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Chapter 3
The Cellular Level of
Organization
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A Generalized Cell
1. Plasma membrane
- forms the cells outer boundary
- separates the cells internal environmentfrom the outside environment
- is a selective barrier
- plays a role in cellular communication
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A Generalized Cell
2. Cytoplasm
- all the cellular contents between the plasmamembrane and the nucleus
- cytosol - the fluid portion, mostly water
- organelles - subcellular structures havingcharacteristic shapes and specific functions
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A Generalized Cell
3. Nucleus
- large organelle that contains DNA
- contains chromosomes,each of which
consists of a single molecule of DNA and
associated proteins
- a chromosome contains thousands ofhereditary units called genes
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Fig. 3.1 Generalized Body Cell
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Plasma Membrane
Flexible yet sturdy barrier
The fluid mosaic model- the arrangement ofmolecules within the membrane resembles a
sea of lipids containing many types ofproteins
The lipids act as a barrier to certain
substances The proteins act as gatekeepers to certain
molecules and ions
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Structure of a Membrane
Consists of a lipid bilayer - made up ofphospholipids, cholesterol and glycolipids
Integral proteins - extend into or through the
lipid bilayer
Transmembrane proteins - most integralproteins, span the entire lipid bilayer
Peripheral proteins - attached to the inneror outer surface of the membrane, do notextend through it
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Structure of the Plasma Membrane
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Structure of aMembrane
Glycoproteins - membrane proteins with acarbohydrate group attached that protrudesinto the extracellular fluid
Glycocalyx - the sugary coatingsurrounding the membrane made up of thecarbohydrate portions of the glycolipids and
glycoproteins
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Functions of Membrane Proteins
Some integral proteins are ion channels
Transporters - selectively move substancesthrough the membrane
Receptors - for cellular recognition; a ligandis a molecule that binds with a receptor
Enzymes - catalyze chemical reactions
Others act as cell-identity markers
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Figure 3.3
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Membrane Permeability
The cell is either permeable or impermeableto certain substances
The lipid bilayer is permeableto oxygen,
carbon dioxide, water and steroids, butimpermeableto glucose
Transmembrane proteins act as channels
and transporters to assist the entrance ofcertain substances, for example, glucose andions
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Passive vs. Active Processes
Passive processes - substances move acrosscell membranes without the input of anyenergy; use the kinetic energyof individual
molecules or ions Active processes - a cell uses energy,
primarily from the breakdown of ATP, to
move a substance across the membrane, i.e.,against a concentration gradient
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Diffusion
Steepness of
concentrationgradient
Temperature
Mass of diffusingsubstance
Surface area Diffusion
distance
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Simple Diffusion, Channel-mediated
Facilitated Diffusion, and Carrier-mediated
Facilitated Diffusion
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Channel-mediated Facilitated Diffusion of
Potassium ions through a Gated K+ Channel
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Carrier-mediated Facilitated Diffusion of
Glucose across a Plasma Membrane
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Glucosetransporter
Glucosegradient
Glucose
Extracellular fluid Plasma membrane Cytosol
1
Glucosetransporter
Glucosegradient
Glucose
Extracellular fluid Plasma membrane Cytosol
1
2
Glucosetransporter
Glucosegradient
Glucose
Glucose
Extracellular fluid Plasma membrane Cytosol
1
2
3
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Osmosis
Net movement of water through aselectively permeable membrane from anarea of high concentration of water (lowerconcentration of solutes) to one of lower
concentration of water Water can pass through plasma membrane
in 2 ways:
1. through lipid bilayer by simple diffusion
2. through aquaporins, integral membraneproteins
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Tonicity and its effect on RBCS
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Active TransportSolutes are transported across plasma membranes with the use of energy, from an area
of lower concentration to an area of higher Concentration Sodium-potassium pump
1
3 Na+
K+gradient
Na+gradient
Na+/K+ ATPase
Extracellular fluid
Cytosol
1
3 Na+ expelled
3 Na+
ADP
P
K+gradient
Na+gradient
Na+/K+ ATPase
Extracellular fluid
CytosolATP
21
3 Na+ expelled
3 Na+
ADPP
P
K+gradient
Na+gradient
Na+/K+ ATPase
Extracellular fluid
Cytosol
2K+
ATP2 31
3 Na+ expelled
3 Na+
ADPP
P
2 K+
imported
K+gradient
Na+gradient
Na+/K+ ATPase
Extracellular fluid
Cytosol
2K+
ATP2 3 4
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Transport in Vesicles
Vesicle- a small spherical sac formed by budding offfrom a membrane
Endocytosis - materials move into a cell in a vesicleformed from the plasma membrane
three types: receptor-mediated endocytosisphagocytosisbulk-phase endocytosis (pinocytosis)
Exocytosis - vesicles fuse with the plasmamembrane, releasing their contents into theextracellular fluid
Transcytosis - a combination of endocytosis andexocytosis
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Receptor-Mediated Endocytosis
1 Binding
1 Binding
1 Binding
1 Binding
1 Binding
1 Binding
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1
Clathrin-coatedpit
Binding
Receptor
Receptor-LDLcomplex
LDL particle Plasmamembrane
Invaginated plasmamembrane
1
Clathrin-coatedvesicle
Clathrin-coatedpit
Binding
Vesicle formation
Receptor
Receptor-LDLcomplex
LDL particle Plasmamembrane
Invaginated plasmamembrane
2
1
Clathrin-coatedvesicle
Clathrin-coatedpit
Binding
Vesicle formation
Receptor
Receptor-LDLcomplex
Uncoated vesicle
Uncoating
LDL particle Plasmamembrane
Invaginated plasmamembrane
2
3
1
Clathrin-coatedvesicle
Clathrin-coatedpit
Binding
Vesicle formation
Receptor
Receptor-LDLcomplex
Uncoated vesicle
Fusion with
endosome
Uncoating
LDL particle Plasmamembrane
Invaginated plasmamembrane
Endosome
2
3
4
Transportvesicle
1
Clathrin-coatedvesicle
Clathrin-coatedpit
Binding
Vesicle formation
Receptor
Receptor-LDLcomplex
Uncoated vesicleTransportvesicle
Fusion with
endosome
Recyclingof receptorsto plasmamembrane
Uncoating
LDL particle Plasmamembrane
Invaginated plasmamembrane
Endosome
2
3
4
5
Transportvesicle
1
Clathrin-coatedvesicle
Clathrin-coatedpit
Binding
Vesicle formation
Receptor
Receptor-LDLcomplex
Uncoated vesicle
Degradationin lysosome
Lysosome
Transportvesicle
Transportvesicle
Fusion with
endosome
Recyclingof receptorsto plasmamembrane
Uncoating
LDL particle Plasmamembrane
Invaginated plasmamembrane
Endosome
Digestiveenzymes
2
3
4
5
6
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Phagocytosis
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Bulk-phase Endocytosis
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Cytoplasm - 2 components
1. Cytosol - intracellular fluid, surrounds 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
The cytoskeleton - network of protein filaments throughout the
cytosol-provides structural support for the cell
-three types according to increasing size: microfilaments,intermediate filaments, and microtubules
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The Cytoskeleton
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Organelles
Centrosome - located near the nucleus,consists of two centrioles and pericentriolarmaterial (Fig. 2.7)
Cilia - short, hair-like projections from the cellsurface, move fluids along a cell surface
Flagella - longer than cilia, move an entire
cell;only example is the sperm cells tail (Fig.2.8)
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The Centrosome
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Cilia and Flagella
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Organelles
Ribosomes - sites of protein synthesis
Endoplasmic reticulum - network of membranes inthe shape of flattened sacs or tubules
- Rough ER -connected to the nuclear envelope, aseries of flattened sacs, surface is studded withribosomes, produces various proteins
-Smooth ER- a network of membrane tubules, does
not have ribosomes, synthesizes fatty acids andsteroids, detoxifies certain drugs
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Ribosomes
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Endoplasmic Reticulum
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Organelles
Golgi complex - consists of 3-20 flattened,membranous sacs called cisternae
- modify, sort, and package proteins for
transport to different destinations
- proteins are transported by variousvesicles
Lysosomes - vesicles that form from the Golgicomplex, contain powerful digestive enzymes
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Golgi Complex
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Processing and Packaging
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Lysosomes
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Organelles
Peroxisomes - smaller than lysosomes,detoxify several toxic substances such asalcohol, abundant in the liver
Proteasomes - continuously destroyunneeded, damaged, or faulty proteins, foundin the cytosol and the nucleus
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Organelles
Mitochondria - the powerhouses of the cell
Generate ATP
More prevalent in physiologically active cells: muscles, liverand kidneys
Inner and outer mitochondrial membranes
Cristae- the series of folds of the inner membrane
Matrix- the large central fluid-filled cavity
Self-replicate during times of increased cellular demand or
before cell division
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Mitochondria
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Organelles - Nucleus
Spherical or oval shaped structure
Usually most prominent feature of a cell
Nuclear envelope- a double membrane thatseparates the nucleus from the cytoplasm
Nuclear pores- numerous openings in the nuclear
envelope, control movement of substances betweennucleus and cytoplasm
Nucleolus- spherical body that produces ribosomes
Genes- are the cells hereditary units, control
activities and structure of the cell Chromosomes- long molecules of DNA combined
with protein molecules
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Packing of DNA into a Chromosome of a
Dividing Cell
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Overview of Gene Expression
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Transcription
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Translation
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Somatic Cell Division - Mitosis
The cell cycle is a sequence of events inwhich a body cell duplicates its contents anddivides in two
Human somatic cells contain 23 pairs ofchromosomes (total = 46)
The two chromosomes that make up eachpair are called homologous chromosomes
(homologs) Somatic cells contain two sets of
chromosomes and are called diploid cells
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Cell Division
Interphase - the cell is not dividing
- the cell replicates its DNA
- consists of three phases, G1, S, and G2,
replication of DNA occurs in the S phaseMitotic phase - consists of a nuclear division(mitosis) and a cytoplasmic division
(cytokinesis) to form two identical cells
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The Cell Cycle
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DNA Replication
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Nuclear Division: Mitosis
Prophase- the chromatin fibers change intochromosomes
Metaphase - microtubules align the centromeres ofthe chromatid pairs at the metaphase plate
Anaphase- the chromatid pairs split at thecentromere and move to opposite poles of the cell;the chromatids are now called chromosomes
Telophase- two identical nuclei are formed aroundthe identical sets of chromosomes now in theirchromatin form
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Cytoplasmic Division: Cytokinesis
Division of a cells cytoplasm to form twoidentical cells
Usually begins in late anaphase
The plasma membrane constricts at itsmiddle forming a cleavage furrow
The cell eventually splits into two daughtercells
Interphase begins when cytokinesis iscomplete
1
Pericentriolar materialCentrioles
Centrosome:
1
Pericentriolar materialCentrioles
Centrosome:
1
Pericentriolar materialCentrioles
Centrosome:
1
Pericentriolar materialCentrioles
Centrosome:
1
Pericentriolar materialCentrioles
Centrosome:
1
Pericentriolar materialCentrioles
Centrosome:
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NucleolusNuclear envelopeChromatinPlasma membrane
Cytosol
(a) INTERPHASE
all at 700xLM
LateEarly
NucleolusNuclear envelopeChromatinPlasma membrane
Cytosol
Chromosome(two chromatidsjoined atcentromere
(a) INTERPHASE
(b) PROPHASE
Fragments ofnuclear envelope
Mitotic spindle(microtubules)
Kinetochore
2
all at 700xLM
Centromere
NucleolusNuclear envelopeChromatinPlasma membrane
Cytosol
Metaphase plate
(a) INTERPHASE
(c) METAPHASE
2
3
LateEarly(b) PROPHASE
Fragments ofnuclear envelope
Mitotic spindle(microtubules)
Kinetochore
all at 700xLM
Chromosome(two chromatidsjoined atcentromere
Centromere
EarlyLate(d) ANAPHASE
NucleolusNuclear envelopeChromatinPlasma membrane
Cytosol
Chromosome
(a) INTERPHASE
(c) METAPHASE
2
3
4
Cleavagefurrow
LateEarly(b) PROPHASE
Fragments ofnuclear envelope
Mitotic spindle(microtubules)
Kinetochore
Metaphase plate
all at 700xLM
Chromosome(two chromatidsjoined atcentromere
Centromere
EarlyLate(d) ANAPHASE
NucleolusNuclear envelopeChromatinPlasma membrane
Cytosol
(a) INTERPHASE
Cleavage furrow
(e) TELOPHASE
(c) METAPHASE
2
3
4
5
Cleavagefurrow
LateEarly(b) PROPHASE
Fragments ofnuclear envelope
Mitotic spindle(microtubules)
Kinetochore
Metaphase plate
Chromosome
all at 700xLM
Chromosome(two chromatidsjoined atcentromere
Centromere
EarlyLate(d) ANAPHASE
NucleolusNuclear envelopeChromatinPlasma membrane
Cytosol
(a) INTERPHASE
(f) IDENTICAL CELLS IN INTERPHASE
Cleavage furrow
(e) TELOPHASE
(c) METAPHASE
Cleavagefurrow
2
3
4
5
6
LateEarly(b) PROPHASE
Fragments ofnuclear envelope
Mitotic spindle(microtubules)
Kinetochore
Metaphase plate
Chromosome
all at 700xLM
Centromere
Chromosome(two chromatidsjoined atcentromere
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R d i C ll Di i i
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Reproductive Cell Division
During sexual reproduction each new organism isthe result of the union of two gametes (fertilization),one from each parent
Meiosis - reproductive cell division that occurs inthe gonads (ovaries and testes) that producesgametes with half the number of chromosomes
Haploid cells - gametes contain a single set of 23
chromosomes Fertilization restores the diploid number of
chromosomes (46)
R d i C ll Di i i
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Reproductive Cell Division
Meiosis occurs in two successive stages:meiosis I and meiosis II
Each of these two stages has 4 phases:prophase, metaphase, anaphase, and
telophase Summary - Meiosis I begins with a diploid cell
and ends with two cells having the haploidnumber of chromosomes; in Meiosis II, each
of the two haploid cells divides, the net resultis four haploid gametes that are geneticallydifferent from the original diploid starting cell
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Leptotene stage condensation of chromatin
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Leptotene stage- condensation of chromatinfibers into long, threadlike structures, similarto what occurs at the beginning of mitosis.
Zygotene stage- individual chromosomesbecome distinguishable and homologouschromosomes become closely paired with
each other via synapsis.
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Pachytene stage crossing over or the
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Pachytene stage- crossing over, or theexchange of DNA segments between thehomologous chromosomes.
This accounts for genetic recombination.
Each chromosome undergoes compactingprocess where the size is reduced to less
than of its previous length.
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Diplotene stage- homologous chromosome
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Diplotene stage- homologous chromosomebegin to separate from each other particularlynear the centromere. They however, remainattached by connections known aschiasmata.
Chiasmata regions where exchange of DNA
materials has occured. Diakinesis- chromosomes recondense to
their maximally compacted state. Nucleoli
disappear, the spindle forms and the nuclearenvelope breaks down.
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Cellular Diversity
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Cellular Diversity
The average adult hasnearly 100 trillion cells
There are about 200different types of cells
Cells come in a varietyof shapes and sizes
Cellular diversity
permits organization ofcells into more complextissues and organs
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End of Chapter 3
Copyright 2009 John Wiley & Sons, Inc.All rights reserved. Reproduction or translation of this work beyond that
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