5.2.1 - Photosynthesis

Question 1

What is photosynthesis?

Answer 1

The synthesis of complex organic molecules using light

General equation:
Carbon dioxide + water -> glucose + oxygen
6CO2 + 6H20 -> C6H12O6 + 6O2

Question 2

What are the features of a chloroplast?

Answer 2

• thylakoid
• granum
• lamella
• ribosome
• chloroplast DNA
• starch granule
• drop of lipids
• inner membrane
• outer membrane
• intermembrane space

• stroma

Question 3

What are thylakoids?

Answer 3

• membranes surrounding lumens
• provides large SA for attachments of photosynthetic pigments + enzymes (including ATP synthase enzymes)
• pigments held in place to form photosystems

Question 4

What is a granum?

Answer 4

• pl grana
• stack of thylakoids
• joined by membranous channels called lamellae

Question 5

What is the stroma?

Answer 5

• site of many chemical reactions that result in the synthesis of complex organic molecules eg glucose, lipids
• contains other structures (eg ribosomes, starch granules)

Question 6

What are starch granules and drops of lipids?

Answer 6

• starch grains = storage of glucose in an insoluble form

* drops of lipids = storage of lipid molecules for membrane formation

Question 7

What are the stages of photosynthesis?

Answer 7

1) light dependent (takes place within/across thylakoids)

2) light independent (takes place in the stroma)

Question 8

What are photosynthetic pigments?

Answer 8

• molecules that absorb specific wavelengths of light

* absorb light energy needed for photosynthesis

Question 9

What are primary and accessory pigments?

Answer 9

PRIMARY:
• chlorophyll a
• absorbs mainly blue and green light

ACCESSORY:
• carotenes, xanthophylls and chlorophyll b
• absorbs some light wavelengths not absorbed by chlorophyll a

Question 10

What are photosystems?

Answer 10

• funnel shaped protein and pigment complexes responsible for the absorption of light and electron transfers in photosynthesis
• made of light harvesting systems (antenna complexes) and a reaction centre (chlorophyll a)

Question 11

What is the antenna complex?

Answer 11

• aka light harvesting system
• formed from accessory pigments
• absorbs light energy of various wavelengths and transfers it the the reaction centre

Question 12

How is light harvested in chloroplast membranes?

Answer 12

• light energy absorbed by accessory pigments excites electrons within them.
• excited electrons have more energy
• excited electrons become delocalised and then return to the pigment, releasing energy and passing it on to the next pigment
• energy is passed from one pigment molecule to the next in the photosystem
• energy reaches chlorophyll a (primary pigment)
• excited electrons with enough energy can only be ejected by pigment molecules in reaction centres (chlorophyll a), not by accessory pigment molecules
• ejected electrons are transferred to acceptor molecules

Question 13

How are chloroplasts adapted for photosynthesis?

Answer 13

STROMA:
•gel-like fluid contains enzymes that catalyse the reactions of the light-dependant stage
•surrounds grana and membranes, making transport of products from the light dependant stage into stroma quick (for light independent stage)

GRANA:
• stacks create large surface area, increasing space for photosystems and maximising absorption of light
• provides more membrane space for electron carriers and ATP synthase enzymes

DNA:
• chloroplast DNA (cpDNA) contains genes thwart code for some of the proteins and enzymes used in photosynthesis

RIBOSOMES:
• allow for the translation of proteins coded for by cpDNA

INNER MEMBRANE:
• selective transport proteins in inner membrane of chloroplast envelope controls flow of molecules between stroma and cytosol

Question 14

What are the different photosystems?

Answer 14

Photosystem 1 (PSI):
• aka P700
• chlorophyll a has maximum absorption of light with wavelength 700nm

Photosystem 2 (PSII)
• aka P680
• chlorophyll a has maximum absorption of light with wavelength 680nm

Question 15

What happens during the light-dependent stage of photosynthesis?

Answer 15

• light energy is used to break down water (photolysis) into H+, e- and O2 in the thylakoid lumen
• There is a high conc of H+ ions in the thylakoids lumen, resulting in concentration gradient
• electrons travel through an electron transport chain of proteins within the membrane
• reduced NADP (NADPH) is formed when H+ in stroma and electrons from electron transport chain combine with the carrier molecule NADP
• ATP is produced during photophosphorylation

Question 16

What is photophosphorylation?

Answer 16

The overall process of using light energy and the electron transport chain to phosphorylate ADP to ATP

Question 17

Describe the process of non-cyclic photophosphorylation

Answer 17

1) a photon of light is absorbed by a pigment in PSII and the energy passes through the light harvesting centre to the reaction centre. Energy is transferred to the primary pigment (P680), releasing an electron from the reaction centre of PSII.
2) The high-energy electron is passed to an acceptor molecule. It is replaced with an electron from photolysis (splitting of water into H+, e- and oxygen)

3) The high-energy electron travels down an electron transport chain, losing energy as it goes.
Some of the released energy drives pumping of H+ from the stroma into the thylakoid interior, building a gradient (H+ from photolysis also add to this)

4) As H+ flow down the concentration gradient via facilitated diffusion, they pass through ATP synthase, driving ATP production (chemiosmosis)

5) Electron arrives at PSI and joins the P700 special pair of chlorophylls in the reaction centre.
When light energy is absorbed by pigments and passed inward to the reaction centre, the electron in P700 is boosted to a very high energy level and transferred to an acceptor molecule.

6) High energy electron travels down an electron transport chain. At the end, the electron is passed to NADP+ (along with a second electron from the same pathway and 2H+ from photolysis) to make NADPH.