Photosynthesis Flashcards
Describe the structure of a chloroplast.
Chloroplasts consist of the chloroplast envelope (outer an inner membrane) and the inter-membrane space. Inside this is the stroma, starch grains, lipid droplets, DNA, ribosomes and a system of thylakoid membranes, which stack up to give grana. Membranes between grana are called lamellae.
Where does the light-dependent reaction occur?
This happens on the thylakoid membranes and in the thylakoid space.
How are thylakoids adapted for the light-dependent stage of photosynthesis?
The thylakoids contain pigments, enzymes and electron carriers required for the light dependent reaction. The grana create a large surface area to increase the number of light-dependent reactions that can occur. There are many different photosynthetic pigments (e.g. chlorophyll a, chlorophyll b, carotene, xanthophyll etc.), to ensure as much light as possible is absorbed.
Describe the light-dependent reaction of photosynthesis
- light is absorbed by chlorophyll in photosystem 2, exciting electrons in chlorophyll to a higher energy level (photoionisation)
- the excited electrons move down a collection of membrane proteins called the electron transport chain
- light energy is used to break down water (photolysis), into hydrogen ions, oxygen and electrons (catalysed by the enzyme oxygen-evolving complex)
- the electrons removed from water replace those lost from photosystem 2
- as electrons move down the electron transport chain, they lose energy. This energy can be used to actively transport H+ from the stroma to the thylakoid space, across the thylakoid membrane (chemiosmosis), creating an electrochemical gradient
- H+ move down a concentration gradient form the thylakoid space to the stroma, via the transmembrane protein ATP synthase. This allows the production of ATP from ADP and Pi (photophosphorylation)
- electrons in photosystem 1’s chlorophyll are excited and move down the ETC. the electrons lost from PS2 replace those lost from PS1. The electrons combine with H+ and NADP to produce NADPH (reduced NADP).
Describe cyclic photophosphorylation
Light hits chlorophyll in PS1, exciting electrons. These electrons move down the electron transport chain, providing just enough energy to synthesise ATP and NADPH, but they then return to PS1. This occurs when there is very little light available.
Describe the light-independent stage of photosynthesis (Calvin cycle).
The cycle begins with ribulose biphosphate (RuBP), which has 5 carbon atoms. CO2 enters the leaf through the stomata then diffuses across the chloroplast envelope into the stroma. The CO2 combines with RuBP in a process called carbon fixation, catalysed by the enzyme rubisco (large quantities of rubisco are needed as this process is slow). This produces two molecules of glycerate 3-phosphate (GP). 2 molecules of reduced NADP and 2 ATP molecules are hydrolysed, this reduces GP to two molecules of triose phosphate (TP).
- one molecule of ATP is used to regenerate RuBP from TP (5/6 of all TP made is converted back to RuBP)
- 1/6 of TP made is converted into organic products, like glucose, lipids, amino acids etc.
How many Calvin cycles produce one glucose molecule?
6
Where does the light-independent reaction take place?
In the stroma of chloroplasts
What is the compensation point?
The point at which O2 used up in respiration is equal to O2 produced in photosynthesis, so there is no net gas exchange.
What is a limiting factor?
A limiting factor is a factor which affects the rate of a reaction, whereby the rate is proportional to the value of this factor (so increasing the value (making the conditions more favourable) increases the reaction rate).
Why does light intensity affect the rate of photosynthesis?
Low light intensity means a shortage of the products of the light-dependent reaction (ATP and reduced NADP), so the rate-limiting step of the Calvin cycle is the reduction of glycerate 3-phosphate to triose phosphate as this requires the hydrolysis of ATP and NADPH.
Why does temperature affect the rate of photosynthesis?
Low temperature means the enzymes catalysing the Calvin cycle are slowed (particularly the activity of rubisco). This means that the carbon fixation step (RuBP to GP) is the rate-limiting step.
Why does carbon dioxide concentration affect the rate of photosynthesis?
Low CO2 concentration means that RuBP can’t be converted into GP, so this is the rate-limiting step. RuBP, NADPH and ATP accumulate, levels of GP reduce
What is the method for separating photosynthetic pigments?
- grind a leaf using a pestle and mortar with acetone, obtaining a leaf solution
- draw a line on TLC (thin layer chromatography) paper 1cm from the bottom, using a pencil
- draw another line 1cm from the top (solvent front)
- use a paintbrush to spot some leaf solution onto the TLC paper (on the origin line)
- add running solvent to a beaker and place the TLC paper in the beaker, ensuring that the solvent line does not start above the origin line.
- leave the reaction to progress. The pigments will move different distances up the paper depending on their solubility in the solvent.
- calculate the Rf values of each spot and determine which pigment caused which one
What is the purpose of the acetone used in the chromatography practical?
The acetone dissolves the phospholipid bilayer that makes up the thylakoid membranes, the cell membranes, and breaks down the cell wall, allowing the pigments out of their cells and suspending them in solution.
