Tag Archives: biology

Sugars

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“Sugar” is one of those terms that scientists and the general public use in different ways. A scientist would be far more likely to refer to “sugars”, as sugar is actually a group of different molecules. All sugars are carbohydrates, i.e. they contain carbon, hydrogen and oxygen, but the arrangement of these atoms differs between sugars.

Monosaccharides

monosaccharide-glucose

The monosaccharides are the simplest sugars. Shown above is the straight-chain form of glucose.

Glucose (aka dextrose)

Glucose is what fuels cellular respiration in living things, and as such is an essential nutrient. It is one of the main products of photosynthesis, created when carbon dioxide and water are combined (using energy from the Sun’s light), producing oxygen as a by-product.

Glucose is found in most foods that contain carbohydrates. It can be created through the breakdown of starch and glycogen (see below), or in more complex disaccharide sugars like sucrose and lactose.

The name dextrose comes from the scientific name for one of glucose’s isomers, D-glucose. The D- prefix comes from the fact that a solution of D-glucose rotates the plane of polarised light to the right (i.e. from the Latin for “right”, which is “dexter”).

Fructose

Fructose is a sugar that is commonly found in many plants, especially in fruits. It is the sweetest of the naturally occurring carbohydrates.

Galactose

Galactose is less sweet than glucose and fructose, and is found in dairy products and sugar beets.

Disaccharides

disaccharide-lactose

Dissaccharides are formed when two monosaccharides are combined. Shown above is lactose.

Sucrose (aka “Table Sugar”)

Sucrose is the sugar that sits on your kitchen table. It comes from cane and beets, and is formed when glucose and fructose are combined.

Maltose

Maltose is formed of two glucose molecules combined together. It is created in the mouth when starch (a polysaccharide) is broken down by amylase in saliva. You can demonstrate this by chewing a piece of bread for a long time – after a while the starch will be broken down into maltose and it will begin to taste sweet. Maltose is contained in cereals, pasta and potatoes.

Lactose

Lactose is a combination molecule of glucose and galactose, and is found in milk, and therefore in milk products like cheese. A dietary intolerance to lactose is found in more than half of the world’s population.

Oligosaccharides

Oligosaccharides are formed when between three and nine monosaccharides are combined.

Fructo-oligosaccharides are formed from combinations of fructose molecules, and are found in vegetables, and are an important component of fibre. Galacto-oligosaccharides are formed from combinations of galactose molecules, and are found in soybeans; they stimulate the action of “friendly” bacteria in the gut, but cannot be digested by humans.

Polysaccharides

A section of a cellulose molecule

Polysaccharides are longer chains (more than ten units) of monosaccharide molecules; common polysaccharides include starch, glycogen, cellulose and chitin. Starches are broken down in the body into more “useful” glucose molecules, and glycogen is used as secondary store of energy (after fat tissue) in humans (the same role that starch plays in plants). Cellulose and chitin are structural, used in the creation of the structure of a plant or animal.

Is the Portuguese Man o’ War an Organism?

man-o-war

Despite its appearance, the Portuguese Man o’ War is not a jellyfish.

It is technically known as a siphonophore, and although it might appear to be one single organism, a man o’ war is actually a colony of four smaller individual organisms known as zooids, which could not survive outside of the colony.

The most noticeable feature of a man o’ war is the “sail” or pneumatophore, a gas-filled balloon filled with carbon monoxide generated by the man o’ war’s gas gland, and nitrogen, oxygen and argon from the atmosphere. The sail allows the man o’ war to trail it’s tentacles through surface water, allowing it to feed, and can be deflated in case of attack from the surface, allowing the man o’ war to escape by submerging itself temporarily.

man-o-war

The sail is the clear “bag” at the top of the image, and the dactylzooids are the bluish tentacles below.

The remainder of a man o’ war is composed of three groups of zooids: the gastrozooids, the gonozooids and the dactylzooids. The dactylzooids make up the familiar tentacles, up to ten metres long, that trail through the water and which are covered in venom-filled nematocysts that are responsible for the tentacles’ highly painful stinging effect. The tentacles capture and guide food to the gastrozooids, which ingest and digest it. The gonozooids are responsible for reproduction.

Sapwood and Heartwood

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Until recently, I didn’t realise that there was more than one type of wood inside a tree. The difference was brought to my attention by Earth Science Photo of the Day‘s photo from April 4th.

ebony-sapwood-heartwoodSource: David K. Lynch

The photograph above shows a cross-section through a branch from an ebony tree. The heartwood in the centre is what we traditionally think of as being ebony – almost dark black in colour, whilst the sapwood surrounding it is the more “usual” pale brown colour.

All wood begins as sapwood, and it is sapwood that grows just under the surface of the bark, forming growth rings in the process. Sapwood, as its name suggests, carries sap (transported in tubes called xylem) which the tree uses to store and transport water, sugars (maple syrup is made by reducing xylem sap from maple trees to concentrate the sugars), hormones and nutrients.

In young trees all wood is sapwood, but in older trees, as the tree grows in diameter, less cross-sectional area is required for the transport of sap, and greater structural support is required to keep the tree upright. The sapwood in the centre of the tree dies, forming heartwood, and as the cells die they release chemicals that change the colour of the wood, as well as making the wood stronger and more resistant to attack by insects.

sapwood-comparison

The ratio of sapwood to heartwood depends on how many leaves the tree has and how fast it grows: more leaves and faster growth require more water and therefore more sapwood, and not all trees form any heartwood at all. In the photograph above, a cross-section of a maple tree is on the left and a cross-section of a black locust tree on the right: maple trees have very large leaves, and the black locust trees have small leaves, hence the very obvious difference in their sapwood to heartwood ratios.

 

Breathing Underwater

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When you are diving underwater, the pressure of the gases in your lungs must be the same as the ambient pressure under water: if it were lower your lungs would implode, and if it were higher they would explode. As you go deeper and deeper, this increased pressure in your lungs forces nitrogen to dissolve into your bloodstream and from there into your tissues.

If you surface too quickly, this nitrogen is released from the tissues in the form of bubbles, and these large bubbles cause a variety of symptoms, most commonly pain in large joints (e.g. shoulders, hips an knees) and itching in the skin. (Bending these joints lessens the pain, giving the condition its colloquial name: “the bends”.) This decompression sickness can be avoided by surfacing slowly (as calculated by decompression tables or dive computers), which allows the nitrogen to leave blood and tissues in a controlled manner.

In order to lessen the risk of the bends, divers can use a different mix of gases, known as nitrox, in their scuba equipment. Nitrox contains less nitrogen (and therefore more oxygen), and this reduces the amount of nitrogen available to dissolve into the blood.

air-nitrox-comparison

The difference between air (L) and nitrox (R). Nitrogen is coloured blue, oxygen is coloured red and other gases (carbon dioxide, etc.) are coloured orange.

For diving below about thirty metres nitrox is not suitable, for reasons best explained by the ideal gas law:

pV=nRT

p is the pressure in your lungs, V is the volume of your lungs, n is the number of gas molecules (or rather the number of moles of gas), R is a constant known as the molar gas constant (R=8.31 \mathrm{J/K/mol} and T is the temperature of the gas. Only the pressure and the number of gas molecules are variable, so as you dive deeper and the pressure increases this means that you will have more and more molecules of each gas in your lungs.

At a depth of fifty metres the pressure is about five times what it is at the surface, and your lungs would therefore contain five times as many nitrogen and oxygen (and carbon dioxide, argon, etc.) molecules as they normally would. This can lead to nitrogen narcosis, an impairment of cognitive function thought to be caused by disruption of nerve impulses. At this depth it’s not enough to simply replace the nitrogen with more oxygen, as in the case of nitrox; the extra oxygen in the lungs will lead to oxygen toxicity, a condition characterised by altered vision, drowsiness and disorientation.

To dive at this depth both the nitrogen and oxygen must be replaced by an inert gas that does not have narcotic effects. In most cases this is helium, producing a gas mix known as trimix. Different trimix “recipes” are used depending on dive depth: a high-oxygen variety for shallow depths (30-60 metres) and a lower-oxygen variety for deeper dives (below sixty metres).

trimix-comparison

Shallow-dive (L) and deep-dive (R) trimix recipes. Nitrogen is shown in blue, oxygen in red and helium in green.

Other breathing gases are also used. During very deep dives (below 150 metres) high-pressure nervous syndrome (also known as helium tremors) is a serious risk, and thus helium is either partially or entirely eliminated, to form hydreliox or hydrox respectively. These gases are highly dangerous, as they contain both hydrogen and oxygen, which forms an explosive mixture.

Zahavi handicaps

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In 1975 biologist Amotz Zahavi proposed his handicap principle.

“It is suggested that [characteristics] which develop through mate preference confer handicaps on the selected individuals in their survival. These handicaps are of use to the selecting sex since they test the quality of the mate. The size of [characteristics] selected in this way serve as marks of quality.”

peacock-plumage

In some populations of animals some of those animals will display a costly (in terms of survival) handicap. The classic example of this is the peacock’s tail: The presence of the tail makes it more likely that the peacock will fall prey to predators; the fact that it has survived despite this handicap suggests to peahens that it possesses superior genes, and is therefore worthy of mating with.

The handicap has to be expensive (in terms of survival) in order for it to be a reliable signal as to reproductive fitness. If it is not, then all males or females could display the same signal, and the signal would serve no purpose. Other examples of Zahavi handicaps include courtship dances, the extravagant nests built by Bowerbirds and the bouncy “stotting” behaviour of gazelles.

It has been suggested that certain activities which humans engage in, such as bungee jumping or conspicuous consumption (especially of Veblen goods) are examples of Zahavi handicaps – attempts by humans to demonstrate reproductive fitness to possible mates.