basic components of living systems Flashcards
What 3 things does Cell Theory state?
• Both plant and animal tissue is composed of cells • Cells are the basic unit off all life • Cells only develop from existing cells
Define ‘compound light microscope
A light microscope which uses two lenses to magnify an object; the objective lens, which is placed near to the specimen and an eyepiece lens, through which the specimen is viewed.
List the 4 ways of preparing a sample for examination by light microscopy
List the 4 ways of preparing a
sample for examination by light
microscopy
Describe the dry mount sample preparation method
• Solid specimens are viewed whole or cut into very thin slices (sectioning) • Specimen placed in centre of slide and cover slip is placed over the sample • e.g. hair, pollen, dust, insect parts; muscle tissue or plants can be sectioned and viewed this way
Describe the wet mount sample preparation method
• Specimen are suspended in liquid e.g. water or oil • Cover slip is placed on from an angle • E.g. aquatic samples and other living organisms
Describe the squash slides sample preparation method
• Wet mount is prepared first • Lens tissue is used to gently press down cover slip • Squash sample between 2 microscope slides to avoid damage to coverslip • Good technique for soft samples • e.g. root tip squashes are used to look at cell division
Describe the smear slides sample preparation method
Edge of slide is used to smear sample creating thin even coating on another slide • Cover slip then placed over sample • E.g. for sample of blood to view cells in the blood
Give 3 reasons why stains are used in samples in light
microscopy
• Images tend to have low contrast as most cells don’t absorb lots of light • The cytosol (aqueous interior) of cells and other cell structures are often transparent • Stains increase contrast as different components in the cell take up stains to different degrees
How do you prepare a sample for staining?
• Place the sample on a slide and allow it to air dry • Heat fix the sample by passing through a flame • The specimen will adhere to the microscope slide and will then take up stains
List and describe 4 different types of stains
Crystal violet & Methylene blue are \+ve dyes attracted to and staining cell components • Nigrosin & Congo red are negatively charged dyes that are repelled by the -ve cytosol so stay outside cells
Define differential staining
Using specific stains to distinguish
different types of cell
Describe the gram stain technique
• Used to separate bacteria into gram-positive and gram-negative bacteria • Crystal violet is first applied to a bacterial specimen on a slide, then iodine which fixes the dye, then the slide is washed with alcohol
What are the 2 types of bacteria in gram staining?
• Gram negative bacteria - Bacteria with cell walls that stain red with Gram stain • Gram positive bacteria - Bacteria with cell walls that stain purpleblue with Gram stain
What is the acid-fast technique?
• Used to differentiate species of Mycobacterium • Lipid solvent is used to carry carbolfuchsin dye into the cells, which are then washed with dilute acid-alcohol solution • Mycobacterium are not affected by the acid-alcohol and retain the carbolfuchsin stain which is bright red
Define magnification
The number of times larger an image
appears, compared with the size of
the object
Define resolution
The clarity of an image; the high the
resolution, the clearer the image
What is the formula for calculating magnification?
Magnification = size of image /
. actual size of object
In a light microscope, what is resolution limited by?
The diffraction of light as it passes through samples (and lenses). • Diffraction is the tendency of light waves to spread as they pass close to physical structures • Light reflected from individual structures can overlap due to diffraction • Structures are no longer seen as separate entities and detail is lost • In optical microscopy, structure closer than 1/2 the wavelength of light can’t be seen separately
Why does using electron beams instead of light produce a better resolution?
Beams of electrons have a wavelength thousands of times shorter than light, so they are still diffracted, but individual beams can be a lot closer before they overlap. Therefore objects that are smaller and closer together can be seen separately without diffraction blurring the image
What is an eyepiece graticule?
A glass disc marked with a fine scale of 1 to 100. The scale has no units. The scale on the graticule at each magnification is calibrated using a stage micrometer
What is a stage micrometer?
A microscope slide with an accurate scale in micrometers marked on it. The scales usually 100 divisions = 1mm, so 1 division = 10 micrometers
What are the steps in using a graticule to calibrate a light microscope?
For each objective lens: 1. Put stage micrometer in place and the eyepiece graticule in the eyepiece 2. Get the scale on the micrometer slide in clear focus 3. Align the micrometer scale with the scale in the eyepiece. Take a reading from the 2 scales
Give the equation that links measurement (micrometers), graticule divisions, and
magnification factor
Measurement (micrometers) =
Graticule divisions x magnification
factor
Define ‘electron microscopy’
Microscopy using a microscope that uses a beam of electrons to illuminate a specimen. As electrons have a smaller wavelength than light, they produce images with higher resolutions.
Describe electron microscopes
• Use beam of fast-travelling electrons with wavelength < 1nm to illuminate the specimen • Electrons fired from a cathode and focused by magnets onto screen or photographic plate • Produce images with magnification up to x500 000 and still have clear resolution • Show more detail of cell ultrastructure because electrons have shorter wavelengths
What are the 2 types of electron microscope? Describe
both.
Transmission Electron Microscope • Beam of electrons transmitted through a specimen and focused to produce an image • Specimen must be chemically fixed by being dehydrated and stained then thinly sliced • 2D grey scale image is produced • Best resolving power (0.5nm) • Can produce magnification up to 2 million times Scanning Electron Microscope • Beam of electrons sent across surface of specimen; reflected electrons focused onto a screen • 3D grey scale image • Resolving power (3-10nm) • Magnification x15 to x200 000
Summarise light microscopes/microscopy
• Inexpensive to buy and operate • Small and portable • Simple sample preparation • Sample preparation doesn’t usually lead to distortion • Vacuum is not required • Natural colour of sample is seen (or stains are used) • Up to x2000 magnification • Resolving power is 200nm • Specimens can be living or dead
Summarise electron microscopes/ miscroscopy
• Expensive to buy and operate • Large and needs to be installed • Complex sample preparation • Sample preparation often distorts material • Vacuum is required • Black and white images produced (but can be coloured digitally) • Over x500 000 magnification • Resolving power of TEM is 0.5nm and of SEM is 3-10nm • Specimens are dead
What are artefacts?
Objects or structures seen through a microscope that have been created during the processing of the specimen e.g. air bubbles trapped under coverslip in light microscopy
How is electron microscopy affected by artefacts?
When preparing specimen, changes in the ultrastructure of cells are inevitable in the processing that sample must undergo. e.g. loss of continuity in membranes, distortion of organelles and empty spaces in the cytoplasm of cells
Define a ‘laser scanning microscope’
A microscope that employs a laser
beam and a pin-hole aperture to
produce an image with a very high
resolution
Describe the structure of mitochondria
• Surrounded by two membranes (double membrane) with a fluidfilled space between them • Inner membrane is highly folded into cristae, and the fluid interior is called the matrix • Membrane forming the cristae contains the enzymes used in aerobic respiration
Describe the structure of chloroplasts
• Large organelles • Found only in plant cells and in some protoctists • Surrounded by a double membrane • Internal network of membranes which form flattened sacs called thylakoids that contain chlorophyll • Each pile of thylakoids is called a grana (pl. granum) • Fluid enclosed in the chloroplast is called the stroma • Contain loops of DNA and starch grains
Describe the structure of lysosomes
• Specialised forms of vesicles that contain hydrolytic (digestive) enzymes • Each is surrounded by a single membrane
What is the function of the cytoskeleton?
• Network of fibres necessary for the shape and stability of a cell • Holds organelles in place, controls cell movement, and the movement of organelles within cells • Microfilaments - Responsible for cell movement and cell contraction during cytokinesis • Microtubules - Form a scaffold-like structure that determines the shape of the cell. Act as tracks for the movement of organelles, including vesicles around the cell. Spindle fibres are made of microtubules • Intermediate fibres - Give mechanical strength to cells ad help maintain their integrity
Describe the structure of vesicles
• Membranous sacs
• A single membrane with fluid
inside
1. Proteins are synthesised on the ribosomes bound to the ER 2. They then pass into the cisternae and are packaged into transport vesicles 3. vesicles containing newly synthesised proteins move towards the GA via the transport function of the cytoskeleton 4. Vesicles fuse with cis face of GA and the proteins enter 5. The proteins are structurally modified before leaving GA in vesicles from its trans face 6. Secretory vesicles carry proteins tone taken out of the cell; the vesicles move towards and fuse with cell surface membrane releasing their contents by exocytosis 7. Some vesicles form lysosomes that contains enzymes for use in the cell
Summarise the differences between prokaryotic and eukaryotic cells: • Nucleus • DNA • DNA organisation • Extra chromosomal DNA • Organelles • Cell wall • Ribosomes • Cytoskeleton • Reproduction • Cell type • Cell-surface membrane
