WATER The note blank templates are available here. The first cells originated in water. Liquid water is essential for life as we know it. There is evidence that water on Earth originated around 4.5 billion years ago (A1.1.7) and that life first emerged at least 3.8 billion years ago. In order for life to form and maintain itself, the molecular ingredients of life need to react with each other in a liquid solvent - water. There is scientific debate around whether the water was in a pond, hydrothermal pool or sea - but there is no debate that water was present. Harvard Magazine Atomic Structure An atom is the smallest unit of matter that is unique to a particular element. Our current model of the atom can be broken down into three constituents parts – ● ● ● Protons- carry a positive charge Neutrons - possess no net charge Electrons - have a negative charge Atoms are electrically neutral if they have an equal number of protons and electrons. Atoms that have either a deficit or a surplus of electrons are called ions. Atomic Bonding Electrons may be transferred to other nearby atoms or shared between atoms. By this mechanism, atoms are able to form bonds. A chemical bond is an attraction between atoms, ions or molecules that enables the formation of chemical compounds. In IB Biology, the types of “bonds” to know are: ● ● ● ● Nonpolar covalent bond Polar covalent bond Ionic bond Hydrogen bond (intermolecular force, not a true bond) Covalent Bond A covalent bond holds together two atoms that share one or more pair of electrons between atoms. Nonpolar Covalent Share electrons equally. ⚫ is the nucleus of each atom Polar Covalent Share electrons unequally blue is the cloud of electrons shared equally between the two atomic nuclei Because electrons have a negative charge and there are MORE ELECTRONS around this nucleus, there is a slight negative charge (ઠ) on this nucleus Because there are LESS ELECTRONS around this nucleus, there is a slight positive charge (ઠ+) on this nucleus Nonpolar covalent bonds are common within the many carbon compounds upon which life is based (B1.1.1), such as proteins (B1.2.9) and DNA (A1.2.3). Polar covalent bonds can occur in amino acid R-groups (B1.2.6) which impacts the tertiary structure of a protein (B1.2.10). ⚫ This nucleus has more pull on the electrons, so it has a bigger electron cloud surrounding it Ionic Bond An ionic bond is an attraction between a positively charged ion and an negatively charged ion. This atom has lost an electron, making it a positively charged CATION. Ionic bond Example: Na+ This atom has gained an electron, making it a negatively charged ANION. Ionic bonds can occur between amino acids as a polypeptide folds into its tertiary structure to become a functional protein (B1.2.9). Example: Cl- Hydrogen Bond A hydrogen is an attraction between two polar molecules. A polar molecule is a molecule in which one end of the molecule is slightly positive, while the other end is slightly negative. The attraction between the slightly positive (ઠ+) and slightly negative (ઠ-) regions of two different polar molecules is called a hydrogen bond. H-bonds are represented with a dotted line. one polar molecule another polar molecule Hydrogen bonds form between strands of DNA (A1.2.6), allowing for allowing genetic information to be easily replicated (A1.2.8) and expressed (D1.2.2). Hydrogen bonds also maintain the structure of cellulose (B1.1.6) and proteins (B1.2.8 and B1.2.9) H-bond Water Structure A water molecule consists of 2 hydrogen and 1 oxygen atom, hence…H2O. Within a water molecule, electrons are shared through polar covalent bonding between the atoms. These electrons🟡 are being shared between the O and H, so it’s a covalent bond. Water molecules are polar The shared electrons (and their negative charge) are pulled towards the oxygen. HYDROGEN OF WATER MOLECULE the 1 proton in the nucleus of the H-atom has less pull on the shared electrons the electrons are pulled towards the oxygen atom OXYGEN OF WATER MOLECULE the 8 protons in the nucleus of the Oatom have more pull on the shared electrons Hydrogen bonds form between water molecules The partially positive hydrogen atoms of one water molecule are attracted to the partially negative oxygen atom of a different water molecule, forming a hydrogen bond. Each water can form up to H-bonds with up to 4 other water molecules The H-bonds are made and broken quickly as the molecules move, however the large numbers of bonds contribute to the stability and retainment of water on Earth (A1.1.7*) Water’s ability to attract polar and charged molecules grants it a number of emergent properties such as cohesion (A1.1.3), adhesion (A1.1.4), solvency (A1.1.5 & D2.3.1) and a high specific heat (A1.1.6). Collectively Online these property make water theChemistry primary medium of life (A1.1.1). Hydrogen bonds form between water molecules Water’s ability to make hydrogen bonds with itself (A1.1.2) causes water molecules to stick together, a property called cohesion. The cohesion of water molecules: ● Allows plants to move water under tension in xylem (B3.2.7) ● Retains water on Earth’s surfaces to serve as habitats (A1.1.7*) ● Contributes to the physical properties of water important to living organism (A1.1.6) Chemistry Online Longitudinal Cross Section of Xylem There are parallel tubes of xylem with rings of “lignin” that provide extra support for the plant as it grows up against gravity. Transport of water under tension in xylem The cohesion-tension hypothesis is the most widely-accepted model for movement of water in vascular plants (B3.2.7). The cohesion-tension model works like this: 1. Transpiration (evaporation) occurs because stomata are open to allow gas exchange for photosynthesis (B3.1.9). As transpiration occurs, it creates negative pressure (also called tension or suction). 2. The tension created by transpiration “pulls” water in the plant xylem, drawing the water upward in much the same way that you draw water upward when you suck on a straw. 3. Cohesion (water sticking to each other) pulls up water molecules in a chain as the top-most water is pulled up and out of the stomata. That’s a long chain of water! If there were no hydrogen bonds between water molecules, the column of water would break and trees would not be able to grow as tall. Hydrogen bonds between water molecules creates a long chain that moves from the roots to the leaves That’s a big tree! Water surfaces as habitats due to surface tension The molecules on the surface are more attracted other molecules of the liquid than to molecules in the surrounding air. The net effect is an inward force that causes water to behave as if its surface were covered with a stretched elastic membrane. At the surface, there are fewer other water molecules to bond to since there is air above (thus, no water molecules). This results in a stronger bond between those water molecules that actually do come in contact with one another. Surface tension allows organisms like water striders to “walk on water” and provides a stable environment for other organisms that live on or near the surface of water. To break through the surface of the water, enough force must be applied to break many hydrogen bonds simultaneously. The ability of water striders to “walk on top of water” stems from their highly adapted legs that distribute weight (B4.1.2). Hydrogen bonds form between water and polar or charged molecules Polar and charged molecules are called “hydrophilic” because they attract water. The attraction of water to other polar or charged molecules is called adhesion. The adhesion of water molecules to other molecules: ● Allows plants to move water using capillary action (B3.2.7) ● Permits water to move through soil, even against the force of gravity ResearchGate Hydrogen bonds form between water and polar or charged molecules Charged ions result from when atoms have different numbers of electrons and protons. ◎ ◎ A cation has a positive charge because it has more protons than electrons (Na+) An anion has a negative charge because it has more electrons than protons (Cl-) Water will be electrostatically attracted to ions, such as phosphate (PO4- ), an ion that is present in nucleotides (A1.2.2, C1.2.1) and phospholipids (B1.1.9). The slight positive charge (δ+) of of water hydrogen atom is attracted to the negatively charged oxygen atom of the phosphate group. It is this attraction that make structures such as the phospholipid head to be hydrophilic. Impact of adhesion for organisms Capillary action is the movement of water in through a narrow space, often in opposition to external forces like gravity. Water sticks to tube Plants and trees couldn't thrive without capillary action. Capillary action helps bring water up into the roots. With the help of adhesion and cohesion, water and dissolved ions (A1.1.5) can move all the way up to the branches and leaves (B3.2.7). Water sticks to water Adhesion of water to the walls of a vessel will cause an upward force on the liquid at the edges and result in a meniscus which turns upward. Impact of adhesion for organisms Hydrogen bonds between water molecules and the cellulose in the xylem cell wall assists in the movement of water from the roots to the leaves of plants Impact of adhesion for organisms In the soil, water adheres to the surface of soil particles. Plants take in this water via osmosis (B2.1.5 and D2.3.3). Impact of adhesion for organisms Capillary action in soil the primary force that enables the soil to retain water. In the same way that water moves upwards through a tube against the force of gravity; water moves upwards through soil pores, or the spaces between soil particles. The height to which the water rises is dependent upon the type of soil. Capillarity is the rate at which water is pulled upward from the water table into pore spaces by capillary action. Different soils have different capillarity rates, which is one of the factors affecting the sustainability of agriculture (D4.2.7) Groundwater is found beneath the soil surface and can move through capillary action up into the soil Water is a solvent. the liquid in which a solute dissolves the substance that dissolves in a solvent a mixture of one or more solutes dissolved in a solvent Water is a solvent. Polar molecules will dissolve in water because they are “hydrophilic” (A1.1.4) and can can form hydrogen bonds with water (A1.1.2). ● ● The slight positive charge (δ+) of of water hydrogen atom is attracted to the negatively charged region of the solute. The slight negative charge (δ-) of of water oxygen atom is attracted to the positively charged region of the solute. The slight negative charge (δ-) of water oxygen atom attracted to the slight positive charge (δ+) of glucose hydrogen atom. The slight negative charge (δ-) of glucose oxygen atom attracted to the slight positive charge (δ+) of water hydrogen atom. Water is a solvent. Charged ions will also dissolve in water because they are “hydrophilic” (A1.1.4). Water is electrostatically attracted to ions. ● The slight positive charge (δ+) of of water hydrogen atom is attracted to the negative charge of an anion . ● The slight negative charge (δ-) of of water oxygen atom is attracted to the positive charge of a cation. Labster Not everything will dissolve in water! Molecules that are nonpolar or do not have a charge atoms will not dissolve in water because they are “hydrophobic”. Hydrophobic molecules can NOT attract water, so they are insoluble (do not dissolve) in water. Hydrophobic molecules are attracted to other hydrophobic molecules, so they will clump together when exposed to water. All lipids are hydrophobic (B1.1.8) so they will not mix with water. Water molecules Because of their hydrophobic tail, phospholipids will form bilayers in water, with the tails not exposed to water (B1.1.12). This is an example of the function of a molecule depending on it being hydrophobic! Hydrophobic tails are not exposed to water. Water molecules Water’s solvent properties allow it to be used as a medium for metabolism. Water is needed for cellular metabolism because it dissolves the reactants and enzymes so they can come together for reactions (C1.1.3). ◎ Catabolic reactions break down larger molecules into smaller molecules ◎ Anabolic reactions build larger molecules from smaller molecules. Scitable Water’s solvent properties allow it to be used as a medium for transport. Dissolved solutes can be transported in solution around the body of an organism. In vascular plants: ● Dissolved mineral ions are transported in the xylem from roots to leaves (B3.2.7) ● Dissolved sugars produced in photosynthesis are transported in the phloem from source to sink (B3.2.18). Water’s solvent properties allow it to be used as a medium for transport. Dissolved solutes can be transported in solution around the body of an organism. Animal blood plasma transports (B3.2.12): ● Salt ions such as Na+ and Cl● ● Amino acids Proteins such as antibodies (C3.2.6) and those used in blood clotting (C3.2.3) ● Glucose which is used in cellular respiration (C1.2.4) and whose concentration must be regulated (D3.3.3) ● Waste products of metabolism such as urea which is later removed from the blood at the kidney (D3.3.8) ● A small amount of dissolved gasses such as CO2 and O2 Coffey, J. What are the parts of an atom? Universe Today (2015). Available from: What are the parts of an atom? © www.biologyforlife.com Before using any of the files available on this site, please familiarize yourself with the Creative Commons Attribution License. It prohibits the use of any material on this site for commercial purposes of any kind. A1.1.6 Physical properties of water and the consequences for animals in aquatic habitats. The note blank templates are available here. The physical properties of water allow it to be the medium for life. The physical properties of water contribute to water being the the medium of life, which means that water is the substance upon which life exists and depends (A1.1.1). A physical property is a measurable behavior or characteristic of matter that exists without the matter reacting or interacting with other things. Science Notes Physical Property: Buoyancy Buoyancy is an upward force applied to an object that is immersed in a fluid. If the buoyant force of the fluid is greater than the object's weight, the object will float. Physical Property: Buoyancy Buoyancy depends on density. ● If the density of the object is lower than the density of the fluid, the buoyant force will be greater than the force due to gravity and the object will float. ● If the density of the object is greater than the density of the fluid, the buoyant force will be less than the force due to gravity and the object will sink. The items dropped into the cup might have the same mass, but they differ in density. The more dense an object (such as iron or obsidian), the less likely the buoyant force will be able to overcome the force of gravity and the more likely the item will sink. The less dense an object (such cork), the more likely the buoyant force will be able to overcome the force of gravity and the more likely the item will float. Physical Property: Buoyancy Bony fish can change their density by changing the size of a structure called the swim bladder. By filling the swim bladder with gases, a fish will become less dense and will be able to move upwards in water. Birds main limb bones are hollow, with struts inside to strengthen them. This makes them strong but not dense. Because they are not dense, the buoyant force of water is able to hold birds up, allowing many birds to float on water. By removing air from the swim bladder with gases, a fish will become more dense and will be able to move downwards in water. Physical Property: Viscosity Viscosity is a measure of a fluid’s tendency to flow. Viscosity is due to the amount of friction the molecules of a liquid experience as they flow over each other. A thick fluid is more viscous and a thin fluid is less viscous. Less viscosity More viscosity Physical Property: Viscosity Less viscosity Water is more viscous than some other substances (such as organic solvents and air) because it can form hydrogen bonds with itself (A1.1.3). The hydrogen bonds increase the friction and reduce water’s tendency to flow. Although still a great medium for transport (A1.1.5), blood does not flow as easily as pure water because cells and dissolved solutes increase viscosity. The formation of hydrogen bonds (purple) reduce water’s tendency to flow. Water can dissolve many solutes. These solute increase viscosity. More viscosity Physical Property: Thermal Conductivity Thermal conductivity is a measure of a material's ability to move heat across a temperature gradient. The thermal conductivity of the material is determined by how easily energy transfers through the material. Physical Property: Thermal Conductivity Less Conductive More Conductive Heat slowly moves through the material. Heat rapidly moves through the material. Better at insulation and preventing heat loss. Better for absorbing and transferring heat. Styrofoam Air Wool Fat Water Copper A thick layer of fat provides insulation from cold ocean temperatures in marine mammals (B1.1.11). A person will more quickly become hypothermic in cold water than in cold air because the water rapidly conducts body heat away from the body. A thick layer of wool traps the body heat generated during metabolism (C1.1.12) of these sheep from escaping on a cold day. The water in blood is carrying heat generated at contracting muscles (C1.1.12) to the skin where it will be lost from the body. This is a form of thermoregulation (D3.3.6) as the body changes blood supply in response to changes in activity (D3.3.11) Physical Property: Specific Heat Capacity Specific heat capacity is the quantity of heat needed to raise the temperature of a chemical per unit mass. Water has the highest specific heat Water’s high heat capacity is caused by its numerous hydrogen bonds (A1.1.2). Each individual “bond” is weak, but there are so many of them that collectively collectively a lot of energy must be added to break them all. capacity of any liquid, which makes it good for temperature Which means it takes a lot of regulation. heat energy to raise the temperature of water. AIR It takes 1,007 J of energy to heat 1 kg of air by 1 degree Kelvin LIQUID WATER It takes 4,183 J of energy to heat 1 kg of water by 1 degree Kelvin A lot more energy to raise the temperature of water compared to air! Physical Property: Specific Heat Capacity As a result of its high specific heat capacity, water heats up or cools down very slowly. This provides for a stable internal environment and habitat of living things. For example: Because living body’s contain a lot of water, body temperature is slow to fluctuate when environmental temperatures change. Water’s high specific heat capacity helps body temperature rise and fall slowly when external temperatures becomes very hot or cold. Physical Property: Specific Heat Capacity For example: The temperature of aquatic habitats (B4.1.1) rise and fall slowly when surrounding air temperature becomes very hot or cold. The water must absorb or lose a lot of heat energy before the temperature changes. Contrasting physical properties of water with those of air Water Air Buoyant Force Viscosity Thermal Conductivity Specific Heat Capacity Higher Higher Higher Higher Water applies more upward force than air, allowing objects to float Water is more resistant to flow Water absorbs and transfers heat It takes more energy to change the temperature of water Lower Lower Lower Lower Air applies less upward force than than water, so most mass does not float in air Air is less resistant to flow Heat is lost slower to the air It takes less energy to change the temperature of air Buoyancy in water allows the seal to stay afloat without expending a lot of energy. However, the water is viscous, so the seal has adaptations for streamlining as it swims through it (B3.3.10). Water has a greater thermal conductivity than air, so the seal needs to insulate itself with blubber to maintain body temperatures (B1.1.11). However, because the water has a high specific heat, the temperature of the water does not change as rapidly as the air around it, providing habitat stability for the seal. Ringed seal (Pusa hispida) Buoyancy in water allows the bird to stay afloat without expending a lot of energy, however when flying through air the bird must expend energy to stay aloft. Air is not viscous, so the loon can easily move through it when flying. The loon doesn’t lose as much body heat to the air because air has low thermal conductivity. However, because the air has a low specific heat, its temperature changes as rapidly. Black throated loon (Gavia arctica) Holzner, S. Measuring Thermal Conductivity in Different Materials. For Dummies (2016). Available from Measuring Thermal Conductivity in Different Materials © www.biologyforlife.com Before using any of the files available on this site, please familiarize yourself with the Creative Commons Attribution License. It prohibits the use of any material on this site for commercial purposes of any kind. A1.1.7* Extraplanetary origin of water on Earth and reasons for its retention. The note blank templates are available here. * HL only Water on Earth Water is essential for all life (A1.1.1). Water covers 71% of the earth’s surface, creating many habitats in which life has evolved. In fact, water is so vital to life that the presence of water is precedent to the search for extraterrestrial life (A1.1.8*). CK12 Origin of Water on Earth Earth formed around 4.5 billion years ago when gravity pulled swirling gas and dust in to become the third planet from the Sun. Studies of rocks from this time suggest that water may have begun to exist on Earth as early as 4.4 billion year ago. But where did the water come from? Currently, the most favored explanation is that Earth acquired water from extraplanetary objects, meaning object from outside Earth’s orbit. An illustration shows a rocky planet like Earth forming in the disk of leftover debris from a star's birth. Such planets take shape as dust and gas coalesce in the disk, and through collisions with other primitive rocky bodies. NASA/JPL-Caltech Origin of Water on Earth There is evidence that numerous planetary bodies, including asteroids and comets, containing large amounts of water. At present, asteroids up to a few hundred kilometers across seem the most likely sources of most of Earth’s water, specifically the types of asteroid that dominate the outer asteroid belt between Mars and Jupiter. Science Notes Origin of Water on Earth An artistic conception of the early Earth, showing a surface pummeled by large impact, resulting in extrusion of deep seated magma onto the surface. At the same time, distal portion of the surface could have retained liquid water. Image Credit: Simone Marchi, NASA Artist's impression of the early Earth. Huge, impact-generated lava lakes coexisted with surface liquid water Water is retained on Earth The distance of the Earth from the Sun ensures that sunlight never raises temperature high enough for water to boil. Liquid water is retained much more easily than water vapor due to cohesion from hydrogen bonding. Earth’s gravity keeps water from escaping the planet. A1.1.8* Relationship between the search for extraterrestrial life and the presence of water. The note blank templates are available here. * HL only Astrobiology Given the size of the universe - there are at least 100 billion stars in our home galaxy alone and perhaps 100 billion galaxies of much the same size scattered throughout deep space - few scientists believe that the Earth is the only home of life. Astrobiology is the study of the origins, distribution, and possibility of life in the universe. Water Water is essential for all life (A1.1.1). Water likely arrived on early Earth from asteroids (A1.1.7*). In fact, water is so vital to life that the presence of water is precedent to the proceeds, search for comes before extraterrestrial life. from outside the Earth Plumes of water vapour that shoot from the surface of Enceladus, a moon of Saturn. The Telegraph Location Astrobiologists search for extraterrestrial objects that fall in a Goldilocks Zone, meaning it is just the right distance from a star for water to remain at least periodically in liquid form on the surface. Water will vaporize Water will exist in liquid form Water will freeze Universe Today https://ny.pbslearningmedia.org/resource/nvap-scigoldilocks/the-goldilocks-zone/ Mars Mars remains the best candidate for discovery of extraterrestrial life. Both the NASA rovers Curiosity and Perseverance have clearly determined that ancient Mars was significantly more wet and warm, and was an entirely habitable place for microbial life. All the ingredients needed for life as we know it – the proper chemicals, a consistent source of energy, and water that was likely present and stable on the surface for millions of years – were present. Mars Curiosity rover’. Curiosity’s mission was to find evidence of past or present habitable conditions on the surface of Mars (NASA) Europa Europa is a moon of Jupiter. Its surface is made of water ice. Analysis of Europa's lineae (dark fractures that crisscross the ice's surface) shows that they're gradually moving, perhaps evidence of tectonic activity or volcanic eruptions underneath. If true, this activity could provide enough heat to generate a liquid ocean underneath the ice. This hypothetical combination of volcanic activity and liquid water has prompted some scientists to speculate that Europa could harbor life. Alexander, C. Where did Earth's water come from? Carnegie Science (2021). Carter, J. Earth’s Water Came From Asteroid Strikes, New Research Suggests. Forbes (2023). Conference, G. Early Earth was bombarded by series of city-sized asteroids. Phys.org (2021) Gray, R. Water carried on asteroids are common . The Telegraph (2010). NASA. Earth; Our Home Planet. Solar System Exploration (2022). USGS. How Much Water is There on Earth? Water Science School (2019). © www.biologyforlife.com Before using any of the files available on this site, please familiarize yourself with the Creative Commons Attribution License. It prohibits the use of any material on this site for commercial purposes of any kind.
