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Chapter 3 The Cellular Level of Organization

An Introduction to Cells
Most of the chemical events that sustain life take place inside cells
...

Cells measure about 0
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Cell Theory concepts summarized
▪

Cells are the building blocks of all plants and animals

▪

All cells come from the division of preexisting cells

▪

Cells are the smallest unit that carry out life’s essential physiological function

▪

Each cell maintains homeostasis at the cellular level
...


Cytology) the study of cellular structure and function, which is part of the broader study of
Cellular biology
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Somatic cells (soma = body)) body cells that include all the other cells in the human body
...
The plasma membrane is extremely thin, ranging
from 6-10 nm in thickness
...
It
controls the entry of ions and nutrients, the elimination of wastes, and the release of secretions
Sensitivity to the Environment: the plasma membrane is the 1st part of the cell affected by
changes in the composition, concentration, or pH of the extracellular fluid
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The plasma membrane contains lipids, proteins, and carbohydrates
Lipids form most of the plasma membrane surface but only make up about 42% of its weight

Phospholipid bilayer) is what the plasma membrane is called because the phospholipid
molecules in it form 2 layers
...

There are 2 general structural classes of membrane proteins:

Integral proteins) are part of the plasma membrane structure and cannot be easily separated
from it without damaging or destroying the membrane
...

Membrane proteins carry out a variety of functions:
1
...
Recognition Proteins (Identifiers): The cells of the immune system recognize other cells as
normal or abnormal based on the presence or absence of characteristic recognition proteins,
many which are glycoproteins
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Enzymes: In plasma membranes they be integral or peripheral proteins
...

4
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Ligand) can be anything from a small ion, like calcium, to a relatively large and complex hormone
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Carrier proteins: Carrier proteins bind solutes and transports them across the plasma
membrane
...

For example, virtually all cells have carrier proteins that bring glucose into the cytoplasm without
expanding ATP
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6
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Ions do not dissolve in lipids, so they cannot cross the phospholipid bilayer
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Membranes are neither rigid nor uniform
...
For example, some enzymes are only found in the inner surface other membrane, and receptors
are found only on the outer surface
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These areas
mark the location of anchoring proteins and some kinds of receptor proteins
...

Glycocalyx) layer formed by large molecules that extend beyond the outer surface of the
membrane
...

Anchoring and Locomotion: Because its components are sticky, glycocalyx can help anchor the
cell in place
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Specificity in Binding: Glycoproteins and glycolipids can function as receptors, binding specific
extracellular compounds
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Recognition: Cells involved with the immune response recognize glycoproteins and glycolipids
as normal or abnormal
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3-2
Organelles with the cytoplasm perform particular functions
Cytoplasm) is a general term for the material between the plasma membrane and the membrane
that surrounds the nucleus
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These proteins make
up 30% of the cell's weight
...

Organelles) are structures suspended within the cytosol that performs specific functions for the
cell
...
The concentration of potassium ions is much higher in the cytosol than in extracellular fluid,
and conversely the concentration of sodium ions is much lower in the cytosol than in the
extracellular
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The cytosol contains a much higher concentration of suspended proteins than does extracellular
fluid
...

3
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The extracellular fluid is transport medium only, and no amino acids or lipids are stored
there
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The Organelles
Organelles are the internal structures of the cells that perform most of the tasks that keep the cell alive
and functioning normally
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Membranous organelles) are isolated from the cytosol by phospholipid membranes, just the plasma
membrane isolates the cytosol from the extracellular fluid
...
Its an internal protein framework that gives the cytoplasm strength
and flexibility
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The cytoskeleton plays a role in the metabolic organization of cells
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Chapter 3 The Cellular Level of Organization

Microfilaments
The smallest element of the cytoskeleton elements
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Typical microfilaments are made up of the protein actin
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Microfilaments anchor the cytoskeleton to integral proteins of the plasma membrane
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Microfilaments, interacting with other proteins, determine the consistency of the cytoplasm
...
Actin can produce movement of a portion of a cell or change the shape of the entire cell by
interacting with the protein myosin
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These filaments arrange from 9-11 nm in
diameter
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Intermediate filaments:
1
...
Stabilize the position of organelles and
3
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Microtubules
Most cells contain microtubules, which are hollow tubes built from the globular protein
tubulin
...

Microtubules have the following functions:
•
•
•
•
•

Form the main portions of the cytoskeleton
Change the shape of the cell, and may assist in cell movement
Serve as a kind of momrail system to move vesicles or other organelles within the cell
During cell division, they distribute duplicated chromosomes to opposite ends of the dividing
cell
Form structural components of organelles

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Chapter 3 The Cellular Level of Organization

Thick filaments
Are relatively massive bundles of subunits made of the protein myosin
...
5 micrometers in length
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Centrosomes and Centrioles
Centrosome) a region of cytoplasm located next to the nucleus in a cell
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There are 9 groups of microtubules with 3 in each group known as triplets
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Motile cilia are found on cells lining both the respiratory and reproductive tracts, and at
various other locations in the body
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Primary cilium(nonmotile) are found on the cells of a wide variety of tissues in the body
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Motile cilia beat rhythmically to move fluids or secretions across the cell surface
...
The only human cell
with a flagellum is a sperm cell
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Approximately 25 nm in diameter
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A functional ribosome consists of 2 subunits that are normally separate and distinct:
One is called a small ribosomal subunit and the other a large ribosomal subunit
...


Fixed ribosomes) are modified and packed for use within the cell or they are secreted from the
cell
...
Their job is to remove proteins from the cytoplasm
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It has 4 major functions:
I
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Storage:
III
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Detoxification: The ER can absorb drugs or toxins and neutralize them with enzymes
...
Some of the proteins are
enzymes that will function inside the endoplasmic reticulum
...
Also known as the Golgi complex
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Continued on next page

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Chapter 3 The Cellular Level of Organization
It has 3 major functions:
1
...
renews or modifies the plasma membrane; and
3
...

These vesicles are produced by the Golgi apparatus, contain digestive enzymes
...

•
•
•
•
•
•

Lysosomes have several functions:
remove damaged organelles
enzymes break down the content of these damaged organelles
the cytosol reabsorbs released nutrients and the remaining material is expelled from the cell
destroy bacteria that enter from the extracellular fluid
break down and release the contents of unusable substances
essential cleanup and recycling inside the cell

Autolysis) the process in which enzymes rapidly destroy the cell's proteins and organelles with
aging
...
They are
produced by the growth and subdivision of existing peroxisomes
...
As they do they produce
hydrogen peroxide, a potentially dangerous free radical
Catalase is most abundant enzyme in peroxisomes, then breakdown it down to oxygen and
water
...
They are present in all cells
...
With the exception of mitochondria, all
membranous organelles in the cell are either interconnected or in communication through the
movement of vesicles
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Chapter 3 The Cellular Level of Organization

Mitochondria
Mitochondria) the organelle that produce the energy required for functions of life
...
They have
an unusual double membrane
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The mitochondria uses ADP to produce ATP and does this back and forth over and over again
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Steps of Energy production in the Mitochondria
1) Glycolysis--- breaks down a glycose molecule into 2 molecules of pyruvate which the
mitochondria then absorbs the pyruvate molecules
2) Citric acid cycle--- an enzymic pathway that breaks down the absorbed pyruvate
...
It is the control center for
cellular operations
...


Nuclear envelop) surrounds the nucleus and separates it from the cytosol
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These pores cover 10% of the nucleus's surface
...
The fluid of the nucleus is called the nucleoplasm
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Nucleoli) dark staining areas of the nuclei
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Nucleoli are composed of RNA, enzymes, and proteins called------histones

Nucleosome) a complex
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Chapter 3 The Cellular Level of Organization

Chromatin) the grainy material visible in the cell nuclei giving a grainy appearance during
interphase
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There are 23 pairs (46 total) of chromosomes in humans

Information Storage in the Nucleus
Genetic code) the language cells use to transfer the chemical information of the building blocks of
cell structures
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Gene) a functional unit of heredity
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3-4
The role of Gene Activation in Protein Synthesis
Gene activation) the process where histones are removed before a portion of a DNA molecule
can be uncoiled
...

Transcription)

This binding is the 1st step in the process
the synthesis of RNA from a DNA template
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It is essential because DNA is too large to leave the nucleus
...
Identical to the coding strand

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Chapter 3 The Cellular Level of Organization

Translation and Protein Synthesis
Protein synthesis) is the assembling of functional peptide in the cytoplasm
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The amino acids are provided by

Transfer RNA (tRNA) a relatively small and mobile type of RNA
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How the Nucleus Controls Cell Structure and Function
2 levels of control are involved
1
...
Indirect control over metabolism through regulation of enzymes

3-5
Diffusion is a passive transport mechanism that assists
membrane passage
The plasma membrane is a barrier that isolates the cytoplasm from the extracellular fluid
...

Nutrients go in and wastes go out of the cell
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Impermeable) a membrane in which nothing can pass through
Freely permeable) a membrane which any substance can pass through without difficulty
Plasma membranes are called Selectively permeable, however, because their permeability lies
somewhere between those extremes (permeable and impermeable)
...


Diffusion) the net movement of a substance from a higher concentration to an area of lower
concentration
...

(example the foaming of the sugar cube as it disintegrates in the water)

3Main types of diffusion:
• Diffusion (passive)
• Carrier-mediate transport (passive or active)
• Vesicular transport (active)

In diffusion
•
•
•
•
•

all molecules are constantly in motion
molecules in solution move randomly
random motion causes mixing
concentration is the amount of solute in a solvent
Concentration gradient
➢ more solute in one part of a solvent than another

➔
➔
➔
➔
➔

Factors that influence diffusion:
Distance particle has to move
Molecular size (smaller is faster)
Temperature (more heat faster motion)
Concentration gradient
• difference between high and low concentrations
Electrical forces (inner surface = +, outer surface= -)
• Opposites attract, like charges repel

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Chapter 3 The Cellular Level of Organization

Osmosis: A Special Case of Diffusion
Osmosis) the diffusion of water across the cell membrane in response to a concentration gradient
...
This means
(solute concentration = water concentration)

Osmotic pressure) indicates the force with which pure water moves into the solution as a result
of its solute concentration
...
It opposes osmotic pressure
...


Tonicity) term used to describe the effects of various osmotic solutions on cells
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Hypotonic) low pressure
Hemolysis) when a cell swells and bursts
Hypertonic) high pressure
Crenation) the dehydration of red blood cells
Saline) usually sodium chloride used for blood loss because it approximates the normal osmotic
concentration of extracellular fluids, and because sodium and chloride are the mosr abundant ions in
the extracellular fluid, and it isotonic to blood cells
...


Of ions and organic substrates
Characteristics of Carrier-mediate transport

• Specificity
• 1 transport protein, 1 set of substrates
• Saturation limits
• rate depends on transport proteins, not substrate
• Regulation
• cofactors such as hormones
Protein Synthesis is in the mitochondria-------95% of energy comes from here

Facilitated Diffusion
Many essential nutrients such as glucose and amino acids are insoluble in lipids and too large to fit
through membrane channels
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Chapter 3 The Cellular Level of Organization

In passive transport”
•

Carrier proteins transport molecules too large to large to fit through channel proteins (glucose,
amino acids)
• molecules bind to receptor cells on carrier protein
• protein changes shape molecules pass through
• receptor cite is specific to certain molecules

In active transport”

Active transport) a high energy bond (in ATP or another high-energy compound) provides
energy needed to move ions or molecules across the membrane
...


Exchange pump) what a carrier protein is called when it countertransports
•

Active transport (Primary or Secondary)
• Active transport proteins
• move substrates against concentration gradient
• requires energy, such as ATP
• ion pumps move ions
• exchange pump countertransports 2 ions at the same time

----- ----- ----- ----- ----- ----- ----- ----Carrier-Mediate Transport
•

Primary Active Transport
• Sodium-potassium exchange pump
• Active transport, carrier mediated
• Sodium ions out, potassium ion in
• 1 ATP moves 3 sodium and 2 potassium

Homeostasis within the cell depends on removing sodium ions and recapturing lost
potassium ions

Vesicular Transport
Vesicular transport) materials move into or out of the cell in vesicles
Vesicles) small membrane sacs that form at, or fuse with, the plasma membrane
...

•

Transmembrane Potential
• charges are separated creating a potential difference
• Unequal change across the plasma membrane is transmembrane potential
• Resting potential ranges from -10 mV to -100 mV, depending on cell type

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Chapter 3 The Cellular Level of Organization

3-8
Stages of a cell's life cycle include interphase, mitosis, and
cytokinesis
Upon maturity the body has about 70 trillion cells as a result of what is called cell dision
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Apoptosis) genetically controlled death of cells
Mitosis) nuclear division
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Meiosis) the process of the production of sex cells
Cell Life Cycle
▪
▪

Most of a cell's life is spent in the nondividing state (interphase)
Body (somatic) cells divide in 3 stages
▪ DNA replication duplicates genetic material exactly
▪ Mitosis divides genetic material equally
▪ Cytokinesis divides cytoplasm and organelles into 2 daughter cells

DNA Replication
DNA polymerase) an enzyme that binds to exposed nitrogenous bases, that promotes bonding
between the nitrogenous bases of the DNA strand and complimentary DNA
nucleotides in the nucleoplasm and links the nucleotides by covalent bonds
...
Promotes bonding between the nitrogenous bases of the DNA strand and
complementary DNA nucleotides dissolved in the nucleoplasm
2
...

Stem cells) special cells that maintain cell populations through repeated cycles of cell division,
their only function is to produce daughter cells
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•

•
•

Rate of cell division
• Slower miotic rate means longer cell life
• Cell division requires energy (ATP)
Muscle cells, neurons rarely divide
Exposed cells (skin and digestive tract) live only days or hours- replenished by stem
cells
Cell Division:
• Normally, cell division balances cell loss
• Increased cell division
• Internal factors
• Extracellular chemical factors
• Decreased cell division
• Repressor genes (faulty repressors cause cancers)
• Worn out telomeres (terminal DNA segments)

3-9
Growth factors) substances that stimulate the division of specific cell types
Telomeres) terminal segments of DNA with associated proteins that regulate how many times a
cell will divide throughout its lifetime
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Primary tumor) the tumor of origin

Invasion) term given to the spreading process
Metastasis) what the dispersion of malignant cells is referred to when the cancer travels to distant
cells or organs
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Chapter 3 The Cellular Level of Organization

Cancer) is an illness that results from the abnormal proliferation of any of the cells in the body
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Oncogenes) modified genes when normal cells begin the path to becoming malignant when
mutation occurs in a gene

Mutagens) agents that cause a mutation (change in DNA)
Carcinogens) cancer-causing agents
Angiogenesis) the growth of new blood vessels

3-11
Differentiation is cellular specialization as a result of gene
activation or repression
Differentiation) the development of specific cellular characteristics and functions that are
different from the original cell

Tissues) specialized cells with limited capabilities/functions
•

*Differentiation
All cells carry complete DNA instructions for all body functions
• Cells specialize or differentiate
• To form tissues (liver cells, fat cells, and neurons)
• By turning off all genes not needed by that cell
• All body cells, except sex cells, contain the same 46 chromosomes (23 pairs)
• Differentiation depends on which genes active and which are inactive

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Chapter 3 The Cellular Level of Organization

Related Clinical Terms:
anaplasia) an irreversible change in the size and shape of tissue cells
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genetic engineering) a general term that encompasses attempts to change the genetic makeup of cells
or organisms, including humans
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hypertrophy) the enlargement of an organ or tissue due to an increase in the size of its cells
...
Often formed artificially
to carry drugs into the tissues
...

Oncologist) physician who specializes in the identification and treatment of cancers
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