Photosynthesis

Learning Intentions - Leaves and Photosynthesis

4. Describe and discuss the life processes of photosynthesis

Photosynthesis

Glucose

Glucose is probably the most important molecule in biology

It is made of just 3 types of atoms = Carbon, Oxygen and Hydrogen

It is made from Carbon and Water

It is a Carbo-hydrate 

The Water component comes from .... water

The Carbon part comes from Carbon Dioxide

The Dioxide part of Carbon Dioxide floats away while the glucose is being made. It floats away as O2

Glucose is important for us as it is the main fuel for respiration

Cellular Respiration turns the Glucose back into Carbon Dioxide and Water, by re-combining the Carbon with O2, thus making CO2. The remaining atoms are then Hydrogen, Oxygen and Hydrogen. These make H2O, or water


The Process to make glucose needs energy. 

The energy comes from light, the light comes from the sun

The process is Photosynthesis

Photosynthesis is the synthesis of a molecules using energy from light, photons (photo)  

To make glucose, you need 24 atoms:

To get these, you could use the REACTANTS

After the reaction, you end up with these PRODUCTS

To break apart the Carbon dioxide and the water, you need Light energy and a catalyst (something that is going to conduct the reaction)

But what is the Catalyst for photosynthesis?

This process needs a little help. this is where Chlorophyll comes in. 

Chlorophyll is the amazing molecule that uses light energy to break apart the carbon dioxide and the water molecules that are then used to make glucose (and molecular oxygen)

Once glucose has been synthesized through photosynthesis, it can then be used as the fuel for cellular respiration

Or it can be combined with other glucoses to make starch

Or it can be combined to make cellulose

Or it can be disassembled, combined with nitrogen and restructured into an amino acid, or even turned into an oil

The Chlorophyll is found in the Chloroplast of Plants 

Chlorophyll = molecule = Catalyst for the photosynthetic reaction

Chloroplast = organelle 

Photosynthesis and Cellular Respiration

Photosynthesis and Cellular Respiration are mirror images of each other

Photosynthesis uses Carbon dioxide and Water to make Glucose and Oxygen

Cellular respiration uses Glucose and Oxygen to make Water and Carbon Dioxide

So, what is the point if it just goes round and round? 

Follow the energy....

Energy enters Biological systems, that we are part of, via the sun. That energy then becomes chemical energy stored in glucose. We eat something. That glucose gets into our cells and is then 'burnt' via cellular respiration back into water and carbon dioxide (this is the reason we breath in oxygen and breath out carbon dioxide). But, then we use the energy released via cellular respiration to do stuff... to do everything that we do. To go for a run, groom the dog, play PS5  and read this. :-)

The purpose of Photosynthesis 

 Food for plants = starting point of the planets food web

Photosynthesis is the N in MRSGREN = Nutrition

Plants are autotrophs, this means they make their own food

In terms of life processes, photosynthesis is the N in MRSGREN

Photosynthesis is how plants get their Nutrition 

The glucose made by photosynthesis is 'burnt' in cellular respiration to obtain the chemical potential energy that is then used throughout the cell (and thus throughout the plant) to power everything that happens in the cell. 

The glucose made by photosynthesis is then used by the plants as a source of atoms for the building of all of the structures within the plant cells and thus the plant

this includes the Starch granules in the cells. The Starch in the Cotyledon and Endosperm in the seed. 

This also includes the cellulose of the cell wall, the cellulose that we can't digest and refer to as dietary fiber

If we eat the glucose from a plant, such as eating oranges, so of the glucose is 'burnt' via cellular respiration for us to get energy to do stuff - hence if we eat glucose we can get a 'sugar high'

Also, if we eat the glucose from a plant, such as eating oranges, some of the atoms will stay as part of our body - in fact if we eat lots of glucose then the atoms will be stored in our body as fat. 

Likewise,  glucose is the source of energy and of atoms in the plant. So photosynthesis is how plants make their food. Thus, photosynthesis is the source of a plants Nutrition

Easy-as overview of photosynthesis

Factors affecting the rate of photosynthesis

Light Intensity

Light provides the energy for photosynthesis - this is the primary limiting factor for photosynthesis - and this is why photosynthesis does not occur at night

The more light there is, the more energy there is, so the faster the rate of photosynthesis. 

As a light dependent reaction, the reaction rate increases proportionally with light intensity 

This increases to a point, it then plateaus, with other factors acting as the limiters.

Carbon Dioxide

As one of the two substrates for photosynthesis, the more carbon dioxide present, the more carbon that is available for construction into glucose.

As carbon dioxide is present at very low concentrations (0.04%) in the atmosphere, it is a key limiting factor. An increase in carbon dioxide results in an immediate increase in the rate of photosynthesis.  

Temperature

The hotter the temperature, the faster carbon dioxide and water can move within the leaf and the faster the enzymes can move around and operate. So,  the faster the reaction. 

Notice very carefully, the dramatic drop in the graph below. Once the temperature gets too hot, the enzymes start to change their shape, they denature, thus the active sites of the enzymes no longer fit the substrates and the reaction stops. 

Chlorophyll

If we give the plant plenty of Carbon Dioxide, Light and Warmth, then the limiting factor of photosynthesis finally becomes the quantity of Chlorophyll in the leaf

Light Intensity Data