Photosynthesis

Question 1

What is photosynthesis?

Answer 1

The process by which green plants and some other organisms use sunlight to synthesize nutrients from carbon dioxide and water.

The leaf is the main photosynthetic structure in eukaryotic plants. Chloroplasts are the cellular organelles within the leaf where photosynthesis takes place.

Question 2

What Is Oxidation?

Answer 2

When a substance gains oxygen or loses hydrogen, the process is called ‘oxidation’.

The substance to which oxygen has been added or hydrogen has been lost is said to be ‘oxidised’.

Looses electrons.

Results in energy being given out.

Oxidation and reduction always take place together.

Question 3

What Is Reduction?

Answer 3

When a substance loses oxygen, or gains hydrogen, the process is called reduction.

Gains electrons.

Results in energy being taken in.

Oxidation and reduction always take place together.

Question 4

What Is Photoionisation?

Answer 4

• A chlorophyll molecule absorbs light energy. This boosts the energy of a pair of electrons within this chlorophyll molecule, raising them to a higher energy level.
• These electrons are said to be in an excited state and they leave the chlorophyll molecule altogether.
• The chlorophyll molecule becomes ionised and this process is called photoionisation.
• Having lost a pair of electrons, the chlorophyll molecule has been oxidised.

Question 5

What Is An Electron Carrier?

Answer 5

The electrons that leave the chlorophyll through photoionisation are taken up by a molecule called an electron carrier.

The electron carrier, which had gained electrons, has been reduced.

The electrons are passed along a number of electron carriers in a series of oxidation-reduction reactions.

These electron carriers form a transfer chain that is located in the membranes of the thylakoids.

Each new carrier is a slightly lower energy level than the previous one in the chain, and so the electrons lose energy at each stage.

Some of this energy is used to combine an inorganic phosphate molecule with an ADP molecule in order to make a ATP

Question 6

Chemiosmotic Theory?

Answer 6

• Each thylakoid is an enclosed chamber into which protons (H+) are pumped from the stroma using protein carriers in the thylakoid membrane called proton pumps.
• The energy to drive this process comes from electrons released when water molecules are split by light. This is photolysis of water.
• The photolysis of water also produces protons which further increases their concentration inside the thylakoid space.
• Overall this creates and maintains a concentration gradient of protons across the thylakoid membrane with a high concentration inside the thylakoid space and a low concentration in this stroma.
• The protons can only cross the thylakoid membrane through ATP synthase channel proteins. The rest of the membrane is impermeable to protons. These channels from small granules on the membrane surface and so are also known as stalked granules.
• As the protons pass through these ATP synthase channels, they cause changes to the structure of the enzyme which then catalyses the combination of ADP with inorganic phosphate to form ATP.

Question 7

Adaptations of the leaf?

Answer 7

Leaves are adapted to bring together water, carbon dioxide and light whilst removing the products: oxygen and glucose.

Adaptations of the leaf:

• large surface area that absorbs as much sunlight as possible,
• arrangement of the leaves on the plant minimises overlapping,
• thin so that diffusion pathway is short for gases and light is absorbed in first few micrometers,
• transparent cuticle and epidermis that let light through to the photosynthetic mesophyll cells beneath,
• long, narrow upper mesophyll cells packed with chloroplasts that collect sunlight,
• numerous stomata for gaseous exchange so that all mesophyll cells are only a short diffusion pathway from one,
• stomata that open and close in repose to changes in light intensity,
• many air spaces in the lower mesophyll layer to allow for rapid diffusion in the gas phase of carbon dioxide and oxygen,
• a network of xylem that brings water to the leaf cells and phloem which carried away the sugars produced during photosynthesis.

Question 8

Photosynthesis equation?

Answer 8

6CO2 + 6H2O —(light)—> C6H12O6 (glucose) + 6O2

Question 9

What is energy from photosynthesis used for?

Answer 9

Plants use energy for active transport, DNA replication, cell division and protein synthesis’s

Animals use energy for muscle contraction, maintenance of body temperature, active transport, DNA replication, cell division and protein synthesis.

Question 10

ATP Recap?

Answer 10

Immediate source of energy in a cell.

A cell can’t get energy directly from glucose and so, in respiration, the energy released from glucose is used to make ATP (adenosine triphosphate).

ATP is synthesised via a condensation reaction between ADP and Pi (inorganic phosphate). Energy for this is used from the breakdown of glucose in respiration.

ATP carries the energy and diffuses to the part of the call that needs energy.

Here, it’s hydrolysed back into ADP and Pi. Chemical energy is released from the phosphate bond and used by the cell. ATP hydrolyse catalyses this reaction.

The ADP and inorganic phosphate are recycled and the process starts again.

Structure:
Rectangle is the adenine, ribose is the pentagon (5 sides) and 3 circles (phosphate groups) with phosphate bonds between them.

Question 11

Why is ATP a good energy source?

Answer 11

1. ATP stores and releases energy in a small, manageable amount at a time. No energy is asked as heat.
2. It’s small and soluble so it can be easily transported around the cell.
3. It’s easily broken down, so energy can be easily released instantaneously.
4. It can be quickly re-synthesised.
5. It can make other molecules more resistive by transferring one of its phosphate groups to them (phosphorylation).
6. ATP can’t pass out of the cell, so the cell has an immediate supply of energy.

Question 12

Structure of chloroplasts?

Answer 12

Photosynthesis takes place in the chloroplasts of plant cells.

1. Chloroplasts are flattened organelles surrounded by a double membrane. The membrane envelope is made of both membrane.
2. Thylakoids (fluid-filled sacs) are stacked up in the chloroplast into structures called grana. A single stack is a granum. Grana is multiple stacks. The grana are linked together by bits of thylakoid membrane called lamellae. Singular = lamella.
3. Chloroplasts contain photosynthetic pigments (chlorophyll a, chlorophyll b, carotene). These are coloured substances that absorb light energy needed for photosynthesis. The pigments are found in the thylakoid membranes - attached to proteins. The protein and pigment is called a photosystem. Photosystems are funnel-shaped.
4. There are two photosystems used by plants to capture light energy. Photosystem I (or PSI) absorbs light best at a wavelength of 700nn and photosystem II (PSII) absorbs light best at wavelength 680nm.
5. In the inner membrane of the chloroplast and surrounding the thylakoids is a gel-like substance called the stroma. It contains enzymes, sugars and organic acids.
6. Carbohydrates produced by photosynthesis and not used straight away are stored as start grains in the stroma.

Question 13

Three main stages of photosynthesis?

Answer 13

1. The light-dependant reaction.

3. The light-independent reaction.

Question 14

Energy In A Light-Dependant Reaction Is Used For?

Answer 14

• To add an inorganic phosphate (Pi) molecule time ADP, thereby making ATP.
• To split water into H+ ions (protons) and OH- ions. As the splitting is caused by light, it is known as photolysis.

Question 15

Stages of the light-dependant stage?

Answer 15

1. Harvesting light using photosynthetic pigments in the photosystems.
2. Photolysis.
3. Photophosphorylation - generating ATP from ADP and Pi using light.
4. Formation of reduced NADP (NADPH) for use in the light independent stages.

Question 16

Steps of the light-dependant stage - light harvesting?

Answer 16

The reaction NEEDS light energy to occur.

Takes place in the thylakoid membranes of the chloroplasts.

1. Light energy is absorbed by chlorophyll and other photosynthetic pigments) in the photosystem II (we start with photosystem 2 because we discovered this second).
2. The light energy excites the electrons in the chlorophyll II, leading to their release from the molecule. This means the chlorophyll has been photo-ionised. The electrons are in a higher energy level.
3. The electrons that have left the chlorophyll need to be replaced. This is where photolysis comes in.
4. The high-energy electrons move down the electron transport chain to photosystem I.

Question 17

Steps of the light-dependant stage - photolysis?

Answer 17

Photolysis replaces the electrons that have left photosystem II.

1. Light energy splits water into protons (H+), electrons (e-) and oxygen.
2. The reaction is H2O —> 2H+ + 1/2 O2.
3. The electrons that are split from water then replaced the electrons that are leaving the chlorophyll in photosystem II.

Question 18

Steps of the light-dependant stage - photophosphorylation?

Answer 18

1. The electrons that have left the photosystem II move down the electron transport chain to PSI.
2. This energy is used to transport protons in the thylakoid so that the thylakoid has a higher concentration of H+ inside the membrane compared to outside the membrane. This is a proton gradient.
3. There is a ATP synthase enzyme in the membrane too. The H+ (protons) move from their high concentration (inside the membrane) to their low concentration (outside the membrane) through the ATP synthase.
4. The energy created from this movement allows the combination of ADP + Pi to make ATP. This occurs outside the membrane.

Question 19

Steps of the light-dependant stage - formation of NADP?

Answer 19

The last stage goes backwards a bit. This is okay, it still makes sense Hav.

1. PSI absorbs light via the chlorophyll. This excites the electrons in the chlorophyll to an even higher energy level and so they move out of the membrane.The electrons that moves down the concentration gradient from PSII to PSI replace these electrons (photolysis does not replace these electrons).
2. The electrons (and the protons that moved through the ATP synthase earlier), are used to reduce NADP to reduced NADP (NADPH).

This all happens outside the membrane (not too important).

Question 20

What is photolysis?

Answer 20

To split water into H+ ions (protons) and OH- ions. As the splitting is caused by light, it is known as photolysis.

The equation:

2H2O —> 4H+ + 4e- + O2

Question 21

The light-independent reaction steps?

Answer 21

The products of the light-dependant reaction of photosynthesis, ATP and reduced NADP, are used to reduce glycerate 3-phosphate in this stage.

Does not require light.

A ph of 8 is optimum for this Calvin cycle. The pumping of the protons into the thylakoid during the light-dependant stage increases the ph to 8.

Takes place in the stoma of the chloroplasts. The details were worked out by Melvin Calvin.

1. Carbon dioxide from atmosphere diffused into the leaf through stomata and dissolves in water around the walls of the mesophyll cells.
2. Then the carbon dioxide diffuses through the cell-surface membrane, cytoplasm and chloroplast membranes into the stroma of the chloroplast.
3. In the stroma, the carbon dioxide reacts with the 5-carbon compound ribulose bisphosphate (RuBP), a reaction catalysed by an enzyme called ribulose bisphosphate carboxylase (also known as rubisco). This reaction causes the RuBP to be carboxylated which forms an unstable, intermediate molecule with 6 carbons.
4. The 6-carbon molecule quickly breaks into two molecules of the 3-carbon glycerate 3-phosphate (GP - glycerate-3- phosphate). STAGES 3 + 4 are carbon fixation.
5. The hydrogen atoms from reduced NADP from the light dependant reaction are used to reduce glycerate 3-phosphate to triose phosphate (TP) using energy supplied by ATP (also produced in the light-dependant stage). Therefore, for each molecule of carbon dioxide, 2 molecules of ADP are produced.
6. Two molecules of TP are needed to synthesise the hexose sugar glucose. TP can also be used to be converted to starch, sucrose, cellulose, lipids, glucose, amino acids and nuclotides.
7. The RuBP is re formed using 10 TP molecules out of every 12 TP molecules formed. 10 TP molecules reform 6 molecules of RuBP so the cycle can continue. The remaining 2 TP are used to make glucose (hexose sugar).
8. NADP goes back to the light-dependant reaction to be reduced again by accepting more protons.
9. Most triose phosphate molecules are used to regenerate ribulose bisphosphate using ATP from the light-dependant reaction.

Question 22

How is the chloroplast adapted to carrying out the light-independent reaction?

Answer 22

• The fluid of the stroma contains all the enzymes needed to carry out the light-independent reactions. Stromal fluid is membrane-bound in the chloroplast which means a chemical environment which has a high concentration of enzymes and substrates can be maintained within in - as distinct from the environment of the cytoplasm.
• The stroma fluid surrounds the grana and so the products of the light-dependant reaction in the grana can readily diffuse into the stroma.
• It contains both DNA and ribosomes so it can quickly and easily manufacture some of the proteins involved in the light-dependant reaction.

Question 23

Carbon fixation?

Answer 23

When carbon dioxide reacts with the 5-carbon compound ribulose bisphosphate (RuBP), a reaction catalysed by an enzyme called ribulose bisphosphate carboxylase (also known as rubisco). This reaction causes the RuBP to be carboxylated which forms an unstable, intermediate molecule with 6 carbons.

The 6-carbon molecule quickly breaks into two molecules of the 3-carbon glycerate 3-phosphate (GP - glycerate-3- phosphate).

STAGES 3 + 4 in light-independent reaction are carbon fixation.

Question 24

What does it take to make 1 molecule of glucose in light-independent stage?

Answer 24

With each turn of the cycle, 
- 1 molecule of CO2, 
- 3 molecules of ATP,
- 2 molecules of reduced NADP 
CAN PRODUCE 1 molecule of glucose. 

However, this assumes a constant supply of RuBP (5C).

Question 25

How is RuBP reproduced in the Calvin cycle?

Answer 25

• 10 of every 12 TP molecules produced (30 carbons) are recycled to produce 10 molecules of RuBP.
• 2 of every 12 TP molecules (6 carbons) are used to form glucose (6C).

Question 26

What does autotroph and hertrotroph mean?

Answer 26

Autotroph - an organism that makes its own food.

Photoautotroph - when an organism makes it own food specifically through photosynthesis (plants).

Hetrotroph - (humans are this). This is when an organism cannot produce its own food. They take energy from other resources.

Question 27

Difference between cyclic and non-cyclic photophosphorylation?

Answer 27

We have so far learnt about non-cyclic photophosphorylation - where the electrons in the PS1 are excited and are used to product NADPH from NADP. Then, the electrons are replaced by the PS2.

Cyclic photophosphorylation is when the electrons are not replaced by the PS2 to make NADPH. Instead, the electrons are replaced by the same electrons that have just been excited. They’re passed back to PS1 via electron carriers. This means the electrons are recycled. So the cycle continues without the need for PS2.

There is a limit to how long this can go on for because there is no NADPH being produced or any O2 being produced. It only produces small amounts of ATP.

Question 28

What products are made from TP and GP?

Answer 28

TP and GP molecules are used to make carbohydrates, lipids and amino acids.

Carbohydrates - hexose sugars (e.g. glucose) are made by jointing two triose phosphate molecules together and larger carbohydrates (e.g. sucrose, starch, cellulose) are made by joining hexose sugars together in different ways.

Lipids - these are made by using glycerol, which is synthesised from triose phosphate, and fatty acids, which are synthesised from GP.

Amino acids - some amino acids are made from GP.

Question 29

Explain how the Calvin cycle turns 6 times to produce a hexose sugar?

Answer 29

Hexose sugar - glucose.

1. Three turns of the cycle produces six molecules of TP, because two molecules of TP are made for every one CO2 molecule used.
2. Five out of the six TP molecules are used to regenerate RuBP.
3. This means that three turns of the cycle only produce one TP that’s used to make a hexose sugar. BUT a hexose sugar is a 6 carbon molecule and so two TP molecules are needed to make a hexose sugar (glucose).
4. This means the cycle must turn 6 times to produce two molecules of TP that can be used to make one hexose sugar.
5. Six turns of the cycle need 18 ATP and 12 reduced NADP from the light-dependant reaction.

Question 30

Limiting factors in photosynthesis?

Answer 30

1. High light intensity of a certain wavelength.
   - the higher the intensity of the light, the more energy that is provided.
   - only certain wavelengths of light are used for photosynthesis. The photosynthetic pigments chlorophyll a and chlorophyll b and carotene only absorb red and blue light in the sunlight. (Green light is reflected, which is why plants look green).
2. Temperature around 25 degrees (optimum for UK conditions).
   - photosynthesis involves enzymes (e.g. ATP synthase, rubisco). If the temperature falls below 10 degrees, the enzymes become inactive, but if the temperature is more than 45 degrees, they start to denature.
3. Carbon dioxide at 0.4% (optimum for UK).
   - carbon dioxide makes up 0.04% of the gases in the atmosphere.
   - increasing this to 0.4% gives a higher rate of photosynthesis, but any higher than this the stomata start to close.

Plants also need a constant supply of water - too little and photosynthesis will stop. Too much and the soil becomes waterlogged (reducing the uptake of minerals such as magnesium, which is needed to make chlorophyll a.

Question 31

Increasing yield increases plant growth?

Answer 31

A yield is basically the potential the plant has to grow (the theoretical value of how much a plant should grow is no limiting factors were present).

Agriculture growers use green houses and poly tunnels (tunnels made of polythene, under which plants can be grown).

Management techniques in greenhouses:

• CO2 is added to the air by burning amounts of propane in the CO2 generator.
• light can get in through the glass. Lamps provide light at night time.
• Glasshouses trap heat energy from the sunlight, which warms the air. Heaters and cooling systems can also be used to keep a constant optimum temperature, and air circulation systems make sure the temperature is even throughout the glasshouse.

Question 32

Practical: investigating pigments in leaves using chromatography?

Answer 32

All plants contain different photosynthetic pigments in their leaves. Each pigment absorbs a different wavelength of light, so having more than one type of pigment increases the range of wavelengths of light that a plant can absorb.

Some plants also have other pigments in their leaves, which plant other roles, e.g. protecting the leaves from excessive UV radiation. Different species of plants contain different proportions and mixtures of pigments.

You can use a thin layer chromatography (TLC) to determine what pigments are present in leaves of a plant.

Question 33

What is TLC?

Answer 33

TLC is thin layer chromatography.

TLC involves a mobile phase (where molecules can move as a liquid solvent) and a stationary phase (where molecules can’t move. In TLC, this consists of a solid (e.g glass) plate with a thin layer of gel (e.g. silica gel) on top).

A sample of pigments can be extracted from the plant and put on the TLC plate. When the plate is placed vertically in the solvent, the solvent moves upwards through the gel, carrying the dissolved pigments with it. Some pigments travel faster or further than others, which separates them out.

It is possible to identify a certain pigment by calculating its Rf value and looking it up in a database. The Rf value is the distance a substance has moved through the gel in relation to the solvent.

Question 34

How to compare TLC?

Answer 34

Wear a lab coat and eye protection.
This example is comparing shade-tolerant plants and shade-intolerant plants.

1. Grind up several leaves from the shade-tolerant plant your investigating with some anhydrous sodium sulfate, then add a few drops of propanone.
2. Transfer the liquid to a test tube, add some petroleum ether and gently shake the tube. Two distinct layers will form in the liquid - top layer is pigments mixed in with the petroleum ether.
3. Transfer some of the liquid from the top layer into a second test tube with some anhydrous sodium sulfate.
4. Draw a horizontal pencil line near the bottom of the TLC plate. Build up a single concentrated spot of the liquid from step 3 on the line by applying several drops and ensuring each one is dry before the next is added. This is the point of origin.
5. Let the point of origin dry. Put the plate (this is just the rectangle thing - kind of like chromatography paper) into a small glass container with some prepared solvent (mixture of propanone, clycohexane and petroleum ether) - just so the point of origin is just above the water line. Put a lid on the container and leave the plate to develop. As the solvent spreads up the plate, the different pigments move with it, but at different rates so they separate.
6. When the solvent has nearly reached the top, take the plate out and mark the solvent front (the furthest point the solvent has reached) with a pencil and leave the plate to dry in a well-ventilated place.
7. There should be several coloured spots on the chromatography plate between the point of origin and the solvent front. These are the separated pigments. You can calculate their Rf values and look them up in the database to find out which ones they are.
8. Repeat the process for shade-intolerant plant and investigate and compare the pigments present in their leaves.

Question 35

Rf Value equation?

Answer 35

Rf Value = distance travelled by the spot / distance traveled by the solvent (the furthest spot).

Question 36

Why are the pigments in shade-tolerant and shade-intolerant leaves different?

Answer 36

Application: Leaves adapt to light conditions in their environment by possessing a different proportion of photosynthetic pigments, which allows the plant to make the best use of the light available to it.

Example: Chloroplasts in shade-tolerant plants are adapted for photosynthesis in low light conditions, but really sensitive to higher levels of light. There plants sometimes produce dark red and purple pigments called anthocyanins, which are thought to protect their chloroplasts from brief exposure to high levels of light.

Question 37

Practical: investigating the activity of dehydrogenase in chloroplasts?

Answer 37

In PS1, during the light-dependant stage, NADP acts as an electron acceptor and is reduced. This reaction is catalysed by a dehydrogenase enzyme.

The activity of this enzyme can be investigated by adding a redox indicator dye to extracts of chloroplasts. The dye acts as an electron acceptor and gets reduced by the dehydrogenase. The dye changes colour when it gets reduced.

The dye DCPIP loses its blue colour when its reduced.

You can measure the rate of dehydrogenase activity by measuring the rate at which the dye changes colour. To do this, you need a colorimeter (measured how much light a solution absorbs when a light source is shone straight through it). A coloured solution absorbs more light than a colourless solution.

Question 38

Step by step of how to do the practical where you investigate the activity of dehydrogenase in chloroplasts?

Answer 38

Measuring the effect of light intensity on dehydrogenase activity in chloroplasts. This experiment used a bench lamp as a light source and involves placing tubes of chloroplast extract mixed with DCPIP at a range of different distances from the light source.

Light intensity should decrease with increasing distance from the lamps

1. Choose the distances your going to investigate (e.g. 15cm, 30cm…).
2. Cut a few leaves into pieces. Remove any stalks.
3. Use a pestle and mortar to grind up the leaf pieces with some chilled isolation solution (source of sucrose, potassium chloride and phosphate buffer at pH7).
4. Filter the liquid into a beaker using a funnel lined with Muslin cloth.
5. Transfer the liquid to the centrifuge tubes and centrifuge them at high speed for 10 mins. This makes the chloroplast gather at the bottom of each tube in a ‘pellet’.
6. Get rid of the liquid from the top of the tubes, leaving the pellets in the bottom.
7. Re-suspend the pelleted in fresh, chilled isolation solution. This is your chloroplast extract. Store on ice for rest of the experiment.
8. Set up the colorimeter with a red filter and zero it by using a cuvette (cuboid shaped vessel) containing the chloroplast extract and distilled water.
9. Set up a test tube rack and add a set volume of chloroplast extract to the tube and a set volume of DCPIP. Mix the contents of the tube together.
10. Immediately take a sample of the mixture from the tube and add it to a clean cuvette.
11. Place the cuvette in your colorimeter and record the absorbance. Do this every 2 mins for the next 10 mins.
12. Repeat steps 9 and 10 for each distance under investigation.

Question 39

Results of the step by step of how to do the practical where you investigate the activity of dehydrogenase in chloroplasts?

Answer 39

If dehydrogenase activity is taking place, the absorbance will decrease as the DCPIP gets reduced and loses it blue colour.

The faster the absorbance decreases, the faster the rate of dehydrogenase activity.

You can plot a graph of absorbance against time for each distance from the light source.

Compare results to determine how light internists affects the rate of the dehydrogenase enzyme.

You should think about using a control cuvette (only contains DCPIP and chilled isolation solution with NO chloroplast extract. This is a control and so the absorbance should not change.