Biology

• Adhesion


  The force by which individual molecules cling to surrounding materials and surfaces
• Hydrophilic


  Materials and substances with an affinity for water
• Water adheres strongly to most surfaces and can be drawn up long columns, for example through narrow tubes such as the xylem vessels of plant stems, without danger of the water column breaking
• Cohesion is a far more significant force in xylem transport and explains how tensions can be resisted. Adhesion is only significant when air-filled xylem vessels refill with aqueous sap under positive pressures, which is something that happens only rarely (no more than once a year)
• The terms 'cohesion' and 'adhesion' are sometimes treated as if their meanings are interchangeable, but this is not the case. Cohesion ('co' means 'together') is attraction between water molecules, while adhesion ('ad' means 'toward') is attraction to a surface
• Viscosity


  A measure of a fluid's resistance to flow
• Adhesion


  The force by which individual molecules stick to surrounding materials and surfaces
• Hydrophilic


  Attracted to water; e.g. hydrogen bonds are readily formed between a molecule and water
• Soil contains many vertical, thin channels known as capillary tubes, in which plant roots are located. When water enters capillary tubes, adhesion between the water molecules and the wall of the capillary draws water up the small tube: this is called capillary action. In this way, plants bring water up from the water table to the roots when the ground becomes dry
• The cell walls of plants are made from a fibrous material called cellulose. Cellulose is polar/hydrophilic to a certain degree. Fibrous materials can act like wicks, drawing water up into the material by capillary action. Cell walls can draw water by capillary action from nearby xylem vessels, keeping water flowing through plant tissue. Cells that are directly exposed to the air, such as those found in the spongy mesophyll tissue of leaves, remain constantly wetted by capillary action into these cells. Water evaporates from the moist, blotting-paper-like cell walls of the mesophyll and then diffuses out of leaves through pores on the surface of the leaf (stomata), enabling water to be transported up the plant
• Capillary tubes


  Channels with a very small internal diameter
• Capillary action


  The tendency of a liquid to move up against gravity when confined within a narrow tube (capillary). Also known as capillarity
• Solute


  Dissolved molecule or ion in a solution
• Solvent


  A liquid in which another substance can be dissolved
• Hydrophobic


  Repelled by water
• Water is a powerful solvent for polar substances such as ionic substances like sodium chloride (Na+ and Cl−). All cations (positively charged ions) and anions (negatively charged ions) become surrounded by a layer of orientated water molecules
• There is a diverse range of hydrophilic molecules that dissolve in water, such as carbon-containing (organic) molecules with ionized groups (for example, amino acids have a negatively charged carboxyl group, –COO−, and a positively charged amino group, –NH3+); soluble organic molecules like sugars dissolve in water due to the formation of hydrogen bonds with their slightly charged hydroxyl groups (–OH). Once they have dissolved, molecules or ions (the solute) are free to move around in water (the solvent) by diffusion and, as a result, are more chemically reactive than when in the undissolved solid
• Non-polar substances are repelled by water, as in the case of oil on the surface of water. Non-polar substances are hydrophobic. The functions of some molecules in cells depends on them being hydrophobic and insoluble. For example, the cell membrane is made from phospholipids, the tails of which are hydrophobic and form the internal structure of the membrane
• Of the common gases, carbon dioxide (CO2), oxygen (O2) and nitrogen (N2), only carbon dioxide is particularly soluble in water; nitrogen and oxygen are only slightly soluble in water. Carbon dioxide is moderately soluble in water because a proportion of it undergoes a chemical reaction to form carbonic acid (H2CO3 (aq)), which immediately ionises or dissociates to form hydrogen ions, H+ (aq), and hydrogencarbonate ions, HCO3− (aq)
• Oxygen and nitrogen have low solubility in water because they are non-polar and do not form hydrogen bonds with water. In addition, they do not undergo dissociation or ionisation. One consequence of the poor solubility of oxygen in water is the evolution of respiratory pigments, for example haemoglobin, which greatly increase the oxygen-carrying capacity of blood relative to that of pure water
• Most enzymes catalyse reactions in aqueous solution. Enzymes require a certain level of water in their structures to maintain enzyme shape and stability, enabling them to function effectively. Most naturally occurring enzymes cannot form their active forms without being immersed in water. Hydrogen bonds often act as bridges between enzyme binding sites and their substrates. Although most enzymes act in aqueous solutions, sometimes an enzyme may be in a fixed position, such as within a cell membrane. In these cases, the location of the enzyme allows reactions to be localized to particular sites
• The physical properties of water depend on the hydrogen bonding between water molecules and include buoyancy, viscosity, thermal conductivity and specific heat capacity
• Buoyancy is the ability of any fluid (liquid or gas) to provide a vertical upwards force on an object placed in or on it. Objects float in water when their average density is less than water and sink when they are denser. The density of a substance is its mass per unit volume
• Do not confuse the terms 'heat capacity' with 'specific heat capacity'. Heat capacity is the amount of heat required to change the temperature of a body by one degree. The amount of heat energy per unit mass is needed to calculate the specific heat capacity. Unlike heat capacity, the specific heat capacity is therefore independent of mass or volume
• The black-throated loon (Gavia arctica) is a diving bird species, catching its prey (mostly fish) underwater. It breeds in the vicinity of deep freshwater lakes throughout northern Europe, the west coast of Alaska, and Asia. From August, it migrates south to areas around the Black Sea and the Mediterranean Sea, and to north-east Atlantic coasts and the eastern and western Pacific Ocean. It returns to its breeding grounds in early April when sea ice in those areas has melted
• Ringed seals (Pusa hispida) live in the Arctic and sub-arctic regions of the North Pole. They live on packs of ice, but also spend much of their time in the sea, under the ice. They are quite small seals, usually less than 1.5 m in length, and have a distinct pattern of dark spots surrounded by light grey rings on its fur – explaining its common name
• A relatively large amount of energy is required to raise the temperature of water, because a lot of energy is needed to break the large number of hydrogen bonds that restrict the movement of water molecules. This property of water is its specific heat capacity. Consequently, aquatic environments (rivers, ponds, lakes and seas) are very slow to change temperature when the surrounding air temperature changes. Aquatic environments have relatively more stable temperatures than terrestrial (land) environments. As organisms, and the cells from which they are made, are largely composed of water, water's ability to absorb and lose heat without undergoing a large temperature change also provides thermal cushioning within the organisms themselves, protecting cells and organisms from large fluctuations in temperature
• The relatively stable sea temperatures enable seals to live and feed throughout the year. The specific heat capacity of air is lower than water, so air temperature tends to fluctuate more. In winter, the very low air temperatures cause surface water to freeze. One of the interesting properties of water is that, unlike many other substances, it floats when it freezes because the density of ice is lower than that of liquid water. This is due to the behaviour of hydrogen bonds and how they make water molecules interact. Water has its lowest density at 4 °C. The ice forms a platform on which seals can live. Ringed seals have claws to dig through ice to produce holes so that they can emerge from their aquatic habitat to breathe. This enables them to live under and on the ice throughout the year
• Water has a higher thermal conductivity than air, with water conducting heat 28 times better than air. By trapping air in its feathers, the black-throated loon forms an effective insulating layer between its skin and the outside air. Feathers also restrict heat loss
• The specific heat capacity of air is lower than water, so air temperature tends to fluctuate more
• In winter, the very low air temperatures cause surface water to freeze
• Water


  Unlike many other substances, it floats when it freezes because the density of ice is lower than that of liquid water
• Water has its lowest density at 4 °C
• The ice forms a platform on which seals can live
• Ringed seals


  • Have claws to dig through ice to produce holes so that they can emerge from their aquatic habitat to breathe
  • This enables them to live under and on the ice throughout the year
• Thermal conductivity


  Water has a higher thermal conductivity than air, with water conducting heat 28 times better than air
• Black-throated loon


  • Forms an effective insulating layer between its skin and the outside air by trapping air in its feathers
  • Feathers also restrict convection currents by trapping a thin layer of air that is not able to move easily, which also helps to maintain the body temperature of the bird
• Seal


  • Relies on thick blubber to insulate its body
• Ice


  Has insulating properties because its thermal conductivity is low, like the thermal conductivity of air, which stops heat being transferred into the surroundings, even when the temperature is very low
• The ice traps thermal energy in the water beneath the ice, increasing sea temperatures