CELLS
Chapter:3
Cell Theory
• The cell is the smallest structural and
functional living unit
• Organismal functions depend on
individual and collective cell functions
• Biochemical activities of cells are
dictated by their specific subcellular
structures
• Continuity of life has a cellular basis
CELLS
Definition: cells are the smallest units that perform
life functions (functional unit of life)
Two classes of cells in the body:
1. Germ cells: reproductive (sex cells);
Examples: sperm
ovum
2. Somatic cells: (body) all other body
cells
Cell Diversity
• Over 200 different types of human cells
• Types differ in size, shape, subcellular
components, and functions
Erythrocytes
Fibroblasts
Epithelial cells
(a) Cells that connect body parts,
form linings, or transport gases
Skeletal
Muscle
cell
Smooth
muscle cells
(b) Cells that move organs and
body parts
Macrophage
Fat cell
(c) Cell that stores (d) Cell that
nutrients
fights disease
Nerve cell
(e) Cell that gathers information
and control body functions
(f) Cell of reproduction
Sperm
CELLS
• Cells have many structures in common;
however, in addition, cells have special
adaptations to help them perform their
functions
• The “generalized” cell helps us learn
those structures that may be present in
any one cell.
• No cell has all the structures of this
“generalized” cell.
Chromatin
Nucleolus
Nuclear envelope
Nucleus
Smooth endoplasmic
reticulum
Mitochondrion
Cytosol
Lysosome
Centrioles
Centrosome
matrix
Cytoskeletal
elements
• Microtubule
• Intermediate
filaments
Plasma
membrane
Rough
endoplasmic
reticulum
Ribosomes
Golgi apparatus
Secretion being
released from cell
by exocytosis
Peroxisome
Generalized Cell
• All cells have some common structures
and functions
• Human cells have three basic parts:
– Plasma membrane - flexible outer
boundary
– Cytoplasm - intracellular fluid containing
organelles
– Nucleus - control center
Plasma (Cell) Membrane
• The plasma membrane or cell membrane
separates the cell contents from the
extracellular fluid.
• The main structural components of the
plasma membrane are phospholipids,
proteins, and carbohydrates
• The phospholipid molecules are arranged in a
double layer (phospholipid bilayer)
Extracellular fluid
(watery environment)
Polar head of
phospholipid
molecule
Cholesterol
Glycolipid
Glycoprotein
Carbohydrate
of glycocalyx
Outwardfacing
layer of
phospholipids
Integral
proteins
Filament of
cytoskeleton
Peripheral
Bimolecular
Inward-facing
proteins
lipid layer
layer of
containing
phospholipids
Nonpolar
proteins
tail of
phospholipid
Cytoplasm
molecule
(watery environment)
Membrane Lipids
• 75% phospholipids (lipid bilayer)
– Phosphate heads: polar and hydrophilic
– Fatty acid tails: nonpolar and hydrophobic
• 5% glycolipids
– Lipids with polar sugar groups on outer membrane
surface
• 20% cholesterol
– Increases membrane stability and fluidity
Membrane Proteins
• Integral proteins
– Firmly inserted into the membrane (most
are transmembrane)
– Functions:
• Transport proteins (channels and carriers),
enzymes, or receptors
Membrane Proteins
• Peripheral proteins
– Loosely attached to integral proteins
– Include filaments on intracellular surface and
glycoproteins on extracellular surface
– Functions:
• Enzymes, motor proteins, cell-to-cell links, provide
support on intracellular surface, and form part of
glycocalyx
Extracellular fluid
(watery environment)
Polar head of
phospholipid
molecule
Cholesterol
Glycolipid
Glycoprotein
Carbohydrate
of glycocalyx
Outwardfacing
layer of
phospholipids
Integral
proteins
Filament of
cytoskeleton
Peripheral
Bimolecular
Inward-facing
proteins
lipid layer
layer of
containing
phospholipids
Nonpolar
proteins
tail of
phospholipid
Cytoplasm
molecule
(watery environment)
Transport
A protein (left) that spans the membrane
may provide a hydrophilic channel across
the membrane that is selective for a
particular solute. Some transport proteins
(right) hydrolyze ATP as an energy source
to actively pump substances across the
membrane.
Receptors for signal transduction
Signal
Receptor
A membrane protein exposed to the
outside of the cell may have a binding
site with a specific shape that fits the
shape of a chemical messenger, such
as a hormone. The external signal may
cause a change in shape in the protein
that initiates a chain of chemical
reactions in the cell.
Attachment to the cytoskeleton and extracellular
matrix (ECM)
Elements of the cytoskeleton (cell’s
internal supports) and the extracellular
matrix (fibers and other substances
outside the cell) may be anchored to
membrane proteins, which help maintain
cell shape and fix the location of certain
membrane proteins. Others play a role in
cell movement or bind adjacent cells
together.
Enzymatic activity
Enzymes
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 act as a team
that catalyzes sequential steps of a
metabolic pathway as indicated (left to
right) here.
Intercellular joining
Membrane proteins of adjacent cells
may be hooked together in various
kinds of intercellular junctions. Some
membrane proteins (CAMs) of this
group provide temporary binding sites
that guide cell migration and other
cell-to-cell interactions.
CAMs
Cell-cell recognition
Some glycoproteins (proteins bonded
to short chains of sugars) serve as
identification tags that are specifically
recognized by other cells.
Glycoprotein
Membrane Junctions
Three types:
v Tight junction - Impermeable junctions prevent
molecules from passing through the intercellular
space.
v Desmosome - Anchoring junctions bind adjacent
cells together and help form an internal tensionreducing network of fibers
v Gap Junctions - Transmembrane proteins form
pores that allow small molecules to pass from cell to
cell
– For spread of ions between cardiac or smooth
muscle cells
Tight junctions
Plasma membranes
of adjacent cells
Microvilli
Intercellular
space
Basement membrane
Interlocking
junctional proteins
Intercellular
space
(a) Tight junctions: Impermeable junctions prevent molecules
from passing through the intercellular space.
Desmosomes
Plasma membranes
of adjacent cells
Microvilli
Intercellular
space
Basement membrane
Intercellular space
Plaque
Intermediate
filament (keratin)
Linker glycoproteins
(cadherins)
Gap junctions
Plasma membranes
of adjacent cells
Microvilli
Intercellular
space
Basement membrane
Intercellular
space
Channel
between cells
(connexon)
Gap junctions: Communicating junctions allow ions and small molecules to pass from one cell to the next for intercellular communication.
Membrane Transport
Transport through the plasma membrane
can be
1.Passive ( No energy required )
• Passive processes:
üSimple Diffusion
üFiltration
2. Active ( requiring energy and ATP )
3.Carrier mediated transport (active and
passive)
• Filtration: the process by which
water and solutes are forced through a
membrane by fluid pressure.
Water and urea
To urinary
bladder
Diffusion
• Diffusion is the net movement of molecules
from an area of relatively high concentration
to an area of relatively low concentration
– molecules mix randomly
– Solute spreads through solvent
– Solutes move down a concentration
gradient
Diffusion
1. Simple diffusion
2. Facilitated diffusion
– Carrier mediated facilitated diffusion
– Channel mediated facilitated diffusion
Passive Processes: Simple
Diffusion
• Nonpolar lipid-soluble (hydrophobic)
substances diffuse directly through the
phospholipid bilayer
Extracellular fluid
Lipidsoluble
solutes
Cytoplasm
(a) Simple diffusion of fat-soluble molecules
directly through the phospholipid bilayer
Figure 3.7a
Facilitated diffusion
• Special type of diffusion that involves a
carrier molecule
• Carrier molecules transport down a
concentration gradient
• Energy is not required
• Example: movement of glucose
from the blood into the cells
Passive Processes: Facilitated Diffusion
• Certain lipophobic molecules (e.g., glucose,
amino acids, and ions) use carrier proteins or
channel proteins, both of which:
• Exhibit specificity (selectivity)
• Are saturable; rate is determined by number of
carriers or channels
• Can be regulated in terms of activity and
quantity
Copyright © 2010 Pearson Education, Inc.
Facilitated Diffusion Using Carrier Proteins
• Transmembrane integral proteins transport
specific polar molecules (e.g., sugars and
amino acids)
• Binding of substrate causes shape change in
carrier
Copyright © 2010 Pearson Education, Inc.
Lipid-insoluble
solutes (such as
sugars or amino
acids)
(b) Carrier-mediated facilitated diffusion via a protein
carrier specific for one chemical; binding of substrate
causes shape change in transport protein
Copyright © 2010 Pearson Education, Inc.
Figure 3.7b
Facilitated Diffusion Using Channel Proteins
• Aqueous channels formed by transmembrane
proteins selectively transport ions or water
• Two types:
• Leakage channels
• Always open
• Gated channels
• Controlled by chemical or electrical signals
Copyright © 2010 Pearson Education, Inc.
Passive Processes: Osmosis
• Movement of solvent (water) across a
selectively permeable membrane
• Water diffuses through plasma
membranes:
– Through the lipid bilayer
– Through water channels called aquaporins
(AQPs)
Water
molecules
Lipid
billayer
Aquaporin
(d) Osmosis, diffusion of a solvent such as
water through a specific channel protein
(aquaporin) or through the lipid bilayer
Figure 3.7d
Passive Processes: Osmosis
• Water concentration is determined by
solute concentration because solute
particles displace water molecules
• Osmolarity: The measure of total
concentration of solute particles
• When solutions of different osmolarity
are separated by a membrane, osmosis
occurs until equilibrium is reached
(a)
Membrane permeable to both solutes and water
Solute and water molecules move down their concentration gradients
in opposite directions. Fluid volume remains the same in both compartments.
Left
compartment:
Solution with
lower osmolarity
Right
compartment:
Solution with
greater osmolarity
Both solutions have the
same osmolarity: volume
unchanged
H 2O
Solute
Membrane
Solute
molecules
(sugar)
Figure 3.8a
(b)
Membrane permeable to water, impermeable to solutes
Solute molecules are prevented from moving but water moves by osmosis.
Volume increases in the compartment with the higher osmolarity.
Left
compartment
Right
compartment
Both solutions have identical
osmolarity, but volume of the
solution on the right is greater
because only water is
free to move
H 2O
Membrane
Solute
molecules
(sugar)
Figure 3.8b
Importance of Osmosis
• When osmosis occurs, water enters or
leaves a cell
• Change in cell volume disrupts cell
function
Tonicity
• Tonicity: The ability of a solution to
cause a cell to shrink or swell
• Isotonic: A solution with the same solute
concentration as that of the cytosol
• Hypertonic: A solution having greater
solute concentration than that of the
cytosol
• Hypotonic: A solution having lesser
solute concentration than that of the
cytosol
(a)
Isotonic solutions
Cells retain their normal size and
shape in isotonic solutions (same
solute/water concentration as inside
cells; water moves in and out).
(b)
Hypertonic solutions
Cells lose water by osmosis and
shrink in a hypertonic solution
(contains a higher concentration
of solutes than are present inside
the cells).
(c)
Hypotonic solutions
Cells take on water by osmosis until
they become bloated and burst (lyse)
in a hypotonic solution (contains a
lower concentration of solutes than
are present in cells).
Figure 3.9
Osmosis
• A special type of diffusion
– Osmosis is the diffusion of water across
the cell membrane
– Water molecules diffuse across membrane
toward solution with more solutes
Tonicity
• Tonicity : The ability of a solution to
change the shape or tone of cells by
altering the cell’s internal water volume.
– Isotonic
– Hypertonic
– Hypotonic
Tonicity
• Isotonic: Solutions that have the same
concentration of solute outside of the cell
• Hypertonic: Solution that has a greater
concentration of solutes
• Hypotonic: Solution where the solute
concentration is low
Tonicity
§ If a cell is placed in a hypertonic
solution it will shrink and lose its
water volume
§ More solutes → gains water by
osmosis
Tonicity
• If a cell is placed in a hypotonic
solution it will swell and gain the
water volume
• Lysis- rupture of a cell
Active transport
• Atoms, ions and molecules move from LOW
concentration to high concentration through a
cell membrane
• The cells USE energy (ATP) to perform active
transport
• Active transport also require a protein carrier
molecule
– Example: Sodium - Potassium “Pump”
Vesicular Transport
• In vesicular transport, fluids containing
large particles and macromolecules are
transported across the cellular
membrane inside membranous sacs
called vesicles.
• Requires cellular energy (e.g., ATP)
Vesicular Transport
• Functions:
– Exocytosis - transport out of cell
– Endocytosis - transport into cell
– Transcytosis - transport into, across, and
then out of cell
– Substance (vesicular) trafficking - transport
from one area or organelle in cell to
another
Endocytosis and Transcytosis
• Involve formation of protein-coated
vesicles
• Often receptor mediated, therefore very
selective
Vesicular Transport
Endocytosis
• Phagocytosis—pseudopods engulf
solids and bring them into cell’s interior
– Macrophages and some white blood cells
Phagosome
(a) Phagocytosis
The cell engulfs a large
particle by forming projecting pseudopods (“false
feet”) around it and enclosing it within a membrane
sac called a phagosome.
The phagosome is
combined with a lysosome.
Undigested contents remain
in the vesicle (now called a
residual body) or are ejected
by exocytosis. Vesicle may
or may not be proteincoated but has receptors
capable of binding to
microorganisms or solid
particles.
Figure 3.13a
Endocytosis
• Fluid-phase endocytosis (pinocytosis)—
plasma membrane infolds, bringing
extracellular fluid and solutes into
interior of the cell
– Nutrient absorption in the small intestine
(b) Pinocytosis
The cell “gulps” drops of
extracellular fluid containing
solutes into tiny vesicles. No
receptors are used, so the
process is nonspecific. Most
vesicles are protein-coated.
Vesicle
Figure 3.13b
Endocytosis
• Receptor-mediated endocytosis—
clathrin-coated pits provide main route
for endocytosis and transcytosis
– Uptake of enzymes low-density
lipoproteins, iron, and insulin
Vesicle
Receptor recycled
to plasma membrane
(c) Receptor-mediated
endocytosis
Extracellular substances
bind to specific receptor
proteins in regions of coated
pits, enabling the cell to
ingest and concentrate
specific substances
(ligands) in protein-coated
vesicles. Ligands may
simply be released inside
the cell, or combined with a
lysosome to digest contents.
Receptors are recycled to
the plasma membrane in
vesicles.
Figure 3.13c
Exocytosis
• Examples:
– Hormone secretion
– Neurotransmitter release
– Mucus secretion
– Ejection of wastes
Vesicular Transport
PARTS OF THE CELL
Plasma (Cell) Membrane
Outside cell
Inside cell
Functions:
1. Physical barrier between the contents of the cell
and the extra-cellular fluid.
Functions of the cell membrane cont’d
2. Regulates
exchange of chemicals into
and out of the cell (transport)
•
Nutrients, gases (oxygen) go into the cell
• Wastes, gases (carbon dioxide), cell
secretions go out of the cell
Functions of the cell membrane
cont’d
• 3. Sensitivity: Responds to the
extracellular environment.
Ø Receptors on the membrane allow
the cell to respond to hormones
Functions of the cell membrane cont’d
4. Structural support: some membranes
have connections to other cells to help
attach cells to other structures (cells or
molecules)
5. Cell Identity: Special proteins help the
immune system identify a cell as
belonging to you.
v Concept of “SELF”.
PARTS OF THE CELL
CYTOPLASM
DEFINITION: Material located inside the plasma membrane and
outside the nucleus.
nucleus
Plasma membrane
CYTOPLASM cont’d
2 SUB-DIVISIONS:
1. Cytosol = intracellular fluid.
Ø
Its chemical nature is colloid (contains proteins, thickness similar
to jello).
Contains:
• Carbohydrates, water
• Small amounts of lipids
• Large amounts of amino acids
PARTS OF THE CELL
CYTOPLASM cont’d
2. Organelles:
• Have a specific structure to perform a specific
function
2 Types:
• Not enclosed in a membrane
• Enclosed in a membrane (membranous)
TYPES OF ORGANELLES
MITOCHONDRIA (plural) MITOCHONDRION (singular)
“power plant (house) of the cell”
• Function: Continues the breakdown of glucose
to release energy - then stores the energy in the
ATP molecule.
• This process requires oxygen.
Mitochondrion
(enlarged)
Mitochondrion
(cut in half, enlarged)
MITOCHONDRIA
Structure: Two membranes
1. Smooth outer membrane
2. Inner membrane forms folds called cristae
Ø The inner membrane contains enzymes that
are arranged in a special sequence to carry
out the chain reactions required to form ATP.
Ø Having the folds increases the surface area,
thus increases the amount of enzymes in each
mitochondrion
Ø Enzymes are special proteins that speed
up the rate of chemical reactions
Ø ATP is a molecule that stores energy
cristae
MITOCHONDRIA
Self-replication: Each mitochondrion has a small amount
of DNA so they can replicate
1. Just before cell division
2. When the cell needs more energy (on a regular basis).
Example: when a person increases the regular amount
of exercise more ATP is needed so mitochondria in
skeletal and cardiac muscle will multiply.
RIBOSOMES
Structure: Small granules composed of RNA and protein.
Location: 1. Free floating in the cytoplasm
2. Attached to membranes called
Endoplasmic reticulum
RIBOSOMES
Function: Site of protein synthesis.
Nickname: protein factory of the cell.
ENDOPLASMIC RETICULUM
ENDOPLASMIC RETICULUM
“ER”
Structure: An extensive system of interconnected
tubes made of membranes that extends from the
nucleus to the plasma membrane.
ENDOPLASMIC RETICULUM “ER”, cont’d
Two varieties:
1. Rough ER = The exterior surface contains ribosomes
Function: its ribosomes make proteins for secretion
2. Smooth ER = does not have ribosomes.
Functions:
• Contains enzymes for lipid metabolism
• In liver cells, for detoxification of toxins, medication
GOLGI APPARATUS
(Golgi cell, Golgi Body)
Golgi
Apparatus
GOLGI APPARATUS
Only one per cell
Structure: stacked membranous sacs
Golgi Apparatus, cont’d
Function: modifies, concentrates, packages the proteins and
membranes made in the Rough ER. Proteins are packaged into
membranous sacs called vesicles which will have one of three fates:
1. Vesicle moves to plasma membrane
and proteins are secreted from the cell.
2.Vesicle moves to plasma membrane
and the membrane of the vesicle
becomes incorporated into the plasma
membrane.
3
2
1
3. The vesicle becomes a lysosome that contains digestive
enzymes which will break down structures in the cell.
LYSOSOMES
(many per cell)
Lysosomes
cont’d
Structure: a vesicle made of membrane from the Golgi complex; can
contain up to 40 different digestive enzymes (proteins).
Functions:
1. Digest old cell organelles
2. Help digest damaged, dying or dead cells
3. In white blood cells that have “eaten”
bacteria, lysosomes digest the bacteria
Nicknames: “Suicide organelle”
“Demolition organelle”
CENTRIOLES with CENTROSOME
Centrosome
Centriole
CENTRIOLES
Structure: Each centriole is a bundle of tubules placed at
right angles to each other.
Function: Make the spindle fibers for cell division
(mitosis)
CENTRIOLES
• Not found in:
– Red blood cells
– Adipose (fat cell)
– Skeletal and Cardiac muscle
– Neuron (nerve cells)
• These cells do not divide so they don’t
need centrioles.
CYTOSKELETON:
Internal protein framework of the cell
Structure: Filaments (threads) and Tubules (tiny pipes)
CYTOSKELETON:
Internal framework of the cell
Functions: Provides strength and structural support for
the cell and its organelles; helps move organelles and
change shape of cell.
Examples of Filaments: Contractile protein in muscle
Examples of Tubules: Centrioles, cilia,
spindle fibers (for mitosis)
CYTOSKELETON:
Internal protein framework of the cell
Structure: Filaments (threads) and Tubules (tiny pipes)
Contractile fibers in muscle
Provides shape to cell
Centrioles
MICROVILLI
a special feature of some cells
Structure: Small finger shaped projections of the cell
membrane
microvilli
microvillus (singular)
MICROVILLI
• Function: Increase surface area of the
membrane.
• They function in absorbing materials
from the extracellular fluid
Ø They are most often found on the
surface of absorptive cells such as the
intestinal and kidney tubule cells.
MICROVILLI
Villi in the
lining of the
small intestine
absorb the
digested
nutrients into
the blood
stream
CILIA
Structure : Tiny hair-like projections of the cell
membrane that contain microtubules
(part of the cytoskeleton)
CILIA
Function: Cilia “beat” rhythmically to move
particles along the surface of the cells.
Ex: Cilia that line the respiratory tract move
mucus and trapped dust particles upward away
from the lungs.
FLAGELLA (plural)
FLAGELLUM (singular)
Structure: similar to cilia but much longer.
Function: to move cells through liquid.
NUCLEUS
Function: Control center of the cell.
Ø
It contains the DNA that has the instructions for
synthesizing all proteins of the cell (structure and function).
nucleus
NUCLEUS cont’d
NUCLEUS cont’d
• STRUCTURE:
– Nuclear Envelope: Double membrane.
– Outer membrane is continuous with the rough ER
and is covered with ribosomes.
– It has large pores called nuclear pores
• Nucleoli (pl.) / Nucleolus (singular)
– One or two per cell, dark staining spherical bodies
– No membrane
– Form ribosomes from rRNA and proteins
NUCLEUS cont’d
• Chromatin: long threads of DNA and
proteins called histones.
• Just before cell division, the chromatin coils
condenses into rods called chromosomes.
PROTEIN SYNTHESIS
Terms to Review:
Amino acids: subunits of protein; 20 different amino acids
Protein: composed of amino acids bonded in a specific order
Purposes: structure and function
Examples: antibodies, hormones, blood clotting factors,
enzymes
Polypeptide: three or more amino acids bonded; a short
chain of amino acids
Peptide bond: the bond formed between two amino acids.
DNA contains the genetic code because it has the
instrucitons for the sequence of amino acids in
proteins.
Gene: the portion of DNA that codes for one
protein.
RNA: ribonucleic acid
mRNA = formed in the nucleus
(messenger)
carries instructions to the ribosome
tRNA = located in the cytoplasm
(transfer)
brings amino acids to the ribosome
Ribosome = the protein factory of the cell
RNA Molecules
• Single strand of nucleotides
• Each nucleotide contains: ribose,
phosphate, base (A, G, C, and Uracil
instead of Thymine)
• Shorter than DNA
• Different types: mRNA (carries code from
DNA to ribosome), tRNA (brings amino
acids to ribosome), rRNA (a component of
ribosomes)
RNA Molecules
Protein Synthesis involves 2 steps, each
requiring RNA and enzymes
• Transcription occurs in the nucleus and is
the process of copying DNA information
into an RNA sequence
• Translation occurs at the ribosomes in the
cytoplasm as the code is transferred to a
growing chain of amino acids
PROTEIN SYNTHESIS
1. DNA (in the nucleus) contains the code for the sequence of
amino acids in the protein. DNA cannot leave the nucleus, so it
needs a messenger to carry the code to the ribosome, or
protein factory of the cell
PROTEIN SYNTHESIS
2. In the nucleus, the DNA code is transcribed into the RNA form
as messenger RNA (mRNA)
PROTEIN SYNTHESIS
3. mRNA leaves the nucleus and attaches to a ribosome.
PROTEIN SYNTHESIS
4. Transfer RNA (tRNA) brings a specific amino acid to the
ribosome.
tRNA
tRNA
tRNA
tRNA
tRNA
PROTEIN SYNTHESIS
5. tRNA (with the amino acid attached) transcribes or “reads” the
mRNA code and places the amino acid in the correct spot.
The process is
called “translation”
tRNA
tRNA
tRNA
tRNA
tRNA
PROTEIN SYNTHESIS
6. Another tRNA brings the next amino acid to its correct spot (next to the
first amino acid) and a peptide bond is formed between the two amino
acids.
tRNA
tRNA
tRNA
tRNA
PROTEIN SYNTHESIS
6. Another tRNA brings the next amino acid to its correct spot (next to the
first amino acid) and a peptide bond is formed between the two amino
acids.
tRNA
tRNA
tRNA
PROTEIN SYNTHESIS
7. This process is repeated until the entire chain of amino acids is
assembled to form the protein.
PROTEIN SYNTHESIS
8. A “stop” message on the mRNA will tell the tRNA that the protein is
completed. The protein chain will be released from the ribosome into the
cytoplasm or a vesicle made from the membrane of the ER.
9. Identical proteins will be formed until the cell has enough of that
type.
protein
ribosome
Endoplasmic
reticulum