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Cell Structure and
Function
Ms. Taylor
The Plasma Membrane Has Many and Various
Functions
5-2
The plasma membrane is a phospholipid bilayer with
embedded proteins
• Plasma membrane is a fluid phospholipid bilayer
– Polar head of a phospholipid is hydrophilic
– Nonpolar tails of a phospholipid are hydrophobic
• Fluid-mosaic model
– Mosaic - proteins embedded in the membrane have a
pattern
– Fluid - pattern varies according to the particular
membrane and also within the same membrane at
different times
5-3
The plasma membrane is a phospholipid bilayer with
embedded proteins. The proteins have various functions.
plasma
membrane
carbohydrate
chain
glycoprotein
glycolipid
Outside of cell
hydrophobic
hydrophilic
tails
heads
phospholipid
bilayer
peripheralprotein
filaments of cytoskeleton
cholesterol
Inside of cell
5-4
Proteins in the plasma membrane have
numerous functions
• Plasma membrane of different cells and
membranes of various organelles have specific
proteins
– Channel proteins
– Carrier proteins
– Cell recognition proteins
– Receptor proteins
– Enzymatic proteins
– Junction proteins
5-5
Channel proteins
• In some cases the channnel proteins simply
act as a passive pore. Molecules will randomly
move through the opening in a process called
diffusion. This requires no energy, molecules
move from an area of high concentration to an
area of low concentration.
Carrier proteins
• Carrier proteins assist molecules across the cell membrane. This
process is called carrier mediated transport. The makeup of the
amino acid chains in the protein determines the size and shape of
the carrier protein. This, in turn, determines what molecule can be
received by the carrier protein to cross the cell membrane.
Remember that each molecule has a specific carrier protein that
assists the molecule cross the cell membrane.
• That molecule can only cross the cell membrane if the particular
carrier protein is available. There are two types of carrier proteins.
The first type does not require energy to move the substance across
the cell membrane - this is called facilitated diffusion and is
described above. The second type uses energy and the process is
called active transport.
Cell recognition proteins
• Recognition proteins are a protein in the
plasma membrane that provides a fingerprint
on the outside-facing surface of the cell,
making it recognizable to other cells.
Receptor proteins
• Usually found embedded within the plasma
membrane surface of a cell that receives
chemical signals from outside the cell.
Enzymatic proteins
• Catalyze (speed up) the biochemical reactions
that occur in cells.
Junction proteins
Homework
• Write a two page essay on a disease that can be caused by a
malfunctioning membrane protein.
• Must be two FULL pages, typed (1.5 spaced, 12 pt Times New
Roman font).
• Include: What is the problem protein, what is the disease,
how is it diagnosed (at what age usually?), what are the long
term effects? What has been done to treat this disease?
• MUST have proper grammar, proper capitalization and MUST
have sources cited. No citations=no credit.
• Due in class: Monday, November 2, 2015. No late papers will
be accepted for any reason.
Suggested Diseases
• Cystic Fibrosis
• PKU
• QT Syndrome
The Plasma Membrane Regulates the Passage of
Molecules
Into and Out of Cells
5-14
Diffusion across a membrane requires
no energy
• Diffusion
– Molecule moves from a high concentration to a
low concentration until it is distributed equally
– Concentration gradient
• Passive form of transport
– Requires no energy to be added
• Very few molecules can simply diffuse through
the hydrophobic portion of the plasma
membrane
5-15
Some molecules can move freely by simply
diffusing across a membrane.
red dye
H2O
membrane
Time
5-16
Diffusion across a membrane requires
no energy
• Facilitated diffusion
– Requires a transporter but no energy
– Carrier protein or channel protein
– Transporter is specific
carrier
protein
Outside
Inside
During this type of facilitated diffusion, a carrier protein combines with and assists
solute movement across the membrane.
5-17
Diffusion across a membrane requires
no energy
• Osmosis
– Diffusion of water across the plasma membrane due
to concentration differences
– Which way the water moves is dependent on the
solute versus water concentration on both sides of the
membrane
• Isotonic solution – cell neither gains or loses water
• Hypotonic solution – cell gains water
– Hemolysis
– Turgor pressure
• Hypertonic solution – cell loses water
– Plasmolysis
5-18
When cells are in an isotonic solution, they remain
normal. When cells are in a hypotonic solution, they gain water,
and when they are in a hypertonic solution, they lose water to the
environment.
Solution
Isotonic
Hypotonic
Hypertonic
cell swells, bursts
cell shrivels
Animal
cells
normal cell
Plant
cells
normal cell
Normal
turgid cell
cytoplasm shrinks
from cell wall
5-19
Active transport across a membrane requires a
transporter and energy
• Active transport
– Molecules or ions move across the plasma
membrane, accumulating on one side of the cell
– Movement of molecules against their
concentration gradients requires both a carrier
protein and ATP
• Sodium-potassium pump undergoes a change
in shape when it combines with ATP allowing
it to combine alternately with sodium ions and
potassium ions
5-20
During active transport, a protein carrier assists solute movement toward a
higher concentration, and energy is required.
carrier
protein
ATP
Outside
Inside
P
P
5-21
Bulk transport involves
vesicle formation
• Bulk transport
– Occurs when fluid or particles are brought
• Into a cell by vacuole formation - endocytosis
• Out of a cell by evagination – exocytosis
– Phagocytosis occurs when the material taken in is
large
– Pinocytosis occurs when vesicles form around a liquid
or around very small particles
– Receptor-mediated endocytosis - receptors for
particular substances are found in the plasma
membrane
5-22
Bulk transport into the cell is by endocytosis
Endocytosis
5-23