Cell Structure

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课程笔记

Cell Theory

Cell theory states that all living organisms are made up of one or more cells, cells are the basic functional unit of life, and new cells are produced from pre-existing cells.
The term 'cell' was coined by Robert Hooke in the 1660s after examining cork.
Matthias Schleiden and Theodor Schwann proposed in 1837 that all living organisms are made of cells.
All cells share common features: they are surrounded by a membrane, contain genetic material, and have enzyme-catalysed reactions occurring within them.
Deductive reasoning can be used to predict that a newly discovered organism will consist of one or more cells, based on cell theory.

Hooke's microscope and cork cells

Microscopy Skills

Magnification is how many times bigger the image is compared to the actual size; it is calculated as: magnification = image size ÷ actual size.
Resolution is the ability to distinguish between two objects that are close together.
When preparing a temporary mount, gently press the coverslip to remove air bubbles.
Start with the low power objective lens to locate the specimen, then switch to higher power lenses, using only the fine focus at high power.
A graticule is a small disc with an engraved scale placed in the eyepiece; it must be calibrated using a stage micrometer.
Measurements using microscopes are quantitative observations (numerical data), while observations of colour or presence of structures are qualitative.
When using a scale bar, measure its length in mm, convert to the same units as the scale bar, then use the magnification formula.

Parts of a light microscope

Microscopes: Light vs Electron

Light microscopes use light to form an image; they have a maximum resolution of about 0.2 μm (200 nm) and maximum useful magnification of about ×1500.
Electron microscopes use electrons; they have a much higher resolution of about 0.0002 μm (0.2 nm) and maximum useful magnification of about ×1,500,000.
Transmission electron microscopes (TEMs) pass electrons through a thin specimen, producing high-resolution 2D images of internal structures.
Scanning electron microscopes (SEMs) scan electrons across the surface, producing 3D images of external structures.
Electron microscopes require specimens to be dead and do not produce colour images; light microscopes can observe living specimens and produce colour images.
Cryogenic electron microscopy involves flash-freezing proteins and using electron beams to produce 3D reconstructions of molecules.
Freeze fracture involves rapid freezing with liquid nitrogen and fracturing to view internal membrane organisation.

Magnification equation triangle

General Cell Structure

All cells contain DNA as genetic material, cytoplasm, and a plasma membrane.
DNA stores genetic information and controls the production of enzymes and other proteins.
Cytoplasm is a water-based fluid (cytosol) containing dissolved substances where many cellular reactions occur.
The plasma membrane is a bilayer of lipids that controls the exchange of materials between the cell and its environment.
Membrane proteins have functions including cell recognition, cell communication, and transport into and out of the cell.

A typical animal cell

Prokaryotic Cell Structure

Prokaryotes are divided into two domains: Bacteria and Archaea; they are small cells ranging from 0.1 μm to 5.0 μm.
Prokaryotic cells lack a nucleus and membrane-bound organelles; their cytoplasm is not compartmentalised.
They have 70S ribosomes, a single circular DNA molecule (not associated with histone proteins) located in the nucleoid, and often plasmids (small loops of DNA).
The cell wall of most prokaryotes contains murein (peptidoglycan); it provides protection, maintains shape, and prevents bursting.
Some prokaryotes have a capsule (slime capsule) for protection against drying out and immune attack, flagella for movement, and pili for attachment and conjugation.
Gram-positive bacteria retain crystal violet dye and appear blue/violet; Gram-negative bacteria do not retain the dye.
Prokaryotic cells are believed to have been the first living organisms on Earth.

A typical bacterial cell

Eukaryotic Cell Structure

Eukaryotic cells have a more complex ultrastructure than prokaryotes, with membrane-bound organelles that compartmentalise the cytoplasm.
Compartmentalisation allows enzymes and substrates to be at higher concentrations, separates damaging substances, and maintains optimal conditions.
The nucleus is surrounded by a double membrane (nuclear envelope) with pores; it contains chromatin (DNA wound around histones) and a nucleolus where ribosomes are made.
Rough endoplasmic reticulum (RER) is covered with 80S ribosomes and processes proteins; smooth ER lacks ribosomes and synthesises lipids.
Ribosomes are complexes of rRNA and protein, found free in the cytoplasm or on the RER; they are the site of translation.
Mitochondria have a double membrane with inner folds called cristae; they are the site of aerobic respiration and contain their own circular DNA and ribosomes.
The Golgi apparatus consists of flattened sacs (cisternae) that modify and package proteins and lipids into vesicles for transport.
Lysosomes are specialised vesicles containing hydrolytic enzymes that break down waste materials and worn-out organelles.

Plant Cell Structures

Chloroplasts are surrounded by a double membrane and contain thylakoids stacked into grana joined by lamellae; they are the site of photosynthesis.
The light-dependent stage of photosynthesis occurs in the thylakoids, and the light-independent stage (Calvin cycle) occurs in the stroma.
Chloroplasts contain small circular DNA and ribosomes for protein synthesis.
Plant cells have a large permanent vacuole surrounded by the tonoplast, a selectively permeable membrane.
The cell wall is made of cellulose in plants and peptidoglycan in bacteria; it provides structural support and is freely permeable.
Plasmodesmata are narrow threads of cytoplasm connecting neighbouring plant cells.

A typical plant cell

Functions of Life

All living organisms carry out: metabolism, reproduction, homeostasis, growth, response, excretion, and nutrition.
Metabolism includes all enzyme-catalysed reactions in a cell, including cell respiration.
Homeostasis is the maintenance of internal conditions within tolerable limits.
Excretion is the disposal of metabolic waste products, such as carbon dioxide.
Unicellular organisms carry out these functions using organelles: mitochondria for energy, ribosomes for protein synthesis, vacuoles for digestion and storage, and cilia/flagella for movement.

Eukaryotic Comparisons & Atypical Examples

Animal cells lack a cell wall and chloroplasts, have small temporary vacuoles, and contain centrioles for microtubule organisation during cell division.
Plant cells have a cellulose cell wall, large permanent vacuoles, chloroplasts, and lack centrioles.
Fungal cells have cell walls made of chitin and glucans, small non-permanent vacuoles, and lack chloroplasts and centrioles.
Animal and fungal cells store carbohydrates as glycogen; plant cells store starch.
Atypical examples that challenge cell theory include striated muscle fibres (long, multinucleated), aseptate fungal hyphae (multinucleated with continuous cytoplasm), red blood cells (lack nucleus), and phloem sieve tubes.

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练习题

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1.Which of the following ideas are part of cell theory?
Easy
- AI only
- BII only
- CI and III only
- DII and III only
2.Which of the following can not be viewed using a light microscope?
Easy
- ANucleus
- BCell wall
- CChloroplasts
- DRibosomes
3.Which row correctly compares the magnification and resolution of an electron microscope with a light microscope?
Medium
- AMagnification: Lower in LM; Resolution: Higher in LM
- BMagnification: Higher in LM; Resolution: Lower in EM
- CMagnification: Higher in EM; Resolution: Higher in EM
- DMagnification: Lower in EM; Resolution: Lower in LM
4.The diagram shows a bacterium. Which three structures are found in both an animal cell and this bacterial cell?
Medium
- ACell membrane, cell wall and DNA
- BCell membrane, DNA and ribosome
- CCapsule, DNA and ribosome
- DCapsule, cell membrane and cell wall
5.Which row correctly describes the features of prokaryotic chromosomal DNA?
Medium
- AAssociated with histone proteins; Circular
- BNot associated with histone proteins; Linear
- CNot associated with histone proteins; Circular
- DContains a few hundred genes; Circular
6.Which of the following are functions of life that all organisms must carry out? (select all that apply)
Medium
- AMetabolism
- BReproduction
- CHomeostasis
- DPhotosynthesis
- EExcretion
7.The cell theory states that all living organisms are made up of one or more cells.
Easy
True or false?
8.Prokaryotic cells contain membrane-bound organelles.
Easy
True or false?

更多题目

9.Match each organelle with its main function.
Medium
- Nucleus
- Ribosome
- Golgi apparatus
- Lysosome
- Contains genetic material and controls cell activities
- Site of protein synthesis
- Modifies and packages proteins and lipids
- Contains hydrolytic enzymes to break down waste
10.Place the steps of the endosymbiotic theory in the correct order.
Medium
- A large prokaryote engulfs a smaller prokaryote
- The smaller prokaryote survives inside the larger cell
- The smaller prokaryote becomes a mitochondrion or chloroplast
- The host cell and endosymbiont become interdependent
11.Which of the following statements correctly describe the resolution of a microscope? (select all that apply)
Medium
- AIt is the ability to distinguish between two objects that are very close together
- BIt is how many times bigger the image is than the real-life object
- COptical microscopes have a maximum resolution of around 0.2 μm
- DElectron microscopes have a maximum resolution of around 0.2 nm
- EIt is measured in units of times (×)
12.Match each type of microscope with the correct description of how it forms an image.
Medium
- Transmission electron microscope (TEM)
- Scanning electron microscope (SEM)
- Optical (light) microscope
- A beam of electrons is transmitted through a very thin specimen; denser parts appear darker
- A beam of electrons is scanned across the surface and bounces off, producing a 3-D image
- Light is directed through the specimen and focused by objective and eyepiece lenses
13.Place the following steps in the correct order for preparing a temporary mount slide from a solid tissue specimen.
Medium
- Cut a small sample of the tissue with scissors
- Peel away or cut a very thin layer of cells using a scalpel or forceps
- Treat the tissue with chemicals to kill/make it rigid and add a stain if required
- Gently place a coverslip on top and press down to remove air bubbles
- Place the slide on the microscope stage and fix it with stage clips
14.Which of the following are advantages of using a transmission electron microscope (TEM) compared with a light microscope? (select all that apply)
Medium
- AIt gives higher resolution images, allowing internal structures within cells to be seen
- BIt can be used to observe living specimens
- CIt produces a colour image
- DIt can be used to observe small organelles such as ribosomes and the endoplasmic reticulum
- EIt has a much higher maximum useful magnification
15.Match each prokaryotic structure with its correct function.
Medium
- Plasmid
- Capsule
- Flagellum
- Pili
- Small loop of DNA separate from the main circular DNA, often carrying genes such as antibiotic resistance
- Outer slime layer that protects against drying out and attack by immune cells
- Long tail-like structure that rotates to enable movement
- Short, thin structure that assists with adherence to surfaces and conjugation
16.Which of the following statements about the endosymbiotic theory are correct? (select all that apply)
Medium
- AMitochondria and chloroplasts contain their own circular DNA
- BMitochondria and chloroplasts have 70S ribosomes
- CMitochondria and chloroplasts are surrounded by a double membrane
- DMitochondria and chloroplasts can be seen clearly with a light microscope
- EThe theory states that these organelles evolved from engulfed prokaryotes
17.A cell is supplied with radioactive amino acids. Which cell component will show an increase in radioactivity first?
Medium
- AMitochondria
- BGolgi apparatus
- CRough endoplasmic reticulum
- DNucleus
18.Which of the following organelles does NOT directly help an exocrine gland cell in the pancreas to secrete digestive enzymes?
Medium
- ARough endoplasmic reticulum
- BGolgi apparatus
- CLysosomes
- DMitochondria
19.Which of the following statements about ribosomes are correct? (select all that apply)
Hard
- AThey are composed of protein and ribosomal RNA
- BThey are found in both eukaryotes and prokaryotes
- CRibosomal RNA provides structure
- DThey consist of two equal-sized subunits
- EThey are the site of translation
20.In prokaryotes, transcription and translation can proceed simultaneously. Which key feature allows this?
Medium
- ACircular chromosomal DNA
- BFree ribosomes
- CThe lack of a nucleus
- DThe presence of introns in prokaryotic DNA
21.Which of the following statements about eukaryotic chromosomes are correct? (select all that apply)
Medium
- AChromosomes are long, linear molecules that can be condensed for compact storage
- BAfter DNA replication, the two strands of DNA are held together at the centromere
- CDNA wraps around histone proteins to form chromatids
- DChromosomes are circular and not associated with histone proteins
- EChromosomes are found freely in the cytoplasm
22.Which cell structure(s) would enable biologists to classify Epulopiscium fishelsoni as prokaryotic?
Hard
- ACircular DNA and 70S ribosomes occurring freely in the cytoplasm, and a cell wall made of murein
- BA pair of centrioles close to a nuclear pore, and 70S and 80S ribosomes occurring freely in the cytoplasm
- CSmooth endoplasmic reticulum throughout the cytoplasm, and a cell wall made of murein
- DA cellulose cell wall outside the plasma membrane, 70S ribosomes, and circular DNA occurring freely in the cytoplasm
23.The scale bar on a drawing of a cell measures 45 mm in length and represents an actual length of 10 μm. What is the magnification of the drawing?
Medium
- A×45000
- B×4.5
- C×4500
- D×450
24.A prokaryotic cell has a diameter of 1 μm. It is magnified ×50,000 by an electron microscope. What is the diameter of the cell in the electron micrograph?
Medium
- A5 × 10⁰ mm
- B5 × 10^-1 mm
- C5 × 10² mm
- D5 × 10¹ mm
25.An electron micrograph has a scale bar that is 1.5 cm long and represents 5 μm. What is the magnification of the micrograph?
Medium
- A×7000
- B×7.5
- C×3000
- D×300
26.An electron micrograph of a root vascular system has a magnification of ×200. A student measures the diameter of a cell as 10 mm on the image. What is the actual diameter of the cell?
Medium
- A100 μm
- B50 μm
- C10 μm
- D5 μm
27.Erythrocytes (red blood cells) have a diameter of 7,000 nm. Pancreatic cells have a diameter of 35 μm. Which statement about their relative sizes is correct?
Medium
- AErythrocytes are 5 times smaller
- BErythrocytes are 50 times smaller
- CErythrocytes are 5 times larger
- DErythrocytes are 50 times larger
28.A student uses a stage micrometer with divisions of 0.1 mm to calibrate an eyepiece graticule. They find that 10 graticule units align with 5 stage micrometer divisions. What is the length of one graticule unit in micrometres?
Hard
- A5 μm
- B50 μm
- C0.5 μm
- D500 μm
29.A student observes a cell under a light microscope and records: presence of a nucleus, presence of chloroplasts, and a cell wall. Which type of cell are they most likely observing?
Hard
- AAnimal cell
- BPlant cell
- CProkaryotic cell
- DFungal cell
30.A student measures the length of a scale bar on a micrograph as 2 cm. The scale bar represents 5 μm. What is the magnification of the micrograph?
Medium
- A×4000
- B×400
- C×40
- D×40000
31.A giant bacterium, Epulopiscium fishelsoni, was discovered in 1985. Which cell structures would enable biologists to classify it as prokaryotic?
Hard
- ACircular DNA and 70S ribosomes occurring freely in the cytoplasm, and a cell wall made of murein
- BA pair of centrioles close to a nuclear pore, and 70S and 80S ribosomes occurring freely in the cytoplasm
- CSmooth endoplasmic reticulum throughout the cytoplasm, and a cell wall made of murein
- DA cellulose cell wall outside the plasma membrane, 70S ribosomes, and circular DNA occurring freely in the cytoplasm
32.Which set of observations would indicate that a student is observing a eukaryotic cell?
Medium
- ACytoplasm contains endoplasmic reticulum, protein molecules associated with DNA, diameter <1 μm
- BCytoplasm lacks endoplasmic reticulum, protein molecules associated with DNA, diameter >1 μm
- CCytoplasm contains endoplasmic reticulum, protein molecules associated with DNA, diameter >1 μm
- DCytoplasm contains endoplasmic reticulum, no protein molecules associated with DNA, diameter <1 μm
33.Which of the following is evidence for the endosymbiotic theory?
Hard
- A70S ribosomes can be found in prokaryotic cells
- BMitochondria contain their own DNA
- CMeteorites have been found that contain organic molecules
- DIn certain cases, gene transfer from prokaryotic cells to eukaryotic cells via plasmids has been found to occur
34.Which statements best describe ribosomes? (I) They are composed of protein and ribosomal RNA. (II) They are found in both eukaryotes and prokaryotes. (III) Ribosomal RNA provides structure. (IV) They consist of two equal-sized subunits.
Medium
- AI only
- BI and II
- CI, II and III
- DI, II and IV
35.Which of the following statements about eukaryotic chromosomes are correct? (1) Chromosomes are long, linear molecules that can be condensed for compact storage. (2) After DNA replication, the two strands of DNA are held together at the centromere. (3) DNA wraps around histone proteins to form chromatids.
Medium
- AI and II only
- BII only
- CII and III only
- DI, II, and III
36.What is the main purpose of gently pressing down on the cover slip after placing it on the slide?
Medium
- ATo increase magnification
- BTo remove air bubbles
- CTo remove excess stain
- DTo kill the specimen
37.Which structure is responsible for protecting the bacterium from drying out and from attack by immune cells?
Medium
- ACapsule
- BCell wall
- CPlasma membrane
- DPili
38.Which structure enables the bacterium to move?
Medium
- APili
- BFlagellum
- CCapsule
- DCell wall
39.Pili are shorter and thinner than flagella and assist with adherence to surfaces.
Easy
True or false?
40.A student is preparing a temporary mount of onion epidermis. They peel a thin layer of tissue and place it on a slide. Before adding the coverslip, they add a drop of iodine solution. What is the main purpose of the iodine solution?
Medium
- ATo kill the cells so they remain still
- BTo make the cell structures visible by staining
- CTo remove air bubbles from the specimen
- DTo increase the magnification of the microscope
41.Name the polysaccharide that provides structural support in plant cell walls.
Easy
42.Which organelle is the site of aerobic respiration in eukaryotic cells?
Medium
43.Complete the sentence about cell theory.
Easy
Cell theory states that all living organisms are made of one or more ____, and that new cells are produced from ____ cells.
44.A cell is 10 μm in diameter. It is magnified 5000 times. What is the diameter of the cell in the image, in millimetres?
Medium
45.A cell is 20 μm in diameter. It is magnified 2000 times. What is the diameter of the cell in the image, in millimetres?
Medium
46.A cell is 30 mm in diameter in a drawing. The drawing has a magnification of ×3000. Calculate the actual diameter of the cell in micrometres (μm).
Medium
47.A specimen is 5 μm in actual length. It is magnified ×2000. What is the length of the image in millimetres (mm)?
Medium
48.Complete the sentence about the resolution of microscopes.
Medium
Resolution is the ability to distinguish between two objects that are ____ together.
49.Complete the sentence about prokaryotic cell walls.
Easy
Most prokaryotes have a cell wall containing ____.
50.Name the organelle that is the site of aerobic respiration in eukaryotic cells.
Medium
51.Complete the sentence about preparing a temporary mount.
Easy
When preparing a temporary mount from a liquid specimen, a few drops are added to the slide using a ____, and then a ____ is placed on top and gently pressed to remove air bubbles.
52.A student uses a stage micrometer to calibrate an eyepiece graticule. They find that 50 graticule units align with 20 stage micrometer divisions, where each division is 0.01 mm. What is the length in micrometres (μm) represented by one graticule unit? Give your answer to 1 decimal place.
Hard
53.Explain how the structure of the rough endoplasmic reticulum is adapted to its function.
Hard
54.Describe two steps the student should take to improve the clarity of the image.
Medium
55.Explain how the structure of the rough endoplasmic reticulum (RER) is adapted to its function.
Hard
56.Describe two steps the student should take to improve the clarity of a blurry image when using a light microscope.
Medium
57.Describe two precautions that should be taken when preparing a temporary mount from a solid tissue specimen, and explain why each is necessary.
Hard
58.A cell is 10 μm in diameter. It is magnified 5000 times. What is the diameter of the cell in the image, in millimetres?
Medium
- A45 mm
- B50 mm
- C60 mm
- D55 mm
59.A cell is 20 μm in diameter. It is magnified 2000 times. What is the diameter of the cell in the image, in millimetres?
Medium
- A40 mm
- B44 mm
- C36 mm
- D48 mm
60.A cell is 30 mm in diameter in a drawing. The drawing has a magnification of ×3000. Calculate the actual diameter of the cell in micrometres (μm).
Medium
- A9 μm
- B12 μm
- C11 μm
- D10 μm
61.A specimen is 5 μm in actual length. It is magnified ×2000. What is the length of the image in millimetres (mm)?
Medium
- A9 mm
- B11 mm
- C10 mm
- D12 mm
62.A student uses a stage micrometer to calibrate an eyepiece graticule. They find that 50 graticule units align with 20 stage micrometer divisions, where each division is 0.01 mm. What is the length in micrometres (μm) represented by one graticule unit? Give your answer to 1 decimal place.
Hard
- A6 μm
- B4 μm
- C3 μm
- D5 μm

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