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Example summary 2015-2016
Table of contents
PREFACE .................................................................................................. 1
TABLE OF CONTENTS ............................................................................... 2
A. CELL BIOLOGY ................................................................................... 3
B. FYSIOLOGY ........................................................................................ 7
C.
EXAM QUESTIONS ........................................................................... 11
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A. CELL BIOLOGY
All organisms are composed of cells. These are small units, enclosed within a membrane,
and filled with a concentrated, watery, mixture of chemicals. They have the ability to
replicate their content in order to grow, and then divide themselves into two new cells.
Cells can communicate with each other and are the fundamental units of life.
Unit and diversity of cells
Cells differ from one another in size, form and chemistry. These variations make it
possible for different cells to perform different functions. Some cells are specialised
“factories” for the production of certain substances such as hormones, starches, fat, latex
or pigments. Others serve a motor function, such as muscle cells, which burn calories to
perform mechanical work. Each of these cells is so specialized that their survival is
dependent on other cells.
However different cell types might be, they all have the same basic chemistry. They
consist of the same molecules, which participate in the same chemical reactions.
Furthermore, all cells carry genetic information, in the form of genes, in DNA molecules.
This information is written in the chemical code itself, composed of the same building
blocks and able to replicate in the same way.
It isn’t by chance that all cells are so similar to each other. All contemporary cells in a
living organism originate from a common stem cell. Cells found in one organism at a
certain time point can also differ due to mutations, which change and diversify their DNA.
Mutations can be advantageous, providing cells with an advantage in survival and
reproduction, compared to previous cells. On the contrary, mutations can also be a
disadvantage, leading the mutated cells to living and reproducing less than non-mutant
cells. It is also possible that, sometimes, a mutation doesn’t positively or negatively
influence the fate of the mutated cell. In sexual reproduction, the resulting cell doesn’t
originate from the division of another cell. Rather, the resulting cell originates from two
cells, one from each parent, which fuses to form a new cell. Mutations and sexual
reproduction lead to both genetic variability and natural selection and are the basis of
evolution.
The genome of a cell is the whole set of nucleotides in the DNA of an organism. This
directs a cell on how it should behave. A fertilized egg cell can give rise to a wide variety
of cells such as muscle cells and neurons. These cells are different, but contain the same
DNA. Different cells express different genes, depending on their internal information and
environmental influences.
The prokaryotic and eukaryotic cell
Organisms whose cells contain a nucleus are called “eukaryotic”, whereas organisms
whose cells have no nucleus are called “prokaryotic”. Bacteria and Archea (single-cell
microorganisms) are prokaryotes. Human cells are an example of “eukaryotic” cells.
Organelles in the eukaryotic cell
Figure 1 shows a eukaryotic cell. The most important organelle is the eukaryotic cell is
the nucleus. It is enclosed between two concentric membranous layers, which form the
nuclear envelope. It contains DNA molecules – large polymers encoding the genetic
information of the whole organism.
Mitochondria have a worm-like structure. Individual mitochondria (i.e. the
mitochondrion) are enclosed within two separate membranes. The innermost membrane
is formed by folds, which stick out inside the organelle. Mitochondria are responsible for
generating usable energy by oxidizing foodstuff to produce ATP, the fundamental
chemical fuel for most of a cell’s activities.
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Mitochondria use oxygen and release carbon dioxide during their activity. Thus, this
process is called cellular respiration. Furthermore, mitochondria possess their own DNA
and reproduce via binary fission (dividing themselves in two).
Figure 1: The eukaryotic cell:
1: Nucleus
2: Nuclear envelope
3: Endoplasmic reticulum
4: Golgi apparatus
5: Mitochondrion
6: Lysosome
7: Peroxisome
8: Transport vesicle
Chloroplasts are big, green organelles exclusively found in cells in plants and algae.
They contain two adjacent membranes which themselves contain membranous stacks
filled with the green pigment chlorophyll. Chloroplasts resort to photosynthesis: their
chlorophyll molecules gather energy from sunlight and use this energy in other
processes.
The Endoplasmic Reticulum (ER) is an irregular, maze-like structure of mutually
connected spaces, enclosed within a membrane. It’s here where most parts of the cell
membrane and a cell’s export products are formed.
The Golgi apparatus is composed of flattened, stacked membranes, resembling discs.
These modify and package ER molecules to be transported to another cell or organelle.
Lysosomes are small, irregularly shaped organelles where intracellular digestion takes
place: they break down food particles into smaller foodstuffs, as well as breaking down
unwanted molecules, which are recycled or excreted.
Peroxisomes are small, membrane-enclosed vesicles, responsible for providing a safe
environment for several reactions. Toxic molecules are inactivated inside peroxisomes
using hydrogen peroxide.
Molecules are continuously transported between the ER, the Golgi apparatus and the
lysosomes by means of transport vesicles. This happens via the processes of endo- and
exocytosis. Exocytosis and endocytosis are also used to transport materials into and out
from the cell.
The cytosol is the fraction of the cytoplasm found outside the intracellular membrane. It
contains many big and small molecules, which gives it more of a gel-like rather than fluid
consistency. It is where many of the fundamental, vital chemical reactions in the cell take
place (for example, it’s where the first step in the breaking down of nutrient molecules
occurs).
The cytoplasm and cytoskeleton of the cell
The cytoplasm’s structural integrity is dependent on the cytoskeleton. This is a system of
protein filaments frequently anchored to the plasma membrane and the nucleus. There
are 3 kinds of protein filaments:
 Actin filaments: the thinnest filaments are abundantly found in muscle cells.
These filaments are the central pieces in the mechanism responsible for muscle
contration;
 Microtubuli: the thickest filaments resemble hollow tubes. These are intimately
involved in cell division;
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
Intermediate filaments: responsible for strengthening the cell.
Other proteins are also often found along these three filaments. Together, they form a
system of beams, ropes and motors, which grant the cell its form and mechanical power,
and assure its motility. The interior of a cell is in constant motion. Motor proteins use
energy stored in ATP to transport organelles and other proteins through the cytoplasm.
Eukaryotic cell
Unicellular or multicellular
Diameter 5 – 100 um
Nucleus contains genetic information in
the form of complexly organized
chromosomes composed of DNA +
protein.
RNA-synthesis in the nucleus, protein
synthesis in the cytoplasm, nucleoli
present in the nucleus.
Cytoplasm containing cytoskeleton
composed of protein.
Division through mitosis or meiosis.
Mostly possess aerobic metabolism.
Prokaryotic cell
Exclusively unicellular
Diameter 0,5 – 10 um
Genetic information stored in circular DNA
molecules in the cell (nucleoid).
RNA and protein synthesis takes place in
the same compartment, no nucleoli.
No cytoskeleton, organelles poorly or not
developed.
Division through binary fission.
May possess aerobic or anaerobic
metabolism.
Table 1: Differences between the eukaryotic and prokaryotic cell.
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Example summary 2015-2016
B. FYSIOLOGY
The global function of the Central Nervous System
Many kinds of amazingly fast processes take place in the human body with the goal of
transmitting a signal. This transmission occurs via neurons to the central nervous
system, and in a special way. To understand it, it’s important to have knowledge of the
concepts of resting membrane potential and action potential.
Resting membrane potential
There is always voltage in neurons, even when they’re not transmitting any impulses.
This is called the resting membrane potential and is generated by an uneven trade of
Na+ and K+ ions. There are more K+ ions than Na+ ions inside the cell, and more Na+
ions than K+ ions outside the cell. The resting membrane potential is around -90mV. This
potential is maintained by means of leak channels, which allow the entrance of much
more K+ ions than Na+ ions into the cell. Leaks channels function passively, obeying the
concentration gradient. In addition to it, cells possess active Na+/K+ pumps. When “fed”
ATP, these active pumps transport two K+ ions into the cell, exchanging them for three
Na+ ions, which are pumped to the outside. This is what causes the negative potential on
the inner side of the membrane.
Action potential
In response to a stimulus, cells (such as muscle cells and neurons) can alter the
permeability of their cell membrane to ions. Stimulating a neuron, thus, leads to a
localised increase in the membrane’s permeability to Na+ ions: an action potential. An
action potential is generated every time the membrane potential reaches a certain,
critical value. That threshold is usually 8-12 mV greater than the resting membrane
potential.
As with leak channels for keeping the resting membrane potential, there are specific
channels playing a crucial role in the propagation of an action potential: voltage-gated
channels. The voltage-gated Na+ channels are responsible for depolarization, whereas
voltage-gated K+ channels are responsible for repolarization. The Na+/K+ pump,
conversely, is responsible for the reestablishment of the original potential during the
spread of an action potential.
When an action potential starts, the voltage-gated Na+ channels open, allowing Na+ ions
to diffuse into the cell and trigger a change in the membrane potential (a depolarization).
After just a few milliseconds, these sodium channels close again. The voltage-gated K+
channels open somewhat more slowly and stay open for a longer period, allowing for
repolarization to occur. Once the membrane potential becomes negative again, the
voltage-gated K+ channels close. When this happens with a slight delay,
hyperpolarization is said to occur.
The refractory period is the period during which the voltage-gated Na+ channels
cannot open. During this period, cells cannot process new action potentials (i.e. no new
action potentials can be elicited in that cell). The refractory period ends when the
membrane potential is reset to the basal value (resting membrane potential).
The local depolarisation of a nerve fiber sends a wave of electrical current from the
depolarised point to the adjacent micro-region. This happens because there is a change
in electrical charge next to the depolarised point, so that the certain point on the inner
side of the membrane is positively charged. This creates an even greater difference in
potential with regard to the adjacent regions, which are “more negative”, thus causing
the displacement of the excess positive charges to those more negative areas. The action
potential spreads as this occurs repeatedly and continuously along the whole membrane
of a nerve fiber.
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Myelin
A special characteristic of neurons that allows them to transmit signals as fast as they do
is their isolation. Long neurons are myelinated, while short neurons are demyelinated.
Myeline is responsible for the isolation that allows action potentials to spread the way
they do. That is, myeline allows a much faster conduction along the neuron’s axon. The
action potential “jumps” through the myeline sheath, from one node of Ranvier to the
next, spreading much faster this way.
The somatic nervous system
The somatic nervous system, also named the voluntary nervous system, commands all
interactions with the outside world. Sensory neurons bring a message from organs, which
perceive multiple stimuli (for example, the skin). Motor sensors activate skeletal muscle.
The somatic nervous system is composed of the somatic part of the central and
peripheral nervous systems and is responsible for the general sensory and motor
innervation of all parts of the body, apart from the intestines in the abdominal cavity,
smooth muscle and the glands.
The autonomous (visceral) nervous system is the part of the peripheral nervous system
responsible for all reactions taking place unconsciously, such as breathing and the
function of organs like the liver and the kidneys.
Muscle tissue
Each muscle fiber is composed of hundred to thousands myofibrils. Each myofibril is, in
turn, composed of a mesh alternating around 1500 myosin filaments and 3000 action
filaments. A-bands contain thick myosin filaments, which overlap with thin actin filaments
at the end portion of a sarcomere. In the middle, there is no overlap of the myosin
filaments with any actin filaments, and this are is called the H-band. The I-band contains
myosin and actin filaments, which don’t overlap. That is what makes this band often
lighter in colour. In the middle of the I-band is the Z-line, where actin filaments attach.
The before mentioned sarcomere is the fraction between two consecutive Z-lines.
The side-by-side position of actin and myosin filaments is easy to understand. They are
kept in place by the protein titin, which is thread-shaped and thus very elastic. Titin
keeps actin and myosin filaments in place, making it possible for the sarcomere to
perform its contractile function. Titin is bound to the Z-line and changes in length upon
contraction and relaxation.
Mitochondria can be found next to the myofibrils, to which they provide energy. The
space between myofibrils is filled with a liquid: sarcoplasm.
Initiation and execution of a muscle contraction takes place in the following steps:
 An action potential spreads along a motor neuron until it reaches the proximity of
a muscle fiber;
 Each neuron releases the neurotransmitter acetylcholine from vesicles in its endextremity;
 Acetylcholine is responsible for the opening of ion channels in the muscle fiber
membrane;
 Opening these Ach-dependent channels leads to the inflow of large amounts of
sodium ions. This originates a depolarization of the sarcolemma, the membrane of
the muscle fiber which, in turn, causes voltage-gated sodium ion channels to
open;
 The depolarization of the sarcolemma spreads to the myofibrils via T-tubuli;
 The depolarization of the myofibrils leads to the activation of the sarcoplasmic
reticulum, which releases large amounts of the calcium ions it keeps stored;
 These calcium ions will bind to the troponin complex, causing it to undergo a
shape change, and exposing its free binding sites. The head of the myosin
filament can, then, bind to these free binding sites;
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

ATP is released everytimeevery time the head of a myosin filament becomes
bound to troponin. The heads can then flex (pull back in a stroking motion),
leading to the pulling of the actin and myosin filaments along each other. This is
what causes muscle contraction;
Contraction ends when calcium ions are pumped back into the sarcoplasmic
reticulum.
A myosin filament is composed of six polypeptide chains; two heavy and four light
chains. The two heavy chains form a double helix, and are the tail of the myosin
molecule. One end of one of these chains folds bilaterally and possesses a ball-shaped
polypeptide structure: it’s called the myosin head. There are, thus, two myosin spheres
atop the end of a double helix. The four light chains are part of these spheres; two in
each myosin head.
In a resting state, actin and myosin filaments overlap each other only partially. During
contraction, these filaments interdigitate, so that the myosin filaments glide between the
actin filaments. During a contraction, all the myosin and actin filaments in a sarcomere
interdigitate as described above. This happens in the A-band. When a muscle is at rest,
the binding site between actin and myosin becomes blocked by the troponin-tropomyosin
complex in the actin filament, preventing any contraction from happening.
Skeletal muscle tissue
Cardiac muscle
tissue
Smooth muscle
cells
Observable
characteristics
Long, cilinder-shaped
giant cells with many
nuclei. Are arranged in
parallel to each other
Single nucleus,
transversely
striated. May have
a “dendritic-like”
form
Spindle-shaped
Formation/Structure
Myoblasts (stem cells)
divide and fuse to form a
multi nucleated muscle
cell
Bifurcate and
attach to each
other’s end
ramifications
Dense bodies
attach muscle
cells to each
other. Each
muscle cell has a
single nucleus
Striation
Transverse striations with
parallel distribution
Transverse
striations
No striation. Actin,
myosin and dense
bodies are present
Location of the
nucleus
Directly under the
sarcolemma
Central in the cell
In the middle of
the cell
Regeneration
Through satellite cells (a
single nucleated stem cell
with the ability to divide
and to fuse with each
other or with another
muscle cell to augment
existing muscle fibers)
Can’t regenerate
Possible by cell
division (also
possess satellite
cells)
Table 2: Different types of muscle tissue
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Two different substances can affect the normal function of the motor end plate:
 Cholinesterase inhibitors: these substances delay the breakdown of
acetylcholine in the synaptic cleft, thus prolonging the neurotransmitter’s effect.
This may cause a muscle to cramp;

Curare: this substance blocks the binding of acetylcholine to the post-synaptic
membrane receptors. This results in the opening of only an insufficient number of
ion channels in the post-synaptic membrane, not enough to generate/transmit an
action potential. The ultimate effect is that there is no contraction.
An isometric contraction is a contraction in which force is exerted without any change
in muscle length. An isotonic contraction is a contraction with unchanged tension but
where there is a change in muscle length.
The amount of force exerted during a contraction depends on several factors. It’s
essential to know whether the muscle is stimulated by small, spaced stimuli (“shocks”) or
if it is continuously stimulated. This is the difference between twitch and tetanus. A
twitch is the time between the motor end-plate potential, the contraction and the
relaxation of a muscle fiber, making twitch contractions intermittent. Conversely, in
tetanic muscle contraction, the muscle fiber is continuously contracted and there is no
relaxation.
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C. EXAM QUESTIONS
1. Which event triggers the breakage of the actin-myosin bond?
A. ATP hydrolysis
B. Binding of Ca2+ to Troponin C
C. Pumping of Ca2+ back into the Sarcoplasmic Reticulum
D. Binding of ATP to myosin
2.
A mutation always alters the end product of that mutated gene.
A. True
B. False
Answers: 1=D; 2=B
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