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Overview of Photosynthesis

Biology: IB Diploma Higher Level, topic 2.8.1

Photosynthesis

Photosynthesis is a series of reactions that use energy from the Sun to convert carbon dioxide (CO2) into carbohydrates. These reactions take place in chloroplasts of plant and algal cells.

The importance of photosynthesis

The importance of photosynthesis

  • Photosynthesis is one of the most important reactions for life.
  • The reaction converts light energy into chemical energy.
  • This chemical energy is stored in carbohydrates (e.g. glucose) and other biological molecules.
  • Chemical energy can then be transferred to all other organisms by consuming plants.
Photosynthesis equation

Photosynthesis equation

  • The equation for photosynthesis is:
  • Carbon dioxide + water → oxygen + glucose
  • 6CO2 + 6H2O → 6O2 + C6H12O6
  • The reaction requires light energy to take place.
Stages of photosynthesis

Stages of photosynthesis

  • Photosynthesis can be broken down into two main stages:
  • Stage 1: light-dependent reaction.
  • Stage 2: light-independent reaction.
Light-dependent reaction

Light-dependent reaction

  • The light-dependent reaction is the first stage of photosynthesis.
  • The reaction is initiated when light energy is absorbed by chlorophyll in the chloroplasts.
Light-independent reaction

Light-independent reaction

  • The second stage of photosynthesis does not require light energy to take place. It is also called the Calvin cycle.
  • The Calvin cycle cannot happen without the light-dependent reaction.

Chloroplasts

Chloroplasts are flattened organelles with a double membrane. The process of photosynthesis happens in chloroplasts.

Diagram of a chloroplast

Diagram of a chloroplast

Stroma

Stroma

  • The space within chloroplasts is called the stroma.
  • Starch grains in the stroma are where carbohydrates produced during photosynthesis are stored.
Thylakoids

Thylakoids

  • Thylakoids are fluid-filled sacs that are stacked up inside chloroplasts.
  • These stacks of thylakoids form structures called grana (singular = granum).
  • Grana are linked together by thylakoid membranes. The resulting structures are called lamellae (singular = lamella).
Lamellae

Lamellae

  • Thylakoid membranes (lamellae) contain photosynthetic pigments.
  • There are different kinds of photosynthetic pigments. Each pigment has evolved to absorb specific wavelengths (colours) of visible light.
  • Examples of photosynthetic pigments include:
  • Chlorophyll a.
  • Chlorophyll b.
  • Carotene.
Photosystems

Photosystems

  • Within the thylakoid membranes, photosynthetic pigments are attached to proteins. Together they are called a photosystem.
  • There are two types of photosystems used in photosynthesis:
  • Photosystem I (PSI).
  • Photosystem II (PSII).
  • Both photosystems play an important role in the light-dependent reaction.

Test yourself on Overview of Photosynthesis

  • Which of the following statements are true?

  • Where does photosynthesis take place?

  • What two molecules are produced by the light-dependent reaction?

  • Which of the following are NOT a feature of chloroplasts?

  • Which of the following are true of thylakoids?

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