Microscopy and cells

Question	Answer
What dictates a molecule's solubility in water?	Its polarity.
How do lipids behave in water?	Lipids have a polar, hydrophilic 'head' and a non-polar, alkyl, hydrophobic 'tail'. The 'head' interacts with the polar water molecules, while the hydrophobic tails are attracted to each other, and droplets or vesicles are formed to avoid contact between hydrophobic regions and water.
How do proteins behave in water?	As macromolecules, proteins contain both polar and non-polar regions, and so they these tend to attract areas of the same polarity which dictates how a protein folds so that only the polar regions face towards the water in the cytosol.
Can molecules be resolved by any microscope?	No. No they can't.
How can molecules be detected and quantified?	Their properties can be used to display their presence, such as a UV or visible light absorbance characteristic of their structure and bonding. This would be detected by photospectrometry, in which there is a linear concentration-absorbance relationship (the Beer-Lambert law) or a calibration curve of known standards can be used. Proteins absorb UV at 280nm depending on their concentration of aromatic amino acids, or can be stainly fairly uniformly with Coomassie blue dye.
How is the concentration of an enzyme measured?	The loss of a reactant or the formation of a product is measured, and the rate at which this occurs, along with the concentrations in either case, can be used to establish enzyme activity. This is easiest if a substrate or product absorbs light.
What is a metabolic pathway?	A series of enzyme-catalysed reactions resulting in catabolism or anabolism.
What is the Beer-Lambert law?	A = E x C x L. Absorbance = the extinction coeffient x concentration x length. The absorbance is that of light by a sample; the extinction coefficient is the maximum/fixed absorptivity of a substance; the concentration is that of the sample in M, and the lenth is the pathway that the light has to travel e.g. a 1cm3 cuvette. The Beer-Lambert law describes the physical relationship between the absorbance of an analyte and its concentration.
How can the Beer-Lambert law be used to calculate enzyme activity?	The Beer-Lambert law can be used to calculate the concentration difference over time as a rate. This can be converted to um/min, the same as the international units for enzyme activity (IU).
Describe the cytoskeleton.	The cytoskeleton, consisting of thin actin filaments, thick microtubules (which play a part in cell replication) and intermediate filaments (which mechanically strengthen the cell), is important for compartmentalisation, transportation, cell shape and movement.
Which enzyme acts on NADH?	Lactate dehydrogenase.
List some differences between plant and animal cells.	Plant cells have a cell wall (which is crucial for cell shape and allows high turgor pressure, important for positioning for photosynthesis) and often contain chloroplasts and a large central vacuole, unlike animal cells. Animal cells have centrioles which plant cells lack.
Aqueous compartments in cells include...	...the cytosol, the mitochondrial matrix, chloroplast stroma, endoplasmic reticulae lumen, vesicular lumens, and Golgi body cisternae lumens.
The vast majority of a cell's components are...	Macromolecules.
Polymers are important in cells in what 3 ways?	1) Structurally - fibres of different lengths can be produced to be re-modelled and form polysaccharides and proteins. 2) Storage - polymers can be condensed for convenient storage and then hydrolysed when the monomers are needed, which also reduces the osmotic effects of sugars/ions. 3) Functionally - information can be encoded by the sequence of different types of monomers.
What is the main component of cells?	Water!
What makes H2O a liquid even though other molecules of its size (e.g. NH3) are gases?	Hydrogen bonding between H2O molecules require a greater amount of energy than covalent bonds or London forces to be overcome and to separate the molecules, so water exists as a liquid.
To increase the size of an image, one must...	...increase the angle of light between the object and the eye.
The light microscope resolves details as small as...	...0.2um.
List 3 requirements for viewing cells using a light microscope.	1) A bright light must be focused on to the specimen by lenses in the condenser; 2) The specimen must be prepared to allow light through it (thin); 3) An appropriate set of lenses (objective and occular) must be arranged to focus and image of the specimen in the eye.
What is dark field microscopy?	A type of microscopy which places the specimen against a dark background and exploits the human eye's ability to detect very small amounts of light against a dark background.
How are slides prepared?	The specimen is frozen or embedded in wax before being thinly sliced. If using wax, one must treat and dehydrate the specimen in order for it to work and preserve the features one wants to observe.
How does fluorescence microscopy work?	A fluorescent substance absorbs light of one wavelength and emits light of another (longer) wavelength. Either naturally fluorescent pigments in cells (e.g. chlorophyll) or fluorescent dyes are used to bind specifically to cellular molecules or coupled to antibody molecules. The specimen is illuminated with light at the excitation wavelength and then this is filtered so that only the emitted wavelength is allowed back through when the dye fluoresces.
What is confocal microscopy?	A type of fluorescence microscopy which uses a laser beam to produce an aperture of a multi-layered fluorescing specimen which results in a sharp image of the plane of focus (an optical section) that can be built in a series to form a 3D image.
What is resolution and what are the units?	Resolution is the ability to distinguish two points that are close together as separate. It is measured as a length so units are often in um with regard to light microscopy.
Why are higher resolutions achievable in electron microscopy than light microscopy?	Because electron microscopy accelerates electrons into waves using voltage and these are transmitted through/reflected from areas of varying density, allowing more detail to be seen because the wavelength produced is smaller (wavelength = ~4nm, standard resolution = ~2nm).
What is the calculation for magnification?	Magnification = image size / object size
Looking at an image to work out magnification, how does one do it?	Measure the scale bar using a ruler (in mm) and convert to um (1mm = 1000um; 1mm = 1,000,000 nm). Divide this by the number of um the scale represents (object size; keep units in um as is easier) and this is the magnification (e.g. 28mm = 28000um / 0.5 um = x56000).
How is the wavelength adjusted in electron microscopy?	By accelerating or decelerating the voltage of the stream of electrons emerging from the anode that is focused into a beam.
How are specimens produced for electron microscopy?	The specimens are dehydrated using alcohol solutions of increasing concentrations. These are then embedded in an epoxy resin and frozen before either being fractured down the line of least resistance with a knife in a vacuum, or being coated in carbon and dissolved in NaOH to produce a replica. In both outcomes, the specimen is coated in a heavy metal such as uranium, lead citrate, or gold. The heavy metal interacts with the electron waves to represent the density of areas of the sample as it has a higher atomic number than organic elements. Dense areas appear dark where the electrons are scattered while less dense areas transmit electrons and so appear lighter.
What is positive and negative staining in electron microscopy?	Positive staining involves a heavy metal being sprayed on to a specimen, while negative staining involves a solution of a heavy metal salt coating the specimen.
What is an environmental scanning electron microscope (ESEM)?	A specialised electron microscope that allows visualisation of wet/uncoated specimens by using pumps to both maintain a gaseous environment (rather than a vacuum) and allow the electron beam to still achieve a high resolution amongst all the air atoms.
What is the advantage of critical drying over evaporation?	It dries a specimen more quickly which maintains the delicate structures rather than causing damage and collapse of the membrane.
Give examples of structures that electron microscopy can resolve that light microscopy can't.	The increased numbers of cristae in particularly active mitochondria of different tissues, or the spread of adenoviruses composed of very few proteins within the cytoplasm/nucleoplasm/organelles of a cell.
Give the range of the average size of eukaryotic cells.	Between 10um and 100um.
What is the average size of a prokaryotic cell?	~2um

Next up

Microscopy and cells

Description

Resource summary

Similar

	Created by sophietevans almost 11 years ago