Download Cells: the building block of all living things

Cells: the building block of all living things
 Made mostly of C, H, O, N
 Trace elements are important for cellular function
o Ca for blood clotting, Fe for hemoglobin, I for metabolism, Na and K for
nerve impulse transmission for muscle contraction.
 Water takes up about 66% of cells
 Area between blood and cells is interstitial fluid (saltwater solution)
 Vary in size, shape and function
Anatomy of the cell:
 There are 3 main parts of the generalized cell:
1. Nucleus (near center)
2. Cell membrane (forms outer membrane)
3. Cytoplasm (semifluid, surrounds organelles)
1) Nucleus: the control center of the cell; contains genes.
 DNA controls the building of the whole body and more importantly, the
instructions for building proteins.
o Necessary for reproduction
 Conforms to the shape of the cell
 Has 3 parts:
1. Nuclear membrane: double membrane barrier, between which is space
or “moat”
a. Where the two membranes approach and meet are called
nuclear pores (large)
b. It is selectively permeable
c. Encloses the nucleoplasm, where nucleoli and chromatin are
present.
2. Nucleoli: one or more small, dark-staining, round bodies
a. Where ribosomes are assembled
i. Eventually migrate into cytoplasm (site of protein
synthesis)
3. Chromatin: bumpy threads of non replicating DNA combined with
protein scattered throughout the nucleus
a. At the beginning of replication, the chromatin threads coil and
condense to form Chromosomes
2) Plasma membrane: fragile, transparent barrier that contains cell contents and
separates from environment
 Has a core of two lipid fat layers in which proteins and carbohydrates float.
 Lipid bilayer is formed mostly by phospholipids, but also has a substantial
amount of cholesterol (stabilizes and keeps fluidity)
 Impermeable to most water-soluble molecules.
 Specialized function is a result of proteins
o Some are enzymes, hormone/chemical receptors (binding sites)
o Those that span the membrane are usually for transport functions

Clusters of proteins form pores for ions, water, and small, watersoluble molecules
 Others act as carriers that bind to the molecule and bring it into
the cell.
 Some have sugar groups attached to them, called
Glycoproteins, which are responsible for determining blood type,
receptors that bacteria or viruses can bind to, and for cell-cell
interactions. (*step in cancer cell generation)
o Specializations such as microvilli and membrane junctions are
commonly displayed by epithelial cells (line hollow body organs)
 Microvilli-increase surface area of the membrane for absorption
purposes.
 Membrane junctions- vary depending on their role
1. Tight junctions: adjacent plasma membranes fuse together
like a zipper (leak proof).
2. Desmosomes: anchor cells to prevent cells, such as skin
cells from being pulled apart (adjacent cells are connected
by protein filaments)
3. Gap junctions: allows communication between cells by
having molecules pass from one cell to another.
3) The Cytoplasm: cellular material outside of the nucleus, inside of the plasma
membrane
 Site of most cellular activities, “factory center”
 Has 3 parts:
1. Cytosol: semitransparent fluid that suspends the other elements
 Mostly water and dissolved nutrients and solutes
2. Organelles: the metabolic machinery.
 Each one carries out a specific function
3. Inclusions: chemical substances that may be present, depending on the
cell type.
Organelles: (compartmentalization of each allows it to do its specific fxn)
1. Ribosomes: tiny, round, dark bodies made of ribosomal RNA and proteins.
a) Site of protein synthesis
b) Some float free; others are attached to (rough) Endoplasmic Reticulum.
2. Endoplasmic Reticulum: a system of fluid-filled tubules that coil and twist through
the cytoplasm.
a) ½ of total cell’s membranes
b) Network is to carry substances, primarily proteins from one part of the cell
to another.
c) Rough ER: studded with ribosomes (membrane factory)
i. The proteins made on the ribosomes fold into their 3D shape and
are then dispatched to the rest of the cell.
ii. Size of RER is indicator on the amount of protein the cell makes
(large in pancreatic cells)
d) Smooth ER: continuation of the RER, but has no role in protein synthesis.
3.
4.
5.
6.
i. Functions in cholesterol synthesis and breakdown, fat metabolism,
and detox of drugs. (Large in liver cells, and those that produce
steroid-based hormones-testes)
Golgi Apparatus: a stack of flattened membranous sacs with swarms of vesicles.
a) Found close to nucleus and is “traffic director” for cellular proteins
b) It modifies and packages proteins received from the RER.
i. Proteins “tagged” for export accumulate in the Golgi and cause it to
swell.
ii. The swollen end pinches off (now called transport vesicles) and
travel to the plasma membrane.
iii. Fuses w/ the membrane, part ruptures, and the contents are spilled
outside of the cell (exocytosis).
iv. It also packages proteins and phospholipids to maintain the plasma
membrane, and hydrolytic enzyme sacs called lysosomes.
Lysosomes: “breakdown bodies” that appear in different sizes, and are bags of
powerful digestive enzymes
a. These are the cell’s demolition sites b/c they digest worn-out and
nonusable cell structures and foreign substances
b. Abundant in WBCs
c. The enzymes that they contain are formed by ribosomes and “packaged”
by the Golgi (bud off of Golgi).
Peroxisomes: membranous sacs of oxidase enzymes that use O2 to detoxify a
number of harmful or poisonous substances.
a. “Disarm” Free Radicals- reactive chemicals with unpaired electrons that
can scramble the structure of proteins and nucleic acids. (natural
byproduct/dangerous if accumulate)
b. Convert free radicals to H2O2 (hydrogen peroxide). Catalase (an
enzyme) then turns H2O2 to water.
c. Numerous in kidney and liver cells (detoxification)
d. Do not pinch off of Golgi like lysosomes, they pinch in half
Mitochondria: sausage-shaped organelles that squirm, lengthen, and change
shape continuously
a. They have a double membrane, equal to two plasma membranes placed
side by side
i. Outer is smooth, inner has cristae- shelflike protrusions.
b. Enzymes in the cristae membrane and within the mitochondria carry out
the reactions in which O2 is used to break down food into energy
(respiration)
c. Much of this energy escapes as heat, but some is used to form
ATP molecules (why called the “powerhouse” of the cell)
d. “Busy cells” like liver and muscle have hundreds of mitochondria, inactive
have just a few
7. Cytoskeleton: elaborate network of protein structures that extend throughout the
cytoplasm
a. Act as internal framework that determines cell shape, supports other
organelles, and provides the machinery needed for cell-cell transport and
cellular movements.
b. 3 elements of cytoskeleton:
1) Microtubules: Largest and “tube-like”; determine the overall shape of a
cell and the distribution of organelles (important in cell division)
2) Intermediate filaments: strong, stable “rope-like” structure; help form
desmosomes and provide resistance to pulling forces on the cell
3) Microfilaments: Actin and Myosin; cell motility and produces change in
cell shape (**important in muscle cells)
8. Centrioles: pair of rod-shaped bodies that lie close to the nucleus.
a. Lie at right angles to each other
b. Made of fine Microtubules
c. During cell division, they direct the formation of the “mitotic spindle”
9. Others:
a. Cilia-whip-like cellular extensions that move substances along the cell
surface (respiratory system)
i. Where they appear, there are usually many of them.
ii. Form when Centrioles multiply and line up beneath the plasma
membrane; microtubules then begin to “sprout” from the centrioles
and put pressure on the membrane, forming projections
b. Flagella- when the projections are substantially longer
i. Sperm is the only flagellated cell in the human body
**Cilia propel substances, flagella propels the cell itself.