Download Ch2.Cells.Lecture

Ch 2, CELLS: The Living Units
of Organisms
Human embryonic stem cells, S. Huerta
Sonya Schuh-Huerta, Ph.D.
The Inner Life of the Cell…
Introduction to Cells
• Several important scientists made
discoveries about cells
– Antonie van Leeuwenhoek – one of 1st to observe cells;
father of microscopy & microbiology
– Robert Hooke – coined the term “cell” as the basic unit of life
– Matthias Schleiden & Theodor Schwann
– Rudolf Virchow
• Cells – the smallest living units in our bodies
– Organelles  “little organs” – carry on essential functions
of cells
Introduction to Cells
• Cells have 3 main components
– Plasma membrane = the outer boundary
– Cytoplasm = contains most organelles
– Nucleus = controls cellular activities
Structure of a Generalized Cell
Chromatin
Nuclear envelope
Nucleolus
Nucleus
Plasma
membrane
Smooth endoplasmic
reticulum
Cytosol
Mitochondrion
Lysosome
Centrioles
Rough
endoplasmic
reticulum
Centrosome
matrix
Ribosomes
Golgi apparatus
Cytoskeletal
elements
Microtubule
Intermediate
filaments
Secretion being
released from cell
by exocytosis
Peroxisome
The Plasma Membrane
• Defines the extent of the cell
• Structure of membrane
– Fluid mosaic model (lipid bilayer)
– Types of membrane proteins:
• Integral proteins = firmly imbedded in, or
attached to lipid bilayer
• Short chains of carbohydrates attach to integral
proteins - form the glycocalyx
• Peripheral proteins = attach to membrane
surface
– Support plasma membrane from cytoplasmic side
The Plasma Membrane
Polar head of
phospholipid
molecule
Extracellular fluid
(watery environment)
Glycolipid
Cholesterol
Nonpolar tail
of phospholipid
molecule
Glycoprotein
Carbohydrate
of glycocalyx
Bimolecular
lipid layer
containing
proteins
Outwardfacing layer of
phospholipids
Inward-facing
layer of
phospholipids
Cytoplasm
(watery environment)
Integral
proteins
Filament of
cytoskeleton
Peripheral
proteins
The Plasma Membrane
• Functions – relate to location at the interface of
cell’s exterior and interior
– Acts as barrier against substances outside cell
– Cell-to-cell recognition & communication (receptors)
– Determines which substances enter or leave the cell
• Plasma membrane is selectively permeable!
The Plasma Membrane in Action
Membrane Transport
• Simple diffusion – tendency of molecules to
move down their concentration gradient
• Osmosis – diffusion of water molecules across
a membrane – down their gradient
Membrane Transport Mechanisms
• Facilitated diffusion = movement of molecules
down their concentration gradient through an
integral protein
• Active transport = integral proteins move
molecules across the plasma membrane against
their concentration gradient (often using energy
in the form of ATP)
Membrane Transport Mechanisms
Extracellular fluid
Lipidsoluble
solutes
Water
molecules
Solutes
Lipid
bilayer
Cytoplasm
Simple diffusion
Osmosis
Facilitated diffusion &
active transport
Endocytosis
• Endocytosis
– Mechanism by which particles enter cells
• Phagocytosis = “cell eating”
• Pinocytosis = “cell drinking”
Receptor-Mediated Endocytosis
• Receptor-mediated endocytosis = plasma
proteins bind to certain molecules & bring them
into the cell
• Invaginates and forms a coated pit
– Pinches off to become a coated vesicle
• NOTE: This process is very specific to certain
molecules.
• It is the method by which insulin and cholesterol
enter cells!
3 Types of Endocytosis
Pinocytosis
Phagocytosis
Vesicle
Phagosome
Receptor-Mediated
Endocytosis
Vesicle
Receptor recycled
to plasma membrane
Exocytosis
• Exocytosis = mechanism that moves substances
out of the cell
– Substance is enclosed in a vesicle
– The vesicle migrates to the plasma membrane
– Proteins from the vesicles (v-SNAREs) bind with
membrane proteins (t-SNAREs)
– The lipid layers from both membranes bind & the vesicle
releases its contents to the outside
Exocytosis
Extracellular
fluid
Plasma membrane
SNARE (t-SNARE)
1 The membranebound vesicle
Vesicle SNARE migrates to the
(v-SNARE)
plasma membrane.
Secretory
vesicle
Fused
v- and
t-SNAREs
2 There, proteins
at the vesicle
surface (v-SNAREs)
bind with t-SNAREs
(plasma membrane
proteins).
Molecule to
be secreted
Fusion pore formed
3 The vesicle and
plasma membrane
fuse and a pore
opens up.
Cytoplasm
4 Vesicle contents
are released to the
cell exterior.
Exocytosis in Action
The Cytoplasm
• Cytoplasm = lies internal to plasma membrane
& consists of cytosol, organelles, & inclusions
• Cytosol = jelly-like fluid in which other cellular
elements are suspended
– Consists of water, ions, & enzymes
Organelles – Ribosomes
• Ribosomes – constructed of proteins and
ribosomal RNA; not surrounded by a membrane
– Site of protein synthesis
• Assembly of proteins is called translation
– Are the “assembly line” of the manufacturing plant
Endoplasmic Reticulum
• Endoplasmic reticulum—“network within the
cytoplasm”
Rough ER – ribosomes stud the external surfaces
Smooth ER – consists of tubules in a branching
network
• No ribosomes are attached; therefore no protein
synthesis
The ER & Ribosomes
(b) Electron micrograph of smooth and
rough ER (85,000)
Smooth ER
Nuclear
envelope
Rough ER
Ribosomes
Cisternae
(a) Diagrammatic view of smooth and rough ER
Golgi Apparatus
• Golgi apparatus = a stack of 3–10 disk-shaped
envelopes
– Sorts products of rough ER and sends them to proper
destination
– Products of rough ER move through the Golgi from the
convex (cis) to the concave (trans) side
– “Packaging & shipping” division of manufacturing plant
Golgi Apparatus
Many vesicles in the process of pinching off
from the membranous Golgi apparatus
Electron micrograph of the Golgi
apparatus (90,000)
New vesicles forming
Transport vesicle
from rough ER
Cis face—
“receiving” side of
Golgi apparatus
Cisternae
New vesicles
forming
Transport
vesicle
from
trans face
Secretory vesicle
Trans face –
“shipping” side of
Golgi apparatus
Golgi apparatus
Transport vesicle
from the Golgi
apparatus
Lysosomes
• Lysosomes = membrane-walled sacs containing
digestive enzymes
– Digest substances
– Cells of gut  many lysosomes
Lysosomes
Sequence of Events From Protein
Synthesis to Their Final Distribution
Rough ER
ER membrane Phagosome
Proteins in cisterna
1 Protein-containing
vesicles pinch off
rough ER and migrate
to fuse with membranes
of Golgi.
Pathway C: Lysosome
containing acid hydrolase
enzymes
2 Proteins are
modified within the
Golgi compartments.
3 Proteins are then
packaged within
different vesicle types,
depending on their
ultimate destination.
Plasma
membrane
Vesicle becomes
lysosome
Golgi
apparatus
Pathway A:
Vesicle contents
destined for
exocytosis
Secretory
vesicle
Secretion by exocytosis
Pathway B:
Vesicle membrane
to be incorporated
into plasma
membrane
Extracellular fluid
Mitochondria
• Mitochondria = generates most of the cell’s
energy; most complex organelle; Arose from
early animal cell engulfing a prokaryote – they
then lived together happily ever after!
– More abundant in energyrequiring cells, like muscle
cells & sperm
– “Power plant” of the cell
Mitochondria
Outer
mitochondrial
membrane
Ribosome
Mitochondrial
DNA
Inner
mitochondrial
membrane
Cristae
Matrix
Enzymes
Peroxisomes
• Peroxisomes = membrane-walled sacs of
oxidase enzymes
– Enzymes neutralize free radicals and break down
poisons (ie. alcohol, hydrogen peroxide)
– Break down long chains of fatty acids
– Are the toxic waste removal system
– Are numerous in the liver and kidneys!!!
The Cytoskeleton
• Cytoskeleton = “cell skeleton” – an elaborate
network of protein rods
– Contains 3 types of rods:
• Microtubules = cylindrical structures made of tubulin protein
subunits; form mitotic spindle and are the tracks that motor
proteins transport materials on
• Intermediate filaments = protein fibers; provide structure and
rigidity to cell
• Microfilaments = filaments of the contractile protein actin;
perform contractile functions of cell
Cytoskeletal Elements
(a)
Microfilaments
(b)
Strands made of spherical protein
subunits called actins
Intermediate filaments
Tough, insoluble protein fibers
constructed like woven ropes
(c)
Microtubules
Hollow tubes of spherical protein
subunits called tubulins
Tubulin subunits
Fibrous subunits
Actin subunit
7 nm
10 nm
25 nm
The Cytoskeleton in Action
Centrosome
• Centrosomes & centrioles:
– Centrosome = a spherical structure in the cytoplasm
• Composed of centrosome matrix and centrioles
– Centrioles = paired cylindrical bodies made of tubulin
• Each consists of 27 short microtubules
• Act in forming cilia & flagella
• Gives rise to mitotic spindle - necessary for
karyokinesis (nuclear division)
Centrosome & Centrioles
Centrosome matrix
Centrioles
Microtubules
Cytoplasmic Inclusions
• May consist of pigments, crystals of protein, and
food stores (temporary structures, not present in
all cell types)
– Lipid droplets – found in liver cells & fat cells
– Glycosomes – store sugar in the form of glycogen
The Nucleus
• Nucleus = “little nut” or “kernel” – control center
of cell
– DNA directs the cell’s activities & production of proteins
– ~5 µm in diameter
The Nucleus
• Nuclear envelope = two parallel membranes
separated by fluid-filled space
• Nuclear pores penetrate the nuclear envelope
– Pores allow large molecules to pass in and out of the
nucleus (like messenger RNA)
The Nucleus
• Nucleolus = “little nucleus” – in the center of the
nucleus
– Contains parts of several chromosomes
– Site of ribosome subunit assembly
The Nucleus
Fracture
line of outer
membrane
Nuclear
pores
Nuclear envelope
Nucleus
Chromatin
(condensed)
Nucleolus
Cisternae of
rough ER
Nuclear pore complexes.
Nuclear lamina. The netlike
Each pore is ringed by protein
particles
lamina composed of intermediate
filaments formed by lamins lines
the inner surface of the nuclear
envelope.
Chromatin & Chromosomes
• DNA double helix is composed of a sugar
phosphate backbone and four subunits:
– Thymine (T), adenine (A), cytosine (C), & guanine (G)
• DNA is coiled around proteins (histones)
– DNA + proteins = chromatin
Chromatin & Chromosomes
Hydrogen bond
• Each cluster of
DNA & histone
proteins is a
nucleosome
Nucleotides
Sugar-phosphate
backbone
Adenine (A)
Thymine (T)
Cytosine (C)
Guanine (G)
Deoxyribose
sugar
Phosphate
Chromatin & Chromosomes
• Extended chromatin
– Is the active region of DNA where DNA’s genetic
code is duplicated (in replication) or copied into
messenger RNA (mRNA) in the process of
transcription
• Condensed chromatin
– Tightly coiled nucleosomes
– Inactive form of chromatin – no transcription
occurring
• Chromosomes = highest level of organization
of chromatin
– Contains a long molecule of DNA
– Typical human cell has 46 chromosomes!
Chromatin & Chromosome Structure
1 DNA
double
helix (2-nm
diameter)
Histones
2 Chromatin
(“beads on a
string”) structure
with nucleosomes
Linker DNA
Nucleosome (10-nm diameter;
8 histone proteins wrapped
by 2 winds of the DNA double
helix)
3 Tight helical fiber
4 Looped domain
(30-nm diameter)
structure (300-nm
diameter)
5 Chromatid
(700-nm diameter)
Metaphase
Chromosome = 2 sister chromatids
(at midpoint of cell division)
The Cell Life Cycle
• The cell life cycle = the series of changes a
cell goes through, including dividing & producing
daughter cells
– Interphase:
• G1 phase = Growth 1 or Gap 1 phase
–
–
–
–
The first part of interphase
Cell metabolically active  grows & makes proteins
Variable in length from hours to YEARS! (egg cell)
Centrioles begin to replicate near the end of G1
The Cell Life Cycle
• S (synthesis) phase = DNA replicates itself
– Ensures that daughter cells receive identical copies of
the genetic material (chromatin extended)
• G2 phase = Growth 2 or Gap 2
– Centrioles finish copying themselves (now 2 pairs!)
– Enzymes needed for cell division are synthesized
• During S & G2 phases of interphase  cell
carries on normal activities
The Cell Life Cycle
• Cell division = mitosis
– M (mitotic) phase = cells divide during this stage
• Comes after interphase (G1, S, & G2)
Cell Division
• Cell division involves:
– Mitosis = division of the nucleus
• Chromosomes are distributed to the 2 daughter
nuclei
– Cytokinesis = division of the cytoplasm
• Occurs after the nucleus divides
The Cell Life Cycle
G1 checkpoint
(restriction point)
G1
Growth
Interphase
S
Growth and DNA
synthesis
G2
Growth and final
preparations for
division
M
Mitosis
Mitotic phase (M)
G2 checkpoint
Stages of Mitosis
• Prophase = 1st & longest stage of mitosis
– Early prophase  chromatin threads
condense into chromosomes
• Chromosomes = 2 threads called chromatids
(sister chromatids)
• Chromatids held together by the centromere
• Centriole pairs separate from one another
• Mitotic spindle forms
– Late prophase  centrioles continue moving
away from each other
• Nuclear membrane breaks down
• Chromosomes bound by mitotic spindle
Early & Late Prophase
Interphase
Centrosomes (each
has 2 centrioles)
Early Prophase
Plasma
membrane
Late Prophase
Early mitotic
spindle
Spindle pole
Aster
Nucleolus
Chromatin
Nuclear
envelope
Chromosome
consisting of two
sister chromatids
Polar microtubule
Fragments
of nuclear
envelope
Centromere
Kinetochore
Kinetochore
microtubule
Stages of Mitosis
• Metaphase = 2nd stage of mitosis
– Chromosomes cluster at middle of the cell
• Centromeres are aligned along the equator
• Anaphase = 3rd & shortest stage of mitosis
– Centromeres of chromosomes split apart
Stages of Mitosis
• Telophase = 4th stage of mitosis
–
–
–
–
Begins as chromosomal movement stops
Chromosomes at opposite poles of the cell uncoil
Resume threadlike extended-chromatin form
A new nuclear membrane forms
• Cytokinesis = completes the division of the cell
into 2 daughter cells
Metaphase, Anaphase,Telophase &
Cytokinesis
Metaphase
Anaphase
Nuclear
envelope
forming
Spindle
Metaphase
plate
Telophase & Cytokinesis
Daughter
chromosomes
Contractile
Nucleolus forming ring at
cleavage
furrow
Mitosis in Action
Cellular Diversity
• Cellular Differentiation
– Cells in different regions of developing embryo are
exposed to different chemical signals
– Cells differentiate into different cell types &
become specialized
• Specialized functions of cells relates to:
– Shape of cell morphology
– Arrangement, types, & numbers of organelles
Cellular Diversity
• Cells that connect body parts or line & cover organs:
– Fibroblast = makes and secretes protein component of fibers
– Erythrocyte = concave shape provides surface area for uptake
of the respiratory gases
– Epithelial cell = hexagonal shape allows maximum number of
epithelial cells to pack together
Cellular Diversity
• Cells that MOVE organs & body parts:
– Skeletal & smooth muscle cells
• Elongated & made of actin & myosin
• Contract forcefully!
Skeletal
muscle
cell
Smooth
muscle cells
Cellular Diversity
• Cells that store nutrients:
– Fat cell = stores fat; shape is
Fat cell
produced by large fat droplet in
its cytoplasm
• Cells that fight disease:
– Macrophage = moves through tissue to reach
infection sites & engulfs foreign particles
Macrophage
Cellular Diversity
• Cells that gather & transmit information:
– Neuron = has long processes for receiving &
transmitting messages
neuron
Cellular Diversity
• Cells of reproduction:
*Sperm (male) = specialized cell with a
long tail (flagellum) for swimming to and
fertilizing the egg
*Oocyte (female) = largest cell of the body, contains
extra stores of RNA and protein, specialized outer
shell
Developmental Aspects of Cells
• Aging = complex process that happens over
time; caused by a variety of factors:
– Free radical theory
• Damage from byproducts of cellular metabolism
• Radicals build up & damage essential molecules of cells
– Mitochondrial theory
• A decrease in production of energy by mitochondria weakens
& ages our cells
– Genetic theory = proposes that aging is programmed
by genes
• Telomeres = “end caps” on chromosomes
• Telomerase = prevents telomeres from degrading
Questions…?
What’s Next?
Lab: The Microscope & Cells
Wed Lecture: Embryology &
Tissues
Wed Lab: Cells & Tissues