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Cellular Level of Organization

Chapter 3 – Lecture Notes

Anatomy and Physiology I

Motlow Community College

S. A. Edwards

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Chapter Overview

3.1 Principle Parts of a Cell3.2 Plasma Membrane3.3 Transport Across the Membrane3.4 Cytoplasm3.5 Nucleus3.6 Transcription and Translation3.6 Cell Division

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Essential TermsCell biology Study of cellular structurePlasma membrane Cell membraneTranscription Making RNA from DNA templateTranslation Making protein from RNA templateMitosis cell division, separation of chromosomes

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Introduction1. Cells

• Are the basic, living, structural and functional units of the body

• Are composed of characteristic parts The coordinated function of which allows

each cell type to fulfill a unique biochemical or structural role

2. Cell structure and function are intimately related

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Concept 3.1

Principal Parts of a Cell

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Three Principal Parts of a Cell1. Plasma membrane

Separates the inside of cell from outside

2. Cytoplasm All cellular contents between plasma

membrane and nucleus Two major components

1. Cytosol2. Organelles

3. Nucleus• Large organelle that houses the cell’s

DNA

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Concept 3.2

Plasma Membrane

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Plasma Membrane Surrounds and contains cytoplasm of

cell Fluid mosaic model

Fluid lipids with a mosaic of many different proteins

Lipids allow passage of hydrophobic substances

Protein channels allow the passages of certain ions, eg K+, Na+, Cl-

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Lipid Bilayer Bilayer

“Bi” = two Two back-to-back layers of lipids

Lipids Three types

1. Phospholipids (75%)• Lipid with phosphate attached

2. Cholesterol (20%)• Steroid with attached hydroxyl group

3. Glycolipids (5%) • Lipid with carbohydrate attached

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Phospholipid Portion Phospholipids have both polar and

nonpolar parts Phosphate head is polar

(hydrophillic) Lipid tails are nonpolar (hydrophobic) Lipid tails orient toward one another

Away from water Phosphate heads orient toward

membrane surfaces Near water

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Cholesterol Portion Only hydroxyl group is polar

Forms hydrogen bonds with head of phospholipids and glycolipids

Majority of molecule is nonpolar Steroid rings Hydrocarbon tail Nonpolar regions fit among fatty acids

of phospholipids Away from water

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Glycolipid Portion

Carbohydrate group is hydrophillic Lipid portion hydrophobic Appear only in the layer that faces

extracellular fluid One reason two layers are

asymmetric

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Membrane Proteins

Categorized as integral or peripheral

Integral proteins are firmly embedded

Most are transmembrane Span the entire bilayer

Peripheral proteins are more loosely associated with membrane

With polar heads of membrane lipids Or with transmembrane proteins Can be at inner or outer layer

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Glycoproteins Membrane protein with a

carbohydrate group attached Carbohydrate part on extracellular

surface only Part of the surface coating of cells

called the glycocalyx Sugar portion of glycolipids and

glycoprotein Acts like molecular “signature” for cell-

to-cell recognition Also enables cell attachment

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Membrane Protein FunctionsIntegral proteins Ion channels Transporters Receptors Enzymes Linkers (anchors) Cell-identity markersPeripheral proteins Help support plasma membrane Anchor integral proteins Participate in mechanical activities of

cells

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Membrane FluidityFluids move Most membrane lipids and many

proteins easily rotate and move sideways in bilayer

Do not confuse with flip-flopping sides Lipids may wander completely around a

cell in only a few minutes Fluidity is increased by

Increased number of double bonds in fatty acid tails of lipids

Increased amount of cholesterol in membrane

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Membrane Fluidity Rigid membrane lacks mobility Some fluidity needed for

Interactions of membrane proteins Movement of membrane components

responsible for Cell movement Growth Division Secretion Formation of cellular junctions

But complete fluidity would lack structural organization and mechanical abilities

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Membrane Permeability Permeable

Substances can pass through Impermeable

Substances cannot pass through Selectively permeable

Some substances get through while others do not

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Membrane Permeability Permeablilty of lipid portion

Nonpolar substances can easily pass Polar, charged, large cannot pass Permeable to:

Small uncharged polar molecules such as water and urea

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Membrane Permeability Transmembrane proteins

Can act as channels and transporters Increase permeability Are very selective with each being

specific to particular particles

Transport of very large particles discussed in 3.3

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Gradients Across Membrane Living cells maintain different concentrations

of certain substances on each side of membrane

Concentration gradient A difference between concentrations on each side

of membrane Can be chemical Can be electric (difference between charges on

each side of membrane) Electrochemical gradients

Created by different concentrations of chemicals and charges across membrane

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Concept 3.3

Transport Across the Plasma Membrane

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Transport Across Membrane Essential to the life of cell Classified active or passive

Passive process require no cellular energy

Examples include diffusion and osmosis Active processes require cellular

energy (ATP) Substances generally move “uphill”

against gradient Examples include using transporter

proteins, endocytosis, and exocytosis,

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Diffusion Random mixing of particles in

solution Substances move down

concentration gradient Particles eventually become evenly

distributed Kinetic energy is driving force Can occur across a membrane

given there is no barrier to movement of the substance

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Factors that Influence Diffusion Steepness of the gradient

Steeper = faster Temperature

Hotter = more kinetic energy Mass of diffusing substance

Larger particles move slower Surface area

More area = faster diffusion Diffusion distance

Longer distances = slower diffusion

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Diffusion Through Lipid Bilayer Nonpolar, hydrophobic molecules

move freely through bilayer Oxygen, carbon dioxide, nitrogen

gases, fatty acids, steroids, fat soluble vitamins (A, E, D, K), small alcohols, ammonia

Recall that very small charged particles can also move

Water and urea Important for life processes such as

nutrient, waste, and gas exchange

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Diffusion Through Ion Channels Integral transmembrane proteins

Allow passage of Small inorganic ions too hydrophillic to

pass lipid portion of bilayer Specific ions

Generally slower than diffusion across lipid portion

Less opportunity Some channels are gated and open

and close in regulated processes

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Osmosis Passive process Diffusion of water across a

selectively permeable membrane From high concentration of WATER

to lower concentration of WATER Moves toward higher solute

concentration

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TonicityDescribes relationship of solutions on

each side of membrane Hypertonic

Solution with more solutes Water moves toward hypertonic side

Hypotonic Solution with less solutes Water moves away from hypotonic

side Isotonic

Both solutions have similar concentrations of solutes

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Transporter Proteins Facilitated diffusion

Solute binds to specific transporter on one side of and released on other side

Solutes are too big or too polar or too highly charged

Active transport Solutes moving against concentration

gradient Can be driven by ATP use or via

energy stored in ionic concentration Primary active transport uses ATP

Sodium potassium pump

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Transporter Proteins Secondary active transport

No ATP use needed Energy stored in ion concentrations

used to drive other substances across membrane against concentrations

Usually sodium or hydrogen ions Symporters

Both substances move same direction Antiporters

Each substance moves a different direction

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Transport in Vesicles Endocytosis

1. Receptor mediated2. Phagocytosis3. Bulk-phase

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Receptor Mediated Endocytosis Steps

Binding Vesicle formation Uncoating Fusion with endosome Recycling of receptors to plasma

membrane Degradation in lysosomes

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Phagocytosis Only a few body cells are capable Two main types

Macrophages Neutrophils

Particle binds to plasma membrane receptor on phagocyte

Pseudopods extend and surround particle forming phagosome

Phagosome fuses with lysosomes which destroy invader

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Bulk-phase Endocytosis Most body cells carry out process

Especially absorptive cells in intestines and kidneys

Also called pinocytosis Tiny droplets of extracellular fluid

taken into cell No receptor proteins are involved Lysosomes fuse and degrade

particles into smaller useable particles

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Exocytosis Releases materials form a cell All cells carry out process Especially important in

Secretory cells Release digestive enzymes, hormones,

mucus, or other secretions Nerve cells

Release neurotransmitters Vesicles fuse with plasma

membrane and release contents into extracellular fluid

Balanced with endocytosis

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Transcytosis Active process Vesicles undergo endocytosis

followed by exocytosis Occurs most often across

endothelial cells that line blood vessels

Pregnancy antibodies pass from mother to fetus through this process

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Concept 3.4

Cytoplasm

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Cytoplasm All cellular contents between

plasma membrane and nucleus

Two components1. Cytosol2. Various organelles

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Cytosol Intracellular fluid Fluid portion that surrounds

organelles 55% of total cell volume Varies in composition and consistency 75-90% water 10-25% dissolved and suspended

ions, glucose, amino acids, fatty acids, proteins, lipids, ATP, waste products, and some various aggregations of organic molecules

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Cytosol Intracellular fluid Fluid portion that surrounds

organelles 55% of total cell volume Varies in composition and consistency 75-90% water 10-25% dissolved and suspended

ions, glucose, amino acids, fatty acids, proteins, lipids, ATP, waste products, and some various aggregations of organic molecules

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Organelles Specialized structures within the cell Have characteristic shapes Perform specific functions

Cellular growth Cellular maintenance Cellular reproduction

Each organelle has its own set of enzymes Numbers and types vary in different cells,

depending on function All organelles cooperate to maintain

homeostasis

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Organelles Cytoskeleton

Microfilaments Intermediate filaments Microtubules

Centrosome Two centrioles

Cilia Flagella Ribosomes

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Organelles Endoplasmic reticulum (ER)

Rough ER Smooth ER

Golgi complex Lysosomes Mitochondria Nucleus

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Cytoskeleton Network of protein filaments that

extends throughout cytosol Provides structural framework for cell

Determining cell shape Organizes cellular contents Aids movement of organelles within cell

during cell division Aids movement of whole cells such as

phagocytes Continually reorganizes as changes

shape