Photosynthesis

Photosynthesis in Overview• Process by which plants and other autotrophs

store the energy of sunlight into sugars.• Requires sunlight, water, and carbon dioxide.• Overall equation:

6 CO2 + 6 H20 C6H12O6 + 6 O2

• Occurs in the leaves of plants in organelles called chloroplasts.

Leaf Structure• Most photosynthesis occurs in the palisade layer.• Gas exchange of CO2 and O2 occurs at openings

called stomata surrounded by guard cells on the lower leaf surface.

Palisade

Spongy

Chloroplast Structure• Inner membrane

called the thylakoid membrane.

• Thickened regions called thylakoids. A stack of thylakoids is called a granum. (Plural – grana)

• Stroma is a liquid surrounding the thylakoids.

Pigments• Chlorophyll A is the most important

photosynthetic pigment. • Other pigments called antenna or accessory

pigments are also present in the leaf.– Chlorophyll B– Carotenoids (orange / red)– Xanthophylls (yellow / brown)

• These pigments are embedded in the membranes of the chloroplast in groups called photosystems.

Pigment Absorption

This is an ocean plant. Seaweed

Photosynthesis: The Chemical Process

• Occurs in two main phases.– Light reactions– Dark reactions (aka – the Calvin Cycle)

• Light reactions are the “photo” part of photosynthesis. Light is absorbed by pigments.

• Dark reactions are the “synthesis” part of photosynthesis. Trapped energy from the sun is converted to the chemical energy of sugars.

BIG PICTURE

Light Reactions

• Light-dependent reactions occur on the thylakoid membranes.– Light and water are required for this

process.– Energy storage molecules are formed. (ATP

and NADPH)– Oxygen gas is made as a waste product.

Dark Reactions

• Dark reactions (light-independent) occur in the stroma.– Carbon dioxide is “fixed” into the sugar

glucose.– 18ATP and 12NADPH molecules created

during the light reactions power the production of this glucose.

The Minor Details :P

Photosystem I & II

Non-cyclic Electron Flow

• Uses photosystem I (P700) and Photosystem II (P680)

• Electrons from water replace e- lost from PSII

• Electrons lost from chlorophyll in PSII replace e- lost in PSI

• ATP and NADPH are made

Cyclic Electron Flow

• Uses PSI• Produces ATP but not NADPH• Electron is passed back to the chlorophyll

that it left when excited

No NADPH Made

Only ATP Made

Cyclic Electron Flow

Light Independent ReactionsAKA – Dark Reactions (not really –

just don’t need light)Also called Calvin Cycle or C-3

Pathway• 1. Occurs in the stroma of the chloroplast• 2. Uses ATP and NADPH from the Light Reactions• 3. Accepts carbon from carbon dioxide – this is

called carbon fixation to make sugar. It reduces CO2.

• 4. Produces Glyceraldhyde 3- Phosphate which is a

3-C sugar and is used to make glucose

Details

• 1. Carbon dioxide enters the Calving cycle and become attached to a 5-C sugar – ribulose biphosphate (RuBP). This reaction is catalyzed by Rubisco enzyme.

• 2. 6-C intermediate breaks apart into two 3-C molecules of 3-phosphoglycerate (3-PGA)

• 3. Converted to phosphoglyceraldehyde (PGAL) which is used to make glucose. Uses 18 ATP and 12 NADPH.

• 4. RuBP is regenerated using ATP from Light reactions.

At low CO2 concentration, rate is positively correlated with concentration

CO2 is a substrate in an enyme-catalysed light-dependent reaction.

At Low Light Intensity Rate is Proportional to Light Intensity

plateau

Increased temp. gives increased energy and increased rate of photosynthesis

Optimum temperature

Above the optimum temp., enzymes are denatured and rate drops steeply.

Which enzymes are used in respiration?

C4 Photosynthesis

AP BiologyUnit 4

Review: C3 Photosynthesis

• During “regular” photosynthesis, CO2 is trapped into a 3-carbon compound by Rubisco C3 Photosynthesis

• This 3 carbon compound then goes through the calvin cycle to produce glucose (eventually)

C4 Photosynthesis

• Certain plants go through a slightly modified photosynthesis process (C4 Photosynthesis)

• C4 Photosynthesis is an adaptation that evolved due to the environment these plants are in.

Rubisco

• ALL plants have it and use it for the Calvin Cycle.

• Rubisco usually reacts with CO2, but it can also react with O2 – 2 competing reactions.

Competing Reactions

• Rubisco can react with CO2 (Carboxylase Reaction) – good for glucose output

CH2OPO32-

C O

C OHH

C OHH

CH2OPO32-

Ribulose 1,5-bisphosphate

CO2

CO O-

C OHH

CH2OPO32-

3-Phosphoglycerate

CO O-

C OHH

CH2OPO32-

3-Phosphoglycerate

Competing Reactions• Rubisco can also react with O2 (Oxygenase

Reaction) – not good for glucose output– Even though CO2 is eventually regenerated, it

wastes time and energy (occupies Rubisco)CH2OPO3

2-

C O

C OHH

C OHH

CH2OPO32-

Ribulose 1,5-bisphosphate

O2

CO O-

C OHH

CH2OPO32-

3-Phosphoglycerate

O O-C

CH2OPO32-

Phosphoglycolate

Photorespiration

• When Rubisco reacts with O2 instead of CO2

• Occurs under the following conditions:– High O2 concentrations

– High heat

• Photorespiration is estimated to reduce photosynthetic efficiency by 25%

Why high heat?• When it is hot, plants close their stomata to

conserve water• They continue to do photosynthesis use

up CO2 and produce O2 creates high O2 concentrations inside the plant photorespiration occurs

C4 Photosynthesis

• Certain plants have developed ways to limit the amount of photorespiration – C4 Pathway*– CAM Pathway*

* Both convert CO2 into a 4 carbon intermediate C4 Photosynthesis

Leaf Anatomy

• In C3 plants (those that do C3 photosynthesis), all photosynthesis processes occur in the mesophyll cells.

Image taken without permission from http://bcs.whfreeman.com/thelifewire|

Mesophyll cells

Bundle sheath cells

(no photosynt

hesis

C4 Pathway

• In C4 pathway plants photosynthesis occurs in both the mesophyll and the bundle sheath cells.– Light reactions in

mesophyll– Calvin Cycle in Bundle

sheath

Image taken without permission from http://bcs.whfreeman.com/thelifewire|

C4 Pathway

• CO2 is fixed into a 4-

carbon intermediate first

• Has an extra enzyme– PEP Carboxylase that initially traps CO2

instead of Rubisco– makes a 4 carbon intermediate

C4 Pathway• The 4 carbon

intermediate “smuggles” CO2 into the bundle sheath cell

• The bundle sheath cell is not very permeable to CO2

• CO2 is released from the 4C molecule goes through the Calvin Cycle

C3 Pathway

How does the C4 Pathway limit photorespiration?

• Bundle sheath cells are far from the surface– less O2 access

• PEP Carboxylase doesn’t have an affinity for O2 allows plant to collect a lot of CO2 and concentrate it in the bundle sheath cells (where Rubisco is)

CAM Pathway

• Fix CO2 at night and store as a 4 carbon molecule

• Keep stomates closed during day to prevent water loss

• Same general process as C4 Pathway

• Has the same leaf anatomy as C3 plants

How does the CAM Pathway limit photorespiration?

• Collects CO2 at night so that it can be more concentrated during the day

• Plant can still do the calvin cycle during the day without losing water

Summary of C4 Photosynthesis• C4 Pathway

– Separates by space (different locations)

• CAM Pathway– Separates

reactions by time (night versus day)

Comparison• Light intensity is

directly related to temperature usually

• C4 Plants (CAM and C4 Pathway) are able to do more photosynthesis at high temperatures

• C3 Plants close stomata at high temps Light Intensity (cal/cm2/min)

Ne

t Ph

oto

syn

the

sis

(mg

CO

2 /d

m2

/hr)