The Cell Memebrane and Cell Organelles

Question 1

Cell membranes as selective barriers

Answer 1

1. Separate cytosol from extracellular fluid
2. Enclose individual organelles
3. Prevent mixing of different biomolecules
4. Control required molecules getting into a living cell

Question 2

Composition of cell membrane

Answer 2

Lipid molecules
* Phospholipid (relatively negatively charged)
* Cholesterol

Proteins
* Integral
* Peripheral

Question 3

Membrane asymmetry

Answer 3

Compositions of two layers of a living cell are different

Sugar side chains (glyco-) are always at the outer face of membrane (glycolipid and glycoprotein)

Question 4

Mobility of lipid components in membranes

Answer 4

Rapid rotation
* Rotation around the hydrophobic tail

Flip-flop exchange
* Happens when the aging red blood cells have to be eliminated
* Relatively slow due to the thermodynamic constraints

Rapid lateral diffusion
* Exchange places with neighbouring molecules in the same layer
* Proteins are also free to move laterally

Question 5

Increase the fluidity of membrane

Answer 5

• Increase surrounding temperature
• Increase C=C bonds on the hydrocarbon chain so that they are not closely packed
• Less cholesterol so it’s less sticky

Question 6

Biological functions of membrane proteins

Answer 6

As protein receptors
* Enable the cells to repond to hormones and neurotransmitters
* Insulin (peptide hormone frompancreas) binds with insulin receptor on skeletal muscle

As adhesion molecules
* Connect neighbouring cells
* Provide mechanical strength and support to the cells

As enzyme (biocatalyst)
* Facilitates the biochemical reactions or pathways during metabolism in a living cell
* E.g. synthesis of ATP by mitochondrial ATP synthase

Allow transport of substances across the membrane
* Permeable to gas and small uncharged molecules
* Impermeable to ions carrying charges and large uncharged molecules especially water-soluble in nature
* Glucose and ions can be transported into a living cell by trasporter, ion channel or pump

Question 7

Nucleus

Answer 7

• Information centre to store genetic information by DNA
• Nuclear envelope: double layers of membrane, have pores for exchanging materials
• Nucleolus: synthesis of ribosome
• Nucleoplasm: Contains 23 pairs of chromosomes

Question 8

Cytoplasm

Answer 8

• All materials inside a living cell except the cell nucleus
• Cytosol: jelly-like fluid filling the cell
• Organelles: all with membrane except ribosome and cytoskeleton (rigid proteins which supports the shape of the cell)

Question 9

Endoplasmic retiuclum (ER)
Both are single membrane bounded

Answer 9

Smooth ER
* Ribosomes are absent on its surface
* Synthesize lipid (e.g. cholesterol is synthesized by enzymes at the smooth ER of hepatocytes)
* Carries out drug detoxification (e.g. alcohol is metabolized by enzymes at the smooth ER of hepatocytes)
* For intracellular calcium storage (released during muscle contraction)

Rough ER
* Ribosomes are present on its surface
* Continuous with the outer membrane of nuclear envelope
* Synthesize protein
* Modify newly synthesized protein (e.g. glycolysation to make glycoprotein)

Question 10

Golgi body (Golgi apparatus)
Single membrane bounded

Answer 10

• Participates in protein maturation and targets newly synthesized proteins to their appropriate subcellular destination
• Cis- network (faces rough ER): receiving vesicles from rough ER
• Trans- network (faces plasma membrane to fuse with it): releasing vesicles towards cell membrane for secretion

Question 11

Lysosome

Answer 11

• Contains enzyme (lysozyme) to catalyze the breakdown of intracellular unwanted substances
• Carries H+ ion pumps to deliver intracellular H+ ion into lysozyme
• Lysozyme will be activated under acidic environment (pH4.5-5.5)

Question 12

The lysosomal storage disesase (LSD)

Answer 12

Genetic disorder: accumulation of intracellular unwanted molecules and swelling of cells

E.g. Tay-Sachs Diseases (ganglion cells)
* lose motor skills and grow up with seizures, vision and hearing loss and paralysis
* Eye abnormality: cherry-red spot on retina

Question 13

Mitochondria

Answer 13

• Generates ATP as the energy-carrying molecule in the cells
• Two layers of membranes (inner and outer)
• Inner membrane contains lots of proteins responsible for ATP production
• Matrix is responsible for the fatty acid oxidation (energy release) and protein metabolism (urea cycle)
• Mitochondrial DNA is responsible for encoding proteins for ATP production
• Circular DNA vs double-stranded helix of DNA inside cell nucleus
• Involves in programmed cell death (apoptosis) pathways to eliminate unwanted cells (e.g. turnover of aging rbc)
• Abnormal interdigital apoptosis during embryonic development: fusion of fingers or toes