1. Photosynthesis
2. Where does photosynthesis take place - tree pipe system
3. The opposite of photosynthesis - respiration
4. Does a plant need more than water, sunlight and carbon dioxide to grow?
5. Carbon atom cycle
6. Tasks

Photosynthesis


Above is shown a plant cell. A plant cell contains chloroplasts with chlorophyll.
The green chlorophyll helps the photosynthesis reaction to occur. (Info about picture)

Land plants absorb carbon dioxide (gas) from the air and water plants take up carbon dioxide (gas) dissolved in water. In exchange land and water plants emit oxygen. The chemical reaction describing the reaction is called the photosynthesis reaction:

Carbon dioxide  +  water  +  energy    glucose  +  oxygen

6CO₂  +  6H₂O  +  energy (sunlight)    6O₂  +  C₆H₁₂O₆

Apparently, the plant needs carbon dioxide, water and sunlight (energy) to produce oxygen. Simultaneously glucose is formed. Glucose is needed to build the tree or plant. The glucose builds up leafs, stems but also the fruits of the plant.

Trees / plants / bushes that grow can connect glucose molecules into starch or cellulose.

A plant can use glucose to form starch (potato, wheat, root vegetables). Here are about 300-400 glucose molecules linked together in a row as strings. The strings in starch form spirals.

Glucose molecules linked together as straight strings can form long fibers. The number of glucose molecules linked together can be about 1000-1200 pieces. Cellulose builds up i.e. the stem and leaves of a tree.


 
Where does photosynthesis take place - tree pipe system

Photosynthesis occurs in plant leaves. The cells in the leaves have small units called chloroplasts. Chloroplasts contain chlorophyll. Chlorophyll is a green dye with the task to facilitate the photosynthesis reaction. Therefore, the photosynthesis occurs in the green chloroplasts. Carbon dioxide enters the leaves through tiny openings on the underside of the leaves. These openings are called stomata (see photo to the left). In comes carbon dioxide and out comes oxygen. Out comes also excessive water that the plant has taken up from the soil through its roots. A large birch needs several hundred liters of water a hot summer day.


These stomata are magnified 350 times. (Info about picture)


To provide the entire tree with sugar manufactured during photosynthesis the tree has a pipe system. The sugar produced in the leaves passes down through the bark to the roots. This way also the roots can grow.

To provide the tree with water for the photosynthesis, water is sucked up through the roots. The water is led inside the tree trunk up to the tree's crown. The water is transported to the leaves.


The reason that water travels up through the tree depends on two things. Partly because the water travels up through narrow pipes in the tree - the water is drawn up by something called capillary attraction (see photo to the left). Each water molecule is polar. This means that each water molecule has a positively charged side and a negatively charged side. The pipe wall in the tree pulls either the positively charged side of the water molecule or the negatively charged side. Water contains oxygen and hydrogen (H₂O). Also the capillary water traveling contains hydrogen (H) and oxygen (O). The water is thus drawn up through the tree.

The second reason why water is pulled up to the tree's crown is that when the sun warms the tree, water evaporates. The water leaves the tree through the tree's stomata. As this happens a negative pressure is formed in the pipes carrying water to the tree crown. This vacuum sucks up water just like when you suck up the juice through a straw.

The opposite of photosynthesis - respiration

The trees create oxygen that humans and animals need. We humans use oxygen and glucose. Oxygen and glucose react to form water, carbon dioxide and energy. It is the very energy that we humans need. We thus extract energy that once was stored in plants through photosynthesis. The reaction looks as follows and is called the cellular respiration reaction:

Oxygen  +  glucose    carbon dioxide  +  water  +  energy

6O₂  +  C₆H₁₂O₆    6CO₂  +  6H₂O  +  energy

If we instead burn wood then carbon dioxide, water and energy are created and released. This happens also according to the reaction foresaid above. But there is a difference, this combustion reaction can take place only after the ignition temperature of wood is reached. This temperature is about 300 ^oC. The Cell respiration reaction in humans occurs at only 37 ^oC. This is because the body has substances that lowers the activation energy from 300 oC to only 37 ^oC. The cellular respiration reaction is controlled by the body.

Does a plant need more than water, sunlight and carbon dioxide to grow?

The answer is yes. The plant also needs certain nutrients absorbed by the soil water. These nutrients can be phosphate and nitrate. Also some ions are can be helpful. I.e. sodium ions (Na⁺), calcium ions (Ca²⁺), potassium ions (K⁺) and magnesium ions (Mg²⁺). Chemical fertilizers used by farmers on farmland contain primarily nitrogen (N), phosphorus (P) and potassium (K).

Carbon atom cycle


Animals and plants eaten by animals are transformed through the cellular respiration reaction to carbon dioxide, water and energy.

Tasks:

1. Draw a leaf and show with arrows what is needed and what is emitted when photosynthesis occurs?
2. Tell about the cellular respiration reaction?
3. If you are to grow aquarium plants - what does the water need to contain to let the plants grow extra well?
4. Think of a leaf as a factory. What are the raw materials, products, and machinery?