A2.2 Cell structure

Question 1

when were microscopes first invented?

Answer 1

17th century and led to discovery of cells

Question 2

what is fluorescence?

Answer 2

absorbance of light and re-emission at a longer wavelength

Question 3

what is immunofluorescence?

Answer 3

a development of fluorescent staining

Question 4

how does immunofluorescence work?

Answer 4

antibodies that bind to a specific chemical in the cell are produced, a fluorescent marker is linked to the antibodies, images produced of cells treated with these antibodies show the cell structure overlain with the bright colour of the fluorescent marker where the specific chemical occurs in the cell

Question 5

how can multicoloured fluorescent images be produced?

Answer 5

using multiple types of antibodies with fluorescent markers of different colours

Question 6

what happens when the resolution of the microscope has been exceeded?

Answer 6

magnification can be increased with a microscope until a point beyond which the image can no longer be focused sharply

Question 7

what is resolution?

Answer 7

the ability of a microscope to show 2 close objects separately in the image; the shorter the wavelength, the higher the resolution

Question 8

what did electron microscopes allow for?

Answer 8

to investigate the detailed structure(ultrastructure) of cells

Question 9

what is freeze-fracture electron microscopy?

Answer 9

used to produce images of surfaces within cells

Question 10

how does freeze-fracture work?

Answer 10

sample is plunged into liquefied propane at -190 so it rapidly freezes; steel blade used to fracture the frozen sample; fracture goes thru weakest points of cells; vapour of platinum or carbon is fired onto the fracture surface at angle of 35 to form a coating; creates a replica of the fracture surface; replica is removed from sample and examined w electron microscope; usually 2nm but thickness varies bc of angles at which coating is applied; gives the impression of a 3D image with shadowing

Question 11

where is the weakest point in cells?

Answer 11

usually the middle of membranes between the 2 layers of phospholipid

Question 12

what is cryogenic electron microscopy?

Answer 12

used for researching the structure of proteins

Question 13

how does cryo-EM work?

Answer 13

thin layer of protein solution applied to a grid then flash-frozen with liquid ethane at -183 to create smooth vitreous ice and prevent the formation of water crystals; grid placed in electron microscope and detectors record patterns of electrons transmitted by individual protein molecules

Question 14

what is used to produce a 3D image of the protein molecules in cryo-EM?

Answer 14

computer algorithms

Question 15

what does cryo-EM analyse and what does it allow for?

Answer 15

analyses proteins at the instant in time when the water around them froze, allows scientists to research proteins that change from one form to another as they carry out their function

Question 16

what are the cellular structures found in all living organisms?

Answer 16

DNA as genetic material(needed for producing mRNA by transcription so proteins can be synthesised); cytoplasm composed mainly of water(contains enzymes which catalyse chemical reactions); plasma membrane composed of lipids(controls movement of substances in and out of cell and allows different chemical conditions to be maintained inside the cell from the outside eg pH

Question 17

what is the cell wall made of in prokaryotes?

Answer 17

peptidoglycan

Question 18

which ribosomes are in prokaryote cells?

Answer 18

70s

Question 19

which components of the cell have 1 membrane?

Answer 19

rough ER, smooth ER, golgi, lysosomes, vesicles, vacuoles

Question 20

which components of the cell have 2 membranes?

Answer 20

nucleus, mitochondria, chloroplasts

Question 21

why do animal cells not have cell walls?

Answer 21

allows them to take in food by endocytosis but makes them vulnerable to bursting if too much water enters by osmosis

Question 22

which 2 types of small temporary vacuole occur in some animal cells?

Answer 22

contractile vacuoles that expel excess water by exocytosis and food vacuoles that digest food or pathogens taken in by endocytosis

Question 23

what are plastids and where are they?

Answer 23

family of double-membraned organelles, plant cells have varied types such as chloroplasts(photosynthesis) and amyloplasts(to store starch) animal and fungus cells have no plastids

Question 24

what are centrioles?

Answer 24

hollow cylinders containing a ring of microtubules arranged at right angles to each other, centrioles are involved in producing spindle fibres for cell division

Question 25

cells with no nucleus?

Answer 25

anucleate-cannot transcribe DNA to make mRNA and cant synthesise proteins

Question 26

cell structure in RBCs

Answer 26

no nucleus so there’s more space for haemoglobin; cytoplasm has no nucleus, mitochondria or ribosomes; without source of proteins for repair or other functions they have a limited lifespan usually 4 months

Question 27

cell structure in phloem sieve tube elements

Answer 27

subunits of the tubes that transport sugar-containing sap in plants; initially have a nucleus but breaks down so sap can flow more easily; supplied with proteins by adjacent companion cells which have a nucleus and rough ER

Question 28

cell structure in skeletal muscle

Answer 28

made up of muscle fibres, each fibre is enclosed inside a plasma membrane like a cell

Question 29

why do muscle cells have striated appearance?

Answer 29

regular arrays of protein filaments used in muscle contraction

Question 30

cell structure of aseptate fungi

Answer 30

consist of thread-like structures called hyphae, there are long undivided sections of hypha which contain many nuclei

Question 31

what does endosymbiotic theory explain?

Answer 31

how eukaryotic cells originated

Question 32

what is the endosymbiotic theory?

Answer 32

proposes that some of the organelles within eukaryotes were once free-living prokaryotes, ingested by a larger cell where they stayed and provided a benefit causing the larger cell to eventually become a eukaryote

Question 33

what are the 3 distinct similarities between chloroplasts and mitochondria that proves endosymbiosis?

Answer 33

chloroplasts and mitochondria reproduce by binary fission just like prokaryotes; both contain DNA similar to prokaryotes; both have their own 70s ribosomes just like prokaryotes

Question 34

what is compartmentalisation?

Answer 34

division of parts of the cell into internal structures called organelles

Question 35

eg of prokaryotes

Answer 35

bacteria, archea

Question 36

function of nucleoid in prokaryotes

Answer 36

region containing genetic info-DNA is circular and naked(not associated with protein)

Question 37

function of pili in prokaryotes

Answer 37

protein filaments used for bacterial adhesion and conjugation

Question 38

how does TEM(transmission electron microscope) work?

Answer 38

a beam of electrons passes thru a thin section of a specimen, areas that absorb the electrons appear darker on the electron micrograph

Question 39

how does SEM(scanning electron microscope) work?

Answer 39

in a scanning electron microscope a beam of electrons passes across the surface and scatter; the pattern of scattering builds up a 3D image depending on the contours of the specimen

Question 40

limitations when using electron microscopes?

Answer 40

whole system must be in a vacuum so living specimens cant be seen; a complex staining processes needed which might introduce artefacts into the image; specimens need to be v thin esp for TEM so electrons can pass through;

Question 41

why is a vacuum needed inside the body of an electron microscope?

Answer 41

electrons are deflected when they strike molecules of the gases which make up air making it difficult to focus the electron beam, producing a vacuum in the microscope stops this

Question 42

what is an artefact?

Answer 42

an object observed in an image of a specimen seen in a microscope which is not part of the specimen in real life, result of the preparation of specimen for microscopy

Question 43

mitochondria structure

Answer 43

oval shaped, bound by a double membrane called the envelope, inner membrane is folded to form projections called cristae with a matrix on the inside containing all the enzymes needed for respiration

Question 44

function and structure of RER

Answer 44

series of flattened sacs enclosed by a membrane with ribosomes on the surface,RER folds and processes proteins made on the ribosomes

Question 45

function and structure of SER

Answer 45

system of membrane bound sacs, SER produces and processes lipids

Question 46

why is the resolution of a light microscope low?

Answer 46

because it uses light with a long wavelength