Biology (B2) - Plants: Obtaining Food and Growth

The equation for photosynthesis is:

carbon dioxide + water + (light energy) --> glucose +oxygen

The reason why plants photosynthesise is to get glucose. 

Photosynthesis happens in the chlorophyll which are green pigments found in chloroplasts and absorbs light energy. Chloroplasts are found in the palisade cells in the upper layer of the leaf. Photosynthesis can only take place in the green part of the leaf.

Some algae can photosynthesise as they too have chloroplasts. 

A way to see if a plant has been photosynthesising is testing for starch. If there is any excess sugars they are stored as the insoluble product, starch.

Before starch is tested for the leaves and put into boiling water and decolourised by heating in ethanol

If iodine is added to and it turns blue-black then starch is present and therefore has been photosynthesising, if it turns brown then no starch is present.

Crops grow faster if the rate of photosynthesis is also faster.

There are three main factors to the rate of photosynthesis, light intensity, warmth and the amount of carbon dioxide.

The rate of photosynthesis is slowed even if only one of these is low. 

If you increase the rate of carbn dioxide and light intensity then the rate of photosynthesis will also increase but to a certain point, after that point the rate of photosynthesis will stay the same even if the light intensity and the concentration of carboon dioxide was increased.

If the warmth is increased then the rate of photosynthesis will iincrease but to a certain point. After this point the rate will decrease quickly as a high temperature will denature enzymes in the plant.

Glucose is made in plants during photosynthesis, some of this is then used in respiration to gain energy for growth, rebuilding and repairing cells, chemical reactions and movement in the organism.

Some glucose will be converted into starch which is useful as it is insoluble and doesn't react easily. It is an energy store as it can be converted back into glucose. 

Other glucose is converted into fat or oils which are also energy stores and belong to the group called lipids which has a higher energy content than carbohydrates and therefore provide more glucose for respiration. 

When cellulose synthesises glucose links to form strong fibres making cellulose a useful structural material for plant and some algae cell walls. It prevents the cell from bursting when it absorbs water.

Plant and algae cells can also synthesise proteins using glucose and other raw materials like nitrates which are absorbed in the soil (or water if the plant is aquatic).

If plants are grown in greenhouses or polytunnels it is possible to control the conditions around the plants. If there is a faster rate of photosynthesis then the plant will grow faster.

One of the limiting factors is the light intensity, this can be increased by providing more light and making sure taller plants do not shade the smaller ones.

The concentration of carbon dioxide can also be increase by burning fuels such as propane or adding the gas through a PVC tube. 

The temperature can also be made to 25 degrees celcius, which is the optimum conditions for photosynthesis, by boilers and radiators, however sometimes this temperature is exceeded by heatrays from the sun as well as the radiators being on. This is resolved by vents being opened and blinds covered the area to lower the temperature.

Unfortunately, increasing carbon dioxide, light intensity and warmth costs, there are also one off costs of equipments. The key is finding a balance between the yeild and costs.

There are three ways of getting carbon dioxide to a green house which are:

Propane Burners which is where propane is burnt producing carbon dioxide. Propane is a fossil fuel and contains sulpher as an impurity so by burning propane, sulpher dioxide is also formed which is a pollutant and causes acid rain. The one off cost of equipment is £32,600 and a daily cost of £217 for propane.

Flue Gases which is where natural gas is burned in a microturbine generating electricity, the heat is used to heat water and can either be circulated straight to the greenhouse through pipes or stored in tanks.  The one off cost is £118,000 but £84 per day for the natural gas.

Liquid Carbon Dioxide  which is where liquid carbon dioxide is delivered and stored in special cylinders and then transported to the greenhouses using PVC tubing. The equipment is rented and costs £6900 per year and lasts up to ten years and the daily cost of carbon dioxide is £234.