Cell Membranes: Osmosis and Diffusion

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Cell MembranesOsmosis and Diffusion
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Membrane Functions

• 1. Protect cell
• 2. Guard incoming and outgoing substances
• 3. Maintain ion concentrations of various
  substances
• Selectively permeable
• Allows some molecules in
• Others are kept out

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Lipid Bilayer

• Main component of cell membranes
• Gives the membrane its fluid properties
• Two layers of phospholipids

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Phospholipid Bilayer
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Fluid Mosaic Model
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Membrane Proteins

• Transport proteins
• Receptor proteins
• Recognition proteins
• Adhesion proteins

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Solutions

• Solutions are made of solute and a solvent
• Solvent - the liquid into which the solute is
  poured and dissolved
• Solute - substance that is dissolved or put into
  the solvent. Salt and sucrose are solutes

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Membrane Transport

• Diffusion
• Passive transport
• No energy expended
• Osmosis
• Passive transport
• Water across membrane

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Diffusion

• Movement of molecules from one side of a membrane
  to the other
• Occurs from a region of high concentration of
  substance to a region of lower concentration

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Di f fus ion
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Osmosis
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Turgor Pressure

• Hypotonic Solution
• Inside has higher salt concentration
• Hypertonic Solution
• Inside has lower salt concentration
• Isotonic Solution
• Both solutions have same concentrations of solute

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Plant and Animal Cells in Various Solutions
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Membrane Transport

• Facilitated Diffusion
• Passive transport
• Use of proteins to carry polar molecules or ions
  across
• Active Transport
• Requires energy to transport molecules against a
  concentration gradient
• Energy is in the form of ATP

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Active Transport

• Movement against a concentration gradient (from
  low to high concentration)
• Requires added energy (ATP) and a specific
  membrane carrier (pump)

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Endocytosis

• Ingestion of large molecules or particles
• Cell surrounds and envelops substance forming a
  vesicle which fuses with lysosome
• Phagocytosis
• Endocytosis of organism
• Pinocytosis
• Endocytosis of large, soluble molecules

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Exocytosis

• Cell secretes material
• Package in a vesicle
• Moves in vesicle to cell surface
• Vesicle membrane fuses with cell membrane
• Contents are secreted out
• Specialized animal cells produce and secrete
  digestive enzymes and hormones

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Cell Structure and Function
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Early Discoveries

• Mid 1600s - Robert Hooke observed and described
  cells in cork
• Late 1600s - Antony van Leeuwenhoek observed
  sperm, microorganisms
• 1820s - Robert Brown observed and named nucleus
  in plant cells

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Cell Theory

• Matthias Schleiden
• Theodor Schwann
• Rudolf Virchow

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Cell Theory

• 1) Every organism is composed of one or more
  cells
• 2) Cell is smallest unit of life
• 3) Cells arise from pre-existing cells

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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

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Cells are Microscopic

• Why arent cells larger?
• Animal cells range from 7-50 µm, some are much
  larger
• Nerve cells
• Algae
• Eggs
• Plant cells are generally larger

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Why arent Cells Larger?

• As cells grow larger, volume increases faster
  than surface area
• Large cells do not have sufficient surface area
  for efficient diffusion of nutrients and wastes
• Nuclei control activities of smaller cells more
  easily

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Structure of Cells

• Must Have
• Plasma membrane
• DNA Region
• Cytoplasm

• Two types
• Prokaryotic
• Eukaryotic
• Plant
• Animal

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Structure of Cells

• The plasma membrane and internal cell membranes
  consist of lipids and proteins. The lipids are
  organized as two adjacent layersas bi-layer.

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Fluid Mosaic Model

• Membrane is a mosaic of
• Phospholipids
• Glycolipids
• Sterols
• Proteins
• Most phospholipids and some proteins can drift
  through membrane

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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

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Microscopes

• Create detailed images of something that is
  otherwise too small to see
• Light microscopes
• Simple or compound
• Electron microscopes
• Transmission EM or Scanning EM

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Light Microscope Limitations

• If a structure is less than one-half of a
  wavelength long, it will not be visible
• Light microscopes can resolve objects down to
  about 200 nm in size
• Can magnify 2000x
• Our microscopes magnify 400x at best

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Electron Microscopy

• Uses streams of accelerated electrons rather than
  light
• Electrons are focused by magnets rather than
  glass lenses
• Can resolve structures down to 0.5 nm magnifying
  20,000x-100,00x

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Eukaryotic Cells
Plant cell

• Defined as a cell that starts out life with a
  nucleus
• Have a nucleus and other organelles
• Eukaryotic organisms
• Plants
• Animals
• Protista
• Fungi

Animal cell
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Animal Cell Features

• Plasma membrane
• Cytoskeleton
• Nucleus
• Ribosomes
• Endoplasmic reticulum
• Vesicle
• Mitochondria
• Golgi body

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Plant Cell Features

• Cell wall
• Chloroplast
• Central Vacuole

• Nucleus
• Plasma Membrane
• Ribosomes
• Endoplasmic reticulum
• Golgi body
• Mitochondria

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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

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Components of Nucleus

• Nuclear envelope - double membrane enclosing the
  nucleus
• Nucleoplasm - fluid portion of the nucleus
• Nucleolus - where other organelles are
  constructed
• Chromosome made of chromatin
• Chromatin - DNA with all associated protein

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Nuclear Envelope

• Two outer membranes (lipid bi-layers)
• Innermost surface has DNA attachment sites
• Pores span bi-layer

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Nucleolus

• Dense mass of material in nucleus
• May be one or more
• Cluster of DNA and proteins
• Puts together ribosomal subunits

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Chromatin

• Cells collection of DNA and associated proteins
• Chromosome is one DNA molecule and its
  associated proteins
• Appearance changes as cell divides

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Cytomembrane System

• Group of related organelles in which lipids are
  assembled and new polypeptide chains are modified
• Products are sorted and shipped to various
  destinations

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Cyto-membrane System

• Endoplasmic reticulum
• Golgi bodies
• Vesicles

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Endoplasmic Reticulum

• In animal cells, continuous with nuclear membrane
• Extends throughout cytoplasm
• Two regions - rough and smooth

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Rough ER

• Arranged into flattened sacs
• Ribosomes on surface give it a rough appearance
• Some polypeptide chains enter rough ER and are
  modified
• Cells that specialize in secreting proteins have
  lots of rough ER

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Smooth ER

• A series of interconnected tubules
• No ribosomes on surface
• Lipids assembled inside tubules
• Smooth ER of liver inactivates wastes, drugs

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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

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Vesicles

• Membranous sacs that move through the cytoplasm
• Lysosomes
• Peroxisomes

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Mitochondria

• ATP-producing powerhouses
• Double-membrane system
• Carry out most efficient energy-releasing
  reactions that require oxygen

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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

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Mitochondrial Origins

• Mitochondria resemble bacteria
• Have own DNA, ribosomes
• Divide on their own
• May have evolved from ancient bacteria that were
  engulfed but not digested

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Plant Organelles

• Plastids organelles that specialize in
  photosynthesis
• Central Vacuole store amino acids, sugars, ions
  and toxic wastes.

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Chloroplasts

• Convert sunlight energy to ATP through
  photosynthesis

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Photosynthesis

• First stage
• Occurs at thylakoid membrane
• Light energy is trapped by pigments and stored as
  ATP
• Second stage
• Inside stroma, ATP energy is used to make sugars,
  then other carbohydrates

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Central Vacuole

• Fluid-filled organelle
• Stores amino acids, sugars, wastes
• As cell grows, expansion of vacuole as a result
  of pressure that forces end cell walls to expand
• In mature cell, central vacuole takes up 50-90
  percent of cell interior

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Cytoskeleton

• Present in all eukaryotic cells
• Basis for cell shape and internal organization
• Allows organelle movement within cells and, in
  some cases, cell motility

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Flagella and Cilia

• Structures for cell motility

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Cell Wall
Plasma membrane

• Structural component that wraps around the plasma
  membrane
• Occurs in plants, some fungi, some protista

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Prokaryotic Cells

• Archaebacteria and Eubacteria
• DNA is NOT enclosed in nucleus
• Generally the smallest, simplest cells
• No organelles

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Prokaryotic Structure
pilus
cytoplasm with ribosomes
DNA
flagellum
capsule
cell wall
plasma membrane
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Things to know

• The first cell seen under a microscope was cork.
• One portion of the cell theory states that all
  living organisms are made up of cells.
• The cell theory was proposed by Theodor Schwann
  and Matthias Schleiden.
• The idea that all living cells come from
  pre-existing living cells was proposed by Rudolf
  Virchow.

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Things to know

• The phospholipid molecules of most membranes have
  a hydrophilic head and two hydrophobic tails.
• Hydrophobic reactions of phospholipids may
  produce clusters of their fatty acid tails, which
  form a lipid bilayer.

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Things to know

• The relative impermeability of membranes to
  water-soluble molecules is a result of the
  presence of phospholipids in the lipid bilayer.
• Membranes have holes due to proteins that extend
  through the membrane.
• Some membranes have proteins with channels or
  pores that allow for the passage of hydrophilic
  substances.

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Things to know

• The current concept of a membrane can be best
  summarized by the fluid mosaic model.
• The lipid bilayer serves as a hydrophobic barrier
  between two fluid regions.
• Receptor proteins are responsible for binding
  hormones that can switch on a cell.
• There are 1,000,000,000 nanometers in a meter.

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Things to know

• The maximum power of magnification of a light
  microscope is 4,000.
• The highest magnification generally used to study
  cells is provided by the transmission electron
  microscope.
• The transmission electron microscope has the
  highest magnification.
• The ribosomes are made of two subunits of RNA and
  protein.

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Things to know

• Organelles composed of a system of canals, tubes
  and sacs that transport molecules inside the
  cytoplasm are endoplasmic reticula.
• Mitochondria are the primary cellular sites for
  the recapture of energy from carbohydrates.
• Golgi bodies are the primary structures for the
  packaging of cellular secretions for export from
  the cell.

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Things to know

• The cell part responsible for maintaining cell
  shape, internal organization, and cell movement
  is the cytoskeleton.
• The cell wall is found in plant cells but not in
  animal cells.
• The nucleolus is found in the nucleus.
• The nuclear envelope has two lipid layers there
  are pores in the membrane ribosomal subunits can
  pass out of the nucleus protein filaments are
  attached to inner surface.

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Things to know

• Scientists use the word chromosome to describe
  an individual DNA molecule.
• Endoplasmic reticula are sometimes referred to as
  rough or smooth.
• Ribosomes are the primary cellular sites for the
  production of protein.
• The Golgi body has been likened to a stack of
  pancakes.

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Things to know

• The lysosomes contain enzymes and are the main
  organelles of intracellular digestion.
• Animal cells dismantle and dispose of waste
  materials by several lysosomes fusing with a sac
  that encloses the wastes.
• A tadpoles tail disappears when it changes into
  an adult frog by action of lysosomes.

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Things to know

• The nuclei and mitochondria contain DNA.
• Mitochondria contain enzymes used in the
  breakdown of glucose and generation of ATP.
• Energy stored in carbon compounds is converted by
  mitochondria to a form usable by the cell.
• There are two membranes surrounding the
  mitochondrion.

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Things to know

• The inner membranes of mitochondria are called
  cristae.
• Fluid-filled sacs that may store food or water in
  cells are called vacuoles.
• Plastids in plant cells function in
  photosynthesis or storage.
• Stroma and grana are portions of chloroplasts.

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Things to know

• Vacuoles dramatically increase the cell size and
  surface area.
• Flagellum is compared to a whip and used for
  mobility.
• Bacteria are examples of prokaryotes.
• Prokaryotic cells do NOT have membrane bound
  nucleus.
• Prokaryotes have DNA regions are unicellular
  have cell walls and are bacteria.

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Bibliography

1. http//filebox.vt.edu/users/malsaghi/ 6 Dec 2004,
   Mohannad AL-Saghir, Biology Department, Virginia
   Tech
2. http//pls.atu.edu/biology/, 6 Dec 2004, Arkansas
   Tech University, Biology Department, Beck,