Cells and their organelles and stem cells

Two types of polarity found in cells?

apical and basal polarity

Define apical basal polarity

Apical-basal polarity is the property in epithelial cells where they develop distinct apical (top) and basal (bottom) surfaces, forming an axis that is essential for cell and tissue function

Biomembrane functions

1. Regulate the transport of solutes (import/export)

2. Receive information from microenvironment (receptors)

3. Mediate cell to cell communication

4. Has capacity for expansion/growth (and movement)

Effect of cholesterol affects the fluidity of the

membrane

(more cholesterol reduces fluidity)

4 types of proteins

Transporters, anchors, receptors, enzymes

Example and function: Transporters

Na+ pump

actively pumps Na+ out and K+ into cells

Example and function: Anchors

Integrins - A type of protein found on the surface of cells that helps them attach to, and communicate with, nearby cells

Link intracellular actin filaments to extracellular matrix proteins

Example and function: Receptors 

Binds extracellular PDGF to generate intraceulllar signals that cause cells to grow

Membrane is selectively permeable

Mitochondria

The ‘energy powerhouse’

❑ site of ATP generation

❑ clustered in cells that have

high energy demands e.g.

muscle cells

❑ Outer membrane is highly

permeable

❑ Inner membrane folded into cristae,

where oxidative phosphorylation

takes place

❑ Matrix a mixture of enzymes

Generation of ATP in the mitochondria

Movement of electrons

coupled to pumping of

protons

❑ Creates a proton motive

force

❑ Electrochemical gradient

generates ATP through

ATP synthase

Endoplasmic Reticulum (ER)

❑ Site of protein synthesis,

protein complex assembly

and modification

❑ Rough ER surrounded by

ribosomes

❑ Smooth ER lipid and

steroid hormone synthesis

❑ Transport of materials

within the cell

❑ Where transported in the cell dependent

on 15-60 aa signal sequences (usually

removed afterwards)

Golgi Apparatus

❑ Modification, packaging and sorting of
proteins and lipids - enhancing complexity
❑ Destined either for secretion or for another
organelle or cell membrane via vesicles

Lysosomes

❑ Vesicles packed with

degradative enzymes

❑ Low pH

❑ Digest and destroy

Peroxisomes

❑ Small vesicles for reactive

H2O2 generation

❑ Oxidation reactions and

breakdown of fatty acids

Main function of cytosol

contains many metabolic pathways, protein synthesis

Main function of nucleus

Contains main genome,DNA and RNA synthesis

Endoplasmic reticulum

Synthesis of most lipids

synthesis of proteins

Main function of golgi apparatus 

Modification, sorting, packaging, of proteins and lipids for delivery or secretion

Main function of lysome

Intracellular degradation

Cytoskeleton function

Structural - supports the

plasma membrane

❑ Controls cell shape (and

therefore often function)

❑ Pulls the chromosomes

apart during mitosis

❑ Drives and guides the

intracellular transport of

organelles, proteins

❑ Enables some cells to

move

Three major components of the cytoskeleton

• intermediate filament

• microtubules

• actin filaments

intermediate filaments

❑ 10 nm diameter filaments twisted into ropes to provide tensile strength

❑ Needed to maintain cell shape, flexible, withstand mechanical stress

❑ Made up of a family of fibrous proteins:

- keratin filaments in epithelial cells

- vimentin in many other cells

- neurofilament proteins in neurones

- lamins within the nucleus

Actin filaments

Polymers of actin monomers

❑ Necessary for movement, especially cell surface

❑ Can form contractile bundles and microvilli (strength

in number) – crosslinked bundles and networks

❑ Carry cargo-bearing motor proteins

❑ May associate with myosin to form powerful

contractile structures e.g. Muscle sarcomere

Microtubules

❑ 20 nm diameter polymers of tubulin dimers

❑ Rapidly assemble and disassemble, polar

❑ Organised from structures such as the centrosome

❑ Carry cargo-bearing motor proteins

❑ Important in cell shape and movement (cilia)

❑ Form the spindle in mitosis

Two families of motor proteins

1. Kinesins (out)

2. Dynein (In)

Molecular motors are dependent on

ATP

2 types of motor proteins size and speed

Dynein

0.2- 60 m/sec

Kinesin

(0.02-2 m/sec)

Defining properties of stem cells

❑ They are not terminally differentiated

❑ They can divide without limit

❑ When a stem cell divides, each

daughter can either (1) remain a stem

cell; (2) embark on a course to terminal

differentiation

❑ They are required wherever there is a

recurring need to replace differentiated

cells

❑ They are maintained in stem cell niches

keeping them undifferentiated (wnt

signaling pathway)

Definition of Terminally differentiated

Specialised cells performing a specific

function that cannot proliferate further, will die and will need to be replaced

Definition of stem cell

Relatively undifferentiated self-renewing cell that produces

daughter calls that can either differentiate into more specialized cell types

or can retain the development potential of the parent cell

Definition of pluripotent

Capable of giving rise to any type of cell or tissue (except

the placenta) i.e. an embryonic stem (ES) cell

Definition on multipotent

Capable of giving rise to a discrete range of cells or tissues

i.e. haemopoietic stem cell

Adult multipotent stem cells features and examples

❑ Required to replace dead terminally differentiated cells

(cell turnover)

❑ Cells of the intestine epithelium replaced every 3-6 days

Haemopoietic stem cells features and examples

❑ 2-3 million new erythrocytes are

produced per second in human

adults

❑ Multipotent

stem cells

❑ Huge variety

of cell types

If goes wrong:

❑ anemia (too few red blood cells)

❑ leukopenia (too few white blood

cells)

❑ blood cancers - leukemia,

lymphoma, myeloma - stem cell

transplants possible treatment

option

embryonic stem cells can be used as spare parts

Why and how

Embryonic stem cell (ES) – Undifferentiated cell type derived from the inner cell mass of an early mammalian embryo and capable of differentiating to give rise to any of the specialised cell types in the adult body

❑ Regenerative medicine and

tissue replacement after injury or

disease

❑ From same person, less chance

of tissue rejection

❑ Personalised medicine

❑ Ethical and technological issues,

potential cancer risk

Induced pluripotent stem cells (IPS) summary

Induced pluripotent stem cell (iPS or

iPSCs) – Somatic cell that has been

reprogrammed to resemble and behave

like a pluripotent embryonic stem cell

(ES) through the artificial introduction of

a set of transcriptional regulators

Induced pluripotent stem cells (IPS) advantages

❑ Any cell type could be replaced

❑ Less chance of rejection

❑ Fewer ethical issues

❑ Important for medical research

Induced pluripotent stem cells (IPS) disadvantages

❑ More basic research needed on

developmental pathways

❑ Potential cancer risk

Two ways of cell death

Necrosis, apoptosis

Necrosis (lysis)

❑ Cells simply swell and burst

❑ The membrane is destroyed and cell

contents released into the tissue

❑ Degraded by extracellular enzymes

and phagocytic cells engulf the

remains

❑ Potential damage to neighbouring

cells, trigger inflammatory response

Apoptosis (programmed cell death)

❑ Normal carefully-controlled process

❑ Billions of cells in adult every hour

❑ No damage to neighbouring cells

❑ Internal or external signals activate

Bcl2 family that regulate process

❑ Cell dismantled by caspases, then

phagocytosed