Cells- structural units of all living
Things.
* The human body has 50 to 100 trillion cells
*Most cells are composed of four elements (Carbon,
Hydrogen, Oxygen, Nitrogen)
* All cells have three main regions or
parts—a nucleus, a plasma membrane,
and the cytoplasm.
* For cells, “headquarters,” or the control
center, is the nucleus.
* The genetic material, or deoxyribonucleic acid (DNA),
is a blueprint that contains all the instructions needed
for building the whole body. so, as you might expect,
human DNA differs from frog DNA. More specifically,
DNA has genes, which carry the instructions for building
proteins.
*A cell that has lost or ejected its nucleus (for whatever
reason) is destined to “self-destruct.”
1. Nuclear Envelope- The nuclear boundary is a double
membrane barrier called the nuclear envelope, or
nuclear Membrane.
* The nuclear membrane encloses a jellylike fluid called
nucleoplasm in which other nuclear elements are
suspended.
2. Nucleolus - The nucleus contains one or more small,
darkstaining, essentially round bodies called nucleoli.
* Nucleoli (plural for nucleolus) are sites where cell
structures called ribosomes are assembled.
* Most ribosomes eventually migrate into the
cytoplasm, where they serve as the actual sites of
protein synthesis.
3. Chromatin - When a cell is not dividing, its DNA is
carefully wound around proteins called histones to
form a loose network of “beads on a string” called
chromatin that is scattered throughout the nucleus.
When a cell is dividing to form two daughter cells, the
chromatin threads coil and condense to form dense,
rodlike bodies called chromosomes.
* Plasma membrane- is a fragile,
transparent barrier that contains the cell contents and
separates them from the surrounding environment.
The Fluid Mosaic Model- The structure of the plasma
membrane consists of two phospholipid (fat) layers
arranged “tail to tail,” with cholesterol and floating
proteins scattered among them.
* hydrophilic (“water loving”)- The polar
“heads” of the lollipop-shaped phospholipid molecules
* hydrophobic (“water fearing”)- nonpolar fatty acid
“tails,”
* “sugar-proteins” are called glycoproteins.
Cell Membrane Junctions
main types of junctions:
1. Tight junctions are impermeable junctions
that encircle the cells and bind them together into
leakproof sheets.
2. Desmosomes are anchoring junctions scattered like
rivets along the sides of adjacent cells.
3. Gap junctions, or communicating junctions, function
mainly to allow communication.
>the neighboring cells are connected by hollow
cylinders composed of proteins (called connexons)
The Cytoplasm
Cytoplasm-is the cellular material outside the nucleus
and inside the plasma membrane.
three major components:
1. Cytosol is semitransparent fluid that suspends the
other elements.
2. Inclusions are chemical substances that may or may
not be present, depending on the specific cell type.
3. Organelles (little organs)- are specialized cellular
compartments.
* Mitochondria- tiny, lozenge-like or sausage-shaped
organelles.
*Ribosomes- are tiny, bilobed, dark bodies made of
proteins and one variety of RNA called ribosomal RNA.
*Endoplasmic Reticulum- “network within the
cytoplasm”), or ER, is a system of fluid-filled tunnels (or
canals) that coil and twist through the
cytoplasm.
*Erythrocyte (red blood cell). This cell carries oxygen in
the blood.
There are two forms of ER:
1. Rough endoplasmic reticulum - it is studded with
ribosomes.
2. Smooth endoplasmic reticulum-communicates with
the rough variety, it plays no role in protein synthesis,
because it lacks ribosomes.
Golgi Apparatus- appears as a stack of flattened
membranous sacs that are associated with swarms of
tiny vesicles.
Lysosomes- which appear in different sizes, are
membranous “bags” containing powerful digestive
enzymes.
* Epithelial cell. The hexagonal shape of this
cell is exactly like a “cell” in a honeycomb
of a beehive.
Skeletal, cardiac, and smooth muscle cells.
These cells are elongated and filled with
abundant contractile filaments.
*Fat cell. The huge spherical shape of a fat
cell is produced by a large lipid droplet in
its cytoplasm.
Peroxisomes- sacs containing powerful oxidase
enzymes that use molecular oxygen (O2) to detoxify a
number of harmful or poisonous substances, including
alcohol and formaldehyde.
* White blood cells such as the macrophage (a
phagocytic cell).
> Free radicals are highly reactive chemicals
with unpaired electrons that can damage the structure
of proteins and nucleic acids.
*Nerve cell (neuron). This cell has long pro
cesses (extensions) for receiving messages
and transmitting them to other structures
in the body.
Cytoskeleton- An elaborate network of
structures extends throughout the cytoplasm.
protein
>
(such as actin and myosin) are
most involved in cell motility and in producing changes
in cell shape.
* Oocyte (female). The largest cell in the
body, t
* Sperm (male). This cell is long and stream
lined, built for swimming to the egg for
fertilization.
> intermediate filaments- made up of fibrous
subunits.
> microtubules made up of repeating subunits
of the protein tubulin.
Centrioles- lie close to the nucleus
Cell Extensions
1. Cilia- are whiplike cellular extensions
2. Flagella- If the projections formed by the centrioles
Cell Physiology
*solution- homogeneous mixture of two or more
components.
*solvent- substance present in the largest amount in a
solution. Water is the body’s chief solvent.
*Solutes- Components or substances present in smaller
amounts
* Intracellular fluid -is a solution containing
small amounts of gases (oxygen and carbon dioxide),
nutrients, and salts, dissolved in water.
are substantially longer.
Microvilli- are tiny, fingerlike extensions of the plasma
membrane.
* Extracellular fluid, or interstitial fluid, the
fluid that continuously bathes the exterior of our cells.
* Selective permeability- means that a
barrier allows some substances to pass through it
while excluding others.
* Fibroblast. This cell has an elongated
shape, like the cable-like fibers that it secretes.
* Pressure gradient- actually pushes solutePassive Processes: Diffusion and Filtration
containing fluid (filtrate) from the higherpressure area.
* Diffusion- is an important means of
passive membrane transport for every cell
* Diffusion is the process by which
molecules (and ions) move away from areas
where they are more concentrated (more
numerous) to areas where they are less
Active Processes
* Active- Whenever a cell uses ATP to move
substances across the membrane
concentrated (with fewer of them).
two most important active processes:
However, molecules will diffuse through the
1. Active transport is similar to facilitated
plasma membrane if any of the following
are true:
• The molecules are small enough to pass
through the membrane’s pores (channels
formed by membrane proteins).
• The molecules are lipid-soluble.
• The molecules are assisted by a membrane
carrier.
* Simple diffusion- The unassisted diffusion
of solutes through the plasma membrane
* Osmosis- Diffusion of water through a
selectively permeable membrane such as the
plasma membrane.
* Facilitated diffusion- provides
passage for certain needed substances
* Filtration is the process by which
water and solutes are forced through a
membrane (or capillary wall) by fluid, or
hydrostatic, pressure.
diffusion in that both processes require
protein carriers that interact specifically
and reversibly with the substances to be
transported across the membrane
* sodium-potassium (Na+-K+) pump alter
nately carries sodium ions (Na+ ) out of and
potassium ions (K+ ) into the cell
2. Vesicular transport, which involves
help from ATP to fuse or separate
membrane vesicles
The two types of
vesicular transport
1. Exocytosis- is the mechanism that cells use
to actively secrete hormones, mucus, and
other cell products
2. Endocytosis- includes those ATP-requiring
processes that take up, or engulf,
extracellular substances