue to move but they maintain the same levels at equilibrium.

Have you ever had the pleasant surprise of waking up to the smell of coffee or bacon? In the human body the diffusion of O 2 and CO 2 are critical for gas exchange. O 2 levels are higher in your arterial blood than your tissue cells so O 2 will diffuse out of the blood into your cells.

CO2 has the opposite concentration gradient. CO 2 levels are highest in your cells your mitochondria produce CO2 as a waste product from cellular respiration and CO 2 diffuses out of the cell into the blood. These molecules are small enough to pass through the phospholipid bilayer and are examples of simple diffusion. Remember that anything that increases kinetic energy will also increase the rate of diffusion.

Molecules can be divided into four categories with regard to their ability to cross the plasma membrane. The first category is nonpolar molecules. These hydrophobic molecules can easily cross the membrane because they interact favorably with the nonpolar lipids.

Note that these molecules can accumulate in the membrane because they interact so well with the lipids. The second category is small polar molecules. The third category is large polar molecules. These have difficulty crossing the membrane because of their size and poor interaction with the lipids. The last category is ionic compounds.

Their charge interacts very unfavorably with the lipids, making it very difficult for them to cross the membrane. As three different molecules move, they encounter the lipid bilayer depicted by the horizontal membrane across the center of the stage in the preceding animation. Notice that one type of molecule passes freely through the lipid bilayer while the second type of molecule only occasionally passes through the membrane, and the lipid bilayer is totally impermeable to the third type of molecule.

The size, polarity, and charge of a substance will determine whether or not the substance can cross the cell membrane by diffusion.

The cholesterol was an example of a lipid, and is highly soluble in the nonpolar environment of the lipid bilayer. You saw, in the animation above, the cholesterol freely passing into the hydrophobic environment of the membrane. The cholesterol distributes freely in the membrane and then some fraction will dissolve in the aqueous environment of the cytoplasm. Water, on the other hand, while polar, is small and because of this is able to freely cross the membrane.

The lipid bilayer is much less permeable to the ion, because of its charge and larger size. As a general rule, charged molecules are much less permeable to the lipid bilayer. Cells must be able to move large polar and charged molecules across the lipid bilayer of the membrane in order to carry out life processes.

To allow these molecules, which are not soluble in the lipid bilayer, to pass across the hydrophobic barrier it is necessary to provide ports, channels or holes through the membrane.

The molecules will still move spontaneously down a concentration gradient from high to low concentration. Some of these channels can remain open at all times, allowing the molecules to move freely according to the concentration gradient. The sodium-potassium pump maintains the electrochemical gradient of living cells by moving sodium in and potassium out of the cell.

Describe how a cell moves sodium and potassium out of and into the cell against its electrochemical gradient. The primary active transport that functions with the active transport of sodium and potassium allows secondary active transport to occur. The secondary transport method is still considered active because it depends on the use of energy as does primary transport. Active Transport of Sodium and Potassium : Primary active transport moves ions across a membrane, creating an electrochemical gradient electrogenic transport.

The process consists of the following six steps:. Several things have happened as a result of this process. At this point, there are more sodium ions outside of the cell than inside and more potassium ions inside than out. For every three ions of sodium that move out, two ions of potassium move in. This results in the interior being slightly more negative relative to the exterior. This difference in charge is important in creating the conditions necessary for the secondary process.

The sodium-potassium pump is, therefore, an electrogenic pump a pump that creates a charge imbalance , creating an electrical imbalance across the membrane and contributing to the membrane potential. Define an electrochemical gradient and describe how a cell moves substances against this gradient. Electrochemical Gradient : Electrochemical gradients arise from the combined effects of concentration gradients and electrical gradients.

Simple concentration gradients are differential concentrations of a substance across a space or a membrane, but in living systems, gradients are more complex. Because ions move into and out of cells and because cells contain proteins that do not move across the membrane and are mostly negatively charged, there is also an electrical gradient, a difference of charge, across the plasma membrane. The interior of living cells is electrically negative with respect to the extracellular fluid in which they are bathed.

The situation is more complex, however, for other elements such as potassium. The combined gradient of concentration and electrical charge that affects an ion is called its electrochemical gradient. To move substances against a concentration or electrochemical gradient, the cell must use energy. Active transport mechanisms, collectively called pumps, work against electrochemical gradients. Small substances constantly pass through plasma membranes.

Active transport maintains concentrations of ions and other substances needed by living cells in the face of these passive movements. Two mechanisms exist for the transport of small-molecular weight material and small molecules. Primary active transport moves ions across a membrane and creates a difference in charge across that membrane, which is directly dependent on ATP. Secondary active transport describes the movement of material that is due to the electrochemical gradient established by primary active transport that does not directly require ATP.

An important membrane adaption for active transport is the presence of specific carrier proteins or pumps to facilitate movement. There are three types of these proteins or transporters: uniporters, symporters, and antiporters. A uniporter carries one specific ion or molecule.

A symporter carries two different ions or molecules, both in the same direction. An antiporter also carries two different ions or molecules, but in different directions. All of these transporters can also transport small, uncharged organic molecules like glucose. These three types of carrier proteins are also found in facilitated diffusion, but they do not require ATP to work in that process.

Both of these are antiporter carrier proteins. Uniporters, Symporters, and Antiporters : A uniporter carries one molecule or ion.

A symporter carries two different molecules or ions, both in the same direction. An antiporter also carries two different molecules or ions, but in different directions. In secondary active transport, a molecule is moved down its electrochemical gradient as another is moved up its concentration gradient.

Unlike in primary active transport, in secondary active transport, ATP is not directly coupled to the molecule of interest. Instead, another molecule is moved up its concentration gradient, which generates an electrochemical gradient. The molecule of interest is then transported down the electrochemical gradient.

While this process still consumes ATP to generate that gradient, the energy is not directly used to move the molecule across the membrane, hence it is known as secondary active transport.

Both antiporters and symporters are used in secondary active transport. Co-transporters can be classified as symporters and antiporters depending on whether the substances move in the same or opposite directions across the cell membrane.

Secondary active transport brings sodium ions, and possibly other compounds, into the cell. As sodium ion concentrations build outside the plasma membrane because of the action of the primary active transport process, an electrochemical gradient is created. If a channel protein exists and is open, the sodium ions will be pulled through the membrane. This movement is used to transport other substances that can attach themselves to the transport protein through the membrane.

Many amino acids, as well as glucose, enter a cell this way. This secondary process is also used to store high-energy hydrogen ions in the mitochondria of plant and animal cells for the production of ATP. The potential energy that accumulates in the stored hydrogen ions is translated into kinetic energy as the ions surge through the channel protein ATP synthase, and that energy is used to convert ADP into ATP. Secondary Active Transport : An electrochemical gradient, created by primary active transport, can move other substances against their concentration gradients, a process called co-transport or secondary active transport.

Endocytosis takes up particles into the cell by invaginating the cell membrane, resulting in the release of the material inside of the cell. Endocytosis is a type of active transport that moves particles, such as large molecules, parts of cells, and even whole cells, into a cell.

There are different variations of endocytosis, but all share a common characteristic: the plasma membrane of the cell invaginates, forming a pocket around the target particle. The pocket pinches off, resulting in the particle being contained in a newly-created intracellular vesicle formed from the plasma membrane.

Phagocytosis : In phagocytosis, the cell membrane surrounds the particle and engulfs it. For example, when microorganisms invade the human body, a type of white blood cell called a neutrophil will remove the invaders through this process, surrounding and engulfing the microorganism, which is then destroyed by the neutrophil. In preparation for phagocytosis, a portion of the inward-facing surface of the plasma membrane becomes coated with a protein called clathrin, which stabilizes this section of the membrane.

The coated portion of the membrane then extends from the body of the cell and surrounds the particle, eventually enclosing it. Once the vesicle containing the particle is enclosed within the cell, the clathrin disengages from the membrane and the vesicle merges with a lysosome for the breakdown of the material in the newly-formed compartment endosome. When accessible nutrients from the degradation of the vesicular contents have been extracted, the newly-formed endosome merges with the plasma membrane and releases its contents into the extracellular fluid.

The endosomal membrane again becomes part of the plasma membrane. Pinocytosis : In pinocytosis, the cell membrane invaginates, surrounds a small volume of fluid, and pinches off. A variation of endocytosis is called pinocytosis. In reality, this is a process that takes in molecules, including water, which the cell needs from the extracellular fluid. Pinocytosis results in a much smaller vesicle than does phagocytosis, and the vesicle does not need to merge with a lysosome.

Potocytosis, a variant of pinocytosis, is a process that uses a coating protein, called caveolin, on the cytoplasmic side of the plasma membrane, which performs a similar function to clathrin. The cavities in the plasma membrane that form the vacuoles have membrane receptors and lipid rafts in addition to caveolin. The vacuoles or vesicles formed in caveolae singular caveola are smaller than those in pinocytosis. Potocytosis is used to bring small molecules into the cell and to transport these molecules through the cell for their release on the other side of the cell, a process called transcytosis.

Receptor-Mediated Endocytosis : In receptor-mediated endocytosis, uptake of substances by the cell is targeted to a single type of substance that binds to the receptor on the external surface of the cell membrane.

A targeted variation of endocytosis, known as receptor-mediated endocytosis, employs receptor proteins in the plasma membrane that have a specific binding affinity for certain substances.

If fluid droplets are taken in, the processes is called pinocytosis. Illustration of endocytosis. Note that the particle entered the cell surrounded by a piece of cell membrane. The opposite of endocytosis is exocytosis. Cells use exocytosis to secrete molecules too large to pass through the cell membrane by any other mechanism.

Click on the button above to open a problem solver to help you practice your understanding of membrane transport with the following examples:. A white blood cell engulfs a bacterium as you fight off an infection. Carbon dioxide a small uncharged gas molecule enters the lungs where it is less concentrated from the blood where it is more concentrated.

Cells of the stomach wall transport hydrogen ions through a ATP-dependent membrane protein to the inside of the stomach, producing a pH of 1. The pH of the cytosol fluid inside the cells of stomach wall cells is approximately 7. Recall that a low pH means high hydrogen ion concentrations.

The lung cells of a victim who drowned in fresh water are swollen due to water entering the cells. Salivary gland cells produce the enzyme salivary amylase and secrete it into the salivary ducts to be delivered to the mouth. A Paramecium a single celled organism swims into an area of salty water.. This transport process moves substances against the concentration gradient - the substances move from a low concentration to a higher concentration.

It requires an energy input. Glucose can be taken into cells by active transport if the concentration of glucose inside the cell is already quite high. This requires carrier proteins.