AQA GCSE biology - Bioenergetics

What is photosynthesis?

The process by which plants synthesis glucose into food using light energy from the sun. Light energy is converted into chemical energy.

Where does photosynthesis take place?

Within chloroplasts in leaf palisade cells. They contain chlorophyll, a pigment which absorbs light energy.

State the word equation for photosynthesis

Carbon dioxide + water →(light) glucose + oxygen

State the symbol equation for photosynthesis

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

Is photosynthesis an endothermic or exothermic reaction and why?

Endothermic - energy is transferred from the environment to chloroplast by light

How can you show that a plant gives off oxygen during photosynthesis?

Using a water plant, collect gas bubbles produced during photosynthesis. The gas will relight a glowing splint as it contains oxygen.

Give examples of leaf adaptations which maximise the rate of photosynthesis

• Broad leaves - maximise surface area

• Thin leaves - short diffusion distance

• Chlorophyll present - trap light energy

• Veins - transport water to leaves via xylem, remove photosynthesis products via phloem

• Air spaces - allow carbon dioxide to enter and oxygen to leave

• Guard cells - control opening of stomata for gaseous exchange and prevent water loss

What are the four main factors that affect the rate of photosynthesis?

• Temperature

• Light intensity

• Carbon dioxide concentration

• Amount of chlorophyll

How does temperature affect the rate of photosynthesis?

Increasing the temperature increases the rate of photosynthesis as the kinetic energy of particles is increased. The rate decreases past a certain temperature as enzymes become denatured.

How does light intensity affect the rate of photosynthesis?

Increasing the light intensity increases the rate of photosynthesis until another factor becomes limiting

How does carbon dioxide concentration affect the rate of photosynthesis?

Increasing the carbon dioxide concentration increases the rate of photosynthesis (until another factor becomes limiting) as carbon dioxide is required to make glucose

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

Decreasing the amount of chlorophyll decreases the rate of photosynthesis as chlorophyll is required to absorb light energy

What is a limiting factor?

An environmental factor which can restrict the rate of photosynthesis (e.g. light intensity) so that it’s below the maximum rate

Limiting factors continued

These factors interact and any one of them may be the factor that limits photosynthesis

• Explain how you can calculate the rate of photosynthesis by measuring oxygen production

• Set up bubble potometer apparatus (pondweed in a sealed tube of water, attached to a capillary tube and a gas syringe)

• Oxygen gas produced causes the bubble in the capillary tube to move. The distance moved by the bubble is used to calculate the volume of oxygen produced

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

They can control temperature, light intensity and carbon dioxide concentration to achieve the fastest possible rate of photosynthesis leading to a greater yield. They can do this in greenhouses, by using computer software, electricity and gas which are all very expensive.

Benefits:

• Profits will be high as there is a high turnover of clean produce

• Less staff needed

• Crops will be grown where land is poor

State the law which describes the relationship between the distance of a light source from a plant and light intensity

Inverse square law - light intensity is directly proportional to 1/distance²

State 5 uses of the glucose produced during photosynthesis

• Respiration

• Insoluble starch for storage

• Cellulose to strengthen cell walls

• Production of amino acids for protein synthesis

• Production of fat or oil for storage

What do plants use for protein synthesis?

To produce proteins, plants also use nitrate ions that are absorbed from the soil

What is aerobic respiration?

An exothermic reaction in which glucose reacts with oxygen to release energy which can be used in cells

What is the word equation for aerobic respiration?

Glucose + oxygen → carbon dioxide + water (+ energy)

What is the symbol equation for aerobic respiration?

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O

Where does aerobic respiration take place?

In the mitochondria

Why do organisms require the energy released in respiration?

• Synthesis of larger molecules

• Muscle contraction for movement

• Maintenance of body temperature to keep warm

• Active transport

What is anaerobic respiration?

An exothermic reaction in which glucose is broken down to release energy in the absence of oxygen

What are the equations for anaerobic respiration?

Muscles:

Glucose → lactic acid (+ energy)

Plants and yeast:

Glucose → ethanol + carbon dioxide (+ energy)

Why is anaerobic respiration less efficient than aerobic respiration?

As the oxidation of glucose is incomplete in anaerobic respiration, much less energy is transferred than in aerobic respiration.

Why can anaerobic respiration lead to muscle fatigue?

Lactic acid (product of anaerobic respiration) which is toxic, builds up in muscles, preventing efficient contraction

What is an oxygen debt?

The amount of oxygen needed to convert lactic acid back into glucose after anaerobic respiration

What is fermentation?

A type of anaerobic respiration that occurs in yeast cells

Why is the fermentation reaction important?

It is used in the manufacture of bread and alcoholic drinks

What are the differences between aerobic and anaerobic respiration?

• Aerobic required oxygen; aerobic does not

• Aerobic produced carbon dioxide and water; anaerobic produces lactic acid or ethanol + carbon dioxide

• Aerobic transfers a greater amount of energy

How do muscles store glucose?

As glycogen

What changer take place when muscular activity increases in the body?

• Heart rate increases and arteries dilate - increases flow of oxygenated blood to muscles

• Breathing rate increases and breathing is deeper - increases the rate of gaseous exchange

• Stored glycogen is converted back into glucose

How is lactic acid transported away from the muscles?

Blood flow through the muscles transports lactic acid to the liver, where it is oxidised back to glucose

What happens to the lactic acid in the liver?

• It is oxidised to carbon dioxide and water

OR

• Converted to glucose, then glycogen - glycogen levels in the liver and muscles can then be restored

When does anaerobic respiration take place during exercise?

If insufficient oxygen is supplied anaerobic respiration takes place in muscles, so they no longer contract efficiently. The incomplete oxidation of glucose causes a build up of lactic acid and creates an oxygen debt

What happens during long periods of vigorous activity?

• Lactic acid levels build up

• glycogen reserves in the muscles become low as more glucose is used for respiration, and additional glucose is transported from the liver

What is metabolism?

The sum of all the reactions that take place in a cell or orgaism

How do cells use the energy transferred by respiration?

To continuously carry out enzyme-controlled processes which lead to the synthesis of new molecules

Give 5 examples of 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