MCD Cells Alexandra Burke-Smith 1 1. Cells and Organelles Professor Michael Ferenczi ( [email protected]) 1. Understand what constitutes a cell, and the scale of cells and molecules Cell Biology Definition of a cell- the basic unit from which living organisms are made, consisting of an aqueous solution of organic molecules enclosed by a membrane. All cells arise from existing cells, usually by a process of division. The body is made up or organs and tissues which are made up on cells and extracellular fluid; a dense material often made of protein fibres embedded in a polysaccharide gel. Some cells can live independently (protozoa), whereas some divide to form colonies of genetically identical daughter cells When cells come together, they can ‘specialise’ by differentiating to give the organism an advantage Some protozoa form occasional colonies when individual cells specialise (e.g. slime moulds, dictyostelium) Cells assemble to form tissues These cells specialise: particular genes are switched on, triggered by signals from their immediate environment (developmental biology), which cause production of mRNA, travels out of cells to manufacture a particular protein. There are about 200 different types of cells in the body These genes produce enzymes which induce the formation of specialised cytoskeleton, organelles, cell-cell contacts, secretion and absorption The distribution of organelles within a cell is unsymmetrical, i.e. one end of the cell faces the lumen and the other the basal membrane, hence polarity is established Definition of polarity- refers to a structure such as an actin filament or a fertilised egg that has an inherent asymmetry so that one end can be distinguished from the other. There is an in-built developmental programme which then responds to external factors, and epigenetic modification of proteins then occurs (changes in the gene expression as a result of mechanisms other than the DNA sequence) Cells are attached to neighbouring cells by membrane junctions, which provide mechanical force and let chemicals through Scales Size of Cells: 10-20 micrometres in diameter Volume of a cell is measured in nanolitres Mass of a cell is typically 1 nanogram. Size of a virus: 10 nanometres A small protein: 40 nanometres Size of molecules: 0.2 nanometres in diameter To see the internal structure of cells, stains are used to look at specific organelles which exploit the differences in refractive indexes of the organelles The human eye is able to distinguish dimensions of objects in the millimetre-metre range, a conventional light microscope can be used for objects in the millimetre-micrometre range (however resolving power limited by wavelengths of light- 400-700nm), otherwise light outside the EM spectrum can be used
16
Embed
MCD Cells Alexandra Burke-Smith 1. Cells and Organelles · MCD Cells Alexandra Burke-Smith 1 1. Cells and Organelles Professor Michael Ferenczi ([email protected]) 1. Understand
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
MCD Cells Alexandra Burke-Smith
1
1. Cells and Organelles Professor Michael Ferenczi ([email protected])
1. Understand what constitutes a cell, and the scale of cells and molecules
Cell Biology
Definition of a cell- the basic unit from which living organisms are made, consisting of an aqueous solution of organic
molecules enclosed by a membrane. All cells arise from existing cells, usually by a process of division.
The body is made up or organs and tissues which are made up on cells and extracellular fluid; a dense
material often made of protein fibres embedded in a polysaccharide gel.
Some cells can live independently (protozoa), whereas some divide to form colonies of genetically identical
daughter cells
When cells come together, they can ‘specialise’ by differentiating to give the organism an advantage
Some protozoa form occasional colonies when individual cells specialise (e.g. slime moulds, dictyostelium)
Cells assemble to form tissues
These cells specialise: particular genes are switched on, triggered by signals from their immediate environment (developmental biology), which cause production of mRNA, travels out of cells to manufacture a particular protein. There are about 200 different types of cells in the body
These genes produce enzymes which induce the formation of specialised cytoskeleton, organelles, cell-cell contacts, secretion and absorption
The distribution of organelles within a cell is unsymmetrical, i.e. one end of the cell faces the lumen and the other the basal membrane, hence polarity is established
Definition of polarity- refers to a structure such as an actin filament or a fertilised egg that has an inherent
asymmetry so that one end can be distinguished from the other.
There is an in-built developmental programme which then responds to external factors, and epigenetic modification of proteins then occurs (changes in the gene expression as a result of mechanisms other than the DNA sequence)
Cells are attached to neighbouring cells by membrane junctions, which provide mechanical force and let chemicals through
Scales
Size of Cells: 10-20 micrometres in diameter
Volume of a cell is measured in nanolitres
Mass of a cell is typically 1 nanogram.
Size of a virus: 10 nanometres
A small protein: 40 nanometres
Size of molecules: 0.2 nanometres in diameter
To see the internal structure of cells, stains are used to look at specific organelles which exploit the
differences in refractive indexes of the organelles
The human eye is able to distinguish dimensions of objects in the millimetre-metre range, a conventional
light microscope can be used for objects in the millimetre-micrometre range (however resolving power
limited by wavelengths of light- 400-700nm), otherwise light outside the EM spectrum can be used
Thin sheet of fibres that underties the epithelium or endothelium
Selective barrier for macromolecules, type VI collagen network, laminas, type XV collagen
Type XV collagen- is manufactured within cell, then transported to form membrane. Provides strength. Extracellular fluid Definition of extracellular fluid: complex network of polysaccharides (e.g. glycosaminoglycans, cellulose) and proteins (e.g. collagen) secreted by cells. A structural component of tissues that also influences their development and physiology
4. Identify the essential characteristics of prokaryotic and eukaryotic cells.
Characteristics of all cells
All have a cell membrane that separates the outside from the organised interior
contain DNA as the genetic material (exceptions e.g. RNA virus)
Contain several varieties of RNA molecules and proteins (mostly enzymes).
Are composed of the same basic chemicals: carbohydrates, proteins, nucleic acids, minerals, fats and vitamins.
Regulate the flow of nutrients and wastes that enter and leave the cell.
Reproduce and are the result of reproduction.
Require a supply of energy.
are affected and respond to the reactions that are occurring within them and many of the environmental conditions around them; this information is continually processed to make metabolic decisions
Prokaryotes/Eukaryotes Definition of a prokaryote: major category of living cells distinguished by the absence of a nucleus or other membrane-bound organelles. Single-celled organisms comprising the kingdoms archaea and bacteria. Definition of a eukaryote: living organism composed of one or more cells with a distinct nucleus and cytoplasm. Include all forms of life except archaea, bacteria, and viruses.
Evolved from aggregates of prokaryotic cells that became interdependent and eventually fused to form a single larger cell
Have a higher degree of organisation than prokaryotes, in that they contain many organelles or structures separated from the other cytoplasm components by a membrane
Prokaryotes (Monera and Archaea ) Eukaryotes
No organelles Membrane-bound organelles
No nucleus Nucleus
Have external whip-like flagella for locomotion or hair-like pili for adhesion
May have cilia or microvilli on surface of cell membrane
MCD Cells Alexandra Burke-Smith
4
Simpler, smaller, haploid Cytoskeleton
Cell walls contain PEPTIDO-GLYCAN No cell wall- only cell membrane
Shapes: cocci (round), bacilli (rods), and spirilla or spirochetes (helical cells).
5. Understand that movements of molecules and organelles in cells and that the movement of cells are essential
processes
Cells are dynamic
Molecules move spontaneously by diffusion and Brownian motion, which provides the natural mixing of molecules
Other forms require energy from the hydrolysis of ATP; active transport, movement of organelles, tuning of hair cells in the ear, movement of cell membranes, growth and migration of cells, nerve growth and development from CNS to target organs, cell division and movement of chromosomes, muscle contraction and the heartbeat
All require specialised motor proteins Definition of a motor protein: protein such as myosin or kinesin that uses energy derived from ATP hydrolysis to propel itself along a protein filament or polymeric molecule
6. Explain the relationship of individual cells to the organisation of the whole body.
Why cell biology?
Development and repair is based on programming the cell cycle and turning on differentiation mechanisms
Cancer is when cellular development programs are failing
Infections occur when cellular defence mechanisms fail to prevent bacterial invasion
Viruses take over the chemical machinery of cells
7. Understand that cancer is a disorder of cell division
Definition of cancer: disease caused by abnormal and uncontrolled cell division resulting in localised growths, or
tumours, which may spread throughout the body.
Mutations that can lead to cancer:
Switch on “divide” signals.
Switch off “don’t divide” signals.
Loss of correction mechanism on DNA copying
Loss of escape mechanism from cell division
Loss of limit on number of times a cell can divide
Loss of control keeping cell within tissue boundaries
Ability to evade body defence mechanisms
Ability to recruit blood vessels to growing tumour
Ability to migrate into blood stream of lymph vessels
Ability to establish tumours in the “wrong” tissue.
Not all mutations cause cancer because most mistakes which occur during the copying of the genetic code
are removed/destroyed
Proliferation
Under a microscope, you can see migration and tighter packing of cell nuclei
evidence of division and stacking of cells
Eventually, secondary colonies form within the underlying connective tissue
The apoptotic cells then start dying because the tumour has outgrown the supply of oxygen and nutrients
MCD Cells Alexandra Burke-Smith
5
2. Infectious Agents Professor Christoph Tang ([email protected]) Why are we interested in infectious disease?
Global cause of death: 52.2 million people/year- 17.4% of all deaths in 2006. These patients also tend to be
young, therefore is a selective force from evolution pre-reproduction. Also results in decrease in life
3. Describe the permeability properties of a phospholipid bilayer with respect to macromolecules, ions, water and
organic compounds (including drugs). Distinguish simple diffusion, facilitated diffusion and active transport of ions
and molecules across cell membranes.
Lipid bilayers are permeable to...
Water
Some small uncharged molecules like oxygen and carbon dioxide
The lipid bilayer is more fluid than predicted, as it is coated in components giving water-absorbing properties
so less energy is required for the movement of substances across and within the membrane
Simple Diffusion: the movement of molecules and small particles across the semi-permeable membrane driven by a
difference in the concentration of molecules on either side.
Osmosis: the net movement of water molecules across a semi-permeable membrane driven by a difference in
concentration of solute on either side. The membrane must be permeable to water but not to the solute molecules
Active Transport: the movement of a molecule across a membrane against its concentration gradient driven by ATP
hydrolysis or another form of metabolic energy
Facilitated diffusion: the movement of hydrophilic (charged) molecules down their concentration gradient through
protein pores that hide the ionic charges from the hydrophobic core of the lipid bilayer. Proteins (or protein
assemblies) offer a water-filled channel. The channel can be ‘gated’- specific, i.e. a particular molecule may modify its
structure, opening the channel allowing the substance to pass through the membrane e.g. drugs.
Lipid bilayers are impermeable to...
Cations (K+, Na+, Ca2+) but some do leak through, down the concetration gradient.
Anions (Cl-, HCO3-)
Small hydrophilic molecules like glucose
Macromolecules like proteins and RNA
4. Categorise the functions of membrane proteins.
Definition of a membrane protein: a protein associated with a lipid bilayer; can either be integral (transmembrane,
monolayer-associated, or lipid-linked) or peripheral.
Definition of a membrane transport protein: any protein embedded in a membrane that serves as a carrier of ions or
small molecules from one side to the other
Membrane Proteins
Cell membranes and organelle membranes contain proteins, conferring new properties to the membrane.
They increase the fluidity of the membrane, as their position is not fixed and they diffuse in the plane of the
membrane
Protein composition is different in the inner and outer leaflets of the bilayer. In the core, the proteins have a
predominantly α-helical structure as they cross the lipid bilayer- as result of an orientation relating to their
functions
Protein composition is dependent on the cell/organelle type, i.e. myelin sheath has a lower protein percentage
composition, making it more rigid and a better insulator which makes it more suited to its function
Proteins (or protein assemblies) offer a water-filled channel. The channel can be ‘gated’- these specialised pores
provide a route for hydrophilic substances to move across the membrane down their concentration gradient.
This is known as protein mediated permeability.
MCD Cells Alexandra Burke-Smith
9
Functions of the Proteins
Transport (Sodium-Glucose transport)
Transmission of signals
Anchors to link intracellular actin filaments to extracellular matrix proteins (anchors the membrane to
macromolecules on either side)
Receptors for hormones and growth factors- detect chemical signals in the cell’s environment, and relay
them to the cells interior
Cell recognition and adhesion
Electron carriers in cellular respiration and photosynthesis in mitochondria and chloroplasts
Enzymes
5. Explain the movement of Na+ and K+ ions across the cell membrane against a concentration gradient and the
consequences of failure of such a movement.
Membrane Potential Membrane potential: voltage difference across a membrane due to a slight excess of positive charge on one side and
of negative ions on the other. A typical membrane potential for an animal cell plasma membrane is -80mV (inside
negative), measured relative to the surrounding fluid.
The Sodium-Potassium Pump
Electrostatic force due to the charge separation across the membrane
tends to move ions in a direction determined by its particular charge
The high concentration of fixed anions inside cells (the proteins) and their
accompanying cations (e.g. chloride ions) means that water is drawn into the
cells by the resulting osmotic gradient.
The high concentration of Na+ in the extracellular space means that Na+ will
tend to move down its concentration gradient into the cell.
The sodium-potassium pump (Na+-K+ ATPase) maintains the osmotic
balance and stabilises the cell volume by transporting 2K+ into the cell in exchange for 3Na+. This is
electrogenic (unequal transfer of charge) therefore requires energy.
It also provides a diffusion gradient for chloride ions. The
chloride ions tend to move inward down their concentration
gradient, but excess negative charge inside the cell (from non-
diffusible proteins, lipids, and the unequal distribution of
positive charge of K+ and Na+) tend to push Cl- ions back out of the cell.
The sodium-potassium pump consists of two polypeptide chains, alpha and
beta, with 1000 and 300 amino acids respectively. The alpha-chain spans the membrane 10 times, forming a
hydrophilic pore.
The K+ Na+ exchange is mediated by a series of conformational transitions of the pump molecule, which is
driven by phosphorylation of an aspartyl residue followed by hydrolysis of aspartylphosphate.
There are two consequences: Ionic gradients are created: less Na+ and more K+ inside the cell than outside. A
charge gradient is also created, which results in the inside of the cell being at a more negative potential than
the outside.
The potassium channel consists of four subunits and is highly specific for K+, as it mimics the environment
potassium is usually surrounded by.
The high concentration of potassium inside the cell means there is a tendency to move out of the cell, but
this would accentuate the voltage difference across the cell (making it more negative), therefore an
equilibrium is reached when the rate of inward movement = rate of outward movement.
MCD Cells Alexandra Burke-Smith
10
This equilibrium does not fully compensate for the electrogenic sodium-potassium pump, therefore the
membrane potential of a nerve or muscle cell at rest is about -80mV.
The Nernst equation describes how the distribution of ions leads to a membrane potential
Action potentials occur in elongated cells (nerves, muscle) when the membrane potential is disrupted by a
brief pulse of current which cause a massive influx of Na+ in the cell (depolarisation), which must then use
metabolic energy to reinstate the membrane potential.
Na+ channels become inactivated locally, preventing further Na+ entry.
Voltage-gated K+ channels open, to restore the resting membrane potential.
The process propagates down the nerve/muscle
Specific ion pumps
There are specific pumps for Na+, Ca2+ and H+, which use ATP hydrolysis to provide the energy.
Some pumps can work in reverse and generate ATP from an ion gradient, e.g. the F1-ATPase in mitochondria.
Other mechanisms exist for other substances that need to cross the membrane
6. Explain how the entry of glucose and amino acids into the cell against a concentration gradient is coupled to ATP
dependent Na+ transport.
Glucose transport
Glucose is membrane-impermeant.
Glucose moves down the concentration gradient into the cell
Glucose binds to a specific glucose transporter which functions by a flip-flop mechanism
The transport is ‘facilitated’.
Several different proteins. Some are insulin-sensitive Amino acids
Uses coupled transporters- symporters and antiporters
Move in the opposite direction to Na+ using ATP hydrolysis
Other transport mechanisms:
Pinocytosis: engulfment by the membrane of extracellular solute and small
molecules which end up in small intracellular membrane-bound vesicles.
Phagocytosis: engulfment by the membrane of extracellular objects such as bacteria, cell debris, other cells.
Again these end up in intracellular membrane-bound vesicles.
Exocytosis: movement of proteins and other molecules (e.g. hormones, blood clotting factors) from
intracellular vesicles towards the extracellular space by fusion with the cell membrane.
7. Explain how external chemical signals can be sensed at the interior of a cell.
It is not only substances that need to cross membranes. Signals need to cross membranes too.
Some use exocytosis, e.g. hormones.
Others use lipid-soluble molecules that cross membranes.
But other signals rely on trans-membrane receptors.
8. Be able to calculate the membrane potential from the Nernst equation
Where E is the membrane potential in V R = Gas constant, 8.135 J K-1 mol-1 F = Faraday‟s constant, 9.684 x 104 C mol-1
E x V =RT
zFln
[ X ]o
[ X ]i
MCD Cells Alexandra Burke-Smith
11
T = absolute temperature, -273 °C, At 25°C, T=298
Z = valence of the ion, 1 for Na+
9. Understand the role of membranes in synaptic transmission, using the neuromuscular junction as an example
The Neuromuscular junction The Neuromuscular junction or synapse is a highly complex structure involving pre- and post-synaptic membranes, pre-synaptic vesicles, invagination of the post-synaptic membrane, receptors and enzymes.
Depolarisation of the muscular post-synaptic membrane results in a propagated action potential.
The wave of depolarization extends into the t-tubules (invaginations of the cell membrane) to transmit the
activation signal into the core of each muscle cell in the motor unit.
Close contact with the sarcoplasmic membrane via triadic junctions involving the dihydropyridine (t-tubule
membrane) and ryanodine receptors (sarcoplasmic reticulum membrane) results in calcium release from the
sarcoplasmic reticulum.
Calcium diffusion into the myofilaments lattice and calcium binding the troponin on the thin filaments (actin)
in skeletal and cardiac muscle result in activation of the contractile machinery and contraction.
A2 NOTES (neuromuscular junctions and muscle contraction)
A neuromuscular junction is the point where a motor neurone meets a skeletal muscle fibre. As rapid muscle
contraction is frequently essential for survival there are many neuromuscular junctions spread throughout the
muscle. This ensures that contraction of muscle is rapid and powerful when simultaneously stimulated by action
potentials. All muscle fibres supplied by a single motor neurone act together as a single functional unit, known as a
motor unit.
1. When a nerve impulse is received at the neuromuscular junction, the synaptic vesicles fuse with the pre-
synaptic membrane and release acetylcholine
2. The acetylcholine diffuses to the post-synaptic membrane, altering its permeability to sodium ions,
depolarising the membrane
3. The acetylcholine is broken down by acetylcholinesterase to ensure that the muscle is not over-stimulated
4. The resulting choline and ethanoic acid diffuse back into the neurone, where they are recombined to form
acetylcholine using energy
Muscle stimulation
An action potential reaches many neuromuscular junctions simultaneously, causing calcium ion channels to open and calcium ions to move into the synaptic knob
Calcium ions cause the synaptic vesicles to fuse with the pre-synaptic membrane and release their acetylcholine
Acetylcholine diffuses across the synaptic cleft and binds with receptors on the post-synaptic membrane, causing it to depolarise.
Muscle contraction
The action potential travels deep into the fibre through T-tubules that branch throughout the sarcoplasm
The tubules are in contact with the sarcoplasmic reticulum, which has actively absorbed calcium ions from the sarcoplasm
The action potential opens the calcium ion channels on the sarcoplasmic reticulum, flooding into the sarcoplasm down a diffusion gradient
The calcium ions cause Troponin molecules to change shape, which causes the Tropomyosin molecules to pull away from the actin binding sites
The ADP molecule attached to the myosin heads bind to the actin filament and form a cross-bridge
The myosin heads then change their angle, pulling the actin filament along and releasing a molecule of ADP
An ATP molecule attaches to each myosin head, causing it to become detached
MCD Cells Alexandra Burke-Smith
12
The calcium ions then activate the enzyme ATPase, which hydrolyses ATP to ADP, providing the energy needed for the myosin head to return to its original position
The myosin head with ADP then re-attaches itself further along the actin filament and the cycle is repeated as long as nervous stimulation continue
Subgroups: Cytotoxic T-cells, helper T-cells, Supressor T-cells (no info required)
4. Monocytes: Large, single horse-shoe nucleus. Appear after granulocytes and in tissue become macrophages
(“big eaters”), engulfing micro-organisms, tissue debris and dead polymorphs. Secrete inflammatory
mediators and stimulate angiogenesis (vessel growth = repair). Ingest and store antigens, present modifies
antigen to lymphocytes
MCD Cells Alexandra Burke-Smith
16
Normal Leukocyte count
Total = 3.5-10.0x109/L
Neutrophils 2.5-7.5 (40-75%)
Eosinophils 0.04-0.4 (1-6%)
Basophils 0.01-0.1 (<1%)
Monocytes 0.2-0.8 (2-10%)
Lymphocytes 1.5-4.0 (20-25%)
Leukocytosis = raised leukocytes due to infection, cancer etc. Leukopenis = low leukocytes due to chemotherapy, HIV etc
7. Explain simply the major functions of platelets
Derived from megakaryocytes
2 - 3 µm diameter (small)
Normal platelet count 25 x 104/ml
Life span 8 - 10 days
Granules
Many organelles, no nucleus Haemostasis
Express surface receptors for platelet activators in the presence of collagen in vessels or thrombin from coagulation cascade
They adhere to exposed collagen (in wound or atherosclerosis), and release granules which promotes platelet aggregation
Coagulation cascade- Produce thromboxane A2 from cycloxygenase enzyme, which is involved in clot/thrombus formation
Aspirin inhibits cycloxygenase and is therefore anti-clotting. The vascular endothelium also produces prostacyclin and nitric oxide which inhibit platelet activation.
8. List the major functions of plasma
Fluid component of blood, acts as transport carrier
“Organic and inorganic substances dissolved in water”
Water and proteins
Plasma proteins: exert osmotic pressure to maintain blood volume. Albumins + globulins are carrier
molecules e.g. hormones, bile salts, water insoluble drugs. Fibrinogen is present for clotting. If the balance of
proteins is changed, can lead to blood pressure and kidney function problems
Serum: plasma with proteins removed due to clotting