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Chapter 3
The Cellular Level of Organization
INTRODUCTION
• A cell is the basic, living, structural, and functional unit
of the body.
• Cytology is the study of cell structure
• Cell physiology is the study of cell function.
PARTS of a CELL
• The cell can be divided into three principal parts for
ease of study.
• Plasma (cell) membrane
• Cytoplasm
– Cytosol
– Organelles (except for the nucleus)
• Nucleus
THE PLASMA MEMBRANE
• The plasma membrane is a flexible, sturdy barrier that
surrounds and contains the cytoplasm of the cell.
• Fluid mosaic model (Figure 3.2).
– The membrane consists of proteins in a sea of lipids.
Lipid Bilayer of the Cell Membrane
• Two back-to-back layers of 3 types of lipid molecules
• Cholesterol and glycolipids scattered among a double row of phospholipid
molecules
Phospholipids
• Comprises 75% of cell membrane
• Phospholipid bilayer =
• Each molecule is amphipathic
– polar parts (heads) are hydrophilic and face on
both surfaces a watery environment
– nonpolar parts (tails) are hydrophobic and line up
next to each other in the interior
Cholesterol within the Cell Membrane
• Comprises 20% of cell membrane lipids
• Interspersed among the other lipids in both layers
Glycolipids within the Cell Membrane
• Comprises 5% of the lipids of the cell membrane
Membrane Proteins
Integral versus Peripheral Proteins
Functions of
Membrane Proteins
• Membrane proteins vary in
different cells and functions
(Figure 3.3)
• The different proteins help to
determine many of the functions
of the plasma membrane.
Functions of
Membrane Proteins
• Formation of Channel
• Transporter Proteins
• Receptor Proteins
Functions of
Membrane Proteins
• Cell Identity Marker
• Linker
• Act as Enzyme
Membrane Permeability
• Plasma membranes are selectively permeable.
• The lipid bilayer portion of the membrane is permeable
to
but impermeable to
• Transmembrane proteins
• Macromolecules are unable to pass through the plasma
membrane.
Gradients Across Membrane
• Concentration gradient
• Electrical gradient
TRANSPORT ACROSS THE PLASMA MEMBRANE
• Three types of passive processes
–1
–2
–3
• Active transport requires cellular
• Materials can also enter or leave the cell through
vesicle transport.
Principles of Diffusion
• Diffusion (Figure 3.6)
• Diffusion rate across plasma membranes is influenced
by several factors:
Diffusion
• Crystal of dye placed in a cylinder of
water
• Net diffusion from the higher dye
concentration to the region of lower
dye
• Equilibrium has been reached in the
far right cylinder
Diffusion Through the Lipid Bilayer
• Nonpolar, hydrophobic
molecules such as
respiratory gases, some
lipids, small alcohols, and
ammonia can diffuse across
the lipid bilayer.
• It is important for gas
exchange, absorption of
some nutrients, and
excretion of some wastes.
Diffusion Through Membrane Channels
• Most membrane channels are ion channels, allowing
passage of small, inorganic ions which are hydrophilic.
• Ion channels are selective and specific and may be
gated or open all the time (Figure 3.6).
Osmosis
• Osmosis is the net
movement of a solvent
through a selectively
permeable membrane,
or in living systems, the
movement of water (the
solute) from an area of
higher concentration to
an area of lower
concentration across
the membrane (Figure
3.7).
Tonicity
• In an isotonic solution, red blood cells
(Figure 3.8a).
• In a hypotonic solution, red blood cells
(Figure 3.8b).
• In a hypertonic solution, red blood cells
(Figure 3.8c).
Facilitated Diffusion
• In facilitated diffusion, a solute binds to a specific
transporter on one side of the membrane and is
released on the other side after the transporter
undergoes a conformational change.
Examples:
(Figure 3.6a).
• Rate of movement depends upon
– steepness of concentration gradient
– number of transporter proteins (transport maximum)
Facilitated Diffusion of Glucose
• Glucose binds to transport
protein
• Transport protein changes
shape
• Glucose moves across cell
membrane (but only down
the concentration gradient)
• Kinase enzyme reduces
glucose concentration inside
the cell by transforming
glucose into glucose-6-phosphate
• Transporter proteins always bring
glucose into cell
Active Transport
• Active transport
• In primary active transport, energy
derived from ATP changes the shape
of a transporter protein, which pumps
a substance across a plasma
membrane against its concentration
gradient.
Primary Active Transport
• The most prevalent primary active transport mechanism
is the sodium ion/potassium ion pump (Figure 3.10).
– requires 40% of cellular ATP
– all cells have 1000s of them
– maintains low concentration of Na+
and a high concentration of K+ in the cytosol
– operates continuously
Secondary Active Transport
• In secondary active transport, the energy stored in the
form of a sodium or hydrogen ion concentration
gradient is used to drive other substances against their
own concentration gradients.
• Plasma membranes contain several antiporters and
symporters powered by the sodium ion gradient (Figure
3.11a).
One in & one out.
vs.
Both going in
Digitalis
• Digitalis slows the sodium ion-calcium ion antiporters,
allowing more calcium to stay inside heart muscle cells,
which increases the force of their contraction and thus
strengthens the heartbeat.
Transport in Vesicles
• A vesicle is a small membranous sac formed by
budding off from an existing membrane.
– endocytosis
– exocytosis
Overview: Vesicular Transport of Particles
• Endocytosis = bringing something into cell
– phagocytosis = cell eating by macrophages & WBCs
• particle binds to receptor protein
• whole bacteria or viruses are engulfed & later digested
– pinocytosis = cell drinking
• no receptor proteins
• Exocytosis = release something from cell
• Vesicles form inside cell, fuse to cell membrane
• Release their contents
– digestive enzymes, hormones, neurotransmitters or
waste products
Endocytosis
• In endocytosis,
materials move into a
cell in a vesicle formed
from the plasma
membrane.
• Receptor-mediated
endocytosis is the
selective uptake of
large molecules and
particles by cells
(Figure 3.12).
Exocytosis
• In exocytosis, membrane-enclosed structures called
secretory vesicles that form inside the cell fuse with the
plasma membrane and release their contents into the
extracellular fluid.
CYTOPLASM
• Cytosol is composed mostly
of water, plus proteins,
carbohydrates, lipids, and
inorganic substances.
• Cytosol is the medium in
which many metabolic
reactions occur.
Organelles
• Organelles are specialized structures that have
characteristic shapes and perform specific functions in
cellular growth, maintenance, and reproduction.
Cytoskeleton
• Network of protein
filaments throughout the
cytosol
• Functions:
Microfilaments
(Figure 3.15a)
Intermediate Filaments
(Figure 3.15b)
Microtubules
(Figure 3.15c)
Centrosomes
(Figure 3.16a – 3.16c)
Cilia and Flagella
• Cilia
(Figure. 3.17a – 3.17b).
• Flagella
(Figure 3.17c).
Cilia and Flagella
• Structure
– pairs of microtubules
(9+2 array)
• Differences
– cilia
• short and multiple
– flagella
• longer and single
Ribosomes
• Composed of Ribosomal RNA & protein
• Free ribosomes
– synthesize proteins found inside the cell
• Membrane-bound ribosomes
– synthesize proteins needed for plasma membrane or
for export
Ribosomal Subunits
• Large + small
subunits
– made in the
nucleolus
– assembled in
the cytoplasm
Endoplasmic Reticulum
• (Figure 3.19).
Endoplasmic Reticulum
• Rough ER
• Smooth ER
• The ER transports substances, stores newly
synthesized molecules, synthesizes and packages
molecules, detoxifies chemicals, and releases calcium
ions involved in muscle contraction.
• (Figure 3.20)
Golgi Complex
Golgi Complex
• The principal function of the Golgi complex is to
process, sort, and deliver proteins and lipids to the
plasma membrane, lysosomes, and secretory vesicles
(Figure 3.21).
Lysosomes
• membrane-enclosed vesicles
that contain powerful
digestive enzymes (Figure
3.22).
• Functions
– digest foreign substances
Tay-Sachs Disorder
• Affects children of eastern European-Ashkenazi
descent
– seizures, muscle rigidity, blind, demented and
dead before the age of 5
• Genetic disorder caused by absence of single
lysosomal enzyme
– enzyme normally breaks down glycolipid
commonly found in nerve cells
– as glycolipid accumulates, nerve cells lose
functionality
– chromosome testing now available
Peroxisomes
• Peroxisomes are similar in structure to lysosomes, but
are smaller.
• They contain enzymes (e.g., catalase) that use
molecular oxygen to oxidize various organic
substances.
Mitochondria
• The mitochondrion is bound by a
double membrane.
• The outer membrane is smooth
with the inner membrane
arranged in folds called cristae
(Figure 3.23).
– surface area for chemical
reactions of cellular respiration
– central cavity known as matrix
Mitochondria
• Mitochondria are the site of ATP production in the cell.
• Mitochondria self-replicate using their own DNA.
– increases with need for ATP
– circular DNA with 37 genes
• Mitochondrial DNA (genes) are usually inherited only
from the mother.
NUCLEUS
• The nucleus is usually the most prominent feature of a
cell (Figure 3.25).
NUCLEUS
• Most body cells have a single nucleus.
• The parts of the nucleus include the nuclear envelope
which is perforated by channels called nuclear pores,
nucleoli, and genetic material (DNA),
• Within the nucleus are the cell’s hereditary units, called
genes, which are arranged in single file along
chromosomes.
Function of the Nucleus
• 46 human DNA molecules or chromosomes
– genes found on chromosomes
– gene is directions for a specific protein
• Non-dividing cells contain nuclear chromatin
– loosely packed DNA
• Dividing cells contain chromosomes
– tightly packed DNA
– it doubled (copied itself) before condensing
Chromosomes
• Each chromosome is a
long molecule of DNA that
is coiled together with
several proteins (Figure
3.25).
PROTEIN SYNTHESIS
• The instructions for protein synthesis is found in the
DNA in the nucleus.
• Protein synthesis involves transcription and translation
(Figure 3.26).
Transcription
• Transcription
• (Figure 3.27a).
Transcription
• DNA sense strand is template for the
creation of messenger RNA strand
• Transcription begins at promoter
sequence where RNA polymerase
attaches
• When RNA polymerase reaches the
terminator sequence it detaches and
transcription stops
• Pre-mRNA contains intron region that
are cut out by enzymes
• Exon regions of mRNA will code for
segments of the protein
Translation
• Translation
• (Figure 3.28).
Translation
– sequence of nucleotides on mRNA is “read” by rRNA to
construct a protein (with its specific sequence of amino
acid)
• 3 nucleotide sequences on mRNA are called codons
– specific tRNA molecule carry specific amino acids
– anticodons on tRNA are matched to specific codons on
mRNA so proper amino acids can be strung together to
create a protein molecule
The sequence of
translation
• Messenger RNA
associates with
ribosomes, which
consist of tRNA and
proteins.
The sequence of
translation
• Specific amino acids
attach to molecules
of tRNA. Another
portion of the tRNA
has a triplet of
nitrogenous bases
called an anticodon,
a codon is a
segment of three
bases of mRNA.
The sequence of
translation
• Transfer RNA
delivers a specific
amino acid to the
codon; the ribosome
moves along an
mRNA strand as
amino acids are
joined to form a
growing
polypeptide.
CELL DIVISION
• Cell division is the process by which cells reproduce
themselves. It consists of nuclear division (mitosis and
meiosis) and cytoplasmic division (cytokinesis).
The Cell Cycle in Somatic Cells
• The cell cycle is an orderly sequence of events by
which a cell duplicates its contents and divides in two.
• It consists of interphase and the mitotic phase (Figure
3.30).
Chromosome number
• Human somatic cells contain 46 chromosomes or 23
pairs of chromosomes
• The two chromosomes that make up a chromosome
pair are called homologous chromosomes or homologs.
• A cell with a full set of chromosomes is called a diploid
cell (2N).
• A cell with only one chromosome from each pair is
termed haploid (N).
Interphase Stage of Cell Cycle
•
During interphase the cell carries on every life process except
division. (Figure 3.30).
• Doubling of DNA
• Phases of interphase stage -- G1, S, and G2
– G1 = cytoplasmic increase
– S = replication of chromosomes
– G2 = cytoplasmic growth
Replication of Chromosomes
• Doubling of genetic material
during interphase. (S phase)
• DNA molecules unzip
• Mirror copy is formed along
each old strand.
• Nitrogenous bases pick up
complementary base
• 2 complete identical DNA
molecules formed
Interphase
• A cell in interphase shows a distinct nucleus and the
absence of chromosomes (Figure 3.32a).
Mitotic Phase The mitotic phase consists of
mitosis (or nuclear division) and cytokinesis (or
cytoplasmic division).
• Mitosis is the distribution of two
sets of chromosomes, one set
into each of two separate nuclei.
• Stages of mitosis are
– Prophase
– Metaphase
– Anaphase
– Telophase
Prophase
• Chromatin condenses into visible chromosomes
– pair of identical chromatids held together by a centromere
• Nucleolus & nuclear envelope disappear
• Each centrosome moves to opposite ends of cell
– forms a mitotic spindle
– spindle is responsible for the separation of chromatids to
each new daughter cell
Metaphase
• During metaphase, the centromeres line up at the exact
center of the mitotic spindle (Figure 3.32c).
Anaphase
• Anaphase is characterized by the splitting and
separation of centromeres and the movement of the two
sister chromatids of each pair toward opposite poles of
the cell (Figure 3.32d).
Anaphase
• Sister chromatids move toward opposite poles of cell
– now called daughter chromosomes
– movement is due to shortening of microtubules
• Chromosomes appear V-shaped as they are dragged
towards the poles of the cell
– pull is at centromere region
Telophase
• Telophase begins as soon as chromatid movement
stops; the identical sets of chromosomes at opposite
poles of the cell uncoil and revert to their threadlike
chromatin form, microtubules disappear or change
form, a new nuclear envelope forms, new nucleoli
appear, and the new mitotic spindle eventually breaks
up (Figure 3.32 e).
Cytoplasmic Division: Cytokinesis
• Cytokinesis is the division of a parent cell’s cytoplasm
and organelles. The process begins in late anaphase or
early telophase with the formation of a cleavage furrow
(Figure 3.32e).
Cytoplasmic Division: Cytokinesis
• When cytokinesis is complete, interphase begins again
(Figure 3.32f).
Cell Death
• Cell death, a process called apoptosis, is triggered
either from outside the cell or from inside the cell due to
a “cell-suicide” gene.
• Necrosis is a pathological cell death due to injury.
Reproductive Cell Division
• Meiosis results in
the production of
haploid cells that
contain only 23
chromosomes.
• Meiosis occurs in
two successive
stages: meiosis I
and meiosis II.
Meiosis I
• Meiosis I consists of four phases: prophase I,
metaphase I, anaphase I, and telophase I (Figure3.33a).
Prophase I
• During prophase I, the chromosomes become arranged
in homologous pairs through a process called synapsis
(Figure 3.33b). The resulting four chromatids form a
structure called a tetrad. The tetrads may exchange
genetic material between non-sister chromatids through
a process known as crossing over.
Metaphase I
• During metaphase I, the homologous pairs of
chromosomes line up along the metaphase plate of the
cell, with the homologous chromosomes side by side
(Figure 3.33a).
Anaphase I
• During anaphase I, the members of each homologous
pair separate, with one member of each pair moving to
an opposite pole of the cell.
• The net effect of meiosis I is that each resulting cell
contains only one member of each pair of homologous
chromosomes. It is now haploid in number
Meiosis II
• Meiosis II consists of prophase II,
metaphase II, anaphase II, and
telophase II (Figure3.33d).
• These phases are similar to
those in mitosis, but result in
four haploid cells.
Review
• Figure 3.34
compares the
processes of
mitosis and
meiosis