IB Categories Archives: Topic 8: Metabolism, cell respiration and photosynthesis

8.2 – Photosynthesis

8.2 – Photosynthesis

8.2.1 – Draw and label a diagram showing the structure of a chloroplast as seen in electron micrographs

  • double membrane
  • starch grain
  • grana
  • thylakoid
  • internal membrane – location of the light dependent reaction
  • stroma – location of the light independent reaction, including the Calvin cycle. Often contain large starch grains and oil droplets, products of photosynthesis



8.2.2 – State that photosynthesis consists of light-dependent and light-independent

Light Dependent Reactions
In these reactions, the energy found in photons of light from the sun is used to produce ATP for the light independent reactions. This energy is trapped by the pigment chlorophyll in the thylakoid membranes. It is then used break the bonds in a water molecule, splitting it into O2 and H+ ions, called photolysis. Whilst the oxygen is released as a waste product, the hydrogen is taken by the hydrogen acceptor, NADP+. The light energy is then used to convert ADP and phosphate into ATP, a process called photophosphorylation.

Light Independent Reactions

Whilst the light dependent reactions use light energy, the light independent reactions use the chemical energy stored in ATP and NADPH + H+. These take place in the stroma. The carbon from the atmosphere found in CO2 is fixed in the form of sugars. Although these reactions do not directly rely on the presence of light to occur, the products of light dependent reactions must still be available.

8.2.3 – Explain the light-dependent reactions

In the light dependent reactions, light energy from the sun is converted into chemical energy. This is trapped in the chlorophyll, which are grouped into structures called photosystems. The photosystems are found on the thylakoid membranes of the grana.

There are multiple types of chlorophyll found in each photosystem, each of which absorb a different wavelength of light. Chlorophyll A is in the centre of the photosystem. When light hits the chlorophyll, electrons are excited and lost in oxidation.

Cyclic Photophosphorylation

ATP is produced in a cyclic process when the ratio of NADPH + H+ to NADP+ is high. This occurs when light is not a limiting factor for the reaction. Photosystem 1 does not generate any NADPH +H+, but sends electrons to the proton pump. Photosystem 1 is oxidised by the incoming light, releasing an

Photosystem 1 is oxidised by the incoming light, releasing an excited an electron to reduce the membrane proton pump. Protons, in the forms of H+ are pumped into the thylakoid space. This creates a concentration gradient necessary for the later production of ATP. The electrons are the cycled back to photosystem 1 to reduce it.

Non-cyclic Photophosphorylation

The light energy from the sun is trapped in the chlorophyll, and ATP is produced. The coenzyme NADP+ is reduced to form NADPH + H+. The first photosystem, photosystem two [PS2] is able to absorb light of the wavelength 680nm, which is why it is called P680. The second photosystem, photosystem one [PS1] is activated by wavelengths of 700nm, and called P700.

Light is first absorbed by the chlorophyll A in PS2. This energy is then converted into chemical energy by releasing electrons in oxidation.



The electrons from PS2 then pass along the thylakoid membrane in a series of redox reactions. The protein pumps are reduced to pump H+ ions into the thylakoid space.


In PS1, a different light frequency is absorbed. The photosystem is oxidised to release electrons.



The electrons pass from PS1 to ferrodoxins, which reduce NADP+ to NADPH + H+. The NADPH stays in the stroma to be used in the light independent reactions.


Photosystem 1 is reduced by the electrons from PS2.





Photosystem 2 is then reduced so that it can absorb more light. When water is split through photolysis, electrons are given to the photosystem. This is the source of H+ ions and the waste O2.

Since there is a high concentration of H+ ions in the thylakoid lumen, they can diffuse back into the stroma through the pore in ATP synthase. This process drives the phosphorylation of ADP to
ATP. During these redox reactions in the light dependent reactions, the energy levels of the electrons change.

8.2.4 – Explain photophosphorylation in terms of chemiosmosis

A high concentration of H+ ions accumulates in the thylakoid space due to proton pumping. This results in a proton gradient, causing protons to be pumped across the membrane through the ATPase molecules. This drives the motor mechanism of the structure, reducing ADP into ATP. This is like the process used in respiration.

The excited electrons the move to fill the vacancy in the reaction centre of PS2, then in PS1. They are used to reduce NADP+, in non-cyclic photophosphorylation, in which the reaction pathway is linear.


8.2.5 – Explain the light-independent reactions

The CO2 in the air is fixed to form carbohydrates. This is done using the energy trapped from sunlight in the light dependent reactions in the form of ATP and NADPH. These reactions take place in the stroma.

During fixation of CO2, the carbon dioxide is trapped by RuBP to form two molecules of glycerate-3-phosphate.

The GP is then reduced to TP in the reduction step, using ATP and NADPH + H+ to provide the energy. In the

In the product synthesis step, TP is used to make organic molecules such as glucose phosphate, sugar, starch, lipids, amino acids, etc This is followed by the

This is followed by the regeneration of the acceptor step, where some TP turns back into RuBP. This allows for more CO2 to be fixed from the atmosphere. Ribulose biphosphate is a 5-carbon acceptor. The fixation of carbon itself is catalysed by the

Ribulose biphosphate is a 5-carbon acceptor. The fixation of carbon itself is catalysed by the enzyme RuBisCo [ribulose biphosphate carboxylase], and is the most common protein in the leaves of green plants.

8.2.6 – Explain the relationship between the structure of the chloroplast and its function 

8.2.7 – Explain the relationship between the action spectrum and the absorption spectrum of photosynthetic pigments in green plants

Chlorophyll best absorbs red and blue light. Green light, on the other hand, is reflected, causing plants to appear green.

The rate of photosynthesis is highest and blue and red, and lowest at yellow and green because of the optimum wavelength for chlorophyll.

8.2.8 – Explain the concept of limiting factors in photosynthesis, with reference to light intensity, temperature and concentration of carbon dioxide

At any given time, only one of these factors will be the one limiting the rate of photosynthesis.

Light Intensity

Light is essential for photosynthesis, however it reaches a compensation point when the amount of oxygen being produced is the same as that being consumed in respiration. As a result, the relationship reaches a plateau at high intensities.


Light energy aids in the production of H+ ions from water and ATP. On the other hand, when there is no light, the plant can only respire. Too much light can damage the chlorophyll.



As the surrounding temperature increases, the rate of photosynthesis increases, with each plant reaching an optimum temperature where the rate falls off steeply. The enzymes in the reactions a temperature-sensitive.

Carbon Dioxide Concentration

As the concentration of CO2 increases, the rate of photosynthesis increases, before it plateaus. Each plant has a different optimum concentration.

When the CO2 is the limiting factor, the NADPH simply accumulates in the stroma, stopping the photosystems from operating. ATP is formed through cyclic photophosphorylation.

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8.1 – Cell Respiration

8.1 – Cell Respiration

8.1.1 – State that oxidation involves the loss of electrons from an element, whereas reduction involves a gain of electrons; and that oxidation frequently involves gaining oxygen or losing hydrogen, whereas reduction frequently involves losing oxygen or gaining hydrogen

In the equation above, the glucose molecule is oxidised into CO2. The hydrogen atoms in the molecule are removed, and some of the oxygen atoms from the O2 are added.

The oxygen molecules are reduced to form the H2O molecules. Oxygen atoms are removed [separated], and then hydrogen atoms from the glucose molecule are added.

The reaction above shows ADP + Pi being converted into ATP. The ADP molecule is oxidised. In the reverse, when ATP is converted back into ADP, the ATP is reduced.

8.1.2 – Outline the process of glycosis, including phosphorylation, lysis, oxidation and ATP formation

Glycolysis takes place in the cytoplasm. It takes place in a series of steps, breaking down the glucose molecule into the pyruvate ions.


This is the first step, where the glucose molecule reacts with ATP to form glucose phosphate. It is the converted into fructose phosphate, which reacts with another ATP molecule. In total, two ATP molecules are converted into 2ADP.


In this stage, the fructose biphosphate is split into two molecules of triose phosphate, a
three-carbon molecule.


Hydrogen is removed from the triose phosphate molecules. An enzyme and a coenzyme allow for the reaction to take place. The coenzyme is called NAD [nicotinamide adenine dinucleotide] , and it is a hydrogen and electron acceptor. During the oxidation of triose phosphate, NAD is reduced to form NADH + H+

ATP Formation

When the triose phosphate is converted into pyruvate, ATP is released. This type of ATP formation is different, as it occurs at substrate level. Four molecules of ATP are produced when two molecules of pyruvate are formed. Coupled with the loss of two ATP molecules in phosphorylation, the net gain of ATP in glycolysis is two.

The triose phosphate is oxidised to form pyruvic acid. The phosphate is donated to ADP to form the ATP. Pyruvic acid is also a three-carbon molecule. Under the conditions of the cytoplasm, the pyruvic acid immediately ionises to form the ion pyruvate.

The result of this entire process is:

The NADH + H+ will produce more ATP later. No oxygen is required for this process.

8.1.3 – Draw and label a diagram showing the structure of a mitochondrion as seen in electron micrographs

The mitochondrion is the organelle in which the rest of cellular respiration occurs. All the enzymes necessary for these

All the enzymes necessary for these reactions are located here. The matrix contains enzymes and

The matrix contains enzymes and metabolites. The inner membrane folds in to form

The inner membrane folds in to form cristae, maximising the surface area.

8.1.4 – Explain aerobic respiration, including the link reaction, the Krebs cycle, the role NADH + H+, the electron transport chain and the role of oxygen

Link Reaction

Once the pyruvate has diffused through the membrane of the mitochondrion, it is metabolised. The reaction occurs in the matrix. The pyruvate has one carbon atom removed to form CO2 through decarboxylation. It is also oxidised through the removal of oxygen. Combined, this is referred to as oxidative decarboxylation. One of the products is an acetyl group, which joins to the coenzyme A [CoA] in the link reaction to form acetyl CoA.

It is called the link reaction because it essentially ‘connects’ glycolysis to the Krebs cycle.

Krebs Cycle

This is also called the citric acid cycle. After the link reaction, the acetyl CoA reacts with oxaloacetate [OAA], with the result of CoA and citrate. Citrate then gives off two molecules of CO2 in separate decarboxylation reactions. A

Citrate then gives off two molecules of CO2 in separate decarboxylation reactions. A molecule of ATP is formed at substrate level. NADH2 is converted to NAD and FADH2 is converted to FAD after they donate electrons. Two cycles of the Krebs cycle take place for each molecule of glucose, as there are two

Two cycles of the Krebs cycle take place for each molecule of glucose, as there are two molecules of pyruvate formed.

Terminal Oxidation and Oxidative Phosphorylation

In the Krebs cycle and glycolysis, pairs of hydrogen atoms are removed from the respiratory substrates. Oxidised NADH2 is converted into reduced NAD, except in the Krebs cycle, where FAD is reduced instead. As this happens, H+ ions are pumped into the intermembrane space and build up a proton gradient. These protons then move out into the matrix through ATPase, producing ATP.

Hydrogen atoms or their electrons are transported along a series of carriers in the final stage of respiration. They begin from reduced NAD or FAD, combine with oxygen and form water. Here, oxygen acts as the final electron acceptor. Energy is released during the process as the electrons move along the chain, which is controlled and used by the cell in the form of ATP. For each molecules of NADH2 that is oxidised, 3 molecules of ATP are formed.

In total, aerobic respiration forms 36 molecules of ATP for each molecule of glucose.

8.1.5 – Explain the oxidative phosphorylation in terms of chemiosmosis

Chemiosmosis is the process where ATP synthesis is coupled with electron transport via the movement of protons (H+ ions).

Electron carrier proteins along the mitochondrial wall oxidise the reduced coenzymes. The energy from this is then used to pump the protons into the membrane space.

The protons accumulate in the space to form a gradient in hydrogen ion concentration and a lower pH. Potential energy is stored, and the ions will eventually flow back into the matrix through the channels in ATP synthase enzymes. The flow of protons causes ATP synthesis to occur.

8.1.6 – Explain the relationship between the structure of the mitochondrion and its function

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