chpt . 11: photosynthesis

Chpt. 11: Photosynth

esis

• Autotrophic Organisms: are organisms that make their own food – producers.

• Majority of autotrophs are green plants and they make their food by a process called photosynthesis.

Photosynthesis: is the process by which plants make food and oxygen using carbon dioxide from the atmosphere, water from the soil and energy from the sun.

Equation for photosynthesis:

6CO2 + 6H2O + Light C6H12O6 + 6O2

Carbon Water Glucose OxygenDioxide

Chlorophyll

Photosynthesis:

• cells need energy to carry out their reactions

• light (sun) provides this source of energy.

• energy provided by the breakdown of Adenosine TriPhosphate (ATP).

• Light Energy ATP Glucose

Role of Photosynthesis: plants use photosynthesis to make food.

animals get their food from plants.

photosynthesis produces oxygen (O2) needed for respiration.

responsible for forming fossil fuels.

Priestly’s experiment. A plant in a closed jar died and a mouse in another closed jar died, but when plant and mouse were put together, both lived.

Role of photosynthesis:

Location of Photosynthesis:

• Photosynthesis depends on chlorophyll which is located in structures called chloroplasts.

• Chloroplasts concentrated in cells in the upper surfaces of leaves.

• Chloroplasts are also present in the green stems of some plants.

Factors necessary for photosynthesis:

1. Chlorophyll:- green pigment found in grana of chloroplasts.- made with magnesium.- absorbs light and converts it into glucose.

2. Light:- comes directly from the sun.

3. Carbon Dioxide (CO2):- comes from the atmosphere through the stomata of the leaf.- also comes from plant during respiration.

Factors necessary for photosynthesis:

4. Water:- comes from the soil.- taken up through the roots and transported via the

xylem to the leaves.

5. Temperature:- Photosynthesis is controlled by enzymes that work best at 25oC.

- at approx. 50oC most enzymes are denatured.

Stages in Photosynthesis:

Stage 1: Light is absorbed

Stage 2: Water is split

Stage 3: Products are produced (protons, electrons ad oxygen)

Stage 4: Light energises electrons

Stage 5: Glucose is formed

1. CHLOROPHYLL ABSORBS THE LIGHT ENERGY CAUSING ELECTRONS TO LEAVE THE CHLOROPHYLL AND TO PASS THROUGH A SERIES OF ELECTRON CARRIERS

2. WATER MOLECULES SPLIT INTO H+ IONS (PROTONS) AND OH- IONS. THE H+ IONS ACCEPT THE ELECTRONS TO BECOME H ATOMS, THIS PROCESS IS REFERRED TO AS PHOTOLYSIS

3. THE OH- IONS RELEASE THEIR ELECTRONS, WHICH ARE CARRIED BACK TO THE CHOLROPHYLL

4. THE OH GROUPS COMBINE TO FORM H20 AND 0 ATOMS. THE OXYGEN ATOMS ESCAPE FROM THE LEAF INTO THE ATMOSPHERE AS 02 MOLECULES, OR USED INSIDE THE LEAF FOR RESPIRATION

5. THE HYDROGEN ATOMS ARE COMBINED WITH CO2 TO FORM GLUCOSE

Products of photosynthesis:

Glucose - foodOxygen – used for respiration

sunlight

Chlorophyll

molecule

E-

Electron carrier 1

E-EC2

E-glucose

1.

H2O 2.

H+

OH- 3.

OH (+OH)E-EC3EC4

EC5

E-

4.

H20

0

CO25.

CATALYSTS

SUNLIGHT6CO2 + 6H20 -------------- C6H12O6 + 602

CHLOROPHYLL

What happens to the glucose?

• Can be converted to starch and stored for later use• Moved to another part of the plant to give energy( respiration)• Changed to lipids or protein• Can be used to make cellulose for new cell walls for growth

Environmental Factors

Environmental factors effecting the rate of photosynthesis are:

Light intensity*TemperatureCO2 concentration

* need to know experiment

Experiment: To investigate the influence of light intensity on the rate of photosynthesis.

• Plants can only photosynthesise in the light

• At night photosynthesis cannot take place

• By using artificial light at night we can increase the hours of photosynthesis • In winter daylight is often weak , artificial lamps can supplement the light and increase the rate of photosynthesis

•Example – in green houses


As the light intensity increases the rate of photosynthesis will also increase provided there is no shortage of CO2 and if the temperature is maintained at approx 250C

If you vary the light intensity (by moving the light) the other factors are kept constant because CO2 concentration and temp also effects the rate of photosynthesis

Keep CO2 conc constant by adding sodium bicarbonateKeep temp constant by using a water bath


Elodea ( Canadian pondweed ) used, the rate of photosynthesis is measured by the number of oxygen bubbles produced per unit time ( must allow time for plant to adjust to each condition)


No of bubbles

light

Light saturation pt

Sodium bicarbonate

runs out

* Note: see experiment pg 107/108

Biochemistry of Photosynthesis(Higher Level)

Structure of Chloroplast:

LAMELLA

INNER MEMBRANE

OUTER MEMBRANEDNA

Photosynthesis takes place in 2 stages:

- the LIGHT STAGE( light dependent) occurs in the GRANA of chloroplast

- the DARK STAGE ( light independent—occurs whether light is present or not) occurs in STROMA of chloroplast

LIGHT STAGE

Events in the Light Stage:

A) Light absorption

B) Light energy transferred to electrons

C) Electron flow – Pathway 1

D) Electron flow – Pathway 2

A. Light Absorption:Light is absorbed by a range of pigment clusters found in the chloroplast (almost all colours of light are absorbed but green is normally reflected)

Each cluster consists of:- a variety of pigments- a chlorophyll molecule *(specially placed)- an electron acceptor

Electron acceptor

Chlorophyll molecule

Pigments

Pigment Cluster

B. Light Energy Transferred to Electrons:

The pigment cluster absorbs as much light energy as possible and passes it to the reaction centre chlorophyll molecule. This light energy is trapped by the chlorophyll molecule and passed to an electron.

This energy causes the electron to become energised.

The energised electron is passed to the electron acceptor which can then send it on one of two pathways.

C. Electron Flow – Pathway 1:

In pathway 1 the electrons pass from the first electron acceptor to a series of other electron acceptors (electron carriers) and back again to the chlorophyll

As the electrons are passed around they lose energy

This energy is used to join a phosphate (P) to ADP to form high energy ATP and water (Chpt. 10: energy carriers):

ADP + Energy + P ATP + Water The addition of phosphate to ADP in this way is called photophosphorylation

Because the electron travels in a cycle and returns to its original chlorophyll this process is called Cyclic Photophosphorylation (e- recycling)

D. Electron Flow – Pathway 2:

2 high energy electrons at a time are passed from chlorophyll to the electron acceptor and then along another series of electron acceptors (electron carriers)

In this case the electrons do not return to the original chlorophyll

They lose energy as they pass from electron acceptor to electron acceptor and this energy is used to make more ATP

Eventually the 2 electrons are passed to combine with NADP+ to form NADP- :

NADP+ + 2 electrons (2e-) NADP-

The chlorophyll molecule is now short of electrons and gains more from the splitting of water. Electrons pass, two at a time, from a water molecule to chlorophyll

2H2O 4H+ + 4e- + O2

The splitting of water using light energy is called Photolysis

D. Electron Flow – Pathway 2:

The H+ ions are attracted to NADP- and combine with it to form NADPH which is used in the DARK stage of photosynthesis.

Because the electrons start at a chlorophyll and finish at NADPH and form ATP on their way this pathway is known as Non cyclic photophosphorylation (no e- recycling)

Light Energy

Photolysis The splitting of water

End Products of the Light Stage

There are 3 end products of the light stage

1. ATP - this will provide energy for the dark stage

2. NADPH - this will provide protons and energised electrons for the dark stage

3. Oxygen is made when water is split – can be used for respiration or be released into the atmosphere

* Note: water is also produced to hydrate cytoplasm of cells

Chlorophyll molecule

Electron acceptor

CE I

CE 2

CE 3

e-

Light Stages

ATP

ADP + P

Pathway 1 (cyclic)

e- flow from chlorophyll back to chlorophyll

CE 2 CE 1

ADP + P ATP

excited e-

NADP+

2e-

NADP- NADPH (used in dark stage)

Pathway 2 (non-cyclic)

e- start with water and end up with NADPH

2H2O

4H+ ions

4OH- ions

O2

CYC

LIC

P

HO

TO

PH

OS

PH

OR

YLATI

ON

NO

N-C

YC

LIC

P

HO

TO

PH

OS

PH

OR

YLATI

ON

ADP + P

ATP

DARK STAGE(Calvin Cycle)

This may also be called the light independent stage as it can occur in the light but does not need to use it

It takes place in the stroma of the chloroplast

It is controlled by enzymes and therefore can be affected by temperature

Carbon dioxide from the air enters the chloroplast where it combines with protons (H+) and electrons to form glucose.

This process requires energy, protons and electrons:

- it obtains energy from the breaking down of:ATP (produced in light stage) into ADP + P

- it obtains hydrogen ions and electrons from the breaking down of:

NADPH (produced in light stage) into NADP+

+ 2 electrons

+ H+

Remember the addition of electrons to anything is known as reduction, thus,

Carbon Dioxide is reduced to glucose

The NADP+ , the ADP and P return to the light stageto reform NADPH and ATP for the dark stage

The

Dark Stage

ATP

ADP + P +

ENERGY

C6H1206

Carbohydrate is formed

CO2 from atmosphere

NADPH

From light stage

NADP+ returns to light stage

Summary2 stages

(1) The light dependent stage which converts H2O, NADP+, ADP + P into NADPH, ATP and O2 using chlorophyll and sunlight

(2) The light independent stage which converts CO2 into glucose using NADPH and ATP. This process generates NADP+, ADP + P which can be reused in the light dependent stage

Differences between the light and dark stage

LIGHT STAGE DARK STAGE

• Occurs in the grana Occurs in stroma

• Light dependent Light independent

• ATP produced ATP used up

• NADPH produced NADPH used up

• O2 is produced CO2 is used up

• Chlorophyll molecule Chlorophyll not involvedis involved

chpt . 11: photosynthesis

Documents

role of photosynthesis

location of photosynthesis

energy respiration

light energy atpglucose

electrons stage

reactions light

oh ions

h atoms