Cell Biology: Mitosis & Cell Structures

• Chapter 3 focuses on the structures of a cell.

Parts of a Cell

• Cells consists of three main parts: plasma membrane, cytoplasm, and nucleus.

Structures of a Cell

• Structures of a cell diagrams shows the different organelles
• Flagellum and Cilium aid with cell movement
• Proteasomes and Free Ribosomes aid with protein creation and destruction
• The nucleus stores DNA Chromatin and has a nuclear pore and envelope
• Contains a Nucleolus, and Glycogen granules
• Plasma membrane protects cytoplasm
• Both rough and smooth Endoplasmic reticulum (ER) have ribosomes to synthesize proteins
• Golgi complex aids in modifying and packaging proteins

The Plasma Membrane

• The plasma membrane is a barrier that surrounds and contains the cytoplasm
• It has a flexible and sturdy structure.
• It contains Phospholipids, Cholesterol, Glycolipids and Glycoproteins.
• Contains Integral (transmembrane) proteins, and Peripheral proteins
• Forms a Lipid bilayer

Membrane Proteins

• Two types of membrane proteins are integral and peripheral.
• Integral proteins are also called transmembrane proteins.

Functions of Membrane Proteins

• Ion channels in the membrane form a pore for specific ions.
• Carrier proteins transport specific substances by changing shape.
• Receptor proteins recognize specific ligands and may alter cell function.
• Enzymes catalyze reactions inside or outside the cell.
• Linker proteins anchor filaments to provide stability and participate in cell movement.
• Cell identity markers distinguish cells with MHC proteins .

Membrane Fluidity

• Membranes are fluid structures where lipids and proteins move within the bilayer.
• Lipids and proteins can easily move within their own half of the bilayer.
• Cholesterol stabilizes the membrane and reduces fluidity.

Membrane Permeability

• Plasma membranes are permeable to different substances depending on their properties.
• The lipid bilayer is always permeable to oxygen, carbon dioxide, and steroids.
• It is somewhat permeable to water and urea.
• It is never permeable to glucose and amino acids.
• Transmembrane proteins that act as channels or transporters increase membrane permeability.

Gradients Across the Plasma Membrane

• A concentration gradient is the difference in concentration of a chemical between one side of the plasma membrane and the other.
• An electrical gradient is the difference in concentration of ions between one side of the plasma membrane and the other.
• Together, these gradients make up an electrochemical gradient.

Transport Across the Plasma Membrane

• Transport processes that move substances across the cell membrane include passive and active processes.
• Passive processes includes: simple diffusion, facilitated diffusion, and osmosis,
• Active processes includes primary active transport

Passive Transport: Simple Diffusion

• Diffusion is influenced by several factors
• These factors include: steepness of the concentration gradient, temperature, mass of diffusing substance, surface area, and diffusion distance.
• Simple diffusion involves the movement of compounds from an area of high concentration to low concentration
• Requires a permeable substance like Vitamin E and fatty acids

Passive Transport: Facilitated Diffusion

• Transmembrane proteins help solutes that are too polar or too highly charged to cross the lipid bilayer
• Most membrane channels are ion channels.
• There are two types of facilitated diffusion, one using protein channels and the other using carrier proteins.

Diffusion: A Comparison

• Facilitated diffusion requires the help of proteins, in the form of a channel or a carrier.
• Simple diffusion does not require any help, and relies on gradients pushing a substance across a permeable membrane

Passive Transport: Channel Mediated Facilitated Diffusion

• Channel proteins form a pore that opens and closes via a gate

Passive Transport: Carrier Mediated Facilitated Diffusion

• Carrier proteins require a glucose molecule and glucose transporter carrier proteins

Osmosis

• Osmosis is the net movement of a solvent (WATER) through a selectively permeable membrane
• Movement occurs from an area of high concentration to an area of low concentration.
• Can only occur when the membrane is permeable to water, not solutes.
• Process continues until the equilibrium is reached.

Tonicity

• Tonicity of a solution affects the shape of body cells based on solute concentration.
• In a hypertonic solution, the concentration of solutes is higher outside the cell, leading to cell shrinkage
• The cell stays normal shape in Isotonic solution
• In hyptonic solution the cell will grow undergo hemolysis

Active Transport

• Active transport requires carrier proteins/molecules and energy

Active Transport: Primary

• Primary active transport can move a substance from a low concentration to high concentration gradients
• Energy from ATP changes the shape of transporter proteins to pump substances across the membrane against its concentration gradient.
• An example is the sodium-potassium pump (Na+/K+ ATPase)

Organelles of a Cell

Cytoplasm

• The cytoplasm has two components: cytosol(intracellular fluid portion) and organelles.
• Cytosol makes up 55% of total cell volume
• Organelles are specialized structures with specific shapes and functions.

Cytoskeleton

• The cytoskeleton helps determine cells shape and movement.
• it contains Microvilli and Microfilaments.
• Intermediate filaments provides support to cells
• A Composed of the protein tubulin (involved in cell division)

Cilia and Flagella

• Cilia are hair-like projections that extend from the cell surface.
• They have a power stroke, similar to an oar moving through water.
• Their movement helps move foreign particles away from interfering with proper cell function
• Flagella are much longer than cilia and move an entire cell.

Nucleus

• The nuclear envelope consists of a double membrane (lipid bilayers).
• Nuclear pores are openings that control the movement of substances to the nucleus
• The nucleolus is a cluster of DNA, RNA, and proteins to produce ribosomes.

Nucleus

• Genes are arranged along chromosomes in the nucleus.
• The nucleus contains the hereditary units of the cell called genes
• Chromosomes are made of DNA and a histone protein complex.
• Histones provide structural support for DNA to maintain integrity

Gene Expression

• Genes express themselves through the creation proteins

Ribosomes

• Ribosomes are granules of protein and RNA.
• Ribosomes assemble amino acids into proteins using directions in messenger RNA.

Protein Synthesis: Phase I - Transcription

• In Transcription, the DNA is copied to RNA (called mRNA) to direct protein synthesis
• transcription begins and transcription ends

Transcription

• Transcription produces Messenger RNA (mRNA), Ribosomal RNA (rRNA, and Transfer RNA (tRNA)
• mRNA directs protein synthesis of nucleotides which are specific for certain amino acids
• rRNA joins with ribosomal proteins to make new ribosomes

Protein Synthesis: Part II - Translation

• Translation occurs where the mRNA nucleotide sequence is read to determine the amino acid sequence of a protein.
• mRNA leaves cell to enter ribosome
• Amino acid (t-RNA) will match triplets (codon, anti-codon) to create protein
• Protein goes onto protein synthesis

Protein Synthesis

• DNA transcribed to RNA
• RNA converted to mRNA, which moves to ribosome to initiate translation to protein
• Post-translational degradation can occur as well

Endoplasmic Reticulum

• Rough Endoplasmic Reticulum (RER) are made of flattened membranous sacs covered with ribosomes
• RER creates membrane and cytosolic proteins, which leave the ribosome.
• Smooth Endoplasmic Reticulum (SER) extends from RER and synthesis fat and hormones
• SER are involved in glucose metabolism and muscle contraction

Golgi Complex

• The Golgi complex forms lysosomes.
• Lysosomes function with 60 different digestive and hydrolytic enzymes.
• Autophagy is a mechanism digestion of worn-out organelles and removal of protein aggregates
• Autolysis is the destruction of entire cell

Peroxisomes

• Peroxisomes are structures similar to lysosomes, but smaller and contain enzymes.
• Uses enzymes to oxidize organic/toxic substances like alcohol

Proteasomes

• Proteasomes are barrel-shaped structures that destroy unneeded, damaged, or faulty proteins by cutting long proteins into smaller peptides.
• Peptides broken down via proteases to be re-uses as new proteins

Mitochondria

• The matrix of mitochondria contains ribosomes and circular mtDNA
• Cristae are the inner membranes of mitochondria
• Cristae synthesize ATP from the energy of lipids, carbs etc.

Cell Division

• Cell division has two types:
  • Somatic cell division (mitosis and cytokinesis) for any body cell besides reproduction
  • Reproductive cell division (meiosis) produces gametes such as sperm and oocytes

Cell Cycle and Somatic Cell Division

• Cell cycle consists of interphase and mitotic phase.

Replication of DNA

Mitosis

• Mitosis is separated into 4 phases: prophase, metaphase, anaphase, and telophase.
• When the nucleus of a cell divides
• Results in 2 sets of chromosome being distributed into 2 separate nuclei

Mitosis: Prophase

• Chromatin condenses into chromosomes and the nuclear membrane disappears.
• Centrosomes move to opposite poles.
• Mitotic spindle is formed.

Mitosis: Metaphase

• Chromosomes align in middle of mitotic spindle,

Mitosis: Anaphase

• Chromosomes separates and the sister chromatid begin to move the opposite ends of the cells
• Cytokinesis process occurs.

Mitosis: Telophase

• Mitotic spindle dissolves
• New nuclear membrane forms and the cell divides

Cytokinesis

• The division of cells
• Cleavage furrow forms, and the parent cell splits.
• When complete, interphase begins again

Basic Summary

• Interphase - Period between divisions
• G1 phase - active cell duplicates organelles and cytosolic components and begins replication
• S Phase - DNA replicates
• G2 phase - Cell grows and synthesis completes

Control of Cell Destiny

• There are three possible destinies for a cell:
  • Remain alive and functioning without dividing.
  • Grow and divide
  • Die

Reproductive Cell Division: Meiosis I and II

• Meosis 1 - splitting homologous pairs of chromosomes
• Meiosis 2 - sister chromatids separate

Cellular Diversity

• Cells vary based on size and shape depending on purpose and organism
• Examples include sperm cells, smooth muscle cells, nerve cells etc.

Aging and Cells

• Our cells gradually deteriorate as we age, and become more vulnerable to environmental stressors
• Body cells diminish in number, and the integrity of the extracellular components of our tissues is lost

Telomeres and Aging

• Telomeres are repetitive DNA sequences at the ends of all human chromosomes. Telomere regulate when a cell will divide, and shorten over time • Telomere shrink with time decreasing cell life eventually • Stress decreases length of cells • Can maintain telomere length by reducing stress etc

Cancer

• Cancer is an uncontrolled multiplication that causes death
• Cells spread by blood stream or lymphatic system
• A Tumour has no physiological purpose: can be benign and encased, but most of the time is cancerous

Tumor Development

• Genetic mutation that increases its propensity to divide
• Genetically mutated cell, and descendants reproduce too much (hyperplasia)
• Cells appear abnormal, and are now said to exhibit dysplasia
• Affected cells increase/mutate to exhibit more abnormality
• The tumour can travel and form metastatsis through out the bloody/lymph systems

Metastasis

• Cancer cells move through the body, colonizing other organisms

Types of Cancer

• Begins in epithelium cells or supporting tissues/cells
• Most commonly appears in the form of melanoma etc.

Test your knowledge of cell structures such as endoplasmic reticulum and cell division focusing on mitosis and DNA replication alongside the role of histone proteins and RNA polymerase.