Download Chapter 3: Cells

Chapter 3: The Cellular Level Of
Organization
BIO 137 ANATOMY & PHYSIOLOGY I
The Cell
• Adult humans contain about 70 trillion cells (260 different types)
• Differentiation - Process where a cell becomes specialized
• The cell is the smallest unit of life that can survive on its own
• Highly organized for metabolism
• Exhibits the characteristics of life
• All cells have the same DNA, but
different genes are expressed in different
cell types
Cell Size ands Shape
• Cells vary considerably in size and shape
– 7.5µm - 500µm
A Generalized Cell
• All eukaryotic cells are composed of three main
parts:
1. Plasma membrane or “plasmalemma”
2. Cytoplasm - a gelatin-like substance, plus structural
fibers and organelles (but
not the nucleus)
3. Nucleus - contains the
genetic material of the cell
A Eukaryotic Cell
Eukaryotic vs Prokaryotic Cells
• Eukaryotic Cells
• Prokaryotic Cells
– Plasma membrane
and/or cell wall
surrounds the cell
– Have a cell wall
surrounding a plasma
membrane
– Nucleus contains linear
DNA
– No nucleus
– Support comes from the
cytoskeleton
– Cytoplasm contains
organelles
– Closed circular DNA
located in the cytosol
– Have no organelles
– Cytosol is present
Eukaryotic and Prokaryotic Cells
Pili
Nucleoid: region where
the cell’s DNA is located (not
enclosed by a membrane)
Ribosomes
Plasma membrane
Cell wall
Capsule
(a) A typical
rod-shaped bacterium
Flagella
(b) A thin section through th
bacterium Bacillus coag
(TEM)
Eukaryotic Cells
• One of the most distinguishing features of Eukaryotic
cells is Compartmentalization.
• This is achieved by the endomembrane system and
numerous organelles
A Generalized Cell
The plasma membrane forms the cell’s outer
boundary and separates the cell’s internal
environment from the outside environment.
– It is a selectively permeable barrier,
allowing the passage of some
things and not others.
– It plays a role in
cellular communication.
The Plasma Membrane
• Phospholipid bilayer surrounds a cell, separating it from the
external environment
• Phospholipid Structure
– Hydrophilic head and 2 hydrophobic tails
– Hydrophobic tails aggregate together in water, forming a bilayer
Permeability of the Lipid Bilayer
• Plasma membrane acts as a barrier in a cell
– Selectively permeable
• Hydrophobic molecules
– Are lipid soluble and can pass through the membrane
rapidly
• Hydrocarbons, CO2, O2
• Polar molecules
– Do not cross the membrane rapidly
• Ions, polar molecules (sugars, water)
Fluid Mosaic Model
• The Fluid Mosaic Model describes the
arrangement of molecules within the
membrane:
– A sea of phospholipids with protein “icebergs”
floating in it.
• Membrane is a fluid structure with a “mosaic”
of various proteins embedded in it
– Fluidity allows movement
The Plasma Membrane
Other features of the Plasma Membrane
• Integral proteins - extend into or through the
bilayer.
– Transmembrane proteins (most integral proteins)
span the entire lipid bilayer.
– Peripheral proteins attach to the inner or outer
surface but do not extend through the membrane.
• Glycoproteins and glycolipids for cell
identification
The Plasma Membrane
Membrane Proteins
• Different cells have different proteins embedded in the
membrane
• Membrane function is determined by its membrane
proteins
• Membrane protein functions include:
– Transport (Channel proteins)
– Enzymatic activity
– Communication (Receptor proteins)
– Cell-cell recognition (Cell surface markers)
– Attachment (peripheral proteins)
– Cellular junctions (Cell adhesion molecules)
The Plasma Membrane
Examples of different membrane proteins include
 Ion channels
 Carriers
 Receptors
The Plasma Membrane
Examples of different membrane proteins include
 Enzymes
 Linkers
 Cell identity markers
Plasma Membrane Permeability
• Plasma membrane is selectively permeable
• Allows only hydrophobic substances, CO2 and
O2 to pass through freely
• Other hydrophilic substances must enter a cell
through other means
– Channel or carrier
proteins
– Bulk transport
Movements in and out of a cell
•
•
•
•
•
•
Diffusion
Facilitated Diffusion
Osmosis
Filtration
Active transport
Bulk transport
– Endocytosis
– Exocytosis
Membrane Traffic: Passive Transport
• Diffusion
– The tendency for molecules of any substance to move from a
higher concentration to a lower concentration
• Diffusion is an example of Passive transport
– Movement of a substance across a membrane down a
concentration gradient with no energy investment
• Substances diffuse down their concentration gradient
until equilibrium is reached
Diffusion
Diffusion is affected
by temperature,
surface area,
distance and
concentration
Diffusion of Gases
Facilitated Diffusion
• Diffusion of non-lipid
soluble substances
across the plasma
membrane through a
protein channel or
carrier in the membrane
– From high to low
concentration
– Glucose transport
A Comparison of Types of Diffusion
Osmosis: Passive Transport
• Movement of water across
a selectively permeable
membrane towards a
higher solute
concentration
Figure 7.12
Water Balance of Cells
• Tonicity
– Concentration of salt solutions in blood and body fluids
• Isotonic Solution
– Concentration of solutes is equal on both sides of a cell
• Hypertonic Solution
– Concentration of solutes is greater outside than it is inside the
cell
• Hypotonic Solution
– The concentration of solutes is less outside than it is inside the
cell
Be able to determine which way water
would move when a cell is placed in
each type of solution.
Tonicity of a solution determines whether a cell will gain or
lose water or retain the same volume.
Red Blood Cells in
Different Salt
Concentrations
Isotonic
Hypertonic
Hypotonic
Active Transport
• Moves substances against their concentration
gradient
• Low concentration to a high concentration
• Requires energy, usually in the form of ATP
• Involves carrier proteins
Na+/K+ ATPase pump
Secondary Active Transport Mechanisms
• Antiporters carry two substances across the membrane in
opposite directions.
• Symporters carry two substances across the membrane in the
same direction.
Bulk Transport in Vesicles
• Vesicle – a small spherical sac formed by
budding off from a membrane
• Exocytosis
– Transport vesicles migrate to the plasma
membrane, fuse with it, and release their contents
outside a cell
• Endocytosis
– The cell takes in materials by forming new vesicles
from the plasma membrane
Exocytosis
• Vesicles fuse with the plasma membrane, releasing
their contents outside the cell
2
4
1
3
Endocytosis
• Pinocytosis
– Cell drinking
• Phagocytosis
– Cell eating
– Macrophages (WBC) perform phagocytosis
Bulk-phase Endocytosis
Phagocytosis
Receptor Mediated Endocytosis
RECEPTOR-MEDIATED ENDOCYTOSIS
• Moves substances
specific for a certain
cell surface receptor
• Substance binds
receptor on the
plasma membrane,
a vesicle is formed a
substance is brought
into the cell
•Example
Coat protein
Receptor
Coated
vesicle
Ligand
Coated
pit
A coated pit
and a coated
vesicle formed
during
receptormediated
endocytosis
(TEMs).
Coat
protein
Plasma
membrane
0.25 µm
Cytoplasm - 2 Components
1. Cytosol - intracellular fluid, surrounding the organelles
- The site of many chemical reactions
- Energy is usually released by these reactions.
- Reactions provide the building blocks for cell maintenance,
structure, function and growth.
2. Organelles
- Specialized structures within the cell
Cytoskeleton
• Network of protein
fibers that extend
throughout the cytosol
• Give cells their shape,
internal organization
and capacity to move
• Dynamic system
– Some components are
always being
assembled and
disassembled
Cytoskeleton
• 3 types of fibers
found in the
cytoskeleton
– Microtubules
– Microfilaments
– Intermediate
filaments
Components of Cytoskeleton
• Microtubules
– Composed of the
globular protein tubulin
– Thick hollow rods, 25nm
diameter
• Involved in chromosome
movement in cell division
• Present in cilia and
flagella – Motility
• Maintenance of cell
shape
• Organelle movement in a
cell
Cilia and Flagella
• Cilia and flagella
– Locomotor appendages of
some cells
– Cilia and flagella share a
common ultrastructure
– 9 + 2 arrangement of
microtubules in a
cylindrical pattern
Flagella
• Cell motility
• Feeding
• Movement of Sperm
(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
Cilia
• Free surfaces of some epithelial tissues, Lining airways and
Sensory hairs of human ear
• Functions: Moves water over tissues, Protection
• Occur in large numbers
Centrosome and Centrioles
• Structures involved in
chromosome division
during nuclear division
– Are involved in forming a
spindle apparatus
• Centrioles are composed
of microtubules
Components of Cytoskeleton
• Microfilaments
– Composed of the protein actin
– Involved in muscle contraction
– Cell division, Cytokinesis
– Shape of cells
– Cell motility
Components of Cytoskeleton
• Intermediate Filaments
– Composed of keratin proteins
– Provide mechanical strength
– Prevent excess stretching of cells
– Forms the nuclear lamina
Components of the Cytoskeleton
Endomembrane System
• Group of related organelles where lipids are
assembled and amino acid chains are
modified into proteins
– Divides cells into compartments
• Endoplasmic Reticulum
• Golgi Apparatus
• Vesicles
Endoplasmic Reticulum
• The ER membrane is
continuous with the
nuclear envelope
• 2 types of ER
– Rough Endoplasmic
Reticulum, RER
– Smooth Endoplasmic
Reticulum, SER
Figure 6.12
Rough Endoplasmic Reticulum
• Surface is studded with ribosomes
• RER modifies polypeptides
– Attaches sugar residues – Glycoproteins
• RER makes membrane phospholipids and membrane
proteins
• Sends modified polypeptides in vesicles to the golgi
apparatus
Smooth Endoplasmic Reticulum
• No ribosomes on the surface
• Lipids are assembled in the SER
– Oils, phospholipids, steroids
• Carbohydrate metabolism
• Fat absorption from intestines
• SER of liver inactivates wastes, detoxifies drugs and
alcohols
• SER of skeletal muscle, called the sarcoplasmic
reticulum, stores calcium ions
The RER and the SER send their
proteins and lipids in transport
vesicles to the golgi apparatus for
refining.
Golgi Apparatus
• Consists of flattened membrane sacs called
cisternae
• Functions to put finishing touches on proteins
and lipids received from the ER
• Sorts and packages materials into transport
vesicles
– Will stay in cell or fuse with the plasma membrane
Processing and Packaging
Lysosomes: Digestive Compartments
• Membranous sac of hydrolytic enzymes that
can digest all 4 macromolecules
– Enzymes work at acidic pH
• Arise from the golgi apparatus
1. Lysosomes carry out intracellular digestion
after Phagocytosis
2. Destroy dead or worn out cells, organelles
and cell parts
– Recycle what is still good
Lysosomes
Lysosome containing
1µm
two damaged organelles
1 µm
Nucleus
Mitochondrion
fragment
Peroxisome
fragment
Lysosome
Lysosome contains Food vacuole
fuses with
active hydrolytic
lysosome
enzymes
Lysosome fuses with
vesicle containing
damaged organelle
Hydrolytic
enzymes digest
food particles
Hydrolytic enzymes
digest organelle
components
Digestive
enzymes
Lysosome
Lysosome
Plasma membrane
Digestion
Food vacuole
(a) Phagocytosis: lysosome digesting food
Digestion
Vesicle containing
damaged mitochondrion
(b) Autophagy: lysosome breaking down damaged organelle
Organelles
• Peroxisomes
– Smaller than lysosomes
– Detoxify several toxic substances such as alcohol
– Abundant in the liver
• Proteasomes
– Continuously destroy unneeded, damaged, or
faulty proteins
– Found in the cytosol and the nucleus
Mitochondria
• Energy powerhouse of the cell
• ATP synthesis occurs here (requires oxygen)
– Cellular respiration
• Organs with high energy needs have lots of
mitochondria
• Unique features:
– Has its own closed circular DNA & ribosomes
– Divides on its own, not with the cell
– Is inherited only maternally
Mitochondria
• Mitochondria are enclosed by two
membranes
– A smooth outer membrane
– An inner membrane folded into cristae
Nucleus
• Membrane bound structure containing DNA
• Components of the nucleus include:
– Nuclear envelope
– Nuclear Lamina
– Nuclear pores
– Nucleolus
– Chromosomes
– Chromatin
Nucleus
Nuclear Envelope
• Double phospholipid bilayer surrounding nucleus
• Surface studded with ribosomes
• Nuclear Pores span the two bilayers
– Allow for movement into and out of the nucleus
• Nuclear lamina
– Netlike array of protein filaments that maintain shape of nucleus
• Nucleolus
– Dense mass of material in the nucleus
– Site where ribosomal RNA is made
Chromosomes and Chromatin
• Chromatin
– All of a cell’s DNA and the proteins the DNA is
wrapped around
– Uncoiled, allows regulatory proteins to attach to DNA
• Beads on a string
• Chromosome
– 1 condensed chromatin molecule and its associated
proteins
– Chromosomes are made up of chromatin
Packing of DNA into a Chromosome of a
Dividing Cell
Ribosomes
• Ribosomes
– Particles made of ribosomal RNA and protein
– Carry out protein synthesis
Cell Division
• Cell division is important for many reasons
– Embryonic Development
– Normal Growth
– Repair of damaged tissues
– Replace dead or worn out cells
Nuclear Division
2 types of nuclear division
Mitosis
Meiosis
• Nuclear division in somatic
• Division to produce mature
cells
gametes
• Produces 2 daughter cells
genetically identical to the
parent cell
• Produces 4 haploid
daughter cells
Chromosome Terminology
• Replicated vs. unreplicated Chromosome
• Sister chromatids
• Centromere
• Kinetechore
Chromosome Duplication
0.5 µm
A eukaryotic cell has multiple
chromosomes, one of which is
represented here. Before
duplication, each chromosome
has a single DNA molecule.
Once duplicated, a chromosome
consists of two sister chromatids
connected at the centromere. Each
chromatid contains a copy of the
DNA molecule.
Mechanical processes separate
the sister chromatids into two
chromosomes and distribute
them to two daughter cells.
Chromosome
duplication
(including DNA
synthesis)
Centromere
Separation
of sister
chromatids
Centrometers
Sister
chromatids
Sister chromatids
Eukaryotic Cell Terminology
• Diploid
• Haploid
• Somatic cells
• Sex cells
The Cell Cycle
• The cell cycle is a sequence of events in which a body cell
duplicates its contents and divides in two
• The cell cycle includes interphase and nuclear division, mitosis
– G1, S & G2 represent Interphase
• Growth of parent cell and Organelle number doubles (G1 &
G2)
• DNA is replicated (S)
• Cell divides in mitosis (M)
• Before a cell divides it MUST replicate its DNA
– Mitosis begins with replicated chromosomes
The Cell Cycle
Interphase
Mitosis
• Nuclear division in somatic cells
• 4 phases
• Mitosis is the shortest part of the cell cycle
• Mitosis is followed by cytokinesis
– division of the cytoplasm
• Produces 2 daughter cells genetically identical
to the parent cell
Phases of Mitosis
• Prophase
• Metaphase
• Anaphase
• Telophase
Formation of the Mitotic Spindle
• Mitotic spindle controls chromosome
movement during mitosis
• Begins to form in prophase
• Composed of microtubules and other proteins
– Assembly begins at the centrosome
– Microtubules grow out from centrosome = Mitotic
Spindle
Mitotic Spindle
• Spindle microtubules
attach to kinetechore
proteins located at the
centromere
Sister
chromatids
Metaphase
Plate
Kinetochores
Overlapping
nonkinetochore
microtubules
Kinetochores microtubules
0.5 µm
Prophase
• Formation of mitotic spindle and Centrosomes move to opposite
poles of cell
• Chromatin condenses into chromosomes
• Nuclear envelope breaks down
• Spindle microtubules attach to kinetechores
Metaphase
• Longest mitotic stage
• Chromosomes line up at the metaphase plate
Anaphase
•
•
•
•
Shortest stage
Sister chromatids separate at the centromere
Chromosomes move towards opposite poles of the cell
Cell elongates
Telophase
• Nuclei begin to form in
daughter cells
• Nuclear envelope forms
• Chromosomes
decondense
• Mitosis is complete
Cytokinesis in Animal Cells
• Division of cytoplasm
• A cleavage furrow forms at the
surface of the cell where the
metaphase plate was
Cleavage furrow
Contractile ring of
microfilaments
Figure 12.9 A
100 µm
Daughter cells
(a) Cleavage of an animal cell (SEM)
Centrosome:
Centrioles
Pericentriolar material
1
Mitosis
6
Nucleolus
Nuclear envelope
Chromatin
Plasma membrane
Cytosol
LM all at 700x
(a) INTERPHASE
2
Kinetochore
Centromere
5
Chromosome
(two chromatids
(f) IDENTICAL CELLS IN INTERPHASE
joined at
centromere
Early
Mitotic spindle
(microtubules)
Fragments of
nuclear envelope
Late
(b) PROPHASE
Metaphase plate
3
Cleavage furrow
(c) METAPHASE
4
(e) TELOPHASE
Cleavage
furrow
Chromosome
Late
Early
(d) ANAPHASE
Cell Division
• Some cells actively divide throughout our
lifetime
– Skin cells, cells lining the digestive tract
• Some cells divide during embryonic
development and once formed, never divide
again
– Nerve cells, muscle cells
– Are in a state called G0
Cell Cycle Checkpoints
• Built in checkpoint system that can stop the
cell cycle if it detects a problem
– Example of problem: Chromosomes not properly
separated
• Checkpoints are controlled by signaling
pathways
Cell Cycle Checkpoints
• 3 major checkpoints
• G1/S, G2/M and M
• G1 is the restriction point
– If all is well, cycle proceeds
– If not, cell enters G0
Other External Factors Important in Cell
Division
• Nutrients
– Growth factors that stimulate cells to divide
• Density of Cells
– Density dependent inhibition
• Crowded cells stop dividing
• Anchorage Dependence
– In order for normal cells to divide, they must be attached
to something
• Cancer cells DO NOT exhibit density dependent
inhibition or anchorage dependence
Loss of Cell Cycle Control
• Loss of control leads to overgrowth
• Transformation – process where a normal cell
becomes cancerous
Cancer Cell Characteristics
• Are free of the body’s controls
• Have chromosomal abnormalities
• Have metabolic abnormalities
• Loss of attachment to nearby cells
– Malignancy through blood and lymph
• Often grow their own blood supply
Stem Cells
• Stem cells are unspecialized cells that have the
potential to give rise to other types of
specialized cells
• Differentiation
– Process where a cell becomes specialized
Stem Cells
• 3 main populations of stem cells
• Adult stem cells
• Hematopoietic stem cells
• Embryonic stem cells