2.4 MEMBRANES
IB BIOLOGY (CORE)
THE PLASMA (CELL) MEMBRANE
2.4 ASSESSMENT STATEMENTS
2.4.1 Draw and label a diagram to show the structure of
membranes.
2.4.2 Explain how the hydrophobic and hydrophilic properties of
phospholipids help to maintain the structure of cell
membranes.
2.4.3 List the functions of membrane proteins.
2.4.4 Define diffusion and osmosis.
2.4.5 Explain passive transport across membranes by simple diffusion
and facilitated diffusion.
2.4.6 Explain the role of protein pumps and ATP in active transport
across membranes.
2.4.7 Explain how vesicles are used to transport materials within a
cell (between the rER, Golgi Apparatus, and Plasma
Membrane.
2.4.8 Describe how the fluidity of the membrane allows it to change
shape, break and re-form during endocytosis and exocytosis.
FUNCTIONS OF THE PLASMA (CELL)
MEMBRANE
1. To provide the cell (prokaryotes and eukaryotes)
with a selectively permeable barrier/boundary.
2. To separate the interior of the cell from its external
environment.
3. To provide a ‘layer’ of protection.
4. To regulate what materials/molecules can enter
and exit the cell.
5. To provide points of attachment to other cells.
6. To facilitate cell to cell communication.
7. To allow for some flexibility in the structure of the
cell.
ANY CELL NEEDS MATERIALS IN &
PRODUCTS OR WASTE OUT
IN
food
- sugars
- proteins
- fats
salts
O2
H2O
OUT
waste
- ammonia
- salts
- CO2
- H2O
products
- proteins
Certain substances have a easier time moving through the cell
membrane than others.
PLASMA (CELL) MEMBRANE
STRUCTURE
The Cell Membrane is roughly
7-9 nm thick.
KEY CHARACTERISTICS OF CELL
MEMBRANES
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Cell membranes surround all living cells (both
prokaryotes and eukaryotes)
The Fluid Mosaic Model best describes the structure of
cell membranes.
Cell membranes are made up of a phospholipid
bilayer (2 layers of phospholipid molecules).
The properties of phospholipids are such that the cell
membrane is both flexible and fluid (the plasma
membrane can reorient itself after a disturbance).
The properties of phospholipids allow for selective
permeability.
Embedded in the phospholipid bilayer are other lipids
(cholesterol), protein molecules, protein channels.
LABEL THE DIAGRAM
A TYPICAL PHOSPHOLIPID
A phospholipid has a polar, phosphate head and two
non-polar, tails
Polar = Hydrophilic (water-attracting).
“attracted to water”
Non-Polar = Hydrophobic (water-avoiding). Phosphate head
Inside cell
2 Fatty Acid Tails
outside cell
“repelled by water”
PLASMA MEMBRANE ANIMATION(S)
MEMBRANE PROTEIN FUNCTIONS
(DIFFERENT PROTEINS WITH DIFFERENT ROLES)
1. Hormone-Binding Sites: Hormones are chemical signals that are
produced in one area of the body but have an affect elsewhere.
Hormones binding to protein receptors on the outside of cell
membranes allow for a signal to be transmitted to the inside of
the cell.
2. Enzymes: Enzymes can catalyze reactions on the inside or outside
of the cell, depending on where the enzyme is located.
3. Cell-to-Cell Communication & Adhesion: Glycoproteins (sugarprotein) in the membrane allow cells to communicate with each
other and to stick together to form tissues.
4. Channels for Passive Transport: Passages through protein
molecules that allow very specific substances to pass through.
5. Pumps for Active Transport: Pumps use energy from ATP to move
specific substances across the cell membrane.
MEMBRANE PROTEINS
MEMBRANE PROTEINS
HOW DO SUBSTANCES MOVE IN AND OUT OF THE
CELL? – AN OUTLINE
I. Passive Transport
A. Simple Diffusion
B. Facilitated Diffusion
C. Osmosis
II. Active Transport
A. Pump Proteins
B. Bulk Transport and Vesicle Formation
1. Endocytosis
a. Phagocytosis
b. Pinocytosis
2. Exocytosis
PASSIVE TRANSPORT
• Involves the exchange of solid, liquid and gas particles
between the cell and its environment.
• All that needs to exist is a concentration gradient
(regions of varying concentration of particles).
The cell does not need to use energy (ATP) for this
process to occur.
Cells cannot control the direction of particle movement
during this process. The process is random. Particles
move as a result of kinetic energy.
TYPES OF PASSIVE TRANSPORT AND
DEFINITIONS
Types of Passive Transport & Definitions:
A. Simple Diffusion – The passive/random movement of
particles from a region of higher concentration to a
region of lower concentration.
B. Facilitated Diffusion – Similar to simple diffusion but
those substances unable to pass directly through the
phospholipid bilayer, must cross via protein channels.
Protein channels are highly selective and cells can
control what enters and exits by inserting specific
protein channels into the cell membrane.
ANIMATIONS
C. OSMOSIS
(A SPECIAL CASE OF PASSIVE TRANSPORT)
Osmosis is the passive movement of water molecules
from a region of lower solute concentration to a region
of higher solute concentration.
• Osmosis is different than diffusion because water is a
solvent (substance doing the dissolving) not a solute
(substance being dissolved).
IT IS THE CONCENTRATION OF THE SOLUTE INSIDE THE
CELL THAT DETERMINES WHETHER and HOW MUCH WATER
WILL MOVE INTO OR OUT OF THE CELL. WHY??
OSMOSIS ANIMATIONS
PLASMOLYSIS IN PLANT CELLS
OSMOREGULATION
SUMMARY ANIMATIONS
HOW DO SUBSTANCES MOVE IN AND OUT OF THE
CELL? – AN OUTLINE
I. Passive Transport
A. Simple Diffusion
B. Facilitated Diffusion
C. Osmosis
II. Active Transport
A. Pump Proteins
B. Bulk Transport & the Use of Vesicles
1. Endocytosis
a. Phagocytosis
b. Pinocytosis
2. Exocytosis
ACTIVE TRANSPORT
ACTIVE TRANSPORT IS THE MOVEMENT OF MOLECULES
ACROSS MEMBRANES – THE CELL MUST USE (EXPEND)
ENERGY (ATP) TO ACCOMPLISH THIS.
BECAUSE ATP IS USED, MOLECULES CAN BE
PUMPED/MOVED FROM AREAS OF LOWER
CONCENTRATION TO AREAS OF HIGHER
CONCENTRATION.
Let’s Look At Some Examples of Active Transport. . .
A. PROTEIN PUMPS
BULK TRANSPORT
(THE TRANSPORT OF ‘LARGE’ PARTICLES)
B. Bulk Transport & the Use of Vesicles
1. Endocytosis (Into the Cell)
a. Phagocytosis (“Cellular Eating”)
b. Pinocytosis (“Cellular Drinking”)
2. Exocytosis (Out of the Cell)
BULK TRANSPORT ANIMATIONS
C. VESICLE FORMATION
SUMMARY ACTIVE TRANSPORT
ANIMATIONS
EXTRACELLULAR COMPONENTS
Cells will sometimes, when necessary,
produce/synthesize components and place them
outside the cell membrane. These are called
extracellular components.
Examples:
1. Plant Cell Wall
2. Glycoproteins
1. PLANT CELL WALL
• Plants construct cell walls by synthesizing the
extracellular components (cellulose, etc.) inside the cell
and adding them to the inner surface of the cell wall.
Other substances are synthesized and added to
strengthen and connect the cellulose fibers.
Function of the Plant Cell Wall
1. Maintain the cell’s shape.
2. Add strength to prevent the cell from bursting due to
high pressures inside the cell.
3. High pressure inside plant cells prevents water uptake
by osmosis.
4. High pressure inside plant cells (turgor pressure) makes
cells rigid and helps to support the plant.
2. GLYCOPROTEINS
(PROTEINS WITH A CARBOHYDRATE CHAIN ATTACHED)
• Many animal cells, for example, synthesize glycoproteins
inside the cell and secrete those glycoproteins to the
outside of the cell membrane.
• Glycoproteins can form an extracellular matrix (a gellike glue that can hold cells/tissues in place).
Extracellular Matrix Functions
1. Supporting single layers of thin cells, which might
otherwise easily tear or perforate (capillaries, alveoli,
etc.).
2. Cell to cell adhesion, for example, a basement
membrane helps capillary wall cells to adhere to
alveolus wall cells.
PASSIVE VS. ACTIVE TRANSPORT