Comprehensive and in-depth notes on the biological aspects of this chapter, using specification points as headings to ensure that all required material is included- and no irrelevant content (like many of the textbooks). Created and used by an A-Level Biology student for the NEW SPECIFICATION from ...
Biology Spec Led Revision
Chapter 5 - Photosynthesis
Photosynthesis and Respiration
(a) the interrelationship between the process of photosynthesis and respiration. To include the
relationship between the raw materials and products of the two processes.
Biological processes need energy:
Living things need energy for biological processes to occur:
• Plants need energy for things like photosynthesis and active transport
• Animals need energy for things like muscle contraction and homeostasis
• Without energy, these biological processes would stop and the plant or animal would die
Photosynthesis stores energy in glucose
Plants make their own food (glucose) using photosynthesis:
• Photosynthesis is the process where energy from light is used to make glucose from H2O and
CO2 (light energy is converted to chemical energy)
• Photosynthesis occurs in a series of reactions, but the overall reaction is:
6CO2 + 6H2O + Energy —> C6H12O6 + 6O2
• Energy is stored as glucose until the plants release it by respiration
• Animals cant make their own food so they have to obtain glucose by eating plants or other
animals which have eaten plants
Cells release energy from glucose by respiration:
• Living cells release energy from glucose - called respiration
• This is the energy used to power all the biological processes within the cell
• There are two types of respiration:
• Aerobic respiration - Respiration using oxygen
• Anaerobic respiration - Respiration without oxygen
• Aerobic respiration produces carbon dioxide and releases energy
C6H12O6 + 6O2 —> 6O2 + 6H2O + Energy
Definitions:
• Metabolic pathway - A series of small reactions controlled by enzymes e.g. photosynthesis and
respiration
• Phosphorylation - Adding phosphate to a molecule, e.g. ADP is phosphorylated into ATP
• Photophosphorylation - Adding phosphate to a molecule using light
• Photolysis - The splitting of a molecule using light
• Hydrolysis - The splitting of a molecule using water
• Decarboxylation - The removal of carbon dioxide from a molecule
• Dehydrogenation - The removal of hydrogen from a molecule
• Redox reactions - Reactions which involve oxidation and reduction
Redox:
- If something is reduced it has gained electrons (e-) and may have gained hydrogen or lost
oxygen
,- If something is oxidised it has lost electrons (e-) and may have lost hydrogen or gained oxygen.
Oxidation of one molecule always means another will be reduced.
Photosynthesis and respiration involve coenzyme
• A coenzyme is a molecule which aids the function of an enzyme
• Working by transferring a chemical group from one molecule to another
• A coenzyme used in photosynthesis is NADP. NADP transfers hydrogen from one molecule to
another - this means it can reduce (give hydrogen) or oxidise (remove hydrogen).
• Examples of coenzymes used in respiration are: NAD, coenzyme A and FAD
• NAD and FAD transfer hydrogen from one molecule to another - this means they can reduce
(give hydrogen) or oxidise (remove hydrogen) from a molecule.
• Coenzyme A transfers acetate between molecules
(b) the structure of a chloroplast and the sites of the two main stages of photosynthesis. The
components of a chloroplast including outer membrane, lamellae, grana, thylakoid, stroma and
DNA.
Photosynthesis takes place in the chloroplasts of plant cells
• Chloroplasts are the site of
photosynthesis
• They have a double membrane called
the chloroplast envelope
• Thylakoids are stacked up into grana,
which are linked together by lamellae.
• Chloroplasts contain photosynthetic
pigments (chlorophyll a, chlorophyll b
and carotene)
• These are coloured substances which
absorb light energy for
photosynthesis. They're located in the
thylakoid membranes attached to
proteins.
• The protein and pigment together is called a photosystem.
(c) (i) the importance of photosynthetic pigments in photosynthesis
• A photosystem contains two types of photosynthetic pigments - primary pigments and accessory
pigments.
• A primary pigment is the reaction centre where electrons get excited during light dependent
reactions.
• The accessory pigments make up light harvesting systems. Surrounding the reaction centres and
transferring extra energy for electron excitement.
There is two photosystems used by the plant to obtain light energy:
• Photosystem one (PSI) which absorbs light best at a wavelength of 700nm
• Photosystem 2 (PSII) which absorbs light best at a wavelength of 680nm.
• The stroma which is contained within the inner membrane contains enzymes, sugars and organic
acids.
• Chloroplasts have their own DNA found within the stroma and often circular. There can be
multiple copies in each chloroplast.
, • Carbohydrates produced by photosynthesis and not used straight away are stored as starch
grains in the stroma.
Photosynthesis can be split into two stages:
(d) the light-dependent stage of photosynthesis. To include how energy from light is harvested and
used to drive the production of chemicals which can be used as a source of energy for other
metabolic processes (ATP and reduced NADP) with reference to electron carriers and cyclic and
non-cyclic photophosphorylation.
AND
The role of water.
1) Light dependent reaction
• Requires light energy
• Takes place in thylakoid membranes
• Light energy absorbed by photosynthetic pigments in photosystems and converted into
chemical energy
• Light energy is used to add a phosphate group to ADP to form ATP, and reduce NADP to form
reduced NADP
• (reduced NADP is an energy-rich molecule as it transfers hydrogen, and so electrons to other
molecules)
• ATP transfers energy and reduced NADP transfers hydrogen to light-independent reaction
• H2O is oxidised to O2
(e) The fixation of carbon dioxide and the light-independent stage of photosynthesis. To include
how the products of the light-dependent stage are used in the light-independent stage (Calvin
cycle) to produce triose phosphate (TP) with reference to ribulose bisphosphate (RuBP), ribulose
bisphosphate carboxylase (RuBisCO) and glycerinate 3-phosphate (GP) – no other biochemical
detail is required.
2) The Light-Independent Reaction
• This is also called the calvin cycle and as the name suggests it doesn't use light energy directly
• It takes place in the stroma of chloroplasts
• Here the ATP and reduced NADP from the light-dependent reaction supply energy and
hydrogen to make glucose from CO2.
(c) (ii) practical investigations using thin layer chromatography (TLC) to separate photosynthetic
pigments
Thin Layer Chromatography can Separate Photosynthetic Pigments
Photosynthetic pigments can separate using TLC. Like all types of chromatography there is a
mobile phase (liquid solvent) and stationary phase (solid glass with thin layer of silica on top).
1) Grind up several leaves with anhydrous sodium sulphate and propanone.
2) Transfer the liquid to a test tube, adding some petroleum ether and shake the tube. Forms two
layers - top is pigments mixed with petroleum ether
3) Transfer top layer into second tube with anhydrous sodium sulphate
4) Draw a line on the chromatography plate. Put a spot of the liquid from the second test tube
onto the line, this is the point of origin
5) Once the point of origin is dry put the plate into a beaker with prepared solvent. Just enough so
that the point of origin is above the liquid line. As the solvent absorbs the pigments will move
with it at but at different rates, so they will separate.
6) Several coloured spots will be seen between the point of origin and the solvent front. These are
the pigments.
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