2- Cell Structure and Function Chapter 4. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 4-2 The Cell Theory

2-

Cell Structure and Function

Chapter 4

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.4-2

The Cell Theory

All living things are made of cells. A cell

– The basic unit of all living things.


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


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


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


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


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

15

Prokaryotic Cells


Cell Size

Prokaryotic cells– 1-2 micrometers in

diameter

Eukaryotic cells– 10-200 micrometers in

diameter

Fig. 4.2


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


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”).


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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Page 63

http://www.youtube.com/watch?v=owEgqrq51zY

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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


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


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


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


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)


2. Smooth Endoplasmic Reticulum (SER)– Relatively few associated ribosomes– Functions:

Synthesis of membrane lipids Detoxification of foreign substances

CO 5


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


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.


Functions of Lysosomes

Digestion– Of food taken into

the cell Destruction

– Disease-causing organisms

– Old organelles


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


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


Vacuoles and Vesicles

Fig. 4.16


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


The Endomembrane System = Interconversion of Membranes

Membranes are converted from one membranous organelle to another.


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

53

Mitochondria


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

55

Chloroplasts


Nonmembranous Organelles

Ribosomes Cytoskeleton Centrioles Cilia flagella Inclusions


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


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


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


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


Cilia and Flagella

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Fig. 4.24b

http://www.youtube.com/watch?v=2SoSvup6bGI&feature=related


Inclusions

Collections of miscellaneous materials– Can be called granules

Temporary sites for the storage of nutrients and waste

82

Glycogen Inclusions

Stryer's Biochemistry Fig. 23-2


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


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


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.


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

http://www.youtube.com/watch?v=s0p1ztrbXPY&feature=related


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



Some molecules have to be carried across the membrane.

– Accomplished by carrier proteins

Still involves diffusion– Follows a concentration

gradient– Is passive transport


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


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


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.


Osmotic Influences on Cells

http://www.youtube.com/watch?v=XclGRjnilsk&feature=channelhttp://www.youtube.com/watch?v=dHc05E7BijQhttp://www.youtube.com/watch?v=crpeX8nBgJE

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


Active Transport

http://www.youtube.com/watch?v=STzOiRqzzL4&NR=1


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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98

99

http://www.youtube.com/watch?v=fpOxgAU5fFQ&feature=related


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.

101


Endocytosis and Exocytosis

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2- Cell Structure and Function Chapter 4. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 4-2 The Cell Theory