The Cellular Level of
Organization
A cell is the basic, living, structural and
functional unit of the body.
Cells are measured in micrometers.
Cells vary in size and shape.
Every Eukaryotic cell has three main parts:
Plasma (cell) membrane - separates inside of
cell from external environment.
Nucleus – organelle that contains the cell’s
DNA and is surrounded by a double
membrane.
Cytoplasm – everything from the nuclear
membrane to the plasma membrane
Cytoplasm refers to cytosol plus organelles
and inclusions.
cytosol - contains proteins, enzymes,
nutrients, ions, and other small molecules
organelles - highly organized structures with
characteristic shapes that are specialized
for specific cellular activities.
inclusions - are temporary structures in the
cytoplasm that contain secretions and
storage products of the cell.
Cell Membrane
The plasma membrane does more than just
separate the outside of a cell from the inside;
it controls what enters and leaves the cell, and
much of the activity within the cell.
Most of the cell membrane is made of
phospholipid. Also find glycolipids and
cholesterol.
Amphipathic molecule – phosphate heads on
the outside and inside, and fatty acid tails in
the middle.
The membrane is selectively
permeable – it allows fat soluble
substances to pass through (such as
steroid hormones) and some other
small, uncharged molecules..
Glycolipids are the targets of certain
bacterial toxins, and are important for
cell adhesion and communication.
Cholesterol is a large molecule, and
helps to stabilize the membrane.
Fluid mosaic model - proteins float like
ice burgs in a sea of phospholipids.
Proteins can be integral proteins – go all
the way through the membrane, or may be
peripheral proteins -bound to the inside or
outside membrane.
Integral Proteins can be channels or
transporters.
Peripheral proteins can be receptors,
or can be cell identity markers that
identify a cell as “self” (like UPC
codes). These are often glycoproteins.
They may also mark worn out red blood
cells or cells that have been infected
with a virus.
Intercellular junctions:
Tight junctions – membranes of adjacent cells
come together and fuse, forming a barrier to
substances that want to pass between cells.
Desmosomes are protein “spot welds” in skin
and cardiac muscle.
Gap junctions are tubular channels that connect
the cytoplasm of one cell with that of another.
Cellular Adhesion Molecules help cells form
temporary attachments to other cells. CAMs
Membrane Physiology
• Cell membrane function:
– Cellular communication
– Establish an electrochemical gradient
– Are selectively permeable
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Lipids
Size
Electrical charge
Presence of channels and transporters
Movement of materials
• Passive processes:
– Depend on concentration and kinetic energy
– Do not require energy
– Move substances from an area of high
concentration to an area of low concentration
• Down a concentration gradient
Diffusion
• Passive process
• Reaches equilibrium or
• Physiological steady state
Tonicity
• Concentration of one solution relative to
another
• Isotonic – equal concentrations
• Hypertonic – more concentrated
• Hypotonic – less concentrated
Filtration
-a type of bulk flow where the
movement of water and dissolved
substances across a membrane
is due to gravity or hydrostatic
pressure (water pressure).
Active Transport
• Depends on the use of energy (ATP)
• Moves substances up a concentration
gradient (up hill)
• These systems are often called “pumps”
– Na+ / K+ pump
Vesicular Transport
Exocytosis – moving substances outside the
cell
Endocytosis – taking substances into the cell
Pinocytosis – “cell drinking”
Phagocytosis – “cell eating”
Receptor mediated endocytosis
Cytoplasm
• Cytosol
– Semifluid, mostly water
– Protein, carbohydrates, lipids, and inorganic
substances (ions)
– Many important metabolic reactions take
place here
– Cytoplasm is the cytosol plus the organelles
Organelles
“little organs” – have characteristic
appearance and have specialized functions.
Ribosomes – made of ribosomal RNA and
protein, these are the “work benches” where
proteins are put together.
Endoplasmic reticulum (ER)
Can be rough or smooth
Rough ER has ribosomes, makes proteins
for export outside the cell.
Smooth ER is the site of fatty acid,
phospholipid and steroid synthesis. In
certain cells also detoxifies chemicals,
such as alcohol and pesticides.
Gogli apparatus (body, complex)
Made of flattened sacs called cisternae
Process, sort and deliver proteins and
lipids to the plasma membrane and forms
vesicles and lysosomes.
The “UPS” of the cell
Mitochondria
Mitochondrion – singular
Two membranes – inner folds called cristae.
Main function is the use of oxygen to produce
ATP – cellular (aerobic) respiration
These are the “power plants” of the cell.
Lysosomes
The cell’s “stomach” – vesicles that contain
digestive enzymes.
Tay-Sachs disease
Peroxisomes are vesicles that have enzymes
that use oxygen to digest organic substances
and produce hydrogen peroxide.
Also make catalase to break down H2O2
Centrosomes and Centrioles
Centrioles are 2 cylinders of tubules arranged
at right angles.
Form the microtubules of the mitotic spindle
during cell division, and also make up a part of
cilia and flagella
Flagella
Cytoskeleton
Microfilaments and microtubules
Give the cell shape, and allow it to move – the
Musculoskeletal system of the cell.
Myofilaments are made of the protein actin
Myotubules are made of the protein tubulin
Vesicles
Membrane sacs that transport substances
within the cell.
Vesicle trafficking
Inclusions
Usually contain chemical substances
produced by the cell, these are temporary
structures that are not surrounded by a
membrane.
Melanin, glycogen, triglycerides
ribosomes
Nucleus
• Double membrane called the nuclear
envelope
• Nucleoplasm
• Chromatin granules – unwound DNA
• Nucleoli – puts RNA and protein together
to make ribosomes
• Nucleus is essential for cell survival
The Cell Cycle
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Nuclear division (mitosis or meiosis)
Cytoplasmic division – cytokinesis
Mitosis is somatic (body) cell division
Meiosis is reproductive cell division
Mitosis
Homologous chromosomes
Cell cycle – from one cell division to the next
Interphase – “resting phase”
G1 – Gap1 – growth phase
S – Synthesis – replication of DNA
G2 – Gap 2
Nuclear division – mitosis or
karyokinesis
Prophase
Metaphase
Anaphase
Telophase
PMAT
Cytokinesis
• Contractile ring of actin microfilament
• Cleavage furrow
• Two new daughter cells
Control of cell division
• How many times a cell divides depends on
the type of cell
• Stem cells retain the ability to divide and
differentiate
• Cell senescence
• Telomeres
• Levels of proteins called kinases and
cyclins
Control – contd.
• Cell size
• External factors –hormones and growth
factors
• Contact inhibition
Loss of control over division
• Neoplasm or tumor
• Benign tumors remain in a single location
• Malignant tumors (cancer) can spread or
metastasize
• Oncogenes – want these turned “off”
• Tumor suppressor genes – want these
turned “on”
• Apoptosis