Cell Structure and Function

• This lecture covers fundamentals of cell biology, spanning cell structure and function, gene expression and protein synthesis, and cell division through mitosis and meiosis.

Cell Theory

• A cell is the foundational unit of life; smaller structures aren't considered alive
• All living organisms consist of cells
• New cells originate from pre-existing ones

Structure-Function Relationship

• Cells carry out diverse functions within the body
• A cell's shape, organelles, and subcellular structures dictate its function
• Structure and function being closely interconnected defines relationships

Cell Components and Organelles

• Cell organelles and specialized structures within the cytoplasm facilitate cell survival, growth, and operations.
• Key cell organelles and structures comprise the plasma membrane, cytoplasm, nucleus, endoplasmic reticulum, Golgi apparatus, lysosomes, mitochondria, cytoskeleton, centrosomes, cilia, and flagella.

Plasma Membrane

• Surrounds all human cells
• It separates the cell's interior from the external environment
• Its structural integrity and functionality are vital for cell survival

Plasma Membrane Structure

• Composed of two layers (phospholipid bilayer) of molecules
• It features polar "heads" of phospholipid molecules which are water-loving (hydrophilic) and situated on the membrane's inner and outer surfaces
• It features nonpolar "tails" that are water-fearing (hydrophobic) in the membrane's core
• It consists of diverse embedded proteins
• Cholesterol is present for structural support
• Carbohydrates attached to proteins (glycoproteins) & lipids (glycolipids) are found on the exterior leaflet.

Plasma Membrane Permeability

• Acts as a selectively permeable barrier, permitting certain substances entry while blocking others
• A molecule's ability to pass through depends on its electric charge and size
• Permeable to hydrophobic molecules, gases, and small uncharged molecules, but not permeable to charged molecules and ions
• Vital proteins, large molecules, and ions can be transported into the cell; waste/other substances exit

Movement Across Plasma Membranes

• Molecules move across the plasma membrane via passive or active processes
• Passive processes include simple diffusion, facilitated diffusion (using carriers or channels), and osmosis (using water channels called aquaporins)
• Active processes include Active Transport, Exocytosis, Endocytosis (Phagocytosis, Pinocytosis, Receptor-mediated Endocytosis)

Passive Processes

• Simple diffusion involves fat-soluble solutes

Facilitated Diffusion

• Involves glucose, fructose, galactose, some vitamins, and ions like K+, Cl-, Na+, and Ca2+

Osmosis

• Involves the movement of solvents like water

Facilitated Diffusion - Glucose Transporters

• Facilitated transport with a protein carrier makes it possible to transport glucose to the cells moving down a concentration gradient
• Cellular energy isn't consumed
• Alterations in glucose transporters can lead to Diabetes condition that develops when the body can’t use blood glucose efficiently for energy

Primary Active Transport

• Directly uses ATP energy to actively transport a solute against its concentration gradient and establishes an electrochemical gradient

Secondary Active Transport

• Employs an existing pre-established gradient gradient to actively transport molecules
• Relies on the energy stored by pre-existing electrochemical gradients and utilizes it to facilitate a molecule's movement against concentration gradient

Endocytosis Types

• Phagocytosis (cell eating), pinocytosis (cell drinking) and receptor-mediated endocytosis

Receptor-Mediated Endocytosis

• A mechanism for SARS-CoV-2's entry into cells, where the spike proteins of the virus bind to ACE2 receptors on the cell.
• Individuals with pre-existing cardiovascular conditions show higher ACE2 expression; possibly severe with SARS-CoV-2

Exocytosis vs Endocytosis

• Exocytosis transports vesicles for secretion, while endocytosis brings materials into the cell.

Cytoplasm

• Is the internal fluid containing cell organelles and structures (nucleus, endoplasmic reticulum, mitochondria, Golgi apparatus, vesicles, lysosomes, cytoskeleton, proteasomes, centrosomes, etc.)
• Includes solutes and nutrients (ions, vitamins, nucleotides, proteins, enzymes, minerals, etc.)
• Encompasses inclusion bodies; like lipid droplets, glycogen granules, and pigments

Nucleus

• Encloses DNA in the nucleoplasm; surrounded by a nuclear envelope
• The nuclear envelope contains pores regulating molecular movement
• Chromosomes are visible during mitosis; chromatin is dispersed in interphase
• Nucleoli synthesize ribosomal RNA (rRNA) creating ribosome subunits

Endoplasmic Reticulum (ER)

• Is a continuous membrane network in the cell that appears as rough or smooth
• Rough ER (RER) contains ribosomes; involved in protein synthesis and modification
• Smooth ER (SER) lacks ribosomes; it handles lipid/steroid synthesis, harmful substance detoxification, Ca2+ storage, and carbohydrate metabolism
• Smooth ER synthesizes lipids and steroids like cholesterol, estrogen, and testosterone
• Smooth ER detoxifies harmful substances like poisons, drugs, and toxins

Golgi Apparatus

• Receives ER products via transport vesicles for final processing
• Consists of cisternae (flattened vesicles), it modifies and packages proteins/lipids/macromolecules into vesicles
• Vesicles transport materials inside or outside the cell

Lysosomes

• Are responsible for breaking down unwanted proteins and other macromolecules
• Single-membrane organelles budded from from the Golgi complex with enzymes to digest macromolecules (proteins, lipids, carbohydrates, nucleic acids)
• It fuses with and digests substances that enter a cell via endocytosis
• Functions to perform autography and apoptosis (cell programmed death)

Mitochondria

• Referred to as the "powerhouses" of the cell, produce energy
• Double membrane bound; its inner cristae is extending into the mitochondrial matrix
• Generate ATP through cellular respiration and are key to apoptosis

Cytoskeleton

• Network of proteins in the cell that maintain shape and anchor and/or move organelles
• Made of three fiber types that include microtubules, intermediate filaments, and microfilaments

Centrosomes

• An organelle playing a key role in cell division that contains two centrioles near the nucleus
• Each centriole consists of 9 microtubule triplets
• Duplicates before cell division; then moves to opposite cell ends

Cilia and Flagella

• Plasma membrane extensions for cell movement; cilia are short & many; flagella are long & single
• Cilia are 20x smaller than flagella
• Both cilia and flagellum facilitate movement moving fluids like mucus, in the trachea, and the eggs

Gene Expression: DNA to Proteins

• The process includes transcription in the nucleus to produce mRNA
• Followed by translation in the cytoplasm at the ribosome to produce proteins

Primary Structure of Proteins

• Proteins are large polymers of amino acids linked via peptide bonds to form linear polypeptide chains

Chromatin and Chromosomes

• Chromatin is a cluster of DNA and proteins (histones) where nucleosomes create a bead-string structure
• Chromatin condenses to form chromosomes during cell division;
• The nucleus contains dense nucleoli, responsible for synthesizing ribosomal RNA (rRNA) used to assemble ribosomes

Nucleic Acids

• Nucleic acids store, express, and transmit genetic information and comes in two types
• Deoxyribonucleic Acid (DNA): Stores genetic information
• Ribonucleic Acid (RNA): Transfers genetic information from DNA to form a protein

Deoxyribonucleic Acid (DNA)

• A polymer composed of 2 polynucleotide strands forming a double helix structure
• Consists of nitrogenous base + deoxyribose sugar + phosphate
• Nitrogenous bases are classified as pyrimidines (thymine & cytosine) and purines (adenine & guanine)
• Adenine pairs with thymine; cytosine pairs with guanine
• A pairs with T (two hydrogen bonds) and C pairs with G (three hydrogen bonds)

Ribonucleic Acid (RNA)

• Polymer of one nucleotide strand consisting of nitrogenous base + ribose sugar + phosphate and is of three types
• Messenger RNA (mRNA) guides the process of making proteins utilizing DNA
• Ribosomal RNA (rRNA) is vital for protein synthesis and ribosome component
• Transfer RNA (tRNA) transports specific amino acids during protein synthesis from mRNA

DNA vs RNA

• DNA contains deoxyribonucleic acid; RNA has ribonucleic acid
• DNA contains deoxyribose; RNA contains ribose
• DNA contains thymine (T); RNA has uracil (U)
• DNA uses adenine & guanine & cytosine; RNA uses the same
• DNA is double stranded; RNA is single stranded
• DNA is of 1 type; RNA has 3 types

DNA Transcription

• Genetic information is encoded in the DNA sequence which transcribing to mRNA:
• A strand of DNA acts as a template to synthesize complementary RNA using RNA polymerase, where resulting mRNA takes form of thymine.

DNA Transcription: Where To Start and End

• Transcription begins and binds on the promoter, and ends and terminates on the terminator utilizing various transcription factors.

Codon & Anticodon

• A codon that codes for a specific amino acids, is a three-base mRNA sequence that binds to small RNA molecules.
• A tRNA anticodon is a three-base sequence, complementary to the mRNA codon
• A codon may specify a certain growing incorporation or indicate termination signals to the proteins using signals (stop codon)

Genetic Code

• The sequence of bases in mRNA molecules that determines the sequence of amino acid within polypeptide forming groups of 3 bases or codons
• Each codon specifies a particular amino acid
• The genetic code is often redundant, but it means has more than one codon to specific a particular amino acid.
• • Genetic code has 64 codons, start codons (1), stop codons (3), sense codons (61)

Stages of Translation

• Amino acids join to make polypeptide chain

Cell Cycle Overview

• Cell cycle comprises two parts that includes interphase, growth and divide aspects involving Gap 1/Synthesis/ Gap 2, along with Mitosis, the division phase that encompass prophase, prometaphase, and metaphase

Somatic Cell Division (Mitosis)

• Made of events that includes the division of the nucleus as well as the division of cytoplasm
• Each daughter cell inherits the genetic information of mother; diploid cells contains 2N with 46 chromosomes, inheriting member from each parent and duplicating into two chromatids with centromere

Phases of Mitosis

• Mitosis takes place through Prophase, Pometaphase, Metaphase, Anaphase, and Telophase

Karyotype

• Photographic depiction featuring ones complete collection, typically at chromosomes, offering an overview duplicated or condensed chromosome
• A diploid cell chromosomes with chromosomes that are inherited and are visible

Meiosis and Sexual Reproduction

• Sexual Reproduction events when the gamates of haploid cells fuse and create 23 chromosomes and create a zygote
• Produces germline cells for sorting

Key events in Meiosis during prophase (not mitosis)

• Each sister chromatid from its homologue and synapsis lying next
• Exchange between chromosome pieces during crossover
• Chiasma forms when arms move apart but stay attached during a crossover

Mitosis vs Meiosis

• Mitosis produces two diploid cells, genetic replicas or clones in contrast.
• Meiosis produces offspring chromosomes with half or haploid that vary genetically

Aging and Cell Death

• Every second, ~25 million cells that involves mitosis divides to safeguard body and maintain good health, leading damage removal to apoptosis in the programmed cell death or elimination .

Test your knowledge of cell biology! This quiz covers endocytosis, cellular structures like ACE2 receptors and the nucleus, molecule transport, and the plasma membrane. See how well you understand cell structure and function.