GCSE Biology: Bioenergetics

What do plants use for their source of energy?

Light

How do plants trap light?

Via photosynthesis

Why is photosynthesis an endothermic reaction?

Because it takes in energy

What happens during photosynthesis?

1. The green chemical chlorophyll (which can be found in leaves) takes carbon dioxide and water into the leaf.

2. Light energy is then absorbed by the chlorophyll.

3. This light energy is then used to convert carbon dioxide and water into the sugar glucose. Oxygen is also produced

What is the word equation for photosynthesis?

…………………………………….light……………………….

carbon dioxide + water → glucose + oxygen

…………………………………chlorophyll………………

CO2 + H2O → C6H12O6 +O2

Uses of glucose in photosynthesis

FO- Converted into fats and oils which are used by the plant as a storage form of energy.

R- To release energy in respiration

A- Produce amino acids

C- Make cellulose which strengthens cell wall

S- Produce the insoluble storage molecule starch (can be converted back to glucose when needed)

Amino acids

• Used by the plant to synthesise proteins

• To make amino acids from glucose, plants need to absorb nitrate ions from the soil.

Investigate the effect of light intensity on the rate of photosynthesis

1. Take boiling tube and place it 10cm away from an LED light source (LED light doesn’t release much heat so temperature of experiment isn’t affected).

2. Fill boiling tube with sodium hydrogen carbonate solution as it releases CO2 which is needed for photosynthesis.

3. Place a piece of pondweed into boiling tube with cut end at the top.

4. Leave this for 5 minutes to acclimatise to the conditions in the boiling tube.

5. There should be bubbles of gas being produced from the cut end of the pondweed (oxygen gas).

6. Start a stop- watch and count the number of bubbles produced in one minute.

7. Repeat this 2 more times and calculate the mean number of bubbles produced in one minute.

8. Do this all at different distances from light source.

Issues with photosynthesis practical

• Number of bubbles could be too fast to count accurately.

• The bubbles are not always the same size.

• To fix this, measure the volume of oxygen produced instead.

Inverse square law

• As distance is doubled, the number of bubbles falls by 4 times.

• This is because the light intensity falls by 4 times when the distance is doubled.

If we increase the light intensity and the rate of photosynthesis increases, what does that tell us?

The light intensity was a limiting factor- photosynthesis was not as fast as it could’ve been because there wasn’t enough light.

When the rate of photosynthesis stops increasing as the light intensity increases, what does that tell us about the light intensity?

Light intensity is no longer the limiting factor.

If photosynthesis increases as the level of carbon dioxide in the air increases, what does that tell us?

Carbon dioxide is the limiting factor

When the rate of photosynthesis no longer increases as carbon dioxide levels in the air are increasing, what does that tell us?

Carbon dioxide is no longer than limiting factor

Effect of decreasing the amount of chlorophyll on the rate of photosynthesis

Decreases rate of photosynthesis because the leaves have less chlorophyll, they absorb less light energy than normal leaves.

Effect of temperature on photosynthesis

As we increase the temperature, the enzymes involved in photosynthesis work faster so the rate increases.

However, if we keep increasing the temperature, the enzymes will denature and the rate of photosynthesis falls.

Uses of glucose from photosynthesis

• To release energy in respiration- This transfers energy from glucose which enables the plant to convert the rest of the glucose into various other useful substances.

• To produce the insoluble storage molecule starch- The starch can be converted back into glucose when it is needed to e.g. at night.

• Converted into fats and oils- Fats and oils are used by the plant as the storage form of energy.

• Producing cellulose- Glucose is converted into cellulose which strengthens the cell wall of the plant.

• Producing amino acids for protein synthesis- Glucose is combined with nitrate ions (absorbed from the soil) to make amino acids which are then made into proteins.

What do farmers do to their greenhouses to increase the rate of photosynthesis?

• Light them

• Heat them

• Also add extra CO2

• This is quite expensive so the extra cost has to be justified by the increase in yield.

What do some gardeners use to increase the rate of photosynthesis?

• Oil burners- these release heat and CO2 at the same time.

What do organisms need energy for?

• Movement

• Keeping warm

• Chemical reactions to build larger molecules e.g. proteins are made by chemically joining amino acids.

Where does the energy that we need come from?

A process called cellular respiration

Why is respiration an exothermic reaction?

Because it releases energy

Where and when does respiration take place?

Continually in all living cells

Aerobic respiration

glucose + oxygen → carbon dioxide + water +energy

Aerobic respiration releases a great deal of energy because the glucose molecule has been fully oxidised.

Anaerobic respiration in muscles

When oxygen is limited muscle cells respire anaerobically.

glucose → lactic acid + energy

Anaerobic respiration releases much less energy than aerobic respiration because in anaerobic respiration, the oxidation of glucose is incomplete.

Anaerobic respiration in plant and yeasts cells (fermentation)

glucose → ethanol + carbon dioxide +energy

This reaction is used to make alcoholic drinks and yeast to make bread. The carbon dioxide in this reaction is useful as it creates bubbles in the dough causing the bread to rise.

Why do we not need a great deal of energy when we’re relaxing?

Because we aren’t moving

How does the body react to the increased demand for energy?

During exercise, the body needs a great deal of energy for muscle contraction.

Because the body needs more energy aerobic respiration increases.

This means that the body cells need more oxygen.

To provide this extra oxygen, both the breathing rate and the breathing volume increase. This gets more oxygen into the bloodstream.

The heart rate also increases to pump this oxygenated blood to the body.

However, sometimes not enough oxygen can be supplied to the muscles- especially if we are exercising hard.

At this point anaerobic respiration now takes place in the muscles.

Anaerobic respiration during exercise

During anaerobic respiration, the oxidation of glucose is incomplete This leads to a build up of the chemical lactic acid.

During long periods of vigorous activity, the lactic acid causes the muscles to become fatigued.

This causes the muscles to stop contracting efficiently.

At this point the body has to remove the lactic acid from the muscles. This creates a condition called the oxygen debt.

How is lactic acid transported out of the muscles?

1. By the blood.

2. The lactic acid is then taken to the liver and converted back to glucose in a series of chemical reactions.

However, to do this the cells require oxygen.

What is the oxygen debt?

The amount of extra oxygen the body needs after exercise to deal with the accumulated lactic acid.

That is why some people continue breathing rapidly for some time after finishing exercise.

What happens to the energy released by respiration?

It’s used by enzymes to synthesises new molecules in the cell.

What is metabolism?

The sum of all the chemical reactions in a cell or the body.

Metabolic reactions:

• conversion of glucose to starch, glycogen and cellulose

• the formation of lipid molecules from a molecule of glycerol and three molecules of fatty acids

• the use of glucose and nitrate ions to form amino acids which in turn are used to synthesise proteins

• respiration

• breakdown of excess proteins to form urea for excretion

Conversion of glucose into glycogen (humans + other animals)

Glucose is converted into glycogen- this is a storage from of glucose.

The formation of lipid molecules from a molecule of glycerol and three molecules of fatty acids

One molecule of glycerol is reacted with 3 molecules of fatty acids to form a lipid molecule.

Lipids are found in the cell membrane

Breakdown of excess proteins to form urea for excretion

Humans don’t need to eat a large amount of protein.

Excess proteins are broken down into the chemical urea which is then excreted by the kidneys.