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CELLS: THE LIVING FUNCTIONAL
UNITS\
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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
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Structure of a Generalized Cell
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Plasma Membrane• Separates intracellular fluids from
extracellular fluids• Plays a dynamic role in cellular activity• Glycocalyx is a glycoprotein area
abutting the cell that provides highly specific biological markers by which cells recognize one another
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Fluid Mosaic Model• Double bilayer of lipids with
embedded, dispersed proteins• Bilayer consists of phospholipids,
cholesterol, and glycolipids–Glycolipids are lipids with bound
carbohydrate–Phospholipids have hydrophobic and
hydrophilic bipoles
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Functions of Membrane Proteins
• Transport• Enzymatic activity• Receptors for
signal transduction
Figure 3.4.1
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Functions of Membrane Proteins
Figure 3.4.2
• Intercellular adhesion
• Cell-cell recognition
• Attachment to cytoskeleton and extracellular matrix
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Plasma Membrane Surfaces
• Differ in the kind and amount of lipids they contain• Glycolipids are found only in the
outer membrane surface• 20% of all membrane lipid is
cholesterol
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Membrane Junctions• Tight junction – impermeable
junction that encircles the cell • Desmosome – anchoring junction
scattered along the sides of cells• Gap junction – a nexus that allows
chemical substances to pass between cells
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Cell Adhesion Molecules (CAMs)
• Anchor cells to the extracellular matrix• Assist in movement of cells past
one another• Rally protective white blood
cells to injured or infected areas
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Roles of Membrane Receptors• Contact signaling – important in normal
development and immunity• Electrical signaling – voltage-regulated “ion gates”
in nerve and muscle tissue• Chemical signaling – neurotransmitters bind to
chemically gated channel-linked receptors in nerve and muscle tissue
• G protein-linked receptors – ligands bind to a receptor which activates a G protein, causing the release of a second messenger, such as cyclic AMP
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Operation of a G Protein
• An extracellular ligand (first messenger), binds to a specific plasma membrane protein• The receptor activates a G protein
that relays the message to an effector protein
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Operation of a G Protein....• The effector is an enzyme that
produces a second messenger inside the cell• The second messenger activates a
kinase• The activated kinase can trigger a
variety of cellular responses
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Operation of a G Protein
Figure 3.16
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Cytoplasm• Cytoplasm – material between plasma
membrane and the nucleus• Cytosol – 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
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Cytoplasmic Organelles• Specialized cellular compartments• Membranous –Mitochondria, peroxisomes,
lysosomes, endoplasmic reticulum, and Golgi apparatus
• Nonmembranous–Cytoskeleton, centrioles, and
ribosomes
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Mitochondria• Double membrane structure with
shelflike cristae• Provide most of the cell’s ATP via
aerobic cellular respiration• Contain their own DNA and RNA
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Mitochondria
Figure 3.17
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Ribosomes• Granules containing protein and
rRNA• Site of protein synthesis• Free ribosomes synthesize soluble
proteins• Membrane-bound ribosomes
synthesize proteins to be incorporated into membranes
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Endoplasmic Reticulum (ER)• Interconnected tubes and
parallel membranes enclosing cisternae• Continuous with the nuclear
membrane• Two varieties – rough ER and
smooth ER
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Endoplasmic Reticulum (ER)
Figure 3.18a and c
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Rough (ER)• External surface studded with
ribosomes• Manufactures all secreted proteins• Responsible for the synthesis of
integral membrane proteins and phospholipids for cell membranes
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Smooth ER• Tubules arranged in a looping network• Catalyzes the following reactions in various organs
of the body– In the liver – lipid and cholesterol metabolism,
breakdown of glycogen and, along with the kidneys, detoxification of drugs
– In the testes – synthesis of steroid-based hormones – In the intestinal cells – absorption, synthesis, and
transport of fats– In skeletal and cardiac muscle – storage and release of
calcium
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Golgi Apparatus• Stacked and flattened membranous sacs• Functions in modification, concentration,
and packaging of proteins• Transport vessels from the ER fuse with the
cis face of the Golgi apparatus• Proteins then pass through the Golgi
apparatus to the trans face• Secretory vesicles leave the trans face of the
Golgi stack and move to designated parts of the cell
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Golgi Apparatus
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Role of the Golgi Apparatus
Figure 3.21
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Lysosomes• Spherical membranous bags containing digestive
enzymes• Digest ingested bacteria, viruses, and toxins• Degrade nonfunctional organelles• Breakdown glycogen and release thyroid
hormone• Breakdown non useful tissue• Breakdown bone to release Ca2+
• Secretory lysosomes are found in white blood cells, immune cells, and melanocytes
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Endomembrane System• System of organelles that function to: –Produce, store, and export biological
molecules–Degrade potentially harmful substances
• System includes:–Nuclear envelope, smooth and rough ER,
lysosomes, vacuoles, transport vesicles, Golgi apparatus, and the plasma membrane
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Endomembrane System
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Peroxisomes
• Membranous sacs containing oxidases and catalases• Detoxify harmful or toxic substances• Neutralize dangerous free radicals–Free radicals – highly reactive chemicals
with unpaired electrons (i.e., O2–)
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Cytoskeleton
• The “skeleton” of the cell• Dynamic, elaborate series of rods
running through the cytosol• Consists of microtubules,
microfilaments, and intermediate filaments
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Microtubules
• Dynamic, hollow tubes made of the spherical protein tubulin• Determine the overall shape of
the cell and distribution of organelles
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Microfilaments• Dynamic strands of the protein actin• Attached to the cytoplasmic side of
the plasma membrane• Braces and strengthens the cell
surface• Attach to CAMs and function in
endocytosis and exocytosis
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Centrioles• 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
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Cilia
• Whiplike, motile cellular extensions on exposed surfaces of certain cells• Move substances in one direction
across cell surfaces
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Cilia
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Cilia
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Nucleus• Contains nuclear envelope, nucleoli,
chromatin, and distinct compartments rich in specific protein sets
• Gene-containing control center of the cell• Contains the genetic library with blueprints
for nearly all cellular proteins• Dictates the kinds and amounts of proteins
to be synthesized
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Nucleus
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Nuclear Envelope• Selectively permeable double membrane barrier
containing pores• Encloses jellylike nucleoplasm, which contains
essential solutes• Outer membrane is continuous with the rough ER
and is studded with ribosomes• Inner membrane is lined with the nuclear lamina,
which maintains the shape of the nucleus• Pore complex regulates transport of large
molecules into and out of the nucleus
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Nucleoli
•Dark-staining spherical bodies within the nucleus• Site of ribosome production
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SUMMARYORGANELLE COMPOSITION FUNCTION
Plasma membrane Phospholipid bilayer Selective passage of molecules into and out of the cell
Nucleus Nuclear envelope surrounding nucleoplasm, chromatin and nucleolus
Storage of genetic information
Nucleolus Concentrated area of chromatin, RNA and proteins
Ribosomal formation
Endoplasmic Reticulum (ER)
Membranous saccules and canals
Protein synthesis
Rough ER Studded with ribosomes Protein synthesis
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Summary Contd…….
Smooth ER Having no Ribosomes Lipid synthesis in some cells
Golgi apparatus Stack of membranous saccules
Processing, packaging and distributing molecules
Vacuole and vessicle Membranous sacs Storage of Substances
Lysosome Membranous vessicle containing digestive enzyme
Intracellular digestion
Mitochondrion Inner membrane (cristae) within outer membrane
Cellular respiration
Cytoskeleton Microtubules, actin filaments
Shape of cell and movement of its parts
Cilia and Flagella Microtubules Movement of cell
Centriole Microtubules Formation of basal bodies