Investigating the abiotic factors that affect the size of Ivy leaves in shaded and unshaded habitats.

Authors Avatar

Neha Poshakwale

Investigating the abiotic factors that affect the size of

Ivy leaves in shaded and unshaded habitats.


Ivy is any one of a large number of creeping or climbing vines.  These vines have different botanical names, and the word ivy, as commonly used, does not belong to any one plant.  It often applies to climbing vines, especially to those that are ornamental. The particular one being looked at in this experiment is the common, or English, ivy.  English ivy is the plant that makes such an attractive picture as it climbs over walls and tree trunks in Europe and North America.  Its waxy leaves usually have five points, or angles.  They are dark green in summer and turn bright scarlet in the fall.  The plant retains its leaves all year.  English ivy also bears tiny flowers.  This ivy clings to smooth surfaces with the fine roots on its stems.  It does not grow well in the bright sun of the central, southern, and western United States.  But in shady locations, it can be grown as far north as Ontario, Canada.  It makes an excellent covering for buildings.  Its leaves and berries are poisonous. English ivy belongs to the ginseng family, Araliaceae. The scientific classification would be Hedera helix.

Being an evergreen plant it has the advantage of being able to photosynthesis during the winter months whereas deciduous trees are dormant. The increased light that is available, by the absence of deciduous leaves allow it to grow more rapidly up the trunk of the host tree. The evergreen leaves of the plant also inhibit the leaves of the deciduous tree thereby suppressing the growth of the host tree. The increased openness of the tree crown further stimulates the growth of the vine. As the ivy climbs up the host tree to reach the canopy, the density of the vine as well as the weight of the water and ice on the leaves increases the weight of trees. This often results in branches breaking during heavy winds.

The aim of this experiment is to investigate whether abiotic factors contribute to the size of ivy leaves in a shaded and unshaded habitat.


Organisms capable of synthesising their own food are known as Autotrophs. Of the autotrophs, the phototrophs or 'light feeders' which rely on the sun as their source of energy, and the chemotrophs which rely on the energy from breaking chemical bonds to synthesise their food. Ivy is a phototrophic plant.

Photosynthesis is the process by which phototrophs convert carbon dioxide and water into simple carbohydrates and oxygen in the presence of chlorophyll, using sunlight. Photosynthesis is summarised by the equation:

However this equation is somewhat miss leading, as photosynthesis is a two-stage process. The light-dependent reactions produce materials, which are then used in the light-dependent stages. The whole process takes place all the time during the hours of daylight, but only the light-independent reactions of photosynthesis are sometimes referred as the dark reactions (however this does not mean they only occur in the dark, where as in fact they occur continuously).  

Light energy is trapped in the chloroplast lamellae by photosynthetic pigments which are either chlorophylls e.g. chlorophyll a and b or caratenoids (e.g. carotene and xanthophylls). The chlorophyll a pigment absorbs light energy in the red and blue wavelengths of light in the spectrum. They reflect green light, which is the reason why the leaves appear green. The more light availability there is the more likely light energy is to get absorbed at the correct wavelength. Different pigments absorb different wavelengths of light.

The Light-Dependent Stage

This stage has two main functions. Water molecules are split in a photochemical reaction. This provides hydrogen ions, which can then be used to reduce fixed carbon dioxide and so produce carbohydrates. Also ATP is made, which supplies the energy for the synthesis of carbohydrates. When a photon of light hits a chlorophyll molecule, the quantum of energy is transferred to the electrons of that molecule. The electrons are excited - they are raised to higher energy levels. One may be raised to a sufficiently high energy level to leave the chlorophyll completely. If this happens a carrier molecule will pick up the excited electron, and this can result in the synthesis of ATP by one of two processes - cyclic or non-cyclic photophosphorolation.

Cyclic photophosphorolation

The light-excited electron may be passed along an electron transfer chain, with each member of the chain at a lower energy level, until it is returned to the chlorophyll molecule that is came from. As the electron moves along the chain, down the energy levels, ATP is produced by the phosphorylation of ADP. The electron leaves the chlorophyll and returns to it, so may then be excited in exactly the same way again.

Non-cyclic photophosphorylation

The excited electron may instead be used to provide the reducing power needed in the second, light-independent stage of the photosynthetic process. Water dissociates spontaneously into hydrogen (H+) ions and hydroxide (OH-) ions. As a result there are always plenty of these ions present in the cell, including in the interior of the chloroplasts. Interactions between these ions and chlorophyll molecules bring about the process of non-cyclic photophosphorylation.


There are two types of photosynthetic pigments; primary and accessory pigments. The primary pigments are two forms of chlorophyll a, with a slight difference in their absorption peaks. The accessory pigments include other forms of chlorophyll a, chlorophyll b and the caratenoids. The pigments are arranged in light-harvesting clusters called photosystems. In a photosystem, several hundred accessory pigment molecules surround a primary pigment molecules and the energy of the light absorbed by the different pigments is passed on to the primary pigment. These are said to act as reaction centres. Photosystem I is arranged around a molecule of chlorophyll a with a peak absorption at 700nm. The reaction centre is therefore known as P700. Photosystem II is based on a molecule of chlorophyll a with a peak absorption of 680nm, so is known as P680.

An excited electron from photosystem II passes to an electron acceptor and down an electron transfer chain to photosystem I, which is at a lower energy level than photosystem II. This loss of energy allows the synthesis of a molecule of ATP. Light energy can then excite an electron from photosystem I, and this excited electron passes to another electron acceptor - nicotinamide adenine dinucleotide phosphate (NADP). NADP also takes up a hydrogen ion from water and is thus reduced, forming NADPH. The NADPH is a source of reducing power for the light-independent reactions.

So photosystem I receives electrons via the electron transfer chain from photosystem II. This leaves photosystem II electron deficient. The electron from photosystem II is replaced by an electron from a hydroxide ion:

4OH- - 4e- → O2 + 2H2O

The hydroxide ions are 'left behind' from the hydrogen ions taken up in the reduction of NADP to NADPH. The removal of the electrons from hydroxide ions by the photosystem II results in the by-product oxygen. Thus the reactions of the light-dependent stage of photosynthesis provide a source of reducing power (NADPH) and the universal energy-supplying molecule ATP, with oxygen gas given off as a waste product. To find out how the NADPH and ATP are used to make carbohydrates we must move on and consider the reactions of the light-independent stage.

The Light-Independent Stage

The light-independent reactions are known as the Calvin cycle. This is a cyclic reaction consisting of a series of small steps resulting in the reduction of carbon dioxide to bring about the synthesis of carbohydrates. NADPH and ATP from the light-dependent reactions provide the reducing power and the energy needed for the various steps. The stages of the cycle are controlled by enzymes and are independent of light.

Carbon dioxide from the air combines with ribulose bisphosphate (RuBP), a 5-carbon compound which fixes the carbon dioxide by accepting it and making it part of the photosynthetic reactions. The enzyme ribulose bisphosphate carboxylase is necessary for this step. The result is a theoretical highly unstable 6-carbon compound which immediately splits to give two molecules of glycerate-3-phosphate (GP), a 3-carbon compound. This is reduced to give glyceraldehyde-3-phosphate (GALP), a 3-carbon sugar. The hydrogen for the reduction comes from NADPH and the energy required from ATP, both produced in the light-dependent stage. Some of the glyceraldehyde-3-phosphate is synthesised into the 6-carbon sugar glucose, which is supplied to the cells or converted to starch for storage. However, much of the glyceraldehyde-3-phosphate is passed through a series of steps to replace the RuBP, without which further carbon dioxide cannot enter the cycle.

The products of photosynthesis, although initially carbohydrates, are rapidly fed into other biochemical pathways to produce amino acids and lipids for the requirements of the cells of the plant.

If the leaves have a larger surface area they would absorb a large amount of light energy and this will increase the rate of photosynthesis provided that there are no other limiting factors such as availability of water. The leaves would also have a large number of stomata, which allow gaseous exchange, and this would increase the amount of water vapour lost by the plant through transpiration. Transpiration occurs as the sun warms the water inside the blade.  The warming changes much of the water into water vapour.  This gas can then escape through the stomata.  Transpiration helps cool the inside of the leaf because the escaping vapour has absorbed heat.  

Join now!

Transpiration also helps to keep water flowing up from the roots.  Water forms a continuous column as it flows through the roots, up the stem, and into the leaves.  The molecules of water in this column stick to one another.  As the molecules at the top of the column are lost through transpiration, the entire column of water is pulled upward.  This pulling force is strong enough to draw water to the tops of the tallest trees.  In addition, transpiration is necessary for mineral transport from the soil to the plant for cooling the plant through evaporation, to move ...

This is a preview of the whole essay