Tagged: carbohydrate

Human Diet: Grade 9 Understanding for IGCSE Biology 2.24

All animals are heterotrophic.  This means that they cannot make their own food molecules but need to get them from some external source.  Humans get a variety of different food molecules from what they eat.  Diet is a term for what an animal eats (and in a biological context has no associations with any attempt to lose weight or change body shape).  A balanced diet is a combination of foods that provides the correct proportions of all the various food molecules for any particular individual at any particular stage of their life.

Components of a Balanced Diet



Carbohydrates are a family of molecules that includes sugars, starch and other polysaccharides.  They contain C,H and O atoms only and their main function in the diet is to provide molecules that can be respired to release energy for cells.  Carbohydrates are thus one of the main respiratory substrates in our diet.  All sweet foods will contain sugars of course and starch-rich foods are vegetables like potatoes, pasta and rice.  Starch is a polymer of glucose and so needs to be digested to glucose because it is too large a molecule to be absorbed in the small intestine.


Proteins are a family of macromolecules needed to build new cells and thus for growth.  Like starch, Proteins are also polymers and thus get digested into their constituent monomers, in this case amino acids in the digestive system.  Protein-rich foods include all meat and some pulses and beans.    Proteins in the diet are needed to build muscle tissue, to form some components of cell membranes and to make all the enzymes that catalyse all the metabolic reactions in cells.


Lipid is a general term for all fats and oils.  Despite the popular misconception that fat is “bad” in our diet, in fact lipids are essential molecules in the diet.  We need lipids as a respiratory substrate, for long term energy storage in adipose tissue under the skin and for the electrical insulation of nerve cells.  Foods rich in lipids are red meat, many processed foods, and food containing olive oil or other vegetable oils.


Humans need a wide variety of mineral ions in very low concentrations in our diet.  The most important mineral in our diet is Calcium which is needed for making healthy teeth and bones.  Iron is also needed in relatively high amounts as it is required to make the protein haemoglobin found in red blood cells.  Mineral ions come from eating a wide variety of foods, but the main source of calcium is from milk and other dairy products.  Iron is found in high concentrations in red meat. minerals


Rather like minerals, vitamins are needed in very small amounts in a diet but are absolutely crucial for the healthy functioning of the body.  The diseases associated with a lack of a particular vitamin in the diet are called deficiency diseases.  You need to know about three vitamins – A. C and D vitamin_a Vitamin A is a molecule called retinal found in carrots, red peppers and swede.  It is needed for healthy growth and a functioning immune system.  Vitamin A is also essential for normal vision since it is used to make the pigment found in rod cells in the retina.  Vitamin A deficiency in the diet often causes poor vision, especially at night. Vitamin C is needed for the enzyme that produces the protein collagen in the body.  It is found in all fruit especially citrus fruits. A lack of vitamin C causes the deficiency disease scurvy. Vitamin-C Vitamin D is an unusual vitamin since it can be made in the skin using UV light.  Vitamin D is needed in the small intestine to absorb mineral ions such as calcium, magnesium, etc. into the blood.  A lack of vitamin D often results in a deficiency disease called rickets in which the bones malform. Screen-shot-2012-05-14-at-9.44.44-AM


Dietary Fibre is actually made up from the molecule cellulose.  No mammal including humans possesses a cellulase enzyme and so when plant material passes through the intestines, dietary fibre is never digested.  This means it passes into the large intestine where it helps prevent constipation.  Foods rich in fibre included wholegrain bread, vegetables and some breakfast cereals.


Water is the final component of a balanced diet.  It is needed to replace water lost by sweating and in urine and acts as a solvent of course for all the metabolic reactions that happen in every cell.

Factors affecting rates of Photosynthesis: Grade 9 Understanding for IGCSE Biology (part 1) 2.18 2.19

Photosynthesis is the process occurring in plants in which sunlight is trapped by chlorophyll pigments and used to power the chemical reactions involved in making food molecules such as carbohydrates from carbon dioxide and water.  Oxygen is released as a waste product of these reactions.

(I can’t write a chemical equation as I can’t find a way of writing subscript in WordPress….. Can anyone help?)







In the equation above, the carbohydrate produced is glucose, a six carbon sugar.

The reactions of photosynthesis happen in specialised mesophyll tissue in the leaf of the plant (see previous post)  Inside the palisade and spongy mesophyll cells there are thousands of tiny organelles called chloroplasts in which the reactions of photosynthesis occur.


So what environmental factors could be altered to vary the rate of photosynthesis in a plant?

Light Intensity – light provides the energy for photosynthesis and so the higher the intensity of light, the more energy the chloroplasts receive to make carbohydrates.

Light wavelength – chlorophyll pigments absorb the blue-violet and red parts of the spectrum well but cannot absorb green light.

Carbon Dioxide concentration – this is a reactant for photosynthesis so increasing the concentration makes a collision between the reactant molecule and the enzyme inside the chloroplast that bind it more likely, so the rate will go up.

Temperature – many reactions in photosynthesis are catalysed by enzymes and enzymes are very affected by temperature:  too low temperatures and the enzymes and substrate molecules move very slowly and so there are few collisions, too high temperatures and the enzymes change shape (denature) so the substrate molecules cannot fit into the active site.

NB – water availability is never a factor that can alter rates of photosynthesis even though it is a reactant molecule.  This might seem unusual until one remembers that plants that are dehydrating will close the stomata in their leaves to minimise transpiration.  Closed stomata mean that carbon dioxide cannot get into the air spaces in the leaf so this is ultimately what limits photosynthesis in a dehydrated plant.


The experimental set up above is the best way to measure rates of photosynthesis and so investigate the effect of any of the four factors listed above.  Light intensity can be varied either with a dimmer switch as above or by altering the distance between the lamp and the plant.  The heat shield is transparent to let light through but will absorb the heat from the bulb ensuring the temperature of the water stays constant.  Carbon dioxide concentration can be altered by dissolving different masses of sodium hydrogen carbonate in the water.  The wavelength of light will stay constant so long as the build remains the same.

How to measure rates of photosynthesis in this set up?

Well you could collect the gas produced over a long period of time and measure its volume with a gas syringe.  This might sound more accurate than counting bubbles but in fact it is a less reliable way as you would have to leave the set up for a long time and variables might change.  So it is fine to assume that the bubbles produced are oxygen and that every bubble is the same volume: if you do this, the rate of production of bubbles is directly proportional to the rate of photosynthesis in the Elodea plant.