CP BIO: Ch. 7 The Cell Membrane
and transport across the membrane
Review:
Protein structure
Protein function needs
3-dimensional shape
• Peptide bonds hold
amino acids together
 polypeptide chain
• Hydrogen bonds hold
parts of chain
together  shape
Protein Denaturation
Denature – lose shape (and function)
Chemical or physical changes
- break bonds that hold the 3-D shape
Denaturation can be caused by changes in ion
concentration, pH, temperature, and others
Energy in Chemical Reactions
• Life processes are chemical
• Need energy added
(ACTIVATION ENERGY) to
start reactions
• Cells cannot use or make
heat
• ENZYMES speed up reactions
– lower activation energy
How enzymes work
Enzymes are “biologic catalysts”
catalyst - speeds reaction but is not changed or used up
enzymes are specific – act on only one kind of molecule
Names – for chemical process or substrate;
- often end in -ase
Enzyme must fit its target molecule
• Enzymes function by binding to another molecule
• Shapes must match, or bend to fit together
Enzyme denatured – shapes do not fit
Enzyme binds to Substrate
• Substrate – molecule an enzyme acts upon
• Active site – region on enzyme molecule that binds to
substrate molecule - must fit together!!
Active site
substrate
Enzymes catalyze all reactions
A catabolic reaction
Enzymes catalyze anabolic, too.
ALL cell reactions need help of enzymes
2 models of enzyme fit
1. Lock and key
- perfect match,
no shape change
2. Induced Fit
- shape of enzyme
changes when
substrate attaches
Factors Affecting Enzyme Action
3-dimensional protein molecule - shape is critical
Temperature - heat makes molecules move faster
 more contact between enzyme and substrate
 faster reaction rate
BUT, high temps denature proteins!
Enzymes and pH
Work best at a specific pH
- changes in pH break bonds holding molecule shape
Enzyme Inhibition
Sometimes other molecules stop enzyme action
• Compete for/block the active site
• Change active site by bonding somewhere else
on the enzyme
The Plasma membrane
Boundary between the cell and its environment
FLUID MOSAIC MODEL
- Describes structure of cell membranes
• “mosaic” – sea of lipids with scattered proteins
• “fluid” – molecules float and move around within
the layer
Fluid Mosaic Model
Phospholipid
Phospholipid = lipid molecule with phosphate on one end
• glycerol + two fatty acids + phosphate group
• phosphate group = polar end
(hydrophilic “head”)
• Fatty acids = nonpolar end
(hydrophobic “tails”)
Phospholipids form a double layer
Nonpolar tails are on the inside (away from water)
Polar heads are on the outside ( touch water)
Features of the Cell Membrane
• Semipermeable = some substances can pass through
- some cannot
- depends on: molecule size, charge, polar or nonpolar,
concentration, needs of the cell
• Cholesterol – molecules scattered among the phospholipids
- in animal cells
- keep membrane flexible and stable
• Carbohydrates – glucose chains on outside of cell
- Identification “tags”
Membrane Proteins
have many functions
Transport
Enzyme
Allows a specific
molecule to
pass through
the membrane
Catalyzes a
reaction inside
the cell
Receptor
Site for
messenger
molecule to
attach
Membrane proteins (2)
Junctions
Identification
“SELF” – cell belongs Cells join to
form tissues,
in this organism,
communicate
immunity
Structure
Keeps internal
parts organized
How do membranes keep
homeostasis?
Cell membranes are selectively permeable
• The lipid layer blocks most substances
• Some molecules can cross the membrane
– By passive or active transport
• Some are too big to cross at all
PASSIVE TRANSPORT
USES NO CELL ENERGY
Molecules move randomly – spread out until
evenly distributed
• From an area of higher concentration to an area of
lower concentration
Diffusion across membrane
High
Concentration
Equal
Concentrations
Low
Concentration
Concentration gradient: two adjacent areas
with different concentrations of a solute
DIFFUSION
Diffusion: movement of particles from an
area of high concentration to an area of
lower concentration
• Particles move in all directions (random)
• NET movement is from high
concentration to low
• “Down the concentration gradient”
Diffusion
• Particles spread out until evenly distributed
 equilibrium (homogeneous)
• AT equilibrium, particles continue moving, but
in all directions equally
 NO further change in concentration
Cell Membranes
allow some particles to cross
Particles can diffuse across the lipid bilayer if they are:
• Small
• Nonpolar (lipid-soluble)
Examples: CO2 , O2, fatty acids can diffuse easily
FACILITATED DIFFUSION
Transport proteins “help” particles move across
the membrane – DOWN their gradient
• PASSIVE transport – no cell energy used
• Proteins are SPECIFIC
– each allows only a certain substance to pass
Transport Proteins
• Pores and Channels – open path through membrane
• Carrier proteins – take particle on one side of
membrane and release on the other
What crosses by Facilitated Diffusion?
Particles can cross by facilitated diffusion if they are:
• Small (monomers)
• POLAR (water soluble)
• or CHARGED (ions)
– Examples: H2O, glucose, Ca+2, Cl- , Na+, K+
OSMOSIS
Diffusion of water across a membrane
Important process in cell homeostasis
Water crosses the cell membrane easily
- Small enough to pass between lipid molecules
- Also pass through special proteins, aquaporins
Why osmosis matters
• Water crosses membrane easily, faster than
many solutes
• Will try to reach equilibrium
• If NET water moves into or out of cell 
changes homeostasis
• Unequal water on one side of cell membrane =
osmotic pressure
Osmotic pressure in cells
ISOTONIC
Equal concentrations of solutes inside and outside cell
– Equal concentrations of water
– Water goes in and out of cell at equal rates
Isotonic pressure in cells
• No NET movement of water
into or out of cell
• Normal water pressure in
animal cells
• Wilted – low water pressure
in plant cells
When solute concentration is different
on two sides of a membrane
lower solute concentration = hypotonic
Higher solute concentration = hypertonic
Solutes will move down their gradient IF THEY CAN
CROSS THE MEMBRANE
Water concentration is OPPOSITE of solutes
Low solutes  HIGH WATER concentration
High solutes  LOW WATER concentration
Water WILL diffuse down its gradient and crosses
the cell membrane easily
Goes TO whichever side has more solutes
Cells in hypertonic solutions
Solutes are higher outside cell, water is lower
Water leaves cell by osmosis
Cytoplasm shrinks - “plasmolysis”
- animal cells: shrivel
- plant cells: low pressure
- cytoplasm pulls away from cell wall
- but cell wall does not shrink
Cells in hypotonic solutions
Solutes are lower outside cell, water is higher
Water enters cell by osmosis
Cytoplasm swells
- animal cells: swell, may burst (“lyse”)
- plant cells: high osmotic pressure “turgor”
- won’t burst - cell wall
- “Turgid” – stiff and firm, upright stem
Osmosis in Animal Cells
Animal cells like ISOTONIC conditions best
Osmosis in Plant Cells
Plant cells like HYPOTONIC conditions best
Contractile Vacuoles
Fresh-water protists (like Paramecia or Amoeba)
must constantly remove water that comes into the
cell by osmosis
ACTIVE TRANSPORT – uses cell energy
Two kinds:
1. Molecule transport against the gradient
a. from low concentration to high
b. pushed across by membrane proteins (pumps)
c. uses energy ATP
d. small molecules
and ions
Why would cells use active transport?
1) To concentrate substances:
Examples: kidneys concentrate wastes in urine,
- intestine concentrates nutrients in blood
2) To maintain an ion concentration
- sodium-potassium pump – allows nerve impulses
2) Bulk Transport – uses energy
To bring larger particles into or out of cell
Endocytosis = brings material into cell
- fold cell membrane around it form a vacuole
a. Phagocytosis = “cell eating”
- large particles or whole cells
- examples: amoeba, white blood cells
b. Pinocytosis = “cell drinking”
- small folds of membrane take in liquids
Pinocytosis – “cell drinking”
Example: small intestine absorbs
some water this way
Exocytosis – send OUT of cell
Vacuole containing substance fuses with
cell membrane  opens to outside of cell
- for cell secretions
- Examples: hormones from endocrine glands
digestive juices from pancreas or intestine