2.1 cell structure

bio

Define magnification

The number of times larger an image appears compared to the actual size of the object

Define resolution

The ability to distinguish between two points

The clarity of the image

What are the advantages of a light microscope?

Inexpensive

Easy to use

Portable

Can view in colour

Can view live specimens

Can view larger organelles with staining, eg nucleus

Used in schools, hospitals and research labs

What is the maximum magnification of a light microscope?

Most x1500 (some can magnify up to x2000)

What are the limitations of a light microscope?

Can only resolve an image larger than the wavelength of visible light- 400-700nm

Can only resolve structures that are 200nm apart

Cannot see structure

Describe a laser scanning microscope

Uses laser light to scan an object, point by point

Computer assembles pints into an image

High resolution and contrast

Depth selectivity allows focus on structures within specimen

Can view living specimens

Used in medical profession, eg observations of the eye

Describe an electron microscope

Uses beam of electrons, wavelength 0.004nm

Electron fired from cathode and focussed by magnets onto screen or photographic plate

High resolution, high magnification

Compare and contrast transmission and scanning electron microscopes

TEM:

Specimen is dead

Specimen must be dehydrated to fix

Stained with metal salts (which may be hazardous to user)

Beam of electrons passed through specimen and focussed on screen or photographic plate

2D black and white image

Magnification x 1000 000

Resolution 2 nm

Very expensive

Require training and high level of skill

SEM:

Specimen is dead

Specimen placed in a vacuum

Coated with fine metal film

Electrons bounced off surface of specimen and focussed onto a screen

3D black and white image, false colour can be added

Magnification x 500000

Resolution 20 nm

Very expensive

Require training and high level of skill

How do you calculate the actual size?

Actual size \= images size / magnification

How do you calculate magnification?

Magnification \= image size / actual size

How do you calculate image size?

Image size \= actual size x magnification

remember total magnification is the eyepiece x the objective!

What is differential staining?

Stains bind to specific cell structures

Allows visualisation and identification

Stain and colour for DNA- chromosomes

Acetic orcein - Dark red

Stain and colour for cytoplasm

Eosin - pink

Stain and colour for lipids

Sudan red - orange/ red

Stain and colour for Cellulose in plant cell walls

Iodine (potassium iodide) - Yellow

Stain and colour for Starch granules

Iodine (potassium iodide) - Blue black (violet under light microscope)

Why may specimens appear different under the microscope?

Preparation may distort specimen- slicing, dehydration, staining

Specimen may be sectioned at different planes- transverse or longitudinal

Image is only 2D

Sectioning of specimen may exclude some structures

What is a graticule?

Fits in the eyepiece, transparent ruler

Or on the microscope stage

Scale is arbitrary

Divisions are calibrated according to the microscope being used

Define eukaryote

Nucleus surrounded by a nuclear envelope, contains DNA wound and organised into linear chromosomes

Other membrane bound organelles

Define organelle

Ultrastructure within a cell that carries out a specific function

Not all organelles are membrane bound

Function of Nucleus

Contains linear chromosomes, regulates gene expression and protein synthesis, governs the characteristics of a cell/ organism

Function of Nucleolus

Region inside the nucleus where tRNA and rRNA are formed

Function of Nuclear envelope

Separates contents of nucleus from rest of cell, nuclear pores allow substances to pass between nucleus and cytoplasm

Function of Rough endoplasmic reticulum, RER

Fluid filled cavities continuous with nuclear envelope, large surface area studded with ribosomes for polypeptide synthesis

Function of Smooth endoplasmic reticulum, SER

Fluid filled cavities continuous with nuclear envelope, involved in synthesis of lipids, cholesterol and steroid hormones

Function of Golgi apparatus

Membrane bound flattened sacs, responsible for modification of proteins eg glycoproteins, lipoproteins, proteins packaged in vesicles and transported to plasma membrane for export

Function of Mitochondria

Site of aerobic respiration, production of ATP

Function of Chloroplasts

Site of photosynthesis, light and independent light reactions, contain the photosynthetic pigment, chlorophyll

Function of Vacuole

Permanent vacuole in plants only, surrounded by plasma membrane called tonoplast

Function of Lysosome

Vesicle formed at Golgi contains hydrolytic enzymes, can engulf and destroy old cell organelles, foreign matter and pathogens

Function of Cilia and undulipodia

Protrusions of the cytoskeleton, cilia line airways and waft mucus, sperm have undulipodium to beat tail

Function of Ribosome

2 subunits, large and small, made in nucleolus, site of polypeptide synthesis

Function of Centrioles

Bundles of microtubules, separate the spindle during cell division, involved in formation of cilia, absent from higher plants

Function of Cytoskeleton

Network of protein structures within cell, give cell stability, provides 'tracks' for movement of molecules within cell, extend between cells for communication and stability

Function of Cellulose cell wall

Provides, strength, support and shape to plant cells, absent in animal cells

non-membrane bound organelles

Ribosome

Centrioles

Cytoskeleton

Cellulose cell wall

Describe how organelles work together to synthesize and secrete a protein from a cell

Gene transcribed into mRNA in nucleus

mRNA passes out of nucleus via pores

mRNA translated into polypeptide at ribosomes

Polypeptide enters RER and passes through cisternae

Protein pinches off into vesicles

Vesicle fuses with Golgi, releasing protein

Protein modified in Golgi, pinched off into vesicle

Vesicle moved to cell surface plasma membrane and fuses

Protein released from cell

How are prokaryotes different from eukaryotes?

(Plasma membrane & cytoplasm in both)

Ribosomes 70s- smaller

Usually smaller

Less well developed cytoskeleton