Chapter 3 Cellular Form and Function
Chapter 3: Cellular Form and Function
Introduction
All organisms are composed of cells.
Cells are responsible for all structural and functional properties of a living organism.
Important for understanding:
Workings of the human body
Mechanisms of disease
Rationale of therapy
3.1 Concepts of Cellular Structure
Expected Learning Outcomes
Discuss the development and modern tenets of the cell theory.
Describe cell shapes from their descriptive terms.
State the size range of human cells and discuss factors that limit their size.
Discuss the way that developments in microscopy have changed our view of cell structure.
Outline the major components of a cell.
Development of the Cell Theory
Cytology: The scientific study of cells.
Began when Robert Hooke coined the word "cellulae" to describe empty cell walls of cork in the 17th century.
Theodor Schwann concluded that all animals are made of cells 200 years later.
Louis Pasteur demonstrated in 1859 that "cells arise only from other cells," refuting spontaneous generation (the idea that living things arise from nonliving matter).
Cell Theory
All organisms are composed of cells and cell products.
The cell is the simplest structural and functional unit of life.
An organism's structure and functions are due to activities of cells.
Cells come only from preexisting cells.
Cells of all species exhibit biochemical similarities.
Cell Shapes and Sizes
Types of Human Cells
Approximately 200 types of cells in the human body with varied shapes:
Squamous: Thin, flat, scaly.
Cuboidal: Squarish-looking.
Columnar: Taller than wide.
Polygonal: Irregularly angular shapes, multiple sides.
Stellate: Star-like.
Spheroid to ovoid: Round to oval.
Discoidal: Disc-shaped.
Fusiform: Thick in the middle, tapered toward the ends.
Fibrous: Thread-like.
Note: A cell’s shape can appear different depending on the type of section viewed (longitudinal versus cross-section).
Human Cell Sizes
Most human cells range from 10 to 15 μm in diameter.
Egg cells (very large) measure about 100 μm in diameter.
Some nerve cells can be over 1 m long.
Limitations on Cell Size:
An overly large cell cannot support itself and may rupture.
Volume increases more than surface area for given increases in diameter:
Volume is proportional to the cube of diameter: V \propto D^3
Surface area is proportional to the square of diameter: A \propto D^2.
The Relationship Between Cell Surface Area and Volume
Visual Representation: (Note: Requires figure reference - Figure 3.2)
Basic Components of a Cell
Microscopy Techniques
Light Microscope (LM): Revealed basic components like the plasma membrane, nucleus, and cytoplasm (fluid between nucleus and surface).
Transmission Electron Microscope (TEM): Improved resolution to reveal further details.
Scanning Electron Microscope (SEM): Provided further resolution for surface features.
Cell Composition
Plasma Membrane: Surrounds the cell and defines its boundaries; composed of proteins and lipids.
Cytoplasm: Contains organelles, cytoskeleton, inclusions, and intracellular fluid (ICF).
Extracellular Fluid (ECF): The fluid outside of cells, including tissue (interstitial) fluid.
Structure of a Representative Cell
Components include microvilli, desmosomes, fat droplets, secretory vesicles, organelles like mitochondria, lysosomes, and endoplasmic reticulum.
Detailed component relationships are illustrated in Figure 3.4.
3.2 The Cell Surface
Expected Learning Outcomes
Describe the structure of the plasma membrane.
Explain the functions of lipid, protein, and carbohydrate components of the plasma membrane.
Describe a second-messenger system and its importance in human physiology.
Describe the composition and functions of the glycocalyx.
Describe structures and functions of microvilli, cilia, and flagella.
The Plasma Membrane: Structure and Functions
Definition: The plasma membrane is the border of the cell, appearing as a pair of dark parallel lines under an electron microscope.
Functions include:
Defining cell boundaries
Governing interactions with other cells
Controlling passage of materials in and out of the cell.
Membrane Lipids
98% of membrane molecules are lipids:
Phospholipids: Make up 75% of membrane lipids, arranged in a bilayer with hydrophilic heads facing water and hydrophobic tails directed toward the center. They remain fluid by drifting laterally.
Amphipathic molecules arranged in a bilayer
Cholesterol: Comprises 20% of membrane lipids and helps maintain membrane structure by holding phospholipids still or stiffening the membrane.
Glycolipids: About 5% of the membrane lipids, they are phospholipids with short carbohydrate chains on the extracellular face and contribute to the glycocalyx, which serves as a carbohydrate coating on the cell surface.
Membrane Proteins
Composed of 2% of membrane molecules yet account for 50% of the membrane's weight.
Integral Proteins: Penetrate the membrane, some (transmembrance) passing completely through; have hydrophilic regions interacting with cytoplasm and extracellular fluid, and hydrophobic regions embedded in the lipid layer.
Peripheral Proteins: Adhere to one face of the membrane, often tethered to the cytoskeleton.
Functions of membrane proteins include:
Receptors and second-messenger systems
Enzymatic activity
Channels and carriers for solute transport
Cell-identity markers and cell-adhesion molecules (CAMs).
Membrane Protein Functions include:
Receptors: Bind chemical signals.
Second Messenger Systems: Enable communication within the receiving cell when a chemical message is presented.
Enzymes: Catalyze reactions, including digestion of molecules.
Channel Proteins: Provide passageways for solutes and water to cross the membrane.
Some channels are gated and respond to chemical messengers.
ligand-gated channels: respond to chemical messengers
Voltage-gated channels: respond to charge changes
Mechanically-gated channels: respond to physical stress, such as stretch or pressure, allowing ions to flow across the membrane.
Carriers: Bind solutes to transport them across the membrane.
Cell-Identity Markers: Glycoproteins serving as tags for cellular identification.
Cell-Adhesion Molecules: Prominent in anchoring cells together and aiding in tissue formation. (mechanically linked)
Second Messenger Systems
Chemical first messenger (e.g., epinephrine) binds to a receptor, activating the G protein, which relays the signal to adenylate cyclase, converting ATP to cAMP (the second messenger).
cAMP activates cytoplasmic kinases that phosphorylate other enzymes, triggering varied metabolic effects.
Notably, up to 60% of drugs target G proteins and second messenger pathways.
Glycocalyx
Description: A fuzzy coat external to the plasma membrane composed of carbohydrate moieties of glycoproteins and glycolipids.
Unique Features: Unique to every individual except identical twins, possessing several functions including:
Protective role
Cellular adhesion tendencies
Immunity to infection
Fertilization functions
Cancer defense
Embryonic development
Transplant compatibility considerations.
Microvilli
Extensions of the membrane (1 to 2 μm), provide significant increases in surface area (15 to 40 times more) and are prominent in absorptive cells, sometimes appearing as a (Fringe) brush border due to high density.
Function: Aid absorption, with some microvilli containing actin filaments that tugged toward center of the cell help milk absorbed contents into the cell.
Cilia
Characteristics: Hair-like processes ranging from 7 to 10 μm.
Primary cilium is usually nonmotile, acting as an “Antenna” to monitor near by conditions (e.g., balance in the inner ear and light detection in the retina).
Motile cilia, prolific in tissues like the respiratory tract, exhibit a 9+2 microtubule array, assisting in sweeping material across surfaces in unison.
Ciliopathies: Disorders stemming from structural and functional defects in cilia.
Motile Cilia: ( less widespread) on respiratory tract, uterine tubes (fallopian), ventricles of the brain, and duct of testes.
50-200 per cell surface.
they beat in waves which sweeps material across a surface in one direction.
when they bend, power stroke happens which is follow by recovery strokes. and back to upright position.
Axoneme:
The structural framework of cilia and flagella composed of microtubules arranged in a characteristic "9+2" pattern, crucial for movement and functionality.
Cilia: beats freely w/in a saline layer at cell surface.
chloride pumps, pump Cl- into ECF
Na+ and H20 follows next.
Cystic Fibrosis: hereditary disease in which cells makes chiloride channels that do not function properly, leading to thick, sticky mucus accumulation in various organs, particularly the lungs and pancreas. this results in severe respiratory issues, digestive problems, and recurrent infections due to impaired clearance of pathogens.
inadequate digestion of nutrients and absorption of oxygen
chronic respiratory infections
life expectancy of 30
Flagella
The tail of sperm, identified as the only functional flagellum in humans, features a whip-like structure comparable in axoneme to cilia but longer, facilitating movement via undulating, snake-like propulsion without distinct power and recovery strokes.
Pseudopods
Continually changing extensions utilized for cell locomotion and capturing foreign particulate matter.
3.3 Membrane Transport
Expected Learning Outcomes
Explain the concept of a selectively permeable membrane.
Describe mechanisms for transporting material through cellular membranes.
Define osmolarity and tonicity, elaborating on their importance.
Membrane Transport Mechanisms
Plasma and organelle membranes are selectively permeable, allowing certain substances to pass while blocking others.
Passive Mechanisms: No ATP require; rely on random molecular motions (includes filtration, diffusion, osmosis) for energy.
Active Mechanisms: Consume ATP (includes active transport and vesicular transport).
Carrier-Mediated Mechanisms: Utilize membrane proteins for transporting substances across membranes.
Filtration
Definition: Movement driven by physical pressure,
allows delivery of water and nutrient to tissues
allows removal of waste from capillaries in kidneys.
Examples of filtration:
Water and solute filtration through capillary walls.
Simple Diffusion
Net movement of particles from high to low concentrations due to spontaneous molecular motion.
molecules collide and bounce off each other
Substances diffuse down their concentration gradient
does not require a membrane
substance can diffuse through a membrane if permeable to the substance
- Factors impacting diffusion rate:
Temperature (higher = increased motion)
Molecular weight (larger = slower)
Concentration gradient steepness (greater difference = increased rate)
Membrane surface area (larger area = increased rate)
Membrane permeability (higher permeability = increased rate).
Osmosis
Definition: The net flow of water through a selectively permeable membrane from a more concentrated water side to a less concentrated one.
Importance: Key in IV fluid considerations and osmotic balance, with critical roles in conditions like diarrhea and edemas.
Water can diffuse through phospholipid bilayers while being enhanced by aquaporins (specialized channel proteins).
Osmotic Pressure: Amount of hydrostatic pressure needed to halt osmosis, increasing with the amount of nonpermeating solute.
cells can speed osmosis by installing more aquaporins.
osmotic imbalances underlie diarrhea, constipation, edema.
Reverse osmosis: process of applying mechanical pressure to overide osmotic pressure. '
allows purification of water.
Osmolarity and Tonicity
Osmolarity: (number of osmoles per liter of solution) One osmole (osm) defines one mole of dissolved particles; considers whether solute ionizes in water.
one osmole (osm) = 1 mole of dissolved particles
1 m gluclose = 1 osm/L , while 1 m NaCl = 2 osm/L due to its dissociation into two ions in solution (Na+ and Cl-).
1 m NaCI = 2 osm/L This is because NaCl dissociates into two ions (Na⁺ and Cl⁻) when dissolved in water, effectively doubling the osmolarity. For example, 1 m KCl will also yield 2 osm/L as it dissociates into K⁺ and Cl⁻ ions.
Generally, body fluids have osmolarities of approximately 300 milliosmoles per liter (mOsm/L). blood plasma, tissue fluid, and intracellular fluids.
Tonicity: The influence of a surrounding solution on cell fluid volume and pressure in a cell, affected by the concentration of nonpermeating solutes:
Hypotonic: Causes cells to swell due to low nonpermeating solute concentration
EX: distilled water
is an example of a hypotonic solution, as it has a significantly lower concentration of solutes compared to the interior of the cell, leading to water influx and eventual cell swelling.
Hypertonic: Leads cells to lose water and appear shriveled (crenated) due to high solute concentration
Isotonic: No notable change in cell volume; equal solute concentrations on both sides.
Examples: Normal saline (0.9% NaCl) is isotonic.
Carrier-Mediated Transport
Transport proteins are specific for particular solution.
Specificity: Transport proteins selectively bind particular solutes and release unchanged on the opposite side.
Saturation: As solute concentration rises, transport rate increases until reaching a maximum (transport maximum, Tm), where all carriers are engaged.
Three types of carriers:
Uniport: Transports one type of solute (e.g., calcium pump).
Symport: Carries two or more solutes in the same direction (e.g., sodium-glucose transporter).
Antiport: Transports two or more solutes in opposite directions (e.g., sodium-potassium pump). ( this is countertransport)
Facilitated Diffusion
3 mechanisms of carrier-mediated transport
facilitated diffusion
primary active transport
secondary active transport
Facilitated diffusion: Mediated by carriers, moving solutes down their concentration gradients without ATP consumption.
Primary active transport: Involves the direct use of ATP to transport ions against their concentration gradients.
examples
calcium pump (uniport) uses ATP when removing calcium from cell to where it is already more concentrated.
sodium -potassium pump (antiport) uses ATP while removing sodium and importing potassium into cell.
Secondary active transport: carrier moves solute through membrane but use ATP indirectly.
example: sodium-glucose transporter:
moves glucose into cell while simultaneously carrying sodium down in gradient.
depend on primary tranport performed by Na+-K+ pump
does not use ATP.
A carrier moves solutes via ATP against concentration gradients (e.g., sodium-potassium pump).
Vesicular Transport
How one/more ions or molecules move through plasma membrane.
Moves large particles, fluid particles or volumes using vesicles (membrane bubbles like vesicles of membrane); uses motor proteins powered by ATP.
Endocytosis (3 types): Captures materials in vesicles: bring material into the cell.
Phagocytosis: "Cell eating," engulfing large particles such as bacteria, dust, and cellular debris.
ex: neutrophil (white blood cell) traps bacteria by phagosome (eating them).
Pinocytosis: "Cell drinking," taking in fluid droplets.
Receptor-Mediated Endocytosis: Specific uptake interacting with extracellular receptors. membrane sinks in, creating pit coated call clathrin, which than pinch off to form a clathrin-coated vesicle (serve as “address label), ensuring the selective transportation of molecules into the cell.
selective form of phagocytosis or pinocytosis.
Exocytosis and Transcytosis
Exocytosis (exit): process of discharing material from cell. (when endothelial cells release insulin from tissues.
Transcytosis (in): Moves materials across a cell by capturing them and release on the opposite side of the membrane.
3.4 The Cell Interior
Expected Learning Outcomes
Describe the cytoskeleton and its functions.
List the main organelles within a cell, outlining structures and functions.
Provide examples of cell inclusions and differentiate them from organelles.
The Cytoskeleton
Definition: A network of protein filaments and cylinders that:
Determining cell shape, supporting structural integrity, organizing contents, directing material movements, contributing to cellular locomotion.
Composed of:
Microfilaments: Actin filaments (~6 nm); form terminal web. concentrated through a fibrous mat call terminal web (membrane skeleton)
Intermediate Filaments: (~8 to 10 nm), made of protein keratin in skin cells, resist stress. give cell its shape. form junction that junction that connect cells. responsible for hair strenght and fingernails.
Microtubules: (~25 nm); consists of protofilaments made of protein tublin, that radiate from centrosome (come and go)
maintain cell shape
hold organelles
act as railroad tracks for walking motor proteins
make axonemes of cilia and flagella.
form mitotic spindle
Definition: Internal cellular structures performing specialized metabolic tasks.
Membranous Organelles:
Include nucleus,
mitochondria,
lysosomes,
peroxisomes,
endoplasmic reticulum,
Golgi complex.
Non-Membranous Organelles: Include
ribosomes,
centrosomes,
centrioles,
basal bodies.
Nucleus
Typically the largest organelle (~5 μm in diameter).
Usually singular per cell, with few cell types are (anuclear or multinucleate cells).
Nuclear Envelope: A double membrane around nucleus.
perforated by nuclear pores formed by ring of proteins
pores regulating molecular traffic.
hold two membrane layer together
Nucleoplasm: Material within the nucleus containing chromatin (thread-like) (DNA and protein) and nucleoli masses where ribosomes are produced.\
Endoplasmic Reticulum (ER)
Definition: A network of channels enclosed by membranes.
ER synthesis steroids and other lipids, detoxifies alcohol and other drugs
Rough ER: Ribosome-studded, produce proteins and phospholipids.synthesized large number of protein (antibody-producing cells)
Smooth ER:
Lacks ribosomes,
synthesizes lipids and steriods,
detoxifies substance (drugs and alcohol)
stores calcium.
Cisterns though to be continues with rough ER
Cisterns more tubular and branching.
scanty smooth ER in liver and kidney cell.
smooth and rough ER= same network different part.
Ribosomes
small granules of protein and RNA
found in nucleoli, cytosol, and on outer surfaces of rough ER, and nuclear envelope, playing a crucial role in protein synthesis by translating mRNA into polypeptide chains.
assemble amino acids into protein specified by the code
Golgi Complex
A system of channels that modifies, sorts, and packages newly syntheized proteins received from the rough ER, preparing them for secretion, lysosome formation, or integration into the plasma membrane.
sorts proteins, slices some,
adds carbohydrate moieties to some and packages them into membrane-bound Golgi vesicles.
some vesicles become lysosomes
come vesicles migrates to plasma membrane and fuse to it
some become secretory vesicles that store a protein product for later release.
Lysosomes (generally round, but variable in shape)
package of enzymes bound by membrane
Membrane-bound enzyme packages for intracellular digestion, capable of breaking down proteins, phospholipids, nucleic acids, and other substances.
Functions include:
intracellular hydrolytic digestion of proteins
nucleic acids
complex carbohydrates
phospholipids
Autophagy: Digestion of surplus organelles.
Autolysis: Self-digestion of the cell under certain conditions. “cell suicie
Peroxisomes
Resemble lysosomes but contain different enzymes for oxidizing organic molecules, converting substrates into hydrogen peroxide, and subsequently into water and oxygen.
functions:
reactions produce hydrogen peroxide (H202)
catalase breaks down excess peroxide to H20 and 02.
neutralize free radicals, detoxify alcohol, and some blood-borne toxins
breaks down fatty acids into acetyl groups for mitochondrial use in ATP synthesis.
Contribute to detoxification of harmful substances and fats.
Proteasomes
Hollow structures that degrade surplus proteins by breaking down targeted proteins into short peptides and amino acids.
Mitochondria “powerhouse of cell”
its evolved from bacteria that invaded another primitive cell, but survived in its cytoplasm and became permanent resident.
the bacterium provided inner membrane that host cell’s phagosome in the outer membrane
Mitochondrail ribosomes remembles bacterial ribosomes
mtDNA is inherited through the mother:
hereditary diseases affecting tissues with high energy demands. This includes conditions such as mitochondrial myopathy and Leigh syndrome, which can significantly impact muscle function and neurological health.
Specialized for ATP synthesis, characterized by double membranes, cristae, and a matrix.
Evolutionarily derived from bacteria, showcasing similarities to prokaryotic cells.
continually changes shape from spheroidal to thread-like surrounded by a double membrane.
inner membrane has folds called cristae
spaced b/w cristae is call Matrix
matrix contain ribosomes, enzymes , and other necessary components for the production of ATP, which is essential for cellular energy.
mutiple molecules of mitochondrial DNA (mDNA)
Centrioles
Short cylindrical arrangements of microtubules vital to cell division, organized to assist with the formation of cilia and flagella.
form basal bodies of cilia and flagella
each basel body is a centriole that originated in centriolar organizing center and then migrated to the membrane.
Inclusions
there are two kinds;
Cellular stored products such as glycogen granules or fat droplets and foreign materials (viruses, bacteria, etc.).
Not enclosed in membranes, differ from organelles, and are generally not essential for cell survival.
foreign bodies
viruses
intracellular bacteria
dust particles
other debris phagocytized by a cell