BIOLOGY 100 - Cells & CellMembrane: its Structure & Function
Ch 3 & 4
Anatomy of the Cell
Cells are not all the same, however they all cells share general structures. Cells are organized
into three main regions: 1) Nucleus, 2 Cytoplasm, and 3) Plasma membrane
Plasma Membrane – it is the barrier for the cell contents – it isolates cell contents from external
environment. It consists of a double phospholipid layer:
1. Hydrophilic heads
2. Hydrophobic tails
The plasma membrane also contains protein, cholesterol, and glycoproteins. It has a selective
permeable property that allows it to act as a controlling gatekeeper. This selective permeable plasma
membrane allows some materials to pass while excluding others. This permeability includes movement
into and out of the cell
See diagram of Plasma Membrane and the Phospolipid Bilayer structure.
Thhe Phospolipid Bilayer structure blocks the passage of most molecules. It can isolate cell contents
from the external environment. Very small molecules may pass through freely; such as water and
uncharged lipid-soluble molecules.
The mosaic membrane is embedded with protein molecules that:
1. A id in transport of molecules
2. Play a role in the cell’s responses to substances in its environment. The protein molecules play a
role as: a) Transport proteins, b) Receptor proteins, and c)Recognition Proteins
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
Fluid passing transporting through the membrane are in the form of Solutions or solvents.
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Solution – is a homogeneous mixture of two or more components. A solution may contain
solutes and solvents.
o Solvent – is the dissolving medium
o Solutes – are the components within a solution that are in smaller quantities within a
solution
Intracellular fluid – is the fluid within the cell: in the nucleoplasm and in the cytoplasm (called cytosol)
Interstitial fluid – is the fluid on the exterior of the cell. This can include the fluid in the blood
(intravascular) and other body fluids (like in the eyes, digestive juices, etc.
Cell Membrane Transport – involves movement of substance into and out of the cell. Transport is done
by two basic methods:
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Passive transport – where no energy is required.
Active transport – where the cell must provide metabolic energy
Passive Transport:
1)
Diffusion. This is simple diffusion – where nonpolar and lipid-soluble substances pass through the
membrane from an area of higher concentration to an area of lower concentration. This process is
unassisted. The diffusion directly passes through the lipid bilayer. Diffusion also occurs through the
protein channel.
In diffusion,molecules are disperse evenly. Particles (solutes) tend to distribute themselves evenly
within a solution.
Movement is from high concentration to low concentration, or down
a concentration gradient
Diffusion – occurs when the concentration of a solvent is
different on the opposite sides of a membrane. Diffusion of water
moves down the concentrated gradient of water, from a higher concentration of water to a lower
concentration of water. This allows the passage of some molecules but prevents the passage of other
molecules.
The greater the concentration the faster the rate of diffusion. Diffusion will continue until the
concentration gradient is eliminated
2) Osmosis – is the movement of water across selective permeable membrane from areas of higher
water concentration to areas of lower water concentration. The selective membrane may have
pores large enough for water molecules to pass through and not other molecules
Dissolved substances reduce the concentration of water molecules in a solution. Water moves
down from a high concentration gradient to a low concentration gradient of molecules
3)
Filtration – this passive transport process involves forcing water and solutes through a membrane
by fluid pressure or hydrostatic pressure. A pressure gradient must exist. A solute-containing fluid
is pushed from a high pressure area to a lower pressure area
Active Transport:
This transport process allows substances that are unable to pass by diffusion; usually because the
solutes may be too large. They may not be able to dissolve in the fat core of the membrane. There are
2 common forms of active transport:
1. Solute pumping – chemical exchanges
2. Bulk transport - exocytosis
Active Transport - uses ATP energy to move solutes across a membrane. It also requires the assistance
of carrier proteins.
1) Channel proteins
 Channel proteins: form pores in the lipid bi-layer allowing certain ions to cross the membrane.
These channel proteins are specialized and allow only particular ions to pass: for example,
potassium K+, sodium Na+, and calcium Ca++
 Protein carriers: during active transport, the protein carriers grab onto a specific molecule on
one side of the membrane and carries it to the other side. Protein carriers bind to the specific
protein molecules it carries through – such as amino acid, hormone molecule.
The binding triggers a change in the shape of the carrier that allows the molecule to pass
through the protein and cross the membrane
Active Transport Processes – solute pumping
2) Exocytosis
3) Endocytosis – extracellular substances are engulfed by enclosing in a membranous vescicle.
There are 2 types:
1. Phagocytosis - cell eating
2. Pinocytosis – cell drinking
Cell Membrane Permeability
Most plasma membranes are highly permeable to water. Water flowing through is dependent
upon the concentration (tonicity) on either side of the cell membrane (Tonicity of Water)
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Isotonic – fluid in the cytoplasm is equal to the fluid in the extracellular space
Hypertonic – concentration is higher than the extracellular (shrinks cells)
Hypotonic – concentration is lower than the extracelluar (ruptures cells)
Keep in mind: the types of dissolved particles or solutes are seldom the same inside and outside the
cells. The total concentration of dissolved particles is equal to that outside of cell
CELL STRUCTURE & ORGANELLES
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 – membraneous or non-membraneoud
Membranous organelles - mitochondria, peroxisomes, lysosomes, endoplasmic reticulum, and Golgi
apparatus.
Nonmembranous - cytoskeleton, centrioles, and ribosomes
Mitochondria – has double membrane structure with shelf-like folds – cristae. It provide most of the
cell’s ATP via aerobic cellular respiration. Mitochondria contain their own DNA and RNA
Ribosomes – are Granules containing protein and rRNA. These are the sites of protein synthesis. The
free ribosomes are found sporadic through the cytoplasm, these synthesize soluble proteins. The
membrane-bound ribosomes synthesize proteins to be incorporated into membranes.
Endoplasmic Reticulum (ER) - Interconnected tubes and parallel membranes enclosing cristernae
(cristae). It is continuous with the nuclear membrane. Two varieties –
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Rough (ER) – the external surface studded with ribosomes. Manufactures all secreted proteins
that are 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
Golgi Apparatus - Stacked, flattened membranous sacs that modifies concentration of proteins and
packages them into transport vesicles from the ER and are received by Golgi apparatus
Lysosomes - Spherical membranous bags containing digestive enzymes. They digest ingested bacteria,
viruses, and toxins and degrade nonfunctional organelles; breakdown glycogen and release thyroid
hormone. It performs Autolysis – self-digestion of the cell
Lysosomes - Breakdown nonuseful tissue; like breakdown bone to release calcium Ca2+. Secretory
lysosomes are found in white blood cells, immune cells, and melanocytes
Peroxisomes - “Peroxide bodies” Membranous sacs containing oxidases and catalases that detoxify
harmful or toxic substances and 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
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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
Microfilaments - Dynamic strands of protein Actin. Attached to the cytoplasmic side of the
plasma membrane. Braces and strengthens the cell surface
Intermediate Filaments - Tough, insoluble protein fibers with high tensile strength. Resist
pulling forces on the cell and help form desmosomes
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 – like
mucus.
Flagella - for cellular motility
The 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