Alex's Cells

MCD Cells Alexandra Burke-Smith

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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

mailto:[email protected]


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2. Demonstrate the following on a suitable transmission electron micrograph: nucleus; nucleolus; nuclear envelope;

mitochondrion; rough endoplasmic reticulum; smooth endoplasmic reticulum; ribosomes; Golgi apparatus;

secretory granule; plasma membrane; cytoskeletal components.

Transmission electron microscopy- uses a beam or electrons and magnetic coils to focus the beam. Specimen is inside

a vacuum and very thing. Electron-dense heavy-metal contrast is used to absorb/scatter electrons, removing them

from the beam as it passes through the specimen. Minimum resolving power of 0.2nm.

Definition of organelles- separate, recognizable sub-cellular structures that perform specialized functions within the

cell

Organelles

Nucleus: Most prominent organelle of a eukaryotic cell. Enclosed within two concentric membranes and

contains DNA organized into chromosomes.

Nucleolus: large structure in the nucleus where ribosomal RNA is transcribed and ribosomal subunits are

assembled

Nuclear pores: specialised protein complexes which carefully control and filter the molecules moving

between the nucleus and the cytoplasm. Note: adenoviruses have a an advantage as they can travel

through/block nuclear pores.

Mitochondria: membrane-enclosed organelle, about the size of a bacterium, that carries out oxidative

phosphorylation and produces most of the ATP in eukaryotic cells.

Vesicles: small, membrane-enclosed, spherical organelles in the cytoplasm of a eukaryotic cell

Secretory granules: membrane-enclosed organelle in which molecules destined for secretion are stored

prior to release. Visible as small solid object due to darkly staining contents.

Golgi Apparatus: membrane-enclosed organelle in eukaryotic cells where the proteins and lipids made in the

endoplasmic reticulum are modified and sorted for transport to other sites around the cell

Centriole: short cylindrical array of microtubules, usually found in pairs at the centre of a centrosome in

animal cells. Also found at the base of cilia and flagella (called basal bodies).

Ribosome: particle composed of ribosomal RNAs and ribosomal proteins that associate with mRNA and

catalyses the synthesis of protein (translation)

Cytoskeleton: system of protein filaments in the cytoplasm of a eukaryotic cell that gives the cell shape and

the capacity for directed movement. Its most abundant components are actin filaments, microtubules, and

intermediate filaments.

Lysosome: intracellular membrane-enclosed organelle containing digestive enzymes, typically those most

active at the acid pH found in these organelles.

Liposomes: artificially prepared vesicles made from the lipid bilayer.

Plasma membrane: the membrane that surrounds a living cell.

Cilia: hairlike extension on the surface of a cell with a core bundle of microtubules and capable of

performing repeated beating movements. Cilia, in large numbers, drive the movement of fluid over epithelial

sheets, as in the lungs.

Endoplasmic reticulum: membrane-enclosed component in the cytoplasm of eukaryotic cells, where lipids

and secreted and membrane-bound proteins are made

Smooth endoplasmic reticulum: region of ER not associate with ribosomes, but involved in the synthesis of

lipids.

Rough endoplasmic reticulum: region of the ER associated with ribosomes and involved in the synthesis of

secreted and membrane-bound proteins.

Sarcoplasmic reticulum: specialist form of SER found in smooth and striated muscle.


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3. Describe the predominant types of molecules in a cell

Cytosol Definition of cytosol- contents of the main compartment of the cytoplasm, excluding membrane-enclosed organelles and cytoskeletal components.

Intracellular fluid

Site of many chemical reactions including the manufacture of proteins and Glycolysis

Constituents: soluble proteins, sugars, ions (K+, Na+, Mg2+, Ca2+, PO4 2-, Cl-), nucleotides (ATP, cAMP: cyclic adenosine monophosphate, GTP: guanosine triphosphate), amino acids, mRNA, t-RNA, lipids and peptides

Basement Membrane

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

Ions (Na+, Cl-, PO4 2-, CO3 2-, Mg2+, Ca2+)

Soluble proteins, carbohydrates and sugars

Vitamins, amino acids, hormones, nucleotides (ATP), lipids, cholesterol

Lymph, plasma, saliva, urine, bile, sweat, milk etc

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


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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


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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

expectancy.

Community infection e.g. bacterial meningitis

Hospital infections e.g. clostridium difficile (C. Diff) Methicillin Resistant Staphylococcus aureus (MRSA)

Rise of antimicrobial resistance e.g. mycobacterium tuberculosis

Bio-terror e.g. bacillus anthracis (Anthrax)

Causes of many different diseases, e.g. Helicobacter pylori causes stomach ulcers, Human papillovirus (HPV)

causes cervical cancer

New emerging disease e.g. H1N1- Swine Flu

Improves our understanding of Biology e.g. identification of first whole genome of free living organism was

H. Influenza, and identification of restriction enzymes from E.coli

Why do they continue to cause a problem?

Mutations: point mutation rate of virus= 10^-11, whereas human cells= 10^-5. The mutation rate is

significantly faster in infectious diseases, making successful treatment more difficult

Replication: replication rates of infectious diseases when compared to humans is significantly faster

(E.coli=20 mins, homosapiens=26yrs), which means spread of infectious disease poses a problem

1. List the main types of infectious agents causing disease and name their distinguishing features

Viruses: obligate intracellular parasite consisting of nucleic acid (RNA or DNA) enclosed in a protein coat that

make use of a host cell to undergo intracellular replication, and then divide and bud out of the host cell.

Viruses show host specifity but have the ability to infect almost all other life forms including bacteria.

Bacteria: prokaryotes that replicate by binary fission, and contain a chromosome of DNA but no nucleus. The

DNA is condensed into nucleoids. Bacteria are widely distributed in nature, but some are pathogenic

Fungi: single-celled eukaryotes that exist as yeasts or filaments or both which cause cutaneous, mucosal and

systemic mycoses. Yeasts bud or divide, whereas filaments (hyphae) have cross walls or septa. Systemic

infections with fungi often affect immuno-comprimised people.

Protozoa: unicellular, free-living, non-photosynthetic, motile eukaryotic organisms which include intestinal,

blood and tissue parasites which acquired by ingestion or through a vector. They replicate in the host by

binary fission or by formation of trophosoites inside a cell, and often have complex life cycles involving two

hosts.

Helminth parasites: Multicellular eukaryotes of the Kingdom of Metazoa, which are visible to the naked eye

and have life cycles outside of the human host.

2. Give examples of the agent and the disease it causes

Viruses: SARS, Influenza and HIV (use of reverse transcriptase breaks DNA makes RNA makes protein rule)

Bacteria: Neisseria meningitis, mycobacterium tuberculosis, shigellosis (shigella spp. Is an invasive pathogen-

responsible for 50% of deaths from diarrhoeal disease) and septicaemia (characterised by rapid progression,

septic shock, severe inflammatory response)

Protozoa: malaria (plasmodium spp.- vector is female anophelese mosquito which targets red blood cells)

and leishmania spp. (vector is sand fly- targets white blood cells)

Helminth parasites: tapeworms, flukes and roundworms



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3. List the differences between prokaryotic and eukaryotic cells

Prokaryotes (Monera and Archaea ) Eukaryotes

No organelles Membrane-bound organelles

No nucleus- but rather a condensation of DNA called a nucleoid

Nucleus

Have external whip-like flagella for locomotion or hair-like pili for adhesion

May have cilia or microvilli on surface of cell membrane

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).


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3. Cell Membranes Professor Michael Ferenczi ([email protected])

1. Explain the formation of phospholipid bilayers in an aqueous environment.

Structure of the plasma membrane

In an aqueous environment, phospholipids form micelles/droplets (droplets in cells

are known as liposomes) or bilayers in which their hydrophobic tails pack together

to avoid water

Phospholipids are amphipathic- they have a polar/hydrophilic phosphate head with

hydrophobic aliphatic fatty acid chains which may be saturated or unsaturated

Unsaturated hydrocarbon chains have cis-double bonds, forming a kink in the fatty acid chains. Fully

saturated chains have no kinks and can be packed more densely. The fractions of saturated and unsaturated

fatty acids therefore affects the rigidity/fluidity of the membrane

Common constituents are phosphatidylethanolamine, phosphatidylserine, sphingomuelin and

phosphotidylcholine (also known as lethicin)

Membranes also contain cholesterol which increases the membrane rigidity. It has a

polar head (containing a hydroxyl group) and a rigid planar steroid ring structure.

The membrane is asymmetric as glycolipids

(lipids with carbohydrates attached) are on the

extracellular side of the membrane with negative

charges on the inside of the cell.

The phospholipids rarely “flip-flop” (change sides of the membrane), as

this requires too much energy for the hydrophilic head to pass through the hydrophobic core, but they

rapidly undergo lateral diffusion within the monolayer- moving around the distance of 1 large bacterial cell

(2 micrometres) per second

Properties of the membrane

Selectively permeable

Impermeable to macromolecules, biochemical intermediates.

Permeable to water molecules and a few other small, uncharged molecules like oxygen and carbon dioxide,

nutrients, waste products.

Transfer of information (signal transduction).

2. Draw the structure of phosphatidylcholine and identify the component parts.



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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.


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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.


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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


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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


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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


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4. Blood and Blood Cells Dr Michael Emerson ([email protected])

1. List the main functions of the blood

Connective tissue

Transport (connects every part of body)

Heat distribution

Immunity

Haemostasis (process whereby bleeding stops)

Support (e.g. external genitalia - VIAGRA)

MAINTAIN HOMEOSTASIS (constant internal environment)

Blood volume (5l male; 3.5l female) How blood serves cells

Acts as a transport medium which carries the products of digestion e.g. fatty acids, amino acids and

glucose, hormones, vitamins and oxygen to their target cells

Removes metabolic waste e.g. lactic acid and urea, carbon dioxide, excess heat and water from cells so

that they can be removed from the body

2. List the major components of blood

Erythrocytes- red blood cells

Leukocytes- white blood cells

Platelets- derived from megakaryocytes in bone marrow; involved in coagulation and clot formation

Plasma- fluid component of blood which acts as the carrier for all blood cells

3. Describe the essential features of the erythrocyte and list its major functions

Primary function: respiratory transport. Binds with oxygen to form oxyhaemoglobin for transport to cells.

Binds with carbon dioxide to form bicarbonate from carbonic anhydrase for removal from cells.

Biconcave disc maximises surface area for diffusion of oxygen

No nuclei or organelles

Packed with haemoglobin

7.5micrometres and flexible, important as able to penetrate through to smallest vessels

Molecules on the surface confer blood group

Regulation of erythrocytes

Erythrocytes are regulated by the kidney in a negative feedback loop.

1. Low oxygen

2. kidney (+testosterone- hence higher levels of erythrocytes in men) produces erythropoietin (hormone)

3. bone marrow stem cells differentiate to form erythrocytes (erythropoiesis) blood

4. haemoglobin increases

5. blood oxygen increases

6. negative feedback- back to the beginning

Life Cycle of erythrocytes

Produced in bone marrow from precursors which produce haemoglobin then lose organelles

Immature erythrocytes contain ribosomes: reticulocytes.

High levels of circulating reticulocytes are useful in diagnostics e.g. anaemia, chemotherapy



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Removed through reticulo-endothelial system (phagocytic macrophages in spleen)

Lifespan = 120 days (short, no nuclei; 1% or 250 billion cells per day)

Dependent on dietary iron (meat, egg yolk, nuts), iron deficiency causes anaemia

4. Explain the importance, basic structure and role of haemoglobin

Globular protein consitisting of 4 subunits (polypeptide chains),

each with a prosthetic haem group.

Haem contains a ferrous iron (FE2+), each of which binds with one

molecule of oxygen

Hb = deoxyghaemoglobin. Hb02 = oxyhaemoglobin

100ml blood: 15.8g (male)

13.7g (female)

Due to the compact conformational shape of globin molecules,

haemoglobin has a low affinity for oxygen

However, oxygen binding breaks the conformation and opens up the structure, allowing the second oxygen

molecule to bind more easily

This means in a high oxygen environment (e.g. the lungs), cooperative binding can take place which allows

more oxygen to be carried

Different form in foetus with higher O2 affinity, as foetus must obtain oxygen from mother’s blood

Carbon monoxide combines with haem at a 200X greater affinity to form carboxyhaemoglobin, reducing

oxygen binding, which can deprive cells of oxygen resulting in cell death.

Key red cell Parameters

Concentrations of haemoglobin (g/dl)- men: 13.5-16.5, women: 11.5-14.5

Red cell count- men 5.4x1012/l, women 4.8x1012/l

Haematocrit (packed cell volume PCV): men 0.40-0.54/1.00, women 0.35-0.47/1.00

PCV is the proportion of blood which are erythrocytes

Normal MCV (mean cell volume) 82-99fl

Normal MCH (mean cell haemoglobin) 27-33pg

Normal MCHC (mean cell haemoglobin concentration) 32-34 g/dl

5. Define anaemia and list the major causes and subclasses and relate the types of anaemia to red blood cell

volume

Definition of anaemia: low blood haemoglobin concentration

Three main types:

Microcytic (small MCV): Failure of haemoglobin synthesis (and hence smaller erythrocytes) caused by an

iron deficiency as a result of gradual blood loss e.g. menstruation, GIT lesions or cancers and parasitic

infection

Normocytic: red blood cell production is normal, but is a result of acute blood loss

Macrocytic (large MCV): During erythropoeisis, DNA synthesis and cell division fail and reduced division of

progenitor cells so fewer but larger erythrocytes. Folic acid and vitamin B12 are essential for division. Caused

by pregnancy (lack of folic acid), autoimmune disease, pernicious anaemia (destroys uptake of B12 in gut),

and in vegetarians and vegans- all caused by lack of vitamin B12.


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Sickle Cell Anaemia

Haemoglobin point mutation

Causes distorted erythrocytes, which are destroyed by macrophages in the spleen, resulting in anaemia.

Homozygous Genotype- disease manifests.

Heterozygous Genotype- sickle cell trait, partial malarial resistance.

Polycythemia

Excess erythrocytes, high PCV (haematocrit)

Common problem living at high altitude

Increased viscosity of blood leads to heart problems.

6. Explain simply the major functions of leukocytes and explain simply the concepts of immune responses

and passive immunity

Leukocytes

White blood cells

Use circulation for transport

Travel near capillary wall and invade tissue space to fight infection

Classified by structure and dye binding (you can see the granules and nucleus)

Main Types: 1. Polymorphic granulocytes: Segmented nucleus, full of cytoplasmic granules. First on scene - adhere to blood

vessels in infected area and migrate into tissue. Engulf, kill and digest microorganisms. Main types are

neutrophils (phagocytic), eosinophils (allergic + asthma responses), basophils (histamine-producing). Release

inflammatory mediators: toxic oxygen products, digestive enzymes, vasodilators, chemotaxins.

2. B- Lymphocytes: Mature in bone marrow- involved in Humoral (antibody-mediated) immunity. Foreign

antigen → RNA synthesis → immunoglobulin (antibody) production. Immunoglobulins: IgM; IgG; IgA; IgD;

IgE. Antibody-antigen reactions: assist phagocytosis by precipitation; agglutination (clumping) or coating in

antibody (opsonisation) or prevent attachment of micro-organism to tissues (neutralisation). In the primary

immune response, it is the first exposure, so antibodies appear after latent period, peak then fall. The

Secondary response will be greater, quicker, longer response due to memory cells (long lived B-

lymphocytes). Passive immunity: inject immunoglobulins (vaccine) or cross placenta (colostrum in some

species)

3. T- Lymphocytes: Derived in bone marrow, but migrate to thymus where they aquire surface antigenic

molecules and become immunologically competent. Involved in cellular immunity: Circulate → foreign

antigen → blast transformation (rapid cloning) → with receptors for antigen. Activated T-lymphocytes

release chemotaxins (attract macrophages); lymphotoxin (kills cells); interferon (kills viruses).

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


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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

Key components Nutrients Glucose, Lipids, Amino acids Hormones Thyroxine, Cortisol,

Erythropoietin Proteins Clotting factors, Albumin,

Globulins Inorganic ions Na, K, Ca, PO4, HCO3 Products of metabolism Urea, Lactic acid

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