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Cells
Chapter 3
Cell Theory
• Every organism is composed of one or
more cells
• Cell is smallest unit having properties
of life
• All cells come from pre-existing cells
Cell
• Smallest unit of life
• Can survive on its own or has potential
to do so
• Is highly organized for metabolism
• Senses and responds to environment
• Has potential to reproduce
Structure of Cells
All start out life with:
Two types:
– Plasma membrane
– Prokaryotic
– Region where DNA
is stored
– Eukaryotic
– Cytoplasm
Why Are Cells so Small?
• Surface-to-volume ratio
• The bigger a cell is, the less surface
area there is per unit volume
• Above a certain size, material cannot be
moved in or out of cell fast enough
Lipid Bilayer
• Main component of cell
membranes
• Gives membrane its fluid
properties
• Two layers of phospholipids
– Hydrophilic heads face outward
– Hydrophobic tails in center
Animal Cell Components
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Plasma membrane
Nucleus
Ribosomes
Endoplasmic
reticulum
Golgi body
Vesicles
Mitochondria
Cytoskeleton
Cytoskeleton
• Basis for cell shape and internal
organization
• Enables organelle movement within
cells and, in some cases, cell motility
• Main elements are microtubules,
microfilaments, and intermediate
filaments
Flagella and Cilia
microtubule
• Structures for
cell motility
• 9+2 internal
structure
• Arise from
centrioles
dynein
Plasma Membrane
• Extremely thin
• Mosaic of proteins and lipids
• Lipids give membrane its fluid quality
• Proteins carry out most membrane
functions
Membrane Proteins
• Transport proteins
• Receptor proteins
• Recognition proteins
• Adhesion proteins
Cytomembrane System
• Group of related organelles
• Assembles lipids
• Modifies new polypeptide chains
• Sorts and ships products to various
destinations
Endoplasmic Reticulum
• In animal cells, continuous with nuclear
membrane
• Extends throughout cytoplasm
• Two regions: rough and smooth
Smooth ER
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A series of interconnected tubules
No ribosomes on surface
Lipids assembled inside tubules
Smooth ER of liver inactivates wastes,
drugs
• Sarcoplasmic reticulum of muscle is a
specialized form
Rough ER
• Arranged into flattened sacs
• Ribosomes on surface give it a rough
appearance
• Some polypeptide chains enter and are
modified
• Most extensive in secretory cells
Golgi Bodies
• Put finishing touches on proteins and lipids
that arrive from ER
• Package finished material for shipment to
final destinations
• Material arrives and leaves in vesicles
Vesicles
• Membranous sacs that move
through the cytoplasm
• Lysosomes
• Peroxisomes
Diffusion
• The net movement of like molecules or
ions down a concentration gradient
• Although molecules collide randomly,
the net movement is away from the
place with the most collisions (down
gradient)
Factors Affecting
Diffusion Rate
• Steepness of concentration gradient
• Molecular size
• Temperature
• Electrical or pressure gradients
Osmosis
• Diffusion of water molecules across a
selectively permeable membrane
• Direction of net flow is determined by
water-concentration gradient
• Side with the most solute molecules has
the lowest water concentration
Tonicity
Refers to relative solute concentration
of two fluids
Hypertonic - having more solutes
Isotonic - having same amount
Hypotonic - having fewer solutes
Transport Proteins
• Span the lipid bilayer
• Interior is able to open to both sides
• Change shape when they interact with
solute
• Play roles in active and passive
transport
Passive Transport
• Flow of solutes through the interior of
passive transport proteins down their
concentration gradients
• Passive transport proteins enable
solutes to move both ways
• Does not require any energy input
Active Transport
• Net diffusion of solute is against
concentration gradient
• Transport protein must be activated
• ATP gives up phosphate to activate
protein
• Binding of ATP changes protein shape
and affinity for solute
Bulk Transport
Exocytosis
Endocytosis
Cholera
• Cholera exotoxin raises level of
signaling molecule that causes cells to
dump chloride into the intestine
• Other solutes follow
• Water moves into intestine by osmosis
• Result is nearly clear diarrhea
• Treated with oral rehydration therapy
Functions of Nucleus
• Keeps the DNA molecules of eukaryotic
cells separated from metabolic
machinery of cytoplasm
• Makes it easier to organize DNA and to
copy it before parent cells divide into
daughter cells
Components of Nucleus
Nuclear envelope
Nucleoplasm
Nucleolus
Chromatin
• Cell’s collection of DNA and
associated proteins
• Chromosome is one DNA molecule
and its associated proteins
• Appearance changes as cell divides
Mitochondria
• ATP-producing powerhouses
• Double-membrane system
• Carry out the most efficient energyreleasing reactions
• Reactions require oxygen
Mitochondrial Structure
• Outer membrane faces cytoplasm
• Inner membrane folds back on itself
• Membranes form two distinct
compartments
• ATP-making machinery is embedded in
the inner mitochondrial membrane
ATP
Universal Energy Currency
• ATP is earned in reactions that yield energy, and
spent in reactions that require it
adenine
P
P
P
ribose
Metabolic Pathways
large energy-rich
molecules
ADP + Pi
Anabolic Pathways
Catabolic Pathways
ATP
energy-poor
products
Energy input
simple organic
compounds
Participants in
Metabolic Pathways
• Substrates
• Energy Carriers
• Intermediates
• Enzymes
• End Products
• Cofactors
Enzyme Structure
and Function
• Enzymes speed the rate at which
certain reactions occur
• Nearly all are proteins
• An enzyme recognizes and binds to
only certain substrates
• Reactions do not alter or use up
enzyme molecules
Induced-Fit Model
active sight
• Enzyme-substrate
complex is short
lived
• Enzyme resumes its
prebinding shape as
a product molecule
is released
Factors Influencing
Enzyme Activity
Temperature
pH
Salt concentration
Allosteric regulators
Coenzymes and cofactors
Overview of Aerobic
Respiration
C6H1206 + 6O2
6CO2 + 6H20
glucose
carbon
oxygen
dioxide
water
Glycolysis
• Occurs in cytoplasm
• Reactions are catalyzed by enzymes
Glucose
(six carbons)
2 Pyruvate
(three carbons)
Glycolysis Occurs
in Two Stages
• Energy-requiring steps
– ATP energy activates glucose and its six-carbon
derivatives
• Energy-releasing steps
– The products of the first part are split into three-
carbon pyruvate molecules
– ATP and NADH form
Net Energy Yield
from Glycolysis
Energy-requiring steps:
2 ATP invested
Energy-releasing steps:
2 NADH formed
4 ATP formed
Net yield is 2 ATP and 2 NADH
Mitochondrial Reactions
• Mitochondrial membranes form two
distinct compartments
• ATP-making machinery is embedded in
the inner mitochondrial membrane
• Reactions begin when pyruvate enters a
mitochondrion
Two Parts of Second Stage
• Preparatory reactions
– Pyruvate is oxidized into two-carbon acetyl
units and carbon dioxide
– NAD+ is reduced
• Krebs cycle
– The acetyl units are oxidized to carbon
dioxide
– NAD+ and FAD are reduced
The Krebs Cycle
Overall Reactants
Overall Products
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Acetyl-CoA
3 NAD+
FAD
ADP and Pi
Coenzyme A
2 CO2
3 NADH
FADH2
ATP
Results of the Second Stage
• All the carbon molecules in pyruvate
end up in carbon dioxide
• Coenzymes are reduced (they pick up
electrons and hydrogen)
• One molecule of ATP forms
• Four-carbon oxaloacetate is
regenerated
Coenzyme Reductions during
First Two Stages
• Glycolysis
• Preparatory
reactions
• Krebs cycle
2 NADH
2 FADH2 + 6 NADH
• Total
2 FADH2 + 10 NADH
2 NADH
Electron Transport
• Coenzymes give up electrons to electron
transport system
• Electrons are transported through the system
• The final electron acceptor is oxygen
• H+ is moved from inner to outer compartment
• H+ flow back across membrane drives ATP
synthesis
Creating an H+ Gradient
OUTER COMPARTMENT
NADH
INNER COMPARTMENT
Making ATP
ATP
INNER
COMPARTMENT
ADP
+
Pi
Importance of Oxygen
• Operation of the electron transport
system requires oxygen
• Oxygen withdraws spent electrons from
the electron transport system, then
combines with H+ to form water
Summary of Energy Harvest
(per molecule of glucose)
• Glycolysis
– 2 ATP formed by substrate-level phosphorylation
• Krebs cycle and preparatory reactions
– 2 ATP formed by substrate-level phosphorylation
• Electron transport phosphorylation
– 32 ATP formed
Carbohydrate Breakdown
and Storage
• Glucose is absorbed into blood
• Pancreas releases insulin
• Insulin stimulates glucose uptake by cells
• Cells convert glucose to glucose-6-phosphate
• This traps glucose in cytoplasm where it can
be used for glycolysis
Making Glycogen
• If glucose intake is high, ATP-making
machinery goes into high gear
• When ATP levels rise high enough, glucose6-phosphate is diverted into glycogen
synthesis (mainly in liver and muscle)
• Glycogen is the main storage polysaccharide
in animals
Using Glycogen
• When blood levels of glucose decline,
pancreas releases glucagon
• Glucagon stimulates liver cells to convert
glycogen back to glucose and to release it to
the blood
• (Muscle cells do not release their stored
glycogen)
Energy Reserves
• Glycogen makes up only about 1 percent of
the body’s energy reserves
• Proteins make up 21 percent of energy
reserves
• Fat makes up the bulk of reserves (78
percent)
Energy from Fats
• Most stored fats are triglycerides
• Triglycerides are broken down to glycerol and
fatty acids
• Glycerol is converted to PGAL, an
intermediate of glycolysis
• Fatty acids are broken down and converted to
acetyl-CoA, which enters Krebs cycle
Energy from Proteins
• Proteins are broken down to amino acids
• Amino acids are broken apart
• Amino group is removed; ammonia forms, is
converted to urea, and is excreted
• Carbon backbones can enter the Krebs cycle
or its preparatory reactions