Cell Structure Flashcards
What did Schleiden, 1838 suggest?
All plants are made out of cells
What did Schwann, 1839 suggest?
All animal tissues are compromised of cells
What did Virchow, 1858 suggest?
All cells must arise from preexisting cells
1 cm = ?mm
10mm
1mm = ?um
1000um
1um = ?nm
1000nm
1cm Interm of meters
10^-2m
1mm Interm of meters
10^-3m
1um Interm of meters
10^-6m
1nm Interm of meters
10^-9m
Define magnification
How much bigger a sample appears to be under the microscope than in real life
What is the calculation for total magnification
Total magnification = objective magnification x eyepiece
magnification
Define resolution
The ability to distinguish between two points on the image
How detailed it is
Does increasing the magnification increase the resolution of the image?
No
What is the resolution of a light microscope?
200nm
When can two objects be resolved?
When light can pass between them
When are objects seen as one object?
When the objects are less than 200nm apart
What is a dry mount?
Solid specimens can be viewed whole or as very thin sections
What can be viewed as a dry mount?
Hair, dust, pollen, insects
What is sectioning?
Specimens cut with a sharp blade into very thin sections
What is a wet mount
Specimen suspended in a liquid i.e. water
How is the coverslip placed in a wet mount?
At an angle
What can be viewed as a wet mount?
Aquatic samples etc.
What is a squash slide?
A wet mount prepared and a lens tissue is used to press carefully on the coverslip
What is a squash slide used for?
Soft samples, such as root tips
What are smear slides?
The edge of a slide is used to smear a sample, creating a thin, even coating. The coverslip is then placed on the sample
What are smear slides good for viewing?
Cells in blood
List the ways that a slide can be made permanent?
- Fixing
- Embedding
- Sectioning
- Staining
How/why is fixing used to make a slide permanent?
Chemicals like formaldehyde are used to cross-link proteins, ‘fixing’ the structure of the specimen and preventing chemical breakdown
How/why is embedding used to make a slide permanent?
Some material distorts when you try and cut it into thin sections. Specimens are therefore embedded in wax or resin
How is sectioning used to make a slide permanent?
The tissue is cut using a microtome into very thin sections from 2 to 50um
How/why is staining used to make a slide permanent?
A lot of biological material is not coloured. Some chemicals can bind to the specimen, allowing it to be seen. Some of these ‘stains’ are specific to cell structures e.g. Acetic orcein stains DNA dark red
What stain is used in Gram staining?
crystal violet-iodine complex and safranin counterstain
What colour are Gram-positive bacteria?
Violet
Why are Gram-positive bacteria stained violet?
The presence of a thick layer of peptidoglycan in their cell walls because when they are washed with solvent, the cell prees close, thus becoming less permeable and retain the stain
What colour are Gram-negative bacteria?
Red
Why are Gram-negative bacteria stained red?
The thinner layer of peptidoglycan in their cell walls, and so does not retain the crystal violet whilst in the decolouring process, because the solvent dissolves the lipids, which washes out, or decolourises the stain, safranin, the counterstain is used to stain instead
What is the process of Gram staining?
- Staining with crystal violet dye
- Adding Gram’s iodine solution
- Decolourising the sample
- Counterstaining
What does the first stage of Gram staining involve?
The cells are initially stained with crystal violet dye
What does the second stage of Gram staining involve?
Then a Gram’s iodine solution (iodine and potassium iodide) is added to form a complex between the crystal violet and iodine. This complex is a bigger molecule than the previous crystal-violet stain and iodine and is insoluble in water
What does the third stage of Gram staining involve?
Next, a decolouriser such as ethanol or acetone is added to the sample, which dehydrates the peptidoglycan layer. In a Gram-positive bacteria, the crystal violet-iodine complex cannot penetrate the dehydrated peptidoglycan layer and so is trapped in the cell. However, the external membrane of Gram-negative bacteria is degraded, so the thinner peptidoglycan layer is unable to retain the crystal violet-iodine complex, and the colour is lost.
What does the fourth and final stage of Gram staining involve?
Finally, a counterstain, like safranin, because it is barely water-soluble; is added to the sample, staining it red. The safranin does not affect the violet colouration in Gram-positive cells because it is lighter than crystal violet. Conversely, the decolourised cells are stained red.
What is another name for a laser scanning confocal microscope?
Confocal microscope
What does the laser scanning confocal microscope allow?
Clear observation of whole, living specimens and can focus on structures at different depths within a specimen
What resolution and contrast does the laser scanning confocal microscope images have?
High resolution and contrast
What is the maximum resolution of a laser scanning confocal microscope?
0.8um
How are the images formed in a confocal microscope?
Focusing a single spot of laser light and moving it across the specimen. This causes fluorescence by the components labelled with a dye
What light is detected in a confocal microscope?
Light emitted very close to the focal plane
What type of images (2D or 3D) are produced and how?
3D images are produced by creating images at different focal planes
What fields are confocal microscope generally used in?
Medicine as it makes diagnosis earlier and allows early treatment
What does a Transmission Electron Microscope (TEM) use to form images?
Electrons, not light
How are electrons used in a TEM?
The electrons pass through a very thin prepared sample
How can a TEM give contrast?
It gives some contrast as electrons pass through denser parts of the sample less easily
What type of image (2D or 3D) is produced?
2D
What is the approximate magnification of a TEM?
x500,000
What is the approximate resolution of a TEM?
0.05 - 1 nm
What is an advantage of a TEM?
Can see tiny organelles that are not visible with a light microscope
What is a disadvantage of a TEM?
Black and white images - artificially coloured to highlight structures
What does SEM stand for?
Scanning Electron Microscope
When was the SEM first used?
1963
How does the SEM produce an image?
The electron beam is directed onto the sample, but it doesn’t pass through the specimen
The electrons are ‘bounced’ off the sample and collected by special detectors
What type of image (2D or 3D) is produced?
3D
What is the approximate resolution of an SEM?
3-`10nm
What is the approximate magnification of an SEM
x100,000
How are samples prepared?
Coated with a heavy metal such as gold
What is an advantage of an SEM?
Detailed surface view
Has a depth of field
What is a disadvantage of an SEM?
Black and white images - artificially coloured to highlight structures
What are the ways that can be used to prepare a specimen for TEM?
- Fixing
- Embedding
- Slicing (Sectioning)
Why is fixing used to prepare a sample for TEM?
Preventing breakdown. But it can cause changes in the appearance of some organelles
Why is embedding used to prepare a sample for TEM?
Set the sample in a solid-state e.g. paraffin wax, so it doesn’t crumble when thin sections are cut
Why is slicing/sectioning used to prepare a specimen for TEM?
So a thin specimen that is 1000nm or less is able to be used
What is done to the specimen once it is prepared for TEM?
It is stained with heavy metal ions to repel electrons and make the images visible
Why is fixing used to prepare a sample for SEM?
Preventing breakdown. But it can cause changes in the appearance of some organelles
What is done to the specimen for an SEM?
It is coated in a thin layer of a heavy metal
What is the process of protein synthesis?
- An mRNA copy of the gene is made in the nucleus in a process called transcription
- The mRNA leaves the nucleus through a nuclear pore and attaches to a ribosome on the Rough Endoplasmic Reticulum
- The ribosome ‘reads’ the code to assemble the chain of amino acids that make up the protein in a process called translation
- The protein molecules enter the cisternae of the endoplasmic reticulum where they are ‘pinched off’ in transport vesicles, which travel to towards the Golgi apparatus and fuse with the cis face
- In the cisternae of the Golgi apparatus, the proteins are modified, by adding molecules such as sugars and lipids
- The modified protein molecules move through the Golgi apparatus and are pinched off in secretory vesicles
- The secretory vesicles move towards and fuse with the cell surface membrane and are released into the extracellular environment in a process known as exocytosis
What does the nucleus do?
- Contains genetic material
- Controls the cellular activities through mRNA and tRNA production
- Produces rRNA (ribosomal RNA)
What type of membrane does a nucleus have?
A double membrane. The outer membrane continues to form the endoplasmic reticulum
What are nuclear pores?
Small holes in the nuclear envelope that allow substances to move in and out of the nucleus
How big are nuclear pores?
40 - 100nm
What is nucleoplasm?
Granular, jelly-like material that makes up the rest of the cytoplasm
What is the nucleolus?
A spherical structure that produces ribosomal RNA
What are chromosomes?
Protein-bound, linear DNA
What does the nuclear envelope form?
It continues to form the Rough Endoplasmic Reticulum, which continues to form the Smooth Endoplasmic Reticulum
What is the Golgi apparatus/body?
It is completely separate from the nucleus. It is a stack of membrane-bound, flattened sacks that are responsible for the modification of proteins received from the ER, these proteins are then transported around the cell in secretory/transport vesicles, and has secretory vesicles coming off its edges
What are the roles of mitochondria?
- Carry out aerobic respiration (Krebs cycle and Oxidative Phosphorylation)
What type of membrane do mitochondria have?
A double membrane
Why is the inner membrane of mitochondria folded?
To form cristae
What is the role of cristae in mitochondria?
Provide a large surface area for the attachment of enzymes and proteins involved in respiration
What does the matrix of mitochondria contain?
- Proteins
- Ribosomes
- Lipids
- DNA
- Some enzymes required for photosynthesis (in plants only)
What is the role of the Rough Endoplasmic Reticulum?
Transports proteins that are synthesised in the ribosomes
What is the role of the Smooth Endoplasmic Reticulum?
Synthesises lipids
What are the flattened sacs called that make up the ER?
Cisternae
Why is the RER called the RER?
It has ribosomes on its outer surface
What does the nucleus create?
Ribonucleic Acid (RNA) and Ribosomes
What are lysosomes?
Specialised vesicles, they are membrane-bound spherical sacs which contain digestive enzymes used to break down materials
What is produced in the mitochondria during aerobic respiration?
ATP
What are ribosomes?
Small organelles, composed of two subunits, that make proteins from mRNA
What organelle is found in animal cells only?
Centrioles
What are centrioles?
A pair of short, hollow cylinders, usually found near the nucleus of an animal cell. They are involved in the formation of spindle fibres
What is the cytoskeleton?
A network of fine filaments or fibres that run through the cytoplasm of the cell
What are the three types of fibres in the cytoskeleton?
- Microfilaments
- Intermediate filaments
- Microtubules
What is the size of the microfilament?
7nm in diameter
What is the composition of the microfilament?
Protein: actin
What is the function of the microfilament?
Responsible for all cell movement and contraction
It is contractile
In cytokinesis, it is involved in the cell contracting during division
What is the composition of the intermediate filament?
Made up of a variety of proteins that are expressed in different types of cells
What is the function of the intermediate filament?
Provides mechanical strength and helps to maintain cell integrity
What is the composition of the microtubule?
Tubulin (a globular protein that forms tubes)
What are the functions of the microtubule?
- Transport; forms a trackway which helps the movement of organelles and vesicles
- Spindle fibres - chromosome separation
- Centrioles - found in eukaryotes (not flowering plants)
- orders spindle fibres (cell division)
- Role in the positioning of cilia and flagella
What organelles are found in plant cells only
- Chloroplasts
- Permanent vacuole
- Cell Wall
What is the role of chloroplasts?
Responsible for photosynthesis, they contain the pigments which absorb the suns light energy and carry out the reaction of photosynthesis
What are permanent vacuoles?
A fluid-filled space in the cytoplasm surrounded by a membrane called the tonoplast. It contains a solution of sugars and salts called the cell sap
What is the cell wall?
Is rigid and made of cellulose fibres running through a mixture of other polysaccharides
What type of membrane do chloroplasts have?
A double membrane
What is the stroma in chloroplasts?
A fluid-filled matrix where the second stage of photosynthesis takes place
What are the grana in chloroplasts?
A stack of thylakoids
The granal membranes increase the surface area for photosynthesis pigments
What are thylakoids in chloroplasts?
Contain photosynthetic pigments like chlorophylls and xanthophyll
Which of these is the correct equation for the calculation of an object size?
A. Object size = Image size x Magnification
B. Object size = Magnification / Image size
C. Object size = Image size / Magnification
D. None of the above
C
State the function of mitochondria
Does respiration
State the function of smooth endoplasmic reticulum
Synthesises lipids
Explain why the details of mitochondria cannot be seen clearly with a very good light microscope
The length of a mitochondrion is about 200nm, the maximum resolution of a light microscope, so increasing the magnification of the light microscope would make the image blurry
Describe the advantages of staining specimens to be viewed under a microscope
Staining gives contrast so different organelles can be seen separately
