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biology AS > cell structure > Flashcards

cell structure Flashcards

1
Q

what is the use of microscopes

A

the analyse cell components and observe organelles

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2
Q

definition of magnification

A

how many times bigger the image produced by the microscope is than the real life object you are viewing

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3
Q

definition of resolution

A

the ability to distinguish between objects that are close together

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4
Q

light microscopes

A

uses light to form an image
this limits the resolution as it is impossible to distinguish between two objects that are closer than half the wavelength of light (500-650 nm)

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5
Q

what can light microscopes be used for

A

maximum resolution of 0.2 micrometers

can be used to observe eukaryotic cells, their nuclei and possibly mitochondria and chloroplasts
cannot be used to observe smaller organelles such as ribosomes, endoplasmic reticulum or lysosomes.

maximum useful magnification is about x1500

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6
Q

electron microscopes

A

uses electrons to form an image
this greatly increases the resolution giving a more detailed image

the beam of electrons has a much smaller wave length than light so an electron microscope can distinguish between two objects extremely close together

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7
Q

what can electron microscopes be used for

A

maximum resolution of around 0.0002 micrometers

can be used to observe small organelles such as ribosomes, the endoplasmic reticulum or lysosomes

maximum useful magnification is about x1,500,000

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8
Q

transmission electron microscopes

A

uses electromagnets to focus a beam of electrons which is transmitted through the specimen

denser parts of the specimen absorb more electrons and therefore appear darker on the final image produced

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9
Q

advantages of TEMs

A

gives high resolution images
allows the internal structures within cells to e seen

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10
Q

disadvantages of TEMs

A

can only be used on very thin specimens

cannot be used on live specimens ( there is a vacuum inside TEM, all water must be removed so specimen must be dead)

lengthy treatment required to prepare specimens

they do not produce a colour image

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11
Q

scanning electron microscopes

A

scan electrons across the specimen

beam bounces off the surface of the specimen and the electrons are detected forming a three dimensional image that shows the surface of specimens

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12
Q

advantages of SEM

A

can be used on thick or 3d specimens

allow the external, 3d structure to be observed

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13
Q

disadvantages of SEM

A

they give lower resolution images than TEM

they cannot be used to observe live specimens

they do not produce a colour image

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14
Q

laser scanning confocal microscope

A

relatively new technology
cells being viewed must be stained with fluorescent dyes
a thick section of tissue or small living organisms are scanned with a laser beam, which is reflected by the dyes
multiple depths of the organisms are scanned to produce an image

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15
Q

advantages of laser scanning microscope

A

can be used on thick or 3d specimens
allows the external 3d structure to be observed
very clear images are produced, high resolution due to the fact the laser beam can be focused at a specific depth

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16
Q

disadvantages of laser scanning microscope

A

slow process and takes long to obtain an image
the laser has potential to cause photo damage to the cells

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17
Q

different types of sample preparation

A

dry mount- solid specimens are viewed whole or sectioned into thin slices

wet mount- specimens are suspended in a liquid such as water or immersion oil

squash slides- a wet mount is first prepared then lens tissue is used to gently press down the cover slip, used for soft samples

smear slides- the specimen is smeared onto the slide and a cover slip is placed over

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18
Q

how light microscopes work

A

light is directed through a thin layer of specimen
light is focused through several lenses so an image can be visible
magnifying power of the microscope can be increased by rotating the objective lens

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19
Q

keep components of a light microscope

A

the eyepiece lens
objective lens
the stage
the light source
the course and fine focus

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20
Q

Preparing a slide using a liquid specimen

A

Add a few drops of the sample to the slide using a pipette
Cover the liquid/smear with a coverslip and gently press down to remove air bubbles
Wear gloves to ensure there is no cross-contamination of foreign cells

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21
Q

Methods of preparing a microscope slide using a solid specimen

A

Take care when using sharp objects and wear gloves to prevent the stain from dying your skin
Use scissors to cut a small sample of the tissue
Peel away or cut a very thin layer of cells from the tissue sample to be placed on the slide (using a scalpel or forceps)
The tissue needs to be thin so that the light from the microscope can pass through
Apply a stain
Gently place a coverslip on top and press down to remove any air bubbles

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22
Q

how can you used chemicals or freezing on specimen samples

A

Some tissue samples need to be treated with chemicals to kill/make the tissue rigid
This involves fixing the specimen using formaldehyde (preservative), dehydrating it using a series of ethanol solutions, impregnating it in paraffin/resin for support then cutting thin slices from the specimen using a microtome
The paraffin is removed from the slices/specimen, a stain is applied and the specimen is mounted using a resin and a coverslip is applied
OR

Freeze the specimen in carbon dioxide or liquid nitrogen
Cut the specimen into thin slices using a cryostat
Place the specimen on the slide and add a stain
Gently place a coverslip on top and press down to remove any air bubbles

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23
Q

Why when using an optical microscope do you always start with the low power objective lens

A

It is easier to find what you are looking for in the field of view
This helps to prevent damage to the lens or coverslip in case the stage has been raised too high

24
Q

how can you prevent the dehydration of tissues

A

The thin layers of material placed on slides can dry up rapidly
Adding a drop of water to the specimen (beneath the coverslip) can prevent the cells from being damaged by dehydration

25
Q

how to unblur images

A

Switch to the lower power objective lens and try using the coarse focus to get a clearer image
Consider whether the specimen sample is thin enough for light to pass through to see the structures clearly
There could be cross-contamination with foreign cells or bodies

26
Q

what is a graticule

A

A graticule is a small disc that has an engraved ruler
It can be placed into the eyepiece of a microscope to act as a ruler in the field of view
As a graticule has no fixed units it must be calibrated for the objective lens that is in use. This is done by using a scale engraved on a microscope slide (a stage micrometer)
By using the two scales together the number of micrometers each graticule unit is worth can be worked out
After this is known the graticule can be used as a ruler in the field of view

27
Q

what are limitations of graticules

A

The size of cells or structures of tissues may appear inconsistent in different specimen slides
Cell structures are 3D and the different tissue samples will have been cut at different planes resulting in this inconsistencies when viewed on a 2D slide
Optical microscopes do not have the same magnification power as other types of microscopes and so there are some structures that can not be seen
The treatment of specimens when preparing slides could alter the structure of cells

28
Q

staining in light microscopy

A

Many tissues that are used in microscopy are naturally transparent, they let both light and electrons pass through them
This makes it very difficult to see any detail in the tissue when using a microscope
Stains are often used to make the tissue coloured/visible

Coloured dyes are used when staining specimens
The dyes used absorb specific colours of light while reflecting others; this makes the structures within the specimen that have absorbed the dye visible
Certain tissues absorb certain dyes, which dye they absorb depends on their chemical nature
Specimens or sections are sometimes stained with multiple dyes to ensure the different tissues within the specimen show up - this is known as differential staining

29
Q

types of stains used

A

methylene blue is positively charged dye which is attracted to negatively charge materials in the cytoplasm
dyes such as congo red is negatively charged and are repelled by the negatively charged cytosol so this dye stays outside the cell leaving it unstained but it creates a contrast between the stained background

Toluidine blue and phloroglucinol are common stains used
Toluidine blue turns cells blue
Phloroglucinol turns cells red/pink

30
Q

staining of electron microscopy

A

When using Transmission electron microscopes (TEMs) the specimen must be stained in order to absorb the electrons
Unlike light, electrons have no colour
The dyes used for staining cause the tissues to show up black or different shades of grey
Heavy-metal compounds are commonly used as dyes because they absorb electrons well
Osmium tetroxide and ruthenium tetroxide are examples
Any of the colour present in electron micrographs is not natural and it is also not a result of the staining
Colours are added to the image using an image-processing software

31
Q

gram stain technique

A

used to separate bacteria into two groups gram-positive and gram-negative (differential staining)
crystal violet is applied to the bacteria, then iodine and then the slide is washed with alcohol. the gram positive bacteria retain the stain and appear blue-purple but gram negative bacteria have thinner cell walls and therefore lose the stain. they are then counter stained with safranin dye and appear red.

32
Q

drawing cells guidelines

A

The drawing must have a title
The magnification under which the observations shown by the drawing are made must be recorded
A sharp HB pencil should be used (and a good eraser!)
Drawings should be on plain white paper
Lines should be clear, single lines (no thick shading)
No shading
The drawing should take up as much of the space on the page as possible
Well-defined structures should be drawn
The drawing should be made with proper proportions
Label lines should not cross or have arrowheads and should connect directly to the part of the drawing being labelled
Label lines should be kept to one side of the drawing (in parallel to the top of the page) and drawn with a ruler

plan drawings- lower magnification

33
Q

magnification formula

A

magnification is how many times bigger the image is to the real life object
actual size= image size/magnification

34
Q

formula for total magnification

A

A light microscope has two types of lens:
An eyepiece lens, which often has a magnification of x10
A series of (usually 3) objective lenses, each with a different magnification
To calculate the total magnification the magnification of the eyepiece lens and the objective lens are multiplied together:
eyepiece lens magnification x objective lens magnification
= total magnification

35
Q

resolution limitations

A

Resolution is the ability to distinguish between two separate points
If two separate points cannot be resolved, they will be observed as one point
The resolution of a light microscope is limited by the wavelength of light
As light passes through the specimen, it will be diffracted
The longer the wavelength of light, the more it is diffracted and the more that this diffraction will overlap as the points get closer together
Electron microscopes have a much higher resolution and magnification than a light microscope as electrons have a much smaller wavelength than visible light
This means that they can be much closer before the diffracted beams overlap

36
Q

comparison of electron vs light microscopes

A

Light microscopes are used for specimens above 200 nm
Light microscopes shine light through the specimen, this light is then passed through an objective lens (which can be changed) and an eyepiece lens (x10) which magnify the specimen to give an image that can be seen by the naked eye
The specimens can be living (and therefore can be moving), or dead
Light microscopes are useful for looking at whole cells, small plant and animal organisms, tissues within organs such as in leaves or skin
Electron microscopes, both scanning and transmission, are used for specimens above 0.5 nm
Electron microscopes fire a beam of electrons at the specimen either a broad static beam (transmission) or a small beam that moves across the specimen (scanning)
The electrons are picked up by an electromagnetic lens which then shows the image
Due to the higher frequency of electron waves (a much shorter wavelength) compared to visible light, the magnification and resolution of an electron microscope is much better than a light microscope
Electron microscopes are useful for looking at organelles, viruses and DNA as well as looking at whole cells in more detail
Electron microscopy requires the specimen to be dead however this can provide a snapshot in time of what is occurring in a cell eg. DNA can be seen replicating and chromosome position within the stages of mitosis are visible

37
Q

cell surface membrane

A

all cells are surrounded by a cell surface membrane which controls the exchange of materials between the internal cell environment and the external environment- partially permeable
cell membrane is formed from a phospholipid biliary

38
Q

cell wall

A

found in plants and not in animal cells
cell was is formed outside the cell membrane of the cell and offers structural support to the cell
this structural support is provided by the polysaccharide cellulose in plants
cell wall is freely permeable

39
Q

nucleus

A

the nucleus contains chromatin (complex of linear dna tightly wound around histone proteins) which is what makes chromosomes
present in all eukaryotic cells except erythrocytes
the nucleus is large and separated from the cytoplasm by a double membrane (the nucleus envelope) which has many pores

the nucleus pores are important channels for allowing mRNA and ribosomes to travel out of the nucleus as well as allowing enzymes and signalling molecules to travel in

40
Q

nucleolus

A

area within the nucleus that is responsible for producing ribosomes. it is composed of proteins and RNA. RNA is used to produce ribosomal RNA (rRNA) which is then combined with proteins to form the ribosomes necessary for protein synthesis

41
Q

mitochondria

A

site of aerobic respiration within all eukaryotic cells
surrounded by a double membrane with the inner membrane folded to form cristae
the matrix formed by the cristae contains enzymes needed for aerobic respiration producing ATP
small circular pieces of DNA and ribosomes are also found in the matrix needed for replication

42
Q

chloroplasts

A

found in plant cells
larger than mitochondria also surrounded by a double membrane
membrane bound compartments called thylakoids containing chlorophyll stack to form structures called grana
grana are joined together by lamella (thin and flat thylakoid membranes)

chloroplasts are the site of photosynthesis:
the light-dependant stage takes place in the thylakoids
the light-independent stage takes place in the stroma

43
Q

ribosomes

A

Ribosomes are formed in the nucleolus and are composed of almost equal amounts of RNA and protein
Not membrane bound
Found in all cells
Found freely in the cytoplasm of all cells or as part of the rough endoplasmic reticulum in eukaryotic cells
Each ribosome is a complex of ribosomal RNA (rRNA) and proteins
80S ribosomes (composed of 60S and 40S subunits) are found in eukaryotic cells
70S ribosomes (composed of 50S and 30S subunits) in prokaryotes, mitochondria and chloroplasts
Site of translation (protein synthesis)

44
Q

endoplasmic reticulum

A

Rough Endoplasmic Reticulum (RER)
stacks of membrane bound sacs which form sheets called cisternae
Found in plant and animal cells
Surface covered in ribosomes
Formed from continuous folds of membrane continuous with the nuclear envelope
Processes proteins made by the ribosomes

Smooth Endoplasmic Reticulum (ER)
Found in plant and animal cells
Does not have ribosomes on the surface, its function is distinct to the RER
Involved in the production, processing and storage of lipids, carbohydrates and steroids

45
Q

golgi apparatus

A

Found in plant and animal cells
Flattened sacs of membrane similar to the smooth endoplasmic reticulum
Modifies proteins and lipids before packaging them into Golgi vesicles
The vesicles then transport the proteins and lipids to their required destination
Proteins that go through the Golgi apparatus are usually exported (e.g. hormones such as insulin), put into lysosomes (such as hydrolytic enzymes) or delivered to membrane-bound organelles

folds in the apparatus are called cisternae

46
Q

large permanent vacuoles

A

A sac in plant cells surrounded by the tonoplast, selectively permeable membrane
Vacuoles in animal cells are not permanent and small

47
Q

vesicles

A

Found in plant and animal cells
A membrane-bound sac for transport and storage

48
Q

lysosomes

A

Specialist forms of vesicles which contain hydrolytic enzymes (enzymes that break biological molecules down)
Break down waste materials such as worn-out organelles
Used extensively by cells of the immune system and in apoptosis (programmed cell death)

49
Q

centrioles

A

Hollow fibres made of microtubules
Two centrioles at right angles to each other form a centrosome, which organises the spindle fibres during cell division
Each centriole contains an outer membrane with a ring of 9 protein microtubules inside with 2 microtubules in the middle (9+2 arrangement )
allows movement for locomotion

50
Q

microtubules

A

formed by the globular protein tubulin
they polymerise to form tubes that determine the shape of the cell
they can also act as tracks for organelles moving around the cell

51
Q

microvilli

A

Found in specialised animal cells
Cell membrane projections
Used to increase the surface area of the cell surface membrane in order to increase the rate of exchange of substances

52
Q

cilia

A

Hair-like projections made from microtubules
Allows the movement of substances over the cell surface

53
Q

flagella

A

Hair-like projections made from microtubules
Allows the movement of substances over the cell surface

54
Q

microfilaments

A

fibres made from the protein actin
responsible for the movement of the cell an cytoplasm during cytokinesis

55
Q

intermediate fibres

A

gives strength to cells and helps maintain integrity

56
Q

functions of the cytoskeleton

A

made up of microtubules microfilaments and intermediate fibres

Strengthening and support:
The cytoskeleton provides the cell with mechanical strength, forming a kind of ‘scaffolding’ that helps to maintain the shape of the cell
It also supports the organelles, keeping them in position

Intracellular (within cell) movement:
The cytoskeleton aids transport within cells by forming ‘tracks’ along which organelles can move
Examples of this include the movement of vesicles and the movement of chromosomes to opposite ends of a cell during cell division

Cellular movement:
The cytoskeleton enables cell movement via cilia and flagella
These structures are both hair-like extensions that protrude from the cell surface and contain microtubules that are responsible for moving them

57
Q

protein synthesis

A

The DNA from the nucleus is copied into a molecule of mRNA via a process known as transcription

The mRNA strand leaves the nucleus through a nuclear pore and attaches to a ribosome on the rough endoplasmic reticulum

The ribosome ‘reads’ the genetic instructions contained within the mRNA and uses this code to synthesise a protein via a process known as translation

This protein then passes into the lumen (the inside space) of the rough endoplasmic reticulum to be folded and processed

Cells that produce a large number of proteins, e.g. enzyme- or hormone-producing cells have an extensive rough endoplasmic reticulum

The processed proteins are then transported to the Golgi apparatus (also known as the Golgi body or Golgi complex) in vesicles which fuse with the Golgi apparatus, releasing the proteins

The Golgi apparatus modifies the proteins, preparing them for secretion

Proteins that go through the Golgi apparatus are usually exported (e.g. hormones such as insulin), put into lysosomes (e.g. hydrolytic enzymes) or delivered to other organelles

The modified proteins then leave the Golgi apparatus in vesicles

Finally, these vesicles (containing the final proteins) fuse with the cell surface membrane, releasing the proteins by the process of exocytosis

biology AS (13 decks)

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