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A2 Biology F214 > Photosynthesis > Flashcards

Photosynthesis Flashcards

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1
Q

Define autotrophs

A

Organisms that use light energy or chemical energy and inorganic molecules (carbon dioxide and water) to synthesise complex organic molecules

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2
Q

Define heterotrophs

A

Organisms that ingest and digest complex organic molecules, releasing the chemical potential energy stored in them

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3
Q

What does photosynthesis need to work?

A

Light energy

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4
Q

Why does respiration depend on photosynthesis?

A

Plants and animals release chemical potential from complex organic molecules made in photosynthesis in respiration. They also use up oxygen in aerobic respiration, as it appeared in the atmosphere due to photosynthesis.

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5
Q

Where and how does photosynthesis occur?

A

It is a 2 stage process: light dependent reaction and light independent reaction. It occurs in chloroplasts

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6
Q

How big are chloroplasts?

A

Disc shaped, 2-10um long

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7
Q

What kind of membrane do chloroplasts have?

A

Double membrane, intermembrane space of 10-20nm. Outer membrane is permeable to many small ions; inner membrane is less permeable and has transport proteins embedded in it. It is folded into lamellae

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8
Q

What are lamellae?

A

The inner membrane of chloroplasts is folded into these: they are thin plates and have transport proteins embedded in them.

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9
Q

What are grana?

A

A stack of thylakoids or lamellae, connected by intergranal lamellae. Site of light absorption and ATPsynthesis during the light dependent reaction

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10
Q

What are the 2 regions in a chloroplast?

A

The stroma and the grana

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11
Q

What is the stroma?

A

A fluid-filled matrix. Where the light independent reaction occurs: necessary enzymes are located here. It contains starch grains and oil droplets, as well as DNA and prokaryote-type ribosomes

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12
Q

What are thylakoids?

A

Flattened membrane compartments. Can only be seen using an electron microscope. Site of the light dependent reaction.

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13
Q

What is the function of the inner membrane of a chloroplast?

A

With transport proteins, can control entry and exit of substances between cytoplasm and stroma.

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14
Q

What is the purpose of the grana?

A

Having many grana gives a large surface area for photosynthetic pigments, electron carriers and ATPsynthase enzyme (involved in the light dependent reaction)

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15
Q

Why are the photosynthetic pigments arranged how they are?

A

Arranged in photosystems to allow maximum absorption of light energy. They are held in place by proteins embedded in the grana

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16
Q

What is the function of the stroma?

A

Contains the enzymes to catalyse the light independent reaction. Surrounds the grana so can easily receive products of light dependent reaction required for light independent reaction.

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17
Q

Why do chloroplasts contain their own DNA?

A

They can make some of the required proteins for photosynthesis with ribosomes.

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18
Q

Define photosynthetic pigments

A

Molecules that absorb light energy. Each pigment absorbs a range of wavelengths in the visible region and has its own distinct peak of absorption. Other wavelengths are reflected.

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19
Q

What is the importance of photosynthetic pigments?

A

When light of the wavelength that a pigment absorbs hits a pigment, it excites the electrons inside, causing them to be lost (chlorophyll is oxidised). These electrons then go down the electron transfer chain, from which the energy of the electrons is used to create a hydrogen ion gradient.

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20
Q

What is the structure of chlorophyll?

A

It is a mixture of pigments. All have a long phytol (hydrocarbon) chain and a porphyrin group. The porphyrin group contains a magnesium atom, which is the atom from which electrons are lost when chlorophyll is oxidised.

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21
Q

What are the 2 forms of chlorophyll a? How are they different?

A

P(680), found in photosystem 2 and P(700) found in photosystem 1. They absorb light at slightly different wavelength (680 and 700nm - red light). They both appear yellow-green.

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22
Q

Where is chlorophyll a found?

A

At the primary pigment reaction centre. At the centre of photosystems

23
Q

What are the 2 forms of chlorophyll?

A

Chlorophyll a and chlorophyll b. a absorbs blue light of 450nm and red light. b absorbs light at 500 and 640nm. Appears blue-green.

24
Q

What are carotenoids?

A

Photosynthetic pigments that reflect yellow and orange light and absorb blue light, absorbing blue light. Pass energy on to chlorophyll a at primary pigment reaction centre. Main ones are carotene and xanthophyll

25
Q

Define photolysis

A

The splitting of water in the presence of light. 2H(2)O -> 4H+ + 4e- + O(2)

26
Q

What is the role of water in photosynthesis?

A

It is split by photolysis.
A source of hydrogen ions for use in chemiosmosis to produce ATP. Protons reduce NADP+ to NADPH (for light independent reaction
A source of electrons to replace those lost in chlorophyll
Oxygen is released for use in respiration/to atmosphere

27
Q

Define photophosphorylation

A

The making of ATP from ADP and Pi in the presence of light

28
Q

What happens when light of the correct wavelength strikes chlorophyll?

A

When a photon hits chlorophyll, the energy is transferred to 2 electrons, exciting them. The chlorophyll loses these electrons (to be replaced by some from water), in oxidation. The electrons are captured by electron acceptors and passed along a series of electron carriers in they thylakoid membrane

29
Q

What are electron carriers?

A

Molecules that transfer electrons, containing iron atoms

30
Q

What are electron acceptors?

A

Chemicals that accept electrons from another compound. They are reduced while acting as oxidising agents.

31
Q

What happens as electrons are passed along the chain of electron carriers?

A

Energy is released. The energy is used to pump protons across the thylakoid membrane, into the thylakoid space. A proton gradient is formed across the membrane

32
Q

Why do plants create a proton gradient in photosynthesis?

A

The protons flow down their gradient, from the thylakoid space to the stroma, across the thylakoid membrane through channels associated with ATP synthase enzymes. The force produced is used to make ATP from ADP and Pi.

33
Q

Define chemiosmosis

A

The flow of hydrogen ions (protons) through ATP synthase enzymes. The force of this flow allows the production of ATP. IT occurs across the thylakoid membranes in the light dependent stage of photosynthesis and across the inner mitochondrial membrane during oxidative phosphorylation

34
Q

What is cyclic photophosphorylation?

A

Only uses photosystem 1 (so P(700) chlorophyll a). Excited electrons are passed to an electron acceptor, then back to the chlorophyll molecule they were lost from. Water is not required, and NADPH isn’t produced. ATP is made though, which can be used in the light independent stage or in guard cells.

35
Q

What is the sequence of 7 steps that occurs in non-cyclic photophosphorylation?

A

1: light strikes photosystem 2, exciting a pair of electrons that then leave the chlorophyll molecule
2: electrons pass along a chain of electron carriers; energy released is used to synthesise ATP
3: Light has also struck photosystem 1, and a pair of electrons has been lost
4: these electrons along with protons (from photolysis of water at photosystem 2) make NADPH
5: The electrons from photosystem 2 replace those at photosystem 1
6: water is split in photolysis at photosystem 2, producing oxygen, electrons to replace those lost at photosystem 2 and protons
7: protons from photolysis of water take part in chemiosmosis and in reducing NADP+ in the stroma.

36
Q

What does the light independent stage need to occur?

A

NADPH, ATP, CO(2) and a bunch of enzymes!

37
Q

How is carbon dioxide fixed in the Calvin cycle?

A

Combines with a 5-carbon compound: ribulose bisphosphate, catalysed by ribulose bisphosphate carboxylase-oxygenase, or rubisco, making a 6-carbon compound that quickly splits into a 3 carbon compound: glycerate 3-phosphate.

38
Q

How does carbon dioxide get to the stroma?

A

Carbon dioxide diffuses into the leaf via open stomata, into air pockets in spongy mesophyll to palisade mesophyll. Diffuses through cell wall, plasma membrane, cytoplasm and chloroplast envelope into stroma.

39
Q

What is rubisco short for?

A

Ribulose bisphosphate carboxylase-oxygenase: the enzyme that catalyses the fixing of carbon dioxide.

40
Q

What is RuBP short for?

A

Ribulose bisphosphate (note the s). The 5-carbon compound that carbon dioxide is fixed to

41
Q

What is GP short for?

A

Glycerate 3-phosphate. The molecule that results from the fixing of carbon dioxide to RuBP

42
Q

What happens to the GP in the Calvin cycle?

A

It is reduced and phosphorylated to TP. ATP and NADPH are used in this process. ATP is required for the energy; NADPH as a reducing agent. Some is used to make amino acids and fatty acids

43
Q

What happens to TP in the Calvin cycle?

A

5 out of every 6 molecules are converted back to RuBP, requiring energy and inorganic phosphate, gained from ATP. Some TP is used to make other stuff, such as hexose sugars, glycerol etc.

44
Q

What is TP short for?

A

Triose phosphate: the first carbohydrate in the Calvin cycle

45
Q

What are the complex organic products of the light independent reaction?

A

GP is used to make amino acids and fatty acids
TP can make hexose sugars such as glucose. From these, plants can make fructose (hexose sugar) and sucrose (disaccharide). Hexose sugars could also become starch/cellulose
TP can be made into glycerol, which can combine with fatty acids from GP to make lipids.

46
Q

What are the main limiting factors of photosynthesis?

A

Carbon dioxide concentration, light intensity and temperature

47
Q

What is the effect of carbon dioxide concentration on photosynthesis?

A

Carbon dioxide concentration is approximately 0.04% in the atmosphere. This slows down the rate of photosynthesis, as it reduces the rate of the Calvin cycle: the less CO(2) entering the cycle, the less RuBP is regenerated, so the lower the rate. This reduces the rate of growth, as less glucose and other good stuff is made.

48
Q

What is the effect of light intensity on photosynthesis?

A

Light causes stomata to open so CO(2) can enter leaves; is trapped by chlorophyll to excite electrons and splits water molecules into protons. If this doesn’t happen/happens slowly, no/little ATP and NADPH is formed for light independent stage, so not much products formed.

49
Q

What is the effect of temperature on photosynthesis?

A

The photochemical reactions of the light dependent stage aren’t really effected by the rate of photosynthesis. But the light independent stage has enzyme controlled reactions. Higher temperature means more kinetic energy meaning more collisions meaning more enzyme substrate complexes formed meaning more product. Above certain temperature, enzymes denature.

50
Q

What is a photosynthometer?

A

A piece of apparatus that is used to measure the rate of photosynthesis by measuring the volume of oxygen produced in a given time. A water plant is placed leaves down in a boiling tube of sodium hydrogen carbonate solution in a water bath. A capillary tube is placed over a stem cut underwater, which leads to a scale and a syringe to move air bubbles around.

51
Q

How can you convert distance from a light source to light intensity?

A

Light intensity is proportional to 1/(distance)^2

52
Q

What happens to the levels of GP, TP and RuBP when light intensity falls and why?

A

GP levels rise, as there is no ATP or NADPH to convert it to TP, so the cycle gets stuck here.
TP levels fall, as there is no ATP or NADPH to convert GP into TP
RuBP levels fall, as there is no TP being made which is required for production of RuBP. This reduced carbon dioxide fixation, so eventually GP level rise will flatten

53
Q

What happens to the levels of GP, TP and RuBP when carbon dioxide concentration falls and why?

A

GP levels fall, as GP is made from fixing carbon dioxide with RuBP, and there is no carbon dioxide to be fixed.
TP levels fall, as GP is needed to make TP, and there is no GP
RuBP levels rise, as RuBP is used up in carbon dioxide fixation which isn’t occurring.

A2 Biology F214 (4 decks)

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