Light Dependent Stage of Photosynthesis: Where everything goes

Summary

视频脚本详细解释了光合作用中依赖光阶段的电子、氢和氧等元素的去向。在叶绿体的类囊体膜上，光系统1和光系统2作为蛋白质复合体，含有叶绿素，吸收阳光能量。阳光激发光系统中的电子，使它们成为高能电子，这些电子通过一系列分子传递，最终用于合成NADPH。同时，光系统2通过分解水分子补充失去的电子，释放氧气。此外，通过电子传递过程中的能量损失，建立了跨膜的氢离子梯度，进而通过ATP合酶生成ATP。整个过程中，产生了NADPH和ATP，为光合作用的非依赖光阶段提供能量和还原力。

Takeaways

• 🌿 **光合作用分为两个阶段**：光依赖阶段和光独立阶段，视频中主要讲解光依赖阶段。
• 🏞️ **光依赖阶段发生位置**：在叶绿体的类囊体膜上。
• 🔵 **类囊体膜的结构**：含有多种蛋白质复合体和分子，被类囊体腔和基质包围。
• ☀️ **光系统的作用**：光系统1和光系统2是蛋白质复合体，含有吸收太阳能量的叶绿素。
• 🚀 **电子激发和传递**：阳光激发光系统中的电子，使其具有更高能量，并在分子间传递。
• 💧 **水的光解作用**：光系统2通过水的光解补充失去的电子。
• 🚨 **氢离子梯度的建立**：电子传递过程中，氢离子被泵入类囊体腔，形成氢离子梯度。
• 🔋 **ATP的合成**：氢离子通过ATP合酶的协助扩散，驱动ATP的合成。
• 🍀 **NADPH的生成**：高能电子与NADP+结合，形成NADPH，用于光合作用的光独立阶段。
• 🔵 **氧气的释放**：水分子的光解过程中产生氧气，作为副产品释放。
• 🔄 **光依赖阶段的输出**：产生ATP、NADPH和氧气，为光合作用的光独立阶段提供能量和还原力。

Q & A

光合作用中的光依赖阶段发生在哪里？

光合作用的光依赖阶段发生在叶绿体内的类囊体膜上。

类囊体膜被什么包围？

类囊体膜被一种称为基质的流体基质所包围。

光系统1和光系统2是什么？

光系统1和光系统2是蛋白质复合体，含有叶绿素，能够吸收阳光中的能量。

当阳光照射到光系统2时，会发生什么？

阳光照射到光系统2时，叶绿素吸收能量，激发其中的两个电子，使它们成为高能电子。

光系统2失去电子后，如何补充电子？

光系统2通过分解水分子，从水中获取电子来补充失去的电子。

电子在光系统2中激发后，它们是如何传递的？

激发后的电子会从一个分子传递到另一个分子，最终离开光系统2，进入一个叫做质体醌的较小分子。

ATP合酶是什么？

ATP合酶是一种蛋白质，当氢离子通过它进行协助扩散回到基质时，会导致ATP合酶的一部分旋转，从而帮助合成ATP。

光依赖阶段中，氢离子浓度梯度是如何形成的？

通过细胞色素b6f复合体将电子传递给质体蓝素的同时，将两个氢离子从基质泵送到类囊体腔中，形成了氢离子浓度梯度。

光依赖阶段产生了哪些能量丰富的分子？

光依赖阶段产生了ATP和NADPH这两种能量丰富的分子。

光合作用中产生的氧气来自哪里？

光合作用中产生的氧气来自于水分子的分解。

光依赖阶段产生的ATP和NADPH在光合作用的下一阶段中起什么作用？

光依赖阶段产生的ATP和NADPH在光合作用的光独立阶段中使用，用于固定二氧化碳。

为什么氢离子不能直接穿过磷脂双层？

由于磷脂双层中的脂肪酸尾部是非极性的，它们会排斥带电的粒子，如氢离子，因此氢离子不能直接穿过磷脂双层。

Outlines

00:00

🌿 光合作用光依赖阶段的电子传递

本段视频主要解释了光合作用中光依赖阶段的电子传递过程。光依赖阶段发生在叶绿体的类囊体膜上，涉及到光系统1和光系统2，这两个光系统是蛋白质复合体，含有吸收太阳光能的叶绿素。当阳光照射到光系统2时，叶绿素吸收能量使电子激发，然后这些高能电子离开光系统2，通过一系列分子传递，最终用于合成NADPH。同时，水分子被分解，提供电子以补充光系统2失去的电子。此外，还涉及到质子泵的作用，通过质子梯度产生ATP。

05:03

🔋 光依赖阶段的能量转换与ATP生成

在光合作用的光依赖阶段，除了产生NADPH外，还建立了跨膜的氢离子梯度，导致类囊体空间内氢离子浓度高，基质中氢离子浓度低。氢离子通过ATP合酶进行协助扩散回到基质，此过程中ATP合酶部分旋转，将磷酸基团加到ADP上形成ATP。通过12个水分子的光解过程，最终产生18个ATP分子。同时，还生成了12个NADPH分子和6个氧分子。视频中还解释了氢离子数量的计算，说明了在光依赖阶段结束时，只有12个氢离子被输出。

Keywords

💡光合作用

光合作用是植物、藻类和某些细菌利用光能将水和二氧化碳转化为葡萄糖和氧气的过程。在视频中，光合作用被分为两个阶段：光依赖阶段和光独立阶段，视频主要聚焦于光依赖阶段的解释。

💡光依赖阶段

光依赖阶段是光合作用的第一阶段，发生在叶绿体的类囊体膜上。在这个阶段，通过光系统1和光系统2吸收阳光能量，产生高能电子，进而产生ATP和NADPH，这两种分子在光合作用的光独立阶段中使用。

💡叶绿体

叶绿体是植物细胞中的一种细胞器，主要负责光合作用。它包含类囊体膜，是光依赖阶段发生的地方。在视频脚本中，叶绿体被描述为包含蛋白质复合体和其他分子，是光合作用中电子传递和能量转换的关键场所。

💡类囊体膜

类囊体膜是叶绿体内的一种膜结构，上面布满了光系统1和光系统2等蛋白质复合体。在光合作用的光依赖阶段，这些复合体吸收阳光并激发电子，是能量转换的主要场所。

💡光系统

光系统1和光系统2是叶绿体类囊体膜上的蛋白质复合体，含有叶绿素，能够吸收阳光中的光能。在视频中，光系统2吸收阳光能量激发电子，而光系统1则接收来自光系统2的电子，继续电子传递链。

💡电子传递链

电子传递链是一系列分子，它们能够接收并传递高能电子。在光合作用的光依赖阶段，电子从光系统2激发并通过一系列分子传递，最终到达NADP+，形成NADPH。这个过程中，电子的能量被用来泵送质子，建立质子梯度，用于ATP的合成。

💡ATP合酶

ATP合酶是一种蛋白质复合体，位于类囊体膜上。它利用质子梯度的能量来合成ATP，这是光合作用中能量转换的关键步骤。在视频中，当质子通过ATP合酶回到基质时，会驱动ATP合酶的旋转，从而合成ATP。

💡NADPH

NADPH是一种高能分子，由NADP+和电子以及一个质子反应生成。在光合作用的光依赖阶段，NADPH作为电子供体，参与光合作用的光独立阶段，帮助固定二氧化碳。

💡质子梯度

质子梯度是类囊体膜内外氢离子浓度的差异，由电子传递链中的质子泵活动产生。在视频中，质子梯度是ATP合酶合成ATP的能量来源，质子通过ATP合酶回到基质时，提供了合成ATP所需的能量。

💡氧气

氧气是光合作用的一个副产品，由光系统2中的水分子光解产生。在视频中，当光系统2激发电子时，水分子被分解，释放出氧气和电子。这些氧气随后被释放到大气中。

💡水分子光解

水分子光解是光合作用中水分子被分解成氧气和电子的过程。在光依赖阶段，光系统2吸收阳光能量，触发水分子分解，释放出的电子被用来激发电子传递链，同时产生的氧气则作为副产品释放。

Highlights

光合作用中的光依赖阶段发生在叶绿体的类囊体膜上

类囊体膜被类囊体基质（stroma）所包围

光系统1和光系统2是光依赖阶段中最重要的分子，它们是蛋白质复合体并含有叶绿素

阳光照射到光系统2时，叶绿素吸收能量激发电子

激发的电子通过一系列分子传递，最终到达光系统1

光系统2通过分解水分子来补充失去的电子

水分解过程中释放的电子和质子对光合作用至关重要

细胞色素b6/f复合体利用电子流动的能量将质子泵入类囊体空间

电子最终传递给铁硫蛋白，然后用于合成NADPH

NADPH是高能分子，用于光合作用的光独立阶段

通过建立跨膜的质子梯度，质子通过ATP合酶返回基质，驱动ATP的合成

ATP合酶的旋转驱动ADP与磷酸结合形成ATP

光依赖阶段的输出包括18个ATP分子和12个NADPH分子

同时，光合作用还会产生6个氧气分子作为副产品

光依赖阶段结束后，会生成12个氢离子

NADPH的生成涉及到从基质中借用的氢原子的回收

通过计算12个水分子的分解，可以解释光依赖阶段中氢离子的数量

视频详细解释了光合作用中电子和氢离子的流动路径

光依赖阶段的详细解释有助于理解光合作用的复杂机制

Transcripts

if you're wanting to understand where

all the electrons and hydrogens and

oxygens and everything go in the light

dependent stage of photosynthesis then

you've come to the right place because

that's what i'm going to explain in this

short video i'm assuming that if you're

watching this video you already know the

overall equation for photosynthesis you

already know it happens in two different

stages that we call the light dependent

stage and the light independent stage

but as i said in this video we're just

focusing on the light dependent stage

the light dependent stage of

photosynthesis takes place within a

chloroplast on the membranes of

thylakoids and just notice that those

thylakoids are surrounded by a fluid

matrix called stroma

that's what we're looking at here you

can see across the middle of the screen

is a thylakoid membrane at the bottom of

the screen we're looking at the inside

of a thylakoid disc at the thylakoid

space and at the top of the screen we're

looking at the stroma notice that the

thylakoid membrane has a whole lot of

proteins protein complexes and other

molecules we'll look at each of those in

turn

and notice also that there is water down

in the lower left hand corner of the

screen and three hydrogen ions in the

stroma just above the membrane it's

important to notice those because they

help you to sort of add everything

together and see where everything goes

there are three hydrogen ions there in

the stroma already to begin with

by far the most important molecules here

are photosystem 1 and photosystem 2.

both of those photosystems are a

collection of proteins they're a protein

complex and they contain chlorophyll

that pigment that absorbs the energy out

of sunlight notice that they also have

a couple of electrons as well both

photosystems have some electrons in them

two electrons in there

when sunlight hits photosystem two that

energy is absorbed by chlorophyll and

used to excite the two electrons that

are in photosystem two so those

electrons become higher energy electrons

they absorb that energy and become

excited and in that excited state you

know they have a tendency to lose energy

and go back from being excited to being

less excited and they do that by being

passed from one molecule to another they

leave photosystem two and move into a

smaller molecule called pq or

plastoquinine

but notice at this point that

photosystem 2 has lost its electrons and

remember we said photosystem 2 has to

have a couple of electrons so it needs

to replace them and where do you think

it gets those from well that's where

water comes into things because water is

a really great source of electrons each

of the hydrogens in water is sharing an

electron with oxygen so what photosystem

2 does is it breaks the hydrogens off

the oxygen

and the electrons that were being shared

by the hydrogens with the oxygen and

then released and they go into

photosystem two replacing the electrons

that it now lost to plastoquinine all

right so back to plastoquinone it passes

its electrons to another protein called

cytochrome

and cytochrome as these electrons flow

through cytochrome losing energy as they

go that energy is used by cytochrome to

pump two hydrogen ions

across the membrane into the thylakoid

space the electrons in cytochrome then

get passed to another little molecule

called plastocyanin now at this point

let's look at photosystem

1. just like photosystem 2 photosystem 1

is able to absorb sunlight because it

contains chlorophyll and just like

photosystem 2 photosystem 1 also has 2

electrons in it and when it gets hit by

sunlight the energy from the sunlight is

again used to

to to excite those two electrons and

they get passed to a molecule called

ferrodoxin

but just like photosystem 2 photosystem

1 needs to replace those lost electrons

and it does that by grabbing the

electrons from plastocyanin and sitting

there right next to it so again

photosystem 1 is right back to where it

was before ready to go again but

ferrodoxin now has a couple of extra

electrons which it passes to an enzyme

called

nadp plus reductase now nadp plus

reductase is a water-soluble enzyme

there's nadp plus reductase floating

around in the stroma but it's also

sitting in the membrane of the thylakoid

as well it's in both places

in any case once it gets a couple of

high energy electrons those electrons

because the electrons of course are

negatively charged those electrons are

used to attract two positively charged

molecules namely nadp plus

and a hydrogen ion both of which are

electrically positive so now we've got

two positively charged molecules and two

negatively charged electrons the

electrons go into nadp plus and we say

reduce it

that is

the hydrogen gets added onto it and it

becomes nadp

a high energy molecule that is then used

in the light independent stage of

photosynthesis

okay so at this stage

what we've done is set up well firstly

we've created an nadph molecule but

secondly we've set up

a hydrogen ion gradient across the

membrane you'll notice there's now a

high concentration of hydrogen ions

inside the thylakoid space and a low

concentration of hydrogen hydrogen ions

in the stroma all things being equal

those hydrogens would diffuse back into

the stroma

down their concentration gradient and

down their electrical gradient too for

that matter but because they're

positively charged they're repelled

strongly by

the fatty acid tails of those

phospholipids phospholipids have two

fatty acid tails that are non-polar and

they'll repel anything that's got an

electrical charge on it like these

hydrogens so they can't diffuse across

the phospholipid bilayer so instead

they need to diffuse back into the

stroma by facilitated diffusion through

a protein called atp

synthase and as they move through atp

synthase

they actually cause part of atp synthase

to rotate and as it rotates it

physically grabs a phosphate ion and

joins it onto adp to form atp and from

doing that we get one and a half

atp molecules now you might be saying

well how do we get one and a half atp

molecules

well of course we don't really have one

and a half atp molecules because this

process doesn't start with one water

molecule it starts with 12 water

molecules so if we multiply 1.5 atps by

12 that gives us 18 atp molecules which

is the output the atp output of the

light dependent stage and while we're

multiplying things by 12 because we've

got 12 water molecules going in and not

just one

let's think about nadph

we don't just get one nadph of course we

get 12 nadph and again that's the nadph

output of the light dependent stage of

photosynthesis

let's think about oxygen

we don't just have one oxygen atom if we

have 12

water molecules and not just one then we

can multiply that by 12 as well and that

gives us 12 oxygen atoms or six oxygen

molecules each containing two oxygen

atoms

but what about these four hydrogen ions

up here because you might know that we

don't get

48 hydrogens if we multiply 4 by 12 we'd

have 48 hydrogens but we don't have 48

hydrogens coming out of the light

dependent stage we only have 12. so how

do we account for that

well firstly remember that

nadph was made by using a hydrogen that

was already in the stroma

so let's pay that hydrogen back so that

the light dependent stage is ready to go

again okay so we'll pay that hydrogen

back and remember early on in in the

process

that protein called

cytochrome pumped two hydrogens from the

stroma down into the thylakoid space so

if we pay those back as well that leaves

us with just

one

hydrogen

and that one hydrogen now of course if

we multiply that by 12

gives us 12 hydrogen ions which is the

hydrogen output of the light dependent

stage of photosynthesis

well i hope you found that really

helpful