Cell Structure Flashcards
What are the three types of microscopes?
- Light
- Scanning electron
- Transmission electron
How do light microscopes work to produce an image?
- Light is passed through a thin layer of biological material on a glass slide from beneath
- The light then passes through the objective lens and the eyepiece lens, both of which are convex
- The lenses work to enlarge the image through refraction
- The focus wheels can be turned to change the distance between the sample and the objective lens
- The image will be 2-dimensional but coloured
Having multiple lenses allows it to be classified as a compound microscope
What are the magnifications of the eyepiece and objective lenses in a typical light microscope?
- The eyepiece lens always has a magnification of x10
- There are three objective lenses; one objective lens has x4 magnification, one has x10, and one has x40
- The product of the eyepiece lens’ magnification and the magnification of the objective lens being used is the total magnification
Prioritise calculating magnification using the image and actual length
Labelled diagram of a light microscope
- Ensure you know the function of each component
- The diaphragm, which controls how much light is passed through the sample, is also known as the condenser
How does one prepare a slide for examination with a light microscope?
- The sample may be fixed and dehydrated by chemicals like formaldehyde to maintain a near-natural state and induce rigidity
- If the sample is dry, it is sectioned to ensure light can pass through and placed under a cover slip
- If it is to be examined wet, it is suspended in a liquid and the cover slip is placed over it at an angle to reduce the risk of forming artefacts
- After a wet mount is prepared, the slide can be squashed or smeared depending on the nature of the examined sample
- Before mounting the slide and placing the cover slip on, stains can be applied after heating (which increases adhesion)
Artefacts are structural details which are created through processing the specimen but are not features of the specimen itself; examples of artefacts include distortions caused by the presence of air bubbles
What is the purpose of staining?
- To create a visual contrast between different parts of a sample
- It makes individual cells and organelles identifiable and allows differentiation between them
- It is used in both light and electron microscopy, though electron microscopy typically involves the use of heavy metals as stains which alter electron absorption
What are some examples of positively charged stains and what do these adhere to?
- Crystal violet
- Methylene blue
- They adhere to the cytosol as it is negatively charged
What are some examples of negatively charged stains and what do these adhere to?
- Nigrosin
- Congo red
- They adhere to the outside of cells as they are repelled by the cytosol
This technique is known as negative staining as it involves repulsion between the stain and the sample to be examined
How is the gram stain technique used to differentiate between bacteria?
- It separates bacteria into two groups: gram positive and gram negative
- Crystal violet dye is fixed onto a sample by iodine and washed away by alcohol
- The gram positive bacteria, which have significantly ticker cell walls, retain the stain so can be identified by their blue colour
- A counterstain, such as safranin dye, can be applied to the gram negative bacteria, which are much more dangerous as antibiotics which inhibit the formation of cell walls are less efficacious on them
How is the acid-fast staining technique used?
- It is used to differentiate between Mycobacterium, a bacterial genus, and other species of bacteria
- A lipid solvent carries red carbolfuchsin dye into cells in the sample
- The cells are washed by an acid-alcohol solution and Mycobacterium retain the stain while the other species lose it
- Methylene blue may be added to stain the uncoloured cells to further increase contrast
After the methylene blue is added, it can be considered differential staining as two contrasting stains are present
What are the requirements of an effective biological drawing?
- Includes a title, indicates the magnification and has a scale
- Is drawn with a sharp pencil and consists of smooth, continuous lines only (no shading)
- Uses as much of the available space as posible
- Has clearly defined structures with correct proportions
- Has ruler-drawn label lines which are parallel with the top of the page, have no arrows and do not cross each other
- Includes brief, relevant annotations about the structures in the sample
What is the equation to calculate magnification?
Magnification = Image size/actual size
What is the resolution of a microscope?
Conceptually
- The capacity for distinguishing individual objects
- For example, if the resolution is 5nm, one can distinguish objects as individual provided they are at least 5nm apart
- The resolution of a light microscope is limited by the wavelength of light and consequent diffractional overlap when it passes over and between objects
- Resolution is also known as resolving power
What is a graticule and what is a micrometer?
- An eyepiece graticule is a metre with 100 markings attached to the objective lens of a microscope; it remains visually unchanged regardless of the magnification of the objective lens being used
- A stage micrometer is a slide featuring a metre totalling 10mm in length with number of intermediate markings (usually 100 or 1000); as this length remains constant, the micrometer will appear larger or smaller depending on the magnification of the objective lens
How can a graticule and a micrometer be used to determine the magnification of a microscope?
This is a process known as calibration
- For each objective lens, measure how many eyepiece graticule units correspond to a single unit on the micrometer
- For example, one may find that when using the x4 objective lens, 83 EPGUs correspond to 20 micrometer divisions
- One can then surmise that 1 EPGU is (100x20)/83 micrometers long
- The length of one EPGU is often described as the magnification factor of a certain lens as the number of EPGUs corresponding to the length of the sample can be multiplied by the magnification factor to find the true length of the sample (this is the method one should usually use)
- To determine magnification, calculate how much larger a certain fraction (e.g. 1/100) of the micrometer is in relation to the same fraction of the graticule and x10 to account for the magnification of the eyepiece lens; that said, if you have the image length, prioritise dividing it by the actual length to calculate the magnification
Even though one can calculate the magnification of a microscope by multiplying the stated magnification of the eyepiece lens and the objective lens in use, this often gives an inaccurate value when contrasted with the values gained by calibration due to minor mechanical deviations
How do scanning electron microscopes work to produce an image?
- A beam of electrons is sent across the surface of a sample
- The reflected electrons are received by a detector
- The image will be 3-dimensional but without colour
How do transmission electron microscopes work to produce an image?
- A beam of electrons is passed through a specimen and focused for enlargement
- Density dictates absorption so the electrons are not received uniformly by the detector below the sample
- The mechanism is similar to the mechanism in light microscopy
- The image will be 2-dimensional and without colour
What are the respective magnifications and resolutions for each type of microscope?
The relative size of each is more important than the specific value; you will see varying values from different sources
- Light microscope:
Resolution - 200nm
Magnification - Up to x2000 - Scanning electron microscope: Resolution - 10nm
Magnification - x200,000 - Transmission electron microscope: Resolution - 0.5nm
Magnification - x1,000,000
Why do electron microscopes have higher resolutions and magnifications than light microscopes?
- The wavelength of an electron is much smaller than the wavelength of light
- This means electrons can diffract more without overlap and can allow differentiation between objects much closer to each other than light can
- Theoretically, light microscopes could have the same magnification as electron microscopes; however, resolution would be the limiting factor and the image would be completely blurry
- This is why electron microscopes can have much higher magnifications than light microscopes
What are the advantages and disadvantages of light microscopes?
Advantages:
* They are cheap
* They are easy to operate
* The samples can be alive
* The images are coloured
Disadvantages:
* Low resolution
* Low magnification (though this makes them effective for looking at multiple cells in tissue)
* 2-dimensional image
What are the advantages and disadvantages of electron microscopes?
Advantages:
* High resolution
* High magnification
* TEMs therefore allow for the close examination of a cell’s ultrastructure and individual organelles
* SEMs are the only microscopes that produce 3-dimensional, topographical images
Disadvantages:
* Costly
* Requires specialist equipment and expertise to operate
* Requires the sample to be dead as it must be in a vacuum during examination and fixed beforehand, which involves freezing, staining and dehydration with heavy metals and other chemicals
* Intricate preparation process results in the frequent creation of artefacts
* The images will be without colour
Why is compartmentalisation important in eukaryotic cells and how is it performed?
- The cytoplasm, composed of a fluid called cytosol consisting of water, dissolved salts and other organic compounds, is the site of many cellular chemical reactions
- However, many of these metabolic (catabolic for breaking down and anabolic for building up) reactions require a specific temperature and pH and the presence of enzymes to occur rapidly
- Thus, a series of selectively permeable membranes ensure that there can be specific conditions in certain areas and control the ingress and egress of substances
- The cell surface membranes ensures separation of the intracellular and extracellular environment and plasma membranes circumscribing organelles allow for the maintenance of particular conditions for these organelles
Only eukaryotic organelles are surrounded by membranes
Labelled animal cell
What is the structure and function of the nucleus?
- It contains the genetic information of the organism in the form of DNA
- It therefore controls which proteins are synthesised and therefore the metabolism of the cell as enzymes are proteins
- It is circumscribed by a double membrane called the nuclear envelope which protects it from damage and contains nuclear pores to allow RNA and other substances to pass through
- It is usually the largest organelle in the cell
What is the structure and function of mitochondria?
- They are the site of the final stage of aerobic respiration
- The number of mitochondria in a cell is therefore a reflection of its metabolic requirements
- Mitochondria are circumscribed by a double membrane to ensure the optimal internal conditions for respiration
- The inner membrane folds to give structures called cristae
- The fluid found within a mitochondrion is known as the matrix as contains the enzymes necessary for respiration
- Mitochondria also contain small, circular pieces of DNA ((mt)DNA) and ribosomes so can synthesise proteins and replicate independently
What is the structure and function of centrioles?
- Centrioles are additional features of the cytoskeleton in some eukaryotic cells and are comprised of microtubules
- These microtubules are arranged in a 9 + 0 structure involving a cylindrical structure of 9 microtubule triplets
- During cell division, two associated centrioles will form a centrosome, which is essential in the assembly and movement of spindle fibres
- They also play a role in the positioning and formation of flagella and cilia
What is the structure and function of cilia in eukaryotic cells?
- Cilia are hair-like protrusions from cells which are smaller than flagella and present in greater number
- Stationary cilia are used in the detection of changes in the extracellular environment
- Mobile cilia beat in a rhythmic manner to move fluids and other objects adjacent to the cell, e.g. mucus through the trachea
- Eukaryotic cilia, like flagella, have the 9 + 2 arrangement of microtubules
A long cilia is called an undulipodium; the only human cell to have one is a spermatozoon
What is the structure and function of the rough endoplasmic reticulum?
- It is a large, membrane-bound network found in the cytosplasm continuous with the nuclear envelope
- Its membrane folds around sacs integral to protein packaging and modification called cisternae
- It has a large number of ribosomes bound to its surface, though many ribosomes are still found free in the cytosol
- It is essential for protein synthesis, as a result of the attached ribosomes, and initial protein packaging and modification, as a result of the cisternae and other structures within it
One could consider it to be a network of membranes
What is the structure and function of ribosomes?
- Ribosomes are the site of protein synthesis (specifically translation)
- They are very small structures which can be attached to rough endoplasmic reticulum or found free in the cytosol
- They are not bound by a membrane
- They consist of molecules of ribosomal RNA and protein
- They can be found in all cells as well as in mitochondria and chloroplasts
What is the structure and function of the Golgi apparatus?
- The Golgi apparatus is a membrane-bound organelle similar to smooth endoplasmic reticulum
- Its membrane encloses cisternae
- It is responsible for the packaging and modification of proteins, finalising this process
Labelled diagram of a plant cell
In reality, under a microscope the shapes of cells will never be this regular
What is the structure and function of the cell wall?
- The plant cell wall is a rigid structure surrounding the cell membrane and is comprised of cellulose
- It maintains the plant cell’s rigidity as the cell’s contents push against it, contributing to the overall structure of the plant
- It allows the cell to withstand turgid pressure without bursting when excess water enters the cell and maintains its shape when excess water exits the cell
- It is freely permeable so does not inhibit the passage of substances
- It acts as a defence mechanism against invading pathogens
What is the structure and function of chloroplasts?
- They are the site of photosynthesis so are only present in cells that can receive light
- They have a double membrane surrounding them called the chloroplast envelope
- The membrane encloses fluid called the stroma
- There is an internal network of membranes in the stroma forming flattened sacs called thylakoids, which are where the chlorophyll, a pigment which absorbs light, is located
- A stack of thylakoids is called a granum; grana are joined by membranes called lamellae
- Chloroplasts also have their own DNA and ribosomes so can independently engage in protein synthesis and replication
- As starch is produced in photosynthesis, starch grains are also present in the stroma
- The network of membranes provides the surface area necessary for the enzymes responsible for photosynthesis to work optimally
- Chloroplasts are much larger than mitochondria
Labelled diagram of a prokaryotic organism
Prokaryotic cells are up to 100x smaller than eukaryotic cells
