5.2.1 - Photosynthesis (Set A - light Dependent And Independent Photosynthesis)

Question 1

Outline and explain why organisms need energy - give 3 points?

Answer 1

• active transport - essential for uptake of nitrates by root hair cells, loading sucrose into sieve tube cells - selective reabsorption of glucose and amino acids in kidneys ext.
• anabolic reactions - such as building polymers (proteins, polysaccharides or nuclei acids)
• movement - by cilia, flagella or muscle contraction

Question 2

Outline the transfer of energy regarding photosynthesis?

Answer 2

Light energy is trapped by chlorophyll molecules during photosynthesis - light energy transferred to chemical energy during formation of bonds in ATP which are broken to release energy for formation of bonds and synthesis of glucose

Question 3

Give the symbol equation for photosynthesis?

Answer 3

6CO2 + 6H20 — C6 H12 O6 + 6O2

Carbon dioxide + water - glucose + oxygen

Question 4

Explain briefly what respiration is?

Answer 4

Process by which organic molecules (e.g. glucose) are broken down into smaller inorganic molecules (carbon dioxide/water) - energy stored in the bonds used to synthesise adenosine triphosphate (ATP)

Question 5

Outline importance of bonds in biology - how do bonds differ between organic and inorganic molecules?

Answer 5

energy is used to break bonds and energy is released when bonds are formed (same quantity of energy involved whether broken or formed) - called Bond energy

• atoms in small inorganic molecules like H20 and CO2 contain strong bonds that release lots of energy - organic molecules like glucose contain many bonds which are weaker that inorganic and require less energy to break and release less when formed

Question 6

Referencing bond energy explain how it allows the synthesis of ATP during respiration?

Answer 6

Large organic molecules broken down forming small inorganic molecules (total energy required to break all bonds is less that the total energy released in formation of bonds)

• excess energy released used to synthesis ATP

Question 7

Explain why organic molecules require less energy to break than the energy released during formation during respiration?

Answer 7

Organic molecules contain large numbers of carbon-hydrogen bonds which share electrons equally in the bond between them (non-polar)

• non-polar bonds do not require lots of energy to break

Carbon and hydrogen released then form strong bonds with oxygen atoms releasing lots of energy

Question 8

Explain the process of chemiosmosis?

Answer 8

Involves diffusion of protons from region of high concentration to a region of low concentration through partially permeable membrane - movement of protons down conc gradient releases energy used to attach inorganic phosphate to ADP to form ATP (proton gradient generated by energy from excited electrons)

• ATP produced in respiration and photosynthesis synthesised mainly by chemiosmosis

Question 9

Explain 2 ways electrons are excited/raised to higher energy levels?

Answer 9

• electrons present in pigment molecule (chlorophyll) excited by absorbing light from sun
• high energy electrons released when chemical bonds are broken - eg in glucose

important for providing energy to generate a proton gradient for chemiosmosis

Question 10

Explain how an electron transport chain is established and its role in forming a proton gradient?

Answer 10

Made up of series of electron carriers each with progressively lower energy levels which causes energy to be released as electrons move from carrier to carrier

• energy released used to pump protons across membrane creating a conc difference across membrane and therefore proton gradient (impermeability of membrane to H ions maintains gradient)

Question 11

Explain the role of the enzyme ATP synthase in the synthesis of ATP?

Answer 11

Hydrophilic membrane channels linked to enzyme ATP synthase provide path for protons to move back through membrane down their conic gradient

• flow of protons through channel provide energy to synthesise ATP

Question 12

Define photosynthesis?

Answer 12

Process by which energy (in form of light) is used to build complex organic molecules, glucose through the transformation of light energy into chemical energy trapped in the bonds of complex organic molecules produced

• organisms that photosynthesise are atutrophoic

Question 13

Explain what heterotrophic organisms are?

Answer 13

Organisms like animals which obtain complex organic molecules by eating others (either other heterotrophic or autotrophic)

• both break down complex molecules during process of respiration to release energy to drive metabolic processes

Question 14

Outline the structure and function of chloroplast?

Answer 14

• network of membranes present within chloroplast provide large SA to maximise absorption of light
• membrane form flattened sacs (thylakoids) which are stacked to form grana, which are joined by membranous channels (lamellae)
• fluid enclosed in chloroplast called stroma - site of chemical reactions

contain several different photosynthetic pigments

Question 15

Outline the function of pigment within chloroplast?

Answer 15

Pigment molecules absorb specific wavelengths of light and reflect other (main pigment is chlorophyll which absorbs mainly red and blue light, reflecting green)

• primary pigment is chlorophyll a, also chlorophyll b

Question 16

Explain the role of the light harvesting system?

Answer 16

To absorb, or harvest light energy of different wavelengths and transfer this energy quickly and efficiently to the reaction centre

• chlorophyll a located in the reaction centre where reactions regarding photosynthesis take place

Question 17

Explain the types of pigments?

Answer 17

2 types - chlorophylls (a and b) and carotenoids

These pigments are attached to proteins which are called photosystems

Question 18

Explain the 2 stages of photosynthesis?

Answer 18

• light-dependent stage - energy from sunlight, absorbed and used to form ATP - hydrogen and water used to reduce coenzyme NADP to reduced NADP
• light-interdependent stage - hydrogen from reduced NAPD and carbon dioxide used to build organic molecules (glucose) ATP supplies required energy

Question 19

Outline and explain the 2 photosystems used by plants to capture light energy?

Answer 19

• Photosystem 1 (PS1) - The chlorophyll a in
  this system has a maximum absorption of light at 700nm
• Photosystem 2 (PS2) - The chlorophyll a in
  this system has a maximum absorption of light at 680nm

Question 20

Explain the coenzyme involved in photosynthesis?

Answer 20

NAPD is coenzyme - transfers hydrogen from one molecule to another, means it can reduce (give hydrogen) or oxidise (take hydrogen from) a molecule

• NADP or NADPH (reduced NADP)

Question 21

Outline 4 reasons why ATP is a good energy store?

Answer 21

• stores or releases small amounts of energy at a time - so no energy wasted as heated
• can transfer energy by transferring a phosphate group
• small and soluble - easily transported around the cell
• easily broken down - energy can be released easily and rapidly

Question 22

Outline and explain the light-dependent stage of photosynthesis - focusing on the reaction centre of PS11?

Answer 22

• involves 2 photosystems (PSII and PSI) which absorb light causing electrons at the photosystems reaction centres to excite
• excited electrons are released from reaction centre of PSII and passed to electron transport chain - ATP produced by chemiosmosis
• electrons lost at PSII reaction centre are replaced from water molecules broken down using suns energy

Question 23

Outline and explain the light-dependent stage of photosynthesis - focusing on the reaction centre of PS1?

Answer 23

• excited electrons are released from reaction centre PSI and passed to another electron transport chain (ATP produced by chemiosmosis)
• electrons released from PSII travelling along first electron chain replace electrons lost at the PSI reaction centre
• electrons travelling along transport chain from PSI are accepted along with a hydrogen ion by coenzyme NADP - forms reduced NADP (which provides the hydrogen, required for the production of glucose in light-independent stage)

Question 24

Explain how the electrons lost at the PSII reaction centre during the light-dependent stage are replaced?

Answer 24

Replaced from water molecules broken down using energy from the sun

• during photolysis

Question 25

Explain how the electrons lost at the PSI reaction centre during the light-dependent stage are replaced?

Answer 25

Electrons that have just travelled along the first electron transport chain after being released from the PSII reaction centre replace the electrons lost here

Question 26

Outline the role of the coenzyme NADP during the light-dependent stage of photosynthesis?

Answer 26

Electrons leaving the electron transport chain following PSI are accepted along with hydrogen ion by NADP

• this forms reduced NADP (NADPH)
• reduced NADP provides the hydrogen or reducing power in the production of organic molecules (glucose)

Question 27

Explain the process of photolysis?

Answer 27

Water molecules split into hydrogen ions, electrons and oxygen molecules using energy form sun

• electrons released replace the electrons lost from the reaction centre of PSII
• oxygen gas released as by-product

Question 28

Outline and explain cyclic photophosphorylation?

Answer 28

Electrons leaving the electron transport chain after PSI can be returned to the PSI instead of being used to from reduced NADP

• therefore the PSI can still lead to the production of ATP without any electrons being supplied from PSII

Question 29

Explain the key differences between cyclic and non-cyclic photophosphorylation - give 3?

Answer 29

• cyclic photophosphorylation only involve PSI where non-cyclic involves PSI and PSII
• cyclic photophosphorylation does not involve photolysis of water
• products of cyclic photophosphorylation is ATP where as non-cyclic they are ATP, oxygen and reduced NADP

Question 30

What is the light-independent stage of photosynthesis?

Answer 30

• takes place in stoma of chloroplast
• uses carbon dioxide as a raw material
• products ATP and reduced NADP also required
• glucose produced in series of reactions known as Calvin cycle

Question 31

Outline and explain the Calvin cycle?

Answer 31

• carbon dioxide diffuses into stroma where it combines with ribulose-bisphopsphate (RuBP) a 5 carbon molecule RuBisCO catalyses the reaction
• produced six carbon compound immediately breaks down forming two glycerate 3 phosphate (GP) molecules
• GP molecules reduced into triose phosphate (TP) a 3 carbon molecule - uses hydrogen atom from reduced NADP and energy from ATP (products of light dependent)
• majority of TP (5 out of 6) recycled to continue Calvin cycle - regenerate RuBP which requires ATP - rest used to make useful organic compounds (glucose)

Question 32

Explain how carbon dioxide is obtained for the Calvin cycle?

Answer 32

Co2 enters intracellular spaces within spongy mesophyll of leaves by diffusion - from atmosphere through stomata

• diffuses into cells and into stroma of chloroplast - where it combines with 5 carbon molecule RuBP (catalysed by RuBisCO)

Question 33

Explain where energy and the hydrogen ion comes from for the Calvin cycle?

Answer 33

Energy is supplied by ATP - when each GP molecule is converted into TP

• hydrogen atom comes from reduced NADP

2 x GP + ATP and H (from reduced NADP) = TP

Question 34

Explain what happens to the triose phosphate (TP) during the Calvin cycle?

Answer 34

• triose phosphate is a carbohydrate (3 carbon molecule)
• majority is recused to regenerate RuBP allowing the cycle to continue (requires ATP)
• 1/6 of TP molecules used to produce organic molecules - can condense to become hexose phosphates (6C) used to procure starch, sucrose or cellulose
• can be converted to glycerol - for formation of fatty acids which join forming lipids for cell membranes
• can be used in production of amino acids

Question 35

Outline the substances used and the substances produced during the light dependent stage of photosynthesis?

Answer 35

Substances used
- water
- inorganic phosphate
- ADP
- NADP

Substances produced
- ATP
- reduced NADP
- oxygen

Question 36

Outline the substances used and the substances produced during the light independent stage of photosynthesis?

Answer 36

Substances used
- reduced NADP
- ATP
- Carbon dioxide
- RuBP

Substances produced
- NADP
- ADP
- inorganic phosphate
- carbohydrate

Question 37

How many turns is required to make glucose form the Calvin cycle?

Answer 37

Glucose is a hose sugar - so it has 6 carbons meaning the cycle needs to turn 6 times to make glucose

• 1 carbon produced each turn (rest of the 5 TP molecules reused to reform RuBP)

Question 38

Summarise the Calvin cycle in 3 steps?

Answer 38

• fixation - carbon dioxide is fixed with RuBP
• reduction - GP is reduced to TP by addition of hydrogen from reduced NADP using energy supplied by ATP
• regeneration - RuBP is regenerated from recycled TP

Question 39

outline and explain 3 factors which affect photosynthesis?

Answer 39

• light intensity - as light intensity increases more reduced NADP and ATP produced
• temperature - affects rate of enzymes, increases rare until enzymes denature (eg carbon dioxide fixation - Calvin cycle)
• carbon dioxide concentration - needed as source of carbon, increases rate of carbon fixation in Calvin cycle and thus TP production

Question 40

Explain the effect of reducing light intensity on the Calvin cycle?

Answer 40

• will reduce rate of light-dependent, reducing ATP and reduced NADP produced needed to convert GP to TP
• concentration of GP increases, TP concentration decreases
• concentration of RuBP decreases due to less TP to regenerate it

reverse happens when light intensity increases

Question 41

Explain the effect of carbon dioxide and temperature on the Calvin cycle?

Answer 41

• low concentrations of Co2 lead to low levels of GP and TP, less carbon being fixed (conception of RuBP will increases as its still be in formed from TP)
• low temperature reduces kinetic energy of enzymes like RuBisCO which decreases number of successful collisions and reduces rate of reaction - rate increases with temp until enzyme denatures

Question 42

What is the enzyme involved in the Calvin cycle?

Answer 42

ribulose bisphosphate carboxylase (RuBisCO)

• catalyses fixation of carbon with with the 5 carbon molecule RuBP

Question 43

Explain the impact of water levels on rate of photosynthesis?

Answer 43

• stomata can close to avoid water loss during dry spells - reduces diffusion of carbon dioxide into plant (reduces light-independent reaction and eventually stops photosynthesis)
• water is required for photosynthesis, but its not a limiting factor - if water potential is low enough to limit the rate of photosynthesis, the plant will already have closed stomata and stopped photosynthesising