The Working Cell
• The membrane of a cell (or organelle) control
the flow of substances into and out of the cell (or
organelle)
• Remember, that cell membranes are composed
of a phospholipid bi-layer with a polar exterior
and nonpolar interior which restricts the
movement of molecules across its surface
Phospholipid bilayer
(cross-section)
Hydrophilic
head
WATER
Hydrophobic
tail
WATER
Membrane structure and function
• A diverse collection of proteins are embedded in
the framework of this phospholipid bi-layer
• Different types of cells have different types of
embedded proteins; similarly different types of
organelles have different types of proteins
embedded in their membranes
• Some of these proteins give the membrane
strength = integrins
Membrane structure and function
• Glycoproteins (protein linked to a
polysaccharide), on the other hand, are involved
in cell-to-cell recognition
• The outside surface of the membrane has
carbohydrates which are bonded to proteins or
lipids in the membrane
• Many of these function as identification tags that
are recognized by other cells
Plasma Membrane
Phospholipid
bilayer
Hydrophobic regions
of protein
Hydrophilic
regions of protein
Carbohydrate of
glycoprotein
Glycoprotein
Glycolipid
Integrin
Phospholipid
Microfilaments
of cytoskeleton
Cholesterol
Membrane Structure and Function
• Many membrane proteins function as enzymes,
receptors, and transporters
Enzymes
Messenger molecule
Receptor
Activated
molecule
Membrane Structure and Function
• The cell membrane is commonly referred to as a
fluid mosaic
• A membrane is a mosaic (a surface made of
many, small pieces) in having diverse protein
molecules embedded in the phospholipids
• The membrane is fluid in that most of these
molecules can drift about in the membrane
• The membrane is made up of ~80%
phospholipid, 15% protein, and 5% cholesterol
Transport Across Plasma Membranes
• In order for a cell to survive, nutrients, water, and
gases must move into the cell, and waste
materials must be eliminated out of the cell.
• Materials move back and forth across a cell’s
membrane in several different ways:
– diffusion and osmosis
– endocytosis and exocytosis
– active transport
Images : Copyright © The McGraw-Hill Companies, Inc.
Diffusion
• Random movement of molecules causes them to
move from regions of high concentration to regions of
low concentration via a process known as diffusion.
• Diffusion is passive, in that no energy is required
because the movement of molecules occurs along
their concentration gradient.
• Diffusion attempts to produce a uniform mixture of
molecules.
10
0
Diffusion
• Random movement of molecules causes them
to move from regions of high concentration to
regions of low concentration via a process
known as diffusion.
• Diffusion tends to produce a uniform mixture of
molecules.
5
5
Diffusion
• Molecules move from regions of high to low
concentration, or down the concentration
gradient until they obtain equilibrium.
Images : Copyright © Pearson Education, Inc.
Diffusion
• Small, non-polar molecules like O2
and CO2 can diffuse across the
plasma membrane between the tails
of the phospholipids.
Hydrophilic
head
WATER
Hydrophobic
tail
WATER
Images : Copyright © Pearson Education, Inc.
Diffusion
• In contrast, ions and polar molecules
cannot cross the non-polar
environment within the core of the
membrane bi-layer.
Hydrophilic
head
WATER
Hydrophobic
tail
WATER
Images : Copyright © Pearson Education, Inc.
Facilitated Diffusion
• Facilitated diffusion allows large, polar
molecules to cross the phospholipid bi-layer.
• Facilitated diffusion is accomplished by
transport proteins which are embedded into
the plasma membrane.
Transport proteins create a hydrophilic channel that
allows polar molecules and ions to cross the
membrane.
hydrophobic
hydrophilic
Images : Copyright © Pearson Education, Inc.
Osmosis
• Water molecules may diffuse freely across the
plasma membrane through protein channels
known as aquaporins.
• Water moves into and out of the cell along its
concentration gradient in a process known as
osmosis.
• Osmosis occurs across a selectively-permeable
membrane and is dependent upon the
concentration of molecules dissolved in the
water.
Osmosis
More
water
molecules
water
Fewer water
molecules
Osmosis
Inside of the
cell
Watery environment
Plasma
membrane
Osmosis
More water
molecules
water
Fewer
water
molecules
Polar
molecule
Watery environment
Inside of the cell
Osmosis
Fewer
water
molecules
water
More water
molecules
Osmosis
• The amount of all dissolved molecules, or
solutes, in a solution determines the osmotic
concentration of the solution.
• If the number of solute molecules in two
solutions is equal (the osmotic concentration is
equal), the solution is isotonic.
• If the two solutions have unequal osmotic
concentrations, the solution with the higher
concentrations of solutes is hypertonic; the
solution with the lower concentration of solutes
is hypotonic.
Isotonic
Equal number of solute
molecules
No net movement of water
molecules
1
2
2
1
Hypertonic and Hypotonic
More solutes;
fewer water
molecules
Fewer solutes;
more water
molecules
water
Hypertonic
Hypotonic
1
2
1
3
Hypertonic
More solutes;
fewer water
molecules
Hypotonic
Fewer solutes;
more water
molecules
Images : Copyright © The McGraw-Hill Companies, Inc.
Water always moves from hypotonic (fewer solutes) to
hypertonic (more solutes) solutions.
Hypotonic
Hypertonic
1
3
1
2
Images : Copyright © Pearson Education, Inc.
Water always moves from hypotonic (more water) to
hypertonic (less water) solutions
Hypotonic
3
Hypertonic
4
7
1
6
8
2
12
1
13
15
16
18
19
7
6
21
20
5
14
11
22
4
2
9
10
3
5
17
Images : Copyright © Pearson Education, Inc.
Osmotic Pressure
• Movement of water into cells via osmosis
creates force, called osmotic pressure.
• Most animal cells cannot withstand osmotic
pressure, and burst when placed into a
hypotonic solution.
• Plant cells, however, have reinforced cellulose
cell walls which enable cells to fill with water
without bursting.
Hypertonic
Isotonic
Hypotonic
Images : Copyright © The McGraw-Hill Companies, Inc.
Active Transport
• So how do you move molecules against a
concentration gradient?
• Active transport uses energy to move
molecules against their concentration gradient
(from low to high concentration)
• A cell’s energy molecule ATP supplies the
energy for active transport
• Membrane proteins regulate active transport
ATP
• When the bond joining a phosphate group to the
rest of the ATP molecule is broken by hydrolysis,
the reaction supplies energy for cellular work
• The three phosphate groups of ATP are negatively
charged (PO4-2) and are crowded together; their
mutual repulsion makes the triphosphate chain of
ATP the chemical equivalent of a compressed
spring
Adenosine
Triphosphate (ATP)
Phosphate
group
Adenine
Ribose
Hydrolysis
+
Adenosine
Diphosphate (ADP)
ATP Synthesis
• It takes energy to produce an ATP molecule; this
energy is stored in the bonds linking the
phosphate groups together
ATP Hydrolysis
• The release of a phosphate group releases
energy
ATP Recycling
• ATP is reusable
– the synthesis of ATP stores energy
– the hydrolysis of ATP releases energy
• We use chemical energy in foods (ex. Glucose) to
provide the energy to make ATP
• A working muscle cell can consume and regenerate
10 million ATP molecules every second!!!
Active Transport
• How does active transport work?
• First, a molecule on the outside of the plasma
membrane (either outside of a cell or organelle)
attaches to a specific binding site on the
transport protein
• ATP the transfers one of its phosphate groups
(PO4) to the transport protein
Active Transport
Transport
protein
Protein
changes shape
Solute
1 Solute binding
2 Phosphorylation
3 Transport
Phosphate
detaches
4 Protein reversion
Active Transport
• Remember ATP stands for: adenosine
triphosphate
• The transfer of the phosphate group causes the
transport protein to change shape in such a
way that the molecule is released on the other
side of the membrane
• Then, the phosphate group detaches, and the
transporter protein returns to its normal shape
Active Transport
• Active transport allows cells (or organelles)
to maintain concentrations of small
molecules that are different from their
surroundings
• Hugely important! The generation of nerve cells
(and hence muscle movement) depend upon
these concentration differences
Endocytosis
• Another way to transport molecules against
their concentration gradient is via endocytosis
• Endocytosis occurs when a depression in the
plasma membrane pinches in and forms a
vesicle enclosing material that had been outside
of the cell
• Pinocytosis is a form of endocytosis whereby
the cell ‘gulps’ fluid into tiny vessicles
• Phagocytosis is a form of endocytosis whereby
the cell engulfs a food particle within a
membrane-bound food vacuole
Phagocytosis
Pinocytosis
Exocytosis
• Exocytosis is the process by which the cell
exports bulky material, such as proteins or
polysaccharides outside of the cell
• A macromolecule moving away from the Golgi
apparatus where it budded off, for example,
moves toward the plasma membrane where it
fuses with the membrane and spills outside of
the cell
Energy and the Cell
• A cell is a miniature chemical factory in which
thousands of reactions occur within a
microscopic space
– Some of these reactions break down sugars
and other ‘fuels’ to release energy
– The cell uses this energy to manufacture
cellular parts and products, and to move
cellular components or the entire cell
– To understand how a cell works, you must
have a basic knowledge of ENERGY
Energy
• Energy is the capacity to do work
• All organisms require energy to stay alive
• There are two types of energy: kinetic and
potential
• Kinetic energy is the energy of motion; heat is a
form of kinetic energy associated with the
random movement of atoms or molecules
• Potential energy is stored energy; a bicyclist at
the top of a hill or water behind a dam
Energy
• Potential energy can be converted into kinetic
energy
Energy
• Chemical energy refers to potential energy
stored in the bonds of chemical compounds,
such as sugar and lipids
• Chemical energy is the most important type of
energy for living organisms – it is the energy
available to do the work of the cell
• Life depends on the fact that energy can be
converted from one form to another
Cellular Work
• Cells carry out thousands of chemical reactions;
the total of an organism’s chemical reactions is
called its metabolism
• Cells obtain the energy necessary to perform
this work from sugar and other food molecules
Potential energy!
The Return of Enzymes…
• Carbohydrates and lipids are rich in potential
energy
• However, these compounds do not rapidly break
down into simpler molecules thereby releasing
their energy to be used by the cell to regenerate
ATP
• Enzymes, which speed up reactions, accelerate
the release of this energy quickly enough for the
cell to be able to effectively.
Energy of Activation
• Enzymes speed up reactions by lowering the EA
Reaction
without
enzyme
EA without
enzyme
EA with
enzyme
Reactants
Net
change
in energy
(the same)
Reaction with
enzyme
Products
Progress of the reaction
Enzymes
• A specific enzyme catalyzes each cellular
reaction
• Enzymes are catalysts because they increase
the rate of a chemical reaction, and are not
destroyed (consumed) by the reaction in the
process
– Very important; many of our chemical
processes occur rapidly in our cells
Enzymes
• Enzymes are proteins with unique 3-dimensional
shapes
• The shape of the enzyme determines which
chemical reaction the enzyme catalyzes
• The reactant that an enzyme acts upon is called
the substrate
• The substrate fits into a region on the enzyme
called an active site
1 Enzyme available
with empty active
site
Active site
Glucose
Fructose
4 Products are
released
Substrate
(sucrose)
2 Substrate binds
to enzyme with
induced fit
Enzyme
(sucrase)
Sucrase
catalyzes the
hydrolysis of
sucrose to
glucose and
fructose
Hydrolysis
3 Substrate is
converted to
products