MEMBRANES, DIFFUSION, OSMOSIS, ACTIVE TRANSPORT, ETC.

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CHAPTER 9

• MEMBRANES, DIFFUSION, OSMOSIS, ACTIVE TRANSPORT,
  ETC.

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MEMBRANE STRUCTURE AND FUNCTION
Membranes organize the chemical activities of
cells

• Membranes organize the chemical reactions making
  up metabolism

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Cytoplasm
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• Membranes are selectively permeable

• They control the flow of substances into and out
  of a cell
• Membranes can hold teams of enzymes that
  function in metabolism

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Membrane phospholipids form a bilayer
Head

• Phospholipids are the main structural components
  of membranes
• They each have a hydrophilic head and two
  hydrophobic tails

Symbol
Tails
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• In water, phospholipids form a stable bilayer

• The heads face outward and the tails face inward

Water
Hydrophilicheads
Hydrophobictails
Water
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The membrane is a fluid mosaic of phospholipids
and proteins

• Phospholipid molecules form a flexible bilayer
• Cholesterol and protein molecules are embedded in
  it
• Carbohydrates act as cell identification tags

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• The plasma membrane of an animal cell

Glycoprotein
Carbohydrate (of glycoprotein)
Fibers of the extracellular matrix
Glycolipid
Phospholipid
Cholesterol
Microfilaments of the cytoskeleton
Proteins
CYTOPLASM
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Proteins make the membrane a mosaic of function

• Some membrane proteins form cell junctions
• Others transport substances across the membrane

Transport
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• Many membrane proteins are enzymes

• Some proteins function as receptors for chemical
  messages from other cells
• The binding of a messenger to a receptor may
  trigger signal transduction

Messenger molecule
Receptor
Activated molecule
Enzyme activity
Signal transduction
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Passive transport is diffusion across a membrane

• In passive transport, substances diffuse through
  membranes without work by the cell
• They spread from areas of high concentration to
  areas of lower concentration

Molecule of dye
EQUILIBRIUM
Membrane
EQUILIBRIUM
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Osmosis is the passive transport of water
Hypotonicsolution
Hypertonic solution

• In osmosis, water travels from an area of lower
  solute concentration to an area of higher solute
  concentration

Selectivelypermeablemembrane
Solutemolecule
HYPOTONIC SOLUTION
HYPERTONIC SOLUTION
Water molecule
Selectivelypermeablemembrane
Solute molecule with cluster of water molecules
NET FLOW OF WATER
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Water balance between cells and their
surroundings is crucial to organisms

• Osmosis causes cells to shrink in a hypertonic
  solution and swell in a hypotonic solution
• The control of water balance(osmoregulation) is
  essential for organisms

ISOTONIC SOLUTION
HYPOTONIC SOLUTION
HYPERTONIC SOLUTION
ANIMALCELL
(1) Normal
(2) Lysing
(3) Shriveled
Plasmamembrane
PLANTCELL
(4) Flaccid
(5) Turgid
(6) Shriveled
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Transport proteins facilitate diffusion across
membranes

• Small nonpolar molecules diffuse freely through
  the phospholipid bilayer
• Many other kinds of molecules pass through
  selective protein pores by facilitated diffusion

Solutemolecule
Transportprotein
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Cells expend energy for active transport

• Transport proteins can move solutes across a
  membrane against a concentration gradient
• This is called active transport
• Active transport requires ATP

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FLUIDOUTSIDECELL
Phosphorylated transport protein

• Active transport in two solutes across a membrane

Transportprotein
Firstsolute
1
First solute, inside cell, binds to protein
2
ATP transfers phosphate to protein
3
Protein releases solute outside cell
Second solute
4
Second solute binds to protein
5
Phosphate detaches from protein
6
Protein releases second solute into cell
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Exocytosis and endocytosis transport large
molecules

• To move large molecules or particles through a
  membrane
• a vesicle may fuse with the membrane and expel
  its contents (exocytosis)

FLUID OUTSIDE CELL
CYTOPLASM
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• or the membrane may fold inward, trapping
  material from the outside (endocytosis)

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• Three kinds of endocytosis

Pseudopod of amoeba
Food being ingested
Plasma membrane
Material bound to receptor proteins
PIT
Cytoplasm
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Connection Faulty membranes can overload the
blood with cholesterol

• Harmful levels of cholesterol can accumulate in
  the blood if membranes lack cholesterol receptors

Phospholipid outer layer
LDL PARTICLE
Receptor protein
Protein
Cholesterol
Plasma membrane
Vesicle
CYTOPLASM
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Chloroplasts and mitochondria make energy
available for cellular work

• Enzymes and membranes are central to the
  processes that make energy available to the cell
• Chloroplasts carry out photosynthesis, using
  solar energy to produce glucose and oxygen from
  carbon dioxide and water
• Mitochondria consume oxygen in cellular
  respiration, using the energy stored in glucose
  to make ATP

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Sunlight energy

• Nearly all the chemical energy that organisms use
  comes ultimately from sunlight

Chloroplasts,site of photosynthesis
CO2H2O
GlucoseO2
Mitochondriasites of cellularrespiration

• Chemicals recycle among living organisms and
  their environment

(for cellular work)
Heat energy