Cell Structure and Function
1
Cell Structure
•
In 1655, the English scientist Robert Hooke coined the
term “cellulae” for the small box-like structures he saw
while examining a thin slice of cork under a
microscope.
2
Basic Cell Structure
•
•
•
•
All cells have the following basic structure:
A thin, flexible plasma membrane
surrounds the entire cell.
The interior is filled with a semi-fluid
material called the cytoplasm.
Also inside are specialized structures
called organelles and the cell’s genetic
material.
3
Generalized Eukaryotic Cell
4
Visualizing Cells
5
Prokaryotic Cells
•
Simplest organisms
– Cytoplasm is surrounded by plasma membrane and
encased in a rigid cell wall composed of peptidoglycan.
– No distinct interior compartments
– Some use flagellum for locomotion, threadlike structures
protruding from cell surface
6
Eukaryotic Cells
•
Characterized by compartmentalization by
an endomembrane system, and the
presence of membrane-bound organelles.
– central vacuole
– vesicles
– chromosomes
– cytoskeleton
– cell walls
7
Animal
Cell
Animal cell anatomy
8
Membrane Function
• All cells are surrounded
by a plasma membrane.
• Cell membranes are
composed of a lipid
bilayer with globular
proteins embedded in the
bilayer.
• On the external surface,
carbohydrate groups join
with lipids to form
glycolipids, and with
proteins to form
glycoproteins. These
function as cell identity
markers.
9
Fluid Mosaic Model
• In 1972, S. Singer and G. Nicolson proposed the Fluid
Mosaic Model of membrane structure
Glycoprotein
Extracellular fluid
Glycolipid
Carbohydrate
Cholesterol
Transmembrane
proteins
Peripheral
protein
Cytoplasm
Filaments of
cytoskeleton
10
Phospholipids
• Glycerol
• Two fatty acids
• Phosphate group
Hydrophilic
heads
ECF WATER
Hydrophobic
tails
ICF WATER
11
Phospholipid Bilayer
• Mainly 2 layers of phospholipids; the non-polar tails
point inward and the polar heads are on the surface.
• Contains cholesterol in animal cells.
• Is fluid, allowing proteins to move around within the
bilayer.
Polar
hydro-philic
heads
Nonpolar
hydro-phobic
tails
Polar
hydro-philic
heads
12
Steroid Cholesterol
• Effects on membrane fluidity within
the animal cell membrane
Cholesterol
13
Membrane Proteins
• A membrane is a collage of different proteins
embedded in the fluid matrix of the lipid bilayer
• Peripheral proteins are appendages loosely
bound to the surface of the membrane
Fibers of extracellular
matrix (ECM)
Glycoprotein
Carbohydrate
Glycolipid
Microfilaments
of cytoskeleton
Cholesterol
Peripheral
protein
Integral
protein
14
Integral proteins
• Penetrate the hydrophobic core of the
lipid bilayer
• Are often transmembrane proteins,
completely spanning the membrane
N-terminus
EXTRACELLULAR
SIDE
C-terminus
a Helix
CYTOPLASMIC
SIDE
15
Functions of Cell Membranes
• Regulate the passage of substance
into and out of cells and between cell
organelles and cytosol
• Detect chemical messengers arriving
at the surface
• Link adjacent cells together by
membrane junctions
• Anchor cells to the extracellular
matrix
16
6 Major Functions Of Membrane
Proteins
1. Transport. (left) A protein that spans the membrane
may provide a hydrophilic channel across the
membrane that is selective for a particular solute.
(right) Other transport proteins shuttle a substance
from one side to the other by changing shape. Some of
these proteins hydrolyze ATP as an energy ssource to
actively pump substances across the membrane
2. Enzymatic activity. A protein built into the membrane
may be an enzyme with its active site exposed to
substances in the adjacent solution. In some cases,
several enzymes in a membrane are organized as a
team that carries out sequential steps of a metabolic
pathway.
3.
ATP
Enzymes
Signal transduction. A membrane protein may have a
binding site with a specific shape that fits the shape of a
chemical messenger, such as a hormone. The external
messenger (signal) may cause a conformational change
in the protein (receptor) that relays the message to the
inside of the cell.
Signal
Receptor
17
6 Major Functions Of Membrane Proteins
4.
Cell-cell recognition. Some glyco-proteins serve as
identification tags that are specifically recognized
by other cells.
Glycoprotein
5.
Intercellular joining. Membrane proteins of adjacent cells
may hook together in various kinds of junctions, such as
gap junctions or tight junctions
6. Attachment to the cytoskeleton and extracellular matrix
(ECM). Microfilaments or other elements of the
cytoskeleton may be bonded to membrane proteins,
a function that helps maintain cell shape and stabilizes
the location of certain membrane proteins. Proteins that
adhere to the ECM can coordinate extracellular and
intracellular changes
18
Functions of Plasma Membrane Proteins
Outside
Plasma
membrane
Inside
Transporter
Enzyme
Cell surface identity
marker
Cell adhesion
Cell surface
receptor
Attachment to the
cytoskeleton
19
Membrane Transport
• The plasma membrane is the boundary that
separates the living cell from its nonliving
surroundings
• In order to survive, A cell must exchange
materials with its surroundings, a process
controlled by the plasma membrane
• Materials must enter and leave the cell through
the plasma membrane.
• Membrane structure results in selective
permeability, it allows some substances to cross
it more easily than others
20
Membrane Transport
• The plasma membrane exhibits
selective permeability - It allows some
substances to cross it more easily
than others
21
Passive Transport
• Passive transport is diffusion of a
substance across a membrane with
no energy investment
• 4 types
• Simple diffusion
• Dialysis
• Osmosis
• Facilitated diffusion
22
Solutions and Transport
• Solution – homogeneous mixture of
two or more components
• Solvent – dissolving medium
• Solutes – components in smaller quantities
within a solution
• Intracellular fluid – nucleoplasm and
cytosol
• Extracellular fluid
• Interstitial fluid – fluid on the exterior of the
cell within tissues
• Plasma – fluid component of blood
23
Diffusion
•
•
•
The net movement of a substance from an area of higher
concentration to an area of lower concentration - down a
concentration gradient
Caused by the constant random motion of all atoms and molecules
Movement of individual atoms & molecules is random, but each
substance moves down its own concentration gradient.
Lump
of sugar
Random movement leads to
net movement down a
concentration gradient
Water
No net movement at
equilibrium
24
Diffusion Across a Membrane
•
•
•
The membrane has pores large enough for the molecules to pass
through.
Random movement of the molecules will cause some to pass
through the pores; this will happen more often on the side with more
molecules. The dye diffuses from where it is more concentrated to
where it is less concentrated
This leads to a dynamic equilibrium: The solute molecules continue
to cross the membrane, but at equal rates in both directions.
Net diffusion
Net diffusion
Equilibrium
25
Diffusion Across a Membrane
•
•
•
Two different solutes are separated by a membrane that is
permeable to both
Each solute diffuses down its own concentration gradient.
There will be a net diffusion of the purple molecules toward the left,
even though the total solute concentration was initially greater on
the left side
Net diffusion
Net diffusion
Net diffusion
Net diffusion
Equilibrium
Equilibrium
26
The Permeability of the Lipid Bilayer
• Permeability Factors
• Lipid solubility
• Size
• Charge
• Presence of channels and transporters
• Hydrophobic molecules are lipid soluble and can
pass through the membrane rapidly
• Polar molecules do not cross the membrane
rapidly
• Transport proteins allow passage of hydrophilic
substances across the membrane
27
Passive Transport Processes
• 3 special types of diffusion
that involve movement of
materials across a
semipermeable membrane
• Dialysis/selective diffusion
of solutes
• Lipid-soluble materials
• Small molecules that
can pass through
membrane pores
unassisted
• Facilitated diffusion substances require a
protein carrier for passive
transport
• Osmosis – simple diffusion
of water
28
Osmosis
• Diffusion of the solvent across a
semipermeable membrane.
• In living systems the solvent is
always water, so biologists
generally define osmosis as the
diffusion of water across a
semipermeable membrane:
29
Osmosis
Lower
concentration
of solute (sugar)
Higher
concentration
of sugar
Same concentration
of sugar
Selectively
permeable membrane: sugar molecules cannot pass
through pores, but
water molecules can
Water molecules
cluster around
sugar molecules
More free water
molecules (higher
concentration)
Fewer free water
molecules (lower
concentration)
Osmosis

Water moves from an area of higher
free water concentration to an area
of lower free water concentration
30
Osmotic Pressure
• Osmotic pressure of a solution is the
pressure needed to keep it in
equilibrium with pure H20.
• The higher the concentration of
solutes in a solution, the higher its
osmotic pressure.
• Tonicity is the ability of a solution to
cause a cell to gain or lose water –
based on the concentration of solutes
31
Tonicity
• If 2 solutions have equal [solutes], they are called
isotonic
• If one has a higher [solute], and lower [solvent], is
hypertonic
• The one with a lower [solute], and higher [solvent], is
hypotonic
Hypotonic solution
H2O
Lysed
Isotonic solution
Hypertonic solution
H2O
H2O
Normal
H2O
Shriveled
32
Water Balance In Cells With Walls
(b) Plant cell. Plant cells
are turgid (firm) and
generally healthiest in
a hypotonic environment, where the
uptake of water is
eventually balanced
by the elastic wall
pushing back on the
cell.
H2O
Turgid (normal)
H2O
H2O
Flaccid
H2O
Plasmolyzed
33
My definition of Osmosis
• Osmosis is the diffusion of water
across a semi-permeable membrane
from a hypotonic solution to a
hypertonic solution
34
Facilitated Diffusion
•
•
Diffusion of solutes through a semipermeable membrane with the
help of special transport proteins i.e. large polar molecules and ions
that cannot pass through phospholipid bilayer.
Two types of transport proteins can help ions and large polar
molecules diffuse through cell membranes:
Channel proteins – provide a narrow channel for the substance to pass
through.
• Carrier proteins – physically bind to the substance on one side of
membrane and release it on the other.
•
EXTRACELLULAR
FLUID
Channel protein
CYTOPLASM
Solute
Carrier protein
Solute
35
Facilitated Diffusion
• Specific – each channel or carrier
transports certain ions or molecules
only
• Passive – direction of net movement
is always down the concentration
gradient
• Saturates – once all transport
proteins are in use, rate of diffusion
cannot be increased further
36
Active Transport
• Uses energy (from ATP) to move a
substance against its natural
tendency e.g. up a concentration
gradient.
• Requires the use of carrier proteins
(transport proteins that physically bind
to the substance being transported).
• 2 types:
• Membrane pump (protein-mediated active
transport)
37
Membrane Pump
• A carrier protein uses energy from
ATP to move a substance across a
membrane, up its concentration
gradient:
38
The Sodium-potassium Pump
•
One type of active transport system
[Na+] high
[K+] low
1. Cytoplasmic Na+ binds
to the sodium-potassium
pump.
Na+
Na+
+
NaEXTRACELLULAR
FLUID
[Na+] low
Na+
[K+] high
CYTOPLASM
2. Na+ binding stimulates
phosphorylation by ATP.
Na+
Na+
Na+
Na+
6. K+ is released and Na+
sites are receptive again;
the cycle repeats.
ATP
P
ADP
3. Phosphorylation causes the
protein to change its conformation,
expelling Na+ to the outside.
Na+
K+
P
K+
5. Loss of the phosphate
restores the protein’s
original conformation.
K+
4. Extracellular K+ binds to the
protein, triggering release of the
Phosphate group.
K+
K+
K+
Pi
P
Pi
39
Coupled transport
• 2 stages:
• Carrier protein uses ATP to move a substance across the
membrane against its concentration gradient. Storing energy.
• Coupled transport protein allows the substance to move down its
concentration gradient using the stored energy to move a
second substance up its concentration gradient:
40
Review: Passive And Active Transport Compared
Passive transport. Substances diffuse spontaneously
down their concentration gradients, crossing a
membrane with no expenditure of energy by the cell.
The rate of diffusion can be greatly increased by transport
proteins in the membrane.
Active transport. Some transport proteins act
as pumps, moving substances across a
membrane against their concentration
gradients. Energy for this work is usually
supplied by ATP.
ATP
Diffusion. Hydrophobic
molecules and (at a slow
rate) very small uncharged
polar molecules can diffuse
through the lipid bilayer.
Facilitated diffusion. Many
hydrophilic substances diffuse
through membranes with the
assistance of transport proteins,
either channel or carrier proteins.
41
Bulk Transport
• Allows small particles, or groups of
molecules to enter or leave a cell
without actually passing through the
membrane.
• 2 mechanisms of bulk transport:
endocytosis and exocytosis.
42
Endocytosis
• The plasma membrane envelops
small particles or fluid, then seals on
itself to form a vesicle or vacuole
which enters the cell:
• Phagocytosis
• Pinocytosis
• Receptor-Mediated Endocytosis -
43
Three Types Of Endocytosis
PHAGOCYTOSIS
In phagocytosis, a cell
engulfs a particle by
Wrapping pseudopodia
around it and packaging
it within a membraneenclosed sac large
enough to be classified
as a vacuole. The
particle is digested after
the vacuole fuses with a
lysosome containing
hydrolytic enzymes.
EXTRACELLULAR
FLUID
Pseudopodium
of amoeba
“Food” or
other particle
Bacterium
Food
vacuole
Food vacuole
An amoeba engulfing a bacterium via
phagocytosis (TEM).
PINOCYTOSIS
In pinocytosis, the cell
“gulps” droplets of
extracellular fluid into tiny
vesicles. It is not the fluid
itself that is needed by the
cell, but the molecules
dissolved in the droplet.
Because any and all
included solutes are taken
into the cell, pinocytosis
is nonspecific in the
substances it transports.
1 µm
CYTOPLASM
Pseudopodium
0.5 µm
Plasma
membrane
Pinocytosis vesicles
forming (arrows) in
a cell lining a small
blood vessel (TEM).
Vesicle
44
Process of Phagocytosis
45
Receptor-mediated Endocytosis
Coat protein
Receptor
Receptor-mediated endocytosis
enables the cell to acquire bulk quantities
of specific substances, even though those
substances may not be very concentrated
in the extracellular fluid. Embedded in the
membrane are proteins with specific
receptor sites exposed to the extracellular
fluid. The receptor proteins are usually
already clustered in regions of the
membrane called coated pits, which are
lined on their cytoplasmic side by a fuzzy
layer of coat proteins.
Extracellular substances (ligands) bind
to these receptors. When binding occurs,
the coated pit forms a vesicle containing
the ligand molecules. Notice that there are
relatively more bound molecules (purple)
inside the vesicle, other molecules
(green) are also present. After this
ingested material is liberated from the
vesicle, the receptors are recycled to the
plasma membrane by the same vesicle.
Coated
vesicle
Coated
pit
Ligand
Coat
protein
A coated pit
and a coated
vesicle formed
during
receptormediated
endocytosis
(TEMs).
Plasma
membrane
0.25 µm
46
Exocytosis
• The reverse of endocytosis
• During this process, the membrane of a vesicle
fuses with the plasma membrane and its
contents are released outside the cell:
47
Cell Junctions
•
Long-lasting or permanent connections between
adjacent cells, 3 types of cell junctions:
TIGHT JUNCTIONS
Tight junction
Tight junctions prevent
fluid from moving
across a layer of cells
0.5 µm
At tight junctions, the membranes of
neighboring cells are very tightly pressed
against each other, bound together by
specific proteins (purple). Forming continuous seals around the cells, tight junctions
prevent leakage of extracellular fluid across
A layer of epithelial cells.
DESMOSOMES
Desmosomes (also called anchoring
junctions) function like rivets, fastening cells
Together into strong sheets. Intermediate
Filaments made of sturdy keratin proteins
Anchor desmosomes in the cytoplasm.
Tight junctions
Intermediate
filaments
Desmosome
Gap
junctions
Space
between Plasma membranes
cells
of adjacent cells
1 µm
Extracellular
matrix
Gap junction
0.1 µm
GAP JUNCTIONS
Gap junctions (also called communicating
junctions) provide cytoplasmic channels from
one cell to an adjacent cell. Gap junctions
consist of special membrane proteins that
surround a pore through which ions, sugars,
amino acids, and other small molecules may
pass. Gap junctions are necessary for communication between cells in many types of tissues,
including heart muscle and animal embryos.
48
The Nucleus And The Nuclear Envelope
•
•
•
•
Repository for genetic material called chromatin - DNA and proteins
Nucleolus: holds chromatin and ribosomal subunits - region of intensive
ribosomal RNA synthesis
Nuclear envelope: Surface of nucleus bound by two phospholipid bilayer
membranes - Double membrane with pores
Nucleoplasm: semifluid medium inside the nucleus
Nucleus
1 µm
Nucleus
Nucleolus
Chromatin
Nuclear envelope:
Inner membrane
Outer membrane
Nuclear pore
Pore
complex
Rough ER
Surface of nuclear
envelope.
1 µm
Ribosome
0.25 µm
Close-up of
nuclear
envelope
Pore complexes (TEM).
Nuclear lamina (TEM).
49
Chromosomes
•
DNA of eukaryotes is divided into linear
chromosomes.
– Exist as strands of chromatin, except
during cell division
– Histones associated packaging proteins
50
Ribosomes
•
Ribosomes are RNA-protein complexes composed of two
subunits that join and attach to messenger RNA.
– Site of protein synthesis
– Assembled in nucleoli
Ribosomes
ER
Cytosol
Endoplasmic reticulum (ER)
Free ribosomes
Bound ribosomes
Large
subunit
0.5 µm
TEM showing ER and ribosomes
Small
subunit
Diagram of a ribosome
51
Endomembrane System
•
Compartmentalizes cell, channeling passage
of molecules through cell’s interior.
– Endoplasmic reticulum
 Rough ER - studded with ribosomes
 Smooth ER - few ribosomes
52
Rough ER
•
•
Rough ER is especially abundant in cells that secrete proteins.
–
As a polypeptide is synthesized on a ribosome attached to rough ER, it is threaded into the
cisternal space through a pore formed by a protein complex in the ER membrane.
–
As it enters the cisternal space, the new protein folds into its native conformation.
–
Most secretory polypeptides are glycoproteins, proteins to which a carbohydrate is
attached.
–
Secretory proteins are packaged in transport vesicles that carry them to their next stage.
Rough ER is also a membrane factory.
–
Membrane-bound proteins are synthesized directly into the membrane.
–
Enzymes in the rough ER also synthesize phospholipids from precursors in the cytosol.
–
As the ER membrane expands, membrane can be transferred as transport vesicles to other
components of the endomembrane system.
53
Smooth ER
•
•
•
•
•
The smooth ER is rich in enzymes and plays a role in a variety of metabolic processes.
Enzymes of smooth ER synthesize lipids, including oils, phospholipids, and steroids.
These include the sex hormones of vertebrates and adrenal steroids.
In the smooth ER of the liver, enzymes help detoxify poisons and drugs such as
alcohol and barbiturates.
Smooth ER stores calcium ions.

Muscle cells have a specialized smooth ER that pumps calcium ions from
the cytosol and stores them in its cisternal space.

When a nerve impulse stimulates a muscle cell, calcium ions rush from
the ER into the cytosol, triggering contraction.
54
The Golgi apparatus
•
The Golgi apparatus is the shipping and receiving center for cell
products.
–
Many transport vesicles from the ER travel to the Golgi apparatus
for modification of their contents.
–
The Golgi is a center of manufacturing, warehousing, sorting, and
shipping.
–
The Golgi apparatus consists of flattened membranous sacs—
cisternae—looking like a stack of pita bread.
–
The Golgi sorts and packages materials into transport vesicles.
55
Functions Of The Golgi Apparatus
Golgi
apparatus
cis face
(“receiving” side of
Golgi apparatus)
1 Vesicles move
2 Vesicles coalesce to
6 Vesicles also
form new cis Golgi cisternae
from ER to Golgi
transport certain
Cisternae
proteins back to ER
3 Cisternal
maturation:
Golgi cisternae
move in a cisto-trans
direction
5 Vesicles transport specific
proteins backward to newer
Golgi cisternae
0.1 0 µm
4 Vesicles form and
leave Golgi, carrying
specific proteins to
other locations or to
the plasma membrane for secretion
trans face
(“shipping” side of
Golgi apparatus)
TEM of Golgi apparatus
56
Membrane Bound Organelles
Nucleus
•
•
•
Lysosomes – vesicle
containing digestive
enzymes that break down
food/foreign particles
Vacuoles – food storage
and water regulation
Peroxisomes - contain
enzymes that catalyze the
removal of electrons and
associated hydrogen
atoms
1 µm
Lysosome
Lysosome contains
active hydrolytic
enzymes
Food vacuole
fuses with
lysosome
Hydrolytic
enzymes digest
food particles
Digestive
enzymes
Lysosome
Plasma membrane
Digestion
Food vacuole
(a) Phagocytosis: lysosome digesting food
57
Mitochondria
•
•
•
•
Sites of cellular respiration, ATP synthesis
Bound by a double membrane surrounding fluid-filled matrix.
The inner membranes of mitochondria are cristae
The matrix contains enzymes that break down carbohydrates and
the cristae house protein complexes that produce ATP
58
Cytoskeleton
•
•
The eukaryotic cytoskeleton is a network of
filaments and tubules that extends from the
nucleus to the plasma membrane that support
cell shape and anchor organelles.
Protein fibers
– Actin filaments
 cell movement
– Intermediate filaments
– Microtubules
 centrioles
59
Centrioles
•
•
Centrioles are short
cylinders with a 9 + 0
pattern of microtubule
triplets.
Centrioles may be
involved in microtubule
formation and
disassembly during cell
division and in the
organization of cilia and
flagella.
60
Cilia and Flagella
•
•
•
Contain specialized arrangements of microtubules
Are locomotor appendages of some cells
Cilia and flagella share a common ultrastructure
Outer microtubule
doublet
Dynein arms
0.1 µm
Central
microtubule
Outer doublets
cross-linking
proteins inside
Microtubules
Radial
spoke
Plasma
membrane
Basal body
Plasma
membrane
(b)
0.5 µm
(a)
0.1 µm
Triplet
(c)
Cross section of basal body
61
Cilia and Flagella
•
•
•
Cilia (small and numerous) and flagella (large and single)
have a 9 + 2 pattern of microtubules and are involved in
cell movement.
Cilia and flagella move when the microtubule doublets
slide past one another.
Each cilium and flagellum has a basal body at its base.
62
Cilia and Flagella
(a) Motion of flagella. A flagellum
usually undulates, its snakelike
motion driving a cell in the same
direction as the axis of the
flagellum. Propulsion of a human
sperm cell is an example of
flagellatelocomotion (LM).
Direction of swimming
1 µm
(b) Motion of cilia. Cilia have a backand-forth motion that moves the
cell in a direction perpendicular
to the axis of the cilium. A dense
nap of cilia, beating at a rate of
about 40 to 60 strokes a second,
covers this Colpidium, a
freshwater protozoan (SEM).
15 µm 63