Photosynthesis: WJEC A level Biology cram notes

Lesson 1 of 17

What is photosynthesis and why are chloroplasts called transducers?

Cram notes for topic 3.2, , in WJEC A level Biology Unit 3.

uses light energy to make organic molecules from carbon dioxide and water: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂. are because they convert light energy into chemical energy.

The diagram below links the two stages.

Diagram of a chloroplast containing two boxes. The light-dependent stage on the thylakoid membranes takes in light and water and releases oxygen. An arrow labelled ATP and reduced NADP goes to the light-independent stage in the stroma, which takes in carbon dioxide and produces triose phosphate, glucose and other organic molecules. A return arrow carries ADP, inorganic phosphate and NADP back. — The two stages of photosynthesis are linked by ATP and reduced NADP. Original BioWelsh diagram.

The , on the membranes, uses light and water and releases oxygen.
It makes (energy) and (reducing power) for the .
The (), in the , uses them to turn carbon dioxide into .
, and return to the .

Lesson 2 of 17

How are leaves and chloroplasts adapted to trap light?

Most are in the , just below the transparent upper epidermis, where light is brightest. and guard cells have fewer.

Leaves are thin and flat, with a large surface area.
are long, packed at right angles to the surface, and have a large vacuole that pushes to the edge.
move towards light in dim conditions and away from it in very bright light.
Stacked membranes in give a large surface area for and .
Shade leaves may have more palisade layers and more to absorb more of the little light available.

Lesson 3 of 17

How do chlorophyll and other pigments absorb light?

, and each absorb different . ( and ) widen the of absorbed and pass energy to .

An shows how much light a absorbs at each . An shows the rate of at each .

The graph below compares the with the .

Line graph from 400 to 700 nm with coloured wavelength bands. Chlorophyll a peaks near 430 and 665 nm, chlorophyll b near 455 and 645 nm, and carotenoids between 440 and 490 nm only. All absorb little between 500 and 600 nm. The action spectrum on the right axis is high from 400 to 480 nm, falls to a minimum near 550 nm and peaks again near 675 nm. — Absorption spectra of chloroplast pigments with the action spectrum for photosynthesis. Original BioWelsh graph.

Chlorophylls absorb most strongly in blue and red light; absorb only in the blue region.
Little green and yellow light is absorbed.
The peaks at the same as the absorption peaks, so the light absorbed is the light used in .

Engelmann showed that aerobic bacteria gather next to parts of an alga lit by blue and red light, because most oxygen is released there.

Lesson 4 of 17

How are chloroplast pigments separated by chromatography?

in a leaf extract separate because they differ in solubility in the solvent and in attraction to the paper. = distance moved by ÷ distance moved by .

Specified practical

Separating chloroplast pigments by paper chromatography

Unit 3, section 2, specified practical 1 (BioWelsh entry number)

Aim To separate leaf by paper and identify them from their values.

Five panels in order: a pestle and mortar to grind leaves with sand and propanone; a capillary tube to spot the extract on a pencil origin; a stoppered boiling tube with a developing chromatogram; a pencil and ruler to mark and measure; a paper strip to calculate Rf values. — Separating chloroplast pigments at a glance. Original BioWelsh diagram.

Equipment

Leaves, sand, propanone, pestle and mortar
Leaves, sand, propanone, pestle and mortarExtract the .
Capillary tube
Capillary tubeApplies a small, concentrated spot.
Boiling tube with bung and running solvent
Boiling tube with bung and running solventSeparates the .
Pencil and ruler
Pencil and rulerMark the and front, and measure.

Safety

Flammable, harmful solventsRiskFire or inhaling vapour.ControlNo flames; fume cupboard; eye protection and gloves.

Variables

Independent variablePigment

Each forms a separate spot.

Dependent variableRf value

Distance moved by ÷ distance moved by solvent.

Controlled variablesSolvent composition, Type of paper, Temperature

Solvent composition

Same solvent mixture.

Type of paper

Same batch of paper.

Temperature

Same room.

Variables in brief

Dependent: of each .
Controlled: solvent, paper and temperature.
Comparison: known values.

Method, step by step

1)
Grind leaves with sand and propanone in a fume cupboard.
2)
Draw a pencil 20 mm from the end of the paper and build up a small, concentrated spot of extract.
Why?
Small spots give clear separation.
3)
Stand the paper in solvent below the in a stoppered tube.
Why?
Vapour-saturated air stops evaporation.
Setting up and running a paper chromatogram of leaf pigments. Original BioWelsh diagram.
Open full size
4)
Remove when the solvent is near the top, mark the front at once, and calculate values.

Results and analysis

Example data

Distances moved by from a spinach leaf extract ( 80 mm)

Spot	Colour	Distance from / mm
1	Orange-yellow	75	0.94
2	Yellow	55	0.69
3	Blue-green	48	0.60
4	Yellow-green	35	0.44

A chromatography strip with a dashed origin line near the bottom, a dashed solvent front near the top and a green spot part way up. A blue arrow shows the distance from origin to solvent front, and a black arrow shows the distance from origin to the centre of the spot. A box gives Rf = distance moved by pigment divided by distance moved by solvent. — Measuring the distances needed to calculate an Rf value. Original BioWelsh diagram.

What the results show

moved furthest ( 0.94), then (0.69), (0.60) and (0.44).

more soluble in the solvent travel further.

Worked calculations

of = 48 ÷ 80 = 0.60

Evaluation

Large spots overlap.

Effect on results values are less accurate.

ImprovementBuild up a small, concentrated spot.

The solvent front evaporates.

Effect on results values are too high.

ImprovementMark the front immediately.

Model answer: describe the method

Describe how you would separate and identify the photosynthetic in a leaf using paper .

Original practice response · 6 indicative marks

Grind leaves with sand and propanone in a fume cupboard.
Draw a pencil and apply a small, concentrated spot with a capillary tube, drying between applications.
Stand the paper in running solvent below the in a stoppered tube.
Remove when the solvent is near the top and mark the immediately.
Calculate = distance moved by ÷ distance moved by solvent and compare with known values.

What the answer needs to cover

Extraction with sand and organic solvent
Pencil
Small concentrated spot
Solvent below ; sealed tube
Front marked immediately
calculated and compared

Exam practice

Why is the origin drawn in pencil?Show answer

Graphite does not dissolve in the solvent, whereas ink would dissolve and separate into its own spots.

Pencil does not dissolve
Ink would separate and confuse the results

Lesson 5 of 17

What are photosystems and how is light harvested?

A is an of protein and molecules in the membrane that passes energy from light of a of to at the .

Energy, not electrons, is passed between antenna . At the , an electron is excited and passed to an .

The two photosystems
Photosystem	Reaction centre	Replacement electrons from
	(absorbs best at 700 nm)	, or itself in
	(absorbs best at 680 nm)	of water

Lesson 6 of 17

What happens in non-cyclic photophosphorylation?

uses both and makes and . Electrons travel one way, from water to .

The below shows the energy of the electrons.

Energy level diagram. Light excites two electrons from PSII (P680) at the bottom left up to an electron acceptor. The electrons fall through three electron carriers, where ADP and inorganic phosphate form ATP, to PSI (P700). Light excites them again up to a second, higher electron acceptor, and they pass down to NADP, forming reduced NADP. Water is split into protons, oxygen and electrons that enter PSII. — The Z scheme: the energy of electrons in non-cyclic photophosphorylation. Original BioWelsh diagram.

Light excites electrons in ; an picks them up.
Electrons lose energy along the ; the energy pumps and is made.
Light excites the electrons again at ; a second acceptor passes them to , forming .
of water replaces the electrons lost from .

Lesson 7 of 17

What do photolysis and the reduction of NADP do?

in the splits water: H₂O → 2H⁺ + 2e⁻ + ½O₂. Electrons replace those lost from , add to the gradient, and oxygen diffuses out of the leaf or is used in .

In the , + 2H⁺ + 2e⁻ → . This removes from the .

Three things keep the : the moving into the , releasing there, and removing from the .
diffuse back through , making in the ().

Lesson 8 of 17

How is cyclic photophosphorylation different?

In , from pass along and return to . is made, but no , no and no oxygen.

Cyclic and non-cyclic compared
Feature	Cyclic	Non-cyclic
	only	and
Products		, and oxygen
Electron source		Water

Lesson 9 of 17

How do isolated chloroplasts show the light-dependent stage?

Isolated in light reduce the blue dye , which becomes colourless, because electrons from the reduce instead of .

Ice-cold solution slows activity; solution stops bursting by .
A tube kept in the dark does not change colour: light is needed, and (which respire in the dark) are not responsible.
A tube with no stays blue: the cause the change.
Extra carbon dioxide has no effect, because it is only used in the .

Lesson 10 of 17

What happens in the light-independent stage?

The diagram below summarises the in the .

Cycle diagram. Carbon dioxide joins ribulose bisphosphate (RuBP, 5C), catalysed by rubisco, forming two glycerate-3-phosphate (GP, 3C). GP is reduced to two triose phosphate (TP, 3C) using two ATP and two reduced NADP, which become ADP, inorganic phosphate and NADP. TP regenerates RuBP using ATP, and some TP leaves to make glucose, sucrose, starch, cellulose, amino acids and lipids. A note says for every 6 TP made, 5 regenerate RuBP. — The Calvin cycle in the stroma. Original BioWelsh diagram.

catalyses the reaction of carbon dioxide with (5C), forming two molecules of (3C).
is to (TP) using for energy and for hydrogen.
Most TP regenerates , using ; 5 of every 6 TP are used this way.
The rest of the TP is used to make glucose and other organic molecules.

Lesson 11 of 17

What happens to GP, TP and RuBP when light or carbon dioxide changes?

The graph below shows the effect of switching off the light.

Line graph of concentration against time from 0 to 10 minutes. Before 5 minutes, GP is steady at 4, TP at 6 and RuBP at 5 arbitrary units. After the light is switched off at 5 minutes, GP rises to about 8.6, while TP falls to about 1.5 and RuBP falls to about 0.5. — Concentrations of GP, TP and RuBP in the stroma when the light is switched off at 5 minutes. Original BioWelsh graph.

rises: still combines with carbon dioxide, but cannot be without and .
TP falls: none is made from , but it is still used.
falls: it is used in but not regenerated.

If carbon dioxide concentration falls in the light, the opposite happens: rises, and and TP fall.

Lesson 12 of 17

How does a plant make other molecules from triose phosphate?

Carbohydrates: → hexose sugars → sucrose, starch and cellulose.
Lipids: from ; from via .
Amino acids: from , with the amino group made using nitrogen from .

Lesson 13 of 17

What limits the rate of photosynthesis?

A is the factor in shortest supply: increasing it increases the rate. Light intensity, carbon dioxide concentration and temperature can all be limiting.

The graph below shows how combine.

Line graph with three curves rising from zero together. The curve for 0.1% carbon dioxide at 25 °C levels off highest at about 28 arbitrary units. The curve for 0.04% carbon dioxide at 25 °C levels off at about 17. The curve for 0.04% carbon dioxide at 15 °C levels off lowest at about 11. — Rate of photosynthesis against light intensity under three conditions. Original BioWelsh graph.

At low light intensity all lines rise: light is limiting.
Each line levels off when another factor becomes limiting.
Raising temperature or carbon dioxide raises the plateau, showing which factor was limiting.

Above the optimum temperature, such as denature and the rate falls. The is the light intensity at which the rates of and are equal, so there is no net gas exchange; it is lower in shade plants.

Lesson 14 of 17

How can the rate of photosynthesis be measured?

The rate is measured as the volume of gas released by pondweed per minute, using a . Light intensity ∝ . Volume = πr² × bubble length.

Specified practical