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CELLS: The Living Units
BIO 200 Chp 3
Cell Theory:
The cell is the basic structural and functional unit of life
Organismal activity depends on individual and collective activity of cells
Biochemical activities of cells are dictated by subcellular structure
Continuity of life has a cellular basis
Cell Structure
Plasma Membrane
Separates intracellular fluids from extracellular fluids. Plays a dynamic role in cellular activity
Glycocalyx (a glycoprotein) bordering the cell that provides highly specific biological markers by which
cells recognize one another
Fluid Mosaic Model – a Double bi-layer of lipids with imbedded proteins. It forms the basic “fabric” of
the cell membrane. The Bi-layer consists of phospholipids, cholesterol and glycolipids
Hydrophilic – attracts water (polar head)
Hydrophobic – repel water (nonpolar tails)
Functions of Membrane Proteins
Transports; has Enzymatic activity; utilizes Receptors for signal transduction
Cell Plasma Structure - The plasma aims to maintain homeostasis with Lipid molecules of the by-layer
move freely
Polar-nonpolarity interactions keeps stability
Microvilla – (hairs) increase the plasma membrane surface
Plasma Membrane (Membrane Junctions) – help to knit or adhere cellular tissue (enzymes). It’s a
Tight junction – impermeable junction that encircles the cell, prevents molecules from passing through
Desmosome – anchoring junction scattered along the sides of cells, aid in mechanical stress
Gap junction – a nexus that allows chemical substances (electrical activity) to pass between cells
Functions of Plasma Membrane: Membrane transport
The Cells are surrounded by extacelluar or interstitial fluid. Interstitial fluid is rich and nutritious that
Derives from the blood stream. The Ingredients include: amino acids, sugars, fatty acids, vitamins,
hormones, salts, waste products.
Substances can move continuously across the plasma membrane. It allows some substances to pass and
excludes others. The plasma membrane is a: Selective barrier; Differential barrier; Permeable barrier
Passive Transport:
1. Simple diffusion – nonpolar and lipid-soluble substances
Diffuse directly through the lipid bilayer. Diffuse through channel proteins. Molecules disperse evenly
Passive Difussion: Facilitated diffusion - Allows transport of glucose, amino acids, and ions
Transported substances bind carrier proteins or pass through water-filled protein channels
Carrier Proteins - Are integral transmembrane proteins
Show specificity for certain polar molecules like sugars and amino acids
Molecules too large to pass so they are carried through by transport receptor carriers
Passive Transport: Diffussion through Osmosis
Occurs when concentration of a solvent is different on opposite sides of a membrane
Diffusion of water across a semi-permeable membrane
Osmolarity – total concentration of solute particles in a solution. Tonicity – how a solution affects cell
Active Transport - Uses ATP to move solutes across a membrane. Requires carrier proteins
Types of Active Transport:
1. Primary active transport – hydrolysis of ATP phosphorylates the transport protein causing
conformational change
2. Secondary active transport – use of an exchange pump (such as the Na+-K+ pump) indirectly
to drive the transport of other solutes
3. Vesicular Transport - Transport of large particles and macromolecules across plasma
a. Exocytosis – moves substance from the cell interior to the extracellular space
b. Endocytosis – enables large particles and macromolecules to enter the cell
Vesicular Transport performs the following:
Transcytosis – moving substances into, across, and then out of a cell
Vesicular trafficking – moving substances from one area in the cell to another
Phagocytosis – pseudopods engulf solids and bring them into the cell’s interior
Membrane Potential of the plasma membrane
Voltage (electrical potential) across a membrane
Resting membrane potential – the point where K+ potential is balanced by the membrane potential
range -50 to -100 millivolts (mV). The Cells become polarized which results from Na+ and K+
concentration gradients across the membrane. This is a steady state – maintained by active transport of
Cell Membrane - Cell adhesion molecules - anchor cells to the extracellular matrix, assist in movement,
Membrane Receptors - important in immunity, regulates voltage in nerve and muscle tissue and
Cytoplasm – material between plasma membrane and the nucleus
Cytosol – viscous semi-fluid, largely water with dissolved protein, salts, sugars, and other solutes
Cytoplasmic organelles – metabolic machinery of the cell
Inclusions – chemical substances such as glycosomes, glycogen granules, and pigment
Cytoplasmic Organelles
Membranous - mitochondria, peroxisomes, lysosomes, endoplasmic reticulum, and Golgi apparatus
Nonmembranous - cytoskeleton, centrioles, and ribosomes
Double membrane structure with shelf-like cristae; provides most of the cell’s ATP via aerobic cellular
respiration. Contain their own DNA and RNA
Granules containing protein and rRNA. Site of protein synthesis. Free ribosomes synthesize soluble
proteins. Membrane-bound ribosomes synthesize proteins to be incorporated into membranes
Endoplasmic Reticulum (ER)
Interconnected tubes and parallel membranes enclosing cristernae (cristae). It’s a Continuous with the
nuclear membrane. There are 2 varieties – rough ER and smooth ER
a. Rough (ER) - External surface studded with ribosomes. Manufactures all secreted proteins
Responsible for the synthesis of integral membrane proteins and phospholipids for cell membranes
b. Smooth (ER) - Looping tubule network. Catalyzes the following reactions in various organs of
the body:
1. Liver – lipid & cholesterol metabolism, breakdown of glycogen, detoxification of drugs
2. In the testes – synthesis steroid-based hormones
3. In the intestinal cells – absorption, synthesis, and transport of fats
4. In skeletal and cardiac muscle – storage and release of calcium
Golgi Apparatus
Stacked and flattened membranous sacs. Functions in modification, concentration, and packaging of
proteins. “Traffic director” for cellular protein . Transport vesicles from the ER and are received by
Golgi apparatus
Spherical membranous bags containing digestive enzymes. Digest ingested bacteria, viruses, and toxins
Degrade nonfunctional organelles. Breakdown glycogen and release thyroid hormone
Autolysis – self-digestion of the cell. Breakdown nonuseful tissue and breakdown bone to release Ca2+
Secretory lysosomes are found in white blood cells, immune cells, and melanocytes
Endomembrane System
System of organelles that function to: Produce, store, and export biological molecules; Degrade
potentially harmful substances
Contains the following system: Nuclear envelope, smooth and rough ER, lysosomes, vacuoles,
transport vesicles, Golgi apparatus, and the plasma membrane
Peroxisomes - “Peroxide bodies”
Membranous sacs containing oxidases and catalases. They Detoxify harmful or toxic substances and
Neutralize dangerous free radicals. Free radicals – highly reactive chemicals with unpaired electrons
Cytoskeleton - The “skeleton” of the cell
A Dynamic, elaborate series of rods running through the cytosol that Consists of microtubules,
microfilaments, and intermediate filaments
a. Microtubules - Dynamic, hollow tubes made of the spherical protein tubulin. Determine the
overall shape of the cell and distribution of organelles
b. Microfilaments - Dynamic strands of protein Actin. Attached to the cytoplasmic side of the
plasma membrane. Braces and strengthens the cell surface
c. Intermediate Filaments - Tough, insoluble protein fibers with high tensile strength. Resist
pulling forces on the cell and help form desmosomes Pg 91
Small barrel-shaped organelles located in the centrosome near the nucleus
Pinwheel array of nine triplets of microtubules
Organize mitotic spindle during mitosis
Form the bases of cilia and flagella
Whip-like, motile cellular extensions on exposed surfaces of certain cells
Move substances in one direction across cell surfaces
Cellular Motion
CELIA - Cellular extensions that provide motility in a whiplike motion. Typically found in large numbers
Located in the exposed surface of the cell. Move substances in one direction across cell surface
Flagella – for locomotion of a cells
The control center containing genetic . Largest cytoplasmic organelle - 5µm. Nuclear envelop –dbl
membrane barrier. Nucleoli – DNA & RNA for genetic synthesis.
Chromatin – threadlike coils that form chromosomes in cell division. Genes
Cell Growth and Reproduction: Cell Life Cycle
Cell division – essential for growth and tissue repair.
Cells die and continuously reproduce
Some reproduce faster than others (skin, intestinal vs. liver). Some loose ability to divide @ maturation
(nervous tissue, skeletal muscle, heart, RBCs). The DNA replicates before cell division
Cell Growth and Reproduction - Cell Division - M Phase (Mitotic)
2 phases: Mitosis & Cytokinesis
Phase 1: Mitosis – nuclear division
a) prophase
b) metaphase
c) Anaphase
d) telophase
Phases merge together
Phase 2 – Cytokinesis
Cytokinesis - cytoplasmic division. Cleavage furrow formed in late anaphase by contractile ring.
Cytoplasm is pinched into two parts after mitosis ends. The forming of 2 daughter cells