Introduction

Living organisms need energy to perform vital bodily functions. Tracing the origin of nutrients in food, we will always find that they come from green plants. Green plants get their food through photosynthesis which occurs in the chloroplast.

The conditions for photosynthesis are: sunlight, chlorophyll, carbon dioxide and water.

Structure of the Leaf

Leaves are very significant in photosynthesis. They contain chloroplasts, which contain chlorophyll. This is the green pigment that absorbs light for photosynthesis to occur.

Stomata are pores undernearth the leaves framed by a pair of guard cells. This is where gaseous exchange occurs. Carbon dioxide diffuses into the leaves and oxygen diffuses out of the leaves here.

Testing for Photosynthesis

During photosynthesis, excess glucose is converted into starch for storage. Starch is insoluble and thus suitable as storage.

In the presence of starch, iodine solution changes from brown colour to blue-black.

The Starch Test

To test for the presence of starch in leaves, follow the steps below.

1) Immerse the leaf in boiling water. (softens and kills leaf, preventing consumption of starch)

2) Place leaf in a boiling tube containing ethanol, then place the boiling tube in hot water. (removes chlorophyll to make colour change more visible)

3) Remove leaf from ethanol and immerse in hot water. (removes the ethanol)

4) Place leaf on white tile and add a few drops of iodine solution to the leaf. (determine presence of starch)

Investigating CO2

To determine if carbon dioxide is necessary for photosynthesis to occur, carry out the following steps.

1) Keep two potted plants in the dark for two days prior to experiment.

2) After two days, enclose them in a bell jar and place them under the Sun. One bell jar contains a beaker of sodium hydroxide solution and the other bell jar contains a beaker of sodium hydrogen carbonate solution.

3) After a few hours, remove a leaf from both plants and test for presence of starch.

The leaf from the plant in the bell jar with sodium hydroxide will test negative for starch. The sodium hydroxide solution absorbed the carbon dioxide in the bell jar.

The leaf from the plant in the bell jar with sodium hydrogen carbonate will test positive for starch. The sodium hydrogen carbonate supplies carbon dioxide in the bell jar. This shows that carbon dioxide is needed for photosynthesis.

Investigating Chlorophyll

To determine if chlorophyll is necessary for photosynthesis, carry out the following steps.

1) Keep a plant with variegated leaves in the dark for two days prior to the experiment.

2) After two days, the plant is placed under the Sun.

3) After a few hours, remove a leaf and test for presence of starch.

The green region of the leaf tests positive for starch whereas the non-green region of the leaf tests negative for starch. This shows that chlorophyll is necessary for photosynthesis.

Investigating Sunlight

To determine if sunlight is needed for photosynthesis, carry out the following steps.

1) Keep a plant in the dark for two days prior to the experiment.

2) After two days, cover a portion of the leaf with black paper before it is placed in the Sun.

3) After a few hours, remove this leaf and test for presence of starch.

The green region tests positive for starch whereas the covered region tests negative for starch. This shows that sunlight is needed for photosynthesis.

Ornamental Plants vs Large Scale Crops

The four conditions necessary for photosynthesis play a big part in determining the healthy growth of the plant. This is especially evident from the ways plants in different environments grow.

The table summarises how the different environment affects the growth of ornamental plants and large scale crops, two very different types of plants.

Definitions

Respiration is one of the essential characteristics of all living things.

Respiration is the breaking down of food substances (eg. Glucose) to release energy.

Types of Respiration

There are two types of respiration : aerobic and anaerobic.

Normally, organisms carry out aerobic respiration, which can ONLY be done with oxygen present. This oxygen is taken into the body via breathing. The general process of aerobic respiration can be shown by this equation:
 Glucose + Oxygen -> Carbon Dioxide + Water + Energy

However, cells can respire in the absence of oxygen. This type of respiration is anaerobic respiration. It releases less energy than aerobic respiration.

Showing Respiration at Work

This video demonstrates two different experiments to show that living things carry out respiration and release carbon dioxide. Limewater tests for carbon dioxide as it turns chalky in its presence. On the other hand, potassium hydroxide is commonly used to absorb carbon dioxide.

Breathing

Breathing is the mechanical process that draws oxygen into the body by inhalation and releases carbon dioxide by exhalation. it is a physical process carried out by the gas exchange system.

Inhalation takes air into the lungs whereas exhalation passes air out of the lungs.

Breathing vs Respiration

The terms respiration and breathing are often used interchangeably, but they are referring to two very different processes. This table summarises the key differences.

Gaseous Exchange

Gaseous exchange occurs in the lungs, specifically the alveoli or air sacs, found there.

Alveolar walls are one-cell thick and supported by a network of capillaries. This facilitates faster diffusion of gases to the blood stream. There is also a thin film of moisture since dissolved gas diffuses faster.

Composition of Air

Respiration involves the consumption of oxygen and release of carbon dioxide, a waste product. This means that the composition of air entering and leaving the lungs is different when we breathe.

Common Respiratory Diseases

The following are some common respiratory diseases.

Lung Cancer: Uncontrollable cell division in the lung, forming a tumour, which might invade other tissues
Emphysema: Damage to the alveoli and bronchioles, leading to breathing difficulties
Bronchitis: Inflammation of bronchi leading to narrowing of airway and breathing difficulties

Asthma

Asthma is a very common chronic (long-term) disease.  The airways are inflamed, swollen (narrower than normal) and very sensitive. Foreign particles (e.g. pollen/ haze) in the airway triggers the contraction of surrounding muscles, further narrowing the airway. This leads to difficulties breathing.

Furthermore, the cells lining the airway also secrete mucus to protect the patient from these particles, making the airway even narrower.

Photosynthesis vs Respiration

Photosynthesis and respiration are both extremely essential processes that allow living things to survive. The key features of both processes are summarised in this table.