4.4 Bioenergetics

Question 1

photosynthetic reaction

Answer 1

word equation: carbon dioxide + water -> glucose + oxygen
symbol equation : 6CO2 + 6H2O → C6H12O6 + 6O2
Photosynthesis is an endothermic reaction, where energy is transferred from the environment to the chloroplasts by light. To make glucose molecules by photosynthesis, energy is required as it’s an endothermic reaction (takes heat in), this energy is supplied by sunlight.
Takes place in chloroplasts, which contain chlorophyll that absorbs light.

Question 2

Rate of photosynthesis - factors

How does temperature affect the rate of photosynthesis?

Answer 2

Increasing the temperature increases the rate of photosynthesis, as more energy is provided for the reaction and increases the kinetic energy of particles. The rate of photosynthesis decreases past a certain temperature (45 degrees), as enzymes become denatured.
(enzymes also work more slowly at low temperatures)

Enzymes are proteins that help speed up metabolism, or the chemical reactions in our bodies

Question 3

Rate of photosynthesis - factors

How does light intensity affect the rate of photosynthesis?

Answer 3

Increasing light intensity increases the rate of photosynthesis, as more energy is provided for the reaction. At a certain point, even though light intensity increases, the rate will no longer increase, as another factor becomes limiting factor instead (temperature or CO2 levels).

Question 4

Rate of photosynthesis - factors

How does carbon dioxide concentration affect the rate of photosynthesis?

Answer 4

Like light intensity, increasing the carbon dioxide concentration increases the rate of photosynthesis until another factor becomes the limiting factor. CO2 is needed to make glucose.
(as long as light and CO2 levels are in plentiful supply, the limiting factor must be temperature.)

Question 5

Rate of photosynthesis - factors

How does the amount of chlorophyll affect the rate of photosynthesis?

Answer 5

Chlorophyll is an essential pigment in the leaf, as it is required to absorb light energy. Therefore, decreasing the amount of chlorophyll decreases the rate of photosynthesis. (vice versa)
The amount of chlorophyll can be affected by:
- diseases (tobacco mosaic virus)
- lack of nutrients (eg magnesium)
- loss of leaves (fewer leaves -> fewer chloroplasts)

Question 6

limiting factors

Answer 6

a factor that stops the reaction going any faster

an environmental factor that can restrict the rate of photosynthesis (eg light intensity)

Question 7

inverse square law and light intensity in the context of photosynthesis

Answer 7

The inverse square law shows the relationship between light intensity and distance.
As distance increases, the light intensity decreases. (lamp).
However it’s not that simple, light intensity decreases to the square of the distance.
This means that light intensity is inversely proportional to the square of the distance.

Question 8

inverse square law

Answer 8

light intensity ∝ 1/distance²
∝ means proportion to
putting 1 over distance shows the inverse

The inverse square law means that if you halve the distance, the light intensity will be 4 times greater. If you third the distance, the light intensity will be 9 times greater. Likewise, if you double the distance, the light intensity will be 4 times smaller.

eg. Calculate the light intensity when the lamp is 10cm from the pondweed using the inverse square law.

light intensity = 1/d²
light intensity = 1/10²
= 0.01 a.u.

Question 9

RP6: investigate the effect of light intensity on the rate of photosynthesis using an aquatic organism such as pondweed

Answer 9

1. Place a boiling tube 10cm away from a light source
   (led -> doesn’t release much heat, otherwise it could change the temperature of experiment and affects rate of photosynthesis)
2. Fill the boiling tube with sodium hydrogen carbonate solution. Sodium hydrogen carbonate solution releases CO2, which is needed for photosynthesis for the pondweed.
3. Place a piece of pondweed into the boiling tube with the cut end on top
4. Leave the boiling tube to rest for 5 minutes
   Bubbles of gas should be produced from the cut end of the pondweed -> this gas is oxygen and is produced by photosynthesis
   The number of bubbles produced in a given time is proportional to rate of photosynthesis
5. Start the stopwatch and count the number of bubbles produced in one minute.
6. repeat this 2 more times and calculate the mean number of bubbles produced in one minute
7. repeat the whole experiment for distances 20cm, 30cm, and 40cm
8. Plot a graph of light intensity (distance from light source) (x) against rate of photosynthesis (Bubbles per minute/volume of oxygen per minute) (y)

Question 10

RP6: sources of error

Answer 10

• the number of bubbles can be too fast to count accurately
• bubbles are not always the same size (large bubble would count the same as a small bubble)
• Temperature may also be a factor affecting the rate of photosynthesis and this is not taken account
  of

how to fix this:

• place pondweed under a funnel and catch bubbles in a measuring cylinder w/ water
• then use measuring cylinder to measure the volume of oxygen produced

Question 11

RP6: graph

What is the relationship between the distance from a light source and light intensity?

Answer 11

inverse square law
If we double the distance, the number of bubbles per minute falls by a factor of 4.
If we double the distance, the light intensity falls by a factor of 4
As we need light for photosynthesis, causes the number of oxygen bubbles to also decrease 4 times

Question 12

RP6: variables and risks

Answer 12

• independent variable – distance from the light source/light intensity.
• dependent variable – the number of bubbles produced per minute.
• control variables – concentration of sodium hydrogen carbonate solution, temperature, piece of pondweed

risks
being careful to avoid water being dropped onto the hot light bulb and electrical equipment

Question 13

uses of glucose from photosynthesis

Answer 13

1. used for respiration to provide energy
2. converted into insoluble starch for storage (stored in the stem, leaves, or roots), ready to use when photosynthesis isn’t taking place eg winter
3. used to produce fat or oil for storage -> glucose is turned into lipids for storing in seeds
4. used to produce cellulose -> which strengthens the cell wall
5. used to produce amino acids for protein synthesis -> plants use nitrate ions that are absorbed from the soil, to make amino acids which are then made into proteins
   (nitrate ions are needed as there is nitrogen in amino acids, but not in glucose)
   Proteins are used for growth and enzymes

Question 14

respiration

Answer 14

Cellular respiration is an exothermic reaction (releases heat and energy is transferred to the environment) which is continuously occurring in living cells. The energy transferred in respiration supplied all the energy needed for all living processes. It is the process of transferring energy from the breakdown of glucose. Respiration takes place in every cell in the body, and can take place aerobically (w/ oxygen) or anaerobically (w/o oxygen) to transfer energy.

Question 15

Why do organisms need this energy?

Answer 15

• for chemical reactions to build larger molecules
• movement
• keeping warm

Question 16

aerobic respiration

Answer 16

respiration using oxygen, and is the most efficient way to transfer energy from glucose
this energy released can be used by cells
Most of the reactions in aerobic respiration happen inside mitochondria

glucose + oxygen -> carbon dioxide + water

C6H12O6 + 6O2 -> 6CO2 + 6H2O

Question 17

anaerobic respiration

Answer 17

anaerobic respiration is used if there is not enough oxygen -> glucose is broken down to release energy in the absence of oxygen
Eg. during vigorous exercise and your body can supply enough oxygen to muscles

As the oxidation of glucose is incomplete in anaerobic respiration (glucose is not completely broken down), much less energy is transferred than in aerobic respiration (less efficient)
equation in muscles and in animals:
glucose -> lactic acid

As it doesn’t make much energy, it’s only used in emergencies; during exercise when it lets you keep using your muscles for a little longer

Question 18

Why can anaerobic respiration lead to muscle fatigue?

Answer 18

lactic acid (product of anaerobic respiration) builds up in muscles and prevents efficient contraction (painful)

Question 19

anaerobic respiration in plant and yeast cells

Answer 19

glucose -> ethanol + carbon dioxide
Anaerobic respiration in plant and yeast cells is called fermentation and has economic importance in the manufacture of bread and alcoholic drinks.
1. bread making -> carbon dioxide from fermentation makes bread rise
2. in beer and wine making -> fermentation process produces alcohol

Question 20

response to exercise

Answer 20

During exercise, the human body reacts to the increased demand of energy. More energy is needed for muscles to contract -> increased respiration, which means more oxygen needs to be supplied to cells and more CO2 removed.
The heart rate, breathing rate and breath volume increase during exercise to supply the muscles with more oxygenated blood, and removes CO2 more quickly as well.

Question 21

insufficient oxygen in exercise

Answer 21

If insufficient oxygen is supplied, anaerobic respiration takes place in muscles. The incomplete oxidation of glucose causes a build up of lactic acid and creates an oxygen debt. During long periods of vigorous exercise, muscles become fatigued and stop contracting efficiently.

Question 22

what is an oxygen debt?

Answer 22

Blood flowing through the muscles transports the lactic acid to the liver, where it is converted back into glucose Oxygen debt is the amount of extra oxygen your body needs after exercise to react with the accumulated lactic acid and remove it from the cells.

Question 23

metabolism

Answer 23

metabolism is the sum of all the reactions in a cell or the body
The energy transferred by respiration in cells is used by the organism for the continual enzyme controlled processes of metabolism that synthesise (make) new molecules.

Question 24

examples of metabolism

Answer 24

• 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 is used to make proteins
• respiration
• breakdown of excess proteins to form urea for excretion

Question 25

How can farmers use their knowledge of limiting factors to increase their profits?

Answer 25

Farmers want to increase the rate of photosynthesis, as it increases the yields of crops they produce.
So they light and heat their greenhouses, and also add extra carbon dioxide
Greenhouses trap the Sun’s heat, and makes sure that temperature doesn’t become limiting. In summer, they could use shades and ventilation to cool things down. Using artificial light after the Sun goes down, and increasing CO2 using a paraffin heater, as when it burns, it releases CO2 as a by-product.

Question 26

pros and cons of farmers using limited factors to their benefit

Answer 26

Limiting factors are important in the economics of enhancing the conditions in greenhouses to gain the maximum rate of photosynthesis while still maintaining profit. Keeping plants in a greenhouse is expensive, but the increased yield of the crop and fact that the crop can be harvested more frequently, means that the farmer will make more money.