Chapter 3
The Cellular Level of
Organization
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Fig. 3.1 Generalized Body Cell
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A Generalized Cell
1. Plasma membrane
- forms the cell’s outer boundary
- separates the cell’s internal environment
from 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 plasma
membrane and the nucleus
- cytosol - the fluid portion, mostly water
- organelles - subcellular structures having
characteristic 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 of
hereditary units called genes
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Fig. 3.1 Generalized Body Cell
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Plasma Membrane
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Flexible yet sturdy barrier
The fluid mosaic model - the arrangement of
molecules within the membrane resembles a
sea of lipids containing many types of
proteins
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
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Consists of a lipid bilayer - made up of
phospholipids, cholesterol and glycolipids
Integral proteins - extend into or through the
lipid bilayer
Transmembrane proteins - most integral
proteins, span the entire lipid bilayer
Peripheral proteins - attached to the inner
or outer surface of the membrane, do not
extend through it
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Structure of the Plasma Membrane
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Structure of a Membrane
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Glycoproteins - membrane proteins with a
carbohydrate group attached that protrudes
into the extracellular fluid
Glycocalyx - the “sugary coating”
surrounding the membrane made up of the
carbohydrate portions of the glycolipids and
glycoproteins
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Functions of Membrane Proteins
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Some integral proteins are ion channels
Transporters - selectively move substances
through the membrane
Receptors - for cellular recognition; a ligand
is 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
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The cell is either permeable or impermeable
to certain substances
The lipid bilayer is permeable to oxygen,
carbon dioxide, water and steroids, but
impermeable to glucose
Transmembrane proteins act as channels
and transporters to assist the entrance of
certain substances, for example, glucose and
ions
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Passive vs. Active Processes
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Passive processes - substances move across
cell membranes without the input of any
energy; use the kinetic energy of 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
Speed of diffusion
depends on:
 Steepness of
concentration
gradient
 Temperature
 Mass of diffusing
substance
 Surface area
 Diffusion
distance
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Diffusion Through Lipid Bilayer
Nonpolar, hydrophobic molecules move
freely through bilayer
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Recall that very small charged particles
can also move
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Oxygen, carbon dioxide, nitrogen gases,
fatty acids, steroids, fat soluble vitamins (A,
E, D, K), small alcohols, ammonia
Water and urea
Important for life processes such as
nutrient, waste, and gas exchange
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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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Extracellular fluid
Glucose
1
Plasma membrane
Cytosol
Glucose
transporter
Glucose
gradient
2
3
Glucose
Carrier-mediated Facilitated Diffusion of Glucose across a Plasma Membrane
Osmosis
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1.
2.
Net movement of water through a
selectively permeable membrane from an
area of high concentration of water (lower
concentration of solutes) to one of lower
concentration of water
Water can pass through plasma membrane
in 2 ways:
through lipid bilayer by simple diffusion
through aquaporins, integral membrane
proteins
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Tonicity
Describes relationship of solutions on each
side of membrane
 Hypertonic
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Solution with more solutes
Water moves toward hypertonic side
Hypotonic
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Solution with less solutes
Water moves away from hypotonic side
Isotonic
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Both solutions have similar concentrations
of solutes
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Tonicity and its effect on RBCS
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Active Transport
Solutes 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
Na+
gradient
Extracellular fluid
Na+/K+ ATPase
Cytosol
K+
gradient
3 Na+ expelled
P
3 Na+
1
2K+
ATP
2
ADP
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3
P
+
4 2K
imported
Transport in Vesicles
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Vesicle - a small spherical sac formed by budding off
from a membrane
Endocytosis - materials move into a cell in a vesicle
formed from the plasma membrane
three types: receptor-mediated endocytosis
phagocytosis
bulk-phase endocytosis (pinocytosis)
Exocytosis - vesicles fuse with the plasma
membrane, releasing their contents into the
extracellular fluid
Transcytosis - a combination of endocytosis and
exocytosis
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1 Binding
Receptor-LDL
complex
Plasma
membrane
LDL particle
Receptor
Invaginated plasma
membrane
Clathrin-coated
pit
2 Vesicle formation
Clathrin-coated
vesicle
3 Uncoating
Transport
vesicle
Receptor-Mediated Endocytosis
Uncoated vesicle
4 Fusion with
endosome
5 Recycling
of receptors
to plasma
membrane
Endosome
Transport
vesicle
Digestive
enzymes
Lysosome
6 Degradation
in lysosome
Phagocytosis
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Only a few body cells are capable
Two main types
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Macrophages
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Neutrophils
Particle binds to plasma membrane receptor on
phagocyte
Pseudopods extend and surround particle
forming phagosome
Phagosome fuses with lysosomes which destroy
invader
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Phagocytosis
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Bulk-phase Endocytosis
Most body cells carry out process
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Especially absorptive cells in intestines and
kidneys
Also called pinocytosis
Tiny droplets of extracellular fluid taken
into cell
No receptor proteins are involved
Lysosomes fuse and degrade particles
into smaller useable particles
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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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Cytoskeleton
Network of protein filaments that
extends throughout cytosol
Provides structural framework for cell
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Determining cell shape
Organizes cellular contents
Aids movement of organelles within cell
during cell division
Aids movement of whole cells such as
phagocytes
Continually reorganizes as changes shape
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The Cytoskeleton
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Organelles
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Centrosome - located near the nucleus,
consists of two centrioles and pericentriolar
material (Fig. 2.7)
Cilia - short, hair-like projections from the cell
surface, move fluids along a cell surface
Flagella - longer than cilia, move an entire
cell; only example is the sperm cell’s tail (Fig.
2.8)
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The Centrosome
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Cilia and Flagella
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Organelles
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Ribosomes - sites of protein synthesis
Endoplasmic reticulum - network of membranes in
the shape of flattened sacs or tubules
- Rough ER - connected to the nuclear envelope, a
series of flattened sacs, surface is studded with
ribosomes, produces various proteins
-Smooth ER - a network of membrane tubules, does
not have ribosomes, synthesizes fatty acids and
steroids, 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 various
vesicles
Lysosomes - vesicles that form from the Golgi
complex, contain powerful digestive enzymes
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Golgi Complex
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Synthesized
protein
Ribosome
1
Transport vesicle
Entry face cisterna
2
Medial cisterna
3
Exit face cisterna
9
Transport vesicle
(to lysosome)
4
8
Rough ER
6
Transfer vesicle
4
7
5
Transfer vesicle
Secretory
vesicle
Membrane
vesicle
Proteins in vesicle
membrane merge
with plasma
membrane
Proteins exported
from cell by exocytosis
Plasma
membrane
Processing and Packaging by
Golgi Apparatus
Lysosomes
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Organelles
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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
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Organelles
Mitochondria - the “powerhouses” of the cell
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Generate ATP
More prevalent in physiologically active cells: muscles, liver
and 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
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Spherical or oval shaped structure
Usually most prominent feature of a cell
Nuclear envelope - a double membrane that
separates the nucleus from the cytoplasm
Nuclear pores - numerous openings in the nuclear
envelope, control movement of substances between
nucleus and cytoplasm
Nucleolus - spherical body that produces ribosomes
Genes - are the cell’s 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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Gene Expression
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DNA is the blue print for RNA
RNA is the blue print for protein
Proteins determine the physical and chemical
characteristics of cells
Three RNA nucleotides (codon) code for a
particular tRNA (anticodon) which carries a
particular amino acid
Therefore, the sequence of DNA nucleotides
determines sequence of RNA nucleotides
which in turn determines the sequence of
amino acids
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Overview of Gene Expression
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Transcription
Base pairing
DNA nucleotide binds to RNA nucleotide
Adenine
Uracil
Thymine
Adenine
Guanine
Cytosine
Cytosine
Guanine
 Coding region of gene stops at
terminator site which signals enzyme to
release
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Transcription
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Translation
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Nucleotide sequence is read by ribosome
Occurs in cytosol
Codon on mRNA signals for tRNA
carrying particular amino acid
Small subunit has binding site for
mRNA
Large subunit has two binding sites for
tRNA P site and A site.
See figure 3.26
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Large
subunit
P site
Amino acid
Initiator tRNA
Translation
tRNA
A site
UAC
GG A U G
C
Small
subunit
P site
Anticodon
2 Large and small ribosomal
subunits join to form a functional
ribosome and initiator tRNA
fits into P site.
Amino acid
(methionine)
A site
UAC
GG A U G
mRNA
Codons
Initiator tRNA
3 Anticodon of incoming tRNA pairs
Anticodon
UAC
GG A U G
with next mRNA codon at A site.
mRNA
Small
subunit
mRNA
binding
site
Start codon
UAC U
GG A U G U A G
1 Initiator tRNA attaches to a
start codon.
New
peptide
bond
4 Amino acid on tRNA at P site
forms a peptide bond with
amino acid at A site.
C
U
G G A U G U GC
mRNA
movement
Stop codon
6 Protein synthesis stops when
the ribosome reaches stop
codon on mRNA.
Key:
= Adenine
5 tRNA at P site leaves ribosome,
ribosome shifts by one codon;
tRNA previously at A site is now
at the P site.
Growing
mRNA
protein
tRNA
= Guanine
= Cytosine
= Uracil
Complete protein
Summary of movement of ribosome along mRNA
Somatic Cell Division - Mitosis
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The cell cycle is a sequence of events in
which a body cell duplicates its contents and
divides in two
Human somatic cells contain 23 pairs of
chromosomes (total = 46)
The two chromosomes that make up each
pair are called homologous chromosomes
(homologs)
Somatic cells contain two sets of
chromosomes and are called diploid cells
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Cell Division
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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 phase
Mitotic 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
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Prophase - the chromatin fibers change into
chromosomes
Metaphase - microtubules align the centromeres of
the chromatid pairs at the metaphase plate
Anaphase - the chromatid pairs split at the
centromere and move to opposite poles of the cell;
the chromatids are now called chromosomes
Telophase - two identical nuclei are formed around
the identical sets of chromosomes now in their
chromatin form
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Cytoplasmic Division: Cytokinesis
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Division of a cell’s cytoplasm to form two
identical cells
Usually begins in late anaphase
The plasma membrane constricts at its
middle forming a cleavage furrow
The cell eventually splits into two daughter
cells
Interphase begins when cytokinesis is
complete
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Centrosome:
Centrioles
Pericentriolar material
1
6
Nucleolus
Nuclear envelope
Chromatin
Plasma membrane
Cytosol
LM all at 700x
(a) INTERPHASE
2
Kinetochore
Centromere
(f) IDENTICAL CELLS IN INTERPHASE
5
Chromosome
(two chromatids
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
Early
Late
(d) ANAPHASE
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Cellular Diversity
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The average adult has
nearly 100 trillion cells
There are about 200
different types of cells
Cells come in a variety
of shapes and sizes
Cellular diversity
permits organization of
cells into more complex
tissues and organs
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End of Chapter 3
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