What we are learning: (component knowledge)

In the presence of UV light, a very special reaction takes place in plant cells containing chlorophyll (in chloroplasts). Photosynthesis is the reason that there is life on Earth. The reaction is:
Carbon dioxide + Water → Glucose + Oxygen
6CO2 + 6H2O → C6H12O6 + 6O2
You can experiment on plants to see proof of photosynthesis. If you cover a section of a leaf in aluminium foil and leave it in the sun. That part of the leaf that has no UV light getting to it, will not be able to photosynthesise. If that leaf it treated (ethanol to remove the waxy cuticle) and covered in iodine water. The iodine turns black in the presence of starch. Areas where photosynthesis has taken place turn black. We do not test for glucose as the glucose produced is quickly converted and stored as insoluble starch.

Rate of photosynthesis:
The rate of photosynthesis is of particular interest when we look at growing food. In order to maximise the rate of plant growth, we need to maximise the rate of photosynthesis. In specialised greenhouses, the atmosphere is rich in carbon dioxide, the plants have plenty of water (the two reactants in the equation), the temperature is optimal and there is as much sunlight as there can be.
We can test the importance of light intensity during one of our required practicals

Limiting factors of photosynthesis:
When trying to maximise the rate of photosynthesis, particularly commercially, it is important to understand that there are limits:
• Temperature - just thinking that hotter environment = faster reaction is not entirely true, after a certain temperature, the enzymes in plants become denatured over 50°C.
• Carbon dioxide concentration - over a certain point, the plant cannot process it as fast as you provide it so it is wasted.
• Light intensity - no matter how much you add, if every chloroplast is working to its full capacity, no amount of extra light will increase the rate.

Glucose and its importance:
We have discussed the importance of oxygen, however, from the point of view of plants and the point of view of the earliest cyanobacteria from around 3.5 billion years ago, oxygen is a waste product. The thing that plants want is the glucose. Glucose is used by us in respiration (the reverse of photosynthesis equation), it is converted to starch (God bless potatoes), plants also use it to make oils, fats, and the very fabric of the plants themselves - cellulose. Glucose is essential for making amino acids which are used to make proteins. As we "are what we eat", photosynthesis made the raw ingredients that we are made from. In fact, as we are carbon-based life forms, all of the carbon in us was taken from the atmosphere by photosynthesis and we ate it.
More importantly is the idea of oils and starch coming from photosynthesis which means that we have plants to thank for chips and crisps!

Aerobic respiration:
Aerobic respiration is happening in all cells when there is a plentiful supply of oxygen. There is a misconception that only photosynthesis occurs in plants, plant cells undergo respiration too. The equation for aerobic respiration is:
Glucose + Oxygen → Carbon dioxide + Water
C6H12O6 + 6O2 → 6CO2 + 6H2O
As well as these two products, energy is also transferred from the chemical store into the thermal store. This is how mammals keep warm. This is the reverse of the equation for photosynthesis.

Anaerobic respiration:
If respiration is taking place and there is not enough oxygen, it is anaerobic. Because of the lack of oxygen, lactic acid is produced and not as much energy is transferred. This lactic acid remains in your muscles and can cause cramps. After you have finished your exercise, you will need to pay off the oxygen debt in which this extra oxygen reacts with the lactic acid and it breaks down to carbon dioxide and water.
Although not a good thing in human muscles, anaerobic respiration with yeast in solution of sugars is called fermentation. This is the carbon neutral method of making ethanol (alcohol) for use in alcoholic drinks, solvents and hand sanitiser.
Glucose → Ethanol + Carbon dioxide
C6H12O6 → 2C2H5OH + 2CO2

Metabolism is the term used for all of the reactions happening in an organism. These include:
• Making lipids and fats.
• Making amino acids and turning these into proteins.
• Respiration.
• Converting glucose into insoluble starch for storage.
These reactions (and their rates) go from glucose and nitrates making amino acids then proteins and finally being broken down to waste products like urea which is excreted in urine. This can be absorbed by plants from the soil (if you had a tinkle nearby) as a source of nitrogen.

Key words/terms for this topic

Aerobic      Amino acid      Anaerobic      Carbohydrate      Carbon dioxide      Cellulose      Chlorophyll      Chloroplast      Concentration      Endothermic      Exothermic      Fatty acid      Fermentation      Glucose      Glycerol      Glycogen      Greenhouse gases      Hydroponics      Inverse Square Law      Lactic acid      Light intensity      Limiting Factor      Lipid      Metabolism      Mitochondria      Muscle fatigue      Oxidation      Oxygen debt      Photosynthesis      Protein      Respiration      Starch     

Curriculum Health Check:

Q: The glucose made in photosynthesis is quickly converted to other substances. Which of these is NOT one of them?

A: Fats and oils stored in seeds

B: Cytoplasm

C: Proteins (along with nitrate ions)

D: Cellulose which strengthens cell walls

What you need to know

Biologists' favourite process, photosynthesis, is represented by the equation:

Carbon dioxide + Water ➜ Glucose + Oxygen

6CO2 + 6H2O ➜ C6H12O6 + 6H2O

This is an endothermic reaction and needs uv light for it to happen. The reaction takes place in the chloroplasts of cells. The rate of photosynthesis is affected by:

• Light intensity,

• Temperature,

• Availability of CO2.

You need to be able to both calculate and actually measure the rate of photosynthesis, interpret graphs and explain trends/results using given data.

Required practical 5: investigate the effect of light intensity on the rate of photosynthesis. Using pondweed underwater and collecting oxygen through water.

Glucose, which is produced during photosynthesis is used for the following:

• Respiration,

• Converted then stored as insoluble starch,

• Manufacture of amino acids and proteins (need nitrogen from the soil too),

• Production of cellulose (plant cell walls),

• Making plant oils (olive oil, sunflower oil etc).

There are two types of respiration:

• Aerobic happens in the presence of oxygen and gives out more energy. This process is occurring in cells continuously and the energy is used for living organisms.

• Glucose + Oxygen ➜ Carbon dioxide + Water (reverse of photosynthesis so must be exothermic),

• Used for movement, keeping warm and making larger molecules (storage and growth).

• Anaerobic respiration occurs when there is not enough oxygen produces less energy.

• Glucose ➜ lactic acid

• This lactic acid needs to be later broken down, we call this the oxygen debt. It occurs in muscles after anaerobic exercise.

• In plant and animals cells, another product is formed

• Glucose ➜ Ethanol + Carbon dioxide

• If completed by yeast cells, this process is called fermentation.

When exercising, the heart and breathing rates increase so more oxygen gets to the muscles for respiration, if there is not enough oxygen arriving , the mitochondria in cells are forced to undergo anaerobic respiration.

Metabolism is the sum of all of the reactions happening in your body, you need to know the roles of sugars, amino acids, fatty acids and glycerol in making and breaking carbohydrates, proteins and lipids. Ensure that you know the metabolic process of:

• Glucose ➜ Starch ➜ Glycogen ➜ Cellulose

• Fats ➜ Glycerol + 3 Fatty acids

• Nitrate ions + Glucose ➜ Amino acids

• Amino acids ➜ Proteins

• Respiration

• Excess proteins ➜ urea (excreted)

Extra topics needed for the Higher Tier papers:

Photosynthesis has limiting factors, given variables of the factors that affect the rate, you need to work out which is the limiting factor. You may be given graphs showing the affect of several variables and you need to pick out which is the limiting factor.

In relation to limiting factors, ensure that you can explain the inverse square law in relation to light intensity. These factors are crucial as they affect the production rates of commercial greenhouses and it will have more and more importance with our population growing so that we can maximise food production to feed everyone.

After anaerobic respiration, the lactic acid is transported to the liver where it is converted back to glucose using more oxygen. It is this extra oxygen that is the oxygen debt.

This page was updated on: 10th April 2024