Biology 12-1.3b Passive Transport

What happens during/ effecting the rate of diffusion?

- Smaller, more permeable molecules can slip by polar heads more easily than larger molecules. - The greater the surface area of the cell, the faster diffusion will take place. - The thinner the cell membrane, the faster diffusion will occur.

The Effects of Hypertonic, Isotonic, and Hypotonic Environments on Animal Cells

Cells are very much affected by osmosis. The water concentration of solution surrounding a cell determines the direction of water flow, either into or out of the cell. A solution contains a solute, usually a solid (e.g., NaCl), and a solvent, usually a liquid (e.g., water). Cell environments can be described by three terms, depending on the concentration of solutes outside the cellisotonic solutions, hypotonic solutions, and hypertonic solutions.

Plasmolysis

Contraction of the cell contents due to the loss of water.

Diffusion

Diffusion is the movement of molecules from an area where the molecule is in high concentration (more solute than solvent) to an area of lower concentration (less solute than solvent.) For example, imagine a spray of perfume is released into a closed room. The area where the perfume was originally sprayed is the area of high concentration (in this case the perfume is the solute and the air is the solvent.) Since the perfume and air molecules are in constant random motion, they bump into each other, causing some of the relatively crowded perfume molecules to be pushed away from the crowd. The molecules gradually spread in this way from the area of high concentration and can be sensed by someone standing across the room in an area of low perfume concentration. Eventually the perfume molecules become evenly mixed with other gases in the air so that each area of the room has the same number of perfume molecules. (Some animals like sharks use the concentration gradient to locate prey, 'sniffing' along a line of increasing concentration to locate their prey.) The relative differences in perfume concentration in this case is referred to as their concentration gradient. The process of diffusion allows the cell to obtain nutrients and dispose of carbon dioxide without using energy. Simple diffusion only works for very small, uncharged molecules such as O2, CO2, and H2O.

Osmosis

Diffusion of water through a selectively permeable membrane.

Facilitated Diffusion

Facilitated diffusion/transport is the movement of a molecule from an area of high concentration to an area of lower concentration (diffusion), with the aid of a protein channel or carrier. This process is required for larger molecules or charged ions (e.g., amino acids, glucose, Na+, and K+). This explains why molecules such as glucose and amino acids cross the plasma membrane, even though they are not lipid soluble. Cell membranes have protein carrier molecules specific to each molecule being transported. This is one way the membrane is selectively permeable. Further, the cell can control how much of a particular molecule (or if the molecule) is transported, by varying the amount of the carrier protein is manufactured. The cell's environment is shown to have an effect on this. Cells placed in an environment rich in glucose for example, will make more glucose carrier protein molecules. When the glucose excess diminishes, it slows the manufacture of the glucose carrier protein.

Concentration Gradient

Gradual change in chemical concentration from one point to another.

Hypotonic SolutionsWater Enters Cell

Hypotonic solutions cause cells to swell or even burst due to the intake of water. The prefix hypo means less than and refers to an environment with less solute (and more water) than inside the cell. For example, the fluid outside the cell has 0% solute and 100% water (e.g., distilled water), while the cell has 1% solute and 99% water. This causes osmotic pressure outside the cell to push water into the cell. The cell gets bigger. Lysis, the build-up of pressure in the cell, can cause it to burst. When a plant cell is placed in a similar solution, the swelling creates turgor pressure.. The plant cell will not burst, however, because the plasma membrane pushes against the rigid cell wall. Such turgor pressure is necessary for cells to maintain their shape. When plants wilt, it is from the loss of water from the cells, lessening turgor pressure. Think of a flaccid balloon versus one that is full of air. The air-filled balloon has shape and form, whereas the limp one is not able to maintain a particular shape.

If an animal cell is placed in a ______ solution, the cell will swell and possibly burst. If the cell bursts due to the build of pressure, ________ has occured.

Hypotonic, lysis.

Turgor Pressure

In plant cells, pressure of the cell contents against the cell wall when the central vacuole is full.

Passive Transport

Is a movement of biochemicals and other atomic or molecular substances across cell membranes without need of energy input.

Isotonic Solutions

Isotonic solutions have the same concentration of solute and water, both inside and outside the cell. Iso means the same as and tonicity refers to the concentration of solute in the solution. For example, if the fluid outside of the cell and the cytoplasm inside the cell could both be 1% solute and 99% water. Because the concentrations are equal, there is no net gain or loss of water by the cell. Although water will still move into and out of the cell, there is no net movement of water or resulting change in cell volume. The cell remains the same size.

Hypertonic Solutions

Lower solute (more water) concentration than the cytosol of a cell; causes cell to gain water by osmosis.

Osmotic Pressure

Measure of the tendency of water to move across a differentially permeable membrane; visible as an increase in liquid on the side of the membrane with higher solute concentration.

Diffusion

Movement of molecules or ions from a region of higher to lower concentration; it requires no energy and stops when the distribution is equal.

The passing of water molecules through a cell membrane is an example of _______.

Osmosis.

What is the rate of transfusion determined by?

Size-smaller molecules can slip by the polar heads of the phospholipids but larger molecules can not. Shape-carrier proteins are specifically shaped to carry only certain molecules. For example, glucose carrier proteins can move up to 100 glucose molecules per second but will not transport a single Na+ or an amino acid. concentration—the greater the concentration gradient between the outside and the inside of the cell, the greater rate of diffusion. charge (+/-)—ions and molecules that have a charge can not easily pass though the membrane, unless facilitated; the mechanisms required for this will be discussed in the next lesson lipid solubility—molecules such as steroid hormones (testosterone and estrogen) are considered lipid soluble and can move easily through the lipid bilayer. Non-polar solvents such as benzene, gasoline and toluene also pass easily through living membranes. (These can disrupt the cell in many ways, which is why you should avoid exposure to these chemicals.) temperature—the rate of diffusion increases as the temperature increases because the particles are moving faster (kinetic molecular theory)

Isotonic Solutions

Solutions that are equal in solute concentration to that of the cell; causes cell to neither lose not gain water by osmosis.

Hypotonic

Supposition that is formulated after making an observation; it can be tested by obtaining more data, often by experimentation. The more hypotonic the solution is, the lower the concentration of solute relative to another solution.

Lysis

The disintegration of a cell by rupture of the cell wall or membrane.

Over View

The plasma membrane is selectively permeable because some substances can move freely across it while others can not. There are two ways for molecules to pass through a plasma membrane—one is active, the other is passive. Passive transport requires no chemical energy from the cell. Instead, this process involves facilitated diffusion. Active transport is discussed in the next lesson. In this lesson you will compare and contrast the processes of diffusion, by which small molecules move from areas of high concentration to low concentration, facilitated transport, which requires no energy, and osmosis, the movement of water molecules from areas of high concentration to low concentration. You will also learn about the factors that affect the rate of diffusion across a cell membrane, and you will be able to predict the effects of hypertonic (more solute), isotonic (equal amounts of solute), and hypotonic (less solute) environments on animal cells.

Crenation

The shrinkage of an animal cell placed in this type of solution

Facilitated Diffusion/Transport

Use of a plasma membrane carrier to move a substance into or out of a cell from higher to lower concentration; no energy required.

Plasmolysis

When a plant cell is placed in this solution, the plasma membrane pulls away from the cell wall and plasmolysis a shrinking of the cytoplasm due to osmosis occurs.

Hypertonic SolutionWater Exits the Cell

When animal cells are placed in a hypertonic solution, the cells shrink or shrivel up due to the movement of water out of the cell. The prefix hyper means more than and refers to an environment with more solute and less water than inside the cell. For example, the fluid outside the cell has 5% solute and 95% water, while the cell has 1% solute and 99% water. If a cell is placed in this type of solution, water will move from the area of high concentration and move out of the cell.

When a plant cell is placed in a ________ solution, the plasma membrane pulls away from the cell wall and _____ occurs (a shrinking of the cytoplasm due to osmosis)

hypertonic, plasmolysis.

Osmosis

is the diffusion of water through a semi-permeable membrane. Water moves from an area of high water concentration (or low solute concentration) to an area of low water molecule (or high solute concentration.) Osmotic pressure is the force that moves the water in either direction.

Passive Transport

is the movement of biochemicals and other molecular substances across membranes, without using chemical energy. Instead, passive transport depends on a concentration difference (called a 'gradient') and on the permeability of the cell membrane, which in turn is dependent on the organization and characteristics of the membrane lipids and proteins. The three main kinds of passive transport through the membrane are diffusion, facilitated transport, and osmosis.