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Cell Membrane
Chapter 5
Cell membrane
• Also known as plasma membrane.
• Maintains homeostasis
Fluid Mosaic model
• “mosaic” – surface made of small pieces
• Has diverse protein molecules embedded in a framework
of phospholipids.
• “fluid” – most molecules can drift about in the membrane.
• The double bonds in the unsaturated fatty acid tails of
many phospholipids produce kinks that prevent them
from packing tightly together. (fluid as salad dressing)
• The steroid cholesterol wedged in the bilayer in animal
cells helps stabilize the membrane in warm temps., and
keeps the membrane fluid at lower temps.
Proteins within the cell membrane
• Different cells have different proteins in the
cell membrane.
– More than 50 proteins can be found in red
blood cell membranes alone.
• Integrins - proteins give the cell a stronger
framework
Carbohydrate of
glycoprotein
Glycoprotein
Glycolipid
Integrin
Phospholipid
Microfilaments
of cytoskeleton
Cholesterol
Proteins within the cell membrane
• Glyco-proteins – involved in cell to cell
recognition.
• Carbohydrates outside the surface of the
cell membrane function as “id tags”.
– Cells in an embryo can sort themselves into
tissue & organs
– Immune system to recognize and reject
foreign cells (such as bacteria)
– Form junctions between cells.
Carbohydrate of
glycoprotein
Glycoprotein
Glycolipid
Integrin
Phospholipid
Microfilaments
of cytoskeleton
Cholesterol
Proteins within the cell membrane
• Many membrane proteins are enzymes
which work together to carry out
sequential steps.
Enzymes
Messenger molecule
Receptor
Activated
molecule
Proteins within the cell membrane
• Other proteins work as receptors for
chemical messengers from other cells.
– Has a shape that fits a specific messenger,
such as a hormone.
Enzymes
Messenger molecule
Receptor
Activated
molecule
Proteins within the cell membrane
• Signal transduction – a message-transfer
process activated by the messenger binding to
the receptor triggering a chain of reactions
relaying the message to molecules within the
cell to perform a specific function.
Enzymes
Messenger molecule
Receptor
Activated
molecule
Proteins within the cell membrane
• Transport proteins enable selective
permeability allowing some substances to
cross the membrane more easily than
others.
– The hydrophobic interior
(phospholipid tails) of the cell
membrane makes this possible.
Solute
molecule
Transport
protein
Proteins within the cell membrane
– Nonpolar, hydrophobic molecules can easily
pass through, while polar molecules and ions
are not soluble in lipids.
– Thus essential molecules like glucose and
ions require transport proteins to enter or
leave the cell.
Membranes form spontaneously
• Phospholipids were probably the first
organic molecules that formed in early
Earth.
• Could spontaneously self-assemble into
simple membranes
Membranes form spontaneously
– This can be demonstrated when a mixture of
phospholipids and water are shaken, the
phospholipids organize into bilayers
surrounding water-filled bubbles
Water-filled
bubble made of
phospholipids
• This formation of membrane enclosed
collections of molecules was a critical step
in the evolution of the first cell.
Types of Cellular Transport
• PASSIVE
• ACTIVE
• Does not require
energy.
• Requires energy from
ATP.
• Goes with the
concentration gradient
(high too low).
• Simple & Facilitated
Diffusion
• Goes against the
concentration gradient
(low too high).
• Active Transport,
Endocytosis, Exocytosis.
Passive Transport
• Passive transport – cell performs no work
when molecules move across the
membrane.
• Example: In our lungs, oxygen enters red
blood cells, and carbon dioxide passes out
by passive transport.
– Because they are small nonpolar molecules!
– Polar molecules can also move by passive
transport if they are moving down their
concentration gradient, and have transport
proteins to provide a pathway.
Passive Transport
• Diffusion – the tendency for particles of
any kind to spread out evenly in an
available space, moving from highly
concentrated areas, to low concentrated
areas.
Passive Transport
• Requires NO work, it results from random
thermal motion (vibration & movement
from heat) of atoms and molecules.
• Although movement
is random, there is a
net movement of
particles.
Passive Transport
• Concentration gradient – movement of
particles from high to low concentration
until a equilibrium is reached.
• There is still movement of particles, but no
net change in concentration.
Passive Transport
• Osmosis – is the diffusion of water across
a membrane.
• The net movement of water down its own
concentration gradient!
Water
molecule
Solute molecule with
cluster of water molecules
Net flow of water
Passive Transport
• If a membrane is permeable to water but not a
solute (ex: glucose) then the water will cross the
membrane until the solute concentration is equal
on both sides!
Equal
Higher
concentration
of solute
Lower
concentration
of solute
concentration
of solute
H2O
Solute
molecule
membrane
• The direction of osmosis is determined by
the difference in total solute concentration.
Passive Transport
• Tonicity – the ability of a solution to cause
a cell to gain or lose water.
• Depends on the solution’s concentration of
solutes that cannot cross the membrane,
relative to the concentrations of solutes
within the cell.
Passive Transport
• Solutions of various tonicities can have three
different effects on plant & animal cells.
• Isotonic solutions:
– (iso – the same) (tonos – tension)
• The solute concentration in the external
environment is equal to that of the cell.
– The cell’s volume remains constant. It gains
water at the same rate that it loses water.
• Plasma that transports red blood cells.
• Intravenous fluid administered in hospitals.
• Marine animals are isotonic to seawater.
Passive Transport
• Hypotonic solution:
– (hypo – below)
• The solute concentration in the
external environment is below
that of the cell.
• The cell gains water, swells,
and may burst (lyse).
– The cell’s volume increases. It gains water faster than
it loses water.
Passive Transport
• Hypertonic solution:
– (hyper – above)
• The solute concentration in
the external environment is
above that of the cell.
• The cell loses water, shrivels,
and can die from water loss.
– The cell’s volume decreases. It loses water faster that
it gains water.
Isotonic solution
Hypotonic solution
Hypertonic solution
(A) Normal
(B) Lysed
(C) Shriveled
Animal
cell
Plasma
membrane
Plant
cell
(D) Flaccid
(E) Turgid
(F) Shriveled
(plasmolyzed)
Passive Transport
• Osmoregulation – the control of water
balance.
• Prevents excessive uptake or excessive
loss of water.
– Freshwater fish live in a hypotonic
environment, has kidneys and gills that work
to expel water.
Passive Transport
• Water balance differs slightly for plant cells
vs. animal cells.
• Animal cells prefer isotonic environments.
• Plant cells prefer hypotonic environments.
– The cell wall of plants exerts pressure on the
cell, preventing it from taking in too much
water and bursting.
Isotonic solution
Hypotonic solution
Hypertonic solution
(A) Normal
(B) Lysed
(C) Shriveled
Animal
cell
Plasma
membrane
Plant
cell
(D) Flaccid
(E) Turgid
(F) Shriveled
(plasmolyzed)
Passive Transport
• Facilitated diffusion – when a transport
protein makes it possible for a polar or
large molecule to move down its
concentration gradient, and cross the cell
membrane.
• Does NOT
require energy.
Transport
protein
Solute
molecule
Passive Transport
• Another type of protein binds its
passengers, and changes shape, and
releases its passenger on the other side.
• A transport protein is always specific for
the substance it helps move across the
membrane.
• Substances that use facilitated diffusion:
– Sugars, amino acids, ions, and water.
Active Transport
• A cell expends energy to move a solute
against its concentration gradient – that is
toward the side were there is more solute.
• ATP provides the needed energy.
Active Transport
1. Solute on the inside of the cell binds to
an active site on a transport protein.
2. ATP then transfers one of its phosphate
groups to the transport protein.
Active Transport
3. Causing the protein to change shape, so
that the solute is released on the other
side of the membrane.
4. Then the phosphate group detaches, and
the transport protein returns to its original
shape.
Active Transport
• Active transport allows a cell to maintain
concentrations of small molecules that are
different from concentrations external to
the cell.
Active Transport
• Sodium-Potassium pump: a transport
protein that helps generate nerve signals.
• Creates a higher concentration of K+ and a
lower concentration of Na+ inside the cell.
• The transport protein constantly shuttles
the K+ into the cell, and the Na+ out of the
cell.
Active Transport
•
Exocytosis – (exo – outside) export bulky
materials such as proteins or polysaccharides.
1. A transport vesicle filled with macromolecules buds
from the golgi body.
2. Moves to the cell membrane.
3. Vesicle fuses with the cell membrane.
4. Vesicle contents spill out of the cell.
5. Vesicle membrane becomes part of the cell
membrane.
Active Transport
•
Endocytosis – (endo – inside) a cell
takes in substances.
1. A depression forms in the cell membrane.
2. Material outside the cell sits within this
depression.
3. The depression pinches in and forms a
vesicle (containing materials).
•
3 types of endocytosis.
Active Transport
• Phagocytosis – “cellular eating” A cell
engulfs a particle by wrapping extensions
called pseudopodia around it and
packaging it within a vacuole.
•Vacuole then fuses
with a lysosome,
which digests the
contents.
Active Transport
• Pinocytosis – “cellular drinking” A cell
gulps droplets of fluid into tiny vesicles.
Active Transport
•
Receptor – mediated endocytosis –
1. Receptor proteins for specific molecules are
embedded in cell membrane.
2. These receptors have picked up particular
molecules.
3. Then the cell membrane pinches off to form vesicle
containing receptors and their attached molecules.
• Used to take in cholesterol
(LDL) from the blood.
Active Transport
• Hypercholesterolemia – inherited disease
• The LDL receptor proteins are defective
and cholesterol accumulates to high levels
in the blood.
• Leading to atherosclerosis.
Energy
• Cells use oxygen in cellular respiration, which
harvests chemical energy from food molecules.
• The waste products are CO2 and H2O
• Cells are able to convert about 40% of the
chemical energy stored in foods to energy for
cellular work.
• The other 60% generates heat.
• This explains why exercise makes you hot!
• Sweating helps release this excess heat.
Energy
• Energy - the capacity to do work.
• Kinetic energy - energy of motion.
• Potential energy – stored energy.
• Chemical energy – potential energy
available for release in a chemical reaction
Energy Transformations
• Thermodynamics – the study of energy
transformations.
• 1st law of thermodynamics – (law of energy
conservation) Energy can be transferred,
and transformed, but it cannot be created
or destroyed.
Energy Transformations
• 2nd law of thermodynamics – during every
energy transfer or transformation, some
energy becomes unstable & unusable
(most often it converts to heat).
– Heat is a disordered form of energy, and its
release makes the universe more random.
• Entropy – a measure of this disorder.
– Energy conversions increases the entropy
(disorder) of the universe.
Chemical Reactions
• Exergonic reaction – “energy outward” a
chemical reaction that releases energy.
Example: Wood burning – as the cellulose
(a polysaccharide) burns, each of the
glucose molecules rich in potential energy
release this energy in the form of heat and
light.
Chemical Reactions
• Cellular respiration – an exergonic
chemical process that uses oxygen to
convert the chemical energy stored in food
molecules to a form of chemical energy
(ATP) that the cell can use to perform
work.
Chemical Reactions
• Endergonic reactions – “energy inward”
yield products that are rich in potential
energy.
• Example: Photosynthesis, starts with
energy-poor reactants (CO2 and H2O) and,
using energy absorbed from sunlight,
produces energy-rich sugar molecules.
Chemical Reactions
• Metabolism – (metabole = change) the
total of an organism’s chemical reactions
(exergonic & endergonic).
• Metabolic pathway – a series of chemical
reactions that either builds a complex
molecule or breaks one down.
Chemical Reactions
• Energy coupling – the use of energy
released from exergonic reactions to drive
endergonic reactions.
– All cells have this ability.
– ATP molecules are key to this ability.
ATP drives cellular work
• ATP (adenosine triphosphate) Energy
storing molecule that powers nearly all
forms of cellular work.
– Adenosine = a nitrogenous base adenine +
ribose sugar.
– Triphosphate = a chain of 3 phosphate
groups.
ATP drives cellular work
• The 3 phosphates are negatively charged
causing a mutual repulsion.
• This allows the triphosphate chain to be
easily broken by hydrolysis.
• When a bond between the 3 phosphates is
broken, one phosphate group leaves,
energy is released, and ADP (adenosine
diphosphate) remains.
ATP drives cellular work
• The hydrolysis of ATP is an exergonic
reaction – it releases energy.
• The removed phosphate is transferred to
some other molecule, a process called
phosphorylation.
• Phosphorylation is an endergonic reaction.
• The two together form energy coupling.
3 Types of cellular work
• Chemical – the
phosphorylation of reactants
provides energy to drive
endergonic synthesis of
products.
3 Types of cellular work
• Mechanical – the transfer of
phosphate groups to special
motor proteins in muscle cells
causes the proteins to change
shape and pull on actin
filaments, in turn causing the
cells to contract.
3 Types of cellular work
• Transport – ATP drives the
active transport of solutes
across a membrane against
their concentration gradient by
phosphorylating membrane
proteins.
ATP drives cellular work
• ATP drives all 3 types.
• Work can be maintained because ATP is a
renewable resource that cells regenerate.
– Working muscle cells consume and
regenerate 10 million ATP molecules each
second!