Eukaryotic Cells Flashcards
Who put together one of the first working optical microscopes?
Robert Hooke
What does cell theory state?
That cells are a fundamental unit of structure, function and organisation in all living organisms
What are the two types of microscope and what is their magnification?
- the light microscope/ optical microscope. It can magnify up to 1500 times
- the electron microscope: can give a magnification of up to 500 000 times
How does a light microscope work?
A specimen or thin slice of biological material is placed on the stage of the light microscope and illuminated from underneath, either by sun light reflected with a mirror or by a built-in light source. The objective lens produces a magnified and inverted image, which the eyepiece lens focuses at the eye
How do you calculate the total magnification kf a light microscope?
Magnification of objective lens x maginfication of eyepiece lens = total magnification
How will most of the specimens in a light microscope be prepared?
Most of them will be dead, stained, specially preserved and sectioned (very thinly sliced) before they are mounted on a slide. However you can look at living organisms, tissue and cells
Staining a specimen is used to make it to identify particular types of cell or particular parts of the cell under the microscope. What are some of the stains used and what do they stain?
- haematoxylin - stains the nuclei of plant and animal cells purple, blue or brown
- methylene blue - stains the nuclei of animal cells blue
- acetocarmine - stains the chromosomes in dividing nuclei in both plants and animals
- iodine - stains starch containing material in plant cells blue-black
What are the advantages and disadvantages of a light microscope?
• advantages:
- can see living plants and animals or parts of them directly. This is useful and allows you to compare prepared slides with living tissue
- relatively cheap so are available in schools and universities, hospitals, industrial labs and research labs
- relatively light and portable so we can use them anywhere
• disadvantages:
- preserving and staining tissue can produce artefacts in the tissues being observed, so what we see may be the result of preperation rather than a true representation of the living tissue
- limited powers of resolution and magnification
How do electron microscopes work?
- uses a beam of light to form an image
- the electrons are scattered by the specimen in much the same way as light is scattered in a light microscope
- in an electron microscope the electrons effectively behave like light waves with a very tiny wavelength
- electromagnetic or electrostatic lenses focus the electron beam to form an image.
- resolving power increases as the wavelength gets smaller so electron microscopes can resolve detail down to less than 0.00001 um
- for it to work the specimens have to be in a vacuum
What does the preperation of a specimen for an electron microscope involve?
• the specimen has to be dead as it takes place in a vacuum
• involves:
- chemical preservation
- freeze drying
- freeze fracturing
- removing the water (dehydration)
- embedding
- sectioning
- mounting on a metal grid
• specimens are often stained using heavy metals such as uranium and lead. This is to improve the scattering of the electrons and make greater contrast in the image making it clearer and easier to interpret
• the image is displayed on a monitor or computer screen
What are the two main types of electron micrographs and what are their differences?
- transmission electron micrographs (TEMs): they are 2D
* Scanning electron micrographs (SEMs) have lower maginfication but are 3D and can be very striking
What is added after the image has been taken on an electron micrograph and why?
False colours to make it easier to identify different cells. They are not stains
What are the advantages and disadvantages of using electron microscopes?
• advantages:
- huge powers of magnification and resolution
• disadvantages:
- all specimens are examined in a vacuum (air would scatter the electrons and make the image of the tissue fuzzy) so it is impossible to look at living material
- specimens undergo severe treatment that is likely to result in artefacts. Preparing specimens for the electron microscope is very skilled work
- very expensive
- large, have to be kept at a constant temperature and pressure and need to maintain an internal vacuum
What is maginfication
A measure of how much bigger the image you see is than the real object
What is resolution (resolving power)?
A measure of how close together two objects are before they are seen as one
What are eukaryotes?
Cells with genetic material contained in a membrane bound nucleus and a number of other membrane bound organelles
What membranes do prokaryotes have?
Cell surface membrane
What does the cell surface membrane (outer cell membrane) do?
Forms the boundary of all cells controllint what passes into and out of the cell and allowing the fluids either side of it to have different compositions
What do membranes within cells make possible?
To have the right conditions for a particular reaction in one part of the cell and different reactions to suit other reactions somewhere else in the same cell
What functions do membranes perform apart from the controlling of substances in and out?
- many chemical processes take place on membrane surfaces. The reactions of respiration take place on the inner mitochondrial membrane. Enzymes and other factors are held closely together so the reaction can proceed smoothly
- the cell surface membrane is flexible to allow the cell to change shape very slightly as its water content changes or dramatically e.g. when a white blood cells engulfs a bacterium
- chemical secretions made by the cells are packaged into membrane bags called vesicles so some membranes must be capable of breaking and fusing together readily
What is the membrane mainly made up of?
Lipids and proteins
What type of lipids exist in the membrane?
Polar lipids. They are lipid molecules with one end joined to a polar group
Many of the polar lipids in the membrane are phospholipids. How do they form a unit membrane?
With water or aqueous solution on either side they form a bilayer with their hydrophillic heads pointing into the water while the hydrophobic tails stay protected in the middle
What molecules does a lipid-bilayer allow in?
Fat-soluble organic molecules
Ionic molecules are needed in cells but these can not pass through lipids. How do they get in?
Because the cell membrane also consists or proteins and other molecules
What is the fluid mosaic model?
• the model of the floating proteins (like icebergs) in a lipid sea
What affects how freely proteins float about in the membrane?
The proportion of phospholipids containing unsaturated fatty acids. The more unsaturated fatty acids the more fluid in the membrane
How are proteins arranged in the membrane?
- many have a hydrophobic part which is buried in the lipid bilayer and a hydrophilic part whicn can be involved in a variety of activities
- some proteins penetrate all the way through the lipid whilst others only go part of the way through the bilayer
How do membrane proteins help substances move across the membrane?
- the proteins form pores or channels, some permanent and some temporary that allow specific molecules to move through
- some of these channels can be open or shut depending on the conditions in the cell. These sre known as gated channels
- some of the protein pores are active carrier systems using energy to move molecules
- other protein pored are gaps in the lipid bilayer that allow ionic substances to move through the membrane in both directions
Proteins may act as specific receptor molecules on membranes. What does this mean and give an example
They make cells sensitive to a particular hormone
They may be enzymes, particularly on any internal cell membranes, to control reactions linked to that membrane
What are the cytoplasm and nucleus called together?
The protoplasm
What are the structures that can onlt be observed in detail using the electron microscope known as?
The ultrastructure of the cell
How are membranes important and what do they do?
- they are important both as an outer boundary to the cell and in the multitude of internal membranes
- they localise enzymes in reaction pathways e.g. respiration in mitochondria and photosynthesis in chloroplasts
- they compartmentalise chemicals, for example hydrolytic enzymes in lysosomes
What is the Cytoplasm
A jelly like liquid containing what is needed to carry out rhe day-to-day tasks of living
What is the structure of the nucleus?
- it is the largest organelle in the cell and can be seen with a light microscope
- it is usually spherical in shape and surrounded by a nuclear double membrane containing holes or pores, known as the nuclear envelope
- inside the nuclear envelope the two main substances are the nucleic acuds and proteins
- there is at least one nucleolus- an extra dense area of almost pure DNA and protein. It is involved in the production of ribosomes
How can the nucleus control events in the cytoplasm?
Because chemicals can pass in and out of the nucleus through the pores
When the cell is not actively dividing what is happening in the nucleus?
the DNA is bonded to the protein to form chromatin which looks like tiny granules
What does mitochonrion mean and what does it describe?
- ‘thread granule’
- describes the tiny rod- like structures that are 1um wide by up to 10 um long and seen in the cytoplasm of all eukaryotic cells under the light microscope