Cells Flashcards
What is a microscope?
An instruments that produces a magnified image of an object
What type of lenses are in compound light microscopes?
Convex glass lenses
What is the resolution of most light microscopes?
0.2 micrometers
Which has a high resolution: light or electron microscopes?
Electron
Why do electron microscopes have higher resolutions than light microscopes?
Beams of electrons have shorter wavelengths than beams of light
What is magnification?
How many times bigger the image is compared the object.
Magnification = image size / object size
What is 1km in metres?
1 * 10^3 m
What is 1mm in metres?
1 * 10^-3 m
What is 1μm in metres?
1 * 10^-6 m
What is 1nm in metres?
1 * 10^-9m
What does μm stand for?
micrometre
What does nm stand for?
nanometre
What is meant by resolving power?
The ability of a microscope to distinguish between two close objects
What does the resolving power of a microscope depend on?
The wavelength and form of radiation used
What is meant by the resolution limit?
The point where increasing the magnification will not also increase the detail
What is Cell Fractionation?
The process in which cells are broken up into different organelles and the organelles are separated out
Why do we need cell fractionation?
So we can isolate organelles and study the structure and function of them
What are the features of the solution the sample placed in before cell fractionation?
Cold isotonic buffered solution
What are three properties of cell fractionation buffer solution and why?
- Cold to reduce enzyme activity that may break down organelles
- Same water potential as the sample to prevent organelles bursting/shrinking as a result of osmotic water gain/gain
- Buffered so the pH doesn’t fluctuate and alter organelle structure and/or functioning enzyme activity
What is meant by a buffer solution?
One that can resist pH change upon the addition of an acidic or basic components
What are the two steps to cell fractionation?
- Homogenation
- Ultracentrifugation
What is homogenation?
The process by which cells are broken up in a homogeniser, releasing organelles from the cell. The resultant fluid, the supernatant, is filtered to remove complete cells and debris
What is a homogenate?
The fluid left after a sample is put through a homogeniser
What is Ultracentrifugation?
Process by which the fragments in the filtered homogenate are separated in a centrifuge.
What is the process of ultracentrifugation in animal cells?
- Tube of filtrate is placed in the centrifuge and spun at a low speed
- Heaviest organelles (nuclei) are forced to the bottom and form a tin sediment or pellet
- Supernatant (fluid at the top) is removed, leaving just the sediment of the nuclei
- Supernatant is transferred to another tube and is spun in the centrifuge at a faster speed than before
What is the equation for magnification?
Magnification = image size / object size
What decade were electron microscopes developed?
1930s
What are the 2 main advantages of electron microscopes?
- Higher resolving power
- The beam can be focused used electromagnets
What is the resolution of the best possible modern electron microscopes and how much better is it than light microscopes?
0.1 nm, 2000x better than light microscopes
Why must specimens be kept in a near vacuum to be viewed effectively using an electron microscope?
Because electrons are negative so they can be absorbed or deflected by particles in the air which will disrupt the function of the microscope
What are the two types of electron microscope?
- Transmission Electron Microscope (TEM)
- Scanning Electron Microscope (SEM)
What produces the beam of electrons in a TEM?
An electron gun
What focuses the beam of electrons onto the specimen on a TEM?
A condenser electromagnet
How is an image formed in a TEM?
The beam passes through a section of the specimen. Some parts of the specimen absorb electrons and appear dark and other allow electrons to pass through so appear bright. An image is then produced on a screen
What is the image formed by TEM and captured called?
A photomicrograph
Why can’t the theoretical resolving power of an electron microscope always be achieved?
- Difficulties preparing the specimen can limit the resolution
- A higher energy electron beam is required which could damage the specimen
What are the main limitations of transmission electron microscopes?
- The whole system must be in a vacuum so you can’t observe living things
- A complex staining process is required and the image is still not in colour
- Specimen must be extremely thin
- Image may contain artefacts
What are artefacts and what problems do they bring?
Artefacts are visible details that aren’t part of the specimen being observed and are results of problems during preparation. They make it hard to be sure that what we see on the photomicrograph really exists.
Why do TEMs produce 2D images?
Because the specimen has to be thin so the electron beam can penetrate.
How can we produce a 3D image using a TEM?
Take a series of sections throughout the specimen, then build up a 3d image using the series of photomicrographs produced.
Which limitations of the TEM does the SEM overcome?
- The complications of creating a 3D image
- The specimen doesn’t have to be that thin as electrons don’t have to penetrate it
How does a SEM work?
A beam of electrons is directed onto the surface of a specimen from above and is passed back and forth across a portion of the portion in a regular pattern. The electrons are scattered by the specimen and the pattern of the scattering helps build up a 3D image of the specimen.
What is the resolution of a basic SEM?
20nm
What are the disadvantages of light microscopes?
- Low resolution due to longer wavelength of light
- Low magnification
- Specimen may not represent true specimen
What are the advantages of a light microscopes?
- Cheap
- Easy to use
- Living specimens can be observed
- True colour images, but sometimes staining is required
Give two functions of membranes:
- Barrier between internal contents and of the cell and the external environment
- Barrier between the contents of an organelle and the cytoplasm
- Can be a location for chemical reactions
- Involved in cell signalling
Which molecules can cross the plasma membrane?
- Lipid soluble/non-polar
- Small enough to get through by osmosis
- Steroids, Oxygen, Carbon dioxide
Why can’t polar molecules pass easily through the phospholipid bilayer?
The centre is hydrophobic non-polar, and so won’t easily interact with the polar molecules.
How does the presence of cholesterol contribute to the plasma membrane?
- It increases strength and makes the membrane more stable
- Reduces the sideways movement of the molecules in the membrane, which controls the fluidity of the membrane.
- Reduces the movement of water soluble molecules across the cell surface membrane
Why does cholesterol increase the strength of the membrane?
It is amphipathic, so its polar and non-polar regions interact with the polar and non-polar regions of the phospholipids. This interaction between molecules increases strength and stability.
Why does cholesterol reduce the movement of water soluble molecules across the cell surface membrane?
It fills the spaces between the phospholipids
What is meant by the “Fluid Mosaic Model”?
It describes the plasma membrane structure. “Fluid” describes the flexibility of the membrane due to the fact that the phospholipids can move relative to each other. “Mosaic” refers to the distribution of proteins in the membrane, varying in size, shape and pattern.
Membrane proteins are either…
Intrinsic/Integral or Extrinsic/Peripheral
What is meant by an intrinsic protein?
One that is fully embedded in the membrane from one side to the other
What property do intrinsic proteins have that allows them to interact with fatty acid tails?
They have hydrophobic amino acids on their outside surface
What type of protein is a protein channel and what are its properties?
They are intrinsic proteins. They are lined with hydrophilic amino acids and are filled with water. This allows water soluble molecules to diffuse through the membrane.
What type of protein is a carrier protein and what are its properties?
They are intrinsic proteins, They can change shape or position to transfer molecules or ions from one side of the membrane to the other
What is meant by an extrinsic protein?
Proteins that do not span the full membrane and are only on one side of it. They are sometimes attached to intrinsic proteins
What are some functions of extrinsic proteins?
- Provide structural integrity
- Can act as enzymes
- Receptors for other molecules e.g. hormones
What is a glycoprotein?
Membrane protein with a carbohydrate molecule attached.
What are the functions of glycoproteins?
- Allow cells to attach to each other and form tissues
- Play a role in the immune system, presenting antigens to T-Cells
- Act as receptors for hormones
What are glycolipids?
Phosolipids with carbohydrate molecules attached to them
What are some of the functions of glycolipids?
- Used when cells come into contact. Glycolipids identify each other and determine whether cells can come into contact
- Can also act as antigens, determining blood group
What are the fundamental components of all plasma membranes?
- Phospholipids
- Cholesterol
- Proteins
- Glycolipids
- Glycoproteins
Levels of organisation
Cell > Tissue > Organ > Organ System > Organism
What are the steps to the production and secretion of proteins?
- Nucleus contains the gene for the protein and is the site of transcription
- Protein synthesis takes place in the ribosomes on the rER
- Protein is transported to golgi apparatus in a vesicle
- Protein is modified in golgi apparatus
- Protein is packaged into golgi vesicle
- Vesicle is transported to cell surface membrane
- Exocytosis - vesicle fuses with membrane
What is a specialised cell?
A cell that is adapted to perform a specific function
What is differentiation?
The process by which cells become specialised to perform a specific function
Name 3 key differences between prokaryotic cells and eukaryotic cells
- Prokaryotes contain no membrane bound organelles
- Prokaryote cell walls are made of peptidoglycan and plant cell walls are cellulose, fungal cell walls are chitin
- DNA of prokaryotes is in circular strands not nucleus
- ATP production of prokaryotes occurs in mesosomes
Features of viruses
- Acellular, non living
- Small (20-300nm)
- Can only multiply inside a host cell
- Attachment proteins on the outside allow it to attach to a host
- Uses reproductive system of host to reproduce
Describe how you would use a microscope to find the mean diameter of triglyceride droplets on a slide
- Calibrate eyepiece graticule using stage micrometer
- Measure organelle length
- Repeat measurements and calculate a mean
What is mitosis?
Process by which cells divide into 2 identical daughter cells typically for growth and repair
True or False - Daughter cells produced by mitosis are identical to each other but not the parent cell
False - They are identical to each other and the parent cell
What percentage of the cell cycle is mitosis?
Around 10%
What is the name of the part of the cell cycle that precedes mitosis?
Interphase
What happens during the 3 phases of interphase?
G1: Cells grow
S: DNA replicates
G2: Cells grow more in preparation for mitosis
What stage of mitosis is this and what happens?
Prophases
- Chromosomes condense and become visible
- Centrioles move to their respective poles of the cell
- Nuclear envelope dissolves, leaving chromosomes free in the cytoplasm
- Chromosomes are drawn towards the equator of the cell by spindle fibres
