KEGS Biology Blog


Wednesday 26th September 2012 COVER
September 26, 2012, 8:29 pm
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Water

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Water is bonded together by hydrogen atoms which are covalently bonded to one oxygen atom. The oxygen atom pulls the shared electrons closer towards it, away from the hydrogen. This causes water molecules to be slightly negative near the oxygen and slightly positive near the hydrogen (polar molecule). This causes the water molecules to pull each over in at the surface to create cohesion. This results in the surface tension of water, hence why we see a meniscus in a test tube. This property of water allows for the xylem in plants to function as water is pulled up in transpiration. The water molecules sticking together means that the water can be transported up the plant and used in photosynthesis. Small organisms also make use of this property as they are able to ‘walk on water’. For example, the water spider is able to run across small ponds to get around quickly and efficiently, as well as avoiding certain predators and the need to adapt to aquatic conditions. The cohesion of water is rather essential within nature.

Comment by zeemitha

In pure liquid water, the molecules form hydrogen bonds with each other. They also form a network that allows the molecules to move around, continually making and breaking hydrogen bonds as they do so. As temperature falls, the water molecules have less kinetic energy and therefore move less. More hydrogen bonds form, but they don’t break as easily. As water becomes solid, the hydrogen bonds formed hold the structure in a semi-crystalline form, which is around 9% less dense that liquid water. The density falls because the ‘cooling’ of intermolecular vibrations allows the molecules to form steady hydrogen bonds with their neighbours, so they gradually locking into position.

When water freezes, ice is formed on the surface. Water beneath the surface becomes insulated by the ice so is less likely to freeze.

– Organisms such as polar bears live in an environment of floating ice packs.
– Lakes tend not to freeze completely, so aquatic organisms are not killed as temperatures fall. This is because the ice insulates the water beneath the surface.
– The changes in the density of the water due to freezing are important in the oceans because they set up currents, which circulate nutrients.

Comment by sarahroche12

Hydrogen bonding in water is caused by electronegativity, due to the greater attraction between the oxygen atom and the “shared” electrons than the attraction between the hydrogen atoms and the “shared” electrons. This pulls the electrons nearer to the oxygen atom and away from the hydrogen atoms, leaving the parts of the water molecule formed by the hydrogen atoms slightly positively charged, and the oxygen atom part of the molecule slightly negatively charged. This is why water is called a “polar molecule”.
This property of water makes it a good solvent, since when a polar substance (or ion) is added to water, the water molecules cluster around it (due to electrostatic attraction), separating the molecules of the substance from each other so they dissolve. Once in solution, the molecules can move around and react with each other.
-This is the used for most metabolic reactions in the cytoplasm (which is 75%-90% water) e.g dissolves chemicals so they can take part in photosynthesis and respiration.
-Also organisms use this property of water for transportation of substances, e.g. minerals such as K+ ions through a plant, from the roots to the rest of the plant, or urea out of the body through the kidneys.

Comment by kuranpahwa

One property of water is that it is a liquid at room temperature. This is extremely important for organisms as we rely on water to transport materials around organisms both in cells and on a large scale in multicellular organisms. An example of this is blood in animals use water for transportation which is very important as they need water in order to survive.

Hydrogen bonding occurs between water molecules. Hydrogen atoms attach to one Oxygen atom by covalent bonding. They do not share electrons evenly as Oxygen pulls the electrons towards it therefore resulting in a slightly negative charge. Water is described as polar molecules.

Hydrogen bonds also occur in ice. The hydrogen bonds are constantly making and breaking at liquid form. When we reduce the temperature this reduces the kinetic energy therefore there are more bonds forming but not all of the bonds are breaking as there isn’t enough energy. This becomes solid as bonds form the structure of ice cubes, however solid form of water ice, is less dense than its liquid form water.

Comment by meganbpotter

Cohesion and Adhesion

-Cohesion refers to the fact that water sticks to itself very easily.
-Adhesion means that water also sticks very well to other things, which is why it spreads out in a thin film on certain surfaces like glass for example.
-When water comes into contact with theses surfaces, the adhesive forces are stronger than the cohesive forces, instead of sticking together in a ball, it spreads out.

Hydrogen bonding in relation to cohesion and adhesion

Capillary action is due to adhesion: “When you put a small tube into water, the water likes to stick to each side, with a meniscus on each side. If the tube is so skinny that the meniscus on one side can touch the meniscus on the other side, the water will rise up the tube (each meniscus wants to go up the side, and they chase each other). This is called “capillary action.”

Transport of water in the xylem is due to cohesion: it relies on water molecules sticking to each other as they are pulled up the xylem in the transpiration system.

-“Hydrogen bonding is the attraction of H atoms in the water molecule (slight + charge) to O atoms in a neighbouring water molecule (slight – charge); it is a weak inter-atomic force. But it is still strong enough to help the water molecules “stick” together.It is this cohesive force that allow a “string” of water molecules go a few hundred feet, from the tip of a root to the top leaf of a tree.”

Roles of Water on Living Organisms

-Temperature Buffer- cells host a large amount of chemical reactions, many of these are catalysed by enzymes. Enzyme activity is sensitive to temperature change and so can only occur in a specific range of temperatures. Water helps to buffer temperature changes because of its relatively high specific heat capacity. These properties are essential to living organisms in order to survive.

-Metabolite-Chemical reactions that take place in cells are collectively known as ‘metabolites’. Water is a metabolite in many reactions, either as a reactant or as a product of reaction. For example, it’s involved in photosynthesis, digestion and aerobic respiration which are vital reactions in living organisms.

-Living Environment- In freezing weather, ice forms on the surface of ponds and lakes forming an insulating layer above the water below. This provides a living environment for some organisms until the ice melts. Organisms can also live under the ice.The surfaces of ponds and lakes (and other forms of water) are covered in a ‘skin’ of water molecules. While most objects break through this skin, it is strong enough to support small insects such as pond skaters

-Difficult to compress- This property of water is also helpful to many living organisms such as slugs and worms as it gives creatures like this ‘structural’ support

Comment by roobennithi

In water, the molecules form hydrogen bonds with each other. They also form a complex network within the water that allows the molecules to move, as this process is happening hydrogen bonds are constantly being broken and made. when temperature decreases, the water molecules have less kinetic energy and therefore move less, thus less energy to break the bonds with.. More hydrogen bonds form, but they don’t break as easily. As water becomes solid, the hydrogen bonds created make a semi-crystalline structure, which is less dense that liquid water. The density falls because the ‘cooling’ of intermolecular vibrations allows the molecules to form steady hydrogen bonds with their neighbours, so they gradually lock into position.

When water is frozen, ice is formed on the surface, this is because ice is approximately 9% less dense than water. ice is rarely more than 5 metres thick as the ice insulates the water underneath. this means that organisms living in lakes in the Arctic can survive in the deeper waters. as the density of water changes due to freezing, currents are created which allows nutrients to be circulated throughout the water. As the temperature reduces the availability of light under the ice is reduced as the pore spaces are reduced, and as this happens the salinity of the water increases (in salt water lakes and oceans) which affects what organisms can live within it.

Comment by harrymoseley

to add to what Zee said, cohesion also makes long, thin water columns very strong and difficult to break. A drop of water on a waxy surface, such as a plant leaf, looks almost spherical. It hardly wets the leaf at all. This is because the hydrogen bonds pull water molecules in at the surface. Hence the property of water molecules sticking to each other is called cohesion. Surface tension can be seen at the surface of the body of water.

Comment by stefansoyombo

Water: small molecule consisting of two hydrogen atoms covalently bonded to one oxygen atom
Hydrogen bonds: interactions between the positive and negative charged part of a molecule
In liquid water, molecules form hydrogen bonds with each other which in turn form a network that allows the molecules to move around, continually creating and breaking hydrogen bonds. As the temperature decreases, water molecules move less due to less kinetic energy. More hydrogen bonds form but these do not break as easily. As water becomes solid, the hydrogen bonds formed hold the structure in a semi-crystalline form. This form is less dense than liquid water, so ice forms on the surface of the water as it cools. Beneath the surface, the water becomes insulated and less likely to freeze, allowing aquatic organisms to survive when the temperature falls. Also, organisms such as polar bears live in an environment of floating ice packs.

Comment by charlieshah

This is a simple breakdown of why hydrogen bonding leads to a high heat capacity in water.
When molecules are heated, they vibrate more rapidly and move around faster as they gain more energy. H2O is a polar molecule (has a plus end and a minus end). Positive attracts negative and water molecules join together, making intermolecular dipole-dipole bonds, which are very strong. Because the molecules are being held tightly in place by these bonds, the H2O molecules don’t move much when heated. It takes a lot of heat to move the molecules, causing water to have a high specific heat capacity.

Comment by benpessell

Why hydrogen bonding makes ice float –
as Water cools, the continual making and breaking of hydrogen bonds between molecules slows, so more permenent bonds are made
This gives the ice a semi-crystalline structure, which is less dense than H2O’s liquid state.
This helped a bit http://www.physicsofmatter.com/NotTheBook/Talks/Ice/Ice.html

Comment by kuranpahwa




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