Cell Physiology: Structure and Membrane Function

• Cell physiology is the study of the function and mechanisms that operate within living cells.

Cell Structure and Components

• The cell is the smallest functional unit of a living organism.
• Cells are bound by a plasma membrane.
• Cytoplasm fills the interior of the cell, composed of cytosol (liquid) and organelles.
• Organelles, mostly membrane-bound, have specific functions.
• Key organelles include the nucleus, nucleolus, ribosomes, endoplasmic reticulum, Golgi apparatus, cytoskeleton, mitochondria, lysosomes, and peroxisomes.

Cell Membrane Function

• Maintains physical integrity by separating intracellular from extracellular environments.
• The cytoskeleton is attached to membrane proteins for shape definition and maintenance.
• Offers protection from harmful chemicals and prevents the loss of biological macromolecules.
• Selective permeability allows communication between intracellular fluid (ICF) and extracellular fluid (ECF).
• It maintains ICF and ECF differences by preventing leakage of essential ions/proteins and restricting entry of non-essential ones.
• Active transport moves ions/molecules against concentration gradients.
• Responsible for exocytosis and endocytosis.
• Provides cellular identifiers and receptor sites for hormones, neurotransmitters, and antibodies for communication.
• Has enzymes attached for metabolism.

Nucleus Structure and Function

• Present in cells can reproduce .
• Contains nucleoplasm, karyolymph, nucleolus, and chromatin (DNA + histones).
• Separated from the cytoplasm by a double membrane (nuclear envelope/membrane).
• The nuclear is permeable to ions and small molecules.
• Contains nuclear pores (nuclear pore complexes) of about 100 proteins for mRNA and protein transport.
• Regulates cell differentiation and maturation.
• Responsible for DNA replication and RNA synthesis.

Chromatin Structure and Function

• Creates chromosomes arranged in pairs as 23 pairs (44 autosomes + 2 heterochromosomes).
• Observable during cell division.
• The components of chromosomes include DNA, histones, and non-histone proteins.
• DNA is "wrapped" around a core of histone octamer which makes the nucleosome which forms a basic unit of DNA packaging in eukaryotes.

Nucleus and DNA Replication

• DNA encodes approximately 100,000 proteins.
• DNA replication precedes cell division during the S-phase.
• Initiation occurs at replication origins where DNA strands separate via helicase complexes.
• Elongation begins as primase adds RNA primers to the template strand and DNA polymerase extends the 3' end.
• The leading strand is continuously extended by DNA polymerase with new DNA nucleotides
• The lagging strand is extended discontinuously from primers forming Okazaki fragments, which are completed via ligase.
• Termination of DNA replication does not reach the chromosome's end, stopping at telomeres.
• Telomeres consist of about 11,000 bases of repetitive DNA sequence (TTAGGG).

Nucleus and Transcription

• Transcription of DNA into RNA synthesizes messenger RNA (G, C, U, A).
• Governed by the sequence of bases in DNA (C, G, A, T) under RNA polymerases.
• A base triplet in DNA transcribes to a codon (base triplet in mRNA that encodes one amino acid - AA).
• Transcription starts by removing repressor proteins and binding transcription factors to the promotor.
• The resulting primary transcript (heterogenous RNA) is modified by adding a 5'-cap and a 3'-polyA tail, forming pre-mRNA.
• Pre-mRNA contains exons (coding sequences) and introns (non-coding sequences) and introns are then cut out (splicing).
• mRNA leaves the nucleus where it can be synthesised into proteins.

Nucleolus Features

• Lacks a membrane.
• It is formed around genes encoding rRNA of acrocentric chromosomes (13, 14, 15, 21, 22).
• Responsible for ribosome synthesis.
• Rich in rRNA.
• Disappears during cell division.
• Its possible role is aging.

Ribosomes and Protein Synthesis

• Protein synthesis involves mRNA translation into a protein chain.
• Ribosomes, which are responsible for protein synthesis, are either free in the cytoplasm to produce cytoplasmic and mitochondrial proteins.
• Ribosomes are also attached to the endoplasmic reticulum to create membrane proteins or store proteins in the Golgi apparatus.
• 20 types of amino acids get transported to ribosomes by specific tRNA (transfer RNA).
• tRNA contains an anticodon and can bind amino acids.
• 64 combinations exist.
• Some amino acids (Met, Asp, Glu, Trp) have only one tRNA.

Endoplasmic Reticulum (ER) Types and Functions

• A sophisticated network of membranes in cisterns, tubules, and vesicles within the cytoplasm.
• Rough (Granular) ER has ribosomes attached to the cytoplasmic surface.
• Membrane proteins and proteins stored in GA are secreted by the cell, etc.
• The proteins from the rough ER get modified for the glycosylated and the formation of new polypeptides.
• Rough ER participates in the synthesis and transport of proteins.
• Smooth (Agranular) ER lacks ribosomes.
• Participates in lipid synthesis of phospholipids, cholesterol, triacylglycerols and steroid hormones.
• Smooth ER stores Ca2+ in muscle cells to form sarcoplasmic reticulum.

Golgi Apparatus Function

• Functions as a transport center of the cell.
• Composed of discs and cisterns with convexity toward the nucleus.
• Has communication with the nucleus through transport vesicles separating from the perinuclear cistern and ER.
• Vesicles fuse with the surface of the Golgi Apparatus.
• Protein modification, polysaccharide synthesis, and vesicle sorting.
• Transports to target structures (membrane, lysosomes, secretory vesicles).
• Processes compounds coming from the extracellular fluid (proteins, lipids).

Cytoskeleton Structure and Types

• The cytoskeleton is a network of filaments that maintains cell shape, provides mechanical support, and enables movement.
• Microtubules are long, hollow structures formed from globular proteins α- and β-tubulin.
• Microtubules are a dynamic part of the cytoskeleton and are continuously assembled and disassembled.
• Microtubule organizing centers (centrosomes) are near the nucleus.
• Intermediate Filaments form a flexible support scaffold that resists external pressing.
• Microfilaments are long, firm fibers made up of actin that polymerizes into fibrillar F-actin.

Mitochondria Function

• Semiautonomous organelles produce energy in the form of ATP.
• Protons (H+) accumulate in the intermembrane space being pumped by respiratory complexes of the inner mitochondrial membrane.
• Respiratory complexes transport electrons horizontally to molecular oxygen
• The matrix contain multiple metabolic processes such as the Krebs cycle, β-oxidation of FA, and AA metabolism.
• Contains its own DNA encoding some subunits of respiratory complexes.

Lysosomes and Peroxisomes

• Lysosomes from the digestive system of the cell.
• They are Structures that are limited by a membrane containing acidic fluid (pH 4.8) and digestive enzymes.
• Has about 40 acidic hydrolases like collagenase, glycosidase, deoxyribonuclease, phosphatase.
• Low pH is created by protonic pump in the membrane
• Processes material via phagocytosis, endocytosis, and autophagy (digestion of the cell's organelles)
• Single membrane with transport proteins.
• Matrix enzymes include oxidases, catalase, and dehydrogenase of D-AA, and uricase.
• Degradation of AA, FA, toxic substances (ethanol), and cholesterol conversion are catalyzed by these enzymes.
• Catabolism of fatty acids with long chains through β-oxidation into acetyl-CoA.
• Formed in ER.