Edexcel IAL Biology Unit 2 - Topic 3: Cell Structure

What are all living things composed of?

eukaryotic and prokaryotic cells.

Ultrastructure of eukaryotic cells

Human cells are eukaryotic.

Contain a nucleus and membrane-bound organelle.

Most eukaryotic cells have the same internal organelles, but cell specialisation means cells often differ in number and sometimes types of organelle present.

Structure and function of nucleus

double membrane bound called the envelope containing pores which enable molecules to enter and leave the nucleus.

has linear DNA which dictates protein synthesis
site of transcription of DNA
nuclear Membrane enables compartmentalisation and keeps reactions occuring in the nucleus and its contents separate from the cyptoplasm

Structure and function of nucleolus

Site of ribosome production and rRNA

Structure and function of rough endoplasmic reticulum

ribosomes attached to its outer surface and has single membrane bound cisternae

increases S.A for attachment of ribososmes.
polypeptide chain moves into the cisternae of the RER and folds to assume a tertiary structure.
protein is packaged into vesicles which bud off from the RER and go fuse with the golgi

Structure and function of smooth endoplasmic reticulum

A system of membrane bound sacs.

Produces and processes lipids.

Structure and function of ribosomes

Composed of two subunits made of RNA + protein

Site of translation of mRNA
not membrane bound

Structure and function of mitochondria

bound by a double membrane called the envelope.

Inner membrane is highly folded to increase surface area for attachment

with a matrix on the inside, containing all the enzymes needed for respiration.
70s ribosomes present

site of aerobic respiration which forms ATP providing energy for active transport

Structure and function of centrioles

Hollow cylinders containing a ring of microtubules arranged at right angles to each other.

Involved in cell division lay down the spindle fibres which shorten to separate chromatids

Structure and function of lysosomes

A vesicles containing digestive enzymes bound by a single membrane.
digest bacteria or worn out organelles
carries out autolysis

Structure and function of Golgi apparatus

3 to 4 curved cisternae single membrane bound. many vesicles associated with it

Golgi apparatus modifies proteins by adding a lipid or a carb

Produces lysosomes
packages protein into secretory vesicles

Role of the rER and the Golgi apparatus in the formation of extracellular enzymes

polypeptide chain moves into the cisternae of the RER and folds to assume a tertiary structure where it is packaged into a vesicle and buds off from the RER to fuse with the golgi

in golgi the proteins are modified, it is then packaged in a secretory vesicle which can then fuse with the cell surface membrane to release the enzymes outside of the cell by exocytosis

Ultrastructure of prokaryotic cells

Bacteria cells are prokaryotic cells.

structures present in bacteria

slime capsule
plasmids
pilli
flagellum
mesosme

Function of capsule

Protective slimy layer which prevents dehydration of cell

Function of plasmid

small circular DNA molecules only present in some bacteria

Function of flagellum

A tail like structure which rotates to move the cell

Function of pili

help attach to other bacterial cells.

mesosme

infolding of the cell surface membrane which increases surface area for atachment of enzymes for aerobic respiration

Function of ribosomes

Site of protein production

How can magnification and resolution be achieved using light and electron microscopy?

The electron microscope uses a beam of electrons and their wave-like characteristics to magnify an object's image, unlike the optical microscope that uses visible light to magnify images.

To achieve the maximum resolution in a microscope system, each of the optical components should be of the highest NA available. In addition, using a shorter wavelength of light to view the specimen will increase the resolution

Magnification

The degree to which an image of an object is larger than the object itself.

Image size = Actual size x Magnification

Resolution

The degree to which it is possible to distinguish between 2 points that are close together

Optical microscopes

Uses light to form an image, light has a longer wavelength than electrons, so this microscope has a lower resolution than the electron microscopes.

Advantages of using optical microscopes

Easy to use

Slides are easy to prepare

Can view live specimens

Disadvantages of using optical microscopes

Lowest resolution

Lowest maximum magnification

Only large organelles are visible

Scanning electron microscope

Scans a beam of electrons across the sample which knocks off electrons which then form an image

Advantages of using scanning electron microscopes

Higher resolution than optical microscope

Higher magnification than optical microscope

Forms 3D images

Can be used on thick specimens

Disadvantages of using scanning electron microscopes

Lower resolution than TEMs

Transmission electron microscope

Passes electrons through a thin specimen; denser regions absorb more electrons so less pass through creating a darker area on the image.

Advantages of using transmission electron microscopes

Highest resolution

Highest magnification

Can see internal structures of organelles

Disadvantages of using transmission electron microscopes

Needs very thin specimens

Slides are hard to prepare

Importance of staining specimens in microscopy

Stains and dyes are applied to tissue samples and bind to organelles making them easier to view.

Staining increases the contrast in the image formed, this can make it easier to see apart 2 objects that are close together, so it increases resolution.

different parts of the prokaryotic cells

cell wall
cell membrane
circular DNA
ribososmes (70s)

cell wall

structural su[ppport for cell contents
prevents osmotic bursting

cell membrane

controls what enters and exits the cyptoplasm which is the site of chemical reactions

circular DNA

carries genetic code that dictates protein synthesis

ribososmes 70s

site of translation of mRNA

differences between prokaryotic and eukarotic

prokaryotic cells have circular DNA while eukaryotic has Linear DNA
prokaryotic have 70s while eukaryoric has 80s and 70s
prokaryotes such as bacteria have no membrane bound organelles while eukaryotes have double and single bound organelles

cell

basic unit of structure and function in living organisms

Tissue

group of similar cells of the same origin performing a given function

organ

this is a group if different tissues and the organ can perform 1 or more functions

system

group of different organs working together in a coordinated manner

compare and contrast tissue and organ

same- both are made of cells
different- tissue is specialised similar cells whereas organ is amde of different tissues
tissues are specialized and so they perform 1 function but organ can perform more than 1 function