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Cell Structure and Function
Chapter 4
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The Cell Theory
All living things are made of cells. A cell
– The basic unit of all living things.
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The Historical Context of the Cell Theory
Robert Hooke coined the term “cell.”
– Look at cork cells under a simple microscope.
Anton van Leeuwenhoek– Made better microscopes– Used them to look at a variety
of substances and identified animalcules
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The Historical Context of the Cell Theory
Mathias Jakob Schleiden– Concluded that all plants were made of cells
Theodor Schwann– Concluded that all animals were made of cells
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Initial Observations of Cells
Cell wall– Outer non-living part of
plant cells Protoplasm
– Interior living portion of the cell
– Nucleus Contains the genetic
information of the cell– Cytoplasm
Fluid part of the protoplasm
– Organelles “Little organs” within the
protoplasm
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Different Kinds of Cells
Prokaryotic– Structurally simple cells – Lack a nucleus– Lack most other organelles– Bacteria
Eukaryotic cells– More complex– Have a nucleus– Have a variety of organelles– Plants, animals, fungi, protozoa and algae– Typically much larger than prokaryotic cells
Prokaryotic Cells
Origin: ‘pro’-before; ‘karyote’ - nut Lack a membrane-bound nucleus. genetic material is present in the nucleoid Two types of prokaryotes:
– Bacteria– Archaea
Prokaryotic Cell Characteristics:
Simplest organisms - simple internal organization
Very small (1 to 10 microns across) Genetic material in the nucleoid No membrane-bound organelles Capsules Cytoplasm
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Eukaryotic Cells
Origin: ‘eu’ - true, good; ‘karyote’ - nut Possess a membrane-bound nucleus. genetic material is highly organized within
double-layer nuclear envelope DNA never leaves the nuclear envelope Types of eukaryotes divided into 4
kingdoms:1. Plantae 2. Fungi 3. Animalia 4. Protista
Eukaryotic Cell Characteristics:
More complex organisms highly organized structure Typically larger than prokaryote (10-100
microns) Genetic material in the membrane-bound
nucleus Many membrane-bound organelles Cytoplasm Cytoskeleton
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Prokaryotes vs. Eukaryotes
Eukaryotic and Prokaryotic Characteristics:
DNA, RNA Ribosomes Plasma membrane Cytoplasm Cell walls (plantae, fungi, protista, not
present in animal cells) Flagella
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Eukaryotic Cells
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Prokaryotic Cells
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Cell Size
Prokaryotic cells– 1-2 micrometers in
diameter
Eukaryotic cells– 10-200 micrometers in
diameter
Fig. 4.2
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Cell Size is Limited
Surface Area-to-Volume Ratio – Cells must get all of their nutrients from their environment
through their cell membranes.– Volume increases more quickly than surface area.– Surface area-to-volume ratio must remain small.
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Surface Area-to-Volume Ratio
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The Structure of Cell Membranes
Cell membranes– Thin sheets composed of phospholipids and
proteins Fluid-mosaic model
– Two layers of phospholipids Fluid
– Has an oily consistency– Things can move laterally within the bilayer.
Mosaic– Proteins embedded within the phospholipid bilayer
Phospholipids
Phospholipid Structure (Chapter 3)
– glycerol – a 3-carbon polyalcohol acting as a backbone for the phospholipid
– 2 fatty acids attached to the glycerol
– phosphate group attached to the glycerol
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Phospholipids Chapter 3: Phospholipids are Amphiphilic molecules
Polar Head Group
Nonpolar Hydrocarbon Tail
Phospholipids
The fatty acids are nonpolar chains of carbon and hydrogen.
– Their nonpolar nature makes them hydrophobic (“water-fearing”).
– The phosphate group is polar and hydrophilic (“water-loving”).
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The Phospholipid Bilayer
1. Phospholipid structure– Hydrophobic tails– Hydrophilic heads
2. Bilayer– Hydrophobic tails of each
layer associate with each other.
– Hydrophilic heads on the surface of the bilayer
3. Cholesterol– Hydrophobic1. Found within the
hydrophobic tails – Keeps the membrane
flexible
Phospholipids
The partially hydrophilic, partially hydrophobic phospholipid spontaneously forms a bilayer:
– fatty acids are on the inside
– phosphate groups are on both surfaces of the bilayer
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Membrane Proteins
Some are on the surface Some are partially embedded.
– Protrude from one side
Some are completely embedded.
– Protrude from both sides
Functions– Transport molecules across
the membrane– Attachment points for other
cells– Identity tags for cells
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Organelles Composed of Membranes
Plasma membrane (cell membrane) Different cellular membranous structures serve
different functions Endoplasmic reticulum Golgi apparatus Lysosomes Peroxisomes Vacuoles and vesicles Nuclear membrane
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The Plasma Membrane
Composed of phospholipid bilayer Separates the contents of the cell from the external
environment Important features
– Metabolic activities– Moving molecules across the membrane– Structurally different inside and outside– Identification: Self vs. nonself– Attachment sites– Signal transduction
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The Endoplasmic Reticulum (ER)
Consists of folded membranes and tubes throughout the cell
Provides a large surface area for important chemical reactions
– Because it is folded, it fits into a small space.
Two types of ER1. Rough2. Smooth
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The Endoplasmic Reticulum
The Endoplasmic Reticulum
1. Rough Endoplasmic Reticulum (RER)– System of cytoplasmic membranes that create a
network of channels throughout the cytoplasm– Ribosomes are attached to the outside of the
RER membrane giving it a rough appearance under the microscope
– Synthesis of proteins to be secreted out of the cell, or packaged and sent to lysosomes or plasma membrane
– Proteins are synthesized into the RER channels (cisternal space)
The Endoplasmic Reticulum
2. Smooth Endoplasmic Reticulum (SER)– Relatively few associated ribosomes– Functions:
Synthesis of membrane lipids Detoxification of foreign substances
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The Golgi Apparatus
Stacks of flattened membrane sacs
Functions– Modifies molecules that
were made in other places
– Manufactures some polysaccharides and lipids
– Packages and ships molecules
Fig. 4.12
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Traffic Through the Golgi
Vesicles bring molecules from the ER that contain proteins.
Vesicles fuse with the Golgi apparatus. The Golgi finishes the molecules and ships
them out in other vesicles.– Some are transported to other membrane
structures.– Some are transported to the plasma membrane.– Some vesicles become lysosomes.
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Lysosomes
Vesicles containing enzymes that digest macromolecules
– Carbohydrates– Proteins– Lipids– Nucleic acids
Interior contains low pH– These enzymes only work at pH=5.– The cytoplasm is pH=7.
If the lysosome breaks open, these enzymes will inactivate and will not damage the cell.
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Functions of Lysosomes
Digestion– Of food taken into
the cell Destruction
– Disease-causing organisms
– Old organelles
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Peroxisomes
Not formed from golgi membrane, but from ER membrane
Contain the enzyme catalase– Breaks down hydrogen peroxide– Breaks down long-chain fatty acids– Synthesizes cholesterol and bile salts– Synthesizes some lipids
Fig. 4.15
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Vacuoles and Vesicles
Membrane-enclosed sacs Vacuoles
– Larger sacs– Contractile vacuoles found in many protozoa
Forcefully expel excess water from the cytoplasm
Vesicles– Smaller vesicles
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Vacuoles and Vesicles
Fig. 4.16
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The Nuclear Membrane
Separates the genetic material from the rest of the cell
Filled with nucleoplasm Composed of two
bilayers Contains holes called
nuclear pore complexes– Allow large molecules
like RNA to pass through the membrane into the cytoplasm
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The Endomembrane System = Interconversion of Membranes
Membranes are converted from one membranous organelle to another.
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Mitochondria
Energy Converting Organelles
Mitochondrion– A small bag with a large bag
stuffed inside– Larger internal bag is folded into
cristae Cristae contain proteins for
cellular respiration.– Releases the energy from food – Requires oxygen– Uses the energy to make ATP
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Mitochondria
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Chloroplasts
Energy Converting Organelles Chloroplasts
– Sac-like organelle– Contain chlorophyll– Perform photosynthesis
Uses the energy in light to make sugar
– Contain folded membranes called thylakoids
Thylakoids stacked into grana Thylakoids contain chlorophyll
and other photosynthetic proteins.
– Thylakoids surrounded by stroma
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Chloroplasts
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Nonmembranous Organelles
Ribosomes Cytoskeleton Centrioles Cilia flagella Inclusions
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Ribosomes
Made of RNA and proteins Composed of two subunits
– Large – Small
Are the sites of protein production
Found in two places– Free floating in the
cytoplasm– Attached to endoplasmic
reticulum
Fig. 4.10
Ribosomes
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Cytoskeleton
Network of protein fibers found in all eukaryotic cells
Made up of– Microtubules– Microfilaments
(actin filaments)– Intermediate filaments
Cytoskeleton
Functions:– Supports the shape of the cell – Keeps organelles in fixed locations– Helps move materials within the cell
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Cytoskeleton
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Centrioles
Two sets of microtubules arranged at right angles to each other
Located in a region called the centrosome
– Microtubule-organizing center near nucleus
Organize microtubules into spindles used in cell division
Fig. 4.21
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Cilia and Flagella
Hair-like projections extending from the cell Composed of microtubules covered by plasma
membrane Flagella
– Long and few in number– Move with an undulating whip-like motion
Cilia– Small and numerous– Move back and forth like oars on a boat
9 + 2 arrangement of microtubules Cell can control their activity
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Cilia and Flagella
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Fig. 4.24b
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Inclusions
Collections of miscellaneous materials– Can be called granules
Temporary sites for the storage of nutrients and waste
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Glycogen Inclusions
Stryer's Biochemistry Fig. 23-2
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Nuclear Components
Contains chromatin– DNA + proteins– Becomes condensed during
cell division into chromosomes Surrounded by double layer of
membrane Nuclear membrane contains
pores to control transport of materials in and out of nucleus
Contains one or more nucleoli– Site of ribosome synthesis
Contains nucleoplasm– Water, nucleic acids, etc.
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Nuclear Components
Getting Through Membranes
Membrane Transport – Motion of substances in and out of the cell
Cell membranes are Selectively Permeable
Two Types of Transport Mechanisms:1. Passive Transport
2. Active Transport
Membrane Transport
Passive transport is movement of molecules through the membrane in which no energy is required from the cell
Active transport requires energy expenditure by the cell
1. Passive Transport
Passive transport is movement of molecules through the membrane in which no energy is required from the cell
Molecules move in response to a concentration gradient
– A concentration gradient is a difference between the concentration on one side of the membrane and that on the other side
Passive transport mechanisms only movement substances along the concentration gradient From a higher concentration to a lower concentration
Types of Passive Transport:
1. Diffusion – movement of solute molecules from high
solute concentration to low solute concentration
2. Osmosis– movement of solvent water from high solvent
concentration to low solvent concentration
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1. Diffusion
Molecules are in constant, random motion.
Molecules move from where they are most concentrated to where they are less concentrated.
– Involves a concentration gradient (diffusion gradient)
No concentration gradient=dynamic equilibrium
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The Rate of Diffusion
Depends on– The size of the molecule
Smaller molecules diffuse faster.
– The size of the concentration gradientThe greater the concentration difference, the
faster the diffusion.
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Diffusion in Cells
Diffusion can only happen if there is no barrier to the movement of molecules.
Can only happen across a membrane if the membrane is permeable to the molecule– Membranes are semi-permeable; they only allow
certain molecules through.– Membrane permeability depends on the
molecules size, charge, and solubility.
Diffusion
There are two types of diffusion1. Simple (Direct) Diffusion
2. Facilitated Diffusion
Simple Diffusion
Substances pass directly through the cell membrane
The cell membrane has limited permeability to small polar molecules, water, and ions
Determined solely by the concentration gradient
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Simple Diffusion
Example:– Oxygen diffusion
Simple Diffusion
The rate (molecules/sec.) of simple diffusion depends on the degree of concentration gradient
As the gradient reaches equilibrium, diffusion slows
At equilibrium, substances pass in and out of the membrane at equal rates
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Rate of Simple Diffusion vs Concentration
Concentration
Rate
Facilitated Diffusion
Substances must pass through transport proteins to get through the cell membrane
Facilitated diffusion is movement of a molecule from high to low concentration with the help of a carrier protein.
Facilitated Diffusion:– is specific– is passive– saturates when all carriers are occupied
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Facilitated Diffusion
Some molecules have to be carried across the membrane.
– Accomplished by carrier proteins
Still involves diffusion– Follows a concentration
gradient– Is passive transport
Facilitated Diffusion
Is Specific - a carrier protein transports only certain molecules or ions
Is Passive - the direction of net movement is determined by the relative concentrations on the substances inside an outside the cell
Has a Saturation Point - rate of facilitated diffusion (molecules/sec.) increases with gradient until all protein carriers are in use - saturation point
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Saturation of Facilitated Diffusion
Concentration
Rate
2. Osmosis
Osmosis is the movement of water from an area of high to low concentration of water
Movement of water toward an area of high solute concentration
In osmosis, only water is able to pass through the membrane
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Osmosis
Osmosis vs. Diffusion
The difference between osmosis and diffusion is the semipermeable membrane between the two solutions
If the membrane is permeable to the solute, then diffusion occurs
If the membrane is impermeable to the solute, but permeable to water (solvent) only, then osmosis occurs
Osmosis
Osmotic concentration is determined by the the concentration of all solutes in solution– All solutes displace water
Relative Osmotic Concentrations– Hypertonic solutions: have a higher relative
solute concentration – Hypotonic solutions: have a lower relative
solute concentration– Isotonic Solutions: have equal relative solute
concentrations
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Osmotic Influences on Cells
If a cell has less water (thus more solute) than its environment
– It is hypertonic to its surroundings.
If a cell has more water (thus less solute) than its environment
– It is hypotonic to its surroundings.
If a cell has equal amounts of water (and solute) as its environment
– It is isotonic to its surroundings.
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Osmotic Influences on Cells
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2. Active transport
Able to moves substances against the concentration gradient - from low to high concentration
Requires energy – ATP is used directly or indirectly to fuel active transport– allows cells to store concentrated substances
Requires the use of carrier proteins
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Active Transport
http://www.youtube.com/watch?v=STzOiRqzzL4&NR=1
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Endocytosis
Moves large molecules or sets of molecules into the cell
– Phagocytosis Cell eating Food engulfed by the membrane Material enters the cell in a vacuole.
– Pinocytosis Cell drinking Just brings fluid into the cell
– Receptor-mediated endocytosis Molecules entering the cell bind to receptor proteins first.
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Exocytosis
Moves large molecules or sets of molecules out of the cell
Vesicles containing the molecules to be secreted fuse with the plasma membrane.– Contents are dumped outside the cell.
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Endocytosis and Exocytosis