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1 The Plasma Membrane

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UNIT 3 BIOLOGY
UNIT 3 TOPICS
• Cell membranes and organelles
• Protein synthesis
• Gene regulation
• Enzymes
• Photosynthesis
• Respiration
• Cellular Signals
• Infection, pathogens, and defence mechanisms
• Immunity
UNIT 4 TOPICS
• Mutations
• Changes in populations over time
• Changes in biodiversity over time
• Methods to determine relatedness between species
• Hominin evolution
• DNA manipulation and genetic technologies
RESOURCES AND TASKS TO COMPLETE
• PowerPoints available on Compass
• Learning tasks on Compass
• Edrolo videos
• Douchy’s Biology Podcast
• Past exam questions (one set per topic)
THE PLASMA MEMBRANE
KEY KNOWLEDGE BLAH BLAH
• The fluid mosaic model of the structure of the plasma membrane and
the movement of hydrophilic and hydrophobic substances across it
based on their size and polarity
KEY KNOWLEDGE
• Purpose and function of the plasma membrane
• Structure of the plasma membrane
• Phospholipids
• Membrane proteins
• Methods of crossing the plasma membrane
• Factors affecting transport across the membrane
• Surface-area to volume ratio
PURPOSE OF THE PLASMA MEMBRANE
• Protect the cell
• Keeps the internal contents together
• Keeps out foreign molecules
• Communication with other cells
• Contains markers to identify the cell
• Control the osmotic environment of the cell
• Let certain substances in and out
THE FLUID MOSAIC MODEL
• The fluid mosaic model describes the structure of the plasma
membrane
• The plasma membrane is composed of:
• Phospholipids
• Membrane proteins
• Sterols (cholesterol in animal cells, also other sterols in plants and fungi)
• Fluid because the phospholipids can move around, and the
membrane can break and reform
• Mosaic because proteins embedded within the phospholipids form
mosaic-like patterns
THE FLUID MOSAIC MODEL
PHOSPHOLIPIDS
• Composed of a hydrophilic, polar
head and a hydrophobic, nonpolar tail
• Hydrophilic head contains
phosphate
• Hydrophobic tail is made of fatty
acid chains
MEMBRANE PROTEINS
RECEPTOR PROTEINS
• Receive signal molecules
(ligands) from other cells
• These proteins are usually just
called receptors
• For example, some receptors
receive signals from other cells
in the form of hormones, which
triggers the cell to do
something
• Each receptor is specific for the
signal molecule it receives
RECOGNITION PROTEINS
• Called MHC proteins
• Act as markers to identify
the cell as belonging to the
individual
• This prevent the body’s
immune system from
attacking the cell
• More on that in AOS 2…
ADHESION PROTEINS
• In multicellular organisms, adhesion proteins link cells together
CHANNEL PROTEINS
• Allow for transport
across the cell
membrane
• Form narrow
passageways within the
cell membrane through
which molecules can
move
• Cross the plasma
membrane
CARRIER PROTEINS
• Also allow for transport
across the cell membrane
• Bind to a specific molecule
on one side of the plasma
membrane
• After binding, the protein
changes its shape and
releases the molecule on
the other side
METHODS OF CROSSING THE PLASMA
MEMBRANE
SIMPLE DIFFUSION
• Movement of a substance across the membrane from an area of high
concentration to low concentration
• Moves down its concentration gradient
• No energy required
• No transport proteins required – the molecules freely move through
the phospholipid bilayer
• Small, non-polar, non-charged molecules do this pretty well
OSMOSIS
• Simple diffusion of water
• It’s actually kind of hard for
water to move through the
plasma membrane, as it is
repelled by the hydrophobic
fatty-acid tails
• But as it is a small molecule, if
there is a strong enough
concentration gradient it can
move through
KNOW THESE!
HERE’S A BETTER EXAMPLE
FACILITATED DIFFUSION
• Some substances can’t cross the membrane even though there is a
concentration gradient
• They may be too large, polar, charged, etc
• Facilitated diffusion is the movement of a substance down its
concentration gradient with the assistance of carrier proteins or
channel proteins
• Example: Glucose requires carrier proteins
• Example: Most ions, like magnesium and sodium, require channel
proteins. Sometimes they are referred to as ion channels in this case
ACTIVE TRANSPORT
• The movement of a substance against its concentration gradient
• Requires carrier proteins (always carrier proteins, not channel
proteins, for active transport)
• Requires energy in the form of ATP
• Example: Movement of sodium and potassium ions across the
membrane of nerve cells, to build up an action potential
BULK TRANSPORT
• The movement of molecules that are too large to pass through the
membrane or via a membrane protein
• Cells make vesicles to transport these substances across the
membrane – a vesicle is a small membrane-bound package
• The membrane for the vesicle comes from the cell membrane
• Bulk transport requires energy
TYPES OF BULK TRANSPORT
• Endocytosis is bulk transport
into the cell
• Phagocytosis is bulk transport
of solids into the cell
• Pinocytosis is bulk transport of
liquid into the cell
• Exocytosis is bulk transport of
solid or liquid out of the cell
ENDOCYTOSIS
FACTORS AFFECTING MEMBRANE
TRANSPORT
• Size
• Polarity
• Hydrophobic/hydrophilic
• Charge
• Concentration gradient
MOLECULAR SIZE
Phospholipid tails repel
charged particles
High
concentration
Low
concentration
SUMMARY
• The plasma membrane protects and contains cell contents, denotes
cell identity, and communicates with other cells
• It consists of a phospholipid bilayer, membrane proteins, and sterols
• Facilitated diffusion and simple diffusion do not require energy
• Active transport and bulk transport require energy
• Osmosis is simple diffusion of water
• Size, charge, polarity and size of concentration gradient affect the
ease at which a substance will cross the plasma membrane
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