Chapter: Photosynthesis and Chloroplasts
Welcome to one of the most fundamental chapters in A Level Biology! Photosynthesis powers almost all life on Earth by converting light energy into chemical energy stored in biological molecules. Don't worry if this topic feels detailed at first—we will break it down into manageable, step-by-step pieces so that every stage makes complete sense.
Cross-reference note: This chapter focuses purely on the cellular, organelle, and biochemical mechanics of photosynthesis. For how this trapped energy moves through food webs and ecosystems, see the companion chapter: Ecosystems, Productivity and Succession.
---1. Overview of Photosynthesis and ATP Hydrolysis
At its core, photosynthesis is the process by which autotrophic organisms capture light energy and use it to build complex organic molecules from simple inorganic raw materials (carbon dioxide and water).
The overall balanced equation for photosynthesis is:
\(6\text{CO}_2 + 6\text{H}_2\text{O} \rightarrow \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2\)
Energy Currency: Photophosphorylation and ATP Hydrolysis
Living cells cannot use raw solar radiation directly to drive chemical synthesis. Instead, light energy is converted during photosynthesis into chemical energy in the form of adenosine triphosphate (ATP).
The process of synthesizing ATP from adenosine diphosphate (ADP) and inorganic phosphate (\(\text{P}_i\)) using light energy is known as photophosphorylation:
\(\text{ADP} + \text{P}_i \rightarrow \text{ATP} + \text{H}_2\text{O}\)
When energy is needed to drive endergonic cellular reactions (such as fixing carbon dioxide into sugar), ATP is broken down via ATP hydrolysis:
\(\text{ATP} + \text{H}_2\text{O} \rightarrow \text{ADP} + \text{P}_i + \text{energy}\)
Key Takeaway: Photosynthesis couples the capture of light energy to the photophosphorylation of ADP into ATP, which later undergoes hydrolysis to power the synthesis of organic sugars.
---2. Chloroplast Ultrastructure and Site of Reactions
In eukaryotic plant cells, photosynthesis takes place within specialised double-membrane organelles called chloroplasts. Every part of the chloroplast's internal architecture is adapted to support a specific phase of the process.
Key Structural Features of a Chloroplast
1. Double Membrane Envelope: An inner and outer phospholipid bilayer that regulates the transport of substances in and out of the organelle.
2. Thylakoids: Flattened, fluid-filled membranous sacs. The thylakoid membranes contain photosynthetic pigments (such as chlorophylls and carotenoids), electron transport chains, and the enzyme ATP synthase. This is the exact site of the light-dependent reactions.
3. Grana (singular: granum): Stacks of thylakoid discs. Stacking increases the total membrane surface area available to trap light photons.
4. Stroma: The gel-like, enzyme-rich fluid surrounding the grana. It contains high concentrations of the enzyme RUBISCO, dissolved carbon dioxide, circular DNA, and 70S ribosomes. This is the exact site of the light-independent reactions (Calvin cycle).
5. Thylakoid Lumen: The tiny, enclosed interior space inside thylakoid discs. Because it has a very small volume, protons (\(\text{H}^+\)) pumped into it rapidly build up a steep electrochemical proton gradient.
Analogy: Think of a chloroplast like a professional bakery. The thylakoids are the solar-powered ovens producing immediate power tokens (ATP and reduced NADP), while the stroma is the large mixing floor where master chefs (enzymes like RUBISCO) assemble the raw materials into loaves of bread (sugars).
Key Takeaway: Light-dependent reactions happen on the thylakoid membranes; light-independent reactions happen in the fluid stroma.
---3. Photosynthetic Pigments, Spectra, and Chromatography
Plants capture light using chemical pigments embedded in the thylakoid membranes within clusters known as photosystems.
Absorption Spectra vs. Action Spectra
Students often mix these two terms up in exam questions, but the distinction is straightforward:
Absorption Spectrum: A graph showing the percentage or proportion of light energy absorbed by a specific photosynthetic pigment (such as chlorophyll a, chlorophyll b, or carotenoids) across different wavelengths of visible light.
Action Spectrum: A graph showing the actual rate of photosynthesis achieved by the whole plant at different wavelengths of visible light.
Exam Insight: When you compare the two graphs, they show a remarkably close match. Both display distinct peaks in the blue-violet region (\(400\text{--}450\text{ nm}\)) and the red region (\(650\text{--}680\text{ nm}\)), and a deep trough in the green region (\(500\text{--}550\text{ nm}\)). This close correlation provides clear evidence that the light absorbed by these pigments is the specific energy driving photosynthesis.
Separating Pigments: Chromatography and \(R_f\) Values
Chloroplasts contain a mixture of different pigments, each absorbing slightly different wavelengths of light. We can separate and identify them using paper or thin-layer chromatography:
1. Plant leaves are ground up with a suitable solvent (such as propanone) to extract the lipid-soluble pigments.
2. The concentrated extract is spotted repeatedly onto a pencil baseline on a chromatography strip.
3. The strip is placed into a running solvent, ensuring the solvent level is strictly below the pencil baseline.
4. As the solvent moves up the strip by capillary action, different pigments travel at different speeds depending on their solubility in the solvent and their affinity for the stationary phase.
5. When the solvent almost reaches the top, the solvent front is marked immediately with a pencil.
To identify the separated pigments reliably, we calculate their retardation factor (\(R_f\) value):
\(R_f = \frac{\text{distance travelled by pigment spot}}{\text{distance travelled by solvent front}}\)
Rule of thumb: Because the pigment can never travel further than the solvent itself, the \(R_f\) value is always a decimal number between \(0\) and \(1\), and it has no units.
Key Takeaway: Absorption spectra show light captured by individual pigments; action spectra show actual photosynthetic rates. Chromatography separates these pigments so they can be identified via their characteristic \(R_f\) values.
---4. The Light-Dependent Reactions
The light-dependent stage takes place on the thylakoid membranes and requires light energy directly. Its purpose is to produce two high-energy intermediates needed by the Calvin cycle: ATP and reduced NADP (NADPH).
Step 1: Excitation of Electrons in Chlorophyll
Photons of light hit chlorophyll molecules located in photosystems in the thylakoid membrane. The light energy is absorbed by electrons in the chlorophyll molecule, raising them to a higher energy level. This process is called photoionisation or excitation of electrons. The excited, high-energy electrons leave the chlorophyll molecule and enter an electron transport chain (ETC).
Step 2: Photolysis of Water
Because chlorophyll has lost electrons, it becomes positively charged and oxidised. It must replace these lost electrons to continue absorbing light. An enzyme associated with photosystem II splits water molecules using light energy—a reaction called photolysis:
\(2\text{H}_2\text{O} \rightarrow 4\text{H}^+ + 4\text{e}^- + \text{O}_2\)
Fate of the products of photolysis:
• Electrons (\(\text{e}^-\)): Replace the electrons lost from the photoionised chlorophyll.
• Protons (\(\text{H}^+\)): Accumulate in the thylakoid lumen, contributing to the proton gradient, and eventually reduce \(\text{NADP}\).
• Oxygen (\(\text{O}_2\)): Released as a waste byproduct, either used internally for aerobic respiration or diffused out through open stomata into the atmosphere.
Step 3: Non-Cyclic Photophosphorylation
This is the standard, primary route of electron flow:
1. Excited electrons from chlorophyll pass down a series of electron carrier proteins embedded in the thylakoid membrane in an electron transport chain.
2. As electrons pass from carrier to carrier down an energy gradient, energy is released.
3. This released energy is used to actively pump protons (\(\text{H}^+\)) from the stroma across the membrane into the thylakoid lumen.
4. A steep electrochemical proton gradient builds up across the thylakoid membrane.
5. Protons diffuse down this gradient back into the stroma through channel-linked ATP synthase enzymes. This movement drives the catalytic head of the enzyme, joining \(\text{ADP}\) and \(\text{P}_i\) to synthesize ATP (a process called chemiosmosis).
6. Meanwhile, light also excites electrons in a second photosystem. These electrons, together with protons from the stroma, are accepted by the terminal electron acceptor NADP to form reduced NADP:
\(\text{NADP}^+ + 2\text{H}^+ + 2\text{e}^- \rightarrow \text{reduced NADP}\)
Step 4: Cyclic Photophosphorylation
Sometimes, the plant needs extra ATP without producing additional reduced NADP. In cyclic photophosphorylation:
• Electrons excited from chlorophyll are passed down an electron transport chain, pumping protons to synthesize ATP via chemiosmosis.
• However, instead of being transferred to NADP, these electrons cycle back to the original chlorophyll molecule.
• Key differences: Cyclic photophosphorylation generates ATP only. It does not involve photolysis, does not generate oxygen, and does not produce reduced NADP.
Quick Summary Comparison:
• Non-Cyclic: Produces ATP, reduced NADP, and \(\text{O}_2\); involves photolysis of water.
• Cyclic: Produces ATP only; no photolysis, no \(\text{O}_2\), no reduced NADP.
Key Takeaway: Non-cyclic photophosphorylation uses light energy to split water and excite chlorophyll electrons, creating a proton gradient that drives ATP synthesis and ultimately generates reduced NADP.
---5. The Light-Independent Reactions: The Calvin Cycle
The light-independent reactions take place in the stroma of the chloroplast. Although they do not use light photons directly, they rely entirely on the continuous supply of ATP and reduced NADP generated by the light-dependent reactions.
The cycle proceeds in three central phases:
Phase 1: Carbon Fixation
Carbon dioxide (\(\text{CO}_2\)) diffuses into the stroma via the stomata. Here, one molecule of \(\text{CO}_2\) (1 carbon) combines with a 5-carbon acceptor molecule called ribulose bisphosphate (RuBP). This reaction is catalysed by the enzyme RUBISCO (ribulose bisphosphate carboxylase-oxygenase):
\(\text{CO}_2\ (1\text{C}) + \text{RuBP}\ (5\text{C}) \xrightarrow{\text{RUBISCO}} \text{unstable 6C intermediate} \rightarrow 2 \times \text{GP}\ (3\text{C})\)
The resulting 6-carbon compound is highly unstable and instantly breaks down into two molecules of a 3-carbon compound called glycerate 3-phosphate (GP).
Phase 2: Reduction of GP to GALP
Each molecule of GP is then reduced to a 3-carbon sugar called glyceraldehyde 3-phosphate (GALP). This reduction step requires two inputs supplied directly by the light-dependent stage:
• ATP: Provides the phosphate and energy (via ATP hydrolysis).
• Reduced NADP: Donates hydrogen atoms (electrons and protons) to reduce GP.
Once oxidized, \(\text{ADP}\), \(\text{P}_i\), and \(\text{NADP}\) return to the thylakoid membranes to be recharged.
Phase 3: Regeneration of RuBP
For the Calvin cycle to continue, the starting molecule RuBP must be regenerated. For every \(6\) turns of the cycle (fixing \(6\text{ CO}_2\) molecules):
• \(12\) molecules of GALP (\(12 \times 3\text{C} = 36\text{C}\)) are produced.
• \(10\) of these GALP molecules (\(10 \times 3\text{C} = 30\text{C}\)) are rearranged through a series of reactions requiring energy from ATP hydrolysis to regenerate \(6\) molecules of RuBP (\(6 \times 5\text{C} = 30\text{C}\)).
• The remaining \(2\) molecules of GALP (\(2 \times 3\text{C} = 6\text{C}\)) represent the net gain of organic carbon exported from the cycle.
Uses of the Synthesized Sugars (GALP)
GALP is the versatile building block for almost all biological molecules in the plant:
• Monosaccharides & Polysaccharides: Two molecules of GALP join to form hexose sugars (glucose, fructose). Glucose can be polymerised into starch (for energy storage) or cellulose (for structural cell walls).
• Lipids: GALP can be converted into glycerol, while GP can be converted into fatty acids; together they form triglycerides and membrane phospholipids.
• Amino Acids & Nucleic Acids: GALP, combined with inorganic mineral ions absorbed from the soil (such as nitrates and phosphates), forms amino acids (for protein synthesis) and mononucleotides (for DNA and RNA).
Mnemonic to remember the Calvin Cycle order:
Remember Green Giants Run!
RuBP \(\rightarrow\) GP \(\rightarrow\) GALP \(\rightarrow\) Regeneration of RuBP.
Key Takeaway: The Calvin cycle uses RUBISCO to fix \(\text{CO}_2\) onto RuBP to make GP. GP is reduced to GALP using ATP and reduced NADP. Most GALP regenerates RuBP, while the rest forms glucose, lipids, and amino acids.
---6. Core Practical 10: Investigating Factors Affecting Photosynthesis
In Core Practical 10, you investigate how environmental variables affect the rate of photosynthesis in a suitable aquatic plant (such as Elodea or Cabomba).
The Experimental Setup
A freshly cut sprig of pondweed is placed upside down in a dilute solution of sodium hydrogencarbonate (\(\text{NaHCO}_3\)), which provides a constant, abundant supply of dissolved carbon dioxide. The cut stem releases bubbles of oxygen gas as photosynthesis occurs.
The rate can be quantified in two ways:
1. Counting the number of bubbles produced per minute (semi-quantitative).
2. Measuring the volume of gas collected over a fixed time period using a micro-pipette or gas syringe attached to a capillary tube (more accurate and quantitative).
Investigating the Four Syllabus Variables
1. Light Intensity:
• How to vary: Move a bench lamp to predetermined distances from the plant (e.g., \(10\text{ cm}\), \(20\text{ cm}\), \(30\text{ cm}\), \(40\text{ cm}\), \(50\text{ cm}\)). Light intensity is proportional to \(\frac{1}{d^2}\) (the inverse-square law).
• Key control: Lamps emit heat. Place a transparent glass beaker of cold water between the lamp and the pondweed to act as a heat shield, keeping temperature constant.
2. Light Wavelength (Colour):
• How to vary: Wrap the experimental container in translucent optical filters of different colours (e.g., red, blue, green), keeping distance from the lamp identical.
3. Temperature:
• How to vary: Place the tube containing the pondweed into thermostatically controlled water baths set to a range of temperatures (e.g., \(15^\circ\text{C}\), \(25^\circ\text{C}\), \(35^\circ\text{C}\), \(45^\circ\text{C}\)), keeping light intensity and \(\text{NaHCO}_3\) concentration constant.
4. Carbon Dioxide Availability:
• How to vary: Dissolve different known concentrations of sodium hydrogencarbonate (e.g., \(0.1\%\), \(0.2\%\), \(0.5\%\), \(1.0\%\), \(2.0\%\)) in the surrounding water, keeping light and temperature constant.
Essential Practical Skills and Controls
• Acclimatisation: Always allow the aquatic plant at least 5 minutes to adapt to the new condition before you begin recording measurements.
• Repeats & Reliability: Carry out at least 3 replicate trials at each value of the independent variable, discard obvious anomalies, and calculate a mean rate.
• Assumptions & Limitations: The bubbles collected are assumed to be pure oxygen. However, some oxygen dissolves directly in the water, some is consumed immediately by the plant's own aerobic respiration, and some gas in the bubble contains dissolved nitrogen that diffused out of the solution.
Key Takeaway: Core Practical 10 uses aquatic plants to measure oxygen production while systematically varying light intensity, wavelength, temperature, or \(\text{CO}_2\) supply, always controlling heat with a water bath shield.
---7. Common Exam Traps to Avoid
• Confusing GP and GALP: Remember that GP comes first (formed directly when \(\text{CO}_2\) is fixed onto RuBP). GALP is the sugar formed after reduction with reduced NADP and ATP.
• "The light-independent stage occurs in the dark": While it does not directly absorb light photons, the Calvin cycle requires products of the light-dependent stage (ATP and reduced NADP). In prolonged darkness, these quickly run out, stopping the Calvin cycle completely.
• Writing NAD instead of NADP: Photosynthesis uses NADP (think P for Plant or Photosynthesis). Respiration uses NAD. Mixing them up will cost you marks.
• Stating that photolysis produces ATP: Photolysis only splits water into protons, electrons, and oxygen. The movement of protons through ATP synthase via chemiosmosis generates ATP.