PHOTOSYNTHESIS

Describe what thylakoids are

Disc-shaped fluid-filled sacs made of thylakoid membranes

Describe what the stroma is

A jelly-like fluid that contains enzymes and starch grains (plus DNA and ribosomes)

What is the purpose of starch granules?

To store the products of photosynthesis

Where does light dependent reactions take place?

The thylakoid

Where do light independent reactions take place?

The stroma

What structure links the grana together?

The Lamellae

Describe the adaptations of the thylakoid that help the chloroplast to fulfill its function

• The thylakoid contains photosynthetic pigment to absorb light

• It has a large SA as it is multiple discs stacked together

• Contains ATP synthase for light dependent reactions

What type of membrane does the chloroplast have?

A double membrane

What is found within the inner chloroplast membrane?

Transport proteins to control the passage of molecules

How are photosynthetic pigments arranged?

In structures called photosystems

What light wavelength does photosystem II absorb?

A wavelength of 680

What light wavelength does photosystem I absorb?

A wavelength of 700

What are accessory pigments for?

Accessory pigments are not directly involved in light dependent reactions but they capture more light energy so more electrons in the chlorophyll can be excited.

Where are photosynthetic pigments found?

In the thylakoid membrane

What is the main photosynthetic pigment?

Chlorophyll a

Why does chlorophyll appear green?

Because it reflects green light back into our eyes

What is photoionisation?

The process of using light energy to excite the electrons in chlorophyll enough so that it leaves the molecule

What happens when the electron is excited and leaves the molecule?

It travels down the electron transport chain being taken up by an electron carrier

What is the photosystem’s charge after excited electrons leave?

Positive

What is the equation for photolysis?

H²O → 2H + 2e + ½ O²

How is the positive photosystem II balanced out?

The electrons from photolysis replace the excited electrons that left to balance out the charge

Why can only PSII perform photolysis?

It has the correct enzyme complex to split the water

Describe the properties of ATP

• Soluble

• Easily diffuses around the cell

• It can be broken down and remade with simple reactions

What enzyme synthesises ATP?

ATP Synthase

How is ATP made from ADP?

ATP synthase catalyses the reaction between ADP and an inorganic phosphate(Pi)

Describe the steps of photophosphorylation

1. Photosystem II absorbs light energy, which excites the electrons until it leaves the molecule, travelling down the electron transport chain via an electron carrier

2. As electrons move down the ECT, energy releases to synthesise ATP

3. Excited electrons from PSI and hydrogen ions from the photolysis of water both combine with NADP to form reduced NADP(NADPH)

How does cyclic photophosphorylation differ from non-cyclic?

• The electrons at PSI can’t be replaced from the electrons produced from photolysis so the electrons return and are repeatedly recycled to the PSI.

• No NADH is produced and less ATP is made

The coenzyme made during light-dependent reactions

NADH / Reduced NAD

How do electrons move along the electron transporter chain?

A series of REDOX reactions

Chemiosmosis

The process where charged particles cross a membrane to create an electrochemical gradient to power ATP synthesis

The 3 stages of the Calvin cycle

1. Carboxylation

2. Reduction

3. Regeneration

Describe carboxylation in the Calvin cycle

• CO2 enters the leaf through the stomata

• It combines with RuBP(a 5C compound), using the enzyme rubisco.

• An unstable 6C compound is made, which quickly splits intro x2 3C compound, called GP or G3P

Describe reduction in the Calvin cycle

• GP is reduced into a different 3C compound, TP.

• This is done using ATP and H+ ions (from NADH) provided by the light-dependent reaction

Describe regeneration in the Calvin cycle

• 5/6 molecules of TP are used to regenerate RuBP.

• The rest of the ATP produced in he light-dependent reaction is used.

What are the products of the Calvin cycle and where do they go

• NADP → Back into the light-dependent reaction

• ADP → Back into the light-dependent reaction

• Inorganic phosphate → Back into the light-dependent reaction

• TP → Used to build organic compounds like glucose, amino acids, and lipids

• RuBP → Regenerated and reused in the Calvin cycle

How many turns of the Calvin cycle is needed to produce a molecule of glucose

6 cycles

• 3 turns of the Calvin cycle produces 6 molecules of TP, 5 are needed for RuBP

• Glucose is made by joining 2 molecules of TP

4 limiting factors of photosynthesis

• Light intensity/wavelength

• Temperature

• Carbon dioxide levels

• Availability of water

How does light intensity affect photosynthesis rates?

• The higher the light intensity, the more energy provided to excite electrons.

• At extremely high light intensities, chlorophyll may be damaged

How does temperature affect photosynthesis rates?

• Photosynthesis is a series of reactions that rely on enzymes (ATP synthase and rubisco).

• If the temperature is too low, the enzymes will work too slowly.

• Too high of a temperature, the enzymes may start to denature

How does carbon dioxide levels affect photosynthesis rates?

• photosynthesis work best at 0.4% CO2 concentration

• If the CO2 concentration is increased further, the stomata may begin to close

How does availability of water affect photosynthesis rates?

• Plants need a constant supply of water

• Too much water may cause roots to become waterlogged, preventing the plant from absorbing enough oxygen

How can farmers optimise conditions to increase the rate of photosynthesis?

• Light intensity/wavelength : Using lamps or LEDs

• Temperature : Glasshouses to trap heat energy and using cooling systems

• CO2 concentrations : CO2 generators can be used