test: Flashcards
cell theory:
-states that all living organisms are made of cells, which makes cells the basic unit of life
microscopes:
-these are tools that allow us to see microscopic entities, such as cells, organelles…even DNA stramds or proteins
-magnification: how large the image is made in the microscope, it’s dimesionless, but we can use an x to express it
magnification = size of magnified image/size of the real image
-resolution: how far apart two objects need to be in order to see them separately, the naked eye has a resolution of 0.1 mm
basic units:
m: meter, 1m
cm: centimeter, 10^-2m
mm: millimeter, 10^-3m
μm: micrometer, 10^-6m
nm: nanometer, 10^-9m
light microscopes:
-light microscopes magnify images up to 1500 times
-they’re widely used and relatively cheap
-they have a resolution of around 0.2μm
light microscopes: lenses
light source -> condenser lens -> specimen lens (specimen has to be a very thin layer, no more than one cell thin) -> objective lens (magnifies and inverts image) -> eyepiece lens (focuses the image in the eye, you can change the eyepiece lens to change magnification)
light microscopes: magnification
total magnification = magnification of objective lens x magnification of eyepiece lens
light microscopes - pros and cons
pros:
-we can see living organisms and cells, although some times the preparation of the sample kills them
-relatively cheap
-portable and relatively light
cons:
-artifacts appear from preparation and staining, these are results of the processing of the sample that can be mistaken for actual characteristics of the specimen
-low magnification and resolution (compared to electron microscopes)
light microscopes: stains
-we can use stains to differentially color parts of the cells, or even distinguish between living cells and dying cells
-hematoxylin stains the nuclei of plant and animal cells purple, blue or brown
-methylene blue stains the nuclei of animal cells blue
-acetocarmine stains dividing chromosomes in animal and plant cells
-iodine stains starch-containing organelles in plant cells blue-black
-trypan blue stains dead cells but not living cells
light microscopes: graticules
-graticules help us make very accurate length measurements in samples
-a graticule is a tiny, transparent scale inside the eyepiece of a microscope
-it doesn’t have fixed units, it’s just a scale that helps measure things when you look through the microscope
-the graticule is inserted in the microscope eyepiece
-it can be calibrated with the stage micrometer
-it’s used with a stage micrometer, a slide with a calibrated scale that has units (such as micrometers)
electron microscopes:
-electron microscopes magnify images up to 10 million times
-they have a resolution of less than 0.0001μm (1nm)
-there are two main types:
-Transmission electron microscopes (TEM): similar to optical, but higher resolution
-Scanning electron microscopes (SEM): they produce 3D images
electron microscopes: preparing the sample
-sample needs to be prepared in order to be seen in an electron microscope
-sample must be extremely thin and in vacuum- we slice it with a microtome, therefore, it’s always a dead sample
-we usually stain them with heavy metals to improve electron scattering- lead and uranium
-it involves other processes such as: free-dying, freeze fracturing. dehydration, embedding, sectioning, mounting on a metal grid
electron microscopes: TEM
-the transmission electron microscope works by letting a beam of electrons pass through a sample
-they have a series of electromagnetic lenses that direct the electrons through the sample
-a detector will create the image based on the electrons it receives
-some cell structures, such as the cytoplasm, let the electrons through
-some denser structures, such as the nucleus, don’t
electron microscopes: transmission micrographs
-no color, as no real image is formed
-sometimes color appear, but they’re added later with the computer
electron microscopes: SEM
-scanning electron microscopes have a lower magnification, but they provide 3D images
-the sample is not sliced that much
-they work by adding a second detector that detects the electrons as they collide with the sample
electron microscope VS light microscope:
electron microscope:
-large and installation means it cannot be moved
-vacuum needed
-complicated sample preparation
-over x500000 magnification
-resolution 0.5 nm
-specimens are dead
light microscope:
-small and easy to carry
-no vacuum needed
-easy sample needed
-up to x2000 magnification
-resolution 200nm
-specimens can be living or dead
eukaryotic cells:
-cell shape, structure and function varies a lot depending on the cell type
-some cells are spherical, cylindrical, geometrical, amorphous…
-cells contain two main structures:
-cytoplasm: contains the structures and components needed for cell function
-cell membrane: established the limit between the inside and outside of the cell
protoplasm = cytoplasm + nucleus
animal cell ultrastructure:
-cell ultrastructure: the structures of the cell that can only be seen only with an electron microscope
cell membrane:
-it delimits the inside of the cell
-it also controls transport of substances in an out of the cell
-also membranes bound most of the organelles inside the cell: mitochondria, chloroplasts, vesicles, lysosomes, rough endoplasmic reitculum, smooth endoplasmic reticulum and vacoule
-some of those membranes have enzymes that take part in metabolic processes in the cell
nucleus:
-largest organelles (1-20μm), contains DNA and the RNA formed in transcription
-DNA is attached to proteins called histones
-DNA is most of the times uncondensed, in the form of chromatin
-it has at least one denser region called nucleolus, involved producing ribosomes and controlling cell growth and division
-when cell division is to take place, DNA condenses into chromosomes
membrane of the nucleus:
-a double membrane
-it has nuclear pores
-nuclear pores control entrance and exit of substances to the nucleus
-the nuclear membrane is continuous, as it elongates to form the rough endoplasmic reticulum
mitochondria:
-the ‘powerhouse’ of the cell
-they vary in size from 1 to 10μm
-they can be seen with optical microscopes
-they perform the aerobic respiration, producing ATP from organic compounds
-cell function is related to the number of mitochondria: cells that need more energy have more mitochondria
mitochondrial structure:
-they have a double membrane too
-they have their own DNA, with genes involved in metabolism
-they divide with the cell, so each new cell obstains approximately the same number of mitochondria
-the inner membrane is folded towards the inside, forming cristae, which increases surface area
-this inner membrane has enzymes attached to take part in respiration
-the inside of the mitochondrion is called the matrix and the space bwteen membranes is called inter-membrane space
origin of mitochondria:
-besides their own DNA, mitochondria also have ribosomes
-these are special, smaller type of ribosomes called 70S ribosomes (opposite to cell ribosomes, which as 80S)
-these two facts, plus having a double membrane, made scientists believe that they come from an eubacterium: a bacterium able to produce ATP that was englufed by an early cell and stayed living inside of it, providing energy to the host cell
-this is called the endosymbiotic theory, and was proposed by LYnn Margulis
endosymbiotic theory- evidence
-based on four ideas:
-mitochondria have two membranes, and the external is though to be formed when the cell engulfed the eubacterium
-mitochondria have DNA
-mitochondria have 70S ribosomes, same as bacteria
-mitochondria synthesise their own ribosomes
