Cellular Organization & Plasma Membrane

Cellular Levels of Organization

• A cell is the basic, living, structural, and functional unit of living organisms
• There are about 200 different types of cells in our body
• All cells are produced by the process of cell division
• Cell biology studies cellular structures and their functions
• The structure of a cell is intimately related to its function

Parts of a Cell

• Three major parts include the plasma membrane, cytoplasm, and nucleus
• The cytoplasm has two components: cytosol and organelles
• Cytosol is the fluid portion of the cytoplasm that contains 75-90% water, ions, amino acids, proteins, lipids, ATP, and waste products
• Organelles include ribosomes, endoplasmic reticulum, Golgi complex, mitochondria, and the nucleus

Plasma Membrane

• It is composed of proteins and lipids (phospholipids)
• Held together by non-covalent forces
• Phospholipid molecules have an electrically charged, hydrophilic head and a hydrophobic, uncharged tail
• Embedded in this layer are sugar molecules

Functions of the Plasma Membrane

• Offers protection, and acts as a barrier
• Maintains the cell's shape
• Forms cell junctions and facilitates cell movement
• Selective permeability that allows impulse transmission

Cytoplasm

• Has two components: cytosol and organelles

Cytosol

• It is the fluid portion, containing 75-90% water, ions, amino acids, proteins, lipids, ATP, and waste products

Organelles

• Includes ribosomes, endoplasmic reticulum (smooth & rough), Golgi complex, mitochondria, and nucleus

Ribosomes

• These are tiny granules composed of RNA & protein
• They synthesize proteins from amino acids using RNA
• Ribosomes that are present in free units in the cytoplasm make proteins for use within the cell
• Ribosomes are also found on the outer surface of the nuclear envelope and rough endoplasmic reticulum, where they manufacture proteins for export from the cell

Endoplasmic Reticulum

• It is a series of interconnecting membranous canals in the cytoplasm
• There are two types of endoplasmic reticulum: smooth and rough
• Smooth endoplasmic reticulum lacks ribosomes
• Rough endoplasmic reticulum is studded with ribosomes that synthesize proteins

Golgi Apparatus

• It has a stack of closely folded flattened membranous sacs
• Present in all cells, but larger in cells that synthesize and export proteins
• Proteins move from the ER to the Golgi apparatus, where they are 'packaged' into membrane-bound vesicles called secretory granules
• Vesicles are stored and exported through the plasma membrane when needed

Nucleus

• The nucleus contains the nucleolus, chromatin, nuclear pores, and the nuclear envelope
• Every cell in the body has a nucleus, except mature red blood cells
• Skeletal muscle and some other cells contain several nuclei
• It is the larger organelle in the cell and is contained within the nuclear envelope
• The nucleus contains body's genetic material that directs metabolic activities of the cell
• 46 chromosomes made from DNA make up the nucleus

Mitochondria

• The mitochondria contains the intermembrane space, matrix, cristae, DNA, ribosomes, and granules
• This is also known as the 'Power House'
• Involved in aerobic respiration
• Aerobic respiration is when chemical energy is made available in the cell
• Form of energy is in ATP
• ATP releases energy when the cell breaks down
• ATP synthesis is most efficient in the final stage of aerobic respiration, requiring oxygen

Transport of Material Across the Cell

• The motion of substances in and out of the cell
• Cell membranes are selectively permeable
• There are two types of transport mechanisms: passive and active transport

Membrane Transport

• Passive transport is the movement of molecules through the membrane where no energy is required from the cell
• Active transport requires energy expenditure by the cell

Passive Transport

• Passive transport is the movement of molecules through the membrane where no energy is required from the cell
• Molecules move in response to a concentration gradient
• A concentration gradient is a difference between the concentration on one side of the membrane and that on the other side
• Passive transport mechanisms only movement substances along the concentration gradient
• Substances move from an area of high concentration to an area of low concentration

Mechanisms of Passive Transport

• Diffusion and osmosis

Diffusion

• Movement of solute molecules from high solute concentration to low solute concentration
• There are two types of diffusion: simple and facilitated diffusion

Simple Diffusion

• Substances pass directly through the cell membrane
• Cell membrane has limited permeability to small polar molecules, water, and ions
• Motion of water across the membrane is known as osmosis
• Simple diffusion rate depends on the degree of concentration gradient
• Diffusion slows when the gradient reaches equilibrium
• At equilibrium, substances pass in and out of the membrane at equal rates

Facilitated Diffusion

• Substances must pass through transported proteins to get through the cell membrane
• Cell membrane is selectively permeable
• Carrier proteins bind to the molecule that they transport across the membrane

Selective Permeability

• Integral membrane proteins allow the cell to be selective about what passes through the membrane
• Channel proteins have a polar interior which allows polar molecules to pass through
• Carrier proteins bind to a specific molecule, which facilitates its passage

Osmosis

• Osmotic concentration is determined by the concentration of all solutes in the solution

Active Transport

• ATP is used directly or indirectly to fuel active transport, requiring energy
• It moves substances against the concentration gradient from low to high concentrations
• Allows cells to store concentrated substances
• Requires the use of carrier proteins

Bulk Transport

• It is how bulk transport of substances is accomplished
• Includes Endocytosis and Exocytosis

Endocytosis

• Movement of substances into the cell

Exocytosis

• Movement of materials out of the cell

Bulk Transport

• Endocytosis occurs when the plasma membrane envelops food particles and liquids
• Phagocytosis occurs when the cell takes in particulate matter
• Pinocytosis occurs when the cell takes in only fluid
• Receptor-mediated endocytosis occurs when specific molecules are taken in after they bind to a receptor

Cell Junctions

• Intercellular space in closely packed tissue is about 20nm
• Cells are bound together by a specific adhesive glycoprotein
• Modified cell membranes that contribute to cohesion and communication are called cell junctions
• Epithelial cells adhere to each other through glycoproteins called Cadherins

Types of Cell Junctions

• Tight junctions, adhering junctions, and communicating junctions

Tight Junctions

• These are found in epithelial tissues and are known as "Tight Junctions"
• Do not allow passage of small molecules from an impermeable membrane
• Types include Zonula Occludens and Fascia Occludens

Zonula Occludens

• Encircles the entire cell perimeter and occludes the intercellular space
• It is a series of focal fusions occur as the adjacent cell membranes
• At fusions sites specific trans membranous proteins such as Occludins, and Claudins perform the binding function
• It is less in PCT and more in the intestinal mucosa

Fascia Occludens

• Acts as a strip like tight junction of limited extent
• Found between the endothelial cells of blood vessels

Adhering Junctions

• Acts like Anchoring junctions
• Provides cell-cell or cell to basal lamina adherence

Types of Adhering Junctions

• Zonula adherens
• Fascia adherens
• Macula adherens (Desmosomes)
• Hemidesmosomes

Zonula Adherens

• Acts as a belt-like junction with no fusion of cell membranes
• Trans membranous glycoprotein "E-cadherin" occupies intercellular gap
• E-cadherin links to adherent proteins in the cytoplasm such as Catenin and Vinculin

Fascia Adherens

• Structurally similar to Zonula adherence
• Cell junction is strip-like and is seen in Cardiac muscle cells.

Desmosomes

• Macula adherens are commonly known as desmosomes or "Spot-weld" like junctions
• Randomly distributed along lateral plasma membranes of cells in the simple epithelium
• It is distributed throughout the plasma membrane in stratified epithelium
• Also found in cardiac muscle cells
• Cell membrane in the region of junctions that are farther apart (30mm) than the usual gap
• Electron dense attachment plaques are located opposite to each other on the cytoplasmic aspects
• Intermediate filaments of the cytoskeleton are anchored to the attachment plaques and two types of transmembranes
• Glycoproteins that are named Desmocolins and Desmogleins provide adherence

Hemidesmosomes

• These junctions anchor the epithelial cells to the basal lamina
• A hemidesmosome is a spot-like adhering junction which gives the appearance of a half desmosome
• Transmembrane linker proteins in hemidesmosomes are integrins
• The cytoplasmic intermediate filaments of keratin are inserted in to the attachment plaque

Communicating Junctions

• These are characterized by the presence of minute tubular passageways
• They provide direct cell-to-cell communication
• Tubular passages allow movement of ions and other small molecules between adjacent cells

Gap Junction

• This is called the "Nexus," which is a communication junction frequently found between epithelial cells
• It can also is found in cardiac muscle cells, smooth muscles, neurons, astrocytes, and osteocytes
• A gap of only 2nm occurs between the plasma membranes of adjoining cells
• Intercommunicating channels are located within the gap junction
• average diameter is 1.5nm of the channels of a gap junction
• Permit free passage of ions, sugar, and amino acids
• They provide electrical coupling of the adjacent cells in cardiac and smooth muscles
• Gap junctions are frequently found in the embryonic cells

Cell Cycle

• Cells that belong renewing populations undergo many sequences of repeating events
• This cycle is divided into two parts of M PHASE and INTERPHASE
• M PHASE occurs in (30 to 60 minutes)

Interphase

• Is the intervening period between two cells, consisting of three subphases
• During the G1 Phase: synthesis of RNA and proteins occur, cell size is restored to normal and lasting about 8 hours
• During the S-Phase: synthesis of DNA happens, resulting in creating an exact replica of a genetic material and lasts about 8 hours
• During the Duration of G2 Phase: production and accumulation of energy for mitosis takes place, lasting between 2 to 4 hours

Cell Division

Mitosis

• The process of cell division creates two daughter cells from a single parent cell
• Daughter cells are identical to one another and to the original parent

Stages of Mitosis

• Interphase, prophase, metaphase, anaphase, and telophase

Interphase

• The cell prepares for division by DNA, Organelles, and cell size replicated/increases

Prophase

• The cell prepares for nuclear division with Chromosomes visible
• Nucleoli disappear as Centrioles separate and migrate to each pole
• Threads become shorter/thicker with two chromatids joined by a centromere

Metaphase

• The cell prepares chromosomes for division
• Chromosomes line up the center of the cell
• Spindle fibers attach to chromosomes at the centromere as the Equatorial plate is formed
• Microtubules exert pull on chromosomes

Anaphase

• Spindle fibers pull chromosomes apart
• 1/2 of each chromosome (called chromatid) moves to each daughter cell as chromatids separate and move to independent poles as an independent chromosome
• Two identical sets of 46 chromosomes happen in human cells

Telophase

• The cytoplasm divides as a constriction called the cleavage furrow appears
• Nuclear envelop forms, enclosing chromosomes
• Two nuclei form as the cell wall pinches in to form two new daughter cells

Meiosis

• Meiosis is the type of cell division by which germ cells (eggs and sperm) are produced
• One parent cell produces four daughter cells
• The daughter cells have half the number of chromosomes from the original parent cell
• DNA replicates once during meiosis, but the nucleus divides twice
• There are four stages described for each division of the nucleus

Prophase

• Much longer, consisting of five stages

The five stages

• Leptotene: Chromosomes become visible in the nucleus
• Zygotene: Homologous chromosomes come together along their entire length and synapses are formed
• Pachytene: Chromosomes become thicker and shorter; each chromosome pair is called bivalent
• Diplotene: Chromosomes begin to separate along their length and each bivalent consists of four chromatids
• Diakinesis: Separation of chromosomes continues as the nucleolus disappears
• The nuclear envelop also disappears in Diakinesis

Metaphase

• A spindle of microtubules is produced by centrioles
• The equatorial plate is formed in which the bivalent chromosome pairs align in the center of the spindle

Anaphase

• Chromosomes of homologous pairs completely separate and move to opposite poles
• Centromere division does not happen here, instead the chromosomes move to opposite poles

Telophase

• Nuclei are reconstructed as the parent cell is divided into two daughter cells where each contains haploid (23) chromosomes
• Each chromosome is double structured, consisting of two sister chromatids

Differences in Mitosis & Meiosis

Mitosis

• Asexual
• Cell divides once
• Two daughter cells are produced
• Genetic information is identical

Meiosis

• Sexual
• Cell divides twice
• Four haploid daughter cells are produced
• Genetic information is different