3.2 Photosynthesis uses light energy to synthesise organic molecules

Question 1

Where does photosynthesis take place?

Answer 1

Within chloroplasts

Question 2

How do chloroplasts receive light energy.

Answer 2

Chloroplasts contain photosynthetic pigments that absorb different wavelengths of light energy.

Question 3

Name 3 photosynthetic pigments

Answer 3

Chlorphyll A/B, Carotene and Xanthophylls

Question 4

Where can chloroplasts be found?

Answer 4

Chloroplasts are within the thylakoid membrane.

Question 5

Name some features of a leaf that make them well suited for photosynthesis

Answer 5

• Large surface area to capture as much light as possible
• Chloroplasts can move within the palisade mesohyll e.g.., moving to the bottom of the leaf in periods of high light intensity to avoid bleaching.
• Pigments are a single layer to maximise the chances of light hitting each one.
• In lower light intensity they spread evenly across the cytoplasm to maximise absorption.

Question 6

How can photosynthetic pigments be separated?

Answer 6

Through chromatography

Question 7

How can photosynthetic pigments be identified?

Answer 7

Through their Rf values, how far they’ve travelled during chromatography. Comparing this to a table of know pigments.

Question 8

How do you calculate an Rf value of a photosynthetic pigment?

Answer 8

By dividing the distance the pigment has moved from the origin by the distance the solvent front has moved from the origin

Question 9

What is an absorption spectrum?

Answer 9

An absorption spectrum is graph that shows the amount of light that is absorbed at different wavelengths.

Question 10

At which wavelengths is the most light absorbed?

Answer 10

Blue and Red. Yellows and greens are reflected.

Question 11

What is an action spectrum?

Answer 11

An action spectrum shows the rate of photosynthesis at each wavelength of light.

Question 12

Relationship between absorption and action spectrum

Answer 12

When overlapped show similar trend which suggests that the light energy absorbed by the wavelengths are used in photosynthesis.

Question 13

Outline Thomas Englemanns experiment

Answer 13

He placed a spirogyra on a slide and added aerobic bacteria. He shone light through a prism to represent the different wavelengths of light, the bacteria then moved to areas that photosynthesised as this would produce oxygen.

Question 14

What did Englemanns conclude?

Answer 14

That blue and red regions of the spectrum had the highest levels of photosynthetic activity.

Question 15

What is the process of light absorption called?

Answer 15

Light harvesting

Question 16

Where in the chloroplasts does light harvesting take place?

Answer 16

In the antenna complex

Question 17

Describe the structure of an antenna complex

Answer 17

Reaction centre at the bottom, has a spherical shape and contains chlorophyll A which is a primary pigment. The antenna complex itself contains all the other pigments which allow a range of wavelengths to be absorbed.

Question 18

What is meant by photophosphorylation ?

Answer 18

The idea that energy for the phosphorylation reaction comes from light.

Question 19

What stops the light energy escaping the antenna complex after it hits it?

Answer 19

Proteins within the antenna complex.

Question 20

Outline light dependent stage of photosynthesis

Answer 20

This stage requires the solar energy from light to excite electrons at an electron acceptor in order for them to be excited to a higher level. These electrons can be accepted by an electron acceptor where they are either used for ATP synthesis or used to reduce NADP.

Question 21

What is cyclic photophosphorylation?

Answer 21

Only involves photosystem 1, high energy electrons are accepted by an electron acceptor, the electron is then donated back to the electron transport chain to help generate gradient for chemiosmosis. It then returns back to photosystem 1.

Question 22

What are the products of cyclic photophosphorylation?

Answer 22

ATP

Question 23

What is non-cyclic photophosphorylation? Outline the stages.

Answer 23

Involves both photosystems. Electrons are excited at photosystem 2 and raised to a higher energy level and accepted by electron acceptor 2. The electron is then passed down an electron transport chain to photosystem 1, this process synthesises ATP by providing energy required to maintain proton gradient. The electrons in photosystem 2 are replaced by photolysis. Electrons are then re-excited to another higher energy level at electron acceptor 1. This is where NADP can accept the electron in order to reduce into NADPH. Oxygen is lost as a waste product.

Question 24

What are the products of non-cyclic photophosphorylation?

Answer 24

• NADPH
• ATP

Question 25

What is the light independent stage of photophosphorylation known as?

Answer 25

Calvin Cycle

Question 26

What products are required from the light dependent stage in order for the light independent stage to take place?

Answer 26

- ATP - NADPH

Question 27

Outline the stages of the Calvin cycle.

Answer 27

Carbon dioxide from the atmosphere reacts with RuBP (A 5 carbon compound) to create an unstable 6 carbon compound called GP. This is catalysed by Rubisco. The 6C compound then reacts with the NADPH and ATP ( from light dependent stage) to create TP a 6 carbon compound, this is a redox reaction as hydrogen is added from NADPH (NADPH is oxidised into NADP). One carbon is released from the TP, RuBP is then regenerated using ATP. This has to occur 6 times in order to create a hexose sugar which can be converted to glucose or other useful products.

Question 28

How many times are ATP and NADPH needed in the Calvin Cycle?

Answer 28

- ATP x2 - NADPH x1

Question 29

What factors are required for photosynthesis?

Answer 29

- Efficient light - Water - Carbon dioxide - Optimum temperature

Question 30

What is a limiting factor?

Answer 30

When a required factors becomes too low and the rate of reaction decreases, the factor is known as a limiting factor.

Question 31

What happens if you increase a limiting factor?

Answer 31

The rate of reaction will increase

Question 32

How does carbon dioxide concentration affect the rate of photosynthesis?

Answer 32

The higher the carbon dioxide concentration the higher the rate of the light independent stage of photosynthesis can occur. However, only to a certain extent as Calvin cycle can only occur as much as products are available from light dependent stage. As the rate of reaction levels off carbon dioxide is not the limiting factor anymore.

Question 33

How does light intensity affect the rate of photosynthesis?

Answer 33

If the plant is in darkness then the light dependent reaction can not be carried out and therefore the Calvin cycle can't take place. If light intensity increases so does the rate of reaction as it excites the electrons to a higher energy level. If light intensity exceeds past a specific point then the pigments bleach and die so photosynthesis can no longer occur.

Question 34

What is the light compensation point?

Answer 34

When the rate of photosynthesis is equal to the rate of respiration

Question 35

What effect does temperature have on photosynthesis?

Answer 35

As the temperature increases the rate of reaction also increases due to kinetic energy increasing. However, above a particular temperature enzymes can decrease such as Rubisco in the light independent reaction and therefore rate of reaction decreases.

Question 36

What macronutrients are required by a plant?

Answer 36

- Sodium - Magnesium - Calcium - Nitrate - Phosphate

Question 37

What micronutrients are needed by plants?

Answer 37

- Manganese - Copper

Question 38

How is Nitrogen useful in a plant?

Answer 38

Nitrogen is a component of chlorophyll therefore aids the colouring and healthiness of leaves. It is also used as proteins for growth.

Question 39

What would happen if a plant had a nitrogen deficiency?

Answer 39

You would see a reduced growth of organs and chlorosis (yellowing of leaves)

Question 40

How is magnesium useful in plants?

Answer 40

Magnesium is involved in the synthesis of ATP and is a key component of chlorophyll.

Question 41

What would happen if a plant had magnesium deficiency?

Answer 41

Chlorosis would occur and the plant would become weak due to decreased ATP production.