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Cell Facts
Cells are the basic structural and functional units of
life
• The activity of an organism depends on the
activity of its cells
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• The biochemical activities of a cell are dictated by
its organelles
• The continuity of life has a cellular basis– all cells come from pre-existing cells and
– the survival of life is contingent on the work of
cells
Structure & Function of Cells
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• All cells are surrounded by an outer membrane called
the cell membrane or plasma membrane (PM).
• Cells are classified as either eukaryotes or prokaryotes
• Eukaryotes have:
• A plasma membrane
– A nucleus
• contains genetic information and directs all
functions of the cell.
– Cytoplasm: composed of a gel-like fluid called
cytosol.
– Organelles:
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• Prokaryotes have no nucleus or true organelles
– have a plasma membrane surrounded by a
rigid cell wall.
– genetic material not necessarily enclosed
within nucleus
Cell Structure Reflects Cell Function
All cells carry out defined activities to maintain life:
-Gather raw materials
-- Make macromolecules
-Excrete wastes
-- Grow and reproduce
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• Structural differences between cells generally
reflect functional differences
– Muscle cells
– Nerve cells:
– Some cells have microvilli to increase surface area
• All raw materials, energy, waste and information can
enter or leave the cell only by moving across the plasma
(cell) membrane
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The Structure of the Plasma Membrane
• Constructed of 2 layers of Phospholipids
(called a lipid bilayer) cholesterol, and some
proteins
– Phospholipids: are lipids with a polar head
and nonpolar tail
– Cholesterol: makes the membrane more rigid
– Proteins: are embedded in the phospholipid
bilayer and provide means of transporting
molecules and information across the
membrane; some proteins anchor the cell’s
internal support network
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• The plasma membrane creates a barrier
between the external cell environment and the
internal cell environment.
• By regulating the environment, the PM keeps the
materials needed for growth and reproduction
inside the cell and determines what and when
other materials should be allowed to enter the
cell.
• Molecules cross the plasma membrane in 3
major ways: passive transport (diffusion and
osmosis), active transport, and endocytosis or
exocytosis.
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• Passive Transport: transports a molecule without
requiring the cell to use any energy. It relies on the
mechanism of diffusion.
– Diffusion—movement of molecules from one region
to another as the result of random motion.
– If there are more molecules in one region than
in another, then, simply by chance, molecules
move away or diffuse from the area of high
concentration towards the area of low
concentration.
– So, diffusion requires that there be a difference
in concentration, called a concentration
gradient, between 2 areas.
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Diffusion: Passive Transport
• Passive transport:
no energy required
• Diffusion:
movement from
area of high
concentration to
low
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Figure 3.6
Osmosis is the diffusion of water across a
selectively permeable membrane.
– Water moves from the region of higher
concentration (lower concentration of solutes) to the
area of lower concentration (higher concentration of
solutes)
• All substances do not readily diffuse into and out
of living cells because the plasma membrane is
selectively permeable
– The plasma membrane is highly permeable to
water but not to all ions or molecules.
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Three Forms of Passive Transport
Passive transport always uses the concentration
gradient and thus relies on diffusion without the
using any additional energy.
• Diffusion through the lipid bilayer:
– This structure allows some molecules to move
through the PM and prevents others.
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• Diffusion through channels
– channels are constructed of proteins that
span the entire lipid bilayer.
– size, shape, and electrical charge of the
channel determine which molecules are able
to pass through.
– Some channels are open all the time (i.e.
water channels).
– Some channels are gated
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• Facilitated Transport or Facilitated
Diffusion
– The molecule attaches to a transport or
carrier membrane protein in order to pass
through the channel.
– Once the molecule is attached, a change in
the protein’s shape or orientation occurs
– this change actually transfers the molecule to
the other side of the membrane where it is
released.
– Once the molecule is released, the protein
returns to its original form.
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Active Transport
• requires energy to move substances through the
PM against the concentration gradient.
– Energy source is ATP
• allows a cell to get essential molecules even
when their concentration outside the cell is low
and to get rid of molecules which it doesn’t need.
• Proteins which actively transport molecules
across the PM are called pumps.
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• One of the most important PM pumps is the
sodium-potassium pump. This pump uses
energy from ATP to transport sodium out of the
cell and potassium into the cell.
Naout of cell
K into cell
• Active transport requires energy to move
substances from an area of lower concentration
to an area of higher concentration.
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Endocytosis and Exocytosis
• Some molecules are too big to be transported by
active or passive transport.
• Cells use endocytosis and/or exocytosis to
move large molecules
• Endocytosis moves materials into the cell
• Exocytosis moves materials out of the cells
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Active Transport: The Sodium/Potassium Pump
• Sodium/potassium pump expels unwanted ions,
keeps needed ones, maintains cell volume
• ATP used to expel three sodium ions for every
two potassium ions brought into the cell
• To increase cell volume = increase water in
cytoplasm by decreasing the pump and allowing
more sodium inside cell
• To decrease cell volume = less water in
cytoplasm by increasing the pump and expelling
more sodium ions
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Tonicity of ECF
• Tonicity is the relative concentration of solutes in 2
liquids
• Because water can diffuse across the PM easily, the
ability of the cell to control its volume is dependent upon
the tonicity of the ECF.
• Isotonic: ECF has the same solute concentration as the
ICF (intracellular fluid). The body’s regulatory
mechanisms ensure that the ECF solute concentration
remains relatively constant.
• Hypertonic: When the ECF concentration is higher than
the intracellular fluid, water diffuses out of the cell and
the cell shrinks. This will impair normal function and lead
to cell death.
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Hypertonic
When the fluid outside the cells has a greater concentration
of dissolved substance in it than the fluid inside the cells , it
is called hypertonic. The outside water concentration
(98.5%) is lower than the concentration of water inside the
cells (99.1%). By osmosis water diffuses from high to low
concentration. This results in the water moving out of the
cells faster than the water enters. The cells shrink.
Cell fluid is
99.1% water
and 0.9%
NaCl
Fluid outside cell
contains 98.5% water
and 1.5% NaCl
Water
• Hypotonic: When the ECF concentration is
lower than the intracellular fluid (fluid outside the
cells has a lower concentration of dissolved
substance in it than the fluid inside the cells),
water diffuses into the cell and the cell swells
and may burst. This will impair normal function
and lead to cell death.
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Hypotonic
Water
Cell fluid is
99.1% water and
0.9% NaCl
Fluid outside cell
contains 100%
water and 0% NaCl
Water
Cell
Hemolysis or
cell swells and
bursts
Variations in Tonicity-a Summary
• Isotonic: extracellular and intracellular
concentration equal
• Hypotonic: extracellular concentration less than
intracellular
• Hypertonic: extracellular concentration more
than intracellular
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Structure and Function of the Nucleus
Functions:
• Information center of the cell
• Contains the genetic information of the cell
• Controls the cell
Structural features:
• Outer surface is a double-layered nuclear membrane
• Nuclear membrane has nuclear pores which permit some
small proteins and RNA through but not DNA
• Nucleolus: RNA and proteins which make up ribosomes
are made. The components of the ribosomes pass
through the nuclear pores to the cytoplasm for final
assembly into ribosomes.
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Ribosomes and Endoplasmic Reticulum (ER)
Ribosomes: responsible for protein assembly.
• Float freely in cytoplasm or are attached to the ER.
– Ribosomes attached to the ER release proteins
into the folds of the ER
– Free floating ribosomes produce proteins for use
by the cell
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Endoplasmic reticulum (ER) (and the ribosomes)
manufacture proteins and chemical compounds
produced by the cell.
• Is a folded membranous system
• Some areas of the ER’s surface are dotted with
ribosomes and are called Rough ER.
– Areas without ribosomes are called Smooth
ER.
• Rough ER: protein synthesis and release (to
smooth ER);
• Smooth ER: also synthesizes lipids; packages
proteins and lipids for transfer to the Golgi
Apparatus.
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Golgi Apparatus
• Stack of membranous sacs which receive substances
from ER (usually proteins), refines and packages them
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Figure 3.17 (1/2)
Vesicles
• Are membrane-bound spheres that enclose
something within the cell.
– Ship and store cellular products
– Secretory vesicles
• Contain products that need to be exported from the
cell
• Endocytotic vesicles
• Enclose bacteria and raw materials from the
extracellular environment and bring these into the
cell by endocytosis.
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– Peroxisomes and lysosomes
• Contain powerful enzymes that must be
stored within the vesicle so they don’t
damage the rest of the cell
• Peroxisomes have enzymes that destroy
toxic wastes
• Lysosomes contain digestive enzymes that
digest bacteria; dissolve and remove
cellular debris
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Mitochondria: Provide Energy to the Cell
• “power plant”
• Has a double
membrane; the inner
membrane contains
enzymes that break
down foods
• Liberate energy used to
create ATP
• Manufactures ATP as it
is needed by the cell.
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Figure 3.19a
Krebs cycle (also called the Citric Acid Cycle)
(takes place in the mitochondria)
– Releases high-energy electrons
– Produces two ATP and carbon dioxide
• Electron transport system
• energy rich electrons are transferred from 1 protein
carrier molecule to another so that energy from the
electrons can be released
• Energy from electrons is used to produce ATP
• Produces water and carbon dioxide as a waste byproduct
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Cytoskeleton: has microtubules & microfilaments
to help form the framework for the plasma
membrane; also supports and anchors the other
structures within the cell
Cell Structures for Support and Movement: Cilia,
Flagella, Centrioles
• Cilia and flagella provide movement.
• Centrioles: used in cell division
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Energy Storage
• Cells store energy.
• Can be stored as:
– Fat—long term energy
– Glycogen (carbohydrate storage)—short term
energy
• most cells rely on glycogen rather than on
fat because the energy in the chemical
bonds of glycogen can produce ATP more
quickly
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Cells Use and Transform Matter and Energy: Two
Basic Metabolic Pathways
Metabolism is the sum of all the chemical
reactions used by an organism.
• 2 basic types of metabolic pathways:
Anabolism: Making of larger molecules
– requires enzymes to perform chemical
reaction
– usually requires energy (ATP)
– Used in building up cell components
– Used in storing energy
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• Catabolism:
– Breaking down of molecules; used in breaking
down nutrients/recycling cell components
– requires enzymes to perform chemical reaction
– Releases stored energy from the cell
– Follows the Citric Acid or Kreb’s cycle to release
energy
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Additional Energy Sources
Most of the body’s energy reserves are stored as
fats (78%) and proteins (21%), glycogen (1%)
• When we take in too many calories, some of this
energy replenishes the body’s stores of
glycogen. The rest is converted to fat and
stored in fat tissue
• Fats are stored as triglycerides; fats have twice
the energy of carbohydrates
• Proteins have the same energy as
carbohydrates
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Anerobic Metabolism
• For small periods of time, a small amount of ATP
can be made without oxygen being used.
• However, lactic acid is formed as a waste byproduct.
– This causes a burning sensation in your muscles and
can also cause muscle cramps.
– Muscle fatigue sets in more quickly.
– When oxygen is available, the lactic acid will be
metabolized
– Anerobic metabolism produces only 2 ATP
molecules.
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• Aerobic Metabolism
– Requires oxygen
– Yields 36 ATP; produces carbon dioxide as a
by-product
• After exercise, oxygen metabolizes the lactic
acid (from anerobic metabolism) and
replenishes energy stores
• The term oxygen debt describes the continued
increased metabolism that must occur in a cell to
remove the excess acid that accumulates during
prolonged exercise.
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