Wednesday, October 20, 2021

Photosynthesis Overview

Biology Index

Where are we going with this? The information on this page should increase understanding related to this standard:  Understand how photosynthesis turns light energy into chemical energy.

Article includes ideas, images, and content from Troy Smigielski (2021-10)

Photosynthesis Overview
(I'm feeling your energy already!)

Before we dig into the fine points of photosynthesis, let's back up and look at two dichotomous classifications of living creatures.

An autotroph makes its own food while a heterotroph gets its food from a different source. Examples of autotrophs include grasses, trees, bushes… plants. Heterotrophs, then are all the things that eat plants or eat things that eat plants.

So… just how do autotrophs make their own food?

Let's think about this a little more scientifically… Food is energy for living organisms. And… the Law of Conservation says that energy cannot be created or destroyed. It can change form… but… It can't just show up out of nowhere. 

If food is energy, and autotrophs make their own food (energy), what's going on? How do autotrophs make their own food?


"Photosynthesis is the process by which green plants and some other organisms use sunlight to synthesize foods from carbon dioxide and water. Photosynthesis in plants generally involves the green pigment chlorophyll and generates oxygen as a byproduct" (Source 2021-10)

  • The main goal of photosynthesis is to build sugars (most commonly glucose).
  • It uses light to create glucose and oxygen.
  • In other words, photosynthesis converts light energy into chemical energy.
Using chemical notation, the reaction for photosynthesis looks like this:

6 CO2  +   6 H2O --- Sunlight ---> C6H12O6 + 6 O2
Carbon Dioxide  +   Water                                            Glucose (Energy)    Oxygen

Photosynthesis takes place in plant organelles called chloroplasts

Chloroplasts are a type of plastid, which are just a special type of organelle.

  • Chloroplasts have saclike photosynthetic membranes called thylakoids.
  • A stack of thylakoids is called a granum.
  • The fluid-like region surrounding the thylakoid membranes is called the stroma.

All chloroplasts have pigments in them that make certain colors.

The main pigment in a chloroplast is chlorophyll. It functions to absorb light energy and is located in the thylakoids. Chlorophyll is green.

The pigments that come out in the fall and produce a yellow/orange color are called carotenoids.

The Photosynthesis Process

Photosynthesis occurs in two separate sets of smaller reactions. In other words, there are two reactions in photosynthesis. One reaction will make oxygen; the other will make a sugar.

The reactions can be looked at with regard to if they require light or not. So, we have two types of reactions in photosynthesis: 
  • light dependent (also called the light reactions) 
  • light independent (also called the Calvin cycle or dark reactions)

The light dependent reactions happen first, and they require light. The light dependent reaction is the reaction that…

Creates oxygen (O2). 

And it creates ATP.

And it creates NADPH.

NADPH is a… a… big ol' molecule. For the purposes of understanding photosynthesis, you can think of it as a molecule that can carry around electrons. Its fancy name is nicotinamide adenine dinucleotide phosphate. When it is not carrying the electron, it is written as NADP+. Since the electron has a negative charge, taking it away leave the positive (+) charge on the NADP+.  Let's stop there and get back to photosynthesis!

Light Independent Reactions require ATP and NADPH; so, Light Dependent Reactions have to happen first so that those molecules can be created.

The light independent reactions happen second, and they do not require light. These are also called the Calvin cycle or dark reactions. This is the reaction that creates the sugar, which is normally glucose. 

And it uses the ATP from the light reactions.

The photosynthesis process can be thought of as a cycle of sorts. It goes sort of like this…

The light reaction begins (by taking in the reactants light, H2O, NADP+, ADP, and P).

ATP, Oxygen, NADPH are produced (and passed to the dark reaction).

The Dark reaction begins (by taking in the reactants ATP, NADPH, and CO2).

Sugar, ADP, P, and NADP+ are produced (and go back to be reused by the light dependent reactions).

Revisiting NADPH / NADP+

Electrons are molecules that carry energy. One is attached to a hydrogen atom.
More specifically, electrons are small negatively charged parts of atoms. They can take in energy and be "excited," move from atom to atom, release energy… suffice to say that they are integral parts of chemical reactions. 

Electron carrier molecules are molecules that function like a taxi or energy shuttle that shuttles electrons (energy) around the cell.

The electron carrier molecule in photosynthesis is NADPH. It has an electron attached to it that is located on the hydrogen atom. (Taxi)

Previously, we discussed how NADPH and NADP+ take part in photosynthesis. So, what is the difference between NADP+ and NADPH? 

NADPH has an electron attached while NADP+ does not.

Do the math… If the NADP+ is missing an electron it ends up with a positive charge.

The photosynthesis reaction is, to be honest, complex. But if you focus on just the NADPH and the NADP+ you can see something. Notice how NADPH functions like a shuttle (for the electron).

In the context of photosynthesis, you can think of it like this… Electrons carry energy, which is needed in the Light Independent Reactions. 

Recall the function of photosynthesis: It converts light energy into chemical energy. So these electrons provide the energy necessary to create chemical energy.

Okay… so… NADP+ goes into the light dependent reactions and comes out as NADPH. Which has picked up an electron. Where do the electrons get their energy from?

In photosynthesis, light gives these electrons their energy. This is often referred to as “light excites the electrons”.

  •  Photosynthesis requires water, CO2, and light. 
  •  Photosynthesis makes glucose and oxygen.
  •  Photosynthesis happens in the chloroplast.
    •  Thylakoid = single flattened sac
    •  Granum = stack of thylakoids
  •  Photosynthesis has two main steps: light dependent and light independent reactions
  •  NADPH is the electron carrier in photosynthesis
    •  P stands for Photosynthesis
Go deeper…


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