Photosynthesis cont'd
Monacia Williams, Contributor

Welcome to another in our series of lessons. Last week we continued our look at the theory of photosynthesis. This week we begin our study of some of the practical aspects. Photosynthesis is one of those topics that lends itself to practical investigations. Carrying out practical investigations reinforces what you have learnt in theory.

You can carry out investigations to show that the two conditions, chlorophyll and sunlight must be present for photosynthesis to occur; that the raw material, carbon dioxide, is also necessary; and that the two end products, starch and oxygen, are produced. Your syllabus requires you to carry out experiments to demonstrate that:

• light is necessary for photosynthesis
• chlorophyll is necessary for photosynthesis and to draw and label the external features of a dicotyledonous leaf.

This does not mean that you are to ignore the other experiments since you may be asked to answer questions that might be set on any or all of them.

Today, I will provide you with several pointers that you can use to get the best possible grade. We will look first at the drawing and labelling of the leaf. Please note that this is the time when you need to look back at the checklist for drawing that I gave to you some weeks ago. After you have completed the drawing, go through the checklist and assess your work.

The leaf that we are going to use for our exercise is the hibiscus leaf. You are required to:

• Examine the leaf, making careful note of the main features
• Think of how the leaf is positioned on the tree and try to draw it in that position
• Make a large drawing, remembering that:
• The drawing must be to the left of the page
• It must occupy at least 70 per cent of the page
• Label the drawing, remembering to:
• Check your spelling label in script
• Have your labelling lines touch the object that you are labelling
• Omit all arrowheads and dots from the labelling lines
• Annotate your labels.

Your teacher will probably mark your drawing using the following criteria:

Clarity

• Clean, clear, continuous lines of even thickness
• Veins should be properly attached to the midrib
• There is no shading

Accuracy

Apex should be drawn with a slightly elongated point

• The leaf margin must be slightly wavy
• The pattern of veins must be similar - you need not draw the small veins.
• The proportions must be correct
• The midrib must be thicker than the lateral veins.

Labels must be correctly spelt and should include the following:

• Apex, margin, lamina, midrib, vein, petiole.
• Labelling lines need to be neatly drawn in pencil with a ruler, not crossing.

Annotations should be as follows:

• Lamina - broad and flat, large surface area, dark green in colour indicates a lot of chlorophyll.
• Midrib - contains the xylem which transports water to the photosynthesising cells and phloem which transports the manufactured food from them.
• Network of veins - these go to all the cells of the leaf.
• Petiole - holds leaf in position to get the maximum sunlight.

Title and magnification - in block capitals placed under your drawing for example:

DRAWING OF A HIBISCUS LEAF x 1.5

I guarantee that, if you practise all that has been suggested so far, you will get the maximum points available. I can just hear some of you saying: "But Miss, I can't draw!" Guess what, with every drawing that you do, your skills improve. What are you waiting for? Start drawing!

Next week we will look at the two required experiments as well as some terms that you will need to understand if you are going to successfully write up your experiments. See you then!

This Hydel Group of Schools student blows a trumpet during a 'Save the Children Rally'. Thousands turned out to give their support. - Anthony Minott/Freelance Photographer

Monacia Williams teaches at Glenmuir High School.