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Eukaryotic Cell Structure and Function
Techniques used in Cytology – Cell Fractionation:
1
...
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
...
5

no

Mitochondria 1-10

yes

Ribosome
Lysosome

0
...
11
...
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

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
...
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
...
ATP synthase

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
...
Antigen
Membrane proteins of adjacent cells may adhere together in various kinds of
intercellular junctions eg
...
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
...

Na-K pump)

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



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