Cell structure and composition

Question 1

Recall what constitutes a cell, list the predominant types of molecules in a cell, and recall the scale of cells and their constituent molecules

and

Adentify the following on a micrograph of a cell, and summarise their functions – nucleus, nucleolus, nuclear envelope, mitochondrion, rough endoplasmic reticulum, smooth endoplasmic reticulum, ribosomes, Golgi apparatus, secretory granule, plasma membrane, cytoskeletal components

Answer 1

Cell; is a bag that contains;

1. genetic material (DNA)–> exception RBC’s
2. macromolecules (proteins, RNA’s)
3. produces molecules (ATP, NADH)

Consists of the following organelles:

1. Nucleus
2. Nucleolus
3. Mitochondria
4. Vesicles
5. Golgi apparatus
6. Centriole
7. Ribosome
8. Cytoskeleton; is composed of fibrous proteins of various diameters –> responsible for cellular support, movment and the movement of the orgnelles
9. Lysosome
10. Membranes
11. Cilia
12. Endoplasmic reticulum (ER)
13. Sarcoplasmic reticulum (SR)
14. Adhesion plaques

Cell characeritics;

1. have a cell membrane that separates the outside from the organised interior.
2. contain DNA as the genetic material (exceptions, e.g. RBCs).
3. contain several varieties of RNA molecules and proteins; many of the latter are enzymes.
4. are composed of the same basic chemicals: carbohydrates, proteins, nucleic acids, minerals, fats and vitamins.
5. regulate the flow of nutrients and wastes that enter and leave the cell.
6. reproduce, and are the result of reproduction.
7. require a supply of energy.
8. are affected by 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.)

Dimensions of a Cell

1. Volume of a cell ? (~ nanolitres)‏
2. Weight of a cell: (density = 1.06)‏

Predominant types of molecules in a cell;

• Extracellular matrix (ECM):
  e. g. Basement membrane:

1. selective barrier for macromolecules
2. type IV collagen network, laminins, type XV collagen

• Extracellular fluid (ECF):

1. ions (Na+, Cl-, PO43-, CO32-, Mg2+, Ca2+)
2. soluble proteins
3. soluble carbohydrates, sugars
4. vitamins
5. amino acids
6. hormones
7. nucleotides (ATP)‏
8. lipids
9. Cholesterol

• Lymph
• Plasma

Question 2

Define the essential characteristics of prokaryotic and eukaryotic cells, list key differences

Answer 2

Eukaryotic cells;

1. evolved from aggregates of prokaryotic cells that became interdependent upon one another and eventually merged or fused into a single larger cell
2. have a higher degree of organisation than prokaryotic cells, in that they contain many organelles or structures separated from the other cytoplasm
3. Organelles in eukaryotic cells derive from incorporation of prokaryotes (or invasion by..): e.g. mitochondria, and chloroplasts in plants.
4. Mitochondria and chloroplasts contain their own DNA. It is circular DNA, as found in prokaryotes.

Prokaryotes;

1. Include kingdoms of Monera (simple bacteria) and Archaea.
2. They are bags of molecules, held within a membrane and a cell wall and do not contain ‘organelles’.
3. may have photosynthetic pigments, such as is found in cyanobacteria (“blue bacteria”).
4. Some have external whip-like flagellae for locomotion or hair-like pili for adhesion.
5. come in multiple shapes: cocci (round), bacilli (rods) and spirillae or spirochetes (helical cells).

Key differences;

• Prokaryotes do not have internal membranes

(few exceptions of photosynthetic bacteria)

Eukaryotes do have internal membranes that define organelles including nucleus, ER, mitochondria, etc

• Prokaryotes have a single copy of a chromosome (haploid)

Eukaryotes can be haploid or diploid

• Eukaryotes have cytoskeleton

Prokaryotes cytoskeleton is not as well defined.

• Cell wall of prokaryotes contain peptido-glycan

Question 3

Alterations that can lead to cancer

Answer 3

1. switch on of “divide” signals
2. switch off of “don’t divide” signals
3. loss of correction mechanism on DNA copying
4. loss of escape mechanism from cell division
5. loss of limit on number of times a cell can divide
6. loss of control keeping cell within tissue boundaries
7. ability to evade body defence mechanisms
8. ability to recruit blood vessels to growing tumour
9. ability to migrate into blood stream or lymph vessels
10. ability to establish tumours in the “wrong” tissue - metastasis

Question 4

Explain the formation of phospholipid bilayers in an aqueous environment

Answer 4

Phospholipids have a hydrophilic head (polar) and hydrophobic tail. To avoid water, the tails pack together.

Suspended in water they form micelles or droplets. They can also arrange themselves into bilayers (a layer two molecules thick), called “liposomes”.

Question 5

Recall the basic structural components of phospholipids

Answer 5

Phospholipids are amphiphilic - contain both hydrophobic and hydrophilic groups. Hydrophilic groups contain anionic and cationic groups, and can be net anionic or neutral.

Cholesterol; steroid abundant in plasma membrane

1. decreases permeability
2. modulates membrane stiffness
3. affects interactions with cytoskeleton

Glycolipids (with sugar headgroups) on the extracellular side of the membrane – based on similar structure to sphingomyelin

Negative charges (e.g. phosphatidylserine, PS) inside the cell

‘fluid mosaic model’; proteins float in a “sea” of lipids in either leaflet of the bilayer, or span both leaflets of lipids that form the bilayer (Singer and Nicolson, 1972).

Question 6

Describe the permeability properties of a phospholipid bilayer with respect to macromolecules, ions, water and organic compounds (including drugs).

Answer 6

Lipid bilayers are permeable to:

1. water molecules and a few other small, uncharged, molecules like oxygen (O2) and carbon dioxide (CO2) which diffuse freely in and out of the cell (also many drugs).
   
   • Diffusion of water across membrane is called osmosis.
   • Diffusion down the concentration gradient is possible – passive.
   • Diffusion against the concentration gradient requires energy or exchange and is called active transport. What is the energy source?

• Facilitated diffusion is movement of hydrophilic (e.g. 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) provide a water-filled channel. The channel can be ‘gated’.

Lipid bilayers are not permeable to:

1. Cations - K+, Na+, Ca2+ (but some do leak through, down the concentration gradient)
2. Anions - Cl-, HCO3-
3. small hydrophilic molecules like glucose
4. macromolecules like proteins and RNA

Question 7

Membrane Transport

Answer 7

1. Protein-mediated permeability - pores
   
   • The protein provides a route for a substance to move down its concentration gradient, e.g. glucose transporters (GLUT1-10; some are insulin-sensitive) in most tissues where the extracellular concentration of glucose is much higher than intracellularly.
   • Coupled transporters:
     
     • Symporters - sugars and amino acids can be dragged into the cell with Na+, as it moves down its concentration gradient
     • Antiporters - other molecules can move in the opposite direction to Na+ (e.g. H+; Na+-H+ exchanger for intracellular pH regulation)
2. The Na+-K+ ATPase (pump)
   • The high concentration of fixed anions inside cells (proteins and negatively charged lipids) and their accompanying cations means that water is drawn into the cells by the resulting osmotic gradient.
   • The high concentration of ions (Na+ and Cl-) in the extracellular space means that there is an opposing osmotic gradient.
   • Na+ will tend to move down its concentration gradient into the cell.
   • The Na+-K+ ATPase maintains the osmotic balance and stabilises the cell volume by exporting Na+.
   • The Na+ gradient is thus maintained.
   • The Na+ gradient is also used to drive the transport of sugars and amino acids (i.e. Symport).
3. The sodium-potassium pump
   
   • found in the plasma membrane of all cells and consists of two polypeptide chains, alpha and beta, with 1000 and 300 amino acids, respectively.
   • The beta chain is a controller.
   • The alpha chain spans the membrane 10 times, forming a hydrophilic pore through which the cations (X+) can move.(check http://www.bio.davidson.edu/courses/Molbio/26aa/alphabeta.html
4. ​Specific ion pumps;
   
   • There are specific pumps for Na+, Ca2+ and H+, which use ATP hydrolysis to provide the energy. There are also Cl- pumps in some cells.
   • Some pumps can work in reverse and generate ATP from an ion gradient, e.g. the F1-ATPase in the mitochondria using H+ gradient.
   • Other mechanisms exist for other substances that need to cross the membrane.​​

Question 8

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.

Answer 8

1. Transport of 2K+ from left (extracellular) to right (intracellular) in exchange for 3Na+. Therefore it is “electrogenic”, i.e. creates a negative intracellular potential.
2. mediated by successive conformational transitions of the pump molecule
3. driven by phosphorylation of an aspartyl residue (red explosion) using ATP (orange shape)
4. followed by hydrolysis of the aspartylphosphate (blue explosion). The changing colour and shape indicates the changing “conformational energy state” of the pump molecule relative to its substrates.

( http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_the_sodium_potassium_pump_works.html)

Question 9

The electrochemical potential

Answer 9

• The Na+-K+ pump exchanges 3 Na+ ions from inside the cell for two K+ ions on the outside.
• There are two consequences:

1. Ionic gradients are created: less Na+ and more K+ inside the cell than outside.
2. A charge gradient is created, as more positive charges are pushed out than are coming in. This results in the inside of the cell being at a more negative potential than the outside.

Question 10

Membrane potential

Answer 10

• Due to a difference in electric charge on the two sides of a membrane.
• Can result from activity of electrogenic pumps, e.g. Na+-K+ pump.
• Also main contributor to potential across mitochondrial inner membrane (electrogenic H+ pump, leading to ATP synthesis).
• Can result from passive ionic diffusion. A major contributor across plasma membrane of animal cells.
• K+ inside the cells is high, to balance the fixed anions, and is pumped into the cell by the Na+-K+ ATPase (K+ also can travel through K+ leak channels). [K+]i = 166 mM; [K+]o = 5 mM
• Near equilibrium for K+: attracted into the cells by fixed anions, and moving out of the cells down the concentration gradient.
• This imbalance gives rise to a membrane potential: of ~ -70mV

Question 11

Action potentials

Answer 11

• Action potentials occur in elongated cells (nerves, muscle) when the membrane potential is disrupted by a brief pulse of current which temporarily opens voltage-gated Na+ channels.
• Na+ ions enter the cell and cause depolarisation from –70 mV to about +50 mV.
• Na+ channels become inactivated locally because of the voltage reversal, preventing further Na+ entry.
• Voltage-gated K+ channels open, resulting in K+ efflux and help to restore the resting membrane potential.
• The process propagates down the nerve/muscle.

Question 12

Bulk Transport mechanisms

Answer 12

Bulk transport mechanisms:

1. Pinocytosis:
   
   • engulfment by the membrane of extracellular solute and small molecules which end up in small intracellular membrane-bound vesicles.
2. Phagocytosis:
   
   • engulfment by the membrane of extracellular objects such as bacteria, cell debris, other cells, specifically bound to the cell membrane by receptors.
   • Signalling triggers actin cytoskeleton rearrangement. Again these end up in intracellular membrane-bound vesicles.
3. Exocytosis:
   
   • Movement of proteins (e.g. hormones, blood clotting factors) and other molecules from intracellular vesicles into the extracellular space by fusion with the cell membrane.

Question 13

Recall the functions of membrane proteins

Explain how external chemical signals can be sensed at the interior of a cell

Answer 13

Cell signalling:

It is not only substances that need to cross membranes. Signals need to cross membranes too.

• Some signals are lipid-soluble molecules that cross membranes e.g. steroid hormones, prostaglandins, NO (also drugs)
• Many impermeable signals rely on trans-membrane receptors

Question 14

Explain how the entry of glucose and amino acids into the cell against a concentration gradient is coupled to ATP-dependent Na+ transport.

Answer 14

• Glucose is membrane-impermeant
• in the kidney, glucose is reabsorbed from the filtrate in the early proximal tubule lumen (where it is at a low concentration) into the cell where it is at a higher concentration, i.e. against its concentration gradient, and subsequently into the bloodstream on the opposite side of the cell where the concentration is lower
• Glucose binds to a specific glucose transporter which functions by a flip-flop mechanism – symport
• The transport is ‘facilitated’, glucose is cotransported with Na+

(see https://www.khanacademy.org/test-prep/mcat/organ-systems/the-renal-system/a/tubular-reabsorption-article)