Cambridge International A Level Biology Revision Guide

Cambridge International A Level Biology
358
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Myosin heads bind to thin filaments to form crossbridges;
the myosin heads then tilt pulling the thin
filaments together so that each sarcomere decreases
in length as the filaments slide over each other.
The myosin ATPase then hydrolyses ATP, providing
the energy for myosin heads to detach from the
thin filament and flip back ready to bind with actin
again. This process is repeated many times during
a contraction, but it can only be reversed by the
relaxation of the muscle (with no cross-bridges) and
the contraction of antagonist muscle that pulls the
filaments further away lengthening
each sarcomere.
Muscles rely on energy from the small quantity of ATP in
the sarcoplasm, small stores of creatine phosphate,
aerobic respiration in mitochondria and, if oxygen
is in short supply, lactate fermentation (anaerobic
respiration) in the sarcoplasm.
Hormones are chemicals that are made in endocrine
glands and transported in blood plasma to their target
cells, where they bind to specific receptors and so affect
the behaviour of the cells.
The menstrual cycle is controlled by secretions of
FSH and LH from the anterior pituitary gland, and
oestrogen and progesterone from the ovaries. In the
first week of the cycle, while menstruation is taking
place, LH and FSH are secreted, and these cause the
secretion of oestrogen, which causes the endometrium
to thicken. Negative feedback reduces the secretion
of LH and FSH. Halfway through the cycle, a surge of
LH causes ovulation to take place. After ovulation, the
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empty follicle becomes a corpus luteum and secretes
progesterone, which maintains the endometrium. At
the end of the cycle, the corpus luteum degenerates;
the reduction in progesterone and oestrogen causes
menstruation.
Contraceptive pills contain oestrogen and/or
progesterone; these hormones suppress the secretion of
FSH and LH from the anterior pituitary gland to prevent
ovulation.
Plants make limited use of action potentials, but they
are used to coordinate fast responses; for example, the
closing of Venus fly traps.
Plants produce several chemicals known as plant
growth substances that are involved in the control of
growth and responses to environmental changes. Auxin
is synthesised mainly in growing tips of shoots and
roots, and stimulates cells to pump protons into the cell
wall to lower the pH. Proteins in the cell wall known as
expansins respond to a low pH by loosening the links
between cellulose microfibrils and the matrix of the
cell wall so allowing microfibrils to slide apart. Plant
cells absorb water by osmosis and the internal pressure
potential causes the walls to stretch and the cells
to elongate.
Gibberellin is synthesised in young leaves and in seeds.
It stimulates growth of stems and germination of seeds
such as those of wheat and barley. In germinating seeds,
gibberellins activate the synthesis of amylase enzymes
for the breakdown and mobilisation of starch.
End-of-chapter questions
1 Which of the following is an incorrect statement about the endocrine system?
A All hormones bind to receptors on the cell surface of their target cells.
B Endocrine glands are ductless.
C Endocrine glands secrete hormones into the blood.
D Hormones are transported in the blood plasma. [1]
2 Which statement describes the biological action of the oestrogen/progesterone contraceptive pill?
A Increased concentrations of the hormones increase the thickness of the endometrium of the uterus.
B Increased concentrations of the hormones prevent the shedding of the endometrium of the uterus.
C Increased concentrations of the hormones stimulate a surge in the secretion of LH from the anterior
pituitary gland.
D Increased concentrations of the hormones suppress the secretion of FSH and LH from the anterior
pituitary gland. [1]