Photosynthesis Overview (College Board AP® Biology): Study Guide

Photosynthesis & energy

• Photosynthesis allows organisms to capture and store energy for use in biological processes

• During photosynthesis light energy enables the conversion of simple, inorganic compounds into carbohydrates

  • Carbon dioxide (CO₂), water (H₂O) and light energy make carbohydrates and

    oxygen (O₂)

  • The energy captured is stored within the bonds of these new organic compounds

  • The carbohydrates produced can be used in other biological processes (e.g. celluar respiration) or stored (e.g. as starch)

• The simple equation for photosynthesis is:

The evolution of photosynthesis

• Photosynthesis first evolved in prokaryotic organisms, such as cyanobacteria

  • Prokaryotic photosynthetic pathways provided the foundation for the evolution of eukaryotic photosynthesis

Oxygenation of the atmosphere

• Evidence indicates that Earth's atmosphere changed due to photosynthesis in early cyanobacteria

  • The first life forms emerged around 4 billion years ago; at the time, there was no oxygen in the atmosphere

  • About 3.5 billion years ago, cyanobacteria became the first organisms to carry out photosynthesis, beginning the release of oxygen into the atmosphere

  • Banded Iron Formations (BIFs): Dissolved iron II ions (Fe²⁺) in ancient oceans were oxidised to iron III ions (Fe³⁺) and precipitated as iron oxides; this requires free oxygen, which was gained from photosynthesis

  • Ancient stromatolites (layered microbial mats) are abundant and are often linked to cyanobacterial communities

  • The molecular structures of oxygenic photosynthesis (e.g. two photosystems, chlorophyll) are shared by cyanobacteria and modern chloroplasts

• This evidence suggests that cyanobacterial oxygenic photosynthesis produced oxygen faster than sinks at the time (e.g., Fe²⁺) could consume it, leading to an oxygenated atmosphere

Prokaryotic photosynthesis

• Prokaryotic photosynthetic pathways were the foundation of eukaryotic photosynthesis

  • An early eukaryote engulfed a cyanobacterium that became the chloroplast

  • The evidence suggests that chloroplasts are modified cyanobacteria inside eukaryotic cells, which matches what the endosymbiotic theory proposes

• Key evidence for chloroplast evolution from a modified cyanobacterium

  • Double membrane

    • Engulfment would leave the cyanobacterium with its own inner membrane and an outer membrane from the eukaryote's engulfing vesicle

  • The features of a chloroplast are similar to those of prokaryotes:

    • Chloroplast genomes are circular, their ribosomes are 70S (not eukaryotic 80S), and they divide like bacteria

    • Photosystems I & II, thylakoid membranes, chlorophyll a closely match cyanobacteria

Chloroplasts & energy

• Chloroplasts contain specific structural features that allow organisms to capture and store light energy

• Two key structures are the stroma and thylakoids

  • The stroma is the fluid within the inner chloroplast membrane and outside the

    thylakoid and is the site of carbon fixation (Calvin cycle) reactions - the light-independent reactions of photosynthesis

  • Thylakoids are organized in stacks called grana. The light reactions of photosynthesis occur in the grana

Chlorophyll and light absorption

• The thylakoid membranes contain chlorophyll pigments organized into two photosystems, as well as electron transport proteins

• Chlorophylls are photosynthetic pigments that absorb energy from light

• Pigments are arranged in structures called photosystems, which are embedded in the internal thylakoid membranes of chloroplasts

  • Within a photosystem, different pigment molecules are positioned in funnel-like structures to absorb as much light energy as possible

• There are two different types of photosystem:

  • photosystem I (PSI), also referred to as P700

    • The chlorophyll a in this system has a maximum absorption of light at 700 nm

  • photosystem II (PSII), also referred to as P680

    • The chlorophyll a in this system has a maximum absorption of light at 680 nm

• As the chlorophylls absorb energy from light, their electrons are boosted to a higher energy level within photosystems I and II

• High energy electrons are transferred between photosystems via the electron transport chain in the thylakoid membrane

  • PSII’s excited electrons travel along carriers toward PSI

  • Water then splits (photolysis), supplying electrons to replace those lost from PSII