UNIT 1 Module 3 Photosynthesis

Photosynthesis transfers light energy to chemical potential energy and is used to synthesise large organic molecules from inorganic substances.

Autotrophs are organisms that use light energy or chemical energy and inorganic molecules (carbon dioxide and water) to synthesise complex organic molecules.

Heterotrophs are organisms that ingest and digest complex organic molecules, releasing the chemical potential energy stored in them.

Animals, fungi and some bacteria are heterotrophs. They cannot make their own food but digest complex organic molecules into simpler soluble ones from which they synthesise complex molecules such as lipids, proteins and nucleic acids.

Both autotrophs and heterotrophs can release the chemical potential energy in complex organic molecules. This is respiration. They can also use oxygen for aerobic respiration.

Organisms that can photosynthesise are described as photoautotrophs. Their source of energy is sunlight and the raw materials are inorganic molecules - carbon dioxide and water. Plants, some bacteria and some protoctists are photoautotrophs.

Photosynthesis:

6CO₂ + 6H₂O (+ light energy) → C₆H₁₂O₆ + 6O₂

Aerobic respiration:

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O (+ energy, some as ATP)

Photoautotrophs have organelles within their cells called chloroplasts. The process happens in two main stages.

The structure of chloroplasts:

• Disc-shaped.
• Double membrane - an envelope.
• Intermembrane space - between inner and outer membrane.
• Outer membrane is permeable to many small ions.
• Inner membrane less permeable - transport proteins embedded in it. Folded into lamellae which are stacked up. Each stack is called a granum.
• Between the grana are integranal lamellae.

Two distinct regions - the stroma and the grana. Both seen under light microscope.

• Stroma is a fluid-filled matrix. Reactions of light-independent stage of photosynthesis occur in stroma where necessary enzymes are located.

• The grana are stacks of flattened membrane compartments, called thylakoids. Sites of light absorption and ATPsynthesis during the light-dependent stage of photosynthesis. Only seen using electron microscope.

How chloroplasts are adapted to their role:

• The inner membrane, with its transport proteins, can control entry and exit of substances between cytoplasm and the stroma inside chloroplasts.
• The many grana, consisting of stacks of thylakoid membranes, provide a large surface area for photosynthetic pigments, electron carriers and ATP synthase enzymes - all involved in light-dependent reaction.
• Photosynthetic pigments arranged into structures called photosystems, which allow maximum absorption of light energy.
• Proteins embedded in grana hold photosystems in place.
• Fluid-filled stroma contains enzymes needed to catalyse the reactions of the light-independent stage of photosynthesis.
• Grana surrounded by the stroma - products of light-dependent reaction which are needed for light-independent reaction can readily pass into stroma.
• Chloroplasts can make some of the proteins they need for photosynthesis using genetic instructions in the chloroplast DNA, and the chloroplast ribosomes to assemble the proteins.

Photosynthetic pigments are substances that absorb certain wavelengths of light and reflect others. They appear to us as the colour of the light wavelengths that they are reflecting. They are in thylakoid membranes, arranged in funnel-shaped structures called photosystems, held in place by proteins.

   Fig 2

Chlorophyll is a mixture of pigments. All have a similar molecular structure, consisting of a long phytol (hydrocarbon) chain and a porphyrin group containing a magnesium atom.

• Light hitting chlorophyll causes a pair of electrons associated with the magnesium to become excited.
• Two forms of chlorophyll a - P₆₈₀ and P_700. Both appear yellow-green.
• Each absorbs red light at a slightly different wavelength (absorption peak).
• Both are found at the centre of photosystems and are known as the primary pigment reaction centre.
• P₆₈₀ is found in photosystem 2 and its peak of absorption is light at a wavelength of 680nm.
• P₇₀₀ is found in photosystem 1 and its peak of absorption is light at a wavelength of 700nm.
• Chlorophyll a also absorbs blue light, of wavelength around 450nm.
• Chlorophyll b absorbs light wavelengths around 500nm and 640nm. It appears blue-green.

Accessory pigments:

• Carotenoids reflect yellow and orange light and absorb blue light.
• They do not contain a porphyrin group and are not directly involved in the light-dependent reaction.
• Absorb wavelengths that are not well absorbed by chlorophylls and pass the energy associated with that light to the chlorophyll a at the base of the photosystem.
• Carotene (orange) and xanthophyll (yellow) are the main carotenoid pigments.

Fig 3 Fig 4

The limiting factor for a metabolic process is the factor that is present the lowest or least favourable value.

As light intensity increases, the rate of photosynthesis increases. Light has three main effects:

• It causes the stomata to open so that carbon dioxide can enter the leaves.
• It is trapped by chlorphyll where it excites electrons.
• It splits water molecules to produce protons.

The rate of photosynthesis will vary throughout the day as the level of light intensity increases and decreases.

The reactions of the light-dependent stage are not very influenced by temperature.

The enzyme-catalysed reactions of the Calvin cycle are influenced by temperature.

Above 25°C the rate of photosynthesis levels off and then falls as enzymes stop working as efficiently and as oxygen more successfully competes for active site of rubisco.

High temperatures will also cause more water loss from stomata, leading to a stress response which the stomata close, limiting the availability of carbon dioxide.

6CO₂ + 6H₂O →

(light energy and in the presence of chlorophyll) 

→  C₆H₁₂O₆ + 6O₂

The chlorophyll is present in the plants chloroplasts. Other factors such as light, supplies of carbon dioxide and water are present in environment.

They may influence the rate at which photosynthesis proceeds.

Enhanced levels of carbon dioxide will increase the rate of photosynthesis.

An increase in light intensity will alter the rate of the light dependent reaction.

• More light energy is available to excite more electrons.
• The electrons take part in photophosphorylation, so increased light intensity means that more ATP and more reduced NADP will be produced.
• These are both used in the light independent stage, as sources of hydrogen and energy, to reduce glycerate phosphate to triose phosphate.

If there is no or very little light available, the light dependent stage will cease. This will stop the light independent stage as it needs the products of the light dependent stage.

•  Glycerate phosphate cannot be changed to Triose phosphate, so Glycerate phosphate will accumulate and levels of Triose Phosphate will fall.
• This will lower the amount of Ribulose bisphosphate, reducing the fixation of carbon dioxide and the fomration of more Glycerate phosphate.

The light-dependent stage of photosynthesis takes place on the thylakoid membranes of the chloroplasts.