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BIOL 186 - Plants > Photosynthesis - Light Reactions (Lecture 4) > Flashcards

Photosynthesis - Light Reactions (Lecture 4) Flashcards

1
Q

Why study photosynthesis?

A

-It is the energetic basis of life in the biosphere, driven by solar energy
-It has shaped our planet’s atmosphere and biosphere
-It is a key driver in the global carbon cycle, as carbon sinks
(in fact, it removes carbon dioxide, which could offset emissions and alleviate issues)
-It is a potential source of sustainable energy and biofuels (solution to global warming?)

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2
Q

What is the net reaction of photosynthesis?

A

6 CO2 + 12 H2O + Light –> C6H12O6 + 7 H2O + 6 O2

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3
Q

Where do the light-dependent reactions of photosynthesis take place?

A

In thylakoids, the lumen and the stroma of chloroplasts.

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4
Q

What are chloroplasts, and where are they found?

A

Chloroplasts are chemical factories powered by solar energy. They are found mainly in mesophyll cells, being the tissue in the interior of leaves.

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5
Q

What are the three components of chloroplasts?

A

Thylakoids, lumen and stroma

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6
Q

Define: Thylakoids

A

Stacked (grana) membrane-bound vesicles, suspended within the stroma. They are the sacs segregating the stroma from their insides (lumen).

Thylakoids capture light and are where the light reactions of photosynthesis occur.

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7
Q

Define: Lumen

A

The inside of the thylakoids, being the fluid-filled space inside the sacs.
Lumen is involved in the proton gradient aspect of photosynthesis.

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8
Q

Define: Stroma

A

The ‘juice’, a dense fluid, inside the chloroplasts and surrounding the thylakoids.
Stroma is involved in the Calvin cycle, being the dark reaction piece of photosynthesis.

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9
Q

What are the two stages of photosynthesis?

A
  1. LIGHT-DEPENDENT REACTIONS: Light energy is captured in the thylakoid membranes, and used to generate ATP and NADPH.]
  2. LIGHT-INDEPENDENT REACTIONS (dark reactions): Using ATP and NADPH, sugars are synthesized.
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10
Q

What are ATP and NADPH?

A

ATP: Energy currency of cells
NADPH: Source of electrons, acting as reducing power. It passes electrons to an electron acceptor.

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11
Q

What organic molecule do the dark reactions incorporate into photosynthesis?

A

Carbon fixation – Carbon dioxide

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12
Q

What are the three key elements that a plant needs in order to photosynthesize?

A
  • Membranes and compartments: Thylakoids
  • Molecules that can absorb lights: Pigments such as chlorophyll carotenoids
  • Proteins within the photosystems, for electron transport and biosynthesis
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13
Q

What is the chlorophyll pigment?

A

Chlorophyll absorbs violet-blue and red wavelengths while transmitting and reflecting green.

Chlorophyll a is the key light-capturing pigment, participating directly in light reactions.

Chlorophyll b is an accessory pigment, working in conjunction with chlorophyll a.

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14
Q

What is the role of carotenoids?

A

They broaden the spectrum that drives photosynthesis.

In other words, they are conjugated lipids to help absorb additional wavelengths.

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15
Q

How is the chlorophyll pigment structured?

A

It is composed of a porphyrin ring, which absorbs light, and a hydrocarbon (lipophilic) tail.

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16
Q

What are the two types of chlorophyll in land plants?

A

Chlorophyll a & b.

17
Q

How does chlorophyll work?

A

It absorbs specific wavelengths of light, then transfers the energy.

When a photon hits the chlorophyll molecule, the electron goes into its excited state. As the electron falls back down to its ground state, it emits heat and a resulting photon.

18
Q

What is the absorption spectrum?

A

It shows which wavelengths are absorbed by a pigment. The set of wavelengths that the pigment does not absorb are reflected, and that reflected light is the colour we see.

19
Q

What is the action spectrum?

A

It shows which wavelengths are used for photosynthesis, aka where the pigment is active.

20
Q

Only a _________ part of the light and radiation spectrum is used.

A

Small

21
Q

How are photosynthetic pigments organized?

A

They are organized into photosystems (PSII, PSI), which contain the pigments & other proteins, among other molecules.

22
Q

What is the role of proteins in the photosystem?

A

The proteins hold chlorophyll in place and organize reactions.

23
Q

What are the two primary ‘centres’ within a photosystem?

A
  1. REACTION-CENTRE COMPLEX:Organized association of proteins holding a special pair of chlorophyll A molecules - the primary electron acceptor
  2. LIGHT-HARVESTING COMPLEX: Pigment molecules bound to proteins (range)
24
Q

What are the specific special chlorophylls, in PS II and PS I?

A

In PS II: RCC = P680 (best absorbs a wavelength of 680 nm)

In PS I: RCC = P700 (best absorbs a wavelength of 700 nm)

25
Q

How is the energy captured within photosystems?

A

The energy absorbed by the chlorophyll can be captured

26
Q

Where are the photosystems located?

A

In the phospholipid bilayer within the thylakoid membrane, nested between the stroma and the lumen.

27
Q

What is the linear electron flow (non-cyclic photophosphorylation)?

A

Electrons are removed from water and passed through PSII and PSI before ending up in NADPH.

This process requires light to be absorbed twice, once in each photosystem, and it makes ATP.

28
Q

What are the basic steps of linear electron flow?

A
  1. LIGHT ABSORPTION IN PSII
    - Light absorbed by pigments in PSII
    - Energy passed from pigment to pigment, until it reaches the reaction center
    - At the reaction center, energy is transferred to P680, boosting an electron to a high energy level
    - The high-energy electron is passed to an acceptor molecule, replaced with an electron from water
    - Water splits, releasing the oxygen we breathe
  2. ATP SYNTHESIS
    - The high-energy electron travels down an electron transport chain, losing energy as it goes
    - Some of the released energy drives the pumping of hydrogen ions from the stroma into the thylakoid interior (lumen), building a gradient.
    - As the ions flow down their gradient, into the thylakoid lumen, they pass through ATP synthase, driving ATP production.
  3. LIGHT ABSORPTION IN PSI
    -The electron arrives at PSI, and joins the P700 special pair of chlorophyll in the reaction center
    -P700 boosts the electron to a very high energy level and transfers the electron to an acceptor molecule
    The special pair’s missing electron is replaced by a new electron from PSII, arriving via the electron transport chain.
  4. NADPH FORMATION
    - The high-energy electron travels down a short second leg of the electron transport chain
    - At the end of the chain, the electron is passed to NADP+ to make NADPH.
29
Q

What is the net effect of light-dependent reactions?

A

They convert light energy into chemical energy, in the form of ATP and NADPH.

30
Q

Why do we need to use two photosystems?

A

One photosystem gives insufficient energy!

31
Q

How are the electron transfers made possible?

A

The absorption of energy from light, by which the electrons in P680 and P700 can then be boosted to a very high energy level and travel downhill, releasing energy spontaneously. `

32
Q

What are the key proteins involved in the linear electron flow?

A

Cytochrome complex; ferredoxin; NADP+ reductase

33
Q

What is the role of ATP synthase in capturing chemical energy?

A

Thanks to the gradient it creates, with the pH of 5 in the lumen, the pH of 8 in the stroma.
There is a 1000x gradient!

The movement of protons OUT provides energy for the ATP synthase, similar to mitochondria.

Basically, as the hydrogen ions are pumped into the lumen, which has a high H+ concentration, they then pass through the other ‘side’ of the thylakoid membrane through the ATP synthase, diffusing out, and thus generating ATP.

34
Q

Chloroplasts convert _________ energy into _________ energy.

A

Light; chemical

35
Q

What is the cyclical electron flow of photosynthesis?

A

It is a ‘short circuit’ that produces ATP and a proton gradient, but not NADPH, by which PSI transports electrons in a cyclical fashion.

36
Q

What are NADPH and ATP used for?

A

Chemical synthesis in the dark reactions of photosynthesis!

37
Q

What is the immediate source of energy used in ATP synthesis?

A

The ATP synthase has a key role in capturing chemical energy. By accumulating H+ in the lumen (thylakoid), there is a strong gradient between that and the stroma.

38
Q

Why is oxygen released during the light-dependent reactions in photosynthesis?

A

Water splits at PSII, releasing an electron transport chain; as protons move into the lumen of the thylakoid, oxygen is a by-product.

This allows for the generation of ATP.

BIOL 186 - Plants (10 decks)

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