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All cells are composed of similar kinds of molecules, and are enclosed by a
cell membrane. Prokaryote and eukaryote cells differ; eukaryote cells (with
occasional exceptions) have a nucleus, membrane-based organelles and a
cytoskeleton; prokaryote cells do not.
Summary of Section 1.1
Despite their underlying uniformity of molecular and intracellular organization,
eukaryote cells are extremely diverse in overall structure and in function.
Summary of Section 1.1
Cells form communities. Some unicellular organisms form colonies. In
structurally complex organisms, cells that perform similar functions are often
grouped together into tissues. In the most complex animals, different cells
and tissues are often grouped together into an organ, or system, which is
specialized to perform a specific function(s).
Summary of Section 1.1
Animal tissues are usually composed of a mixture of cell types, which are
classified as epithelial, muscle, nervous, connective, blood and immune
system, hormone-secreting and germ cells, although some cells, such as the
hormone-secreting cells of the intestinal epithelium, fall into more than one
of these categories.
Summary of Section 1.1
Cells become specialized, or differentiate, as a result of differential gene
expression, leading to the synthesis of specific proteins.
Summary of Section 1.1
Except when in a dormant state (e.g. spores), all living cells are dynamic. They
interact with their environment in order to obtain a source of energy and
molecular building blocks, and respond to biochemical environmental changes.
Individual cells, particularly in multicellular organisms, receive and respond
to ‘signals’ from other cells. Many cells move within their environment. All
these processes require a myriad of biochemical events within the cell.
Summary of Section 1.1
Our understanding of the chemical nature and organization of cells, and how
individual cells function has come from microscopy, biochemistry, and cell
biology techniques such as cell culture.
Summary of Section 1.2
Light microscopy (LM) provides valuable information about the organization
of tissues, but initially did not allow precise localization of specific molecules
to particular parts of a tissue or cell.
Summary of Section 1.2
The internal organization of cells is studied by electron microscopy (EM),
which allows cell organelles to be visualized.
Summary of Section 1.2
Cell fractionation techniques allow cell organelles to be separated by
ultracentrifugation. Subsequent biochemical analysis allows functions such as
the activity of particular enzymes to be pinpointed to specific cell organelles.
Summary of Section 1.2
Advances in LM and EM methods and the advent of immunohistochemistry
(which makes use of the specificity of antibody binding) and autoradiography
(which utilizes radioactively labelled precursor molecules), have allowed
localization of specific molecules to particular cell types and to cell
organelles.
Summary of Section 1.2
Antibodies are used in a variety of techniques, including
immunohistochemistry, immunopurification, immunoassay, and others.
Summary of Section 1.2
Cell culture allows living cells to be studied. Individual cells can be observed
and the effects of specific molecules on cells and cell processes, such as cell
division, can be analysed.
Summary of Section 1.2
Prokaryotes do not exhibit great structural diversity, although they do have a
range of sizes, shapes and some structural specializations.
Summary of Section 1.3
The cytoskeleton is composed of three types of proteins: microfilaments (also
known as actin filaments), microtubules and intermediate filaments. The
cytoskeletal proteins provide shape, support, an internal transport system and
are responsible for cell movements.
Summary of Section 1.4
Prokaryotes do not have a nucleus, cytoskeleton or internal membrane-bound
organelles.
Summary of Section 1.3
Most bacteria have a wall that lies outside the inner membrane and is
composed of proteoglycan; many have an outer membrane too.
Summary of Section 1.3
Two structural specializations associated with the cell membrane of some
bacteria are flagella, which are involved in movement, and pili, which are
involved in adhesion. Some bacteria have membrane specializations.
Summary of Section 1.3
Eukaryote cells contain cytoskeletal proteins and organelles.
Summary of Section 1.4
DNA, which is packaged by binding to histone proteins to form chromatin, is
located in the nucleus, where transcription of DNA into RNA, and the
assembly of ribosomes takes place. RNA and ribosomes leave the nucleus,
and structural and ribosomal proteins and the enzymes needed for DNA
replication and transcription enter it, through pores in the nuclear envelope.
Summary of Section 1.4
Messenger RNA is translated into protein at the ribosomes in the cytoplasm.
Cytosolic proteins and many proteins of the mitochondria and chloroplasts
are synthesized by ribosomes that are free in the cytoplasm. Proteins destined
for export or for other cell organelles are synthesized by ribosomes attached
to the rough ER. These proteins are targeted to the ER by specific amino acid
signal sequences.
Summary of Section 1.4
The smooth ER is the site of detoxification, phospholipid assembly, and some
protein modification.
Summary of Section 1.4
Proteins and lipids pass from the ER to the Golgi apparatus for further
processing (e.g. glycosylation), and sorting and packaging into vesicles,
which deliver specific proteins to specific parts of the cell, e.g. to different
regions of the cell membrane and to lysosomes.
Summary of Section 1.4
Vesicles move short distances within the cell by diffusion, but are transported
longer distances along microtubules.
Summary of Section 1.4
Movement of molecules and vesicles to specific sites in the cell is achieved
because they have molecular ‘address labels’ that may be specific signal
sequences of nucleotides in the case of mRNAs, or amino acids in the case of
proteins.
Summary of Section 1.4
Substances are secreted from cells by exocytosis. One way in which
substances are imported into cells is by engulfment into a cell membranebound
vesicle. This process is known as endocytosis.
Summary of Section 1.4
Ingested materials and old organelles are digested within lysosomes. Some
fatty acids are broken down in peroxisomes.
Summary of Section 1.4
Mitochondria are the site of the majority of ATP production. They have a double
membrane, the inner membrane of which is folded, and their own DNA.
Summary of Section 1.4
Plant cells have a cell wall and two organelles not found in animal cells, the
chloroplast and the vacuole.
Summary of Section 1.4
New organelles form by growth of pre-existing organelles, followed by
division. These processes are independent of nuclear division.
Summary of Section 1.4
The cells of animal tissues are held together by the extracellular matrix and
by cell junctions.
Summary of Section 1.5
The extracellular matrix in animals is composed of a hydrated mixture of
polysaccharides, proteoglycans, and glycoproteins, in which various proteins,
especially collagen, are embedded. The nature of the proteins determines the
properties of the matrix, which vary from tissue to tissue.
Summary of Section 1.5
Cells can be linked by several types of cell junctions, formed by specialized
proteins.
Summary of Section 1.5
Proteins are very diverse, and a cell may contain several thousand different
ones. Proteins have a wide range of functions, including structure,
transport, storage, signalling, recognition and gene regulation. A major
class of proteins is the enzymes, which are biological catalysts.
Summary of Section 2.1
Gel filtration is a technique used to separate proteins in a mixture on the
basis of size differences. The gel acts as a molecular sieve: small
molecules enter its pores, so their passage down the column is retarded,
whereas larger molecules, unable to enter the pores, pass more quickly
down the column between the beads.
Summary of Section 2.1
Proteins can be separated and their Mr measured by SDS-PAGE (SDSpolyacrylamide
gel electrophoresis).
Summary of Section 2.1
Proteins have a four-tier hierarchy of structure: primary, secondary, tertiary
and quaternary. All except primary constitute higher-order structure.
Summary of Section 2.2
Primary structure is the sequence of amino acids, which are joined via
peptide bonds to form a linear polypeptide chain, which has an N-terminus
and a C-terminus.
Summary of Section 2.2
Secondary structure consists of two types of regular folding patterns, α-helix
and β-sheet, which are stabilized by hydrogen bonds.
Summary of Section 2.2
A protein consisting of more than a single polypeptide chain is described as
having quaternary structure, in which each polypeptide chain is called a
subunit. Weak interactions maintain these structures. Subunits can be either
identical or non-identical.
Summary of Section 2.2
Tertiary structure is the three-dimensional arrangement of the entire polypeptide
chain. Domains are independently folded globular units with intervening
irregular loops. Globular proteins are roughly spherical in shape, whereas fibrous proteins are long and thin. Tertiary structure is maintained
by weak interactions between amino acid R groups. Disulfide bridges can
form between two non-adjacent cysteine residues. Ligands attach at specific
binding sites. Tertiary structure can usually be determined by X-ray crystallography.
Summary of Section 2.2
Proteins consisting of a single polypeptide chain, such as RNAase, can be
reversibly denatured by treatment with appropriate reagents, indicating that
the primary structure determines the tertiary structure. Proteins with separate
polypeptide chains linked by disulfide bridges, or multisubunit proteins, often
do not reform after denaturation, and can lose all of their biological activity
with such treatment. Correct folding of some proteins is mediated by chaperone
proteins. Misfolded proteins in a eukaryote are degraded to their constituent
amino acids in proteasomes.
Summary of Section 2.2
Some proteins that are exported from cells undergo post-translational modification,
which can involve the addition of short sugar chains to form glycoproteins,
or the removal by proteolysis of sections of polypeptide chain from
precursor zymogens.
Summary of Section 2.2
A given cell produces a wide diversity of different proteins, ranging from a
few thousand synthesized by a prokaryote to an estimated 104 produced by a
human cell. In multicellular eukaryotes, different proteins can be produced in
different cell types.
Summary of Section 2.3
Proteins often form families, comprising groups closely related both in
structure and function—for example, the serine proteases.
Summary of Section 2.3
The vast majority of proteins in living organisms are enzymes, biological
catalysts. Enzyme names usually but not always end in -ase.
Summary of Section 2.4
The substrate binds at the active site of an enzyme, forming the enzyme–
substrate complex. Catalysis occurs and products are released. Conformations
of the active site and substrate are complementary; hence their interaction is
described as a ‘lock and key’ mechanism.
Summary of Section 2.4
Enzymes usually exhibit substrate specificity, and bind only a single or
limited range of substrates.
Summary of Section 2.4
Catalytic power describes the degree to which enzymes speed up the rates of
reaction, which can be up to 1014 times the rate of the uncatalysed reaction.
Summary of Section 2.4
The study of the rates of enzyme-catalysed reactions is known as enzyme
kinetics. Two key parameters are usually calculated for a given enzyme: vmax
(maximum rate of reaction) and KM (the Michaelis constant), which are most
conveniently calculated from a Hofstee–Eadie plot of v against v/[S]. vmax is
then the intercept on the vertical axis. KM can be calculated from either the
intercept on the horizontal axis (vmax/KM), or from the slope (−KM). K can be M
a measure of the affinity of an enzyme for its substrate: the lower the value of
KM, the greater the affinity, and vice versa.
Summary of Section 2.4
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