Green_Plants_Concept_1
Plant growth
Plant growth
Green plants are solar-powered machines, They have the ability to use the energy in sunlight to build carbohydrates the food source used, either directly or indirectly, by every animal on the planet. This process is called photosynthesis, and is also performed by some algae and bacteria.
Photosynthesis is a chemical process (see Figure l) in which energy (sunlight) is used to convert two common substances, carbon dioxide and water into two different substances: glucose (a simple sugar) and oxygen,T he same chemicals that enter the process come out the other end, but they are arranged differently. This reaction does not occur spontaneously whenever carbon dioxide and water happen to meet on a sunny day: it requires a chemical catalyst to make it happen.The catalyst is chlorophyll, a pigment which gives leaves their green colour. This substance absorbs light energy and uses it to drive the chemical conversion.
The by-product of the process, oxygen, is released back into the atmosphere, while the glucose is retained within the plant Figure 2 shows a cross-section of a leaf. Vessels (similar to those which bring water to the leaf take the glucose solution to the rest of the plant! where it is used as an energy source to build leaves, flowers, fruit and seeds. Some glucose is converted to other forms of carbohydrate: cellulose (to give ceil walls their structure) or starch (a means of storing the energy). Stored energy is mainly contained in the root system, the leaves, the seeds or fruits around the seeds — these are the most nutritious parts of the plant, as far as most animals are concerned Some animals, such as the cow, are also able to derive energy by breaking down cellulose. The glucose used to build fruits provides their sweet taste and energy content.
Rates of photosynthesis
Enzymes (biological catalysts) within the plant control the rate of photosynthesis, There are several factors which affect the rate at which green plants are able to convert carbon dioxide and water into oxygen and glucose, the main ones being the availability of carbon dioxide, light and water and the temperature:
When the amount of carbon dioxide in the atmosphere is increased, the rate of photosynthesis (for a given level of light) increases until the enzymes are not able to work any faster This means that as levels of atmospheric carbon dioxide rise, so does the rate at which plants convert it into glucose and oxygen.
The brighter the sunlight, the greater the rate of photosynthesis — up to the maximum that the enzymes are capable of. Beyond that, there is very little effect.
There is an optimum temperature for photosynthesis. Different plants have evolved different optimum temperatures to match their environment: desert plants have a higher optimum than, say pine trees that grow in the north of Scotland Neither would grow particularly wen if they swapped places. Both above and below the optimum temperature, the rate of photosynthesis will decrease.
There is also an optimum level of soil moisture, This level depends on the type of plant: water lilies prefer a damper soil than cacti, for example. Too much water will engorge the plant cells and cause them to burst, Water use is also dependent on other factors, such as the temperature and the rate of photosynthesis. Plants transpire (release water by evaporation from their leaf surfaces) in order to draw more water (containing dissolved mineral nutrients) into the plant through the roots. The rate of transpiration (like perspiration in animals) increases with temperature. The need for water itself increases with the rate of photosynthesis, since water is a vital part of the photosynthetic process (carbon extracted from carbon dioxide is combined with water to make carbohydrate) and is also needed to transport the products of photosynthesis from the leaf ceils to the rest of the plant.
Vegetable protein
As animals require protein for growth so do plants. Seeds (particularly grains and beans) are particularly rich in protein, due to their role as food stores for growth in the life cycle of the plant Plants are not able to ingest proteins: they have to make them from basic chemicals. Glucose provides most of the chemicals, but nitrogen, phosphates and potassium are also required. Most plants usually extract these from the soil.
Nitrogen is necessary to make the proteins used in cell membranes, DNA enzymes and chlorophyll. Plants lacking sufficient nitrogen have yellowing leaves and stunted growth. Some plants, living in regions where the soil nitrogen level is particularly poor, have evolved into 'carnivores': they trap insects and other small animals to ingest their nitrogen.
A potassium-deficient plant will produce new leaves that are yellow, due to the lack of a particular enzyme that is important in the control of photosynthesis,
Insufficient phosphate will result in a plant having stunted root growth and new leaves that are purple-tinged, again due to an inability to synthesise necessary enzymes.
At KS I and KS2, a difficult line must be walked Children are expected to understand the conditions that plants require for healthy growth without being given an explanation of photosynthesis, the key process responsible for growth. They receive information from many sources concerning the importance of plants in the production of oxygen, which all animals need in order to breathe, They are constantly being warned of the dangers of cutting down rainforests and the overproduction of carbon dioxide through car exhausts and industry. But why do plants produce oxygen? Children need to realise that it is a by-product of a process that leads to plant growth. An understanding of the other factors that lead to plant growth will enable them to gain a better appreciation of the crucial role that plants play in the environment.
Catalyst — a substance that enables a chemical reaction to take place.
Chlorophyll — the chemical that makes the photosynthesis process work
Photosynthesis — the process by which light energy is used to convert carbon dioxide and water into glucose and oxygen,
Transpiration — water loss from plant leaves, causing more water to be drawn up through the roots.
Every year, photosynthesising organisms produce about 1 70 billion tonnes of new carbohydrates. That's about 30 tonnes for every person on Earth — but of course, we do have to share it with other animals!
The largest known leaf belongs to the Amazonian bamboo palm. It is 20 metres long,
Wood is made from the soil.
This is a common mistake; it seems to arise from a difficulty in understanding that a solid (cellulose) can be made by combining a liquid (water) with a gas (carbon dioxide). The child assumes that the solid must have had its origin in another solid medium: the soil. This belief shows that the child has not fully understood the effects of photosynthesis.
More water means a bigger plant.
This is an almost inevitable conclusion of poorly managed 'with and without' science experiments from the primary school of years gone by. No water and the plant dies; provide water and it lives — from which children were often left to infer that the more water a plant has, the better it grows! More effective experiments would show that there is an optimum level of soil moisture for the growth of any given plant. Children need to be aware that over-watering a pot plant can damage it.
Plants give off carbon dioxide at night.
Plants give off carbon dioxide all the time: they need 0xygen to respire, converting the food they have made into usable energy. (There are similarities with animal respiration here .) The waste product of this respiration is carbon dioxide, During the day, when plants are photosynthesizing, they are using up more carbon dioxide than they are producing through respiration; there is thus a net increase in the oxygen level in the air around them. At night when no photosynthesis is taking place but the plant is still respiring, there is a net increase in carbon dioxide and a reduction in the oxygen level around them. For this reason, it used to be common practice to remove pot plants (and even cut flowers) from hospital wards at nightfall for the patients' benefit
Why are plants green?
Because that's the colour of the chlorophyll in the leaves and stalks, It also has some important implications for photosynthesis.
Plants appear green because green light is reflected from them. As photosynthesis works on the light absorbed into cells, the colour of this light must be a mixture of red and blue, A plant grown under green light will reflect rather than absorb the light energy, and so will not be able to photosynthesise. Note that the colour of natural light from the sky may be red or blue, but is never green!
Basic tips about plant investigations
Avoid binary 'with and without' experiments: they are an oversimplification and can lead to inappropriate findings.
Avoid having one plant per child: nobody wants their' plant to die, so fair tests may not be fairly conducted! Few children will be particularly happy about being placed in charge of the plant that will not be receiving any water as part of an investigation into the water requirements of plants, and may end up cheating to protect their plant,
If you want to grow plants for the children to take home at the end of term, do this separately from fair tests.
Use more than one plant in each 'condition' when testing: repetition makes the test more accurate,
Plant growth provides opportunities for a range of related investigations to be carried out by groups, as in the examples below.
Growing plants — light (planning, testing, recording)
Different groups can address different aspects of plant growth: colour of light ('glass houses' made from different-coloured plastic drinks bottles); light intensity (wattage of bulb); duration (hours per day the plant is exposed to the light); frequency (number of times the light goes on and off in a day). They should use the same type of plants under the same conditions over a number of weeks, and record the changes using a video or other camera or by direct measurements (recorded on a computer spreadsheet). Each group can present their findings to the others.
Growing plants — water I (planning, testing, recording)
Do all plants need the some amount of water? The children can try giving plants of different types (but of roughly the same size) the same amount of water to see the effect. (NB Avoid spiny cacti or other potentially dangerous plants.)
Growing plants — water 2 (planning, testing, recording)
How does watering affect growth? Using the same type of plants, different groups can try altering: the amount of water given; the frequency of watering (for example, 500ml once a week or 100ml per day); the time of watering (morning, noon or afternoon); the method of watering (in the pot, on the leaves); the type of water given (tap, distilled, salty); the temperature of the water. Each group should record its results. The activity could be repeated for different types of plant and the results compared.