3.6 Phototsynthesis 2: The dark reactions and sugar synthesis

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From the course by Tel Aviv University

Understanding Plants - Part II: Fundamentals of Plant Biology

375 ratings

Tel Aviv University

Understanding Plants - Part II: Fundamentals of Plant Biology

375 ratings

This class is aimed at people interested in understanding the basic science of plant biology. In this four lecture series, we'll first learn about the structure-function of plants and of plant cells. Then we'll try to understand how plants grow and develop, making such complex structures as flowers. Once we know how plants grow and develop, we'll then delve into understanding photosynthesis - how plants take carbon dioxide from the air and water from soil, and turn this into oxygen for us to breathe and sugars for us to eat. In the last lecture we'll learn about the fascinating, important and controversial science behind genetic engineering in agriculture. If you haven't taken it already, you may also be interested in my other course - What A Plant Knows, which examines how plants see, smell, hear and feel their environment: https://www.coursera.org/learn/plantknows. In order to receive academic credit for this course you must successfully pass the academic exam on campus. For information on how to register for the academic exam – https://tauonline.tau.ac.il/registration Additionally, you can apply to certain degrees using the grades you received on the courses. Read more on this here – https://go.tau.ac.il/b.a/mooc-acceptance Teachers interested in teaching this course in their class rooms are invited to explore our Academic High school program here – https://tauonline.tau.ac.il/online-highschool

From the lesson

Photosynthesis

3.1 Chloroplast structure1:30

3.2 Chemical currency: ATP and NADPH4:38

3.3 Light and Photosynthesis3:25

3.4 The light-dependent reactions and electron transfer2:08

3.5 The Light-dependent Reactions and ATP2:16

3.6 Phototsynthesis 2: The dark reactions and sugar synthesis3:50

3.7 Discussion with Prof. Nir Ohad: The Carbon Cycle7:11

3.8 Inside a photosynthesis lab6:15

Meet the Instructors

Professor Daniel Chamovitz, Ph.D.
Dean, George S. Wise Faculty of Life Sciences
Director, Manna Center for Plant Biosciences

In the second stage of photosynthesis, what I call the dark reactions,

the plant utilizes ATP and NADPH from the light reactions to

capture carbon from the air and to convert it into usable sugars.

It's called the dark reactions not because it happens at night, actually,

they usually happen during the day.

But because the reactions themselves do not directly use the light,

like the reactions we learned about in the previous section.

Carbon is the basis of all organic matter on Earth, sugars,

proteins, DNA, fatty acids.

All the building blocks of life contains chains of carbon atoms.

The dark reactions take carbon dioxide from the atmosphere, and turn it into

sugar, a molecule that our bodies, and also plants knows how to utilize.

Turning atmospheric carbon dioxide into sugar takes a lot of energy, and the dark

reactions utilize the molecules made in the light reactions to power this process.

The energy carrier ATP moves energy from the light-dependent reactions in

the thylakoids to the dark reactions which occur in the stroma of the chloroplasts.

ATP fuels the assembly of sugar molecules.

Let's see what needs to happen to convert carbon dioxide into sugar.

This process is called carbon fixation.

In the series of chemical reactions called the Calvin cycle, a single carbon atom

from CO2 is first incorporated into a three carbon molecule.

And from there,

it becomes a six carbon sugar molecule, which we all know as glucose.

The combining of carbon dioxide into long carbon chains,

there is not a spontaneous process.

It needs a lot of energy.

This energy comes from the ATP.

During the dark reactions, 18 molecules of ATP transfer their energy and

are converted back to ADP to make one molecule of glucose.

The ADP is then used again in the light reactions to renew the ATP.

A big difference between carbon dioxide and sugar is that carbon dioxide has

no hydrogen atoms in it while sugars contain multiple hydrogens.

The source of these hydrogens is the NADPH.

In the process of converting carbon dioxide to sugar,

protons from NADPH are transferred to the sugar,

leaving NADP+, which is then ready to be recharged by the light reactions.

All in all, 12 molecules of NADPH are needed to make one molecule of glucose.

In a nutshell, photosynthesis is summarized in the following slide.

In the light reactions, light powers an electric current which results in one,

the production of NADPH.

Two, a proton gradient which powers the formation of ATP.

And three, the splitting of water into protons, electrons and oxygen.

The dark reactions utilize the ATP and

NADPH to fix atmospheric carbon dioxide into sugars.

A waste product of this process is ADP and

NADP+ which is then recycled through the light reactions.

The cycling of ATP to ADP and back to ATP, and

NADPH to NAD+ back to NADPH links the dark and the light reactions.

Sugar can be further converted to other molecules that build plant cells

such as carbohydrates, proteins and lipids.

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