Selectively Permeable Definition and Examples

Selectively Permeable Definition and Examples Selectively permeable means a membrane allows the passage of some molecules  or ions and inhibits the passage of others. The capacity to filter molecular transport in this manner is called selective permeability. Selective Permeability Versus Semipermeability Both semipermeable membranes and selectively permeable membranes regulate the transport of materials so that some particles pass through while others cant cross. Some texts use terns selectively permeable and semipermeable interchangeably, but they dont mean exactly the same thing. A semipermeable membrane is like a filter that allows particles to pass or not according to size, solubility, electrical charge, or other chemical or physical property. The passive transport processes of osmosis and diffusion permit transport across semipermeable membranes. A selectively permeable membrane chooses which molecules are allowed to pass based on specific criteria (e.g., molecular geometry). This facilitated or active transport  may require energy. Semipermeability can apply to both natural and synthetic materials. In addition to membranes, fibers may also be semipermeable. While selective permeability generally refers to polymers, other materials may be considered to be semipermeable. For example, a window screen is a semipermeable barrier that permits the flow of air but limits the transit of insects. Example of a Selectively Permeable Membrane The lipid bilayer of the cell membrane is an excellent example of a membrane which is both semipermeable and selectively permeable. Phospholipids in the bilayer are arranged such that the hydrophilic phosphate heads of each molecule are on the surface, exposed to the aqueous or watery environment inside and outside of cells. The hydrophobic fatty acid tails are hidden inside the membrane. The phospholipid arrangement makes the bilayer semipermeable. It allows the passage of small, uncharged solutes. Small lipid-soluble molecules can pass through the hydrophilic core of the layer, such hormones, and fat-soluble vitamins. Water passes through the semipermeable membrane via osmosis. Molecules of oxygen and carbon dioxide pass through the membrane via diffusion. However, polar molecules cannot easily pass through the lipid bilayer. They can reach the hydrophobic surface, but cant pass through the lipid layer to the other side of the membrane. Small ions face a similar problem because of their electrical charge. This is where selective permeability comes into play. Transmembrane proteins form channels that permit the passage of sodium, calcium, potassium, and chloride ions. Polar molecules can bind to surface proteins, causing a change in the configuration of the surface  and gaining them passage. Transport proteins move molecules and ions via facilitated diffusion, which does not require energy. Large molecules generally dont cross the lipid bilayer. There are special exceptions. In some cases, integral membrane proteins allow passage. In other cases, active transport is required. Here, energy is supplied in the form of adenosine triphosphate (ATP) for vesicular transport. A lipid bilayer vesicle forms around the large particle and fuses with the plasma membrane to either allow the molecule into or out of a cell. In exocytosis, the contents of the vesicle open to the outside of the cell membrane. In endocytosis, a large particle is taken into the cell. In addition to the cellular membrane, another example of a selectively permeable membrane is the inner membrane of an egg.

5 Causes of the Death of a Tree

5 Causes of the Death of a Tree Trees have an extraordinary ability to withstand many damaging agents that are ever-present in their environment. Trees have evolved over millions of years to ward off many stressors that bite and burn and starve and rot their roots, trunk, limbs, and leaves. It is amazing how a tree compartmentalizes itself to seal off dead wood and disease, defoliates to reduce the effect of drought and bleeds to extract harmful insects. We know that all trees do eventually die. There are many hundreds of seedlings and saplings that succumb for every mature tree left in the forest. All ages of trees eventually die to the same agents and only the most adaptive (and often lucky) individuals make it to old age. There are 5 factors to which a tree eventually succumbs: death from its environment, death from harmful insects and diseases, death from a catastrophic event, death from age-related collapse (starvation) and of course, death from harvest. In most cases, death is the result of several, if not all of these conditions taking place simultaneously. Lets take a look at each of these. Adverse Environment Ground and site conditions on which a tree lives ultimately determines the environmental stressors placed on that tree. If a drought-sensitive tree lives on a dry site during drought conditions, it may indeed die from lack of water. But that same tree can also be more susceptible to every other life-threatening factor placed upon it. For example, a disease that appears to be killing the tree may in effect be only a secondary issue to the initial environmental problem. Examples of adverse environments to trees are poorly draining soils, salty soils, droughty soils, air and ground pollution, extreme sun heating or cold spots and many, many others. It is particularly important to understand a tree species genetic tolerance to environmental conditions when planting. Many trees adapt very well to poor sites, but you need to understand which species fits where. Harmful Insects and Disease Virulent diseases like Dutch elm disease and the chestnut blight have caused sudden death to entire forests in North America. However, the most common diseases are more subtle in their work, killing many more trees in total than virulent types and cost forest and yard tree owners billions of dollars in forest product and specimen tree value. These common diseases include three bad ones: Armillaria root rot, oak wilt, and anthracnose. These pathogens invade the tree through leaves, roots and bark wounds and damage a trees vascular system if not prevented or treated. In natural forests, prevention is the only economic option available and is a significant part of a foresters silvicultural management plan. Harmful insects are opportunistic and often invade trees under stress from environmental problems or disease. They not only can directly cause tree death but will spread harmful disease fungi from a host tree to surrounding trees. Insects can attack a trees cambial layer by boring for food and nesting cavities, or they can defoliate a tree to the point of death. Bad insects include pine beetles, the gypsy moth, and emerald ash borers. Catastrophic Events A catastrophic event is always possible in a vast forest as well as in an urban setting. All property, including trees, are subject to being damaged or destroyed. In many cases, trees are not killed but are damaged to the point where their vigor is lost, and insects and disease take advantage of a trees loss of resistance. Significant tree losses can occur during a forest fire or when exposed to tornado-strength winds. Trees take a terrible hit when heavy ice is deposited on species sensitive to limb weight which results in breakage. Floods that do not recede quickly can cause root oxygen levels to diminish to the point where tree damage can occur. Extraordinary drought makes quick work of moisture-loving tree species and can harm all trees when extended over a long period. Old Age For trees who beat the odds and live through maturity to old age, there is a slow dying process that may take centuries to complete (in long-lived species). The modular tree compartmentalizes around damage and infected areas and continues to grow. Still, growth starts slowing after a tree matures, the ability of the plant to support itself diminishes and incurs the loss of adequate foliage for hydration and food. New immature branches, called epicormic sprouts, try to assist in maintaining an old trees vigor but are weak and are insufficient to sustain life for very long. A mature tree slowly collapses under its weight and crumbles to become the nutrients and topsoil for future trees. Timber Harvests Well remind you that trees do die to the ax. Trees via their wood have supported humankind and civilization for millennia and continue to be a necessary part of the human condition. The practice of forestry through professional foresters work continually with much success to provide a sustained flow of available wood volume and at the same time, ensure a surplus of trees. Some consider deforestation a growing global crisis.