Chapter 3 Cells

1
Chapter 3Cells

• vary in size
• possess distinctive shapes
• measured in micrometers

2
A Composite Cell

• hypothetical cell
• major parts
• nucleus
• cytoplasm
• cell membrane

3
Cell Membrane

• outer limit of cell

• controls what moves in and out of cell

• selectively permeable

• phospholipid bilayer
• water-soluble heads form surfaces
• water-insoluble tails form interior
• permeable to lipid-soluble substances

• cholesterol stabilizes the membrane

• proteins
• receptors
• pores, channels, carriers
• enzymes
• CAMS
• self-markers

4
Cell Membrane
5
Intercellular Junctions

• Tight junctions
• close space between cells
• located among cells that form linings

• Desmosomes
• form spot welds between cells
• located among skin, epithelial
• and muscle cells including heart

• Gap junctions
• tubular channels between cells
• located in cardiac muscle cells

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Tight Junctions

• Some background 1st
• Epithelia are sheets of cells that provide the
  interface between masses of cells and a cavity or
  space (a lumen).
• The portion of the cell exposed to the lumen is
  called its apical surface.
• The rest of the cell (i.e., its sides and base)
  make up the basolateral surface.
• Tight junctions seal adjacent epithelial cells in
  a narrow band just beneath their apical surface.
• Tight junctions perform two vital functions
• They prevent the passage of molecules and ions
  through the space between cells. So materials
  must actually enter the cells (by diffusion or
  active transport) in order to pass through the
  tissue. This pathway provides control over what
  substances are allowed through.
• They block the movement of integral membrane
  proteins (red and green ovals) between the apical
  and basolateral surfaces of the cell. Thus the
  special functions of each surface, for example
• receptor-mediated endocytosis at the apical
  surface
• exocytosis at the basolateral surface
• can be preserved.

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Desmosomes

• If the desmosomes connecting adjacent epithelial
  cells of the skin are not functioning correctly,
  layers of the skin can pull apart and allow
  abnormal movements of fluid within the skin,
  resulting in blisters and other tissue damage.
• Blistering diseases such as Pemphigus vulgaris
  can be due to genetic defects in desmosomal
  proteins or due to an autoimmune response.

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Clinical appearance and pathohistology of various
human desmosomal disorders
Clinical appearance and pathohistology of various
human desmosomal disorders. Pemphigus vulgaris (A
and B) is characterized by the loss of
intercellular adhesion between basal and
suprabasal keratinocytes (A) and by skin
blistering and erosions (B). The hallmark of
palmoplantar keratoderma (C and D) is massive
thickening of the stratum corneum (C), resulting
in dramatically thickened skin on palms and soles
(D). Arrhythmogenic right-ventricular
cardiomyopathy (ARVC) is characterized by
fibrofatty replacement () of the myocardium (E)
Kottke, M. D. et al. J Cell Sci 2006119797-806
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Gap junctions

• intercellular channels that permit the free
  passage between the cells of ions and small
  molecules.
• Because ions can flow through them, gap junctions
  permit changes in membrane potential to pass from
  cell to cell.
• Examples
• The action potential in heart (cardiac) muscle
  flows from cell to cell through the heart
  providing the rhythmic contraction of the
  heartbeat.
• At some synapses in the brain, gap junctions
  permit the arrival of an action potential at the
  synaptic terminals to be transmitted across to
  the postsynaptic cell without the delay needed
  for release of a neurotransmitter.
• As the time of birth approaches, gap junctions
  between the smooth muscle cells of the uterus
  enable coordinated, powerful contractions to
  begin.
• Several inherited disorders of humans
• are caused by mutations in gap junction
• proteins, such as
• certain congenital heart defects and
• certain cases of congenital deafness

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Cell Adhesion Molecules

• guide cells on the move

• selectin allows white blood cells to anchor

• integrin guides white blood cells through
  capillary walls

• important for growth of embryonic tissue

• important for growth of nerve cells

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Cytoplasmic Organelles

• Endoplasmic Reticulum
• connected, membrane-bound sacs, canals, and
  vesicles
• transport system
• rough ER
• studded with ribosomes
• protein synthesis
• smooth ER
• lipid synthesis
• added to proteins arriving from rough ER
• break down of drugs

• Ribosomes
• free floating or connected to ER
• provide structural support

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Cytoplasmic Organelles

• Golgi apparatus
• stack of flattened, membranous sacs
• modifies, packages and delivers proteins

• Vesicles
• membranous sacs
• store substances

• Mitochondria
• membranous sacs with inner partitions
• generate energy

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Cytoplasmic Organelles

• Lysosomes
• enzyme-containing sacs
• digest worn out cell parts or unwanted substances

• Centrosome
• two rod-like centrioles
• used to produce cilia and flagella
• distributes chromosomes during cell division

• Peroxisomes
• enzyme-containing sacs
• break down organic molecules

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Cytoplasmic Organelles

• Cilia
• short hair-like projections
• propel substances on cell surface

• Flagellum
• long tail-like projection
• provides motility to sperm

15
Cytoplasmic Organelles

• Microfilaments and microtubules
• thin rods and tubules
• support cytoplasm
• allows for movement of organelles

• Inclusions
• temporary nutrients and pigments

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Cell Nucleus

• control center of cell

• nuclear envelope
• porous double membrane
• separates nucleoplasm from cytoplasm

• nucleolus
• dense collection of RNA and proteins
• site of ribosome production

• chromatin
• fibers of DNA and proteins
• stores information for synthesis of proteins

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Movements Into and Out of the Cell

• Passive (Physical) Processes
• require no cellular energy
• simple diffusion
• facilitated diffusion
• osmosis
• filtration

• Active (Physiological) Processes
• require cellular energy
• active transport
• endocytosis
• exocytosis
• transcytosis

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Simple Diffusion

• movement of substances from regions of higher
  concentration to regions of lower concentration
• oxygen, carbon dioxide and lipid-soluble
  substances

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Facilitated Diffusion

• diffusion across a membrane with the help of a
  channel or carrier molecule
• glucose and
• amino acids

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Osmosis

• movement of water through a selectively
  permeable membrane from regions of higher
  concentration to regions of lower concentration
• water moves toward a higher concentration of
  solutes

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Osmosis
Osmotic Pressure ability of osmosis to generate
enough pressure to move a volume of water
Osmotic pressure increases as the concentration
of nonpermeable solutes increases

• hypertonic higher osmotic pressure
• hypotonic lower osmotic pressure
• isotonic same osmotic pressure

22
Filtration

• smaller molecules are forced through porous
  membranes
• hydrostatic pressure important in the body
• molecules leaving blood capillaries

23
Active Transport

• carrier molecules transport substances across a
  membrane from regions of lower concentration to
  regions of higher concentration
• sugars, amino acids, sodium ions, potassium
  ions, etc.

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Endocytosis

• cell engulfs a substance by forming a vesicle
  around the substance
• three types
• pinocytosis substance is mostly water
• phagocytosis substance is a solid
• receptor-mediated endocytosis requires the
  substance to bind to a membrane-bound receptor

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Endocytosis
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Exocytosis

• reverse of endocytosis
• substances in a vesicle fuse with cell membrane
• contents released outside the cell
• release of neurotransmitters from nerve cells

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Transcytosis

• endocytosis followed by exocytosis
• transports a substance rapidly through a cell
• HIV crossing a cell layer

28
The Cell Cycle

• series of changes a cell undergoes from the time
  it forms until the time it divides
• stages
• interphase
• mitosis
• cytoplasmic division

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Interphase

• very active period
• cell grows
• cell maintains routine functions
• cell replicates genetic material to prepare for
  nuclear division
• cell synthesizes new organelles to prepare for
  cytoplasmic division
• phases
• G phases cell grows and synthesizes structures
  other than DNA
• S phase cell replicates DNA

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Mitosis

• produces two daughter cells from an original
  somatic cell
• nucleus divides karyokinesis
• cytoplasm divides cytokinesis
• stages
• prophase chromosomes form nuclear envelope
  disappears
• metaphase chromosomes align midway between
  centrioles
• anaphase chromosomes separate and move to
  centrioles
• telophase chromatin forms nuclear envelope
  forms

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Mitosis
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Cytoplasmic Division

• also known as cytokinesis
• begins during anaphase
• continues through telophase
• contractile ring pinches cytoplasm in half

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Control of Cell Division

• cell division capacities vary greatly among cell
  types
• skin and blood cells divide often and
  continually
• neuron cells divide a specific number of times
  then cease

• chromosome tips (telomeres) that shorten with
  each mitosis provide a mitotic clock

• cells divide to provide a more favorable surface
  area to volume relationship

• growth factors and hormones stimulate cell
  division
• hormones stimulate mitosis of smooth muscle
  cells in uterus
• epidermal growth factor stimulates growth of new
  skin

• contact (density dependent) inhibition

• tumors are the consequence of a loss of cell
  cycle control

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Tumors

• Two types of tumors
• benign usually remains localized
• malignant invasive and can metastasize
  cancerous

• Two major types of genes cause cancer
• oncogenes activate other genes that increase
  cell division
• tumor suppressor genes normally regulate
  mitosis if inactivated they are unable to
  regulate mitosis
• cells are now known as immortal

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Stem and Progenitor Cells

• Stem cell
• can divide to form two new stem cells
• self-renewal
• can divide to form a stem cell and a progenitor
  cell
• totipotent can give rise to every cell type
• pluripotent can give rise to a restricted
  number of cell types

• Progenitor cell
• committed cell
• can divide to become any of a restricted number
  of cells
• pluripotent

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Stem and Progenitor Cells
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Clinical Application
Diseases at the Organelle Level

• MELAS mitochondrial encephalomyopathy, lactic
  acidosis, and stroke-like episodes
• mitochondria are missing a gene necessary to
  carry out important energy producing reactions
• usually inherited by mother
• causes strokes, severe headaches, muscle
  weakness and numb hands

• ALD adrenoleukodystrophy
• peroxisomes are missing enzymes
• causes dizziness, weakness, darkening skin, and
  abnormal heart rhythms

• Tay-Sachs Disease
• lysosomes are abnormally large and lack one
  enzyme
• causes nervous system failure and early death