Extra U Flashcards
Unconditioned stimulus
A stimulus which
normally produces a response in a simple reflex.
For example if a dog receives food, the presence
of food in the mouth is an unconditioned
stimulus. It causes the dog to respond by
salivating.
Uterus
Part of the reproductive system in a
female mammal in which the fetus develops. It
has an outer layer consisting mainly of muscle
and an inner lining or endometrium which
contains many glands and blood vessels. During
pregnancy, the uterus increases enormously in
size. In humans, its cavity may increase by
almost five hundred times
Vaccine
A preparation which is given to a person
either as an injection or orally and stimulates the
immune system to respond in the same way as
it does to the disease. A vaccine must contain
one or more antigens that are also found on the
pathogen. These stimulate the B cells and T
cells to produce the immune response.
Vacuole
A region in a plant cell containing cell
sap and surrounded by a cell membrane. A
large vacuole is one of the characteristic features
of a plant cell and distinguishes it from an
animal cell. The cell sap in the vacuole contains
dissolved substances such as sugars and mineral
ions. Both of these are usually at a higher
concentration than in the surrounding
cytoplasm. Because of this, water moves into
the cell and through the cytoplasm by osmosis.
This keeps the cell turgid and helps it to support
the plant.
Vagina
Passage leading from the uterus to the
outside in the female reproductive system of a
mammal. The vagina has two functions. When
mating takes place, it receives the penis of the
male. It also acts as the birth canal when a young
mammal is born
Vegetative propagation
A type of asexual
reproduction that does not involve the fusion of
gametes or sex cells. Vegetative propagation is
common in plants and includes the growth of
new plants from tubers and bulbs. Growth of
new plants from runners or suckers is another
example.
Vas deferens
One of a pair of tubes in a male
mammal which carries sperms from the testis
to the urethra when ejaculation takes place.
Ventilation centre
An area situated in the
medulla oblongata of the brain which controls
breathing. It sends impulses every two or three
seconds to the diaphragm and external
intercostal muscles. These muscles contract,
resulting in inhalation. Stretch receptors then
send impulses back to the ventilation centre
which inhibit muscle contraction and allow
exhalation.
Ventral
The belly or under-surface of an animal
or a structure associated with it. The ventral
blood vessel in a worm, for example, is a blood
vessel that runs along the under-surface of the
animal.
Vertebrate
Animals may be divided into
vertebrates and invertebrates. Vertebrates are
animals which have a skull and vertebral
column. They include fish, amphibia, reptiles,
birds and mammals. Animals such as worms,
snails and insects which do not have a skull and
backbone are known as invertebrates.
Vesicle
A small sac in the cytoplasm of a cell,
surrounded by a cell membrane. The Golgi
apparatus is an organelle consisting of a series
of flattened sacs, each one enclosed by a
membrane. These membranes are continually
being formed on one side and pinched off into
vesicles on the other. The vesicles contain
substances which have been processed and
packaged by the Golgi apparatus and are being
transported to the cell surface membrane.
Other vesicles are formed when substances are
taken into the cell.
Water potential
A measure of the ability of
water molecules to move. Water always moves
from a higher water potential to a lower water
potential. A solution in which there is a higher
concentration of free water molecules has a
higher water potential than one which has a
lower concentration of free water molecules. In
osmosis, water moves from the solution with the
higher water potential through a partially
permeable membrane into the solution with the
lower water potential.
Weed
A plant growing in a place where it is not
wanted by humans. Weeds are very important
economically as they compete with crop plants
for resources such as light, water and mineral
ions. As a result, the presence of weeds results in
a lower crop yield.
White blood cell
The blood contains several
types of white blood cell, all involved in helping
the body to combat infection. The term white
blood cell is a little misleading, as some are able
to leave the blood and are found in the
lymphatic system and in other tissues. They are
larger than red blood cells, from which they also
differ by possessing a nucleus and transparent
cytoplasm.
Wilting
A condition in which the leaves and
young stems of a plant droop. It occurs when the
amount of water lost through transpiration is
greater than the amount which is absorbed
through the roots. Under these conditions the
cells of the plant lose water and are no longer
turgid.
