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The Cellular Level of
Organization
Antonie van Leewenhoek - first person to
see cells.
Robert Hooke coined the term “cell” and
illustrated what he saw.
Cell Theory
1. All organisms are composed of one or
more cells
2. Cells are alive, and are the basic living
unit of organization of all organisms
3. All cells come from other cells
Every cell has three main parts:
1. Plasma (cell) membrane – a boundary that
separates inside of cell from external
environment.
2. A set of genetic instructions In
eukaryotes this is in the Nucleus; in
prokaryotes there is no membrane, stays in a
section of the cell is called the nucleoid.
3. A cell body - the cytoplasm
• Cytosol – gel like substance not contained
in organelles
Cells are measured in micrometers or
microns.
Eukaryotic cells are bigger than prokaryotic
cells.
Advantages of being made of cells:
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Specialization
Surface area
Obtaining nutrients
Separating chemical processes
A multicellular organism can survive the
loss of single cells.
Cytoplasm refers to cytosol plus organelles
and inclusions.
cytosol - contains proteins, enzymes,
nutrients, ions, and other small molecules
organelles -“little organs” - 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.
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
Ribosomes – made of ribosomal RNA and
protein, these are the “work benches” where
proteins are put together.
Made up of two subunits that come together
when needed.
Found free in cells
Also in association with endoplasmic
reticulum
Endoplasmic reticulum (ER)
Can be rough or smooth
Smooth ER is the site of fatty acid,
phospholipid and steroid synthesis. In
certain cells also detoxifies chemicals,
such as alcohol and pesticides.
Rough ER has ribosomes, makes proteins
for export outside the cell.
May add sugars to proteins and make
glycoproteins.
Gogli complex (apparatus, body)
Made of flattened sacs
Process, sort and deliver proteins and lipids
to the plasma membrane and forms vesicles
and lysosomes.
The carbohydrates of the glycoproteins may
be modified to act as “shipping labels”
The “UPS” of the cell
Lysosomes
The cell’s “stomach” – vesicles that contain
digestive enzymes.
Fuses with vesicles containing food or
microorganisms.
Can be released outside the cell – “frustrated
phagocytosis”
Mitochondria
Mitochondrion – singular
Two membranes – inner membrane folded.
Main function is the use of oxygen to produce
ATP – cellular (aerobic) respiration
These are the “power plants” of the cell.
Believed to once be free living organisms –
have unique DNA and RNA
Plastids
• Found in plants and some protista
• Surrounded by a double membrane
• Function photosynthesis – chloroplasts
– Contain chlorophyll
• Chromoplasts – contain red, orange or
yellow pigments
• Amyloplasts – used to store starches
Cytoskeleton
• Made of protein tubules
• Gives structure and support
• Also acts as muscle in moving the cell and
transporting materials within it.
Musculoskeletal system of the cell.
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
Vesicles
Membrane sacs that transport substances
within the cell.
Vesicle trafficking
Vacuoles
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Larger than vesicles
Found in plant cells
Can be storage areas
Can contain enzymes or waste products
Central vacuole helps plants maintain
turgor pressure
Inclusions
Usually contain chemical substances
produced by the cell, these are temporary
structures that are not surrounded by a
membrane.
Melanin, glycogen, triglycerides
ribosomes
Plasma 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 a
phospholipid bilayer.
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.
Cholesterol is a large molecule, and helps to
stabilize the membrane in animals.
Plants use phytosterol.
Fluid mosaic model - proteins float like
icebergs 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 or
recognition proteins 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.
• All cells have a plasma membrane
• Some cells, such as bacterial, fungal and
plant cells, also have a cell wall.
Cell Physiology
• Cell membrane function:
– Are selectively permeable
• Lipids
• Size
• Electrical charge
• Presence of channels and transporters
Movement of molecules depends on:
1. Kinetic energy
Temperature
2. Concentration
gradient – more of something in
one area than another
Diffusion
• Passive process
– Depends on concentration and kinetic energy
– Does not require energy
– Moves substances from an area of high
concentration to an area of low concentration
• Down a concentration gradient
The most concentrated form of water is
pure water.
To make water less concentrated, we
dissolve substances in it.
Tonicity
• Concentration of one solution relative to
another
• Isotonic – equal concentrations
• Hypertonic – more concentrated
• Hypotonic – less concentrated
Tonicity – relative concentrations of
solutions
• Isotonic – two solutions contain the same
amount of substance dissolved in themequal concentrations
• Hypertonic – a solution containing a
greater amount of dissolved substancemore concentrated
• Hypotonic – a solution containing a lesser
amount of dissolved substance –
less concentrated
Osmosis
• The movement of water across a
selectively permeable membrane, down a
concentration gradient.
• Dialysis is the diffusion of a solute across
a selectively permeable membrane.
Cell in an
isotonic
solution
Cell in a
hypertonic
solution
Cell in a
hypotonic
solution
Facilitated diffusion
• Passive process
• Substances can move through protein
channels
• Substances may be shuttled across by
carrier proteins
• Substances still move down a
concentration gradient
Filtration
• Movement of water and solutes across a
membrane due to physical pressure:
– Gravity
– Hydrostatic pressure (water pressure or
blood pressure)
Active Transport
• Depends on the use of energy (ATP)
• Moves substances up a concentration
gradient (up hill)
• These permease proteins are often called
“pumps”
– Na+ / K+ pump
Vesicular Transport
Exocytosis – moves substances outside the
cell
Endocytosis – takes substances into the cell
Phagocytosis – “cell eating”
Pinocytosis – “cell drinking”
Receptor mediated endocytosis
Human Immunodeficiency Virus
• Originally developed in apes or monkeys
• Probably transmitted to man in central
Africa before 1931
• First cases reported 1980’s in male
homosexuals
• Transmitted by sexual intercourse, sharing
needles, blood products
• Incidence increasing most rapidly in
heterosexual females