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Biology OCR A level Energy for biological process Summary Notes
Biology OCR A level Energy for biological process Summary Notes
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1 of 8B17- Energy for biological processes
5.2.1 Photosynthesis
(a) The interrelationship between the process of photosynthesis and respiration.
To include the relationship between the raw materials and products of the two processes.
Key words:
Autotrophic nutrition- organisms that synthesis complex organic molecules from inorganic
molecules via photosynthesis.
Compensation point- where the rate of photosynthesis in an organism , equals the rate of its
respiration. There is no net loss or gain of carbohydrate.
Photon- is a particle of light; each photon contains an amount (a quantum) of energy.
Carbon fixation- the process by which carbon dioxide is converted into sugars.
Heterotrophs- obtain energy by digesting complex organic molecules of food to smaller
molecules that they can use as respiratory substrates. They obtain energy from the products of
digestion by photosynthesis.
• Photosynthesis is a physiological process used by plants, algae and some types of bacteria to
convert light energy from sunlight into chemical energy. Organisms use this chemical energy to
synthesise large organic molecules, to form the building block of living cells, from simple
inorganic molecules- autotrophic nutrition.
• Organisms that photosynthesis are called ‘photoautotrophs’. These organisms are also
described as ‘producers’ as they are at the beginning of a food chain and provide energy and
organic molecules to others.
• The main product of photosynthesis is a monosaccharide sugar which can be converted to
disaccharides for transport and then to starch for storage.
• Photosynthesis is an example of carbon fixation.
• Carbon fixation is endothermic and requires the addition of electrons (reduction reaction).
Respiration
• Organisms that photosynthesis oxidise the organic molecules that they have synthesised by
photosynthesis and stored, releasing chemical energy.
• Non-photosynthetic organisms are heterotrophs.
C6H12O6+ 6CO2 = 6H2O + 6CO2 + energy
How photosynthesis and respiration interrelate
• Both photosynthesis and aerobic respiration are important in cycling carbon dioxide and oxygen
in the atmosphere.
Compensation point
• When photosynthesis and respiration proceed at the same rate, so that there is no net gain or
loss of carbohydrates, the plant is at its compensation point.
• The time a plant takes to each its compensation point is called the compensation period.
• When exposed to light after being in darkness, shade plants reach their compensation point
sooner than sun plants, which require a higher light intensity to achieve their optimum rate of
photosynthesis.
(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.
Key words:
Granum- inner part of chloroplasts made of stacks of thylakoid membranes, where the light-
dependent stage of photosynthesis takes place.
Photosynthetic pigments- pigment that absorbs specific wavelengths of light and traps the
energy associated with the light; such pigments include chlorophyll a, during the light-dependent
stage of photosynthesis.
Photosystem- system of photosynthetic pigments found in thylakoids of chloroplasts; each
photosystem contains about 300 molecules of chlorophyll that trap photons and pass their energy
to a primary pigment reaction centre, a molecule of chlorophyll a, during the light-dependent
stage of photosynthesis.
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Stroma- fluid-filled matrix of chloroplasts, where the light-independent stage of photosynthesis
takes place.
Thylakoid- flattened membrane-bound sac found inside chloroplasts; contains photosynthetic
pigments/photosystems and is the site of the light-dependent stage of photosynthesis.
• Most plant chloroplasts are disc shaped and around 2-10μm long
• Each is surrounded by a double membrane with an intermembrane space.
• The fluid filled matrix inside a chloroplast is called the stroma.
• The grana consists of stacks of thylakoid membranes
• Chloroplasts have three distinct membranes (outer, inner and thylakoid) which create three
separate internal compartments (intermembrane space, stroma and the thylakoid space)
Grana
• Light dependent stage occurs in the grana in the thylakoids.
• Thylakoids within a granum may be connected to thylakoids within another granum by
intergranal lamellae.
• Thylakoid membrane is less permeable and is folded into thylakoids that form sacks, each
called a granum.
• Grana are surrounded by stroma, so the products of the light dependent stage can easily pass
to the stroma to be used in the light independent stage.
Stroma
• Is the fluid-filled matrix which contains enzymes need to catalyse the reactions of the light
independent stage.
• The loop of DNA contains genes that code for some of the proteins needed for photosynthesis.
These proteins are assembled at the chloroplast ribosomes.
(c) (i) The importance of photosynthetic pigments in photosynthesis.
To include reference to light harvesting systems and photosystems.
Key words:
Photosynthetic pigments- pigment that absorbs specific wavelengths of light and traps the
energy associated with the light; such pigments include chlorophyll a, during the light-dependent
stage of photosynthesis.
Photosynthetic pigments
• Within the thylakoid membranes of each chloroplast are funnel shaped structures called photo
systems.
• These photo systems contain photosynthetic pigments. Each pigment absorbs light of a
particular wavelength and reflects other wavelengths of light.
• The energy associated with the wavelengths of light captured is funnelled down to the primary
pigment reaction centre, consisting of a type of chlorophyll, at the base of the photosystem.
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