2.1.1 cell structure and microscopy

Light microscopes- how to prepare a dry mount and what it's used for

-used for hair, pollen, dust, insect parts, muscle tissue, plants

-solid specimens are cut thinly with a sharp blade (sectioning)

-hold with tweezers

-covered with a cover slip

Light microscope- how to prepare a smear slide

-used for blood samples to view cells

-edge of slide is used to smear sample on another slide 

-cover slip place

light microscopes

-poor resolution due to long wavelength of light (200nm)

-living samples can be examined

-a colour image is obtained

-medium is air

-magnification up to x2,000

Transmission electron microscopes 

-high magnification (x 1 000 000) and resolution (0.2nm)

-electrons pass through specimen to create an image

-2D image created

-thin specimens are placed in a vacuum

-beam of electrons are focused by the electromagnet

-some parts of specimen absorb electrons, making them appear darker

Scanning Electron Microscope

-high resolution (2nm) and magnification x500 000

-electrons reflect off surface of specimen to create an image

-electrons are scattered in different ways depending on contours

-producing a 3D image

-specimen doesn't need to be thin

-source is electron beam

-medium is vacuum

Laser Scanning Confocal microscope

-high resolution 

-3D images

-uses laser beam

-a type of fluorescent microscope

-image created using high light intensity to illuminate specimen stained with a fluorescent dye

-light is filtered through a pinhole aperture

-enables scientists to view sections of tiny structures that would be hard to physically section off, like embryos 

-as light is emitted from specimen it causes fluorescence

What is resolution?

-the ability to distinguish 2 objects from one another

-(determined by wavelength if light or wavelength of beam of electrons)

-determined by quality of equipment used, medium used (better in vacuum, no particles), source (light, laser beam)

What is magnification and the equation?

-How many times larger an image is under microscope compared to the actual size of the object

-image size= actual size x magnification

-convert units so they're the same, mm to micrometers, x1000

Light microscopes-How to prepare a wet mount

-when specimens are added to water or a stain

-coverslip is lowered/tilted with a mounted needle at an angle to prevent air bubbles froming

-used for viewing aquatic organisms

Light microscopes-How to prepare a squash slide

-they are wet mounts which you then push down on the coverslip to squash sample

-to ensure you have a thin layer to enable light to pass through

-used when creating a root tip squash sample to view chromosomes in mitosis

How do you calibrate the eyepiece graticule?

1- line up stage micrometer and eyepiece graticule

2- count how many divisions on epg fit into one division of stage micrometer

3- each small division on micrometer is 100 micrometers / 0.1mm

-use this to calculate what one division on epg is at that current magnification

One eyepice division= number of micrometers/number of eyepiece divisions

What is differential staining?

-a technique involving many chemical stains being used to stain different parts of a cell in different colours and distinguish between different organelles

What are crystal violet and methylene blue?

-stains used in light microscopy

-they're positively charged, so are attracted to and stain negatively charged materials in a cell

What are congo red and nigrosin?

-stains that are negatively charged

-can't enter cells as cytosol repels them

-this creates a stained background

-making unstained cells stand out

What is gram staining and what is it used for?

-a use of differential staining to prescribe an appropriate antibiotic

-used to identify the type of bacteria

-2 different stains are used, crystal violet and safranin  

-crystal violet is added, then iodine to fix the stain, then alcohol to wash any unfixed stain

-gram-positive bacteria appear blue/purple as stain is retained due to thicker peptidoglycan cell wall absorbing the dye

-gram-negative bacteria can't absorb crystal violet as cell wall is thin so they can't retain it

-safranin is used as a counterstain, turning them red

-gram positive are susceptible  to penicillin, which inhibits cells wall formation

What are the rules of a scientific drawing?

-draw with a sharp pencil

-title to indicate what the specimen is

-state magnification or add a scale

-annotate cell components, cells and sections of tissue visible

-don't sketch, use solid lines that overlap

-don't colour/shade

 

-aim is to show size, location and proportion

Describe Electron microscopes

-a beam of electrons has a very short wavelength = high resolution, small organelles can be visualised

-image is created using an electromagnet to focus the beam of negative electrons

-vacuum is used as air would absorb beam of electrons so living samples can't be used

-image is black and white so stain has to be added

What is the struture and function of the nucleus?

-nuclear envelope, double membrane structure

-nuclear pores

-nucleoplasm,

-chromosomes, protein-bound, linear DNA

-nucleolus, site of rRNA and ribosome production

-site of DNA replication and transcription (making mRNA)

-contains genetic information for each cell in forms of DNA, whcih directs synthesis of proteins

-controls metabolic activities of cell

What is the function and structure of flagella

-found in some eukaryotic cells

-whip-like tail structure

-for mobility and sometimes a sensory organelle for chemical stimuli

What is the structure and function of cilia?

-found on some eukaryotic cells

-hair-lke projections out of cells

-mobile cilia help move substances in a sweeping motion (e.g. in trachea to move mucus)

-stationary cilia are more important in sensory organs like the nose

What is the struture and function of centrioles?

-made of microtubules

-occur in pairs to form a centrosome

-involved in production of spindle fibre to separate chromosomes in cell division

What is the struture and function of the cytoskeleton?

-a network of fibres for mechanical strength, cell movement, transport within cell

-consists of microfilaments, microtubules and intermediate fibres

-many organelles are bound to hold them in place

-movement of vesicles

-changing cell shape

What is the structure and function of the endoplasmic reticulum?

-Rough and smooth ER both have folded membranes calles cisternae made from phospholipid bilayer for compartmentalisation

-rough ER cisternae hold ribosomes in place

-function of rough ER is protein synthesis

-function of smooth ER is synthesis and storage of lipids and carbohydrates

What is the structure and function of golgi apparatus and vesicles?

-folded membranes called cisternae 

-secretary vesicles pinch off from cisternae

-adds carbohydrate to proteins to make glycoproteins

-produce secretory enzymes

-secrete carbohydrates

-transport, modify and store proteins/lipids

-form lysosomes 

-finished products are transported to cell surface in golgi vesicles where they fuse with the cell membrane and contents are released

What is the structure and function of lysosomes?

-bags/vesicles of digestive enzymes

-break down pathogens ingestes by phaocytic cells

-break down dead cells (autolysis) and worn out organelles for reuse of material

-exocytosis -release enzymes to outside of cell to destroy material

What is the structure and function of mitochondria

-double membrane bound

-inner membrane called cristae

-fluid centre called the mitochondrial matrix

-contain loop of mitochondra DNA

-site of aerobic respiration and ATP production

-DNA to code for enzymes needed in respiration

What is the structure and function of ribosomes?

-small, made up of proteins and rRNA

-80s, large ribosomes found in eukaryotic cells (25nm)

-70S, smaller found in projaryotic cells, mitochondra and chloroplasts

-site of protein synthesis

What is the structure and function of chloroplasts?

-surrounded by a double membrane

-contains thylakoids (folded membranes embedded with a pigment)

-stroma contains enzymes for photosynthesis

-site of photosynthesis

What is the structure and function of the cell wall?

-in plants and fungi cells

• (plants) made of microfibrils of the cellulose polymer

• (fungi) made of chitin, a nitrogen-containing polysaccharide

• (bacteria) made of peptidoglycan

-provides structural strength for cell

What is the structure and function of plasma (cell surface) membrane?

-phospholipid bilayer- molecules embedded within and attached on outside (proteins, carbohydrates, cholesterol)

-controls entrance and exit of molecules

Which organelles are involved in production and secretion of proteins?

-Instructions fro protein syntehsis in genes is transcribed/copid into mRNA

-mRNA leaves nucleus via nuclear pores and travels to ribisomes on RER

-polypeptide chains are synthesised on rough endoplasmic reticulum

-these chains are packages into transport vesicles to be sent to golgi apparatus via cytoskeleton

-in golgi apparatus, the proteins are modifies and packaged into the vesicles

-secretory vesicles carry the proteins to cell surface membrane, where it fuses and releases the protein by exocytosis

What are the key differences between prokaryotic cells and eukaryotic cells?

-prokaryotic cells are much smaller (2micrometres vs 10-100)

-no membrane-bound organelles

-smaller ribosomes, 70S

-DNA isn't contained within a nucleus and is circular rather than linear (not plasmids)

-a cell wall made of polysaccharide, vs no cell wall in animals, chitin in fungi, cellulose in plants

-they may also contains plasmids and flagella

-don't contains mitochondria, chloroplasts, endoplasmic reticulum or golgi apparatus

light microscope structure

Why is staining cells necesaary?

-increases contrast as different components within cells take up stain to different degrees

-so can identify organelles

What is the acid-fast technique?

-used to differntiate species of mycrobacterium from other bacteria

-a lipid solvent is used to carry carbol fuchsin dye into cells

-cells are washed with dilute acid-alcohol solution

-mycrobacterium arent's affected by it and retain carbolfuchsin stain (red)

-other bacteria lose stain

-other bacteria are counterstained with methylene blue

What are artefacts and how are they created?

-visible structural details caused by processing the specimen

-not a feature of the specimen, e.g. air bubble

compartmentalisation

-It's created by the use of internal membranes to rpovide distinct environments

-it allows each area to have different conditions for different enzymes