Building It Up and Breaking It Down: Photosynthesis vs. Cellular Respiration

Photosynthesis and cellular respiration are two biochemical processes that are essential to most life on Earth. Both of these processes involve multiple complex steps and many of the same molecules—oxygen (O2), carbon dioxide (CO2), water (H2O), glucose (C6H12O6), and adenosine triphosphate (ATP).

Today, we’ll briefly go over the main steps of photosynthesis and cellular respiration. We’ll explore their similarities and differences, and we'll also discuss how they interact with each other to create an “energy cycle” in living organisms.


What is photosynthesis?

Most plants are autotrophs, meaning they make their own food. Photosynthesis is the process these plants use to synthesize sugar molecules from sunlight, water, and carbon dioxide. During photosynthesis, plants release oxygen as a waste product.

Here is the basic chemical formula for photosynthesis: 

6CO2 + 6H2O + Sunlight → C6H12O6 + 6O2

Photosynthesis has two main series of reactions, which can (but don’t have to) take place simultaneously: light-dependent reactions and light-independent reactions.


1. Light-dependent reactions

The light-dependent reactions make up the first few steps of photosynthesis. These reactions occur in the thylakoid membranes of the chloroplasts within plant cells. The goal of this series of reactions is to convert photons, or light energy (from the sun), into chemical energy. During the light-dependent reactions, the plant absorbs sunlight, breaks down water molecules, assembles the energy-storing molecules ATP and NADPH (the reduced form of nicotinamide adenine dinucleotide phosphate, or NADP), and releases oxygen as a waste product.

Photosynthesis: light-dependent reactions

Goal

Convert light energy into chemical energy

Location

Chloroplasts - thylakoid membranes

Input

Sunlight, H2O, NADP

Output

NADPH, ATP, O2 (waste product)

 

The light-dependent reactions of photosynthesis go a little something like this. Sunlight hits a chlorophyll molecule in one of the thylakoid membranes, exciting an electron, which leaves the chlorophyll molecule. Carrier proteins move this electron along the thylakoid membrane. 

Chlorophyll is a pigment—a light-capturing molecule—that absorbs light from the sun. Chlorophyll can be found in structures called thylakoid membranes, which are located inside a plant cell’s chloroplasts. See those little stacks inside the chloroplast? Those are stacks of thylakoids, called grana (sing. granum).

plant-cell

The thylakoid membranes are located within the chloroplasts of plant cells. Image from the 3D plant cell model on Visible Body’s Biology Learn Site.

The chlorophyll molecule—specifically chlorophyll a, in this case—is part of a complex called photosystem II. When the energy from sunlight excites an electron in chlorophyll a enough for it to leave and be passed on to another molecule, that departure leaves an “energy vacuum” in its wake. This vacuum is powerful enough that photosystem II splits a water molecule to restore the electron. Humans can't split water in a lab the same way plants can, so the light-dependent reactions of photosynthesis are truly remarkable and unique!

Plants primarily get water from the soil. In vascular plants, tissue called xylem brings water from the roots to the leaves (the main site of photosynthesis).

dicot-root-interior

Vascular tissues are located at the center of dicot roots. Image from the 3D dicot root model on Visible Body’s Biology Learn Site.

Water molecules are composed of two hydrogen atoms and one oxygen atom. After a water molecule is broken down, its hydrogen ions are used to create ATP. These hydrogen ions help an enzyme called ATP synthase add another phosphate group to ADP (adenosine diphosphate). 

The oxygen atom from each disassembled water molecule joins up with another to form O2 (oxygen gas), which is released as a waste product through openings in the leaves called stomata.

monocot-leaf-screenshot-stomata-2

Stomata can be found on the upper and lower surfaces of monocot leaves. Image from the 3D monocot leaf model on Visible Body’s Biology Learn Site.

The electron that has been moving along the thylakoid membrane eventually arrives at another chlorophyll-containing protein complex called photosystem I. At this point, it joins forces with another excited electron. An enzyme called NADP+ uses these electrons and a passing hydrogen ion to build the energy-carrying molecule NADPH. 

Once the light-dependent reactions are complete, energy from sunlight has successfully been converted into chemical energy, which will be used in the next series of steps in photosynthesis—the light-independent reactions—to assemble sugar molecules.


2. Light-independent reactions (aka the Calvin cycle)

The next phase of photosynthesis is a series of reactions that don’t require light energy from the sun (photons). Therefore, they’re widely referred to as light-independent reactions or the Calvin cycle. (The old term “dark” reactions can be misleading, since light-independent reactions don’t have to take place in the absence of light, or at night—they just aren’t fueled by light like the light-dependent reactions.)

Photosynthesis: light-independent reactions

Goal

Use stored chemical energy to “fix” CO2 and create a product that can be converted into glucose

Location

Chloroplasts - stroma

Input

CO2, NADPH, ATP

Output

C6H12O6, G3P, NAD

 

The goal of the light-independent reactions is to “fix” carbon from carbon dioxide into a form that can be used to build carbohydrates (sugars), such as glucose. 

An enzyme called RuBisCo combines a molecule of carbon dioxide with a molecule called ribulose biphosphate (RubP), which contains five carbon atoms. The result is a 6-carbon molecule, which is broken down into two 3-carbon molecules (3-phosphoglycerate).

With the help of ATP and NADPH, each 3-phosphoglycerate molecule gets a hydrogen atom, becoming glyceraldehyde-3-phosphate, or G3P.

Two molecules of G3P are used to make one molecule of glucose (which, if you recall, has six carbon atoms). Typically, one “instance” of the Calvin cycle uses six molecules of carbon dioxide at once, meaning that twelve G3P molecules are produced. Two of these are used to produce a molecule of glucose and the rest are recycled back into RubP, so the cycle can continue.

 

What is cellular respiration? 

Humans, like other animals, are heterotrophs. We can’t make our own food via photosynthesis, so we have to eat other organisms to gain glucose, which powers the process of cellular respiration in our bodies. Cellular respiration is the process that breaks down glucose and produces ATP (a form of stored energy that cells use to carry out essential processes). 

Here is the basic chemical formula for cellular respiration:

C6H12O2 + 6O2 → 6CO2 + 6H2O + (approximately) 38 ATP

In organisms that carry out aerobic cellular respiration—that is, cellular respiration that uses oxygen—there are three main steps involved in breaking down glucose to produce ATP: glycolysis, the citric acid cycle (Krebs cycle) and the electron transport chain (ETC). We have a more detailed blog post dedicated to cellular respiration, but we’ll also quickly go over each step of aerobic cellular respiration in the following sections.


1. Glycolysis

The first phase of cellular respiration, glycolysis, is the initial breakdown of glucose into pyruvate—one molecule of glucose produces two molecules of pyruvate. On its own, glycolysis doesn’t generate very much ATP. In fact, two ATP molecules are required to begin glycolysis in the first place. What’s really important about glycolysis in aerobic respiration is that it provides the material needed for the next step of cellular respiration: the citric acid cycle, also known as the Krebs cycle.

Cellular respiration: glycolysis

Goal

Break down glucose into pyruvic acid (pyruvate)

Location

Cytoplasm of cell

Input

C6H12O6, ATP

Output

ATP, Pyruvate (C3H4O3), NADH

 

Note: Since glycolysis doesn’t require oxygen, it’s also part of anaerobic cellular respiration. You can read more about metabolism in the absence of oxygen in this chapter from OpenStax Biology (2e). 

Glycolysis takes place in the cytoplasm of animal and plant cells, whereas the subsequent steps of cellular respiration take place in the mitochondria.

animal-cell-cytoplasm

The cytoplasm is the jelly-like substance filling the inside of the cell. Image from the 3D animal cell model on Visible Body’s Biology Learn Site.


2. Citric acid cycle (Krebs cycle)

Before the citric acid cycle can begin in earnest, the pyruvate produced during glycolysis lose their carboxyl groups and combine with coenzyme A to form acetyl-CoA. The carbon molecules that are removed during this process are released as carbon dioxide.

The citric acid cycle takes place twice per molecule of glucose that was broken down in the previous step—one “turn” of the citric acid cycle occurs for each molecule of acetyl-CoA.

During each of these two turns, the molecule of acetyl-CoA goes through a series of chemical reactions. The energy from these reactions (in the form of electrons) is captured in the “energy carrier” molecules NADH and FADH2. Two more molecules of carbon dioxide and another molecule of ATP are also produced.

Cellular respiration: citric acid cycle

Goal

Capture energy from chemical reactions, produce a little bit of ATP 

Location

Mitochondria - matrix

Input

2 Acetyl-CoA

Output

ATP, NADH & FADH2 (energy carriers), CO2 (waste product)

 

3. Electron transport chain (ETC)

The electron transport chain is the part of aerobic cellular respiration that produces most of the ATP. The electron transport chain uses high-energy electrons from FADH2 and NADH to pump hydrogen ions (H+) across the inner membrane of the mitochondrion, into the outer compartment. 

mitochondria-screenshot

Mitochondria. The “membrane” label in this image refers to the outer membrane. The inner membrane is the yellow structure surrounding the matrix. Image from Visible Body’s 3D mitochondria model.

As a result of the electron transport chain, there are more positively charged ions on one side of the membrane than the other. As these ions travel back across the membrane to restore equilibrium, they pass through (and “power”) an enzyme called ATP synthase, which turns molecules of ADP into ATP by adding a third phosphate group. 

Cellular respiration: electron transport chain

Goal

Use stored energy from the citric acid cycle to power ATP synthase and generate ATP

Location

Mitochondria - inner membrane

Input

NADH, FADH2

(Note: Oxygen’s role is important here—it is the final acceptor for “spent” electrons!)

Output

Lots of ATP, H2O (waste product)

 

How are photosynthesis and cellular respiration connected? 

When I think about the connections between photosynthesis and cellular respiration, I can’t help but start singing “Circle of Life” from The Lion King in my head. Why? Because the products of photosynthesis are required for cellular respiration, and the products of cellular respiration can be used to power photosynthesis.

Putting the chemical formulas for these processes side-by-side shows this quite clearly:

Photosynthesis: 6CO2 + 6H2O + Sunlight → C6H12O6 + 6O2

Cellular Respiration: C6H12O2 + 6O26CO2 + 6H2O + (approximately) 38* ATP

*The number of ATP molecules produced can vary. 38 ATP is the theoretical maximum yield for the metabolism of one molecule of glucose.

The food that plants make (glucose) and the waste product from producing that food (O2) give animals like us the materials we need to carry out aerobic cellular respiration. We breathe in the oxygen from the air and either eat plants or other animals—either way, plants and their delicious glucose are at the root of our food web. In return, humans and other organisms that carry out aerobic respiration put the waste products from this process (mainly CO2) back into the atmosphere. 

Plants carry out both photosynthesis and cellular respiration. They make their own food, and then break down those glucose molecules later, generating ATP to power their cellular processes. 

Fun fact! Photosynthesis by microorganisms called cyanobacteria is what put oxygen into the Earth’s atmosphere in the first place. These organisms first produced oxygen between 2.7 and 2.8 billion years ago, and oxygen became a significant portion of the atmosphere by around 2.45 billion years ago. This paved the way for oxygen-breathing animals like us to evolve later.

Before we go, here’s a handy chart comparing photosynthesis and aerobic cellular respiration. Happy studying! 

 

Photosynthesis

Cellular Respiration (Aerobic)

Chemical equation

6CO2 + 6H2O → C6H12O6 + 6O2

C6H12O2 + 6O2 → 6CO2 + 6H2O + (approximately) 38 ATP

Input

Carbon dioxide, water, sunlight

Glucose, Oxygen

Steps

1. Light-dependent reactions
2. Light-independent reactions (Calvin cycle)
1. Glycolysis
2. Krebs cycle
3. Electron transport chain

Output

Glucose, oxygen

ATP, carbon dioxide, water

Associated organelle

Chloroplasts

Mitochondria

Function for the organism

Use light, water, and carbon dioxide to create food for the organism in the form of sugar (glucose)

Use glucose to make a form of energy the organism can use in cellular processes (ATP)


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