Cellular physiology

Question 1

What does the endoplasmic reticulum do?

Answer 1

• Processes molecules made by the cell, and transports them in/out of the cell via vesicles.
• The granular/rough endoplasmic reticulum has ribosomes attached to it, the smooth ER does not.
• The smooth ER synthesis and processes lipids and other processes reliant on the intrareticular enzymes.
• Substances are commonly transported to the nearby Golgi body by transport vesicles.
• The ER is believed to be responsible for making peroxisomes (contain oxidases, can create H2O2, which combines with catalase to destroy toxic substances (e.g. EtOH in liver cells)). Its major function is breakdown of long chain fatty acids.
• The ER is continuous with the nuclear envelope/membrane.
• The ER also produces the enzymes required for control of glycolysis, and for processes of detoxification.

Question 2

What is the role of the Golgi apparatus?

Answer 2

• It is important in secretory cells.
• It creates lysosomes, which break off from the Golgi body and disperse in the cytoplasm. The lysosomes contain digestive enzymes (hydrolases).
• The golgi body is important in synthesis of certain carbohydrates (e.g. hyaluronic acid and chondroitin).
• Transport vesicles from the ER fuse with the golgi body and release substances. Further synthesis of carbohydrates will occur.
• The golgi body also is responsible for compacting secretions into concentrated ‘packages’.
• In secretory cells, secretory vesicles travel from the golgi body to the cell membrane, where exocytosis occurs.

Question 3

What is the role of mitochondria?

Answer 3

• They are the powerhouse of the cell, produce ATP.
• Oxidation of nutrients by mitochondrial enzymes produces CO₂, H₂O and energy, which is used to produce ATP.
• Glucose is converted into pyruvate in the cytoplasm (glycolysis). 5% of the cell’s energy metabolism is contributed to conversion of ADP to ATP in this process; the remaining 95% occurs in the mitochondria.

Pyruvic acid is converted into Acetyl-coenzyme A/CoA in the mitochondria. This and other processes make up the Krebs cycle. Acetyl CoA is split into hydrogen + CO₂; the hydrogen combines with O2 to produce lots of energy, used by the mitochondria to produce ATP!

(this process, which involves production of H2O, needs ATP synthetase, which uses the energy from hydrogen ions to produce ATP from ADP).

Question 4

What is the role of the nucleolus?

Answer 4

• It contains RNA and ribosomal protein found in ribosomes. - The nucleoli increase in size when the cell is producing proteins. - The nucleoli are formed in the nucleus, starting with production of RNA from chromosomal genes. Most of these are transported out to the cytoplasm, where they form ribosomes. The remainder is stored in the nucleoli.

Question 5

Explain the processes of pinocytosis & phagocytosis

Answer 5

• A large macromolecule binds to specific receptors on the cell surface membrane, which are found in coated pits. - Clathrin is found on the inside of the membrane beneath the coated pit, and is responsible for the process of invagination of the membrane to create a vesicle with the macromolecule inside. - This process requires ATP - Phagocytosis is the same process, but involved large particles instead of large macromolecules.

Question 6

What is the function of the lysosome?

Answer 6

• Lysosomes attach to the ingested vesicles following phago/pinocytosis.
• Hydrolases from the lysosome digest macromolecules within the vesicle.
• The remaining indigestible substances are excreted by exocytosis of the residual body.
• Lysosomes are also responsible for removal of damaged cells, or parts of cells. Damage of cells results in rupture of lysosomes, releasing hydrolases -> autolysis.
• Lysosomes also contain bactericidal agents - lysozyme (dissolves bacterial cell wall) & lysoferrin (binds to iron).

Question 7

What is ATP?

Answer 7

• ATP is a nucleotide, composed of adenine, ribose and three phosphate radicles.
• The last 2 phosphates are connected to the molecule by high energy phosphate bonds.
• When ATP releases its energy, a phosphoric acid radicle is split away and ADP is formed.
• To produce ATP from ADP, energy from cellular nutrients causes ADP and phosphoric acid to recombine.

Question 8

Describe the process of ameboid locomotion of cells.

Answer 8

• This involves continual formation of new cell membrane at the leading edge of the pseudopodium, and absorption of the mid to rear portion of the cell.
• The leading membrane attaches to surrounding tissues by receptor proteins, which line the exocytic vesicles of the membrane. On the other end of the cell, receptors detach (from their ligands) and form new endocytic vesicles which move towards the pseudopodial end of the cell. Here, they are used to form the new cell membrane.
• Actin, myosin and ATP are crucial for locomotion in these cells.

Question 9

Describe the process of ciliary movements

Answer 9

• These movements are exhibited by cilia, sperm and the fallopian tubes.
• The axoneme is a complex of cross linking and tubules.
• Large amounts of ATP, and appropriate levels of Mg²⁺ and Ca²⁺ are needed.
• During forward motion, the tubules on the front edge slide outward towards the tip (of the cilium) and the tubules on the back edge remain in place.
• Multiple protein arms are composed of dynein, which has ATPase activity.

Question 10

What is DNA composed of?

Answer 10

• Deoxyribose (sugar molecule), phosphoric acid, and four bases (2 purines: adenine and thymine, and 2 pryimidines: cytosine & guanine).
• The DNA strands are held together loosely by weak hydrogen bonds between the purine-pyrimidine bases. This means they can pull apart easily.

Question 11

Explain the process of DNA transcription.

Answer 11

• Note that thymine does not exist in RNA molecules (uracil instead).
• RNA polymerase is responsible for assembly of the RNA molecule.
• In the DNA, the promoter is recognised by and becomes bound to, RNA polymerase. This binding results in unwinding of the start of the gene. A new RNA nucleotide is added to the RNA molecule at each step. The DNA nucleotide becomes bound to the complementary RNA nucleotide through a hydrogen bond. The RNA builds the RNA molecule until it encounters the chain-terminating sequence. The weak hydrogen bonds between RNA and DNA molecules break, as the DNA is more attracted to its complementary strand.

Question 12

Describe the different types of RNA.

Answer 12

• Precursor messenger RNA (pre-mRNA): a large RNA strand processed in the nucleus to form mature mRNA. The pre-mRNA includes introns and exons. The introns are removed, but the exons are retained.
• Small nuclear RNA (snRNA): responsible for the splicing of pre-mRNA to mRNA.
• Messenger RNA (mRNA): carries the genetic code to the cytoplasm to control formation of proteins.
• Transfer RNA (tRNA): transports activated amino acids to the ribosomes.
• Ribosomal RNA: responsible for forming ribosomes.
• Micro RNA (miRNA): a single-stranded RNA molecule that can regulate gene transcription & translation.