Topics 5-6: Eukaryotic Cell
Eukaryotic Cell Structure and Function
Techniques used in Cytology – Cell Fractionation:
1. Homogenisation: disruption of cells and release of the individual cellular components
2. Differential Centrifugation: Purification/separation of cell components
Pellets: Nuclei > mitochondria/chloroplasts/lysosomes/peroxisomes > Plasma
membrane/fragments of ER/large polyribosomes > Ribosomal subunits/small polyribosomes >
cytosol
3. Autoradiography: used to identify the synthesis and cellular distribution sites of metabolic
products by tagging specimen molecule with radioisotopes
Plant Animal
Rough
Endoplasmic
Reticulum
Smooth
Endoplasmic
Reticulum
Cell Surface
Membrane
Nucleolus
Nuclear
Envelope
Size/ LM Structure
Function
µm
?
no  Sheet-like
 Ribosomes bound to rER produce
 Cytosolic face studded by polypeptides
ribosomes- rough
 As polypeptide chain grows, it is
appearance
threaded into ER lumen, where it
 Lumen is continuous with folds into its functional conformation
perinuclear space
 Proteins synthesised are destined for
export/targeted to various cellular
organelles
no  Network of tubules
Functions in diverse metabolic
processes:
lacking ribosomessmooth appearance
 Lipid synthesis
 Single membrane
 Carbohydrate metabolism
 Detoxification of drugs and poisons
 Storage of Calcium ions
no  “Trilaminar” appearance  A partially permeable barrier
due to phospolipid bilayer controlling exchange btw the cell and
its environment
 Fluid mosaic- proteins
randomly distributed in or
loosely attached to the
membrane
no  Dense mass in the centre  Composed of DNA carrying rRNA
of nucleus
genes functioning to synthesise rRNA
no  Double membrane
 Nuclear envelope is perforated by
separating the contents of nuclear pores(~100nm), made up of a
the nucleus from
large protein complex, allowing
cytoplasm
macromolecules such as mRNA and
rRNA and proteins to enter and exit
 Double membrane
the nucleus
 Inner and outer
membranes are
continuous with each
other
 Region in btw them is
perinuclear space
Topics 5-6: Eukaryotic Cell
Nucleus
5-20
yes  Spherical or oval
Golgi Body
2.5
no
Mitochondria 1-10
yes
Ribosome
Lysosome
0.03
0.11.2
no
no
Chloro-plast
5-10
yes
Cell Wall
yes
Vacuole
yes
 Contains most of the genetic material
in cell
 Regulate cell activity
 Stack of flattened,
 Cis face located near ER and receives
membrane-bound sacs
transport vesicles from ER
called cisternae
 GA is involved in modification and
packaging of ER products
 At trans face, membrane buds off to
form secretory vesicles containing
materials to be transported to
extracellular matrix
 Elongated and spherical
 Sites of cellular respiration where
 Smooth outer membrane, catabolic process generates ATP by
oxidising food molecules
highly convoluted inner
membrane has infoldings
called cristae (double
membrane)
 Small, spherical
 Site of protein synthesis
 Appear homogeneously  Contains hydrolytic enzymes to digest
electron-dense under EM biological macromolecules
 Digestion of materials taken into cells
 Autophagy of wron-out organelles
 Autolysis- self-destruction: apoptosis
 Lens-shaped
 A type of plastids found only in plants
and algae
 Surrounded by a double
 Sites of photosynthesis
membrane called
chloroplast envelope
 Convert solar energy to chemical
 Photosynthetic pigments
energy by absorbing sunlight and
and some proteins
using it to drive the synthesis of
required for
organic compounds from carbon
photosynthesis are
dioxide and water
embedded in thylakoid
membrane
 Relatively tigid and
 Supports and defines the shape of
inflexible, made up of
plant tissues
cellulose
 Protecting the cells from both
 Freely permeable
mechanical injury and invasion
 Withstanding the hydrostatic
pressure exerted by uptake of water
by the cell to prevent excessive
uptake of water due to high tensile
strength of cellulose
 One or two big, central
 Contains cell sap
vacuoles, surrounded by  Versatile, able to regulate substances
tonoplast
it concentrates
 Storage of organic compounds and
inorganic ions
 Disposal site for toxic metabolic by-
Topics 5-6: Eukaryotic Cell
Vacuole
Centriole
products – plant protection
 Containment of pigments
 Cell growth and elongation as water
accumulates in vacuole
no  Numerous tiny vacuoles  House and transport substances
no  Found in pairs at right
 Before cell division, each centriole
angles to each other,
replicates itself and move to opposite
each member of the pair
poles of the cell
consisting of nine triplets  Centrioles act as microtubule
of microtubules arranged organizing centres for the spindle
in a ring
apparatus that separates the various
cell components
Cell Membrane & Transport Across Membrane
Fluid: phospholipid bilayer is (asymmetrical; amphipathic) free to move laterally due to weak
hydrophobic interaction between hydrocarbon tails
Mosaic: collage of proteins randomly distributed in or loosely attached to phospholipid bilayer
Function
Define cell’s boundaries
Explanation
Physically separates cell interior from surrounding environment due to
selectively permeability of phospholipid bilayer
Organisation & localization Specialized molecules/structures are embedded in membranes or
of function
localized within organelles (eg. Thylakoid membrane, inner
mitochondrial membrane)
These serve to organise and compartmentalize functions within cells
Regulation of cell’s
Membranes regulate the transport of substances into and out of
contents
cell/organelles by endocytosis, exocytosis, confining materials
Signal Transduction
Specific protein receptors on cell membrane detect specific signals by
drug/hormones etc to trigger specific responses
Cell-to-cell communication Membrane proteins bind the extracellular matrix or cell surface
constituents to mediate adhesion and communication between adjacent
cells
Factor
Temperature
Effect on
Membrane
Decrease/
increase
membrane
fluidity
Explanation
 At low temperature, the kinetic energy of hydrocarbon chains
decreases and hydrocarbon tails are tightly packed, resulting in
increased hydrophobic interactions btw phospholipid molecules, thus
restricting their motion -> Bilayer is in semisolid state
 At high temperature, kinetic energy and thus motion of hydrocarbon
chains increase, thus allowing for increased lateral movements of
individual molecules, flexing the chains and transverse flipping
 Thus overcoming hydrophobic interactions btw phospholipids,
resulting in increased space btw adjacent phospholipid molecules
 Bilayer exists in fluid state
Topics 5-6: Eukaryotic Cell
Length of
fatty acid
chains
Degree of
saturation of
fatty acid
chains
Amount of
cholesterol
Membrane
proteins
Anchorage
Transport
Enzymatic
activity
Decrease/
increase
membrane
fluidity
As degree of
saturation
increases,
membrane
fluidity
decreases
Increase
stability &
regulate
fluidity
 As length of fatty acid chains increases, membrane fluidity decreases
 The longer the hydrocarbon chains, the higher the M.P. due to
increase hydrophobic interaction btw hydrocarbon chains
 Saturated lipids with long, straight hydrocarbon chains allows close
packing and thus membrane solidification
 Unsaturated lipids have kinks, which prevent the hydrocarbon chains
from packing closely together, thus enhancing membrane fluidity
 Rigid steroid ring is intercalated into lipid monolayers, interfering with
the motions of the hydrocarbon chains of phospholipids and
enhancing the mechanical stability of the membrane
 At high temp, cholesterol restrains the movements of phospholipids,
decreasing membrane fluidity
 At low temp, cholesterol prevents the hydrocarbon chains from
packing closely, thus decreasing the tendency of the membrane to
freeze upon, resulting in increased membrane fluidity -> dual effects
 Presence of cholesterol decreases lipid bilayer permeability to ions
and small polar molecules by plugging transient gaps through which
ions and small molecules may otherwise pass
 contain hydrophobic/philic regions
unilateral
Integral
 Held in place by hydrophobic
(intrinsic)
transmembrane interactions with hydrocarbon tails
proteins
 Usually insoluble
 Rich in hydrophilic aa
Peripheral
 Usually soluble
(extrinsic)
proteins
Function of membrane proteins
Anchorage proteins attach the cell membrane to other substances to stabilise the
position of the cell membrane and can help maintain cell shape eg. Integrin
Bound to microfilaments of cytoskeleton (cytoplasmic side) or fibres of
extracellular matrix (extracellular matrix)
Carrier Proteins
Channel Proteins
Bind to solutes,
Some contain a water-filled central pore/hydrophilic
inducing a
channel, forming a passageway which permits the
conformational change movement of water, ion and small hydrophilic solutes
that transports solutes across the cell membrane
across the membrane
Leak channels (eg aquaporin)- permit movement at all
ATP may or may not be times
required
Gated channels (eg voltage gated Na+ or K+ channels)can open/close to regulate ion passage
Enzymes embedded on the membrane catalyse reaction in the extracellular fluid
or within the cytosol, depending on the location of active site
In some instances, several enzymes may be grouped together to carry out
sequential steps in a metabolic pathway eg. ATP synthase
Topics 5-6: Eukaryotic Cell
Signal
transduction
Cell-to-cell
recognition
Intercellular
joining
The specific shapes of these proteins make them ideal receptor molecules for
chemical signalling between cells
Ligand binds to the receptor protein, triggering changes in the cell eg. G proteincoupled receptors
Usually glycoproteins, due to the wide array of possible shapes of carbohydrate
side chains
Enables cells to recognise other cells, and provides a means for foreign markers to
be recognised and attacked by immune system eg. Antigen
Membrane proteins of adjacent cells may adhere together in various kinds of
intercellular junctions eg. Gap junctions and tight junctions
Carbohydrates




Usually short, branched chains bonded as glycolipids or glycoproteins
Maintain orientation of glycolipids/proteins
Highly hydrophilic
Important recognition components involved in:
o Binding extracellular signal molecules in antibody-antigen reactions
o Intercellular adhesion to form tissues
o Cell-to-cell recognition
Transport across membranes





To maintain suitable pH and ionic concentrations within cell for enzymatic activity
Obtain food supplies for energy and raw materials
Excrete toxic substances
Secrete useful substances
Generate the ionic gradients essential for nervous and muscular activity
Simple
Facilitated
Osmosis
diffusion
diffusion
Concentration Occurs down a
From less
gradient
concentration gradient ie. to more
from region of higher to
negative Ψ
lower concentration
Energy
No cellular energy expenditure required
requirement
Molecules have intrinsic kinetic energy
Substance
Small polar Larger
water
moved
uncharged/ hydrophilic
hydrophobic molecules
molecules
Example
O2, CO2,
Aa, ions,
H2O
glycerol
glucose
Facilitator
Channel/
carrier
proteins
Active
Endocytosis Exocytosis
transport
Occurs against a concentration gradient ie.
from region of lower to higher
concentration
ATP required
Small
hydrophilic
molecules
Macromolecules -> bulk
transport
Ions
Proteins, polysaccharides,
enzyme complexes
vesicles
vesicles
Carrier
proteins (eg.
Na-K pump)
Topics 5-6: Eukaryotic Cell
Phagocytosis
Molecules
Large solid particles
transported
Description Pseudopodia extend outwards
from the cell to enclose the
solid particles
Vesicles formed usually fuse
with lysosomes containing
hydrolytic enzymes to digest
these macromolecules
Pinocytosis
Extracellular fluid
Receptor-mediated endocytosis
Specific molecules
Droplets of
extracellular fluid
are incorporated
into small vesicles
Coated pits form vescicles when
specific molecules bind to receptor
proteins on cell surface
Coated pits are reinforced by
clathrin
Selective form of endocytosis allows
cells to take up specific molecules