Photosynthesis - Light Dependent Reactions and the Calvin Cycle

Summary

This educational video delves into photosynthesis, explaining it as the process where plants convert light energy into chemical energy. It outlines the light-dependent reactions occurring in the chloroplast's thylakoids, where water is split to produce oxygen and electrons, leading to ATP and NADPH formation. The light-independent reactions, or Calvin cycle, take place in the stroma, fixing carbon dioxide into glucose. The video highlights the role of chlorophyll in light absorption and the significance of the electron transport chain, emphasizing the interplay between photosynthesis and cellular respiration.

Takeaways

• 🌞 Photosynthesis uses light energy to convert water and carbon dioxide into carbohydrates, specifically glucose, with oxygen as a byproduct.
• 🪴 Water is absorbed through the roots, while carbon dioxide enters the leaves through stomata, and oxygen is released through the same openings.
• 🍃 The chloroplast is the organelle responsible for photosynthesis, while the mitochondria handle cellular respiration.
• 🌱 Photosynthesis and cellular respiration are opposite processes: photosynthesis builds glucose and oxygen, while respiration breaks down glucose and oxygen into carbon dioxide and water.
• 🌈 Chlorophyll, the pigment in the chloroplast, absorbs blue and red light but reflects green light, giving plants their green color.
• 🔆 Photosynthesis consists of two stages: light-dependent reactions (in the thylakoids) and light-independent reactions (Calvin cycle in the stroma).
• ⚡ Light-dependent reactions produce oxygen, ATP, and NADPH by oxidizing water and using sunlight energy.
• 🌀 The Calvin cycle converts carbon dioxide into glucose using ATP and NADPH, with NADPH being oxidized back to NADP+.
• 🔋 The products of light-dependent reactions fuel the Calvin cycle, enabling the reduction of CO2 into sugars.
• 🔗 The electron transport chain in the thylakoid membrane generates a proton gradient, driving ATP synthesis through chemiosmosis.

Q & A

What is the meaning of the word 'photosynthesis'?

Photosynthesis is derived from the Greek words 'photo' meaning light, and 'synthesis' meaning to build. It refers to the process by which plants use light energy to convert carbon dioxide and water into glucose and oxygen.

What are the net reactants and products of photosynthesis?

The net reactants in photosynthesis are six water molecules (H2O) and six carbon dioxide molecules (CO2), and the products are glucose (C6H12O6) and oxygen gas (O2).

How do plants intake water and carbon dioxide, and release oxygen during photosynthesis?

Water is taken up by the roots from the soil, while carbon dioxide enters through the stomata in the leaves. Oxygen is released back into the atmosphere through the same stomata.

What is the role of chloroplasts in photosynthesis?

Chloroplasts are the organelles within plant cells that carry out photosynthesis. They contain chlorophyll, which is essential for capturing light energy.

How are photosynthesis and cellular respiration different?

Photosynthesis converts carbon dioxide and water into glucose and oxygen using light energy, whereas cellular respiration converts glucose and oxygen back into carbon dioxide and water, releasing energy in the process.

What is the function of chlorophyll in photosynthesis?

Chlorophyll is the pigment that absorbs light energy, particularly in the blue and red parts of the spectrum, and is essential for the light-dependent reactions of photosynthesis.

Why do most plants appear green?

Most plants appear green because chlorophyll reflects green light while absorbing blue and red light, which is why the green light is seen by our eyes.

What are the two stages of photosynthesis?

The two stages of photosynthesis are the light-dependent reactions, which occur in the thylakoids and involve the conversion of light energy into chemical energy, and the light-independent reactions, also known as the Calvin cycle, which occur in the stroma and involve the fixation of carbon dioxide into glucose.

What happens during the light-dependent reactions of photosynthesis?

During the light-dependent reactions, water is oxidized to produce oxygen, and light energy is used to generate ATP and NADPH, which are energy carriers for the subsequent reactions.

What is the role of ATP and NADPH in the Calvin cycle?

ATP provides the energy required for the reduction of carbon dioxide into sugars like glucose, and NADPH provides the electrons needed for this reduction process during the Calvin cycle.

How many ATP and NADPH molecules are required to produce one molecule of glucose in the Calvin cycle?

To produce one molecule of glucose, the Calvin cycle requires 18 ATP molecules and 12 NADPH molecules.

Outlines

00:00

🌿 Introduction to Photosynthesis

This paragraph introduces photosynthesis, explaining the meaning of the term, which combines 'photo' (light) and 'synthesis' (building). It describes how plants use light energy to convert water and carbon dioxide into carbohydrates like glucose, while also producing oxygen as a byproduct. The reactants and products of photosynthesis are detailed, with water and carbon dioxide entering the plant and oxygen being released. The role of chloroplasts in carrying out photosynthesis is highlighted, contrasting them with mitochondria, which are responsible for cellular respiration. The paragraph also discusses the pigment chlorophyll, which is found in the thylakoids of chloroplasts, and its role in absorbing light energy. The process of photosynthesis is divided into two stages: light-dependent reactions and light-independent reactions (Calvin cycle), with the former occurring in the thylakoids and the latter in the stroma of the chloroplasts.

05:01

🔬 Light-Dependent Reactions

The second paragraph delves into the light-dependent reactions of photosynthesis, which take place in the thylakoid membranes of chloroplasts. It explains the role of photosystem II (PSII) in the electron transport chain, where light particles excite electrons in chlorophyll, leading to the oxidation of water into oxygen gas and the production of ATP and NADPH. The paragraph outlines the journey of electrons from PSII, through plastoquinone and the cytochrome b6f complex, to plastocyanin and finally to photosystem I (PSI). Here, electrons gain energy from another photon of light and are transferred to ferredoxin and then to NADP reductase, which reduces NADP+ to NADPH. The paragraph also describes chemiosmosis and the role of ATP synthase in producing ATP from ADP and phosphate, driven by the flow of hydrogen ions across the thylakoid membrane.

10:02

🌱 The Calvin Cycle

The third paragraph focuses on the Calvin cycle, the light-independent reactions of photosynthesis, which occur in the stroma of chloroplasts. It details the three stages of the cycle: carbon fixation, reduction, and regeneration of ribulose-1,5-bisphosphate (RuBP). The fixation of carbon dioxide by RuBP, catalyzed by the enzyme Rubisco, results in the formation of 3-phosphoglycerate (PGA). The subsequent phosphorylation of PGA to 1,3-bisphosphoglycerate (1,3-BPG) using ATP is catalyzed by PGA kinase. The reduction of 1,3-BPG to glyceraldehyde 3-phosphate (G3P) by NADPH is facilitated by the enzyme G3P dehydrogenase. The paragraph explains that one G3P molecule is used to produce sugars, while five G3P molecules are used to regenerate RuBP, maintaining the cycle. The summary emphasizes the role of ATP and NADPH in the Calvin cycle and the net conversion of three CO2 molecules into one G3P molecule.

15:08

🌳 Summary of Photosynthesis

The final paragraph summarizes the key points of photosynthesis, highlighting the light-dependent and light-independent reactions. It reiterates the role of PSII in converting water into oxygen and the production of ATP and NADPH during the light-dependent reactions. The Calvin cycle is reviewed, emphasizing the conversion of three CO2 molecules into one G3P molecule, requiring nine ATP and six NADPH molecules. To produce one glucose molecule, six CO2 molecules are needed, which results in the formation of two G3P molecules. This process requires double the amount of ATP and NADPH, totaling 18 ATP and 12 NADPH molecules. The paragraph concludes by reinforcing the understanding of the two processes that make up photosynthesis: the light-dependent reactions in the thylakoid membrane and the light-independent reactions (Calvin cycle) in the chloroplast stroma.

Keywords

💡Photosynthesis

Photosynthesis is a process used by plants, algae, and certain bacteria to convert light energy into chemical energy. In the video, it is described as the process of using light to build carbohydrates, such as glucose, from water and carbon dioxide. The script emphasizes that photosynthesis is essential for life on Earth as it is the primary source of oxygen and organic compounds.

💡Chloroplasts

Chloroplasts are the organelles within plant cells that carry out photosynthesis. The video script explains that chloroplasts contain chlorophyll, the pigment responsible for absorbing light energy, and are the site of both light-dependent and light-independent reactions of photosynthesis. They are depicted as the 'engines' of the plant cell, converting light energy into chemical energy.

💡Stomata

Stomata are tiny openings on the surface of leaves that facilitate gas exchange. As mentioned in the script, carbon dioxide enters the plant through these openings, and oxygen is released back into the atmosphere through the same. They play a crucial role in the process of photosynthesis by regulating the intake of CO2 and the release of O2.

💡Chlorophyll

Chlorophyll is the green pigment found in chloroplasts that is essential for photosynthesis. The video describes how chlorophyll absorbs blue and red light but reflects green light, which is why plants appear green. It is central to the light-dependent reactions of photosynthesis, where it captures light energy and initiates the process of converting light into chemical energy.

💡Light-dependent reactions

These reactions occur within the thylakoids of chloroplasts and are dependent on light energy. The script explains that during these reactions, water is oxidized to produce oxygen, and ATP and NADPH are generated. These products are essential for the subsequent light-independent reactions, also known as the Calvin cycle.

💡Calvin cycle

The Calvin cycle, also referred to as the light-independent reactions or dark reactions, takes place in the stroma of the chloroplasts. As described in the script, this cycle uses the ATP and NADPH produced in the light-dependent reactions to fix carbon dioxide into glucose and other sugars. It can proceed without light, hence the term 'light-independent.'

💡ATP

ATP, or adenosine triphosphate, is the primary energy currency of cells. The video script details how ATP is produced during the light-dependent reactions of photosynthesis through a process called chemiosmosis. ATP is then used to power the Calvin cycle, where it is converted back into ADP, releasing energy that drives the reduction of carbon dioxide.

💡NADPH

NADPH is an electron carrier molecule produced during the light-dependent reactions of photosynthesis. As explained in the script, NADPH is used in the Calvin cycle to reduce carbon dioxide into sugars. It donates electrons and hydrogen ions, which are crucial for the synthesis of glucose from CO2.

💡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 mentions that Rubisco facilitates the reaction between carbon dioxide and ribulose-1,5-bisphosphate (RuBP), leading to the formation of 3-phosphoglycerate, which is a key step in the cycle.

💡Electron transport chain

The electron transport chain is a series of protein complexes in the thylakoid membrane that facilitate the movement of electrons during the light-dependent reactions. As described in the script, this chain is involved in the production of ATP and the generation of NADPH. The movement of electrons through the chain is coupled with the pumping of protons, creating a gradient that drives ATP synthesis.

Highlights

Photosynthesis uses light energy to build carbohydrates like glucose.

The net equation for photosynthesis combines water and carbon dioxide to produce glucose and oxygen.

Water enters the plant through the roots, while carbon dioxide enters through stomata in the leaves.

Oxygen is released through the same stomata openings in the leaves.

Chloroplasts are the organelles that carry out photosynthesis, while mitochondria are responsible for cellular respiration.

Photosynthesis and cellular respiration are exact opposites, converting each other's products and reactants.

Chlorophyll, found in thylakoids, is the pigment that absorbs light energy for photosynthesis.

Chlorophyll absorbs blue and red light but reflects green light, which is why plants appear green.

Photosynthesis is divided into light-dependent reactions in the thylakoids and light-independent reactions in the stroma.

Light-dependent reactions produce oxygen, ATP, and NADPH, while consuming water, NADP+, ADP, and phosphate.

The Calvin cycle, or light-independent reactions, reduces carbon dioxide into sugars like glucose.

The Calvin cycle involves carbon fixation, reduction, and the regeneration of ribulose biphosphate (RuBP).

Carbon dioxide reacts with RuBP to form 3-phosphoglycerate (PGA) in the carbon fixation step.

PGA is phosphorylated by ATP to form 1,3-bisphosphoglycerate (1,3-BPG), requiring kinase enzymes.

NADPH reduces 1,3-BPG into glyceraldehyde 3-phosphate (G3P), which is used for sugar production and RuBP regeneration.

The net result of the Calvin cycle is the conversion of three CO2 molecules into one G3P molecule.

To produce one glucose molecule, six CO2 molecules are needed, resulting in two G3P molecules.

The light-dependent reactions and Calvin cycle together make up the two processes of photosynthesis.

Transcripts

in this video we're going to talk about

photosynthesis just a brief introduction

into it

now what is photosynthesis

let's think about the word photo and

synthesis photo means light

synthesis means to build something

and that's what we're doing we're using

the energy provided by light to build

something in this case carbohydrates

here we have the net equation for

photosynthesis we're combining six water

molecules with six carbon dioxide

molecules

and using light

to build

simple sugars like glucose c6h1206

and we're also going to get oxygen gas

as a product as well

now on the left side of this equation

are the reactants

and on the right side are the products

the picture on the left tells you

where these molecules

enter and leave the plant

water

enters the plant

through the roots

so the plant pulls up water and minerals

that it need

through the roots from the soil

carbon dioxide

enters the leaves of the plant through

tiny openings known as stomata

and oxygen

it leaves the plant

through those same

openings

so it's important to understand that

water and carbon dioxide they go into

the plant and the plant releases oxygen

during photosynthesis

now let's move on to the chloroplasts

the chloroplast is the organelle that

carries out photosynthesis

on the other hand the organelle that is

responsible for cellular respiration is

the mitochondria

now these two processes

they are exact opposites of each other

photosynthesis

converts carbon dioxide and water with

the help of light energy into glucose

and oxygen gas

on the other hand

cellular respiration takes glucose and

oxygen gas

and converts it back into carbon dioxide

and water releasing energy in the

process

so you need to be familiar with these

two

processes

now the pigment that is responsible for

absorbing light energy

is known as chlorophyll

and chlorophyll is found in the

thylakoids

which you can see in this picture

one stack of thylakoids is known as the

granum

if you have multiple stacks

this is called

grana

so that's the plural form of the word

granum

the lumen is the fluid inside of the

thylakoid and the stroma

is the fluid inside of the chloroplast

the chloroplast has two membranes the

inner membrane and the outer membrane

and between that you have the

intermembrane space

so make sure you understand that the

chloroplast is the organelle in which

photosynthesis is carried out

now you also need to know that

chlorophyll absorbs blue light

and it absorbs the red light however

it reflects green light this is why most

plants appear green

photosynthesis can be broken down into

two stages

the light dependent reactions

and the light independent reactions

the light dependent reactions

occur inside of the thylakoids within

the chloroplast

the light independent reactions

which is basically the calvin cycle also

known as the dark reactions

they occur

in the stroma

of the chloroplasts

now the reason why they're called light

independent or dark reactions is because

they can proceed without the assistance

of light energy

the light dependent reactions

oxidizes water

into oxygen gas

and remember oxidation involves the loss

of electrons

so whenever an oxidation reaction occurs

there has to be a reduction reaction

nad plus i said that wrong nadp plus

rather

picks up those electrons and is reduced

into nadph

now some of the energy that is

transferred by light

is used

to

make atp

from adp and phosphate

atp is produced by chemiosmosis using an

enzyme called atp synthase

so for the light dependent reactions you

need to know that the products

are

oxygen gas atp

and nadph

the reactants are water

nadp plus

adp and phosphate

now let's move on to the light

independent reactions or the calvin

cycle

the calvin cycle takes in carbon dioxide

and reduces it

into

sugars such as glucose

now because that's a reductive process

oxidation has to happen somewhere

nadph

is oxidized

back into

nadp plus

so nadph it gives up its electrons

turning into nadp plus

carbon dioxide ultimately receives those

electrons and eventually turn it into

glucose

atp

is used to power that process

as atp converts back into adp and p

it energizes the calvin cycle giving it

the energy it needs to convert co2 into

glucose

so the reactants

of the calvin cycle are

carbon dioxide

atp and nadph

the products

are

sugars such as glucose

nadp plus

adp and p

so let's begin our discussion now with

the light

dependent reactions

so what we have here is the electron

transport chain inside of the thylakoid

membrane

now the first thing that's going to

happen

is a light particle will strike

photosystem 2

and it's going to excite the electrons

in chlorophyll so chlorophyll is going

to lose its electrons

which will flow into this mobile

electron carrier called

plastoquinone

now because chlorophyll lost electrons

it needs to replenish those lost

electrons

and so it's going to take the electrons

from water

oxidizing it

into oxygen gas

so one water molecule

produces one oxygen atom

and it's going to give off two hydrogen

ions

and also two electrons

so water will ultimately lose two

electrons to photosystem two

photosystem two is also called 680

because

that's the wavelength of light that has

the highest absorption it's 680

nanometers

now plastoquinone is going to carry the

electrons

to cytochrome b6f complex

and as the electrons pass through that

complex

what's going to happen next is

it's going to pump protons

from the stroma that is from outside of

the thylakoid

into the lumen or the inner thylakoid

space

so the proton concentration in the

stroma

is going to decrease

while the proton concentration

inside the thylakoid is going to

increase

producing a concentration gradient

now the electrons will continue the

journey from cytochrome b6f

to

this particular peripheral protein

plastocyanin

this is a copper containing protein

and it's going to transfer the electrons

to photosystem one

now

from this journey the electrons have

lost some of its energy

and so what's going to happen here is

the electrons

will gain more energy

by being struck by another photon of

light in photosystem one

also known as p700 so this particular

photosystem has its maximum absorption

at a wavelength of 700 nanometers

so after the electron

is struck by a photon of light

that photon will impart its energy to

the electron the electron will get

excited

and having more energy it's going to go

to

another peripheral protein called

ferrodoxin which is an iron sulfur

protein

and that's going to carry the electrons

to nadp reductase

now

napd reductase

it's a peripheral protein but it's also

an enzyme

you can see the word ace

this enzyme is going to reduce nadp plus

so the electrons

are going to leave

and they're going to

meet up with nadp plus

once nadp plus

accepts the electrons

along with a hydrogen ion

it's going to be reduced into nadph

so this reaction also reduces

the hydrogen ion concentration inch in

the stroma

which

will favor the production of atp soon

so because there's a buildup of hydrogen

ions on the inner thylakoid space

and there's

very little hydrogen ions in the stroma

what's going to happen here now is these

hydrogen ions

due to this concentration gradient

they will begin to flow through an

enzyme called atp synthase in the

process known as chemiosmosis

as those hydrogen excuse me as those

hydrogen ions flow through that enzyme

this protein will rotate in such a way

that it's going to combine adp and

phosphate to make

atp

so that's how the electron transport

chain works

in photosynthesis

so as we can see here water is oxidized

into oxygen gas

nadp reductase

produces

the electron carrier nadph

and atp synthase is used to produce atp

so those are the major products of the

light dependent reaction and keep in

mind that photosystem 2 not photosystem

1 but photosystem two is what

converts water into oxygen gas you might

be tested on that

now let's talk about the other part of

photosynthesis that is the calvin cycle

or the light independent reactions

the calvin cycle can be broken down into

three parts

the first part is the fixation of carbon

dioxide

the second part is reduction and the

third part is the regeneration of rubp

ribulose 1 5 biphosphate

so the first thing that happens is that

carbon dioxide enters the cycle

carbon dioxide is going to react with

ribulose biphosphate

catalyzed by the enzyme rubisco

and that's going to turn into

3-phosphoglycerate represented by the

symbol pga

so that's carbon fixation

rubp

is a molecule with five carbon atoms

on carbon 1

we have a phosphate group and on carbon

5 we have another phosphate group so

that's rubp

now there's three of them and they're

going to react with three co2 molecules

now when one molecule of co2 reacts with

one molecule of rubp

we're going to get initially

a6

carbon molecule but that's going to be

broken up into two

three carbon molecules

so pga is a three carbon molecule

and it's going to have one phosphate

group

so if we keep track of a total number of

carbons

if each rubp has five carbons three of

them has 15 carbons

and then we're going to add three

carbons from the three co2 molecules

so we should have a total of 18 carbons

so thus there's six

three phosphoglycerate molecules

now

the enzyme kinase when you see that that

enzyme is used to transfer a phosphate

group

to a molecule

and so this next step requires atp

we're going to use six atp molecules

to phosphorylate three phosphoglycerate

and so now we have one three by

phosphoglycerate

so we still have a three carbon molecule

but now we have a phosphate group on

carbon one

and carbon three

so as we could see here pga kinase is an

enzyme that catalyzes the conversion

of

3-phosphoglycerate into 1-3 by

phosphoglycerate

so atp

gives up a phosphate in order to become

adp

and that phosphate is transferred to

this molecule as you can see we now have

two phosphate groups so anytime you see

a kinase enzyme it catalyzes the

transfer of a phosphate group from one

molecule to another

now in the next step

we are going to use

nadph

i put it p first

we're going to use this molecule to

reduce 1 3 by phosphoglycerate into g3p

glyceraldehyde 3-phosphate

so nadph is going to convert into nadp

plus

now we need six

molecules of nadph to do this

this reaction will be catalyzed by the

g3p dehydrogenase enzyme

this enzyme

as it suggests the words dehydrogenase

it removes hydrogen from nadph

now at this point we're going to get six

g3p molecules

one

of the six g3p molecules is going to be

used to produce sugars like glucose and

fructose and things like that

the other five g3p molecules

is used to regenerate the three

ribulose biphosphate molecules

so keep in mind the total number of

carbons here is 15.

each g3p molecule

has three carbon atoms

and it has a phosphate group on carbon

3.

so 5 times 3 gives us a total of 15

carbons which is what

we started with

so notice the net result

the calvin cycle converts three

molecules of co2

into one molecule of g3p

and that is the net result of the carbon

cycle

now let's summarize what we've just

considered

in the calvin cycle we saw that three

molecules of co2 was converted into one

molecule of g3p

and that required

the use of nine atp molecules

and

six

nadph molecules

now in order to make one molecule of

glucose

we're going to need six molecules of co2

and once that enters into the carbon

cycle that's going to produce two

molecules of g3p

which can be used to produce one

molecule of glucose

and so we need to double the numbers

that's going to require

18 molecules of atp

and 12 molecules

of nadph

we just got to double everything

and so that's a simplified review of

photosynthesis

and the two process

the two processes rather that make it up

that is the light dependent reactions

which occur

inside of the thylakoid membrane and the

light independent reactions or the

calvin cycle which occurs in the stroma

of the chloropaths so keep that in mind

and that's it for this video thanks

again for watching and don't forget to

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