PHOTOSYNTHESIS:

Can it make sense?

It looks so innocent…

Role of Photosynthesis

Photosynthetic organisms (like plants, algae, and some bacteria) occupy the lowest level of a food chain

The most common chemical energy produced during photosynthesis is

Glucose fuelsglucose

Cell respiration

Behavior of Light Photosynthetic pigments respond to visible light, a type of

electromagnetic energy ranging from 400nm to 740nm Sunlight is made up of many different colors of light that

can be seen after separation by a prism and is absorbed in particles called

ANY substance when struck by light either absorbs it for use (color vanishes) or reflects it (color can be seen)

Example: A red T-shirt absorbs wavelengths but reflects Light energy absorbed in photosynthesis is used to form

covalent bonds between sugar molecules to make glucose

Fun fact: Because almost no green is absorbed, plants can’t be grown in green light

photons

Blue and green

red

Chlorophylls and Carotenoids , which is a green pigment, reflects

green and absorbs other wavelengths of light to be used in photosynthesis

Plant leaves appear green because of the chlorophyll found in the chloroplasts of their cells. Plants also contain various other photosynthetic pigments in their leaves like xanthophylls and carotene

are the major photosynthetic pigment groups

These are found on the membranes of the These are organized into photosystems used during

the first stage of photosynthesis

Chlorophyll

Chlorophylls and carotenoids

thylakoids

Spectra Measurements A spectrophotometer measures absorption of light at

various wavelengths to measure a plant’s absorption spectrum

An is the combination of the absorption spectra of all the pigments in the chloroplast

An is the rate of photosynthesis at a particular wavelength of light—more specifically, it is the percentage use of wavelengths of light in photosynthesis

Action spectrums can be measured via oxygen production; if its high, there is likely a high rate of photosynthesis

Absorption and action spectra vary between plants due to differences in pigment and their amounts

Absorption spectrum

Action spectrum

Two Stages of Photosynthesis The first stage of photosynthesis traps light

energy and converts it into chemical energy ATP The second stage is a series of reactions that

uses ATP to bond carbon dioxide and water molecules together to create a sugar, such as glucose

Overall, photosynthesis is an process-complex molecules are built from simpler ones

Photosynthesis occurs in autotrophic organisms so they can make their own food

anabolic

First Stage of Photosynthesis

This can also be called the Light energy is absorbed in chlorophyll and

used to 1.) Convert light energy into chemical energy in the form of ATP and

2.) split a water molecule into hydrogen and oxygen in a process called . Not all the absorbed light energy is used.

In photolysis, oxygen is released as a waste product and will be used in aerobic cell respiration

Light-dependent reactions

photolysis

Photosystems Light-dependent reactions occur in the thylakoids or grana of the

chloroplast A stack of thylakoids is called a Photosystems exist on the thylakoid membranes and are made of

1) chlorophyll a molecules, 2) accessory pigments, and 3) a protein matrix

The within the photosystem contains 1) a pair of chlorophyll molecules, 2) a matrix of protein, and 3) a primary electron receptor

TERMINOLOGY: Photosystems are only types of complexes decorating the thylakoid; there are more than two per thylakoid

Photosynthetic bacteria have only one type of photosystem, but plants have a photosystem I and a photosystem II

Each photosystem is most efficient at absorbing a certain type of light; photosystem I responds best to light at 700 nanometers and photosystem II responds best to light at 680 nanometers

granum

Reaction center

Steps of Light-Dependent Reactions

1) A photon of light is absorbed by a photosystem II pigment and transferred along other pigment molecules to reach a chlorophyll a molecule in the reaction center. The photon one of the chlorophyll a electrons to a higher energy state. The chlorophyll is referred to as

2) The electron is captured by the primary acceptor of this reaction center

3)Water is split by an enzyme to produce electrons, hydrogen ions, and an oxygen atom through a reaction controlled by light energy. This is called photolysis. The electrons are sent to the reaction center’s chlorophyll a molecules

4)The excited electrons pass from the primary acceptor down an electron transport chain, losing energy at each exchange. The first of the three carriers is plastoquinine (PQ). The middle carrier is a cytochrome complex

5)Energy lost from the electrons moving down the transport chain drives to phosphorylate ADP and produce ATP (a proton is pumped into the thylakoid space via the electron’s energy and travels back through ATP synthase). This is non-cyclic photophosphorylation as opposed to cyclic phosphorylation.

excites

photoactivated

chemiosmosis

Steps of Light-Dependent Reactions (Continued)

6) A photon of light is absorbed by pigment in photosystem I. The energy is again transferred along accessory pigments to a reaction center’s chlorophyll a molecule. One of the chlorophyll a’s electrons is excited and transferred to the reaction center’s primary acceptor. The void left behind is filled with the de-energized electron from

7) Photosystem I’s electron is passed down a second electron transport chain involving the carrier

8) Enzyme NADP reductase catalyzes the transfer of the electron from ferredoxin to the energy carrier NADP+. Two electrons are required to fully reduce NADP+ to NADPH, as well as a H+ ion from the stroma.

Review: OIL RIG: OIL=Oxidation is loss (of electrons) and RIG=Reduction is gain (of electrons)

The final products of the Light-Dependent Reactions are some which supply chemical energy for the light-independent reactions. Oxygen was also produced.

Photosystem II

ferredoxin

NADPH and ATP

ATP: Cellular Respiration VS Photosynthesis

SIMILAR: Energy is released when electrons are exchanged from one carrier to another in an electron transport chain in the involved membranes

Electron transport chain is in the cristae membranes in cell respiration and the thylakoids in photosynthesis

Released energy is used to actively pump hydrogen ion into the intermembrane space in cell respiration; in photosynthesis it is pumped into the thylakoid space

Hydrogen ions originate in the matrix in cell respiration, but in the stroma in photosynthesis

ATP synthase allows hydrogen ions to diffuse back into the matrix in cell respiration or the stroma in photosynthesis

ATP synthase catalyzes the oxidative phosphorylation of ADP to form ATP in cell respiration. In photosynthesis, ATP synthase catalyzes the photophosphorylation of ADP from ATP

In photosynthesis the b6-f cytochrome complex pumps hydrogen ions into the thylakoid space to increase the concentration gradient between the thylakoid space and the stroma

Second Stage of Photosynthesis

Also called the ATP and hydrogen are used to convert CO2 and water into

useful organic molecules like glucose. Glucose is made through the following reaction:

6CO2 + 6H2O C6H12O6 + 6O2

is the conversion of an inorganic form of an element (like CO2) into an organic form (like glucose). It can also be described as the conversion of carbon from a gas to a solid.

Photosynthesis can be described as a series of reactions fixing CO2 and water to glucose, with water produced as a byproduct (meaning this is a condensation reaction)

Glucose is not the only organic molecule possible to form during photosynthesis

Light-independent reactions

Fixation

Light-Independent Reaction In-Depth Occurs in the stroma of the chloroplast This involves the Calvin Cycle 1) Ribulose biphosphate (RuBP), a 5-carbon compound, binds to a CO2

in carbon fixation to produce an unstable 6-carbon compound. catalyzes this reaction

2) The 6-carbon compound breaks into two 3-carbon compounds called glycerate-3-phosphate

3) ATP and NADPH act on the glycerate-3-phosphate and form new (TP) compounds in a reduction reaction

4) TP can either leave the cycle and become sugar phosphates that may become more complex carbohydrates. Most stay in the cycle to reproduce the originating compound RuBP

5)ATP is used to recreate RuBP from TP For every 12 TP molecules, 18 ATPs, and 12 NADPH: one 6-carbon

sugar and six RuBPs are created TP can be used to produce simple sugars (glucose), disaccharides

(sucrose), or polysaccharides (cellulose or starch).

Rubisco

Triose phosphate

Summary of Photosynthesis Light-dependent reactions occur in the light independent reactions occur in the Light-dependent reactions use light energy to make ATP

and NADPH; light-independent reactions use ATP and NADPH to make TP

Light-dependent reactions split water in photolysis and release oxygen into the atmosphere; light-independent reactions return ADP, inorganic phosphate, and NADPH to the light-dependent reactions

Light-dependent reactions include photosystems I and II and two electron transport chains; light-independent reactions involve the Calvin cycle

Thylakoids;stroma

Photosynthesis vs. Cell Respiration Photosynthesis has CO2 as a reactant and

oxygen as a product; cell respiration has oxygen as a reactant and CO2 as a product

Plants do both photosynthesis and cell respiration but the two processes DO NOT cancel each other out. Plants do not have muscle or so many other ATP-demanding tissues like animals and have a consistently low rate of cell respiration because of this.

Photosynthetic rate is not constant and responds to many environmental factors

Cyclic Phosphorylation Another way that a light-dependent reaction of photosynthesis can

produce ATP Can only occur is light is not the limiting factor and when NADPH

has built up in the chloroplast This is when light-energized electrons from photosystem I flow

back into the cytochrome complex of the electron transport chain between photosystems I and II instead of the chain involving ferredoxin

From the cytochrome complex, electrons move down the remaining electron transport chain to allow chemiosmosis to produce ATP

This means the electrons don’t flow to the second transport chain to produce NADPH, of which there is already an abundance (causing the photophosphorylation to occur in the first place)

The extra ATP goes to the Calvin cycle and causes it to take place faster

Photosynthesis and Limiting Factors A limiting factor is a factor that controls the rate of a process like

photosynthesis At any time in the life of a plant, one of the following may be the

plant’s limiting factor—the one that alone alters photosynthetic rate regardless of how the others behave

Only the limiting factor has effect because photosynthesis has many steps with many end-products. When a limiting factor slows one step, the rest are also impeded—that one step is called the rate-limiting step

Stronger light intensity and higher CO2 concentration increase the rate of photosynthesis—more CO2 is taken in and more oxygen is produced up to maximum capacity. Higher temperatures also increase photosynthetic rate until the plant’s enzymes begin to denature

At night as temperatures cool and light intensity drops, photosynthesis may halt entirely, although cell respiration continues

Measuring Photosynthetic Rate

Photosynthetic rate can be measured by oxygen production or CO2 intake, provided a correction is made for cell respiration. It can also be measured by keeping track of a change in biomass of experimental plants (massing is less reliable as many other factors affect biomass)

Example: oxygen production can be seen in the volume of air bubbles released from aquatic plants

Example: CO2 uptake can be measured in aquatic plants by a rising water pH as CO2 is absorbed

Chloroplast Structure All of photosynthesis occurs in the Chloroplasts and mitochondria have an extra outer

membrane, their own DNA, and are close to the size of a prokaryotic cell

Chloroplasts are generally in the leaves of a plant, but not exclusively

Extensive membrane surface area of the thylakoids allows photosystems to absorb more light

Small lumen within the thylakoids allows faster proton accumulation to create a concentration gradient

Stroma region similar to a cell’s cytosol allows an area for the Calvin Cycle’s enzymes to work

Double membrane on the outside isolates working parts and enzymes of the chloroplast from the surrounding cytosol

chloroplast

Chloroplast Diagram