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Biology: The Cell Theory (Organelles and Membranes) (Cells (Units of…
Biology: The Cell Theory (Organelles and Membranes)
The Cell Theory
describes the properties of cells
principles!
The cell is the
smallest unit of life
and is the
basic unit
of structure and organisation in organisms.
each cell can maintain its own
metabolic
activity.
All cells come from
pre-existing cells.
each cell contains its own
hereditary materials
which are passed from parent to daughter cell.
All living organisms are
composed of cells
.
Can be
unicellular
or
multi-cellular
Cells
Study of living organisms at a cellular level, especially microscopy, is referred to as cytology
are
3D
in nature
Dynamic
in nature
Examples:
movements of vesicles
spindle fibres during nuclear division (mitosis and meiosis)
transport of mRNA across nuclear pore during protein synthesis in eukaryotes
Units of measurement
micrometre
: to describe whole cells or large cell structures like chloroplasts or the nucleus
nanometre:
to describe smaller cell structures known as ultrastructures like ribosomes in the cell and large organic molecules like proteins and lipids.
REMEMBER:
1cm= 10^-2m
1mm= 10^-3m
1 micrometre: 10^-6m
1nm= 10^-9m
Microscopy
Light Microscope
An instrument that uses visible light and magnifying lenses to examine small objects not visible to the naked eye.
Electron Microscope
used to study intra-cellular structures in the cell with higher degree of resolution
uses electrons to bombard specimen to generate images on fluorescent screen for a black and white photograph (
electron micrograph
) to be taken.
TYPES of cells
Prokaryotic cell
e.g. bacteria and archae (<- not in syllabus)
Characteristics:
DNA
not enclosed by a nuclear membrane
DNA located in the
nucleoid
, which is not bound by membrane
DNA is
not
complexed with histones
histones are proteins that associate with eukaryotic DNA to help it condense it into chromatin.
do
NOT
have membrane-bound organelles
Typical structure
shape and size
wide diversity of shapes
rod
spherical
sizes too
small
1/10 size of eukaryoic cells
typically 0.5 to 5 micrometres in length
unicellular
cell wall
composed of
peptidoglycan
a network of modified-sugar polymers cross-linked by short polypeptides
gives the cell shape
prevents rupture
serves as an anchorage point for flagella
capsule
sticky layer of
polysaccharide or protein
enable bacteria to
adhere to their substrate
or to each other to
form a colony
may offer protection against dehydration
shield
pathogenic bacteria from attacks by their host's immune system
pili (or fimbriae)
hair-like appendages
, also known as
attachment
/adhesion pili
enable bacteria to stick to their substrate (e.g. teeth surface) or to other bacteria
sex pili
usually longer
and lower in numbers, compared to attachment pili
are appendages that pull 2 bacteria together prior to DNA transfer from one cell to the other (during conjugation process)
flagellum (only for some types)
one or many may be present
responsible for mobility of many bacteria
lack of membrane-bound organelles
e.g. mitochondia, chloroplasts
plasma membrane
may form in-foldings which carry out many of the specialised functions like ATP production (by membrane in-foldings called memosome) and photosynthesis (only in cyanobacteria)
aerobic prokaryote
photosynthetic prokaryote
70S ribosomes
made up of 2 ribosomal subunits
small subunit (30S_
large subunit (50S)
smaller than 80S eukaryotic ribosomes
bacterial chromosome
single
chromosome
circular
, double-stranded DNA which is not enclosed within a nuclear membrane
located in nucleoid
DNA
NOT
complexed with histones
plasmids
extra-chromosomal DNA
small, circular, double-stranded DNA which
replicate independently
of the bacterial chromosome
one/serveral may be present
Eukaryotic cell
e.g. animal/plant cells, fungi, protists
a cell which possesses a distinct
nucleus
bounded by a
double membrane
Typical components
plasma membrane
defines the
boundary
of a cell and
retain
its contents
nucleus
contains the
genetic material
(chromosomes) that directs
cellular activities
nucleolus
non
-membrane bound structure composed of proteins and nucleic acids found within the nucleus
where
rRNA
is transcribed from
rRNA gene
(a short sequence of DNA) and assembled within the nucleolus. thus large amounts of rRNA can be found in nucleolus.
cytoplasm
cytosol
aqueous solution of
ions
and
organic compounds
examples:
sugars
amino acids
proteins
organelles
ribosomes
ER
golgi body
lysosome
mitochondrion
chloroplast
vacuole
centriole
cytoskeleton
network of:
microtubules
intermediate filaments
microfilaments
help cells maintain their
shape
and
internal organisation
provide
mechanical support
to help cells carry out functions such as division and movement.
Nucleus, Nucleolus & Centrioles
Nucleus
found in all eukaryotic cells
except
mature sieve tube elements
RBC
usually
uninucleated
, but some cells can be
binucleated
(e.g. paramecium) or
multinucleated
(e.g. skeletal muscle fibres)
typically spherical
Consists of:
nuclear envelop
composed of 2 membranes
outer membrane
inner membrane
membranes are
separated by a fluid-filled space
(perinuclear space)
each membrane is made up of a phospholipid bilayer (fluid mosaic model)
nuclear envelop is perforated by
nuclear pores
lined by a
protein octet
(pore complex)
function:
facilitates exchange of substances
(e.g. ribosomal subunits, mRNA, macromolecules) between the nucleus and the cytoplasm
nucleoplasm
structure
semi-fluid matrix
that fills nucleus
serves as a
medium for diffusion
of metabolites and
large macromolecules
chromosome/chromatin
structure
in eukaryotes, most of the cell's DNA is located in the nucleus, wrapped around
histone
proteins and packaged into
chromosomes
.
prokaryotes: DNA is wrapped around DNA binding proteins
when not dividing: genetic material is too dispersed and entangled to be identified. they appear under light/electron microscopes as highly elongated chromatin threads (chromatin).
during cell division: chromatin becomes
visible
as they
coil and condense
to become
chromosomes
.
no. of chromosomes in a cell varies according to the individual species
function
contains genetic information
(in the form of DNA nucleotide base sequences) which is involved in the transmission of hereditary information from one generation to the next.
NUCLEOLUS
structure
most visible structure within nucleus when the cell is not undergoing cell division
non-membrane bound structure
composed of a network of
densely stained granules and fibres
, which are made up of proteins and nucleic acids
one or more nucleoli can be found within the nucleoplasm. it depends on the stage in the cell's reproductive cycle.
function
site of transcription of rRNA genes
or synthesis of rRNA which is assembled within the nucleolus
function:
contains the hereditary material/chromosomes of an organisms
essential for cell division
directs protein synthesis by
synthesising mRNA
using the genetic information of genes on chromosomes, and sending
mRNA
to the cytoplasm via nuclear pore for
translation into a polypeptide chain
.
Centrioles
found in
animals
cells and
lower plant
cells
absent in higher plant cells
located next to nucleus
region occupied by a pair of centrioles=Centrosome
structure
exist as a
pair
of rod-like structures (cylinders), positioned with their longitudinal axis at
right angles
to each other.
each cylinder is made up of
9 triplets of microtubules
arranged in a
ring.
function
play a role in
nuclear division
in animal cells
organises the synthesis and assembly of spindle fibres
during nuclear division
position of the pair of centrioles at each pole during nuclear division is important in determining the polarity of the cells
Mitochondria & Chloroplasts
Mitochondrion
found in nearly all eukaryotic cells, including those of plants, animals, fungi and most protists.
no. of mitochondria correlates with the cell's level of metabolic activity (some cells have a single large mitochondrion, but often a cell has hundred or even thousands of mitochondria).
structure:
vary in width and length
elongated cylinder/rod-shaped
organelle; constantly changes shape
double-membrane
(inner and outer membrane, each made up of a phospholipid bilayer and its embedded mosaic of proteins)
smooth
outer membrane
inner membrane highly folded into cristae
intermembrane space
: space between inner and outer membranes
ETC & ATP synthase
complexes present in abundance on inner membrane for
higher rates OP
double membrane of mitochondria enclose dense fluid called the
matrix
enzymes
present in matrix catalyse reactions in
Krebs' Cycle
circular
DNA and
70S ribosomes
present in mitochondrial matrix for synthesis of proteins and enzymes involved in aerobic respiration
function
involved in
aerobic respiration
which results in the
synthesis of ATP molecules
Chroloplasts
structure:
vary in diameter and thickness
biconvex discs/lens-shaped
organelle
double-membrane
stacks of thylakoids on top of one another are known as
grana
, connected to one another by the
intergranal lamella
third membrane system
known as thylakoid membranes
photosystems, ETC & ATP synthase complexes
present in abundance on thylakoid membranes for
higher rates of light absorption and photophosphorylation
double membrane encloses dense fluid (
stroma
). enzymes in it catalyse reactions in the
Calvin Cycle
.
circular DNA
and
70S ribosomes
are present in stroma for the sythesis of proteins and enzymes involved in photosynthesis
function:
site of photosynthesis i
n plants and algae
light absorbing pigments
of photosystems on thylakoid membranes are involved in the conversion of light energy to chemical energy during the
light-dependent stage.
sugar is formed
in the stroma during the
light-independent stage/Calvin cycle
Ribosomes, Lysosomes, Endoplasmic Reticulum, Golgi Body
Ribosomes
structure
non-membranous
organelle
have a
small
subunit and
large
subunit
each
subunit
contains
rRNA
complexed with proteins
bound ribosomes
found attached to RER membrane
or is free in the cell cytosol of eukaryotic cells (
free ribosomes
)
(maybe) maybe in clusters->
polyribosomes
->faster synthesis
function
site of
translation
of mRNA into polypeptide
contains
binding sites
for both tRNA and mRNA
free ribosomes: generally make proteins that will function
within
the cytosol (e.g. enzymes that catalyse the first steps of sugar breakdown)
bound ribosomes: generally make proteins destined for
insertion
into membranes,
packaging
within organelles such as lysosomes, or
exportation
out of cell
sedimentation coefficient
of ribosomes in eukaryotes and prokaryotes differ in value.
eukaryotes
: 80S, small subunit:40S, big subunit: 60S
prokaryotes
: 70S, small subunit: 30S, bug subunit: 50S
Endoplasmic Reticulum
SER
structure
network
of fine membranous tubules
single-membrane
absence
of ribosomes on surface
function
lipids synthesis
for
structural component
of membranes (e.g. phospholipids)
components of
cell-cell recognition/adhesion
, e.g. glycolipids
synthesis of
hormones
(e.g. steroids or triglycerides)
detoxification
of drugs and poisons
storage of calcium ions
in
muscle
cells: special type of SER stores Ca2+ ions required for
muscle contraction
as it functions as a second messenger
RER
structure
flattened
membrane-bound sacs
single
membrane (with fluid mosaic model)
membrane
continuous
with nuclear envelop
presence of
ribosomes
on its surface
function
protein synthesis
due to ribosomes, which are the site of mRNA translation)
post-translational modifications
of proteins synthesised by ribosomes
glycosylation
phosphoyrlation
serves as
intracellular transport network
of protein within cells, as RER transports protein
a network of membranous tubules and sacs called the cisternae
ER membrane
separates
the internal compartment of ER (cisternal space/ER lumen) from the cytosol
each layer is made of a phospholipid bilayer and proteins (
fluid mosaic mode
l)
Golgi Body
structure
flattened membrane-bound sacs (cisternae) and its associated Golgi vesicles
single-membran
e (FMM)
faces
cis
forming face
cisternae is
formed
from fusion of transport vesicles from RER
trans
maturing face
cisternae is being
broken down
into secretory vesicles/lysosomes
function
further post-translational modifications
of proteins and lipids received from the ER (RER and SER), e.g. formation of glycoprotein/glycolipid
sorting and packaging of proteins
before being targeted to various cellular locations
synthesis and transport of
lipids
formation of secreory vesicles
(containing enzymes or proteins for release outside cell)/lysosomes
involved in
carbohydrate synthesis
for the formation of
cell walls
in plants
Lysosome
formation
lysosomal enzymes are
synthesised at the ribosomes
and transported to the ER lumen
post-translational modifications
are carried out on the enzymes before being transported to the Golgi body
enzymes undergo
further post-translational modifications
within the Golgi body
Golgi vesicles containing functional enzymes bud off the Golgi membrane as
primary lysosome
structure
spherical, small vesicles
single-membrane
contains hydrolytic enzymes
(e.g. proteases/lysozymes)
content:
acidic
, enzymes have low optimum pH of 5
functions
autolysis
self-digestion
of the cell after death or damage
autolysis also occurs in the
development
of many
multi-cellular organism
s
lysosomes in the cells forming the
tail
of the tadpole destroy these cells during the transforming of a
tadpole
into a
frog
.
in plant seeds, lysosome-like vesicles
release enzyme
for the
breakdown of stored food reserves
autophagy
digestion
of worn out cells after
damage
or
death
process by which
primary lysosome fuses with the autophagosome, phagosome, endosome
->formation of
secondary lysosome
worn out organelles are
broken down
by hydrolytic enzymes
products of digestion
are
absorbed
and
assimilated
into the cytoplasm of the cell
compartmentalise hydrolytic enzymes
provide a space
where the cell can
digest macromolecules safely
without general destruction that would occur if hydrolytic enzymes were not kept apart from the rest of the cell
digestion
materials made in/taken from outside
foreign particles (e.g. bacteria)
terms
primary lysosomes
produced at the Golgi body
where
hydrolytic enzymes are incorporated
secondary lysosomes
autophagosome
double-membrane bound vesicle
that
encloses cellular constituents
to be digested in a process called
autophagy
phagosome
membrane-bound vesicle formed in a cell by an
inward folding
of the cell membrane
to hold foreign matter taken into the cell by phagocytosis
endosome
membane-bound vesicles
formed from endocytosis
which involved the
invagination
(turn inside out ) of a cell's plasma membrane
to surround macromolecules
Protein Trafficking
Introduction
many types of cells secrete proteins produced by ribosomes attached to the RER
e.g.
protein insulin
produced by certain pancreatic cells is synthesised at the ribsomes on the RER and secrete this hormone out of the cells into the bloodstream
done by 2
principal processes
:
ER targeting
used to
translocate
nascent/newly synthesised
polypeptides
across the
membrane
of RER through a proteinaceous
channel/pore
protein trafficking
where
proteins
synthesised for
export
out of the cell are
transported
in
vesicles
along the
intracellular transport network
refer to pencil diagram on
pg 40
for understanding
Principal processes:
ER targeting
processes
translation begins on a free ribosome in the cytoplasm
signal recognition particle (SRP_ recognises and binds to signal sequence of newly synthesised polypeptide (or signal peptide)on the free ribosome in the cell cytoplasm. translation is temporarily halted.
SRP recognises and binds to the SRP receptor on the RER membrane
SRP-signal sequence-mRNA-ribosome complex
docks to the SRP receptor on RER membrane
SRP is released
1 more item...
functions of SRP receptor
functions as
receptor
to bind the SRP/signal peptide of the polypeptide
allows
movement
of synthesised polypeptide across the membrane
holds the ribosome
so that polypeptide can be inserted into the ER lumen
protein trafficking (IMPORTANT)
memorise and understand
pg 43
of lecture notes
Membrane Structure and Function
Cell Membranes
Fluid Mosaic Model
Main features of cell membranes
differ in thickness
phospholipid is the main lipid found in cell membranes
the cell membrane has selective permeability to different molecules
proportion of unsaturated hydrocarbon tails in phospholipids in the membrane
amount of cholesterol in the membrane
length of hydrocarbon chains in the fatty acids
fluidity of membranes
cell membranes are asymmetric
peripheral proteins (extrinsic proteins)
integral proteins (intrinsic proteins)
General Functions of Membrane Proteins/Glycoproteins
Transport
simple diffusion
facilitated diffusion
active transport
enzymatic activities
signal transductions
cell-celll recognition
cell-cell adhesion (inter-cellular joining)
attachment to the cytoskeleton and extracellular matrix (ECM)
What kind of molecules move via bulk transport?
Exocytosis
Endocytosis
NOTE: for the portions where detailed notes are not typed out, like the endo/exocytosis and protein trafficking portions, I feel that I would learn faster by doing handwritten notes. That's because the content in those sections require diagrams, and I would understand the topic section better if I draw it out myself.