Cell structure Flashcards
Define magnification
How many times larger the image is compared to the actual size of the specimen
Define resolution
ability to distinguish between 2 points that are close together
What is the equation for magnification
magnification=image size/real size
Light microscopes
Max magnification:1500x
Max resolution:200nm
-resolution is poor due to long wavelength
-use visible light to form an image
-wavelength of visible light is around 500-650nm
-impossible to distinguish between 2 objects that are closer together than half the wavelength of light
-can view tissues and cells. Eukaryotic cells, their nuclei and possibly mitochondria and chloroplasts
-cannot observe smaller organelles
Advantages of Optical(Light) microscope
-cheap to buy and easy to use
-you don’t need to kill the specimen
disadvantages of optical(light microscope
-lower magnification
-lower resolution
-can’t see the organelles
Electron microscopes
-use electrons to form an image
-a beam of electrons have a smaller wavelength compared to light so it has a higher resolution
-maximum resolution of 0.2nm(can be used to observe small organelles such as ribosomes, endoplasmic reticulum or lysosomes)
-maximum magnification of electron microscope is about 1,500,000
Transmission electron microscopes(TEM)
-electrons are fired at the specimen and pass through the material. It is transmitted through the specimen
-denser parts of the specimen absorb more electrons which makes them appear more darker-produces contrast between different parts of the object being observed
Advantages of TEM
-can see transparent organelles
-high resolution and magnification
-internal structures can be seen
Disadvantages of TEM
-specimen must be dead
-image will be in black and white
-expensive
Scanning electron microscope(SEM)
-electrons are fired at the specimen and the beam bounces off the surface of the material and electrons are detected to create a 3D image
Advantages of SEM
-can be used on thick or 3D specimen
-allows 3D structure of specimen to be observed
Disadvantages of SEM
-specimen must die
-image is in black and white
-expensive
Laser scanning confocal microscope
-thick section of tissue or small living organisms are scanned with a laser beam which is reflected by fluorescent dyes
-computer then assembles an image
advantages of Laser scanning confocal microscope
-specimen can live
-high resolution
-3D image is produced
disadvantages of Laser scanning confocal microscope
-slow process
-laser can cause photodamage to the cells
-very expensive
What is an eyepiece graticule?
-An eyepiece graticule is a disc within the eyepiece lens that has a cm ruler. Full length is always shown and length is fixed
What is a stage micrometre?
It is a glass disc with a measurement
How to calibrate an eyepiece graticule:
-identify a region where the divisions of the eyepiece graticule and stage micrometre line up well
-count divisions of eyepiece graticule and SM in that region
-in 2 columns, write down the eyepiece graticule unit and the length of SM in that region
-divide length of SM by the number of EGU and that gives the length that represents 1 EGU
Magnification
-size of cell is usually measured in micrometres(Um) and cellular structures are usually measured in nanometres or micrometres
-all measurements must be the exact same when doing calculations
magnification=eyepiece lens magnification x objective lens magnification
Resolution
-if 2 separate points cannot be resolved, they will be observed as one point
-resolution of light microscope is limited as wavelength of light is too long
-longer the wavelength of light, the more it is diffracted
-electron microscopes have a higher resolution due to smaller wavelength of beam of electrons compared to light
Light microscope extra
-shine light through specimen which is passed through an objective lens and an eyepiece lens which magnify the specimen
electron microscope extra
-fire a beam of electrons
-shorter wavelength-high resolution
-useful for looking at organelles, viruses and DNA
-requires dead specimen
Eukaryotic cells
-all cells are surrounded by a cell surface membrane which controls the exchange of materials between the internal cell environment and the external environment
-membrane is ‘partially permeable’
-cell membrane is formed from a phospholipid bilayer
Cell walls
-found in plant cells and bacterial cells. Not animal cells though
-formed outside the cell membrane and offer structural support to the cell
-structural support is provided by the polysaccharide cellulose in plants and peptidoglycan in most bacterial cells
Nucleus
-present in all eukaryotic cells(except red blood cells)
-nucleus is the largest organelle in the cell and separated by the cytoplasm via a double membrane->nuclear envelope which has many pores
-nuclear pores allow mRNA and ribosomes to travel out of the nucleus and letting enzymes travelling in
-nucleus contains chromatin(material from which chromosomes are made)
-chromosomes are made of sections of linear DNA tightly wound around proteins called histones
Example of exchange in nuclear pores
DNA is too large of a molecule to leave the nucleus to the site of protein synthesis so it is transcribed into smaller RNA molecules which is exported via the nuclear pores
Nucleolus
-produces ribosomes which move out of the nucleus and latch onto the outside of the rough ER where they produce proteins
-composed of proteins and RNA. RNA is used to produce ribosomal RNA which is combined with proteins to from ribosomes for protein synthesis.
Nuclear envelope
-made of a double membrane
-contains nuclear pores to allow molecules to move in and out of the nucleus
Mitochondria-present in plant and animal cells
-energy generating organelle
-site for final stage of cellular respiration where energy stored in the bonds of complex organic molecules is made available for cell to use by production of molecule ATP
-has an inner and outer membrane(double membrane)
-inner membrane folds inwards to form cristae which projects into a fluid called the matrix
-inner membrane has enzymes to catalyse reactions of aerobic respiration to form ATP
Smooth Endoplasmic reticulum
-found in plant and animal cells
-formed from continuous folds of membrane which continues off from the nuclear envelope
-fluid filled cavities
-no ribosomes latched onto surface
-involved in the production, processing and storage of lipids, carbohydrates and steroids
Rough Endoplasmic reticulum
-found in plant and animal cells
-formed from continuous folds of membrane which is continued from the nuclear envelope
-coated with ribosomes
-fluid filled cavities
-processes proteins made from the ribosomes
Golgi apparatus
-found in plant and animal cells
-stack on membrane bound flattened sacs
-modifies proteins and lipids before packaging them into Golgi vesicles which transport them to required destination
Lysosomes
-found in animal cells but not in plant cells
-forms of vesicles
-small bags near a Golgi apparatus
-contains hydrolytic enzymes which break biological molecules down
-break down waste materials such as worn out organelles
-used by cells of the immune system and in apoptosis(programmed cell death)
Ribosomes
-found in all cells
-spheres with ribosome DNA bound to RER
-sites of protein synthesis
-made of rRNA and protein
Centrioles
-only animal cell
-hollow fibres made of microtubules
-2 centrioles at right angles form a centrosome which organise the spindle fibres during cell division
-made of the protein, tubulin
Microtubules
-found in all eukaryotic cells
-makes up the cytoskeleton of the cell
-made of Alpha and Beta tubulin combined to form dimers which join to make protofilaments
-13 protofilaments in a cylinder make a microtubule
Cilia
-found in plant and animal cells
-hair like projections made from microtubules
-protrusions from the cell surface membrane
-when mobile, they help move substances in a sweeping motion
-when stationary, they have an important function in sensory organs
flagella
-found in specialised cells
-enables cell mobility
-sensory organelles detect chemical changes in an environment
chloroplasts
-found in plant cells
-surrounded by double membrane
-found in green parts of the plant-green colour is the result of the photosynthetic pigment chlorophyll
-chlorophyll traps sunlight
Large permanent vacuole
-found in plant cells
-maintains cell stability especially when it is turgid
-provides rigid framework for the cell -contents of cell push against the cell wall.
-membrane is called tonoplast
-selectively permeable so some molecules can pass through and some cannot
-if vacuole does appear in animal cells, it is not large and permanent
What are organelles?
-specialised parts of the cell that carry out a particular function
-some organelles are membrane bound
-nucleus stores DNA which codes for production of proteins-also contains nucleolus which manufactures ribosomes.
Stages of production of protein
-DNA from nucleus is copied into the molecule mRNA via transcription
-mRNA leaves the nucleus through the nuclear pore and attaches to a ribosome on the rough endoplasmic reticulum
-ribosome reads instructions contained within the mRNA and uses this code to synthesise a protein via translation
-protein passes into the lumen of the rough ER to be folded and processed
–processed proteins are transported to the Golgi apparatus in vesicles which fuse with the Golgi apparatus, releasing the proteins via the transport system of the cytoskeleton
-Golgi apparatus modifies the proteins, preparing them for secretion.
-proteins are put into secretory vesicles and fuse with the cell surface membrane which releases contents via exocytosis
-some are put as lysosomes or delivered to other organelles
The cytoskeleton
-network of protein fibres
-made of 2 protein fibres which are microfilaments and microtubules
-organelles and other cell contents are moved along these fibres using ATP to drive this movement
-provides the cell with mechanical strength, forming a kind of ‘scaffolding’ that helps to maintain the shape of the cell
-it aids transport within cells by forming ‘tracks’ along which organelles can move along
-enables cell movement via cilia and flagella which contain microtubules
microtubules
-solid strands made from the protein actin
-fibres can cause some cell movement and movement of organelles within the cells
microfilaments
-hollow strands made from the protein tublin
Intermediate filaments
-give mechanical strength to the cells and help maintain their integrity
Examples of organelles using the movement feature of the cytoskeleton
(intracellular movement)
-movement of vesicles or movement of chromosomes to opposite ends of a cell during cell division
Prokaryotic and eukaryotic cells
Eukaryotic cells-plant and animal cells
prokaryotic cells-bacterial cells
How are prokaryotic cells different to eukaryotic cells?
-smaller than eukaryotic cells
-they have a cytoplasm that lacks membrane bound organelles
-ribosomes are smaller(70s) compared to those in eukaryotic cells(80s)
-no nucleus as they have a single circular DNA molecule free in cytoplasm and is not associated with proteins
-cell wall contains murein( a glycoprotein)
-have plasmids, capsules and flagellum
What are plasmids?
-small loops of DNA which are separate from the main circular DNA molecule
-contain Genes that can be passed between prokaryotes
Capsules in prokaryotic cells
-some prokaryotes are surrounded by a final outer layer called a capsule which is known as a slime capsule which helps bacteria from drying out and from attack by cells of the immune system of the host organism
Flagellum
-long tail like structure that rotate enabling prokaryote to move
Dry mount
-cover slip is necessary to flatten the specimen
-specimen viewed whole sectioned with a sharp blade
-cover slip added
e.g. hair, pollen, dust, sections of plant and muscle
wet mount
-drop of water/oil added to specimen
-cover slip added at an angle, slowly to avoid bubbles
e.g. aquatic organisms
squash slides
-prepare wet mount
-squash specimen with a cover slip/another slide
e.g. soft samples-root tips
smear slides
-drop of liquid specimen is smeared across the slide using another slide-thin sample
e.g. blood
Why is staining so important in microscopy?
-some organelles are transparent so staining allows us to identify them and they have different degrees of affinity to dye
