Where Takes Place in the Chloroplast. Mostly in Leaf Cells, Mostly in the Mesophyll (Middle

Photosynthesis

Where – Takes place in the chloroplast. Mostly in leaf cells, mostly in the mesophyll (middle layer of leaf)

Overall reaction:

6CO2 + 6H2O àC6H12O6 + 6O2

Carbon dioxide comes in and Oxygen goes out through the stoma. (stomata = plural)

Water is absorbed from the roots

Page 178 – great diagram!!

Let’s take a closer look at chloroplasts

·  A typical mesophyll cell has 30-40 chloroplasts Each chloroplast has two membranes around a central space, called the stroma.

·  In the stroma are membranous sacs, called thylakoids.

·  These have an internal space filled with water called the thylakoid lumen or thylakoid space.

·  Thylakoids may be stacked into columns called grana.

page 178 fig 10.2

An Overview of Photosynthesis

Photosynthesis is has 2 parts

·  The light reactions convert solar energy to chemical energy.

·  The Calvin cycle takes CO2 from the atmosphere incorporates it into an organic molecule and uses energy from the light reaction to reduce the new carbon molecule into glucose.

Light Reaction

Where – thylakoid membranes

What – Takes sunlight and uses it to split water.

What results –

The electrons from water are given to an electron acceptor called NADP to make NADPH.

Also, an ATP molecule is made. This is called photophosphorylation (wow!)

The oxygen (from the water) is given off.

What next – The Calvin cycle uses the NADPH and ATP.

NADP?? Are you trying to confuse me

Remember NAD and NADH from cellular respiration…well, NADP is close enough to confuse you.

NADP is very much like NAD, it just has an extra phosphate group

Functions like NAD as well in that it is an electron acceptor. NADP becomes NADPH as it is reduced.

(Just remember that the one with the P is in photosynthesis. )

Chlorophyll a, the dominant pigment, absorbs best in the red and blue wavelengths, and least in the green.

Other pigments
with different
structures have
different
absorption
spectra.

Chlorophyll a, chlorophyll b and others

Only chlorophyll a participates directly in the light reactions but accessory photosynthetic pigments absorb light and transfer energy to chlorophyll a.

Chlorophyll b, with a slightly different structure than chlorophyll a, has a slightly different absorption spectrum and funnels the energy from these wavelengths to chlorophyll a.

Carotenoids can funnel the energy from other wavelengths to chlorophyll a and also participate in photoprotection against excessive light.

Absorption of light

When a molecule absorbs a photon, one of that molecule’s electrons is elevated to an orbital with more potential energy.

The electron moves from its ground state to an excited state.

It will fall back down to it’s ground state very quickly (one billionth of a second).

Absorption of light (cont.)

When it falls to ground state it will give off heat and sometimes light.

In chlorophyll it is prevented from falling down to ground state by having an electron acceptor available to take the electron.

Isolated Chlorophyll

An isolated chlorophyll molecule ( not a part of a chloroplast) when hit with a photon will experience the electron moving to an excited state and then as the electron moves back to ground state, the chlorophyll will glow red and release heat

So instead of isolated…

Chlorophyll is a contained within a photosystem found on the membrane of the thylakoid.

There are 2 kinds of photosystems (photosystem II and photosystem I)

Photosystems

There are two photosystems. See top of page 184 and fig. 10.11 on page 185 to see a photosystem.

Here’s what they are – a complex of proteins that includes a few hundred pigment molecules. One of the pigment molecules is located in the reaction center along with a primary electron acceptor

Here’s how they work – one pigment molecule absorbs a photon. Excited electron is passed from pigment molecule to pigment molecule, until it reaches the reaction center.

At that point to electron is passed to a primary electron acceptor (in the reaction center.)

One more thing about photosystems

Keep this in mind:

Because the electron is captured by the primary acceptor, it leaves an electron “hole” in the pigment molecule in the reaction center that must be filled

Let’s walk through the light reaction occurs:

Use diagram 10.12 on page 186. There are numbers that correspond to the diagram.

Step 1: Photosystem II absorbs light, it bounces until it reaches the reaction center and is passed to primary acceptor.

Step 2: The “hole” left by the electron leaving must be filled. Water is split to use the electrons from hydrogen. The oxygen is given off as a waste product.

Step 3: the electron passes from the primary electron acceptor at photosystem II to photosystem I via an electron transport chain.

Step 4: As the electron falls down the ETC, the thylakoid membrane is used to synthesize ATP (remember, pumping H ions out and letting them back through an ATP synthase molecule)

Step 5: Meanwhile Photosystem I has also absorbed a photon and passed an excited electron around until it reaches the pigment molecule at the reaction center. It is passed to a primary electron acceptor and the electron “hole” in the pigment molecule is filled by the electron from the ETC. (from photosystem II)

Step 6: The primary electron acceptor from photosystem I passes the electrons to a second ETC. The ultimate electron acceptor is NADP and it becomes NADPH.

More notes on light reaction

Steps 1 – 6 : This is called noncyclic electron flow because it requires light and water to start it each time. The ATP made is called noncyclic photophosphorylation

When the cell has too many NADPH, it will switch to cyclic electron flow which just uses photosystem I and only generates ATP. (Makes no NADPH, no Oxygen and uses no water). It still requires light. . This is called cyclic photophosphorylation

Noncyclic Cyclic

One last look at the light reaction

·  Requires light

·  Makes oxygen

·  Uses water

·  Make ATP from ADP and inorganic P

·  Makes NADPH from NADP

·  Takes place in the thylakoid membrane

Calvin Cycle

Where – in the stroma

What – Like the Krebs cycle; has 3 major parts: carbon fixation, reductions and regeneration of CO2 acceptor.

Input– 3 molecules of CO2, 6NADPH, & 9ATP Output – 1 G3P (3-carbon sugar)

G3P is later combined with another G3P to make glucose…but that’s not part of the Calvin cycle

Phase 1: Carbon Fixation

3 molecules of carbon dioxide are attached to RuBP. It uses rubisco as the enzyme for this reaction.

Phase 2: Reduction

Phosphate from ATP is added to the molecule

NADPH reduces the resulting molecule to make G3P

Every 3 molecules of carbon dioxide with the RuBP will make 6 G3P.

5 of them must be recycled to make RuBP. The other one leaves Calvin cycle as the product.

Phase 3: Regeneration of the CO2 acceptor

The 5 G3P that are not given off are reorganized using ATP to result in 3 RuBP.

Count the Carbons!!

Start with 15

(three 5-carbon molecules RuBP)

Add 3 from atmosphere (3 CO2)

After Reductions you have 18 (six 3-carbon molecules) (6 G3P)

3 (one 3-carbon) goes to make ½ glucose and

15 (five 3-carbon) get reshuffled

make three 5-carbon (15)

Adaptations for hot, dry climates

Alternative mechanisms of carbon fixation

These alternatives are metabolic adaptations and have evolved in hot, arid climates.

The problem: CO2 comes in through the stoma

Stomata must close during the hot, dry part of the day to prevent water loss from a plant.

When the stomata close, the plant not only can’t get the CO2 that it needs for photosynthesis but the oxygen builds up.

Photorespiration

Normally, the initial fixation of carbon occurs via rubisco, the Calvin cycle enzyme that adds CO2 to RuBP.

This occurs in most plants, called C3 plants because they first organic product of carbon fixation is a three-carbon compound.

However, rubisco will bind to O2 just as readily as CO2. The product is waste (actually gets broken down by peroxisomes

Neither oxygen, carbohydrates nor ATP and NADPH are produced.

This is called photorespiration.

Preventing Photorespiration

Two methods: C4 plants and CAM plants

C4 plants (location)

mesophyll cells are next to a group of cells called bundle-sheath cells clustered around the vein.

In the mesophyll cells, CO2 is incorporated into a 4 carbon molecule (organic acid).

The resulting organic acid moves into the bundle-sheath cell and releases the CO2 .

In the bundle-sheath cells CO2 builds up and is in high concentration and is then fixed by rubisco

CAM plants (time)

CAM stands for Crassulacean acid metabolism (nothing you need to memorize! J

The stoma are only opened at night. CO2 is picked up at that time and stored in the form of an organic acid.

Later (during the day) the CO2 is then incorporated into the Calvin cycle.

Adaptations for hot, dry climates

Physics of light
A little “light” review (get it?)

Light, like other form of electromagnetic energy, travels in rhythmic waves.

The distance between crests of electromagnetic waves is called the wavelength.

Wavelengths of electromagnetic radiation range from less than a nanometer (gamma rays) to over a kilometer (radio waves).

The entire range of electromagnetic radiation is the electromagnetic spectrum.

The most important segment for life is a narrow band between 380 to 750 nm, visible light.

While light travels as a wave, many of its properties are those of a discrete particle, the photon.

Photons are not tangible objects, but they do have fixed quantities of energy.

The amount of energy packaged in a photon is inversely related to its wavelength.

Photons with shorter wavelengths pack more energy.

While the sun radiates a full electromagnetic spectrum, the atmosphere selectively screens out most wavelengths, permitting only visible light to pass in significant quantities.

When light meets matter, it may be reflected, transmitted, or absorbed.

Different pigments absorb photons of different wavelengths.

A leaf looks green
because chlorophyll,
the dominant pigment,
absorbs red and blue
light, while transmitting
and reflecting green light.

A spectrophotometer measures the ability of a pigment to absorb various wavelengths of light.

It beams narrow wavelengths of light through a solution containing a pigment and measures the fraction of light
transmitted at
each wavelength.

An absorption
spectrum plots a
pigment’s light
absorption versus
wavelength.

The light reaction can perform work with those wavelengths of light that are absorbed.

In the thylakoid are several pigments that differ in their absorption spectrum.