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Cells Membrane: its Structure & Function
Anatomy of the Cell
Cells are not all the same
All cells share general structures
Cells are organized into three main regions
– Nucleus
– Cytoplasm
– Plasma membrane
Plasma Membrane
Barrier for the cell contents
Consists of a double phospholipid layer
– Hydrophilic heads
– Hydrophobic tails
Also contains protein, cholesterol, and glycoproteins
Selective Permeability
Acts as a gatekeeper
The plasma membrane allows some materials to pass while
excluding others
This permeability includes movement into and out of the cell
Plasma Membrane
•Phospolipid Bilayers
–Block the passage of most molecules
–Can isolate cell contents from the external environment
–Very small molecules may pass through freely:
•water and uncharged lipid-soluble molecules
Cell Membrane
The mosaic membrane is embedded with protein molecules that:
1. Aid in transport of molecules
2. Play a role in cell’s responses to
substances in its environment
•Transport proteins
•Receptor proteins
•Recognition Proteins
Plasma Membrane Transport
•Transport proteins – allows water-soluable molecules to cross the
plasma membrane by carrying them across
•Receptor proteins – specific to chemical messages (hormones).
•Recognition Proteins – act as identification tags. Recognize your
own cells from invading disease causing organisms
Solutions and Transport
• Solution – homogeneous mixture of two or more components
– Solvent – dissolving medium
– Solutes – components in smaller quantities within a solution
• Intracellular fluid – nucleoplasm and cytosol
• Interstitial fluid – fluid on the exterior of the cell
Cell Membrane Transport
• Membrane Transport – movement of substance into and out of the
cell
• Transport is by two basic methods
– Passive transport
• No energy is required
– Active transport
• The cell must provide metabolic energy
Passive Transport:
Diffusion
Simple diffusion – nonpolar and lipid-soluble substances
• Unassisted process
• Diffuse directly through the lipid bilayer
• Diffuse through channel proteins
• Molecules disperse evenly
Passive Transport Processes
• Diffusion
– Particles tend to distribute themselves evenly within a solution
– Movement is
from high
concentration
to low
concentration,
or down a
concentration
gradient
Passive Transport Processes
• Types of diffusion
– Facilitated diffusion
• Substances require a protein carrier for passive transport
Passive Transport
Diffusion through Osmosis
• Occurs when concentration of a solvent is different on opposite
sides of a membrane
• Diffusion of water across a selective membrane down reduce the
concentration of water, from a higher concentration of water to a
lower concentration of water.
• Allows the passage of some molecules but prevents the passage of
other molecules
• Osmolarity – Tonicity
Passive Transport Processes
• Filtration
– Water and solutes are forced through a membrane by fluid, or
hydrostatic pressure
– A pressure gradient must exist
• Solute-containing fluid is pushed from a high pressure area
to a lower pressure area
Active Transport Processes
• Transport substances that are unable to pass by diffusion
– They may be too large
– They may not be able to dissolve in the fat core of the
membrane
• Two common forms of active transport
– Solute pumping – chemical exchanges
– Bulk transport - exocytosis
Active Transport Processes
-Solute pumping-
Exocytosis
Endocytosis
THE LIVING HUMAN (ANIMAL) CELL
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Cytoplasm
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
Mitochondrion
• Double membrane structure with shelf-like folds - cristae
• Provide most of the cell’s ATP via aerobic cellular respiration
• Contain their own DNA and RNA
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
Endoplasmic Reticulum (ER)
• Interconnected tubes and parallel membranes enclosing cristernae
(cristae)
• Continuous with the nuclear membrane
• Two varieties –
Rough (ER)
• External surface studded with ribosomes
• Manufactures all secreted proteins
• Responsible for the synthesis of integral membrane proteins and
phospholipids for cell membranes
Smooth (ER)
Catalyzes the following reactions in various organs of the body:
• Liver – lipid & cholesterol metabolism, breakdown of glycogen,
detoxification of drugs
• Testes – synthesis steroid-based hormones
• Intestinal cells – absorption, synthesis, and transport of fats
• Skeletal and Cardiac muscle – storage and release of calcium
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Golgi Apparatus
Stacked, flattened membranous sacs
Modifies concentration of proteins and packages them
Transport vesicles from the ER and are received by Golgi
apparatus
Lysosomes
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
Breakdown bone to release Ca2+
Secretory lysosomes are found in white blood cells, immune cells,
and melanocytes
Peroxisomes
“Peroxide bodies”
• Membranous sacs containing oxidases and catalases
• Detoxify harmful or toxic substances
• Neutralize dangerous free radicals
• Free radicals – highly reactive chemicals with unpaired electrons
Cytoskeleton
• The “skeleton” of the cell
• Dynamic, elaborate series of rods running through the cytosol
• Consists of microtubules, microfilaments, and intermediate
filaments
1. Microtubules
• Dynamic, hollow tubes made of the spherical protein tubulin
• Determine the overall shape of the cell and distribution of
organelles
2. Microfilaments
Dynamic strands of protein Actin
• Attached to the cytoplasmic side of the plasma membrane
• Braces and strengthens the cell surface
Cytoskeleton (cont.’)
Intermediate Filaments
• Tough, insoluble protein fibers with high tensile strength
• Resist pulling forces on the cell and help form desmosomes
3. Centrioles
• Small barrel-shaped organelles located in the centrosome near the
nucleus
• Pinwheel array of nine triplets of microtubules
• Organize mitotic spindle during mitosis
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
Example: paramecium organism, ciliated epithelial cells found in the
trachea.
Flagella
•Projections are longer
• A single propulsive flagellum
• Movement is achieved by propelling itself across the
surface or environment
• Basal bodies in the centrioles form the bases for ceia and
flagella
Example: the sperm
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Nucleus
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
DNA