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BIO111 Lecture 6: Cells, Membrane Potential, Organelles, Cell Cycle, and Protein Synthesis

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Membrane Potential

Resting Membrane Potential (RMP)

The resting membrane potential is the electrical potential energy generated by the separation of oppositely charged particles across the plasma membrane in all cells. This voltage occurs only at the membrane surface, with the inside of the cell being more negative relative to the outside. Typical membrane voltages range from –50 to –100 mV.

  • Voltage: The difference in electrical charge between two points.

  • Polarized cells: Cells with a charge across their membrane.

  • Key Point: The negative sign indicates the inside of the cell is more negative than the outside.

Role of K+ and Na+ in RMP

  • K+ diffusion: K+ diffuses out of the cell through leakage channels, making the cytoplasmic side more negative.

  • Electrochemical gradient: The drive for K+ to leave is balanced by its drive to stay, establishing the RMP (usually around –90 mV).

  • Na+ influence: Na+ is attracted to the inside due to the negative charge, and its entry can bring RMP up to –70 mV.

  • Cl–: Does not influence RMP as its gradients are balanced.

The key role of K+ in generating the resting membrane potential

Active Transport and Electrochemical Gradients

  • Na+-K+ pump: Maintains RMP by ejecting 3 Na+ out and bringing 2 K+ in.

  • Steady state: Rate of Na+ pumping equals rate of Na+ diffusion.

  • Excitable cells: Neurons and muscle cells can "upset" RMP by opening gated channels.

Cell-Environment Interaction

Roles of Plasma Membrane Receptors

Membrane receptor proteins serve as binding sites for chemical signals, facilitating communication and recognition between cells.

  • Contact signaling: Cells recognize each other by unique surface membrane receptors, important in development and immunity.

  • Chemical signaling: Interaction between receptors and ligands (e.g., neurotransmitters, hormones) causes changes in cellular activities.

  • Ligand binding: Can trigger enzyme activation or open ion channels, leading to changes in membrane excitability.

Cytoplasm and Organelles

Cytoplasm

The cytoplasm includes all cellular material between the plasma membrane and the nucleus. It consists of cytosol, inclusions, and organelles.

  • Cytosol: Gel-like solution with water, proteins, salts, sugars.

  • Inclusions: Insoluble molecules (e.g., glycogen granules, pigments).

  • Organelles: Metabolic machinery with specialized functions.

Membranous and Nonmembranous Organelles

  • Membranous: Mitochondria, endoplasmic reticulum, Golgi apparatus, peroxisomes, lysosomes.

  • Nonmembranous: Ribosomes, cytoskeleton, centrioles.

  • Compartmentalization: Membranes allow separation of functions within the cell.

Mitochondria

Mitochondria are the "power plants" of cells, producing ATP via aerobic respiration. They have a double membrane, with the inner membrane folded into cristae, and contain their own DNA, RNA, and ribosomes.

  • Function: ATP production.

  • Structure: Double membrane, cristae, matrix.

  • Division: Capable of fission, similar to bacteria.

Mitochondrion structure and function

Ribosomes

  • Site of protein synthesis.

  • Structure: Made of protein and rRNA.

  • Types: Free (in cytosol) and membrane-bound (on ER).

Endoplasmic Reticulum (ER)

The ER is a network of membranous tubes continuous with the nuclear membrane. It comes in two varieties:

  • Rough ER: Has ribosomes; site of protein synthesis. Proteins are packaged in vesicles and sent to the Golgi apparatus.

  • Smooth ER: Involved in lipid synthesis, storage, and calcium release. The sarcoplasmic reticulum is a specialized smooth ER in muscle cells.

Diagrammatic view of smooth and rough ER

Golgi Apparatus

  • Function: Modifies, concentrates, and packages proteins and lipids from the rough ER.

  • Structure: Series of flattened membranous sacs (cisternae).

Golgi apparatus structure and function

Peroxisomes and Lysosomes

  • Peroxisomes: Detoxify harmful substances and neutralize free radicals.

  • Lysosomes: Contain digestive enzymes; degrade bacteria, viruses, toxins, and nonfunctional organelles.

Cytoskeleton

  • Network of rods: Provides structural support and facilitates movement of cell components.

  • Proteins: Link rods to other cell structures.

Centrosome and Centrioles

  • Centrosome: Microtubule organizing center near the nucleus.

  • Centrioles: Barrel-shaped organelles forming the basis of cilia and flagella.

Centrosome and centrioles

Cellular Extensions

Cilia, Flagella, and Microvilli

  • Cilia and flagella: Aid in movement of the cell or materials across its surface.

  • Microvilli: Fingerlike projections that increase surface area for absorption.

Microvilli structure

Nucleus and Chromatin

Nucleus

The nucleus is the largest organelle, containing the genetic library (DNA) for protein synthesis. Most cells are uninucleate, but some are multinucleate or anucleate.

  • Structures: Nuclear envelope, nucleoli, chromatin.

Nucleus structure

Chromatin and Chromosomes

  • Chromatin: 30% DNA, 60% histone proteins, 10% RNA.

  • Chromosomes: Condensed chromatin, protecting DNA during cell division.

Chromatin structure Chromatin and chromosome structure

Cell Cycle and Division

Cell Cycle

The cell cycle consists of interphase (growth and DNA replication) and the mitotic phase (cell division).

  • Interphase: Cell grows and replicates DNA.

  • Mitotic phase: Cell divides into two daughter cells.

Cell cycle phases

Mitosis

Mitosis is the division of the nucleus, ensuring each daughter cell receives a full copy of DNA. It consists of four stages:

  1. Prophase: Chromatin condenses, spindle forms, nuclear envelope breaks up.

  2. Metaphase: Chromosomes align at the cell's equator (metaphase plate).

  3. Anaphase: Chromatids separate and move to opposite poles.

  4. Telophase: Chromosomes uncoil, new nuclear membranes form, nucleoli reappear.

Metaphase stage of mitosis Anaphase stage of mitosis Telophase and cytokinesis

Cytokinesis

  • Process: Begins during late anaphase, actin ring forms cleavage furrow, two daughter cells are pinched apart.

Control of Cell Division

  • Go signals: Surface-to-volume ratio, growth factors, hormones.

  • Stop signals: Contact inhibition, availability of space.

Protein Synthesis

DNA and Genes

  • DNA: Master blueprint for protein synthesis.

  • Gene: Segment of DNA coding for a polypeptide.

  • Triplet code: Three sequential bases specify an amino acid.

Role of RNA

  • Messenger RNA (mRNA): Transcribes DNA code.

  • Ribosomal RNA (rRNA): Helps translate mRNA message.

  • Transfer RNA (tRNA): Brings amino acids for protein assembly.

Steps of Protein Synthesis

  • Transcription: DNA information is coded in mRNA.

  • Translation: mRNA is decoded to assemble polypeptides from amino acids.

Information flow during protein synthesis

Transcription

  • Process: DNA base sequence is transferred to complementary mRNA sequence.

  • mRNA: Detaches and leaves the nucleus for the ribosome.

Translation

  • Process: Nucleic acid sequence is translated into amino acid sequence.

  • Codons: Three-base sequence on mRNA codes for an amino acid.

  • tRNA: Brings amino acids for assembly.

Translation and codon function

Rough ER Processing of Proteins

  • SRP: Directs mRNA-ribosome complex to rough ER.

  • Protein processing: Proteins are modified, folded, and packaged for transport.

Rough ER processing of proteins

Additional info:

  • Protein synthesis is tightly regulated and essential for cell function and growth.

  • Cell division is controlled to prevent uncontrolled growth (cancer).

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