Topic 8.2: Photosynthesis

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

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

Photosynthesis is a two step process which consists of light dependent (converts light energy into chemical energy) and light independent variable (uses chemical energy to make organic molecules)

8.2.3 Explain the light-dependent reactions

The light dependent reaction occurs on the thylakoid membrane

Chlorophyll in both photosystem I and II absorbs light, which triggers the release of high energy electrons (photoactivation)

• The electrons from photosystem II pass along a series of carriers
• The electrons lost from photosystem II are replaced by electrons generated by the photolysis of water with oxygen as by-product
• Hydrogen pump carriers use the excited electrons to pump hydrogen ions over into the thylakoid compartment, the electrons is then passed on to photosystem I
• Photosystem I re-excites the electrons and then passed on to NADP+ reductase
• The electrons then binds with the NADPH + H+ and is transported somewhere else.
• ATP synthase uses the concentration gradient to produce ATP from ADP and Pi

8.2.4 Explain photophosphorylation in terms of chemiosmosis

Photophosphorylation is simply phosphorylation but required light energy.

As the electrons cycle through the electron transport chain located on the thylakoid membrane, they lose energy. The energy is used to pump H+ ions into the thylakoid compartment to create a concentration gradient. The H+ ions return via ATP synthase.

This process is called chemiomosis

8.2.5 Explain the light-independent reactions

Calvin cycle occurs in the stroma and uses ATP and NADPH + H+ produced by the light dependent reaction. There are three main steps which include carbon fixation, reduction and regeneration of RuBP.

Carbon fixation:

• Enzyme RuBisCo catalyses the attachment of carbon dioxide to the 5 carbon compound ribulose bisphosphate (RuBP)
• The unstable 6 carbon compound that is formed immediately breaks down into two 3 carbon molecules called glycerate-3-phosphate (GP)

Reduction:

• ATP is used to put phosphate onto the GP
• NADPH + H+ reduces the compound by giving a hydrogen.
• This forms G3P

Regeneration

• For every six molecules of G3P produced, only one could be used to form half a sugar
• Th remaining G3P is used to restock RuBP in a reaction that require ATP
• With RuBP regenerated, the plant will use the cycle multiple times and construct long chains of sugars.

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.

Pigments require light as a source of energy.

The absorption spectrum indicates the wavelength of light absorbed by each pigment. The action spectrum indicates the rate of photosynthesis for each wavelength.

There are strong relationships between the two as it shows both the peak and the valleys in the graph.

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

The law of limiting factor states that when a chemical process depends on more than one essential condition to become favorable, its rate will be limited by the factors that is nearest its minimum value.

Light intensity

• Light is required for the light dependent reactions (photoactivation of chlorophyll and photolysis of water molecules)
• Low light intensities results in insufficient production of ATP and NADPH + H+

Temperature

• Primarily affect light-independent reaction as it requires more collisions and enzymes
• High temperatures could damage the enzymes and cause the prohibition of enzymes from occuring

Concentration of Carbon Dioxide

• Carbon dioxide is required for the light independent reaction to occur (carbon fixation of RuBP by RuBisCo)
• At low levels, oxygen will over take the enzyme RuBisCo creating toxic gas instead.

Topic 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.

OILRIG - Oxidation Is Loss, Reduction Is Gain
ELMO - Electron Loss Means Oxidation


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

Glycolysis takes place in the cytoplasm.

The process phosphorylation simple oxidizes the glucose present with phosphate groups. It attaches itself to both ends of the glucose for it to become Hexose Biphosphate. This process requires ATP for the phosphate group to attach itself.

Lysis is the stage when the hexose biphosphate molecule becomes too unstable and breaks down into two triose phosphate.

This is the oxidation/ATP formation step. Each of the triose phosphate is oxidized to become a pyruvate molecule. With the addition of one hydrogen, it is passed on and reduce one NAD+ to NADH. Each Triose phosphate adds a phosphate group to the ADP reducing it to ATP. Note: Each Triose phosphate releases enough energy to form two ADP to ATP.

This is the full sequence of glycolysis in short.

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

This is the structure of a mitochondrion.

It is important to remember that Mitochondrion has a double membrane thus the existence of Cristae is to increase inner membrane surface area. Important for electron transport chain.


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

There are 3 more stages in the aerobic respiration. This includes Link reaction, the Krebs cycle and the electron transport chain.

Link Reaction: Pyruvate is decarboxylated to become Acetyl CoA

The link reaction allows for pyruvate and Co-enzyme A to join together to form a complex. The carbon is decarboxylated (removal of carbon) as carbon dioxide. The remains forms Acetyl CoA. The NAD+ is reduced to NADH + H+.


Kreb's cycle: Oxidative decarboxylation of the acetyl group.

Acetyl CoA joins with a 4 carbon group to form citrate. CoA is then released to transport more pyruvate molecules. The C6 compound formed is called citric acid.

Citric acid is oxidatively decarbonxylated. A C5 group is now formed. The carbon is released as carbon dioxide. NAD+ is reduced to NADH + H+

The C5 grouped is also oxidatively decarbonxylated, forming a C4 group. The carbon is released as carbon dioxide. NAD+ is reduced to NADH + H+.

The final stage in the cycle requires the C4 group to regenerate back to its original form and accept acetyl CoA. This reduces NAD to NADH + H+ and FAD to FADH + H+. ADP also uses this energy to bind with a phosphate group to make ATP.

This is the whole process and shows how it is linked to the electron transport chain.

The hydrogen carriers (NADH + H+ and FADH + H+) provide electrons to the electron transport chain on the inner mitochondrial membrane. As the electrons cycle through the chain, they lose energy to translocate the hydrogen ions to the intermembrane space (creating a gradient). The hydrogen ions return to the matrix through ATP synthase mass producing ATP. Oxygen acts as the final electron acceptors for the electron transport chain to allow new electrons enter the chain. Oxygen combines with the hydrogen ions in the matrix to form water molecules.

8.1.5 Explain oxidative phosphorylation in terms of chemiosmosis

This is the electron transport chain.

• Oxidative phosphorylation describes the production of ATP from oxidised hydrogen carriers
• When electrons are donated to the electron transport chain, they lose energy as they are passed between submissive carrier molecules.
• The energy is used to translocate H+ ions from the matrix to the intermembrane space against the concentration gradient.
• The build up of H+ ions creates an electrochemical gradient, or proton motive force (PMF)
• The protons return to the matrix via a transmembrane exnzyme called ATP synthase.
• As the hydrogen ions return, they release energy which is used to produce ATP (from ADP + Pi)
• This process is called chemiomosis and occurs in the cristae
• The hydrogen ions and electrons bind with oxygen to form water molecules. 

8.1.6 Explain the relationship between the structure of the mitochondrian and its function

There are four structures in the mitochondrion which function to improve the reactions happening. Which include: Inner membrane, external double membrane, matrix and inter-membrane space

Inner membrane: The double folded inner membrane space forms cristae, this allows an increase of ATP synthesis thus more ATP produce
External double membrane: Contains appropriate proteins to allow the transport of molecules
Intermembrane space: Small space to increase the gradient difference.
Matrix: The right pH levels for the reaction and enzymes to work at optimal rate.

Topic 8: Cellular respiration and photosynthesis

Topic 8 of the IB HL Biology syllabus is the Cellular respiration and photosynthesis. IBO recommends to spend 10 hours on this topic.

This topic has 2 sub-chapters: "Cell respiration" and "Photosynthesis". Each are separated with numerical values in order of mentioned.

These are all HL syllabus statements, it is recommended to bring a Casio Graphical Calculator instead of Texas.