Topics 3.8 + 8.2

Photosynthesis

IB Biology

Leaves • Green tissue in the interior

of the leaf (mesophyll) = contains chloroplasts

• CO2 enters the leaf and O2 exits through the stoma

• Stomata = bottom of leaf

Chloroplasts

• similar to mitochondria• outer and inner membrane• membranous sacs: THYLAKOIDS• thylakoids = concentrated in

stacks called GRANA• thylakoids: contain chlorophyll

(capture light)• filled with fluid: STROMA (where

sugars are made from CO2 )

Overview

Electromagnetic Spectrum

The 2 stages of Photosynthesis

• Light Dependent Reactions:– convert light energy to chemical

energy (ATP + NADPH) and produce oxygen gas as a waste product

• Light Independent Reactions (Calvin Cycle):

– make sugar using carbon dioxide and the energy-containing products of the light-reactions (ATP + NADPH)

Light Dependent Reaction

• Light can be..– Absorbed– Reflected– Transmitted

• Occurs in thylakoids

• Chlorophyll A and B = main photosynthetic pigments (also carotenes, anthocyanins)

Light Dependent Reaction• Light energy is used to split water. • H + is released = used by ATP Synthase to produce ATP• NADP + is reduced to NADPH+ • ATP and NADPH = used in light independent reactions• O2 = waste product

Photosystems• Photosystem: place in

thylakoid where light is harvested

• Photon is absorbed by a pigment molecule = electrons gain energy

• Excited electron is passed to a neighboring molecule - the primary acceptor

Photosystems I & II work together

• Both photosystems absorb light

• Electron Transport Chain = electrons go down an energy hill = lose energy at each step - this energy is stored in ATP or NADPH

• Electrons released from PSI is replaced by electrons coming from PSII

Chemiosmosis/Photophosphorylation

Role of Water

• The light energy is used to split water molecules (photolysis)

• H2O → H+ + O + e- The electrons allow

reduction of NADP+ / replace those leaving photosystem II

H + flow allows ATP production

The O2 is released and leaves the plant through the stoma

Mechanical Analogy for Making ATP

• As electrons skip down the ETC, ATP are generated.

Animations

http://www.youtube.com/watch?v=hj_WKgnL6MI

Light Independent Reaction (Calvin Cycle)

• Glucose phosphate is produced = stored as starch• CO2 , ATP and NADPH are used

Simple Calvin Cycle animations:

Summary

Light dependent

reaction

Light independent

reaction

plateau

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 of photosynthesis 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.

Light intensity

Rate

of

phot

osyn

thes

is

What are the conditions of each of these trials?

http://www-pord.ucsd.edu/~sgille/stpa35/geritol.html