Cell Structure
IGCSE Biology Topic 2 13:41 English narration · English + 中文 subtitles burned in
Chapters
Transcript
Zoom in on a leaf, on your skin, on a drop of pond water — and you find the same thing everywhere: cells.
放大观察一片叶子、你的皮肤、一滴池水——你到处都会发现同样的东西：细胞。
A cell is the smallest thing that can carry out all seven life processes.
细胞是能完成全部七种生命过程的最小单位。
Some organisms are a single cell.
有些生物只有一个细胞。
You are made of about thirty trillion.
而你由大约三十万亿个细胞组成。
And every one of them came from the division of a cell that already existed.
它们每一个都来自已有细胞的分裂。
Let's look inside.
让我们看看细胞内部。
Welcome to cell structure.
欢迎来到细胞结构。
Animal, plant and bacterial cells, specialised cells, levels of organisation, and magnification.
动物细胞、植物细胞和细菌细胞、特化细胞、组织层次， 还有放大倍数。
Let's begin.
让我们开始吧。
Compare an animal cell and a plant cell.
比较动物细胞和植物细胞。
Both share five structures: a cell membrane, cytoplasm, ribosomes, a nucleus, and mitochondria.
两者共有五种结构：细胞膜、细胞质、核糖体、细胞核和线粒体。
But a plant cell has three extras.
但植物细胞多出三样。
A cell wall, made of cellulose, giving it a fixed shape.
细胞壁，由纤维素构成，给它固定的形状。
A large vacuole filled with cell sap.
一个充满细胞液的大液泡。
And, in the green parts, chloroplasts.
以及，在绿色部分，叶绿体。
So the rule is simple: cell wall, big vacuole and chloroplasts mean plant.
所以规则很简单：有细胞壁、大液泡和叶绿体就是植物细胞。
An animal cell has none of those three.
动物细胞这三样都没有。
Here is a labelled animal cell — the exam drawing you must recognise.
这是一个带标注的动物细胞——考试中你必须认得出的图。
Around the outside sits the cell membrane, the thin boundary that controls what enters and leaves.
最外面是细胞膜， 这层薄薄的边界控制物质进出。
Inside is the cytoplasm, a jelly-like liquid where many chemical reactions happen.
里面是细胞质，一种胶状液体，许多化学反应在这里发生。
Near the centre is the nucleus, which holds the genetic material and controls the cell.
靠近中央的是细胞核，存放遗传物质并控制细胞。
Scattered in the cytoplasm are mitochondria, where respiration releases energy, and tiny ribosomes, where proteins are made.
细胞质里散落着线粒体， 呼吸作用在此释放能量；还有微小的核糖体，蛋白质在此制造。
Notice what is missing: no cell wall, no large central vacuole, and no chloroplasts.
注意缺少什么： 没有细胞壁、没有中央大液泡、也没有叶绿体。
That absence is how you know it is an animal cell.
正是这些缺失让你认出这是动物细胞。
Now the plant cell.
再看植物细胞。
Start with the outer cell wall, made of cellulose — it gives the cell strength and a fixed shape.
从外层细胞壁开始，它由纤维素构成——给细胞强度和固定形状。
Just inside the wall is the cell membrane.
壁内侧就是细胞膜。
A large central vacuole filled with cell sap takes up most of the space and helps keep the cell firm.
中央有一个充满细胞液的大液泡，占据大部分空间，帮助细胞保持坚挺。
Green ovals are chloroplasts, packed with chlorophyll that traps light for photosynthesis — but only in the green parts of the plant, not in roots.
绿色的卵圆形是叶绿体，含有捕捉光进行光合作用的叶绿素——但只在植物的绿色部分， 根里没有。
You still see a nucleus, cytoplasm, mitochondria and ribosomes, just as in an animal cell.
你仍能看到细胞核、细胞质、线粒体和核糖体，与动物细胞一样。
Wall, big vacuole and chloroplasts are the three plant-only features examiners look for.
细胞壁、大液泡和叶绿体，正是考官要找的三种植物专有结构。
This is real tissue under a light microscope: onion epidermis cells.
这是光学显微镜下的真实组织：洋葱表皮细胞。
Look at the brick-shaped rows — each cell has a clear cell wall, so you know these are plant cells.
注意砖块状的行列——每个细胞都有清晰的细胞壁， 所以你知道这些是植物细胞。
You can often see a nucleus as a darker spot, and the cytoplasm filling the rest of the box.
常能看到细胞核是一个较暗的点，细胞质充满其余空间。
Onion cells from the bulb are not green, so you will not see chloroplasts here — chloroplasts sit in green parts that photosynthesise.
鳞茎里的洋葱细胞不是绿色的，所以这里看不到叶绿体——叶绿体在进行光合作用的绿色部分。
When a question shows a photomicrograph, first ask: wall or no wall?
题目给出显微照片时，先问：有壁还是没壁？
That single check sorts plant from animal.
这一步就能把植物细胞和动物细胞分开。
Build the comparison table in your head.
在脑中建一张比较表。
Cell membrane — yes in animal, plant and bacterial cells.
细胞膜——动物、植物和细菌细胞都有。
Cytoplasm — yes in all three.
细胞质——三者都有。
Ribosomes — yes in all three.
核糖体——三者都有。
Nucleus — yes in animal and plant, but no in bacteria, which have a loop of D N A instead.
细胞核——动植物有，细菌没有，细菌用一环脱氧核糖核酸代替。
Mitochondria — yes in animal and plant, no in bacteria.
线粒体——动植物有，细菌没有。
Cell wall — no in animal, yes in plant, and yes in bacteria but of a different material.
细胞壁——动物没有，植物有，细菌也有但材料不同。
Chloroplasts — only plant, and only green parts.
叶绿体——只有植物，且只在绿色部分。
Large vacuole — only plant.
大液泡——只有植物。
Plasmids — only bacteria.
质粒——只有细菌。
Memorise that table; it scores easy marks.
把这张表记牢；它能轻松得分。
Now what each part does.
现在看每个部分的功能。
The cell membrane controls which substances enter and leave.
细胞膜控制哪些物质进出。
The cytoplasm is a jelly where many chemical reactions happen.
细胞质是一种胶状物， 许多化学反应在其中发生。
The nucleus controls the cell's activities and holds the genetic material.
细胞核控制细胞的活动，并储存遗传物质。
Ribosomes are where proteins are made.
核糖体是制造蛋白质的地方。
Mitochondria are where respiration happens to release energy.
线粒体是进行呼吸作用、释放能量的地方。
The cell wall gives strength and a fixed shape.
细胞壁提供强度和固定的形状。
Chloroplasts contain chlorophyll, which traps light for photosynthesis.
叶绿体含有叶绿素，能捕捉光进行光合作用。
And the large vacuole is filled with cell sap, keeping the plant cell firm.
大液泡充满细胞液，使植物细胞保持坚挺。
A bacterium is different again, and one absence drives everything else about it.
细菌又不一样，而它身上的一切都由一样东西的缺席决定。
It is a prokaryote, which means it has no nucleus.
它是原核生物，意思是它没有细胞核。
With no nucleus there is nowhere to shut the DNA away, so the DNA lies free in the cytoplasm as a single loop, with small extra rings called plasmids beside it.
既然没有细胞核， 就没有地方把DNA关起来，于是DNA以单个环的形式自由地待在细胞质里， 旁边还有叫做质粒的小环。
Notice what that does to your method.
注意这对你的判断方法意味着什么。
A plant or animal cell you identify by what is PRESENT — wall, chloroplasts, vacuole.
植物细胞或动物细胞，你靠"有什么"来认——细胞壁、叶绿体、大液泡。
A bacterium you identify by what is missing.
而细菌，你要靠"缺什么"来认。
So in the exam, go looking for the absences first, and the labelled cell coming up next will show you where everything else sits.
所以考试时，先去找那些缺席的东西， 接下来那张带标注的细胞图会告诉你其余的部分各在哪里。
Study this labelled bacterial cell carefully.
仔细看这个带标注的细菌细胞。
Outside is a cell wall, and just inside it a cell membrane.
外面是细胞壁，内侧紧贴细胞膜。
The cytoplasm fills the cell and holds ribosomes for making proteins.
细胞质充满细胞， 其中有制造蛋白质的核糖体。
There is no nucleus.
没有细胞核。
Instead you see a single loop of D N A floating free, plus small extra rings called plasmids.
取而代之的是一环自由漂浮的脱氧核糖核酸， 外加叫做质粒的小环。
There are no mitochondria and no chloroplasts.
没有线粒体，也没有叶绿体。
Plasmids matter in biotechnology because they can carry extra genes, but for this topic the exam key is simple: prokaryote means no nucleus — DNA as a loop, plus plasmids.
质粒在生物技术中很重要， 因为它们能携带额外基因，但对本专题而言考试要点很简单： 原核生物就是没有细胞核——脱氧核糖核酸成环，外加质粒。
Most cells are specialised — their shape suits one job.
大多数细胞是特化的——它们的形状适合一项工作。
Ciliated cells have tiny hairs on top that sweep mucus along the trachea.
纤毛细胞顶部有细小的毛， 把黏液沿气管扫走。
Root hair cells have a long thin hair, giving a large surface for absorbing water and minerals.
根毛细胞有细长的毛，提供大表面积来吸收水和矿物质。
Palisade mesophyll cells are packed with chloroplasts and sit near the top of the leaf, for photosynthesis.
栅栏叶肉细胞充满叶绿体，位于叶片上部，用于光合作用。
Neurones are very long, to carry electrical impulses far across the body.
神经元非常长， 能把电脉冲传到身体很远的地方。
Red blood cells have no nucleus and a dish shape, so they can carry more oxygen.
红细胞没有细胞核，呈盘状，所以能携带更多氧气。
And the gametes: sperm can swim, while the egg cell stores food.
还有配子：精子能游动，卵细胞储存养分。
Look at three specialised shapes side by side.
并排看三种特化形状。
The red blood cell is a biconcave disc with no nucleus — more room for the oxygen-carrying pigment haemoglobin.
红细胞是双凹圆盘，没有细胞核——为携带氧气的色素血红蛋白留出更多空间。
The root hair cell has that long thin extension that massively increases surface area for absorption from the soil.
根毛细胞有细长的延伸，大大增加从土壤中吸收的表面积。
The nerve cell, or neurone, is drawn out into a long fibre so electrical impulses can travel far across the body.
神经细胞即神经元被拉成细长纤维， 好让电脉冲传到身体远处。
For every specialised cell, examiners want the job and the one feature that suits it.
对每一种特化细胞，考官都要作业和适合该作业的那一个特征。
Always link shape to function in one sentence.
永远用一句话把形状和功能连起来。
Three more specialised cells complete the list.
再有三种特化细胞就补全名单了。
Ciliated cells line the trachea and bronchi; their tiny hairs, the cilia, beat to sweep mucus and trapped dust away from the lungs.
纤毛细胞衬在气管和支气管内壁；顶部的细小纤毛拍打， 把黏液和截获的灰尘扫离肺部。
Palisade mesophyll cells sit near the top of the leaf, packed with chloroplasts so they can photosynthesise efficiently in bright light.
栅栏叶肉细胞位于叶片上部，充满叶绿体， 能在强光下高效进行光合作用。
Sperm and egg cells are the sex cells — the gametes.
精子和卵细胞是性细胞——也就是配子。
Sperm has a tail so it can swim to the egg; the egg stores food for the early embryo.
精子有尾巴，能游向卵细胞；卵细胞为早期胚胎储存养分。
Write each pair as job plus feature — that is the mark scheme pattern.
把每一对写成作业加特征—— 那就是评分标准的写法。
Many-celled organisms are built up in levels, and the reason is division of labour: once there are levels, no single cell has to do everything, so cells can specialise.
多细胞生物是分层次构建的，原因是分工：一旦有了层次， 就没有哪个细胞必须包办一切，于是细胞可以特化。
A cell is the smallest unit of life, and alone it can only do what one cell can do.
细胞是生命的最小单位，单独一个细胞只能做一个细胞能做的事。
Now watch one word change as you climb.
现在注意往上爬时有一个词发生了变化。
A group of SIMILAR cells doing the same job is a tissue.
一群做同样工作的相似细胞构成组织。
But an organ is built from DIFFERENT tissues working together, an organ system from several organs, and the whole organism from all of its systems.
但器官是由不同的组织协同构成的，器官系统由若干器官构成， 而完整的生物体由它全部的系统构成。
Similar cells, then different tissues — that switch is the step students describe wrongly, and the diagram after this walks the chain with an example at every level.
先是相似的细胞，再是不同的组织——这一步的转换正是学生描述错的地方， 接下来的图会带着每一层的例子把这条链走一遍。
Follow the chain on this diagram from small to large.
沿着这张图的链条从大到小看。
Start with a single cell — for example a red blood cell, the smallest unit of life.
先从单个细胞开始——例如红细胞，生命的最小单位。
Similar cells working together form a tissue, such as muscle tissue.
相似的细胞一起工作形成组织，比如肌肉组织。
Different tissues combine into an organ — the heart is an organ, and so is a leaf in a plant.
不同的组织组合成器官——心脏是器官， 植物的一片叶子也是。
Organs that work together form an organ system; the digestive system breaks down food.
一起工作的器官构成器官系统；消化系统分解食物。
All the organ systems together make one organism, the whole living thing.
所有器官系统合起来构成一个生物体，也就是完整的生命体。
Learn the order cold: cell, tissue, organ, organ system, organism.
把顺序背熟： 细胞、组织、器官、器官系统、生物体。
Cells are tiny, so you look at them under a light microscope.
细胞很小，所以你用光学显微镜观察。
At the top are the eyepieces, where you put your eyes.
顶部是目镜，眼睛放在这里。
Below, a revolving turret holds the objective lenses — each gives a different magnification.
下方旋转的物镜转盘装着物镜—— 每个物镜给出不同的放大倍数。
The flat stage holds the slide with the specimen.
平坦的载物台放置带标本的载玻片。
Focus knobs on the side move the stage or the body so the image becomes sharp.
侧面的调焦旋钮移动载物台或镜筒， 使图像变清晰。
A lamp under the stage, with a condenser, lights the specimen from below.
载物台下方的灯和聚光镜从下方照亮标本。
The microscope does not change the real size of the cell — it only makes the image appear larger.
显微镜并不改变细胞的真实大小—— 它只是让图像看起来更大。
That ratio is magnification.
这个比值就是放大倍数。
Cells are tiny, so we use a microscope.
细胞很小，所以我们用显微镜。
How many times larger the image appears is the magnification, and it equals the image size divided by the actual size.
图像看起来放大了多少倍就是放大倍数， 它等于图像大小除以实际大小。
Put a real number through it.
我们代入一个真实的数字。
Say a structure is shown at magnification two hundred and fifty, and its image is five point zero zero millimetres.
假设某结构在二百五十倍 放大下显示，图像大小是五点零零毫米。
Actual size is five divided by two hundred and fifty — nought point zero two millimetres.
实际大小是五除以二百五十——零点零二毫米。
Now look at what you just divided: a length by a length.
现在看看你刚才做的除法：长度除以长度。
The units cancel, so magnification never carries one.
单位相互抵消，所以放大倍数从不带单位。
It is only a number, and writing millimetres after it loses the mark.
它只是一个数字，在后面写上毫米会失分。
Here is the formula triangle from the handout.
这是讲义上的公式三角。
Image size sits on top.
图像大小在顶部。
Magnification and actual size sit side by side underneath.
放大倍数和实际大小并排在下面。
Cover the quantity you want, and the remaining layout shows the formula.
盖住你要求的量，剩下的布局就给出公式。
Cover magnification: you are left with image over actual — so magnification equals image size divided by actual size.
盖住放大倍数：剩下图像除以实际—— 所以放大倍数等于图像大小除以实际大小。
Cover actual size: you are left with image over magnification.
盖住实际大小：剩下图像除以放大倍数。
Cover image size: you multiply actual size by magnification.
盖住图像大小：用实际大小乘以放大倍数。
Image size means the size in the drawing or photo.
图像大小指图或照片上的尺寸。
Actual size means the real size of the specimen.
实际大小指标本的真实尺寸。
Here is the mistake that costs the most marks.
这是最容易失分的错误。
Cells are usually measured in micrometres, and one millimetre equals one thousand micrometres.
细胞通常以微米为单位测量，而一毫米等于一千微米。
So you must convert before you divide.
所以你必须先换算再相除。
Take a cell drawn sixty millimetres wide, whose real width is thirty micrometres.
假设一个细胞画出来宽六十毫米，实际宽度是三十微米。
First convert: sixty millimetres is sixty thousand micrometres.
先换算：六十毫米就是六万微米。
Then magnification is sixty thousand divided by thirty, which is two thousand.
然后放大倍数是六万除以三十，等于二千。
If you forget to convert and just do sixty divided by thirty, you get two — a thousand times too small.
如果你忘了换算，直接六十除以三十，就得到二——小了一千倍。
Always make the units match first.
永远先让单位一致。
Drill the unit link both ways.
把单位关系正反练熟。
One millimetre equals one thousand micrometres.
一毫米等于一千微米。
One micrometre equals nought point zero zero one millimetres.
一微米等于零点零零一毫米。
To change millimetres into micrometres, multiply by one thousand.
毫米换成微米，乘以一千。
To change micrometres into millimetres, divide by one thousand.
微米换成毫米，除以一千。
From earlier: nought point zero two millimetres is twenty micrometres.
前面的零点零二毫米就是二十微米。
In the worked trap, sixty millimetres becomes sixty thousand micrometres; sixty thousand divided by thirty is two thousand — the correct magnification.
在那个易错例题里，六十毫米变成六万微米；六万除以三十等于二千——正确的放大倍数。
Forget the convert step and you lose a factor of one thousand.
忘了换算就会差一千倍。
Convert first, every time.
每一次都先换算。
Three marks to lock in, and each is asked in its own recognisable shape.
要锁住三个分，而每一个都有它自己一眼可辨的题型。
The first is a comparison: you are shown two cells and asked for a difference, and the answer has to name features, not impressions — a plant cell has a cell wall, a large vacuole and chloroplasts, and an animal cell has none of the three.
第一个是比较题：给你两个细胞，要你说出一处不同， 而答案必须点出特征，而不是印象——植物细胞有细胞壁、大液泡和叶绿体， 动物细胞这三样都没有。
"They look different" scores nothing.
写"它们看起来不一样"一分也没有。
The second is an identification, and it tests whether you notice what is ABSENT: no nucleus, but a loop of DNA and plasmids, means a bacterium.
第二个是辨认题，它考的是你有没有注意到什么是缺席的： 没有细胞核，但有DNA环和质粒，那就是细菌。
The third is a calculation, and it is lost on units far more often than on arithmetic — convert to the same unit first, one millimetre being one thousand micrometres, and only then divide.
第三个是计算题，而它丢分在单位上远多于丢在算术上—— 先换算成相同单位，一毫米就是一千微米，然后才做除法。
Recognise the shape of the question and the answer follows.
认出题型，答案自然就跟上来了。
Last checklist before the exam.
考前最后一张核对表。
Plant on the left: wall, large vacuole and chloroplasts mark it out from animal.
左边植物：细胞壁、大液泡和叶绿体把它与动物区分开。
Bacterium on the right: no nucleus, a loop of D N A and plasmids.
右边细菌：没有细胞核，有一环脱氧核糖核酸和质粒。
For any specialised cell, name the job and the single feature that suits it — red blood cell has no nucleus so more oxygen; root hair has a long thin extension for absorption.
对任何特化细胞，说出作业和适合它的那一个特征—— 红细胞没有细胞核所以能多带氧气；根毛有细长延伸用于吸收。
Magnification equals image size over actual size, with matching units.
放大倍数等于图像大小除以实际大小， 单位必须一致。
One millimetre is one thousand micrometres.
一毫米是一千微米。
Keep those links and this topic is secure.
记住这些对应关系，这个专题就稳了。