Nutrients, respiration and photosynthesis

Nutrients: what they are and what counts as one

A nutrient is a substance that an organism requires for growth, repair and to provide chemical energy. Those three purposes are the definition, and they are worth learning as a phrase, because a definition that says only 'something the organism needs' or 'food' does not score. A nutrient is also not the same thing as a food: a food is a material an organism takes in, while a nutrient is a substance inside it that the organism actually uses.

Nutrients turn up in three chemical forms in the sea. The first is gases, of which the important one here is carbon dioxide - it is inorganic, it is dissolved in seawater, and it supplies the carbon that every organic compound is built from. The second is dissolved salts, the mineral salts such as nitrate, phosphate, calcium and magnesium that are dissolved in seawater and are absorbed straight across the body surface or the gills of marine organisms. The third is organic compounds, which are the carbohydrates, lipids and proteins. Organic means the compound contains carbon and was made by a living organism.

The organic nutrients divide into three groups you must be able to name and give examples of. Carbohydrates include starch, which is the storage form in plants and algae, and sugars such as glucose, which is the small soluble molecule that cells actually break down for energy. Lipids are the fats and oils - solid fats in cold-blooded animals and liquid oils in fish and in plankton. Proteins are built from amino acids and are the working and structural molecules of every cell. All three contain carbon, hydrogen and oxygen, and proteins also contain nitrogen, which is why nitrogen is such an important nutrient in the sea.

The reason marine scientists care so much about nutrients is that in the surface water of the open ocean they are in very short supply. Light is usually plentiful near the surface, but the mineral salts that algae need have mostly been used up by the organisms living there and have sunk to the deep water with their remains. Growth in the sunlit zone is therefore limited by the supply of nutrients, not by light, which is the idea that the later blocks on productivity and upwelling explain.

The food tests: proving which nutrients a sample contains

The four food tests are qualitative: each one tells you whether a nutrient group is present or absent, not exactly how much there is. Learn each test as three facts - the reagent, whether or not it needs heating, and the colour change from and to. A positive result is never just 'it changed colour'.

Starch - iodine solution. Add a few drops of iodine solution, which is orange-brown, to the sample at room temperature. If starch is present the iodine turns blue-black. Nothing is heated. A slice of potato or a piece of bread turns almost instantly; fish muscle does not turn blue-black, because it stores no starch.

Reducing sugars - Benedict's solution. Add Benedict's solution, which is blue, to the sample and heat it in a water bath for a few minutes. If a reducing sugar such as glucose is present, a green, yellow, orange or brick-red precipitate forms, the exact colour depending on how much sugar there is: green means a little, brick-red means a lot. This is the only one of the four tests that needs heat, and forgetting the water bath is the most common practical error in this topic.

Proteins - biuret solution. Add biuret solution, which is blue, to the sample at room temperature. If protein is present the solution turns purple or lilac. Lipids - the ethanol emulsion test. Add ethanol to the sample and shake it so that any lipid dissolves, then pour the liquid into water. If lipid is present, a cloudy, milky white emulsion forms on the surface of the water, because the lipid comes out of solution as tiny droplets. Neither of these two needs heating.

Two habits turn a weak practical answer into a full-mark one. First, use a control: run the same test on water so you can show that the colour change came from the sample. Second, describe how you prepare the sample - solid tissue is ground or blended with a little water first, and equal volumes of sample and reagent make the colour easier to see. For a quantitative version of the sugar test you would compare the colour with a series of known standards, but the syllabus only asks you to establish presence.

What each nutrient group does

The functions in this syllabus are deliberately limited, so learn exactly those and do not pad the list. Proteins are used for tissue repair and growth, and also as an energy supply. Growth and repair means building new cytoplasm, new muscle, new enzymes and new cell membranes, which is why a growing young fish or a repairing wound needs protein. Protein is only respired for energy when carbohydrate and lipid reserves have run out, because using it means breaking down the body's own structure.

Carbohydrates are the energy supply. Glucose is the fuel that cells break down in respiration, and starch is how plants and algae store glucose until it is needed. Because carbohydrate is the cheapest and quickest fuel, it is the first reserve used. Lipids have three separate functions in the syllabus: insulation, energy supply, and a source of lipid-soluble vitamins. Insulation matters most in the sea because water conducts heat away about twenty-five times faster than air, so warm-blooded marine mammals such as whales, dolphins and seals carry a thick layer of fat - blubber - just under the skin. Gram for gram, lipid stores more energy than carbohydrate, which is why fish oils and plankton lipids are such a concentrated fuel, and why lipid is the reserve that migrating animals rely on.

Vitamins and mineral salts are the micronutrients: they are needed in very small quantities, they are not broken down to release energy, and they are not building materials in the way that protein is. Instead they are used in the chemical reactions of the body and as components of particular molecules. A vitamin is an organic micronutrient that the organism cannot make for itself in sufficient quantities and must obtain from its food; a mineral salt is an inorganic micronutrient absorbed as dissolved ions from the water or from food. Because so little is needed, a deficiency shows up as poor health and poor growth rather than as an obvious shortage of material.

One point of exam technique follows from the table: three different groups can supply energy - carbohydrate, lipid and protein. So an answer to 'state the function of carbohydrate' must say energy, but an answer to 'why is a mixed diet important' should not say 'for energy' three times. Say what each group is doing that the others cannot do.

Essential elements and their biological roles

Some nutrients matter not as energy or as bulk material but because they supply a particular chemical element that the organism needs to build a particular molecule. An element cannot be made by the organism and cannot be replaced by anything else, so if the supply runs out, the molecule cannot be built and the process that depends on it stops. Five of these are listed for this course, and each one is the answer to a different question.

Nitrogen is used to make proteins. Every amino acid contains nitrogen, so no nitrogen means no protein, which means no growth and no repair. In the sea, nitrogen is absorbed mainly as dissolved nitrate by algae and plants, and it enters animals through their food. Carbon is used to make organic compounds of every kind - carbohydrates, lipids and proteins all contain it - and the source of that carbon is carbon dioxide dissolved in the water, taken in for photosynthesis. Nitrogen and carbon are therefore the two nutrients that most often limit growth in the surface ocean.

Magnesium is used to make chlorophyll. Chlorophyll is the green pigment that absorbs light, and a magnesium atom sits at the centre of every chlorophyll molecule, so a plant or alga that cannot obtain magnesium cannot build chlorophyll and cannot photosynthesise, however bright the light is. Calcium is used to make bones, shells and coral skeletons. In the sea it is absorbed as dissolved calcium ions and deposited as calcium carbonate, which is the material of a mollusc's shell, the hard skeleton of a coral polyp, and the bones and vertebrae of fish. This is why coral reefs and shell-forming animals are so sensitive to the chemistry of seawater.

Iron is used to make haemoglobin in the blood of vertebrates and of some invertebrates, and haemoglobin is the pigment that carries oxygen around the body. Iron is needed in tiny amounts, but without it an animal cannot transport oxygen efficiently from its gills or lungs to its respiring cells. In the open ocean iron is so scarce that adding it can trigger enormous plankton blooms, which shows how completely a trace element can control productivity.

Nutrient cycling, decomposers, marine snow and upwelling

Nutrients are recycled, not used up. The same nitrogen atom can be part of an alga, then of the copepod that eats it, then of the fish that eats the copepod, and then be released back into the water to be taken up by another alga. The cycle has four stages, and you should be able to write them in order. First, uptake: producers absorb dissolved mineral salts from the water and carbon dioxide for photosynthesis. Second, transfer up food chains: when a producer is eaten, its nutrients pass to the consumer, and to the next consumer, and so on. Third, release after death and decay: every organism produces waste and eventually dies, and the nutrients locked in that material are broken down and returned to the water in inorganic form. Fourth, return: material that sinks is brought back to the surface by upwelling, so it can be used again.

Bacteria are the decomposers that make the cycle work. They break down dead organisms, faeces and fine organic debris, digesting the material outside their cells and absorbing the products. They use some of the carbon and nitrogen to build their own cells, and they release the rest as dissolved mineral salts and carbon dioxide into the water, where producers can take them up again. Without decomposers, dead matter would simply accumulate: the elements in it would stay locked in organic form, and since most of it sinks, it would be removed from the sunlit zone altogether. Decomposition is therefore the step that turns 'dead' back into 'available'.

Marine snow is the name for the aggregates of dead plankton, faecal pellets, mucus and fine detritus that sink slowly through the water column. Each flake is small, but together they carry a steady stream of carbon and nutrients from the surface to the deep ocean. Bacteria decompose much of this material as it sinks, and more of it on the seabed, so the deep water becomes enriched in dissolved nutrients while the surface water is stripped of them - which is exactly why surface water in the open ocean is nutrient-poor and deep water is nutrient-rich.

Upwelling closes the loop. Where winds push surface water away from a coast, or where currents diverge, cold water from depth rises to replace it, carrying the nutrients that marine snow delivered to the deep back into the sunlit zone. There, with light also available, photosynthesis can proceed at full rate. This is the single most important link in the topic between nutrients and productivity, and it is why the world's great fisheries sit on the western coasts of continents, where upwelling is strongest.

• Uptake: producers absorb dissolved mineral salts and carbon dioxide from the water.
• Transfer: nutrients pass up food chains as organisms are eaten.
• Release: bacteria decompose dead organisms and waste, releasing mineral salts and carbon dioxide.
• Sink: remains aggregate into marine snow and carry nutrients into deep water.
• Return: upwelling brings the nutrient-rich deep water back to the surface, where producers can use it again.
• Decomposers are essential because they convert nutrients from organic form, locked in dead tissue, back into the inorganic form that producers can absorb.

Respiration: releasing usable energy in every cell

Respiration is the chemical reactions in cells that break down nutrient molecules and release usable energy. It is a chemical process, it happens inside cells, and its purpose is to make energy available in a form the cell can use. The nutrient molecule broken down is usually glucose, and the version you must know is aerobic respiration, which uses oxygen. The word equation is: oxygen + glucose -> carbon dioxide + water. Energy is released by the reaction, and although it is not written as a product in the word equation, releasing it is the whole point of the process.

All cells respire - this is a statement in the syllabus and it is worth taking literally. Muscle cells in a fish, root-like cells of a seagrass, the cells of a diatom, the cells of a cyanobacterium and the cells of a decomposing bacterium all respire, because every living cell needs a continuous supply of usable energy. Energy is needed for growth, for repair, for moving substances across membranes, for movement, and simply for staying alive. A cell that stops respiring dies, so respiration never stops.

The consequence is that a photosynthetic organism respires as well as photosynthesises, at the same time and in the same cells. In daylight a seagrass leaf is doing both: its chloroplasts are making glucose from carbon dioxide and water, while its mitochondria are breaking glucose down and releasing energy. At night, when there is no light, photosynthesis stops and respiration continues, which is why the carbon dioxide concentration in a tank of water rises overnight and falls during the day. If you are asked to explain why a plant needs oxygen, the answer is that its own cells respire, not that it fails to make oxygen.

Respiration releases the energy from food in a series of small, controlled steps, so that the cell can capture and use it rather than losing it all as heat at once. That controlled release is the difference between the energy in a glucose molecule and the energy a cell can actually use, and it is why respiration has to happen in every cell rather than in one organ: the energy is made where it is needed and cannot easily be shipped around the body.

Respiration is not gas exchange, and burning food

Gas exchange is the physical movement of oxygen and carbon dioxide across a surface, by diffusion, between an organism and its surroundings. In a fish it happens at the gills; in a whale it happens in the lungs. Respiration is the chemical breakdown of glucose inside cells. The two are linked but they are not the same event: gas exchange is how the gases get in and out of the body, and respiration is what the cells then do with the oxygen. A fish can be exchanging gases perfectly at its gills while the respiration that matters is happening in the muscle of its tail.

Put the two in a sequence and the difference becomes obvious. Oxygen diffuses from the water into the blood at the gill, is carried around the body by haemoglobin (which needs iron to make), and diffuses into a muscle cell. In that cell, oxygen and glucose are broken down and carbon dioxide and water are produced, and energy is released. The carbon dioxide then diffuses back into the blood, is carried to the gill, and diffuses out into the water. Respiration happens at one point in that chain only - inside the cell.

The syllabus also asks you to investigate the release of energy by burning samples of food. A known mass of dried food is burned beneath a boiling tube containing a known mass of water, and the temperature rise of the water is measured. The energy released is calculated as mass of water (g) × 4.2 × temperature rise (°C), which gives the energy in joules; dividing by the mass of food burned gives the energy per gram, so different foods can be compared. The method is a model of respiration, because both break down the same nutrient molecules and release the energy they contain. The differences are just as important: burning releases all the energy at once as heat and light, needs a flame, and needs oxygen from the air, whereas respiration releases energy in controlled steps, at body temperature, and in a form the cell can use.

The practical has obvious weaknesses, and examiners like them named. Heat is lost to the surroundings and to the apparatus, some food is not completely burned, and some energy is used to vaporise water from the food, so the value obtained is always an underestimate of the energy in the food. Repeat readings and a draught shield reduce the error but do not remove it.

Photosynthesis: building glucose from carbon dioxide and water

Photosynthesis is the process by which some organisms make glucose from carbon dioxide and water, using energy from light, and releasing oxygen as a by-product. The word equation is: carbon dioxide + water -> glucose + oxygen, with light energy and chlorophyll needed for the reaction to happen. Two features of that definition carry marks: carbon dioxide and water are the raw materials, and oxygen is a by-product, not the purpose of the process. The purpose is to make glucose.

Chlorophyll is a green pigment that transfers energy from light into energy in organic chemicals, for the synthesis of carbohydrates. It does not supply energy and it is not a nutrient - it is the pigment that captures light energy and passes it into the chemical reactions that build glucose. Because chlorophyll absorbs red and blue light and reflects green, the organisms that contain it look green. A cell with no chlorophyll cannot photosynthesise, however much light falls on it, which is why only the illuminated parts of a plant or alga are photosynthetic and why deeper water, where red and blue light has been absorbed, supports less photosynthesis.

A producer is an organism that makes its own organic nutrients, generally using energy from sunlight, through photosynthesis. Producers sit at the base of every food chain because they are the only organisms that can turn inorganic carbon dioxide into organic compounds. The glucose they make has three destinations: it is respired to release energy, it is converted to starch for storage, and it is used as the starting material for making other organic compounds - the lipids, proteins and structural carbohydrates of the organism's body. That is the direct link back to the nutrients block: carbon dioxide supplies the carbon, nitrogen supplied from the water makes the proteins, and magnesium in the chlorophyll made the whole process possible.

In a lighted cell, photosynthesis and respiration run at the same time and in opposite directions. Photosynthesis takes in carbon dioxide and releases oxygen; respiration takes in oxygen and releases carbon dioxide. The net exchange you can measure - bubbles of oxygen from a pondweed, or a rise or fall in the carbon dioxide of the water - is the difference between the two rates, which is why a plant in the dark appears to be respiring only and a plant in bright light appears to be photosynthesising only.

Marine producers, productivity and upwellings

On land, photosynthesis is done by plants. In the sea it is done by three quite different groups, and you must be able to name all three. Marine plants, such as the seagrasses - flowering plants with roots, rhizomes and leaves that grow in shallow sediment. Some marine protoctists, which includes the microalgae (the single-celled phytoplankton), the macroalgae or seaweeds such as kelp, the diatoms with their silica shells, and some dinoflagellates. And some marine bacteria, of which the cyanobacteria are the example. Do not write 'plants' for all of them: a diatom is not a plant, and a cyanobacterium is not even a eukaryote.

The distinction matters ecologically as well as taxonomically. The attached producers - seagrasses and macroalgae - can only live where there is a hard or sandy bottom shallow enough for light to reach, so they dominate coastal waters and estuaries. The floating producers - microalgae, diatoms and cyanobacteria, collectively the phytoplankton - drift in the open water of the sunlit zone, the upper layer where enough light penetrates for photosynthesis. In the open ocean they are the producers that matter, because there is no bottom within reach of the light.

Productivity is the rate at which producers transfer energy into carbohydrates. It is a rate, so it always involves time, and it is often expressed as the mass of carbon fixed per unit area of sea surface per year. That word 'rate' is where marks are lost: productivity is not the total mass of producers present, and it is not the number of species. A small standing crop of phytoplankton that is dividing rapidly can have very high productivity, while a large, slow-growing bed of kelp can have lower productivity despite holding far more living material.

Local upwellings are one of the main reasons coastal waters are so productive. When winds and currents move surface water away from a coast, cold water rises from depth to replace it, bringing dissolved nitrate and phosphate that had been out of reach in the dark. Those nutrients arrive in the sunlit zone, where light is also available, so both requirements for photosynthesis are met at once and producers can grow and divide rapidly. The result is a bloom of phytoplankton, which feeds the zooplankton, the fish and the seabirds above it - which is why the most productive fisheries in the world are found where upwelling is strong, and why the same coast can be barren when the winds that drive the upwelling fail.

The syllabus also asks you to investigate the effect of light intensity on the rate of photosynthesis in an aquatic plant or macroalga. The standard method uses a piece of pondweed or a small macroalga in water containing hydrogencarbonate, which supplies carbon dioxide, with a lamp at measured distances. The rate is measured by counting the bubbles of oxygen released per minute, or by collecting the gas in a capillary tube and measuring its volume, and the distance of the lamp is changed to alter light intensity. Everything else must be held constant - the temperature, the carbon dioxide supply, the size of the plant - and a heat screen between lamp and plant stops the lamp warming the water, because temperature would otherwise change as well as light. Plotting rate against distance gives a curve that rises steeply as the lamp approaches and then levels off, because at high light intensity something else - usually carbon dioxide or temperature - becomes the limiting factor.