saltatory conduction The quick transmission of a
nerve impulse along an axon by “jumping” from one
node of Ranvier to another.

samara A dry, indehiscent winged fruit either one
seeded (e.g., Ulmus) or two seeded (Acer). Dispersed by
the wind.

SA node (sinoatrial node) Aspecialized cardiac tis-
sue connected to the autonomic nervous system and
found in the upper rear wall of the right atrium of the
heart, near the superior vena cava. Acts as the heart’s
pacemaker by speeding or slowing down the heart rate
when necessary. It generates a brief electrical impulse of
low intensity around 72 times each minute in a resting
adult, causing the contraction of two atria, which then
pushes blood into the ventricles.

sap The liquid part of a plant, e.g., sap from the
sugar maple.

saprobe Any organism that obtains its nutrients and
energy from dead or decaying matter. Formerly called
saprophyte.

saprophyte SeeSAPROBE.

sarcomere The basic unit, a myofibril, of striated

muscle that causes it to contract. It is composed of

overlapping myosin (thick) and actin (thin) filaments

between two adjacent Z disks, (thin, dark disks that

traversely cross through and bisect the clear zone of a

striated muscle), giving it a characteristic striated look.

sarcophagous Flesh eating, e.g., tiger shark. Another

termfor carnivorous.

sarcoplasmic reticulum A cellular organelle, a mod-

ified endoplasmic reticulum, that is found only in stri-

ated muscle cells and which stores calcium ATPase used

to trigger muscle contraction when stimulated.

saturated fatty acid Fatty acid with a hydrocarbon

chain that contains no double bonds but has the maxi-

mum possible number of hydrogen atoms attached to

every carbon atom; it is “saturated” with hydrogen

atoms. A fatty acid is the building block of fat, and it

exists mostly solid at room temperature. It raises the

blood LDL cholesterol (“bad” cholesterol) level.

savanna A biome that is characterized by a flat

extensive cover of tall grasses, often three to six feet tall,

with scattered trees and shrubs and large herbivores. It

is a transitional biome between forest dominates and

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