Photosynthesis

Even SpongeBob does it!!!

http://www.youtube.com/watch?v=lqEDz2vfhpQ&feature=email

Why is energy needed within cells?

• Allows chemical reactions to take place

• BUILD UP (synthesis) or BREAKDOWN of molecules

• In order to do this, energy is required to make and break bonds

Where does the energy come from?

• The SUN is the ultimate source of energy for nearly all living organisms (the exceptions being a few deep sea chemosynthetic bacteria)

• Autotrophs make their own food (organic compounds) using carbon dioxide

• Heterotrophs assimilate energy by consuming plants or other animals

What provides the energy within cells?

• ATP…Adenosine Tri Phosphate• Common to ALL living things• Any chemical that interferes with the

production or breakdown of ATP is fatal to the cell and therefore the organism

•Chemical energy is stored in the phosphate bonds

How does ATP provide the energy?• Chemical energy is stored in the phosphate

bonds, particularly the last one• To release the energy, a HYDROLYSIS

reaction takes place to break the bond between the last two phosphate molecules

• Catalyzed by ATP-ase• ATP is broken down into ADP and Pi

• For each mole of ATP hydrolyzed, about 34kJ of energy is released

• Some is lost, but the rest is useful and is used in cell reactions

Where does the energy to synthesise ATP come from?

• Catabolic (breakdown) reactions• Redox (reduction/oxidation) reactions• The main way in which ATP is synthesised is by the

removal of hydrogen atoms from intermediate compounds in a metabolic pathway

• When two hydrogen atoms are removed from a compound, they are picked up by a HYDROGEN CARRIER or ACCEPTOR

• We say the hydrogen carrier is reduced• Electrons from the hydrogen atoms are passed along

carriers (Electron Transfer Chain)• When a component of the chain receives one of the

hydrogen atoms, we say it is REDUCED• When a component passes an electron on, we say it is

OXIDISED• Each of these redox reactions releases a small amount

of energy and this energy is used to synthesise ATP

What does this have to do with photosynthesis?

• ATP is both synthesised and broken down during photosynthesis!

6CO2 + 6H2O = C6H12O6 + 6O2

• Light energy is required• Chlorophyll• Stored within chloroplasts• 10-50 chloroplasts per plant cell

Evolution of the chloroplast• It is believed that

photosynthetic bacteria were acquired by eukaryotic cells

• By endocytosis (engulfing)

• To produce the first algal/plant cell

• This is called the “endosymbiont theory”

• They were then passed on to the next generation

Intergranal lamellae

Size and Shape

• Can vary • Usually between 2-10µm in length• Usually disc shaped

Membranes

Double membrane;• Outer membrane – permeable to many

small ions• Inner membrane – less permeable and

has transport proteins embedded in it

Intermembrane space is 10–20nm wide between the inner and outer membrane

Lamellae and thylakoids

• The inner membrane is folded into lamellae (thin plates) aka thylakoids

• The lamellae (thylakoids) are stacked in piles called granum

• Between the grana are intergranal lamellae

Stroma

• Fluid-filled matrix

• The light-independent stage of photosynthesis occurs here

• Contains starch grains, oil droplets, DNA and ribosomes

Grana

• Contains stacks of thylakoids

• Where the light-dependant stage of photosynthesis takes place

• Absorb light and make ATP

How chloroplasts are adapted

Adaptation How it helps

Inner membrane with transport proteins

Controls molecules travelling between the cells cytoplasm and the stroma

Many grana (consisting of up to 100 thylakoids)

Large SA for photosynthetic pigments, electron carriers and ATP synthase enzyme needed in LDR

Photosynthetic pigments

Arranged in photosystems, allows max. absorbtion of light

Proteins embedded in grana

Hold photosystem in place

Fluid-filled stroma Contains enzymes needed for LIR

Grana surrounded by stroma

Products made in LDR in grana can pass into stroma to be used in LIR

Chloroplast DNA and ribosomes

Can make some proteins needed for photosynthesis

Pigments and photosystems

• Chloroplasts contain photosynthetic pigments to absorb the light energy

• Pigments are chlorophyll a, chlorophyll b and carotene

• The pigments are found in the thylakoid membranes attached to proteins.

• The protein and pigment are called a photosystem

• Two photosystems used by plants to absorb light are PSI ( 700nm wavelength) and PSII ( 680nm wavelength)

Chlorophylls• Chlorophyll a

– Is in the “Primary pigment reaction centre”– Two forms

•P680 – in photosystem 2

•P700 – in photosystem 1

– Appears yellow-green– Absorbs red light (and blue at 450nm)– contains a Mg atom – when light hits this, a

pair of electrons become excitedhttp://www.youtube.com/watch?v=0W6JgnCFezo&feature=related

http://www.youtube.com/watch?feature=endscreen&NR=1&v=qfw5-wCVMNM

20

Accessory pigments• Chlorophyll b

– Absorbs light at wavelengths between 500-640nm

– It appears blue-green– Is one of the accessory pigments

• Carotenoids– Absorb blue light– Reflects yellow (xanthophyll) and orange

(carotene) light– They absorb light not normally absorb by

chlorophylls

A quick review of visible light- Visible light is a mixture of

wavelengths

- Each wavelength appears as a different color

- Different colors of light can be absorbed by an object. If they are not absorbed, they are reflected