Photosynthesis | HHMI BioInteractive Video

Summary

Photosynthesis, the process by which plants, algae, and some bacteria convert sunlight into chemical energy, is the cornerstone of life on Earth. This process involves the transformation of light energy into chemical energy through two main reactions: the light reactions and the Calvin cycle. The light reactions occur in the thylakoid membranes, where light energy is converted into ATP and NADPH, while releasing oxygen as a byproduct. The Calvin cycle takes place in the stroma, using ATP and NADPH to fix carbon dioxide into organic molecules like G3P, which can be used for growth and energy. This process not only nourishes the plant itself but also supports the entire living world, producing the oxygen we breathe and the organic compounds that form the basis of the food chain.

Takeaways

• 🌿 Photosynthesis is the process by which plants, algae, and some bacteria convert light energy into chemical energy.
• 💧 Water is the initial electron donor in photosynthesis, and carbon dioxide is the final electron acceptor.
• 🍃 The products of photosynthesis are carbohydrates, such as the three-carbon sugar G3P, and oxygen.
• 🌱 Photosynthesis occurs in chloroplasts, which contain the organelles where light reactions and the Calvin cycle take place.
• 🌱🔍 A leaf's mesophyll cells contain numerous chloroplasts, which are responsible for photosynthesis.
• 🌬️ Carbon dioxide enters and oxygen exits the leaf through stomata, small pores on the leaf's outer layer.
• 🔵 The light reactions in the thylakoid membranes produce ATP and NADPH by transferring energy from light via an electron transport chain.
• 🔵🔄 The Calvin cycle in the stroma uses ATP and NADPH to fix carbon dioxide into organic molecules like G3P.
• 🔄 The Calvin cycle consists of three phases: carbon fixation, reduction, and regeneration of RuBP.
• 🌐 Globally, photosynthesis is responsible for producing a significant amount of carbohydrates and oxygen, essential for life on Earth.

Q & A

What is the primary source of energy for life on Earth?

The primary source of energy for life on Earth is the sun, which is captured by plants, algae, and some bacteria through a process called photosynthesis.

How do organisms like plants and algae convert light energy into chemical energy?

Organisms convert light energy into chemical energy through a series of reactions known as photosynthesis, where they produce carbohydrates from water and carbon dioxide and release oxygen.

What is the role of photosynthesis in the life of plants?

Photosynthesis is crucial for plant life as it nourishes them by providing the energy and building blocks needed for growth and development.

Which molecules are involved in the electron transfer during photosynthesis?

Water is the initial electron donor, and carbon dioxide is the ultimate electron acceptor. The process involves the transfer of electrons from water to carbon dioxide.

What is the significance of the byproduct of photosynthesis?

The byproduct of photosynthesis, oxygen, is significant as it is released into the atmosphere and is essential for the respiration of most living organisms.

Where in a leaf does photosynthesis primarily occur?

Photosynthesis primarily occurs in the chloroplasts, which are found in large numbers in the mesophyll cells of a leaf.

How does carbon dioxide enter a leaf and where does it go?

Carbon dioxide enters a leaf through small pores called stomata and is then used in the chloroplasts for the process of photosynthesis.

What are the two sets of chemical reactions that make up photosynthesis?

The two sets of chemical reactions that make up photosynthesis are the light reactions and the Calvin cycle.

What are the thylakoids and what is their role in photosynthesis?

Thylakoids are membrane-lined discs within chloroplasts where the light reactions of photosynthesis take place, converting light energy into chemical energy.

How are ATP and NADPH molecules formed during the light reactions?

ATP and NADPH molecules are formed during the light reactions when light energy is absorbed by photosystems, which drives the formation of these energy-rich molecules.

What is the Calvin cycle and where does it occur?

The Calvin cycle is a set of chemical reactions that use the chemical energy of ATP and NADPH to fix carbon dioxide into organic molecules. It occurs in the stroma of the chloroplasts.

How does the plant use the organic molecules produced during photosynthesis?

Plants use the organic molecules produced during photosynthesis for growth, to fuel their lives, and to transport to other cells via the vascular system.

Outlines

00:00

🌿 Photosynthesis: The Foundation of Life

This paragraph introduces the concept of photosynthesis, a process where light energy is converted into chemical energy by plants, algae, and certain bacteria. It explains how these organisms use light to produce carbohydrates from water and carbon dioxide, releasing oxygen as a byproduct. The paragraph also delves into the cellular structures involved in photosynthesis, such as chloroplasts, and the entry and exit points for gases like carbon dioxide and oxygen through stomata. The two sets of chemical reactions that make up photosynthesis, the light reactions and the Calvin cycle, are mentioned, along with their locations within the chloroplasts.

05:04

🔬 The Light Reactions: Converting Light into Chemical Energy

This paragraph focuses on the light reactions that occur in the thylakoid membranes of chloroplasts. It describes how light energy is absorbed by chlorophyll in photosystem II, exciting electrons and initiating a series of electron transfers. The process involves the splitting of water molecules to replace lost electrons, resulting in the release of oxygen. It also explains how a proton gradient is established across the thylakoid membrane, which is used by ATP synthase to produce ATP from ADP and inorganic phosphate. The paragraph details the role of photosystems I and II, the cytochrome complex, and the formation of NADPH, both of which store energy derived from light.

10:05

🌱 The Calvin Cycle: Carbon Fixation and Energy Storage

The final paragraph discusses the Calvin cycle, which takes place in the stroma of chloroplasts. It outlines the three phases of the cycle: fixation, where carbon dioxide is incorporated into organic molecules; reduction, where these molecules are energized with electrons from ATP and NADPH; and regeneration, where the cycle is reset with the production of RuBP. The paragraph highlights the role of rubisco, the enzyme that catalyzes the initial reaction, and how G3P, the end product, can be used to create other organic molecules like sucrose and starch. It concludes by emphasizing the global impact of photosynthesis, producing vast amounts of carbohydrates and oxygen, which are essential for life on Earth.

Keywords

💡Photosynthesis

Photosynthesis is the process by which green plants, algae, and some bacteria convert light energy into chemical energy in the form of carbohydrates. It is central to the video's theme as it is the primary process that sustains life on Earth. In the script, it is described as a set of chemical reactions involving the conversion of light energy to chemical energy, with a focus on the role of chlorophyll and the production of oxygen as a byproduct.

💡Chloroplasts

Chloroplasts are the organelles within plant cells that carry out photosynthesis. They are highlighted in the script as the 'center of action' for photosynthesis, containing pigments that give plant cells their green color. The script describes how chloroplasts contain structures called thylakoids, which are crucial for the light reactions of photosynthesis.

💡Light Reactions

The light reactions are a part of photosynthesis that occur in the thylakoid membranes of chloroplasts. They involve the conversion of light energy into chemical energy, which is stored in the form of ATP and NADPH. The script explains how light energy excites electrons, leading to the formation of ATP and NADPH, and the release of oxygen as a byproduct.

💡Calvin Cycle

The Calvin cycle is the second set of chemical reactions in photosynthesis that takes place in the stroma of chloroplasts. It uses the energy from ATP and NADPH, produced in the light reactions, to fix carbon dioxide into organic molecules like G3P. The script details the three phases of the Calvin cycle: fixation, reduction, and regeneration.

💡Electron Transport Chain

The electron transport chain is a series of protein complexes in the thylakoid membrane that transfers electrons from one protein to another, releasing energy to synthesize ATP. The script describes how electrons excited by light are passed along this chain, culminating in the production of ATP.

💡ATP

ATP, or adenosine triphosphate, is a molecule that stores and transfers chemical energy within cells. In the context of the video, ATP is produced during the light reactions of photosynthesis and is used as an energy source in the Calvin cycle. The script explains how ATP is formed from ADP and inorganic phosphate, powered by a proton gradient.

💡NADPH

NADPH is a molecule that carries electrons and energy and is produced during the light reactions of photosynthesis. It is used in the Calvin cycle to power the reduction of 3-PGA to G3P. The script describes NADPH as a molecule that stores energy from light and is essential for the synthesis of carbohydrates.

💡Stomata

Stomata are small pores on the surface of leaves that facilitate gas exchange. They allow carbon dioxide to enter the leaf for photosynthesis and release oxygen as a byproduct. The script mentions stomata as the entry and exit points for gases involved in photosynthesis.

💡Carbohydrates

Carbohydrates are organic molecules produced during photosynthesis, primarily used for energy and as building blocks for other organic molecules. The script explains how plants use carbohydrates for growth and as an energy source, with examples including the three-carbon sugar G3P.

💡Rubisco

Rubisco, or ribulose-1,5-bisphosphate carboxylase/oxygenase, is the enzyme that catalyzes the first step of carbon fixation in the Calvin cycle. The script describes rubisco's role in combining carbon dioxide with RuBP to initiate the cycle of reactions that produce G3P.

💡Vascular Bundles

Vascular bundles are structures within plants that transport nutrients and other molecules. In the script, they are mentioned as the 'plumbing system' that carries organic molecules, such as sugars, from the leaves to other parts of the plant for energy and growth.

Highlights

Almost all life on Earth is solar-powered, relying on photosynthesis to convert light energy into chemical energy.

Photosynthesis involves capturing light energy to produce carbohydrates and releasing oxygen as a byproduct.

Chloroplasts in plant cells are the organelles where photosynthesis occurs, containing pigments that absorb light.

Photosynthesis consists of two main stages: the light reactions and the Calvin cycle, occurring in different parts of the chloroplast.

In the light reactions, light energy is transformed into chemical energy, producing ATP and NADPH.

Water molecules are split during the light reactions, releasing oxygen and providing electrons for the electron transport chain.

The Calvin cycle uses ATP and NADPH to fix carbon dioxide into organic molecules like G3P, which can form sugars and other organic compounds.

Photosystems I and II in the thylakoid membranes absorb light and transfer electrons through the electron transport chain.

The movement of electrons creates a proton gradient across the thylakoid membrane, which drives ATP synthesis via ATP synthase.

Carbon fixation is the first phase of the Calvin cycle, where carbon dioxide combines with RuBP to form 3-PGA.

The reduction phase of the Calvin cycle converts 3-PGA into G3P using ATP and NADPH from the light reactions.

Regeneration of RuBP in the Calvin cycle allows the cycle to continue, enabling continuous carbon fixation.

The ultimate electron acceptor of photosynthesis is carbon dioxide, which helps form carbohydrates like G3P.

Photosynthesis produces vital organic molecules that fuel plant growth and contribute to the global oxygen supply.

Globally, photosynthesis generates around 150 billion metric tons of carbohydrates annually, crucial for life on Earth.

Transcripts

Almost all life on Earth is solar-powered.

Plants, algae, and some bacteria capture light energy

from the sun and convert it to chemical energy

in a series of reactions called photosynthesis.

These organisms produce carbohydrates

from simple building blocks like water and carbon dioxide

from the environment, and in the process they release oxygen.

Photosynthesis nourishes almost the entire living world.

Photosynthesis is a set of chemical reactions

in which light energy is converted to chemical energy.

Light energy enables the movement

of electrons from molecules that donate electrons

to molecules that accept electrons, but which molecules?

Water is the first electron donor.

The carbon in carbon dioxide is the ultimate electron acceptor.

Carbon dioxide combines with other molecules

to form carbohydrates, such as a three-carbon sugar called G3P.

Carbohydrates are used to make other organic molecules

that plants use to grow and as a source of energy

to fuel their lives.

An important byproduct of photosynthesis is oxygen.

We are going to zoom in on a cross section of a leaf

to have a closer look at the center of action

for photosynthesis.

A leaf has many different types of cells,

such as mesophyll cells, epidermal cells, and vascular

bundles.

Most cells in the middle of a leaf

contain large numbers of chloroplasts.

Pigments in chloroplasts make these cells green.

Chloroplasts are the organelles where photosynthesis occurs.

Carbon dioxide from the air enters a leaf

through small pores, called stomata,

on the outer cell layer.

Oxygen formed during photosynthesis

also exits the plant through the stomata.

The plant transports organic molecules

produced in its leaf cells to other cells via the plumbing

system found in vascular bundles.

Lets take a closer look at a chloroplast,

the organelle where photosynthesis takes place.

Photosynthesis consists of two sets of chemical reactions:

the light reactions and the Calvin cycle.

They occur in different regions of the chloroplasts.

Chloroplasts contain stacks of membrane-lined discs

called thylakoids, surrounded by a watery clear fluid,

called stroma.

The light reactions are carried out

by molecules in the thylakoid membranes and the Calvin cycle

reactions by molecules in the stroma.

Lets explore these regions and their functions in more detail.

In the thylakoid membranes, the light reactions

transform light energy to chemical energy.

Light energy drives the formation of ATP molecules from

ADP and of NADPH molecules from NADP+ and electrons.

Along the way, water molecules are split and oxygen is formed,

which can be released into the atmosphere.

In the stroma, the Calvin cycle reactions

use the chemical energy of ATP and NADPH

to combine carbon dioxide from the air with organic molecules

to form new molecules, like the sugar G3P.

ADP and NADP+ are recycled and may be used again in the light

reactions.

A plant increases its biomass through the formation

of these new organic molecules.

The thylakoid membranes contain specialized molecules

that work together to perform the light reactions.

Light is absorbed by protein-pigment complexes

called photosystems.

There are two photosystems: photosystem I and photosystem II.

The photosystems transform light energy to chemical energy

by exciting and then shuttling electrons from molecule

to molecule in a chainlike fashion

on the thylakoid membrane.

This process is called an electron transport chain.

Lets take a closer look to see how this process works.

First, photons of light hit chlorophyll,

a light-absorbing pigment in photosystem II.

Electrons in the chlorophyll are excited to a higher energy

level.

The excited electrons are passed to an electron carrier.

Meanwhile, water splits and releases electrons.

These electrons replace those lost at photosystem II.

The byproduct of this reaction is oxygen, which is eventually

released into the air.

The other products are protons, or hydrogen ions,

which are released into the inside of the thylakoid,

or lumen.

The excited electrons move to the cytochrome complex.

Some of the energy from the electrons

is used by the cytochrome complex

to transport additional protons into the lumen.

The second electron carrier, a protein inside the lumen,

receives the electrons and passes them to photosystem I.

These electrons have now lost most

of the energy they gained from light in photosystem II.

Photons of light hit chlorophyll in photosystem I

and excite the electrons again.

The electrons are then passed to the third electron carrier.

Finally, these electrons are either recycled or they

interact with an enzyme and NADP+,

the final electron acceptor of the light reactions,

to form NADPH.

Some of the energy from light is now

stored in the reduced molecule NADPH.

Some of the energy released from the transfer of electrons

established a proton gradient across the thylakoid membrane.

Protons that accumulated in the lumen diffuse into the stroma

through an enzyme called ATP synthase.

ATP synthase uses the potential energy of the proton gradient

to combine ADP with inorganic phosphate to form ATP.

In this way, the potential energy

is transformed into chemical energy stored as ATP.

ATP and NADPH now have stored energy

from the light reactions.

This energy can be used in the Calvin cycle.

This light driven electron transport chain

is usually continuous in the presence of sunlight.

It encompasses a series of chemical reactions

that involve light absorption, energy conversion and electron

transfer carried out by the photosystems and other enzymes

on the membrane of the thylakoids.

The Calvin cycle takes place in the chloroplasts stroma,

the watery clear fluid surrounding the thylakoids.

Its helpful to think of the Calvin cycle in three

phases: fixation, reduction, and regeneration.

In phase one, inorganic carbon, in the form

of carbon dioxide from the air, is

incorporated into organic molecules, a process known

as carbon fixation.

Three molecules of carbon dioxide

react with three molecules of ribulose bisphosphate (RuBP)

to produce six molecules of a three-carbon molecule called

3-PGA.

The enzyme rubisco catalyzes this reaction.

In the second phase, the organic molecules

accept electrons, a process known as reduction.

The six molecules of 3-PGA use six molecules of ATP

and six molecules of NADPH which store

energy from the light reactions to

generate six molecules of G3P.

The G3P molecules contain more electrons

and are higher in potential energy than 3-PGA.

One molecule of G3P exits the cycle.

It can be used to make other organic molecules.

In phase three, the regeneration phase, a large set of reactions

use the other five molecules of G3P and energy

from three molecules of ATP to produce

three molecules of RuBP.

With RuBP reformed, the process can start again.

Notice that in the Calvin cycle, the energy from ATP and NADPH

produced in the light reactions is

used to generate one G3P molecule from three carbon

dioxide molecules.

In this process, the electrons lost from NADPH

are accepted by the carbons from carbon dioxide molecules,

which are the ultimate electron acceptors of photosynthesis.

G3P, the net product from the Calvin cycle,

can be used to generate other organic molecules,

such as sucrose or starch.

Sucrose produced by leaf cells is transported

through the vascular bundles to other parts of the plant,

like stems and roots.

Leaf cells can also sometimes form starch

for long-term energy storage.

Overall, the molecules generated by photosynthesis

provide fuel and building materials

that allow a plant to grow.

Globally, photosynthesis produces

an estimated 150 billion metric tons of carbohydrate per year

and is responsible for the oxygen in our atmosphere,

making it one of the most important chemical

processes for life on Earth.