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Samenvatting deeltentamen 2 Genen en Cellen (GZW, jaar 1)
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Samenvatting tweede deeltentamen van het vak 'Genen en cellen' (in het Engels)
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Deeltoets 2 Genen & cellenStof cellen:
Herhaling H4.1 t/m 4.5 (nu ook 4.3 gedeelte over cytoskelet)
H6, H7, H9, H10.1 t/m 10.3 (van H10 alleen gedeelte over humane en dierlijke weefsels)
Werkcolleges 2, 3 & 4
College pathologie (van hand-out t/m pagina 22 (pagina 17 niet))
Stof genen:
H14 t/m 17
Cellen
Chapter 4: General features of cells
4.2 Overview of cell structure
Cell structure is primarily determined by four factors: mater, energy, organizaton and informaton. he mater
found in living organisms is composed of atoms, molecules, and macromolecules. Each type of cell synthesizes a
unique set of molecules and macromolecules that contribute to cell structure. Energy is needed to produce
molecules and macromolecules and to carry out many cellular functons. he molecules and macromolecules that
consttute cells are found at specifc sites (organizaton). ll living cells have the ability to build and maintain their
internal organizaton. Proteins ofen bind to each other in much the same way that building blocks snap together.
hese protein-protein interactons create intricate cell structures and also facilitate processes in which proteins
interact in a consistent series of steps. Cell structure requires instructons (informaton). hese instructons are found
in the DN . he genes within each species’ genome contain the informaton to produce cellular proteins with
partcular structures and functons. he proteins are largely responsible for determining cell structure and functon.
Based on cell structure, all forms of life can be place into two categories called prokaryotes and eukaryotes.
Prokaryotc cells have a relatvely simple structure. Prokaryotc cells lack a membrane-enclosed nucleus. From an
evolutonary perspectve, the two categories of organisms that have prokaryotc cells are bacteria and archaea. Both
are microorganisms that are relatvely small. Bacteria are abundant throughout the world. Most bacterial species are
not harmful to humans, and they play vital roles in ecology. However, some species are pathogenic, they cause
disease. rchaea are also widely found throughout the world, though they are less common and ofen occupy
extreme environments.
typical bacterial cell has a plasma membrane, which is a double layer of phospholipids and embedded proteins.
his forms an important barrier between the cell and its external environment. he cytoplasm is the region of the
cell contained within the plasma membrane. In the plasma membrane there are the nucleoid (where the genetc
material is located) and ribosomes (which are involved in polypeptde synthesis). Some bacterial structures are
located outside the plasma membrane. Nearly all species of bacteria and archaea have a relatvely rigid cell wall that
supports and protects the plasma membrane and cytoplasm. Many bacteria also secrete a glycocalyx, an outer
viscous covering surrounding the bacterium. he glycocalyx traps water and helps protect bacteria from drying out.
Certain strains of bacteria that invade animals’ bodies produce a very thick, gelatnous glycocalyx called a capsule
that may help them avoid being destroyed by the animal’s immune system or may aid in the atachment to cell
surfaces. Finally, many prokaryotc cells have appendages such as pili and fagella. Pili allow cells to atach to surfaces
and to each other. Flagella provide prokaryotc cells with a way to move, also called motlity.
ll other species are eukaryotes, which include protsts, fungi, plants, and animals. Eukaryotc cells possess a true
nucleus, where most of the DN is housed. nucleus is a type of organelle (a membrane-bound compartment with
its own unique structure and functon). In contrast to prokaryotc cells, eukaryotc cells exhibit extensive
compartmentalizaton, which means they have many membrane-bound organelles that separate the cell into
different regions. Cellular compartmentalizaton allows a cell to carry out specialized chemical reactons in different
places. Some general features of cell organizaton are found in nearly all eukaryotc cells. Even so, the shape, size,
and organizaton of cells vary considerably among different species and even among different cell types of the same
species. Plant cells possess a collecton of organelles similar to animal cells. dditonal structures found in plant cells
but not in animal cells, include chloroplasts, a central vacuole, and a cell wall.
common feature of most cells is their small size. In general, large organisms atain their large sizes by having more
cells, not by having larger cells. Why are cells usually small? One key factor is the interface between a cell and its
extracellular environment, which is the plasma membrane. For cells to survive, they must import substances across
,their plasma membranes and export waste products. If the internal volume of a cell is large, it will require a greater
amount of nutrient uptake and waste export. he rate of transport of substances across the plasma membrane,
however, is limited by its surface area. herefore, a critcal issue for sustaining a cell is the surface area/volume rato.
4.3 The cytosol
he cytosol is the region of an eukaryotc cell that is outside the membrane-bound organelles but inside the plasma
membrane. he cytosol is a central coordinatng region for many metabolic actvites of eukaryotc cells. Metabolism
ofen involves a series of steps called a metabolic pathway. Each step in a pathway is catalyzed by a specifc enzyme.
Some pathways involve the breakdown of a molecule into smaller components, a process called catabolism. Such
pathways are needed by the cell to utlize energy and also to generate molecules that provide the building blocks to
construct macromolecules. Other pathways are involved in anabolism, the synthesis of (macro)molecules.
he cytoskeleton is a network of three different types of proteins flaments: microtubules, intermediate flaments
and actn flaments. Each type is constructed from many protein monomers. he cytoskeleton is a striking example of
protein-protein interactons. he cytoskeleton is found primarily in the cytosol and also in the nucleus along the
inner nuclear membrane.
Microtubules are long, hollow, cylindrical structures composed of protein subunits called alpha- and beta-
tubulin. he assembly of tubulin to form a microtubule results in a structure with a plus end and a minus
end. Microtubules grow only at the plus end, but can shorten at either end. single microtubule can
oscillate between growing and shortening phases, a phenomenon termed dynamic instability. his
phenomenon is important in many cellular actvites, including the sortng of chromosomes during cell
division. he sites where microtubules form within a cell vary among different organisms. Nondividing
animal cells contain a single structure near their nucleus called the centrosome. Within the centrosome are
the centrioles, a conspicuous pair of structures arranged perpendicular to each other. In animal cells,
microtubule growth typically starts at the centrosome in such a way that the minus end is anchored there.
Microtubules are important for cell shape and organizaton. hey are also involved in the organizaton and
movement of chromosomes during mitosis and in the orientaton of cells during cell division.
Intermediate flaments are another class found in the cells of many but not all animal species. hey are
intermediate in diameter between actn flaments and microtubules. Intermediate flament proteins bind to
each other in a staggered array to form a twisted, ropelike structure. hey functon as tension-bearing fbers
that help maintain cell shape and rigidity. Intermediate flaments tend to be relatvely permanent. By
comparison, microtubules and actn flaments readily lengthen and shorten in cells. Several types of proteins
assemble into intermediate flaments. In additon, intermediate flaments are found inside the cell nucleus.
ctn flaments are also known as microflaments. hey are long, thin fbers. ikike microtubules, actn
flaments have plus and minus ends, and they are very dynamic structures in which each strand grows at the
plus end. his assembly process produces a fber composed of two strands of actn monomers that spiral
around each other. ctn flaments play a key role in cell shape and strength. lthough actn flaments are
dispersed throughout the cytosol, they tend to be highly concentrated near the plasma membrane. he sides
of actn flaments are ofen anchored to other proteins near the plasma membrane.
Motor proteins are a category of proteins that use P as a source of energy to promote various types of
movements. motor protein consists of three domains: the head, hinge and tail. he head is the site where P
binds and is hydrolysed to DP and inorganic phosphate. P binding and hydrolysis cause a bend in the hinge,
which results in movement. he tail region is atached to other proteins or to other kinds of cellular molecules. o
promote movement, the head region of a motor protein interacts with a cytoskeletal flament. When P binds and
is hydrolysed, the motor protein atempts to walk along the flament. he head of the motor protein is initally
atached to a flament. o move forward, the head detached form the flaments, cocks forward, binds to the flament
and cocks backward. Motor proteins cause three different kinds of movements: movement of cargo via the motor
protein, movement of the flament or bending of the flament. In certain kinds of cells, microtubules and motor
proteins facilitate movement involving cell appendages called fagella and cilia. he difference between the two is
that fagella are usually longer than cilia and are typically found singly or in pairs. Both fagella and cilia cause
movement by generatng bends that move along the length and push backwards against the surrounding fuid.
Flagella and cilia have the same internal structure called the axoneme. he axoneme contains microtubules, the
motor protein dynein, and linking proteins. he microtubules in fagella and cilia emanate from basal bodies, which
are anchored to the cytoplasmic side of the plasma membrane. he movement involves the propagaton of a bend,
which begins at the base of the structure and proceeds toward the tp. he bending occurs because dynein is
actvated to walk toward the minus end of the microtubules.
,4.4 The nucleus and endomembrane system
he nucleus is an organelle found in eukaryotc cells that contains most of the cell’s genetc material. small amount
of genetc material is also found outside the nucleus, in mitochondria and chloroplasts. he membranes that enclose
the nucleus are part of a larger network of membranes called the endomembrane system. his system includes not
only the nuclear envelope, but also the endoplasmic retculum, Golgi apparatus, lysosomes, vacuoles, and
peroxisomes. he nuclear envelope is a double-membrane structure. Nuclear pores are formed where the inner and
outer nuclear membranes make contact with each other. he pores provide a passageway for the movement of
molecules and macromolecules into and out of the nucleus.
Inside the nucleus are the chromosomes and a flamentous network of proteins called the nuclear matrix. Each
chromosome is composed of genetc material (DN ), and many types of proteins that help to compact the
chromosome to ft inside the nucleus. he complex formed between DN and such proteins is termed chromatn.
he nuclear matrix consists of two parts: the nuclear lamina, which is composed of intermediate flaments that line
the inner nuclear membrane, and an internal nuclear matrix, which is connected to the lamina and flls the interior of
the nucleus. he nuclear matrix serves to organize the chromosomes within the nucleus. Each chromosome is
located in a distnct, nonoverlapping chromosome territory.
he primary functon of the nucleus is the protecton, organizaton, replicaton, and expression of the genetc
material. nother important functon is the assembly of ribosome subunits – cellular structures involved in
producing polypeptdes during the process of translaton. he assembly of ribosome subunits occurs in the
nucleolus, a prominent region in the nucleus of nondividing cells. ribosome is composed of two subunits: one small
and one large. Each subunit contains one or more RN molecules and several types of proteins. Most of the RN
molecules that are components of ribosomes are made in the vicinity of the nucleolus. he ribosomal proteins are
produced in the cytosol and then imported into the nucleus through the nuclear pores. he ribosomal proteins and
RN molecules then assemble in the nucleolus to from the ribosomal subunits. Finally, the subunits exit through the
nuclear pores into the cytosol, where they are needed for protein synthesis.
he endoplasmic retculum (ER) is a network of membranes that form fatened, fuid-flled tubules, or cisternae. the
term lumen describes the internal space of an organelle. he ER membrane encloses a single compartment called
the ER lumen. here are two distnct, but contnuous types of ER: rough ER and smooth ER. he outer surface of the
rough endoplasmic retculum is studded with ribosomes. Rough ER plays a key role in the sortng of proteins that are
destned for the ER, Golgi apparatus, lysosomes, vacuoles, plasma membrane, or outside of the cell. Proteins are
packaged into membrane vesicles – small spheres composed of membrane – and moved from one locaton in the
endomembrane system to another (protein sortng). nother functon of the rough ER is the inserton of certain
newly made proteins into the ER membrane. third functon is the atachment of carbohydrates to proteins and
lipids (glycosylaton). he smooth endoplasmic retculum which lacks ribosomes, functons in diverse metabolic
processes. he extensive network of smooth ER membranes provides an increased surface area for enzymes that
play important metabolic roles. he smooth ER is actve in for example the alcohol breakdown and carbohydrate
metabolism in liver cells. nother important functon in all eukaryotes is the accumulaton of calcium ions (Ca 2+). he
smooth ER contains calcium pumps that transport calcium ions into the ER lumen. he regulated release of calcium
ions into the cytosol is involved in many vital cellular processes. Finally, enzymes in the smooth ER are critcal in the
synthesis and modifcaton of lipids. In additon, enzymes in the smooth ER are necessary for certain modifcaton of
the lipid cholesterol that are needed to produce steroid hormones such as estrogen and testosterone.
he Golgi apparatus consists of a stack of fatened membranes, with each fatened membrane enclosing a single
compartment. he Golgi compartments are named according to their orientaton in the cell. he cis Golgi is near the
ER membrane, the trans Golgi is closest to the plasma membrane, and the medial Golgi is found in the middle. wo
models have been proposed to explain how materials move through the Golgi apparatus:
Vesicular transport model: materials are transported between the Golgi cisternae via membrane vesicles
that bud from one compartment in the Golgi and fuse with another compartment.
Cisternal maturaton model: vesicles form the ER fuse to form a cisterna at the cis face; the cisterna that was
previously at the cis face becomes a medial cisterna. his additon of a cisterna moves the other medial
cisternae toward the trans face. cisterna at the trans face is lost as a result of the export of vesicle from its
surface.
he Golgi apparatus performs three overlapping functons: processing, protein sortng and secreton. Enzymes in the
Golgi apparatus process, or modify, certain proteins and lipids. Glycosylaton contnues in the Golgi. For this to occur,
a protein or lipid is transported via vesicles from the ER to the cis Golgi. Most of the glycosylaton occurs in the
, medial Golgi. second type of processing event is proteolysis, whereby enzymes called proteases make cuts in
polypeptdes. he Golgi apparatus packages different types of materials into secretory vesicles that fuse with the
plasma membrane, thereby releasing their contents outside the cell. Proteins destned for secreton are synthesized
into the ER, travel to the Golgi, and then are transported by vesicles to the plasma membrane. he vesicles then fuse
with the plasma membrane, and the proteins are secreted to the outside of the cell. he entre route is called the
secretory pathway. In additon to secretory vesicles, the Golgi also produces vesicles that travel to other parts of the
cell, such as the lysosomes.
ikysosomes are small organelles found in animal cells that break down macromolecules. ikysosomes contain many
acid hydrolases, which are hydrolytc enzymes that use a molecules of water to break a covalent bond. his type of
chemical reacton is called hydrolysis. he acid hydrolases functons optmally at an acidic pH. ikysosomes contain
many different types of acid hydrolases that break down carbohydrates, proteins, lipids, and nucleic acids. his
enzymatc functon enables lysosomes to break down complex materials. One functon of lysosomes involves the
digeston of substances that are taken up from outside the cell via a process called endocytosis. In additon,
lysosomes break down intracellular molecules and macromolecules to recycle their building blocks to make new
molecules and macromolecules in a process called autophagy.
Vacuoles are prominent organelles in plant cells, fungal cells, and certain protsts. Vacuoles contain fuid and
sometmes even solid substances. Most vacuoles are made from the fusion of many smaller membrane vesicles.
Vacuoles in animal cells tend to be smaller than those in plant cells and are more commonly used to temporarily
store materials or transport substances, sometmes called storage vesicles. he functons of vacuoles are extremely
varied, and they differ among cell types and environmental conditons. For example a central vacuole in plants
(pressure on cell wall, turgor), contractle vacuoles (expanding when water enters) and degradaton vacuoles.
Peroxisomes are small organelles found in all eukaryotc cells. hey consist of a single membrane that encloses a
fuid-flled lumen. Peroxisomes catalyse a variety of chemical reacton, including some reactons that break down
organic molecules and others that are biosynthetc. Peroxisomes, for example, break down H 2O2 (hydrogen peroxide)
(which is a by-product of the breakdown of toxins) with the enzyme catalase to make water and oxygen gas. side
from the detoxifcaton, peroxisomes usually contain enzymes involved in the metabolism of fats and amino acids. o
initate peroxisome formaton, vesicles bud from the ER membrane and form a premature peroxisome. Following the
import of additonal proteins, the premature peroxisome becomes a mature peroxisome. Once the mature
peroxisome has formed, it may then divide.
he cytoplasm of eukaryotc cells is surrounded by a plasma membrane, which is part of the endomembrane system
and provides a boundary between a cell and the extracellular environment. Proteins in the plasma membrane
perform many important functons that affect the actvites inside the cell.
Membrane transport – Some of the proteins involved functon to transport essental nutrients or ions into
the cell, others are involved in the export of substances. Due to the functoning of these protein
transporters, the plasma membrane is selectvely permeable; it allows only certain substances in and out.
Cell signaling – o survive and adapt to changing conditons, cells must be able to sense changes in their
environment. In additon, the cells of a multcellular organism need to communicate with each other to
coordinate their actvites. he plasma membrane of all cells contains receptors that recognize signals. When
a signaling molecule binds to a receptor, it actvates a signal transducton pathway.
Cell adhesion – Protein-protein interactons among proteins in the plasma membranes of adjacent cells
promote cell-to cell adhesion. his phenomenon is critcal for animal cells to properly interact to form a
multcellular organisms and allows cells to recognize each other.
4.5 Semiautonomous organelles
Mitochondria and chloroplasts are semiautonomous. mitochondrion has an outer membrane and an inner
membrane separated by a region called the intermembrane space. he inner membrane is highly invaginated to
form projectons called cristae. he cristae greatly increase the surface area of the inner membrane, which is the site
where P is made. he compartment enclosed by the inner membrane is the mitochondrial matrix. he primary
role of mitochondria is to make P. Even though mitochondria produce most of a cell’s P, mitochondria do not
create energy. Rather, their primary functon is to convert chemical energy that is stored within the covalent bonds
of organic molecules into a form that can be readily used by cells. Mitochondria are also involved in the synthesis,
modifcaton and breakdown of several types of cellular molecules.
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