Pack 2 - Cell Structure Flashcards
What is the cell theory ?
(1839 by Schleiden & Schwann) the cell is the most basic unit of life, all living organisms are composed of one or more cells, all cells arise from pre-existing living cells
What does the differences between cells due to the addition of extra features give evidence of ?
evolution
What is the function of the plasma membrane ?
it is selectively permeable and so it regulates what can enter / leave the cells
What does eukaryotic mean ?
eukaryotes are organisms whose cells contain a ‘true’ nucleus (surrounded by the nuclear membrane) and other membrane bound organelles / genetic material, or DNA, is contained within the nucleus and is organised in chromosomes
What are organelles ?
separate compartments in a cell
What extra features do plant cells have compared to animal cells (both are eukaryotic cells) ?
chloroplasts, large vacuoles and cell walls
What is the function of a nucleus ?
controls cells activities through production of messenger RNA (mRNA), transfer RNA (tRNA) and hence protein synthesis / holds genetic material of cells in the form of DNA in chromosomes / manufactures ribosomal RNA (rRNA) and assembles ribosomes
What is a nuclear envelope ?
a double membrane surrounding the nucleus (the outer membrane is continuous with the endoplasmic reticulum)
What is the function of a nuclear envelope ?
controls entry and exit of substances / contains reactions inside the nucleus
What is the function of nuclear pores ?
allow passage of large molecules such as mRNA out of nucleus
What is a nucleoplasm ?
granular jelly like material making up the bulk of the nucleus
What is a nucleolus ?
small spherical regions in the nucleus (maybe more than one)
What is the function of a nucleolus ?
rRNA manufactured and ribosomes assembled here
What is a chromosome ?
protein bound, linear DNA
What does protein bound mean ?
the DNA is wound around proteins (histones)
What does linear mean ?
in a line (not a circle)
What are ribosomes ?
small granules found in all cells, either in the cytoplasm or associated with the ER / made of two subunits each made up of ribosomal RNA (rRNA) and many proteins / are not enclosed by a membrane
What is the size of a eukaryotic ribosome in the cytoplasm and associated with the ER ?
80s
What is the size of a eukaryotic ribosome in the mitochondria and chloroplasts ?
70s (small / simple)
What is the size of a prokaryotic ribosome ?
70s (small / simple)
What is the function of a ribosome ?
site of protein synthesis
What is an endoplasmic reticulum (ER) ?
elaborate 3D sheet like membrane system spreading throughout the cytoplasm / continuous with the outer nuclear membrane / membranes enclose a network of tubules and flattened sacs (cisternae) / cells that make and store a lot of carbohydrates, proteins and lipids have a very extensive ER
What are the two types of ER ?
rough ER and smooth ER
What is rough ER ?
has ribosomes present on the outer surface of the membranes
What is the function of rough ER ?
large surface area for synthesis of proteins / pathway for transport of materials, especially proteins through the cell
What is smooth ER ?
no ribosomes and is often more tubular in appearance
What is the function of smooth ER ?
synthesises, stores and transports lipids / synthesises, stores and transports carbohydrates
What types of cells will contain a lot of rough ER ?
cells that need a lot of protein (eg muscle, intestinal epidermis)
How many cisternae will you find in one cell ?
one
Why does there sometimes appear to be more than one cisternae ?
if the cell is cut transversely (cross section) then a 2D photograph would not show where the 3D cisternae join up
What is a golgi apparatus ?
an elaborate 3D sheet like membrane system spreading throughout the cytoplasm / similar to smooth ER but more compact / stack of membranes that make up flatten sacs, or cisternae with small vesicles / the golgi apparatus is well developed in secretory cells (eg epithelial cells lining the intestines)
What is the function of a golgi apparatus ?
adds carbohydrates to proteins to form glycoproteins / produces secretory enzymes (eg in pancreas) / secretes carbohydrates such as those used in plant cell walls / transports, modifies and stores lipids, modifies / processes triglycerides, combines triglycerides with proteins, packaged for release / exocytosis, forms vesicles / forms lysosomes
What is a golgi vesicle ?
small, rounded hollow structure
What is the function of a golgi vesicle ?
transports substances to and from the golgi apparatus / some of theses are lysosomes
What is a lysosome ?
vesicles produced by the glogi apparatus containing molecules such as proteases and lipases / also contain lysozymes / has a size up to 1 μm in diameter
What is a lysozyme ?
enzymes that can hydrolyse some bacterial cell walls
What is the function of a lysosome ?
hydrolyse material engulfed by phagocytic cells / release enzymes to the outside of the cell to destroy material around the cell (exocytosis) / digest worn out organelles so that chemicals may be reused / completely break down cells after they have died (autolysis)
What is a mitochondria ?
enclosed by a double membrane that controls entry and exit of substances / inner membrane is folded to form extensions (cristae) / cristae provide a large surface area for enzymes and other proteins involved in aerobic respiration / the matrix contains protein, lipid, ribosomes and DNA, many enzymes involved in respiration are found here
What is the size of a mitochondria ?
1 - 10 μm
What is the function of a mitochondria ?
site of aerobic respiration / produce ATP (their job)
Why do mitochondria sometimes appear to be round ?
orientation when cut - longitudinal / transverse
What is respiration ?
release of energy from glucose
What molecule is used for energy in cells ?
ATP (adenosine triphosphate)
Which types of cells have many mitochondria (need lots of energy) ?
metabolically active cells (eg muscle cells for contraction, epithelial cells in the intestines) lots of active transport
If cells do a lot of active transport, why do they need many mitochondria ?
mitochondria are the site of aerobic respiration, respiration produced ATP, ATP is needed for active transport
Why can a person with mitochondrial disease only exercise for a short time ?
mitochondria are the site of aerobic respiration, respiration produces ATP / releases energy, energy / ATP is needed for muscle contraction
What is the size of chloroplasts ?
2 - 10 μm
What is the function of a chloroplast ?
site of photosynthesis
What is a chloroplast envelope ?
a double plasma membrane that is highly selective
What is a thylakoid ?
disc like membrane structure containing chlorophyll arranged in stacks (grana)
What is a stroma ?
fluid filled matrix containing amongst other things starch granules, DNA and ribosomes
How is the grana in a chloroplast adapted to its function ?
large surface area for attachment of chlorophyll, electron carriers and enzymes needed in the first stage of photosynthesis
How is the stroma in a chloroplast adapted to its function ?
contains enzymes for second stage of photosynthesis
How is DNA and ribosomes in a chloroplast adapted to its function ?
for quick synthesis of proteins for photosynthesis
What is another type of organism that contains chloroplasts ?
algae
What is a vacuole ?
fluid filled sacs bound by a single membrane / plants usually have one large central vacuole / the membrane of the vacuole is called the tonoplast / contains mineral salts, sugars, amino acids, wastes and sometimes pigments
What is the function of a vacuole ?
support herbaceous (non woody) plants by making cells turgid / sugars and amino acids may act as a temporary food store / pigments may colour petals to attract pollinating insects
What is a cell wall in plants ?
made up of cellulose microfibrils (which are very strong), embedded in a matrix of other polysaccharides such as hemicellulose and pectin / the middle lamella is the boundary between cells and cements cells together
What is the function of a cell wall in plants ?
mechanical strength, prevents cells bursting due to pressure build up as water moves in by osmosis / mechanical strength to the plant as a whole / allows water to move along it, contributing to water movement through the plant
What other types of eukaryotic cells have cell walls ?
fungi (made of chitin, glycan, glycoproteins) and algae (made of cellulose and/or glycoproteins)
What is cell fractionation used for ?
uses large numbers of isolated organelles to study the structure and function of organelles
What is the process of cell fractionation ?
cells being broken up and the different organelles being separated out
What are the two stages of cell fractionation ?
homogenisation and ultracentrifugation
What is homogenisation ?
tissue to be used is chopped up and placed in an isotonic buffer solution and kept at between 2 ° and 4 °C / the buffered tissue is placed in a blender and the tissue is blended or homogenised / this breaks up the cells in the tissue and releases the organelles from the cells / the homogenate is then filtered to remove complete cells and large debris
What is an isotonic (buffer) solution ?
of the same water potential as the tissue
What is a homogeniser ?
a blended used in homogenisation
Why does the solution have to be cold when homogenised ?
reduces the enzyme activity, lysosome enzymes might hydrolyse the other organelles
Why is a solution of the same water potential as the tissue used in homogenisation ?
prevents organelles from bursting / shrinking due to osmotic effects (gain / loss of water)
Why is the solution buffered in homogenisation ?
maintains a constant pH, change in pH could alter the structure of the organelle or the function of their enzymes
What is the principle of ultracentrifugation ?
the heavy organelles will fall to the bottom first in the sediment leaving the clear supernatant which will be removed and respun for longer and faster
What is the process of ultracentrifugation ?
homogenised tissue is placed in centrifuge tubes and spun at low speed for 10 minutes, after this a sediment forms at the bottom of the tube with a clear liquid, the supernatant, at the top, sediment 1 will contain the heaviest organelles (eg nuclei) / supernatant 1 is then taken off and spun at medium speed, sediment 2 will contain the next heaviest organelles (eg mitochondria) / supernatant 2 is then taken off and spun at high speed, sediment 3 will contain the next heaviest organelles (eg lysosomes)
What can sediment 1 be used for ?
to be studied under the microscope or used in experiments
What would be found in the sediment if supernatant 3 were centrifuged at an even faster speed (what is the fourth heaviest organelle ?) in ultracentrifugation ?
the next heaviest organelles (eg ribosomes)
Where do all cells come from in a mature multicellular organism ?
produced by mitosis from the fertilised egg and therefore contain exactly the same alleles as each other
What are the first group of cells in an embryo ?
identical
What happens to the cells when the embryo develops ?
the cells specialise to carry out specific functions and differ in size, shape and the number and type of organelles present (eg muscle cells and sperm cells will have many mitochondria, white blood cells many lysosomes)
If cells have the ability to develop into any cell type, why do they develop into one specific type ?
all cells contain all the genes necessary to develop into any cell type only some of these genes are switched on or expressed
How are similar cell grouped together in multicellular organisms ?
into tissues, tissues into organs, organs into organ systems (for increased efficiency)
What is a tissue ?
a collection of similar cells that performs a specific function (eg epithelial tissue in animals, consist of sheets of cells that line the surface of organs and often have a protective or secretory function, and xylem tissue in plants)
What is an organ ?
a combination of tissues that perform one major function (eg stomach in animals, muscle churns and mixes stomach contents, epithelium produces secretion and protects stomach wall, connective tissue holds together the other tissues, and leaf in plants, palisade mesophyll carriers out photosynthesis, spongy mesophyll adapted for gaseous diffusion, epidermis protects leaf and allows gaseous diffusion, phloem transports organic material away from leaf, xylem transports water and ions to the leaf)
What is an organ system ?
a number of organs working together (eg the digestive system, stomach, intestine, pancreas, gall bladder, the respiratory system, trachea, bronchus, lung, diaphragm, and the circulatory system, heart and blood vessels)
What does prokaryote mean ?
prokaryotes are organisms whose cells lack a nucleus and other membrane bound organelles / genetic material is in the form of a molecule of circular double stranded DNA / (pro means before, ie cells that evolved before cells had nuclei)
What is the size of prokaryotes compared to eukaryotic cells ?
prokaryotic cells are smaller than eukaryotic cells, bacteria are between 0.1 - 10 μm
What protection do prokaryotes have for their organelles (in terms of membranes) ?
no membrane bound organelles
What protection do prokaryotes have ?
all prokaryotic cells have a cell wall, the cell wall is made of murein (peptidoglygan), which is a polymer of polysaccharides and proteins
What is the function of a cell wall in prokaryotes ?
physical barrier, excludes some substances / protections against mechanical damage and osmotic lysis
Where is the cell surface membrane in prokaryotes ?
inside the cell wal, encloses the cytoplasm
What is the function of a cell surface membrane in prokaryotes ?
differentially permeable layer, controls entry and exit of chemicals
What is the size of ribosomes in prokaryotes ?
70s (small / simple)
What is the function of ribosomes in prokaryotes ?
protein synthesis
What is prokaryotic DNA in prokaryotes ?
prokaryotic DNA consists of a long circular strand of DNA that is not associated with proteins, unlike that of eukaryotes
Where is prokaryotic DNA in prokaryotes found ?
found in a central area of the cell (nucleoid), but is not surrounded by a membrane
What is the function of prokaryotic DNA in prokaryotes ?
carriers genetic information for replication of bacterial cells by the process of binary fission
What is a plasmid in a prokaryote ?
small circular strand of DNA (not all species)
What is the function of a plasmid in a prokaryote ?
contains genes that aid survival (eg antibiotic resistance)
What are plasmids used as in prokaryotes ?
vectors (carriers of genetic information) in genetic engineering
What is a capsule in a prokaryote ?
layer of mucilaginous slime around the outside of the cell wall (some bacteria have this)
What is the function of a capsule in a prokaryote ?
protects from other cells / helps bacteria stick together for protection
What species of prokaryotes have flagellum in prokaryotes ?
only certain species, may be more than one
What is the function of a flagellum in a prokaryote ?
used for locomotion
What are viruses ?
acellular, non living particles / contain nucleic acids such as DNA or RNA as genetic material enclosed in a protein coat or capsid / can only replicate inside living host cells / may be surrounded by a lipid envelope / capsid or envelope have attachment proteins that allow the virus to identify and attach to host cells
What is seen when an object is viewed using a microscope ?
a magnified image of the object
What is magnification ?
how many times bigger the image is than the actual object
What is the equation for working out magnification ?
magnification = size of image (or drawing) / size of real object (actual size) (this can be used to workout the magnification, given the actual size of the object and measuring the size of the image, and the actual size of the object, by measuring the size of the image and given the magnification)
What does resolution (resolving power) mean ?
the minimum distance apart two objects can be in order to appear as separate items, any closer than this, two objects will be seen as one
What does resolving power depend on ?
wavelength or form of radiation, the shorter the wavelength, the greater the resolution
How far apart do two objects need to be to be seen as separate objects for the human eye ?
0.2 mm
How far apart do two objects need to be to be seen as separate objects for a light microscope ?
0.2 μm
How far apart do two objects need to be to be seen as separate objects for the best electron microscope ?
0.1 nm
What is a limit of resolution ?
up to this limit, increasing magnification reveal more detail but beyond this the image becomes larger by more blurred (greater resolution means a clearer image), every microscope has a limit of resolution
How is a light microscope used ?
specimens can either be thin sections (2 - 5 μm) of prepared tissue that have been stained with coloured pigments, or small live specimens (eg Daphnia or Amoeba) / a beam or light is shone through the specimen (object) and through a series of lenses, the resulting magnified image being viewed by the human eye / light microscopes have poor resolution because of the relatively long wavelength of light / very intracellular detail can be seen
Why do sections need to be thin for a light microscope ?
for light to pass through the specimen, and so that you only focus on one layer of the cell
Why would sections of specimens need to be stained for a light microscope ?
thin sections are transparent, need to selectively stain cell components to be able to see them
How would you prepare a specimen to view starch grains in plant cells on a light microscope ?
add a drop of water to a (glass) slide / obtain a thin section (of plant tissue) and place on the slide or float on drop of water / stain with or add iodine in potassium iodide / lower cover slip using mounted needle
What is an eyepiece (or ocular graticule) in a light microscope ?
a disc of glass or plastic with a scale etched or printed onto it, it fits inside the eyepiece of a microscope
How is an eyepiece graticule used in a light microscope ?
the scale is usually approximately 10mm long and divided into 100 subdivisions, when you look down the microscope, you will see the scale and will be able to rotate it by twisting eyepiece lens
What is a stage micrometer ?
an ordinary looking glass slide but it has a very precise scale engraved on it, usually only 1mm in total length and very difficult to see with the naked eye, it goes on the specimen stage of the microscope but cannot be used at the same time as an ordinary microscope slide and therefore can’t be used directly as a ruler
How do you calibrate an eyepiece graticule ?
the graticule is in the eyepiece, the scale appears constant whatever the magnification on the objective lens, so for each magnification you use you have to calibrate the scale on the graticule first using the micrometer slide before being able to measure the object accurately, having now calibrated your eyepiece graticule at each objective lens magnification, you can now remove the stage micrometer slide, place your specimen under the microscope and measure objects using the eyepiece graticule scale
How is an electron microscope used ?
uses a beam of electrons rather than light / this electron beam has a shorter wavelength than light which means that an electron microscope has a higher resolving power than the light microscope / electron beams are focused using electromagnets as electrons are negatively charged / use of electron microscopes and specimen preparation requires a significant amount of training
What are the two types of electron microscopes ?
transmission electron microscope (TEM) and scanning electron microscope (SEM)
How is a transmission electron microscope used ?
ultra thin section (50 - 100 nm thick) that have been stained using stains containing heavy metals are needed / an electron beam is fired at the specimen producing an image on a screen, where electrons pass through the specimen the image appears bright, where electrons are absorbed or reflected by the specimen the image is dark / a near vacuum is required as electrons are deflected by air molecules, this means living specimens cannot be observed / produces a flat 2D image in black and white / artefacts (resulting from the preparation of the specimen) may be present on the image
How is a scanning electron microscope used ?
electrons do not penetrate the specimen, the beam is passed to and fro over the specimen surface in a regular pattern / electrons are scattered depending on the contours on the surface of the specimen / a 3D image is built up by computer analysis of these scattered electrons / the resolution is lower than that of the TEM at around 20 nm, but still better than the light microscope / images are still primarily in black and white, colour is added by a computer program
How do images contain artefacts ?
due to all the processing specimens must undergo to be examined under light and electron microscopes
What can artefacts appear as ?
breaks in membranes, empty spaces in cells and particularly with TEM where heavy metal stains are used, granular deposits may be seen, so it is therefore not always possible to be sure that what you see on an image is actually part of the natural specimen, microscopists can learn to tell the difference between artefact and true structure with lots of experience comparing images of similar specimens in different ways
