Cell structure

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レッスンノート

Eukaryotic and Prokaryotic Cells

Eukaryotic cells have their genetic material (DNA) enclosed within a nucleus; they include animal, plant, fungal and protoctist cells.
Prokaryotic cells (e.g. bacteria) do not have a nucleus; their DNA is a single loop in the cytoplasm, and they may also contain plasmids (small rings of DNA).
All cells have a cell membrane, cytoplasm and ribosomes; prokaryotic cells also have a cell wall, but it is made of peptidoglycan, not cellulose.
Prokaryotic cells are much smaller than eukaryotic cells, typically about 1 μm compared to 10–100 μm for eukaryotic cells.
Some bacteria have a flagellum (a tail-like structure) that allows them to move.
Plasmids contain extra genes not found in the main chromosomal DNA.

Diagram of a prokaryotic cell with labeled parts.

Animal and Plant Cells

Animal cells contain a nucleus, cytoplasm, cell membrane, mitochondria and ribosomes.
Plant cells have all the structures found in animal cells, plus a cell wall made of cellulose, a permanent vacuole filled with cell sap, and (in leaf and stem cells) chloroplasts.
The nucleus contains genetic material and controls the cell's activities.
Mitochondria are the site of aerobic respiration, releasing energy for the cell.
Ribosomes are the site of protein synthesis.
Chloroplasts contain chlorophyll and carry out photosynthesis.
The cell wall provides support and defines the cell's shape; the cell membrane controls what enters and leaves the cell.

Plant cells (left), packed with green chloroplasts, next to animal cells (right), which have no chloroplasts or cell wall.

Cell Specialisation

Specialised cells have adaptations (in shape or sub-cellular structures) that help them carry out a particular function.
Sperm cells: the tail allows movement, the mid-piece has many mitochondria to release energy, and the head contains enzymes to digest the egg's outer layer.
Nerve cells: elongated to conduct electrical impulses over long distances; dendrites connect to other cells.
Muscle cells: contain protein filaments that slide to allow contraction, and many mitochondria to release energy.
Root hair cells: have an extension that increases surface area for water absorption, and mitochondria for active transport of mineral ions.
Xylem cells: dead, hollow tubes with no end walls, strengthened by lignin, for transporting water and minerals.
Phloem cells: living cells with sieve plates, for transporting dissolved sugars and amino acids.

Root hair cell

Cell Differentiation

Differentiation is the process by which a cell changes to become specialised, developing the structure and sub-cellular components needed for its function.
Almost all cells in a multicellular organism contain the same genes, but only some genes are switched on in each cell type.
In animals, most cells differentiate early in development; mature animals mainly use cell division for repair and replacement.
Some animal cells (adult stem cells) retain the ability to differentiate throughout life.
Plant cells retain the ability to differentiate throughout the plant's life, in regions called the meristem.

Microscopy

Light microscopes use light and lenses to magnify specimens; they can show cells and large structures like nuclei, often with the help of stains.
Electron microscopes use beams of electrons, giving much higher magnification and resolution, so they reveal smaller structures such as mitochondria and ribosomes.
Magnification = image size ÷ actual size. Rearranged: actual size = image size ÷ magnification; image size = magnification × actual size.
Total magnification of a light microscope = eyepiece lens magnification × objective lens magnification.
Magnification has no units and is written as, for example, ×100.
Resolution is the ability to distinguish two points that are close together; it is not the same as magnification.

Required Practical: Using a Light Microscope

Prepare a slide of animal cells (e.g. cheek cells) or plant cells (e.g. onion cells).
For a solid specimen: cut a thin layer of tissue, place it on the slide, add a stain if needed (e.g. iodine for onion, methylene blue for cheek cells), then lower a coverslip to avoid air bubbles.
For a liquid specimen: add a drop to the slide, cover with a coverslip, and press gently to remove air bubbles.
Always start with the low-power objective lens to find the specimen and avoid damaging the slide or lens.
When drawing, use a sharp pencil, continuous lines, no shading, and include a magnification or scale bar.
Add a drop of water to the specimen to prevent dehydration, and take care with sharp instruments and stains.

Parts of a light microscope

Culturing Microorganisms

Bacteria can be grown in a nutrient broth or on an agar gel plate.
Aseptic technique is used to avoid contamination: sterilise Petri dishes and media before use, and pass the inoculating loop through a flame before transferring bacteria.
After adding bacteria, secure the lid with adhesive tape and store the plate upside down.
In school laboratories, cultures should be incubated at a maximum of 25 °C to avoid growing harmful human pathogens.
Bacteria divide by binary fission; under ideal conditions they can divide as often as every 20 minutes.
To calculate the number of bacteria after a given time, use the mean division time and express the answer in standard form if required.

Required Practical: Investigating Antibiotics and Antiseptics

Place paper discs soaked in different antibiotics or antiseptics on an agar plate that has been inoculated with bacteria.
Incubate the plate for a set time, then measure the zone of inhibition (clear area) around each disc.
The area of a clear zone is calculated using the formula: area = πr², where r is the radius (half the diameter).
The larger the zone of inhibition, the more effective the substance is at killing bacteria.
Control variables include the concentration of the substance, the size of the disc, and the incubation temperature and time.
To make results valid, use aseptic technique, repeat the investigation, and calculate a mean.

Units and Standard Form

Cells are measured in micrometres (μm): 1 mm = 1000 μm, 1 μm = 1000 nm.
Convert units before doing magnification calculations: e.g. 1 μm = 0.001 mm = 1 × 10⁻³ mm.
Standard form is written as a × 10ⁿ, where 1 ≤ a < 10.
For large numbers, n is positive; for small numbers, n is negative.
Order of magnitude is a difference of a factor of 10; e.g. a 10 μm cell is one order of magnitude larger than a 1 μm cell.

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練習問題

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1.Which of the following structures is not found in a prokaryotic cell?
Easy
- ACellulose cell wall
- BRibosomes
- CPlasmids
- DA circular loop of DNA
2.Bacteria reproduce by a process called binary fission. Which of the following best describes binary fission?
Easy
- AA cell divides to produce four daughter cells
- BA cell divides to produce two identical cells
- CTwo cells join together to form one larger cell
- DA cell produces a copy by budding
3.One bacterium divides five times. How many bacteria are present in the population now?
Easy
- A6
- B32
- C16
- D64
4.A prokaryotic cell contains its genetic material enclosed within a nucleus.
Easy
True or false?
5.Plant cells do not need mitochondria because they can carry out photosynthesis.
Easy
True or false?
6.Which sub-cellular structure controls the activities of the cell?
Easy
- AThe cell membrane
- BThe cytoplasm
- CThe nucleus
- DThe ribosome
7.A student says the cell wall controls which substances enter and leave a plant cell. Which statement corrects this misconception?
Medium
- AThe cell wall is the site of protein synthesis
- BThe cell membrane controls what enters and leaves the cell
- CThe vacuole controls what enters and leaves the cell
- DThe cytoplasm controls what enters and leaves the cell
8.In which part of a prokaryotic cell is the circular loop of DNA found?
Easy
- AThe nucleus
- BThe cytoplasm
- CThe plasmid
- DThe cell wall

もっと問題

9.Which of the following structures are found in a bacterial cell? (select all that apply)
Medium
- ACell wall made of peptidoglycan
- BMitochondria
- CPlasmids
- DA nucleus
- ERibosomes
10.Which of the following are features of eukaryotic cells but not prokaryotic cells? (select all that apply)
Medium
- ADNA enclosed in a nucleus
- BCell membrane
- CMembrane-bound organelles such as mitochondria
- DCytoplasm
- ERibosomes
11.Magnification has no units and is written as a value such as × 500.
Easy
True or false?
12.Resolution is the ability to distinguish two points that are close together.
Medium
True or false?
13.Which statement best explains why electron microscopes reveal more detail than light microscopes?
Medium
- AElectron microscopes use a larger lens
- BElectron beams have a shorter wavelength than visible light, giving higher resolution
- CElectron microscopes use brighter light
- DElectron microscopes magnify by × 1000 only
14.A light microscope has an eyepiece lens of × 10 and an objective lens of × 20. What is the total magnification?
Easy
- A× 30
- B× 200
- C× 2
- D× 2000
15.Match each specialised cell to its function.
Medium
- Sperm cell
- Ciliated epithelial cell
- Root hair cell
- Nerve cell
- Conduction of electrical impulses
- Transfer of genetic material to an egg cell
- Absorption of water and mineral ions from soil
- Movement of mucus in the trachea and bronchi
16.Match each sub-cellular structure to its function.
Medium
- Nucleus
- Mitochondria
- Ribosome
- Chloroplast
- Site of protein synthesis
- Contains genetic material that controls the cell
- Site of photosynthesis
- Site of aerobic respiration
17.Place the steps for preparing a slide of onion cells in the correct order.
Medium
- Place a thin layer of onion tissue on the slide
- Add a drop of iodine stain to the tissue
- Lower a coverslip and gently press down to remove air bubbles
- View the slide under the low power objective lens first
18.Which of the following is a rule for producing a biological drawing?
Medium
- AShade the drawing to show depth
- BUse broken lines with gaps
- CInclude a magnification or scale bar
- DDraw the cell as small as possible
19.Which structures would the student expect to see in the plant cell? (select all that apply)
Medium
- AChloroplasts
- BCell wall made of cellulose
- CA permanent vacuole
- DPlasmids
- EA nucleus
20.Which of the following best describes the function of the permanent vacuole in a plant cell?
Medium
- AIt controls what enters and leaves the cell
- BIt contains cell sap and helps support the shape of the cell
- CIt is the site of photosynthesis
- DIt is the site of protein synthesis
21.Plasmids are small rings of DNA found in the cytoplasm of prokaryotic cells.
Easy
True or false?
22.A student calculates the magnification of a cell image as 2500. Which statement about this value is correct?
Medium
- AThe image is 2500 times larger than the real cell and has units of mm
- BThe image is 2500 times larger than the real cell and has no units
- CThe image is 2500 μm larger than the real cell
- DThe image is 2500 times smaller than the real cell
23.Why must an inoculating loop be passed through a flame before use?
Medium
- ATo cool the loop so bacteria survive
- BTo sterilise the loop and prevent contamination
- CTo make the loop easier to hold
- DTo provide nutrients for the bacteria
24.Which of the following statements about the size of cells is correct?
Medium
- AProkaryotic cells are about 1000 times larger than eukaryotic cells
- BEukaryotic cells are usually between 10 and 100 μm in size
- CProkaryotic cells are usually between 10 and 100 μm in size
- DEukaryotic and prokaryotic cells are always the same size
25.Place the following in order of size, from smallest to largest.
Medium
- Nanometre
- Micrometre
- Millimetre
- Centimetre
26.Which of the following is the correct way to write 250 micrometres in millimetres using standard form?
Medium
- A2.5 × 10⁻¹ mm
- B2.5 × 10¹ mm
- C25 × 10⁻¹ mm
- D2.5 × 10⁻³ mm
27.Which of the following is a feature of animal cells but not of plant cells?
Medium
- AThey contain a nucleus with a distinct membrane
- BThey store carbohydrates as glycogen
- CTheir cells have cell walls made of cellulose
- DTheir cells contain chloroplasts
28.Cilia and microvilli are the same structure and both can move by themselves.
Medium
True or false?
29.Which of the following structures is found in plant cells but not in animal cells?
Easy
- AMitochondria
- BRibosomes
- CChloroplasts
- DCell membrane
30.Which of the following best explains why a bacterial cell cannot carry out photosynthesis?
Medium
- AIt has no nucleus
- BIt has no chloroplasts
- CIt has no cell wall
- DIt has no ribosomes
31.A student views a specimen using the low power objective lens first. Why is this the correct technique?
Medium
- AIt gives the highest magnification of the specimen
- BIt is easier to find the specimen in the field of view and prevents damage to the lens or coverslip
- CIt prevents the specimen from drying out
- DIt removes the need for a stain
32.Cells with high rates of metabolism have significantly more mitochondria than cells with fewer reactions taking place.
Medium
True or false?
33.Which of the following statements about the xylem are correct? (select all that apply)
Medium
- AXylem is made of dead cells
- BXylem transports dissolved sugars and amino acids
- CXylem cells have no end walls, forming continuous hollow tubes
- DXylem walls are thickened with lignin
- EXylem cells contain many mitochondria
34.Place the stages of binary fission in the correct order.
Medium
- The cell divides into two daughter cells
- The circular DNA and plasmids are replicated
- The cell grows and the DNA loops move to opposite ends of the cell
35.Which statement best describes a plasmid?
Medium
- AA small ring of DNA carrying extra genes, found in the cytoplasm of prokaryotes
- BThe single circular chromosome that carries all the genes of a bacterium
- CA membrane-bound organelle that releases energy in eukaryotic cells
- DA ring of cellulose that strengthens the bacterial cell wall
36.Which of the following are found in both animal and plant cells? (select all that apply)
Medium
- ANucleus
- BCell membrane
- CChloroplast
- DMitochondria
- ECell wall
37.The cell wall of a plant cell is made of cellulose.
Easy
True or false?
38.Prokaryotic cells are about 1000 times smaller than eukaryotic cells.
Easy
True or false?
39.Which of the following is a feature of fungal cells?
Medium
- AThey have a cell wall made of chitin
- BThey contain chloroplasts for photosynthesis
- CThey are always multicellular
- DThey lack a nucleus
40.Which of the following are adaptations of a sperm cell? (select all that apply)
Hard
- AA tail that rotates to propel the cell forward
- BA mid-piece packed with mitochondria
- CAn acrosome containing digestive enzymes
- DMany chloroplasts to provide energy
- EA large permanent vacuole for storage
41.Name the process by which a cell changes to become specialised.
Easy
42.Name the green pigment found in chloroplasts that absorbs light energy for photosynthesis.
Medium
43.A typical prokaryotic cell is 2 μm in length. How many nanometres (nm) is this?
Easy
44.Complete the sentences about preparing an uncontaminated culture.
Easy
Before use, the Petri dishes and culture media must be ____ to kill any unwanted microorganisms. After adding the bacteria, the lid of the Petri dish should be ____ with adhesive tape.
45.Name the three parts that all cells have in common.
Easy
All cells have a cell ____, a ____ and genetic material.
46.The clear zone around an antibiotic disc has a diameter of 14 mm. Using area = πr², calculate the area of the zone of inhibition. Give your answer to 2 decimal places.
Medium
47.Complete the sentence about the magnification equation.
Medium
Magnification = size of ____ ÷ size of ____.
48.Complete the sentence about culturing bacteria.
Medium
Bacteria can be grown in a nutrient ____ solution or as colonies on an ____ gel plate.
49.A bacterial cell divides by binary fission once every 20 minutes. Starting from one cell, how many bacteria will be present after 2 hours? Give your answer as a whole number.
Medium
50.Complete the sentences about aseptic technique.
Easy
Before use, the Petri dish and culture media must be ____ to kill any unwanted microorganisms. After adding the bacteria, the lid of the Petri dish should be secured with ____ tape.
51.Name the sub-cellular structure that controls the activities of the cell.
Medium
52.Name the sub-cellular structure that is the site of protein synthesis.
Easy
53.State two differences between a prokaryotic cell and a eukaryotic cell, other than size.
Medium
54.Describe how the xylem is adapted for carrying water and mineral ions around a plant.
Medium
55.Describe what happens when a cell differentiates.
Hard
56.Explain why bacterial cultures in school laboratories should be incubated at a maximum of 25 °C rather than 37 °C.
Hard
57.Explain two adaptations of a sperm cell that enable it to carry out its function.
Hard
58.A typical prokaryotic cell is 2 μm in length. How many nanometres (nm) is this?
Easy
- A1800 nm
- B2400 nm
- C2000 nm
- D2200 nm
59.The clear zone around an antibiotic disc has a diameter of 14 mm. Using area = πr², calculate the area of the zone of inhibition. Give your answer to 2 decimal places.
Medium
- A153.94 mm²
- B169.33 mm²
- C184.72 mm²
- D138.55 mm²
60.A bacterial cell divides by binary fission once every 20 minutes. Starting from one cell, how many bacteria will be present after 2 hours? Give your answer as a whole number.
Medium
- A76 bacteria
- B70 bacteria
- C58 bacteria
- D64 bacteria

フラッシュカード

What are the four kingdoms of eukaryotic organisms?

Animals, plants, fungi and protoctists. They can be multicellular or single-celled, and their cells contain a nucleus with a distinct membrane.

What is the defining feature of a prokaryotic cell?

Its genetic material is not enclosed in a nucleus — it is found as a single loop of DNA floating in the cytoplasm. Prokaryotes are always single-celled.

Which structures are found in both eukaryotic and prokaryotic cells?

Cell membrane, cytoplasm, ribosomes and genetic material. Prokaryotes also always have a cell wall, but only some eukaryotes do.

What is a plasmid?

A small ring of DNA found in the cytoplasm of prokaryotic cells. It carries extra genes in addition to those on the main circular chromosome.

Name the four structures that every cell must have.

Cell membrane, cytoplasm, DNA (genetic material) and ribosomes.

What is the function of the cell membrane?

It holds the cell together, separating the inside from the outside, and controls which substances can enter or leave the cell.

What is the function of the cytoplasm?

A jelly-like substance where many chemical reactions take place, including anaerobic respiration. It also supports the internal cell structures.

What is the function of mitochondria?

The site of the reactions of aerobic respiration, where energy is released for cellular processes. Cells with high rates of metabolism have many mitochondria.

What is the function of ribosomes?

The site of protein synthesis. They are found in the cytoplasm.

What is the function of the nucleus?

It contains the genetic material (DNA) which controls the activities of the cell.

Which structures are found only in plant cells (not animal cells)?

A cellulose cell wall, a permanent vacuole filled with cell sap, and (in leaf and stem cells) chloroplasts.

What is the function of the plant cell wall?

Made of cellulose, it gives the cell extra support and defines its shape.

What is the function of chloroplasts?

They contain chlorophyll which absorbs light energy, and the enzymes needed for photosynthesis.

What is the permanent vacuole filled with, and what is its function?

It is filled with cell sap — a solution of sugars and salts dissolved in water. It stores materials and helps support the shape of the cell.

Why do plant cells still contain mitochondria even though they photosynthesise?

Mitochondria are the site of aerobic respiration, which releases energy for cellular processes. Photosynthesis makes glucose, but respiration is still needed to release energy from it.

What is a specialised cell?

A cell that has a structure (shape or combination of sub-cellular structures) that aids its specific function within a tissue, organ or organism.

What is differentiation?

The process by which a cell changes to become specialised, developing the structure and sub-cellular structures needed to carry out a certain function.

How does differentiation differ between animals and plants?

Most animal cells differentiate early in development and then lose the ability to differentiate (except adult stem cells used for repair). Plant cells keep the ability to differentiate throughout the plant's life, in the meristem.

How is a nerve cell adapted to its function?

It is elongated so it can conduct electrical impulses over long distances. Dendrites let it communicate with other cells, and a fatty sheath around the axon speeds up impulse transmission.

How is a muscle cell adapted for contraction?

It contains many mitochondria to release energy for contraction, and protein filaments that slide over each other to allow the muscle to contract.

How is a sperm cell adapted to its function?

The tail rotates to propel it forward, the mid-piece is packed with mitochondria to release energy, and the acrosome contains enzymes to break down the egg's outer layer. The head nucleus is haploid.

How is a root hair cell adapted for absorption?

The root hair extension increases surface area so water is absorbed faster by osmosis. Thin walls shorten the diffusion distance, and mitochondria release energy for active transport of mineral ions.

How is a xylem vessel adapted for transporting water?

It is made of dead cells with no end walls, forming continuous hollow tubes. Cells contain no organelles or cytoplasm, allowing free water passage, and walls are thickened with lignin for support.

How is a phloem cell adapted for transporting sugars and amino acids?

It is made of living cells joined end-to-end with pores in the end walls (sieve plates), forming tubes. Cells have very few sub-cellular structures to aid flow.

What is the equation for magnification?

Magnification = size of image ÷ size of real object. Magnification has no units and is written as, for example, × 10.

How do you calculate the total magnification of a light microscope?

Multiply the magnification of the eyepiece lens by the magnification of the objective lens.

How many micrometres are in 1 millimetre?

1000 μm = 1 mm, so 1 μm = 0.001 mm (1 × 10⁻³ mm).

What is the difference between magnification and resolution?

Magnification is how many times larger the image is than the object. Resolution is the ability to distinguish two points that are close together, which lets you see finer detail.

Why can electron microscopes reveal more detail than light microscopes?

They use a beam of electrons, which has a much smaller wavelength than visible light. This gives much higher resolution and magnification, so smaller sub-cellular structures can be seen.

What is the maximum magnification of a light microscope compared with an electron microscope?

A light microscope reaches about 1000–2000×, while an electron microscope reaches about 2,000,000×.

Why are stains such as methylene blue and iodine used when preparing slides?

To make the structures of the cells visible under the light microscope. Methylene blue is used for cheek cells and iodine for onion cells.

Describe how to prepare a slide of onion cells for a light microscope.

Cut a small sample and peel a very thin layer of cells using a scalpel or forceps. Place it on the slide, add a drop of water and a stain such as iodine, then lower a coverslip and press gently to remove air bubbles.

Why should you always start with the low power objective lens on a light microscope?

It is easier to find what you are looking for in the field of view, and it prevents damage to the lens or coverslip if the stage has been raised too high.

Give two rules for producing a biological drawing.

Use continuous lines with no gaps and do not shade. Also include a magnification or scale bar, draw cell walls wide enough to recognise, and label the parts.

Why must Petri dishes and culture media be sterilised before use?

To kill any unwanted microorganisms so that only the bacteria being investigated grow, keeping the culture uncontaminated.

Why must an inoculating loop be passed through a flame before use?

To sterilise it, so that no unwanted microorganisms are transferred to the culture.

Why is the lid of a Petri dish secured with adhesive tape?

To prevent contamination from microorganisms in the air, and to stop the lid falling off. The dish is also stored upside down to stop condensation dripping onto the agar.

Why are bacterial cultures in school laboratories incubated at a maximum of 25 °C rather than 37 °C?

To reduce the risk of growing harmful microorganisms that could cause disease in humans. 37 °C is human body temperature and would allow pathogens to thrive.

What is binary fission?

The process by which a bacterial cell divides to produce two identical daughter cells. Bacteria can divide as often as once every 20 minutes with enough nutrients and a suitable temperature.

How do you calculate the number of bacteria after a given time?

Work out how many divisions occur (time ÷ mean division time), then multiply the starting number by 2 for each division. Answers may be required in standard form.

How do you calculate the area of a zone of inhibition?

Use area = πr². First halve the diameter to get the radius, then substitute into the formula.

Why are doctors advised not to overprescribe antibiotics?

Overuse allows bacteria that are resistant to antibiotics to survive and reproduce, so resistant strains spread and the antibiotics become less effective.

Why do bacterial cells not contain mitochondria?

They are prokaryotic and lack membrane-bound organelles. They are also much smaller than eukaryotic cells, so there is no space for structures like mitochondria.

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