What is a cell?

Some basic concepts about cells:1. Fundamental survival unit of life, may

reproduce (replicate), most carry genetic information

2. A machinery which performs function(s). Complicated components inside the cell to achieve homeostasis

3. Differentiated cells form tissue. Different tissues form an organ (larger, more complex machines). Different organs “communicate” with each other in an organism to achieve multi-functions and overall homeostasis

Homeostasis: a very important concept in Physiology

Maintain a relatively stable internal environment despite fluctuations (changes) in the external environment.

The stable internal environment is for the health of the cell or the body.

Therefore, blood glucose, blood pressure, body temperature, blood Na level etc can be maintained in a predictably and relatively stable level.

Neuron neurotransmitter Neuron

Target eg. muscle

Endocrine cells Hormones Endocrine cells

Target cells

Inter-cellular signaling

eg kidney cellsHormones (autocrine)

Adjacent Cells(paracrine)

Body: 60% water

2/3 of the body water is inside the cell

Therefore, 1/3 of body water is extracellular (outside the cell)

Extracellular fluid: 80% is interstitial fluid (fluid between cells); 20% is plasma

Life begins/proliferate in aqueous (water) environment.

But how to “house” or keep a cell’s own stuff from losing into the environment?

How to prevent external unwanted things from going into the cell?

Water and Oil Do NOT MIX!!!

water

oil

Saline solution

cell

A film oflipid

Anaqueous(“watery”)interior

The cell’s “properties”are kept inside

oil

But there has to be some exchange between the cell and the environment!

K+

Ca2+

glucose

Amino acid

O2CO2

“channels”

transporters

Product eg. Insulin in -cells

OrganicSubstancesEasily go thru

Charged SubstancesDifficult to gothru

waterOnly slightly permeable

Free diffusion

Down the gradientDown the gradient

The cell has to sense and respond to the environment!

receptorreceptor

Signal eg. hormone, neurotransmitter, chemical, nutrient, drug

Signal: eg. Light, odour, mechanical stress

Response!!! Contractionsecretion

Shape changesCell division(growth)

Cell death (necrosis,apoptosis)

Intracellular Signaling

A cell is always exchanging materials and information with the environment, and will take actions accordingly in a specific and desirable manner.

The cell is like a fortress, and the membrane is like the fortress wall.An unhealthy membrane will result in permeability to substances that otherwise would not enter the cell.Trypan blue exclusion (cell viability) test: dead cells are stained with the dye trypan blue, easily observed under microscope.

The cell membrane has “workers” that, like soldiers at the fortress wall, identify, select and transport who/what can enter/leave the cell.

Here, we look closely at some basic machines which work in a piece of healthy membrane.

Note: role of cholesterol? Keeping membrane rigid. Should not be too much, not too less.

Different cell types differ in their lipid-to-protein ratio and theirunique set of membrane proteins

glycolipid

Protein molecules can move around but never “flip-flop”

Phosphatidyl choline Phosphatidyl ethanolamine

Phosphatidyl inositol Phosphatidyl serine

Bilayer is asymmetricEg. In red blood cellsPPC at outer leafletWhile PPI, PPS and PPE at inner leaflet

receptor

G protein

Phosphatidyl inositol 4,5 bisphosphate (a phosphorylated derivative of PPI and a minorlipid at the inner leaflet cleaves, upon hormone stimulation, into inositol 1,4,5 trisphosphate (IP3) and Diacylglycerol (DG). These two are released into the

cytosol as important INTRAcellular messengers

IP3Ca

DG

Protein kinase C

Protein phosphorylation

PLC

No ATP Involved.But ATP hasalready been spent in main-taining the gradient shownin red.

(facilitated transport)

K

Na

Ca

Ca

ATP-driven active transport (pumps)

K

Na

Ca ER in non-muscle cells

SR (sarcoplasmic reticulum) in muscle cells

Models showing how active transport might operate.

The transported solute binds to the proteinas it is phosphorylated (ATP expense).

The opening and closing of ion channels results from conformational changes in integral proteins.Discovering the factors that cause these changes is key to understanding excitable cells.

Figure 4-7

Difference between passive diffusion & facilitated transport

1.Facilitated transport is much faster as the transported molecules never traverse the hydrophobic core of the membrane.

2.Facilitated transport is specific.

3.Facilitated transport shows saturation. Has maximum transport rate.

In simple diffusion,flux rate is limited only by the concentration gradient.

In carrier-mediated transport, the number of available carriersplaces an upper limit on the flux rate.

Figure 4-9

(facilitated transport)

Glut1 is an example of uniporter

Insulin promotes glucose uptake into cells such as skeletal muscle cells, hence lowering glucose level in blood. Here we see the cross-talk between receptor and transporter. Insulin receptor failure causes type II diabetes.

Na glucose

Sodium/glucose Symporter

Na

Ca

Sodium/Ca antiporter (exchanger)

Secondary active transport uses the energy in an ion gradient to move a second solute.

Figure 4-13

Figure 4-15

Diverse examples of carrier-mediated transport.

                                        

                                           Fig 1. Agre’s experiment with cells containing or lacking aquaporin. Aquaporin is necessary for making the 'cell' absorb water and swell.

Water channels are only in certain cells, notably red blood cells and epithelial cells of the renal collecting duct.

The concept of osmolarity: hypoosmolarity and hyperosmolarity.

Cells in hypertonic solutions shrink, while in hypotonic solution swell. Only true in certain cells having water channel!

Hypotonic solution

Generation of membrane potential:

1.Driven by ATP (energy), Na/K ATPase (Na/K pump) establishes

the gradients for Na and K.

2. Membranes are most permeable to K, only slightly permeable to Cl and Na.

3. Negatively charged protein (A-)

are immobile and therefore do not cross the membrane.

4. A negative charge is established at the cytosolic side of membrane.

5. Further K outward movement will increase the negative charge. The latter will become eventually big enough to counteract K movement, and an equilibrium potential is reached: membrane potential

*Concept of electrochemical gradientMolecular Cell Biology 4th edition

Neuron

(K leakchannels)

2.3RT/zF: ~60 mV at biological temperatures (monovalent cations).

For K at physiological setting: Ek = -92 mVIf [K] is the same at both side (ie. No gradient), Ek = 0 mV

Adopted from Stephen Wright, Ph.D

Note: If membrane only permeable to K, then membrane potential will be VK

If membrane only permeable to Na, then membrane potential will be VNa

If membrane only permeable to Cl, then membrane potential will be VCl

Goldman equation

However, membranes are usually permeable to Na, K and Cl

Note that permeability to these 3 ions differ: For example, in neurons, PK is about 10 times of PNa or PCl

Cl channels are so few at the membrane that they do not contribute much to the resting membrane potential

Some predictions:

Opening of K channels causes hyperpolarization

Opening of Na channels causes depolarization

Opening of Cl channels causes hyperpolarization

What is the purpose of the membrane potential?

The K and Na gradients represent a form of STORED ENERGY

Sodium channel (voltage-gated)opening initiates the action potential (AP). AP is theactivation signal that spreads along the neuron.

Symporter

Modified from Dr Tomoko KamishimaDepartment of Human Anatomy and Cell BiologyUniversity of Liverpool

1-2 mM Ca

Ca

[Ca] = 50-200 nM--

- - - -

The Ca ion gradient is extremely steep!!!

Ca conc. in cytosol very low. Indeed an intracellular signal.

Two important second messengers(intracellular signals):

1. Ca2+: activates Ca2+/calmodulin-dependent protein kinasewhich phosphorylate a number of proteins.

2. cAMP Some hormone receptors are coupled to Gs (stimulatory G protein), which in turn activates adenylate cyclase (AC).AC converts ATP to cyclic AMP. cAMP activates protein Kinase A, which cause protein phosphorylation.

Binding of ligands to membrane-spanning receptorsactivates diverse response mechanisms.

Figure 5-5

Binding of the ligand to the receptoralters the receptor’s shape, which then opens (or closes) an ion channel.

Figure 5-5a

Binding of the ligand to the receptor alters the receptor’s shape, which activates its enzyme function, phosphorylating an intracellular protein.

Figure 5-5b

Binding of the ligand to the receptor alters the receptor’s shape, which activates an associated enzyme function, phosphorylating an intracellular protein.

Figure 5-5c

Binding of the ligand to the receptor alters the receptor’s shape, which activates an associated G-protein, which then activates effector proteins,i.e., enzyme functions or ion channels.

Figure 5-5d

The cyclic AMP second messenger system.

Figure 5-6

Figure 5-8

The cAMP system rapidly amplifies the responsecapacity of cells: here, one “first messenger” ledto the formation of one million product molecules.

Amplification is the key concept here!!!

Cells can respond via the cAMP pathwaysusing a diversity of cAMP-dependentenzymes, channels,organelles, contractile filaments, ion pumps, and changes in gene expression.

Figure 5-9

This receptor-G-protein complex is linked to and activates phospholipase C, leading to an increase in IP3 and DAG, which work together to activate

enzymes and to increase intracellular calcium levels.

Not all responses to hydrophilic signals are immediate:

Increases in gene expression can occur, and the resulting proteins can increase the target cells’ response.

Figure 5-4

… but at the target cellthe signal moves easily through the membraneand binds to its receptor.

This hydrophobic signal requires a carrier protein while in the plasma …

Let’s look at the major organelles

The nucleus is the largest organelle of the cell.Double-membraned: Two lipid bilayers. Nucleus keep the genetic material: DNA. The nuclear pore allows exchange between nuclear content and the cytosol eg. mRNA, made thru transcription of the DNA, is exported from the nucleus thru the nuclear pore into the cytosol for protein synthesis. Nucleolus is the place where rRNA is made and ribosomal proteins are added to rRNA.

NUCLEUS

The DNA code is “transcribed” into mRNA.

RIBOSOMES

The mRNA is “translated” to give instructions for proteins synthesis.

Figure 3-16

GENES “CODE FOR” PROTEINS

The “triplet code” of DNA determineswhich amino acid will be placed in

each position of the protein.

(note: mRNA intermediate not shown)

Figure 3-17

Glucose

GlycolysisIn the cytosol

Pyruvate

Oxidative phophorylation

ATP

ATP

“Cleaner” inside the cell

The lumen is acidic which facilitates the digestion. Digestion of engulfed foreign particles.Digestion of protein and peptides by proteasesDigestion of RNA, DNA by nucleases.

Cytoskeleton

Function: Scaffolding (structural stability)Cell shape changes, Movements

(microfilament)

(tubulin as subunits)

The cytosol itself is composing of a lot of proteins, a lot being enzymes responsible for various metabolism. In some cells glucose is stored as polymer (glycogen). In specialized cells (adipocytes) fat is stored in large amount as triacylglycerides.