Chapter 3: The Cellular Level of Organization BSC 2085C

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Chapter 3 The Cellular Level of OrganizationBSC
2085C

• Instructor Jeff Laborda

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Parts of a Cell

• Plasma Membrane
• Cytoplasm
• Cytosol
• Organelles
• Nucleus
• A cell is the basic, living, structural, and
  functional unit of the body.
• Cytology is the study of cell structure, and cell
  physiology is the study of cell function.

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The Plasma MembraneThe Phospholipid Bilayer
with Bobbing ProteinsFluid Mosaic Model

• The Lipid Bilayer composed of phospholipids,
  cholesterol, and glycolipids

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The Plasma Membrane

• The Lipid Bilayer
• Phospholipids

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Amphipathic
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The Plasma Membrane

• The Lipid Bilayer
• Cholesterol classified as a lipid the most
  abundant steroid in animal tissues used for the
  synthesis of steroid hormones and bile salts

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The Plasma Membrane

• The Lipid Bilayer
• Glycolipids lipids with an attached carbohydrate
  group

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The Plasma MembraneBobbing Proteins Fluid
Mosaid Model

• Arrangement of Membrane Proteins
• Integral Proteins extend into or across the
  entire (transmembrane) lipid bilayer
• They are amphipathic.
• Many are glycoproteins.
• The combined glycoproteins and glycolipids form
  the glycocalyx which helps cells recognize one
  another, adhere to one another, and be protected
  from digestion by enzymes in the extracellular
  fluid.
• Peripheral Proteins located on the inner or
  outer surface of the lipid bilayercan be
  stripped away without disturbing the integrity of
  the plasma membrane

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Arrangement of Membrane Proteins
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The Plasma Membrane

• Functions of Membrane Proteins
• Membrane proteins vary in different cells and
  function as channels (pores), transporters,
  receptors, enzymes, cell-identity markers, and
  linkers (Figure 3.3).
• The different proteins help to determine many of
  the functions of the plasma membrane.

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The Plasma Membrane

• Functions of Membrane Proteins
• Formation of channel
• Passageway to allow specific substance to pass
  through
• Transporter proteins
• Bind a specific substance, change their shape,
  and move it across the membrane
• Receptor proteins
• Cellular recognition site bind to substance

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The Plasma Membrane

• Functions of Membrane Proteins
• Cell identity marker
• Allow cell to recognize other similar cells
• Linker
• Anchor proteins in cell membrane or to other
  cells
• Allow cell movement
• Cell shape structure by anchoring filaments
• Act as Enzyme
• Speed up reactions (e.g., lactase splits lactose
  into glucose and sucrose)

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The Plasma Membrane

• Membrane Fluidity
• Membranes are fluid structuresmost of the
  membrane lipids and many of the membrane proteins
  easily move in the bilayer.
• Membrane lipids and proteins are mobile in their
  own half of the bilayer.
• Cholesterol serves to stabilize the membrane and
  reduce membrane fluidity.

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The Plasma Membrane

• The plasma membrane is selectively permeable
  some things can pass through and others cannot.
• The lipid bilayer portion of the membrane is
  permeable to small, nonpolar, uncharged molecules
  (e.g., O2, CO2) but impermeable to ions and
  charged or polar molecules.
• The membrane is also permeable to water.
• Transmembrane proteins that act as channels or
  transporters increase the permeability of the
  membrane to molecules that cannot cross the lipid
  bilayer.
• Macromolecules are unable to pass through the
  plasma membrane except via a vesicle (a type of
  cell organelle).

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The Plasma Membrane

• Gradients Across the Plasma Membrane
• A concentration gradient is the difference in the
  concentration of a chemical between one side of
  the plasma membrane and the other.
• Oxygen and sodium ions are more concentrated
  outside the cell membrane with carbon dioxide and
  potassium ions more concentrated inside the cell
  membrane (Figure 3.4a).

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Gradients Across Membrane

• Concentration gradient
• Electrical gradient

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The Plasma Membrane

• Gradients Across the Plasma Membrane
• The inner surface of the membrane is more
  negatively charged and the outer surface is more
  positively charged. This sets up an electrical
  gradient, also called the membrane potential.
• Maintaining the concentration and electrical
  gradients are important to the life of the cell.
• The combined concentration and electrical
  gradients are called the electrochemical
  gradient.

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Transport Across the Plasma Membrane

• Principles of Diffusion
• A net movement down a concentration gradient
• Movement from an area of high concentration to an
  area of low concentration
• Depends on molecule mass, size, temperature,
  distance, and concentration gradient

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Transport Across the Plasma Membrane

• Osmosis
• Movement of water across a semipermeable membrane
  from an area of low solute concentration to an
  area of high solute concentration
• Water molecules penetrate the membrane by
  diffusion through the lipid bilayer or through
  aquaporins, transmembrane proteins that function
  as water channels.

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Transport Across the Plasma Membrane

• Osmotic pressure of a solution is proportional to
  the concentration of the solute particles that
  cannot cross the membrane (Figure 3.7c).

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Transport Across the Plasma Membrane

• Osmosis
• Tonicity The ability to change the volume of
  cells inside a solution by moving water into our
  out of the cell

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Transport Across the Plasma Membrane

• Diffusion Through the Lipid Bilayer
• Nonpolar, hydrophobic molecules such as
  respiratory gases, some lipids, small alcohols,
  and ammonia can diffuse across the lipid bilayer.
• It is important for gas exchange, absorption of
  some nutrients, and excretion of some wastes.

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Transport Across the Plasma Membrane

• Diffusion Through Membrane Channels
• Most membrane channels are ion channels, allowing
  passage of small, inorganic ions which are
  hydrophilic.
• Ion channels are selective and specific and may
  be gated or open all the time (Figure 3.5).

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Transport Across the Plasma Membrane

• Facilitated Diffusion/Transport
• Substance binds to carrier protein embedded in
  plasma membrane
• Carrier protein changes shape and releases the
  substance on the other side
• Requires no ATP
• Transport occurs down a concentration gradient
• Ex. glucose, amino acids

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Facilitated Diffusion of Glucose

• Glucose binds to transportprotein
• Transport protein changes shape
• Glucose moves across cell membrane (but only
  downthe concentration gradient)
• Kinase enzyme reduces glucose concentration
  inside the cell by transforming glucose into
  glucose-6-phosphate
• Transporter proteins always bring glucose into
  cell

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Transport Across the Plasma Membrane

• Active Transport
• Active transport is an energy-requiring process
  that moves solutes such as ions, amino acids, and
  monosaccharides against a concentration gradient.
  
• In primary active transport, energy derived from
  ATP changes the shape of a transporter protein,
  which pumps a substance across a plasma membrane
  against its concentration gradient.

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Transport Across the Plasma Membrane

• Active Transport Primary Active Transport
• The most prevalent primary active transport
  mechanism is the sodium ion/potassium ion pump
  (Figure 3.8).
• requires 40 of cellular ATP
• all cells have 1000s of them
• maintains low concentration of Naand a high
  concentration of K in the cytosol
• operates continually

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Transport Across the Plasma Membrane

• Active Transport Secondary Active Transport
• In secondary active transport, the energy stored
  in the form of a sodium or hydrogen ion
  concentration gradient is used to drive other
  substances against their own concentration
  gradients.
• Plasma membranes contain several antiporters and
  symporters powered by the sodium ion gradient
  (Figure 3.9).

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Digitalis

• Digitalis slows the sodium ion-calcium ion
  antiporters, allowing more calcium to stay inside
  heart muscle cells, which increases the force of
  their contraction and thus strengthens the
  heartbeat.

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Transport Across the Plasma Membrane

• Transport in Vesicles
• A vesicle is a small membranous sac formed by
  budding off from an existing membrane.
• endocytosis
• exocytosis

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Transport Across the Plasma Membrane

• Transport in Vesicles
• Endocytosis bringing something into the cell
• Phagocytosis cell eating by macrophages
  (derived from a monocytes WBC) and other WBCs
• Particle binds to receptor protein
• Whole bacteria and viruses are engulfed and later
  digested
• Pinocytosis nonselective cell drinking (no
  receptor proteins)
• Exocytosis releasing something from cell
• Vesicles form inside cell, fuse to cell membrane
• Release their contents digestive enzymes,
  hormones, neurotransmitters, or waste products
• Replace cell membrane lost by endocytosis

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Review

• Table 3.1 summarizes the processes by which
  materials are transported into and out of cells.

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Cytoplasm

• Cytosol
• Intracellular fluid mostly water, inorganic
  ions, proteins, carbohydrates, lipids, ATP, and
  wastes
• Functionally, cytosol is the medium in which many
  metabolic reactions occur.

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Cytoplasm

• Organelles (small organs)
• Organelles are specialized structures that have
  characteristic shapes and perform specific
  functions in cellular growth, maintenance, and
  reproduction.

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Cytoplasm

• Organelles
• Cytoskeleton
• The cytoskeleton is a network of several kinds of
  protein filaments that extend throughout the
  cytoplasm and provides a structural framework for
  the cell.
• It consists of microfilaments, intermediate
  filaments, and microtubules.

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Cytoplasm

• Organelles
• Cytoskeleton
• Microfilaments Most microfilaments are composed
  of actin and function in movement and mechanical
  support (Figure 3.13a).

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Cytoplasm

• Organelles
• Cytoskeleton
• Intermediate filaments are composed of several
  different proteins and function in support and to
  help anchor organelles such as the nucleus
  (Figure 3.13b).

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Cytoplasm

• Organelles
• Cytoskeleton
• Microtubules are composed of a protein called
  tubulin and help determine cell shape and
  function in the intracellular transport of
  organelles and the migration of chromosome during
  cell division. (Figure 3.13c)