Photosynthesis

The Main Ideas

1. Cells need energy in the form of ATP so they can carry out essential jobs.

2. Cells make their own ATP and plant cells can make it during photosynthesis as well as respiration.

3. ATP is made using the enzyme ATPsynthase (ATPase).

4. Plant cells use NADP to move H⁺ and electrons.

Photosynthesis has 2 parts - BOTH happen in chloroplasts

1.LIGHT DEPENDENT (NEEDS LIGHT ENERGY) This happens on chloroplast THYLAKOID
MEMBRANES. (grana and lamellae).

2. LIGHT INDEPENDENT (HAPPENS IN DAYLIGHT BUT DOESN`T NEED LIGHT)This
occurs in the STROMA where there are lots of enzymes and it uses energy from ATP.

LIGHT DEPENDENT STAGE

1. Energy from sunlight (PHOTONS) used to split water (PHOTOLYSIS) at PHOTOSYSTEM II.
(Photosystems are a collection of light sensitive chemicals that will trap the energy from different
wavelengths of light.) Photosystem II can split water molecules releasing H⁺, electrons and oxygen

2. When the water is split the electrons from chlorophyll a are excited and escape from their electron
shell (PHOTOIONISATION). They are picked up by electron carriers and are passed along the
thylakoid membrane to PHOTOSYSTEM I. These electrons replace ones at photosystem I that
have already been excited by light energy and have escaped.

3. When water is split at photosystem II the hydrogen ions produced (H⁺) are pumped into the
THYLAKOID SPACE. The build up of H⁺ in the space means more H⁺ inside the space
when compared with the stroma. This makes an ELECTROCHEMICAL GRADIENT

4. H⁺ diffuse out of the space through specialized protein channels connected to ATP synthase. The
movement is called CHEMIOSMOSIS and results in ATP production.

5. Moving H⁺ and electrons provides enough energy to convert ADP + P_i to ATP

6. Electrons and H⁺ are picked up from photosystem I by NADP and this reduces the NADP.

7. The reduced NADP and ATP are now used in the LIGHT INDEPENDENT stage in the stroma.

LIGHT INDEPENDENT STAGE

1. Carbon dioxide from the air diffuses into plant cells and into chloroplasts. It joins with a
5 CARBON compound called RIBULOSE BISPHOSPHATE (RUBP). This is catalysed by the
enzyme RUBISCO

2. The 5 carbon RUBP and CO₂ make a 6 carbon compound that quickly breaks down to two
3 carbon molecules called GLYCERATE 3 PHOSHATE (GP).

3. The GP is reduced by reduced NADP which donates H⁺ and electrons. ATP provides energy for
this.

4. Reduction of GP forms TRIOSE PHOSPHATE (TP) which is used in 2 ways:

a) 5/6 of the TP remakes RUBP to keep the cycle going.

b) 1/6 is used to convert to glucose and then to make other carbohydrates, lipids and
proteins.

The overall process

USES Light energy + Water (light dependent stage)
Carbon dioxide (light independent stage)

PRODUCES Oxygen (light dependent)
Glucose (light independent)

The process requires the presence of chlorophyll and other light sensitive chemicals to harvest the
initial energy from the sunlight and electron carriers and NADP to allow the formation of the energy rich molecule ATP which powers the Calvin cycle and also releases some energy for other processes in the cell.

LIMITS ON PHOTOSYNTHESIS

Light The intensity (brightness), wavelength(colour) - plants mainly use red and blue light and
daylength will all affect the rate of photosynthesis.

Temperature There are lots of enzymes involved in the process so plants are adapted to optimum
temperatures for where they live.

CO₂ concentration This is always low in normal air and plants are used to that, but an increase can
help under certain circumstances.

A greenhouse can be used to: maximize light (use daylight bulbs in winter), control temperature (heater in winter and vents in summer) and provide extra CO₂ (gas cylinders or paraffin heaters can release extra CO₂).

PRIMARY PRODUCTIVITY of a site is improved by maximizing the photosynthesis rate.