Complete AQA A-Level Biology Guide to Photosynthesis

Chloroplast Structure and Light Dependent Reactions

Think of chloroplasts as tiny green factories inside plant cells where all the photosynthesis magic happens. They've got two main parts: the thylakoid membranes (where the light reactions occur) and the stroma (where sugar gets made later).

The light-dependent reaction is like the power generation stage. When sunlight hits chlorophyll in the thylakoids, it gets electrons so excited they literally jump out! Meanwhile, water molecules get split up in photolysis - this gives us oxygen (which we breathe), hydrogen ions, and more electrons to keep the system running.

Here's where it gets clever: those excited electrons travel through a chain of proteins, releasing energy as they go. This energy pumps hydrogen ions across membranes, creating pressure that drives ATP synthase - imagine a tiny turbine powered by hydrogen ions flowing through it. This process, called chemiosmosis, produces ATP and NADPH, which are like rechargeable batteries for the next stage.

Quick Tip: Remember that the light-dependent reaction needs actual sunlight and produces the "fuel" (ATP and NADPH) for making sugar later!

Light Independent Reaction (Calvin Cycle)

The Calvin cycle is where plants actually make sugar - and it doesn't even need direct sunlight! This happens in the stroma using those ATP and NADPH "batteries" from the light reactions, plus carbon dioxide from the air.

RuBisCO (probably the most important enzyme you've never heard of) grabs CO₂ and sticks it onto a molecule called RuBP. This creates two molecules of GP, which then get reduced to triose phosphate using energy from ATP and hydrogen from NADPH. Some of this triose phosphate becomes sugar, whilst the rest regenerates RuBP to keep the cycle spinning.

Think of it like a recycling factory - CO₂ goes in, sugar comes out, and the machinery (RuBP) gets rebuilt to do it all again. The whole process is temperature-sensitive because enzymes are running the show.

Remember: This cycle can happen in the dark as long as there's ATP and NADPH from the light reactions - that's why it's called light-independent!

Limiting Factors in Photosynthesis

Understanding limiting factors is crucial for predicting how fast photosynthesis occurs - it's always the factor in shortest supply that determines the rate, not the abundant ones.

Light intensity affects the light-dependent reactions directly. More light means more excited electrons and more ATP/NADPH production. However, once you've got enough light, something else becomes limiting and the rate plateaus - there's no point having loads of fuel if other parts of the system can't keep up.

Temperature only affects the Calvin cycle because it's enzyme-controlled, whilst the light reactions aren't really temperature-dependent. As temperature rises, RuBisCO works faster until it gets too hot and starts denaturing - then photosynthesis crashes.

CO₂ concentration directly impacts the Calvin cycle since carbon dioxide is a raw material. More CO₂ means RuBisCO can work faster, but eventually other factors become limiting. Interestingly, too much CO₂ can actually poison plants, so there's definitely such a thing as too much of a good thing!

Exam Tip: Look for the point where graphs level off - that's when a different factor becomes limiting, and it's a favourite exam question!