3. Photosynthesis Flashcards
What is an autotroph?
Organisms that use light energy or chemical energy and inorganic molecules to synthesise complex molecules.
What is photosynthesis?
Photosynthesis is the process by which light energy from the sun is transformed into chemical energy and used to synthesise large organic molecules from inorganic molecules. This energy is then available to consumers and decomposers.
What are heterotrophs?
Organisms that ingest and digest complex organic molecules, releasing the chemical energy stored in them.
Where does photosynthesis take place?
In organelles in plant cells called chloroplasts.
Describe the structure of a chloroplast.
- Chloroplasts are small flattened organelles found in plant cells.
- They have a double membrane called the chloroplast envelope.
- Thylakoids (fluid filled sacs) are
stacked up in the chloroplast into structures.called grana. The grana are linked by lamellae. - Chloroplasts contain photosyntetic pigments such as chlorophyll a, b, carotene and xantophyll . These are all coloured substances that absorb light energy needed for photosynthesis. These pigments are found in the thylakoid membrane attached to proteins forming complexes known as photosystems.
- Contained within the inner membrane of the chloroplast and surrounding the thylakoids is a gel like substance called the stroma.
What are the two types of photosynthetic pigment and what are the differences between them?
Two types of photosynthetic pigment i) primary pigments - reaction centres where electrons are excited during the light dependant reaction. ii) accessory pigments - surround the primary pigments and transfer light energy to them.
What are the two photosystems and what are the optimum wavelength of each?
Photosystem 1 - 700nm
Photosystem 2 - 680nm
What sort of substances are contained within the stroma?
Enzymes, sugars and organic acids.
What are starch grains and why do they exist?
Starch grains occur when carbohydrates produced by photosynthesis and not used straight away are stored as starch grains in the stroma.
Where does the light dependant stage take place?
The light dependant stage takes place on the thylakoid membranes of the chloroplasts
Why does the light dependant stage take place in the thylakoid membranes?
Photosystems are embedded in these membranes. The photosystems help to trap light energy so that it can be converted to chemical energy form ATP.
What is water used for in the light dependant stage?
Photosystem 2 contains an enzyme which in the presence of light can split water into H+ ions, electrons and water in a process called photolysis.
Some of the oxygen produced is used for respiration however much of it diffuses out of the stomata.
The hydrogen ions produced are used in chemiosmosis to produce ATP. The H+ ions are then accepted by the coenzyme NADP to be used in the light independent stage.
The electrons produced replace those lost when chlorophyll is oxidised.
Describe the stages of photo phosphorylation.
When a photon hits a chlorophyll molecule the energy of the proton is transferred to two electrons. These electrons are excited. The excited electrons are captured by electron acceptors and are passed onto electron carriers. As the electrons pass along the chain of electron carriers energy is released. This energy is used to pump protons into the thylakoid space. The protons accumulate and form a proton gradient. The protons eventually flow down the gradient through protein channels associated with the enzyme ATP synthase. As h+ ions flow through the channels enough force is produced to join ADP and Pi molecules to form ATP, the flow of protons is called chemiosmosis. ATP is use in the light-independent stage.
The protons then join with NADP molecules (proton acceptors) to be used in the light-independent stage.
What is the difference between cyclic and non cyclic photophosphorylation.
Cyclic photophosphorylation: photons excite electrons in only Photosystem 1. The excited electrons pass to an electron acceptor and back to the chlorophyll molecule to produce small amounts of ATP possibly used to open the stomata by pumping K+ ions into the leaf.
Non cyclic photo-phosphorylation: light strikes PS2 which excites a pair if electrons that leave the chlorophyll molecule from the primary pigment reaction centre. Electrons then pass along a chain of electron carriers and the energy released is used to synthesise ATP. Light also hits Photosystem 1 exciting a pair of electrons which join with NADP to reduce it. The electrons from PS2 replace the electrons lost from PS1. Electrons from photolysed water take the place of those lost from Photosystem 2.
Where does the light independent stage take place?
The light independent stage or Calvin cycle, takes place in the stroma of the chloroplasts
