3. Discovery of Cell
3
• Robert Hooke (mid-1600s)
– Observed sliver of cork
– Saw “row of empty boxes”
– Coined the term cell
4. Cell Theory
4
• (1839)Theodor Schwann & Matthias Schleiden
“ all living things are made of cells”
• (50 yrs. later) Rudolf Virchow
“all cells come from cells”
5. 5
• All living things are made of cells
• Smallest living unit of structure and
function of all organisms is the cell
• All cells arise from preexisting cells
(this principle discarded the idea of
spontaneous generation)
Principles of Cell Theory
8. 8
Characteristics of All Cells
• A surrounding membrane
• Protoplasm – cell contents in thick fluid
• Organelles – structures for cell function
• Control center with DNA
10. Plasma Membrane
10
• In 1895, Ernest Overton proposed that cell
membranes were made of lipids. The lipid
bilayer hypothesis, proposed in 1925 by Gorter
and Grendel, Boundary that separates the
living cell from it’s non-living surroundings.
• Phospholipid bilayer
• Amphipathic - having both:
hydrophilic heads
hydrophobic tails
• ~8 nm thick
• Is a dynamic structure
Phospholipid
11. 11
Membrane Structure
The fluid mosaic model of membrane structure has two
components:
1. Phospholipids arranged in a bilayer
2. Globular Proteins inserted in the lipid bilayer
15. 15
Membrane Structure
• Membrane structure is visible using an
electron microscope.
• Transmission electron microscopes (TEM)
can show the 2 layers of a membrane.
• Freeze-fracturing techniques separate the
layers and reveal membrane proteins.
20. 20
1. 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
21. 21
1. 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”).
22. 22
1. 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
24. 24
1. Phospholipids
•Phospholipid bilayers are fluid:
- Hydrogen bonding of water holds the 2
layers together
- Individual phospholipids and unanchored
proteins can move laterally through the
membrane
25. 25
1. Phospholipids
•Phospholipid bilayers are fluid:
- Saturated fatty acids make the membrane
less fluid than unsaturated fatty acids
- Cholesterols make the membrane more rigid
- Warm temperatures make the membrane
more fluid than cold temperatures
32. 32
Membrane Proteins
• Two types of membrane proteins
- Classified by how they are associated with
the membrane
1. Peripheral membrane proteins
2. Integral membrane proteins
33. 33
Membrane Proteins
1. Peripheral membrane proteins
• Anchored to a phospholipid in one layer
of the membrane
• on the intracellular or extracellular face of the
membrane
• Possess nonpolar regions that are
inserted in the lipid bilayer
• Free to move throughout one layer of the
bilayer
35. 35
Membrane Proteins
2. Integral membrane proteins
• Span the lipid bilayer (transmembrane
proteins)
• Nonpolar regions of the protein are
embedded in the interior of the bilayer
- Transmembrane Domain
• Polar regions of the protein protrude from
both sides of the bilayer
37. 37
Membrane Proteins
• Integral proteins possess at least one
transmembrane domain
- Region of the protein containing hydrophobic
amino acids
- Spans the lipid bilayer
- Usually alpha-helices
- Many receptors are integral proteins
39. 39
Membrane Proteins
• Extensive nonpolar regions within a
transmembrane protein can create a pore
through the membrane.
• sheets in the protein secondary structure
form a cylinder called a -barrel
• -barrel interior is polar and allows water and
small polar molecules to pass through the
membrane
41. 41
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
42. 42
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
43. 43
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
44. 44
1. Passive Transport
• Passive transport mechanisms only
movement substances along the
concentration gradient:
- Substances move from an area of high
concentration to an area of low
concentration
44
45. 45
1. Passive Transport
• Mechanisms 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
46. 46
1. Passive Transport
• Diffusion is movement of solute
molecules from high concentration to low
concentration
47. 47
1. Passive Transport
• There are two types of diffusion
1. Simple Diffusion
2. Facilitated Diffusion
48. 48
1. Passive Transport
1. Simple Diffusion
• Substances pass directly through
the cell membrane
• The cell membrane has limited
permeability to small polar
molecules, water, and ions
• The motion of water across the
membrane is known as osmosis
49. 49
1. Passive Transport
1. Simple Diffusion
• The rate (molecules/s) 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
51. 51
1. Passive Transport
2. Facilitated Diffusion
• Substances must pass through
transported proteins to get through
the cell membrane
• The cell membrane is selectively
permeable
52. 52
Passive Transport
Carrier proteins bind to the molecule that
they transport across the membrane.
Facilitated diffusion is movement of a
molecule from high to low concentration
with the help of a carrier protein.
-is specific
-is passive
-saturates when all carriers are occupied
53. 53
Facilitated Diffusion
• Selective permeability: integral
membrane proteins allow the cell to be
selective about what passes through the
membrane.
- Channel proteins have a polar interior
allowing polar molecules to pass through.
- Carrier proteins bind to a specific molecule
to facilitate its passage.
54. 54
Channel Proteins
• Channel proteins include:
- ion channels allow the passage of
ions (charged atoms or molecules) which
are associated with water
- gated channels are opened or closed
in response to a stimulus
– the stimulus may be chemical or electrical
56. 56
Channel Proteins
• Ion channels allow the passage of ions
(charged atoms or molecules) across the
membrane
• A concentration gradient of ions across the
membrane creates a membrane potential
- a membrane potential is a charge
difference between the two sides of the
membrane
59. 59
1. Passive Transport
2. Facilitated Diffusion
• 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/s) depends on gradient
until all protein carriers are in use - saturation point
61. 61
Passive Transport
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 moves water through aquaporins
63. 63
Osmosis
• Osmotic concentration is determined by
the the concentration of all solutes in
solution
• 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
67. 67
Osmosis
• Organisms can maintain osmotic balance
in different ways:
1. Some cells use extrusion in which
water is ejected through contractile
vacuoles.
2. Isosmotic regulation involves
keeping cells isotonic with their
environment.
3. Plant cells use turgor pressure to
push the cell membrane against the cell wall
and keep the cell rigid.
69. 69
2. Active Transport
Active transport
• Requires energy – ATP is used directly or
indirectly to fuel active transport
• Able to moves substances against the
concentration gradient - from low to high
concentration
- allows cells to store concentrated substances
• Requires the use of carrier proteins
70. 70
Active Transport
• Carrier proteins used in active transport
include:
-uniporters – move one molecule at a
time
-symporters – move two molecules in
the same direction
-antiporters – move two molecules in
opposite directions
71. 71
Active Transport
Sodium-potassium (Na+-K+) pump
• An active transport antiport mechanism
• Uses an antiporter to move 3 Na+ out of
the cell and 2 K+ into the cell
• ATP energy is used to change the
conformation of the carrier protein
• The affinity of the carrier protein for either
Na+ or K+ changes so the ions can be
carried across the membrane
72. 72
Active Transport
Sodium-potassium (Na+-K+) pump
• Used by animal cells to maintain a high
internal concentration of K+ ions and a low
internal concentration of Na+ ions
• Maintains a concentration gradient that is
used to power many other important
physiological process
77. 77
Active Transport
Coupled transport
• Uses the energy released when a
molecule moves by diffusion to supply
energy to active transport of a different
molecule
• A symporter is used
• Glucose-Na+ symporter captures the
energy from Na+ diffusion to move glucose
against a concentration gradient
79. 79
Bulk Transport
• Bulk transport of substances is
accomplished by
1. Endocytosis – movement of
substances into the cell
2. Exocytosis – movement of
materials out of the cell
80. 80
Bulk Transport
• Endocytosis occurs when the plasma
membrane envelops food particles and
liquids.
1. phagocytosis – the cell takes in
particulate matter
2. pinocytosis – the cell takes in only
fluid
3. receptor-mediated endocytosis –
specific molecules are taken in after they
bind to a receptor
84. 84
Bulk Transport
• Exocytosis occurs when material is
discharged from the cell.
• Vesicles in the cytoplasm fuse with the cell
membrane and release their contents to the
exterior of the cell
• Used in plants to export cell wall material
• Used in animals to secrete hormones,
neurotransmitters, digestive enzymes
