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CJ
• Lesson: Begin 4.1 Cell Membrane
Structure
• Understand components and
chemistry of cell membrane
• Calculate surface area to volume
ratio
• Complete 4.1 In-Class Activity
2020-2021
Do Now
• Explain how the structure of a
phospholipid is correlated with the
structure of the cell membrane
• Are phospholipids the only type of
molecules found in the cell
membrane? If no, what other
molecules can be found?
Foulds| Biology 8
1
Cellular Exchange with
Environment
Unit 4
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Lesson 4.1
Plasma Membrane Structure
2020-2021
Foulds | Biology 8
3
Big Picture – Context Going in to Unit 4
• In the last unit, we touched on the idea that the cell membrane separates
the highly delicate biochemical environment inside of the cell from the
chaos and disorder of nature outside of the cell
• But the cell is not a “closed system” – the cell needs to exchange materials
with it’s environment in order to maintain homeostasis
• Thus the cell must carefully pick and choose what materials can enter and
leave the cytoplasm.
• Our cells take advantage of physics and chemistry in order to successfully
transport materials
Cell Membrane Structure
• Cell membranes = flexible, doublelayered sheets called a phospholipid
bilayer.
• Cell membrane contains a variety of
different molecules embedded among
the many phospholipids
Phospholipid
The Lipid Bilayer
• The lipid bilayer forms
spontaneously when
phospholipids are in water
• This is the most stable
configuration of
phospholipids in water
• Thus, the structure of the
membrane itself abides
by laws of physics
• How does phospholipid
structure correlate with
membrane structure?
Amphipathic – both
hydrophobic and hydrophilic
parts
The Lipid Bilayer
• Phospholipids = Amphipathic
• Hydrophilic head groups
point outwards, toward
water
• Hydrophobic tail groups
point inwards, away from
water
• Middle of the membrane is
extremely hydrophobic. It is a
hydrophobic barrier
• Thus, water and other polar
molecules can not pass
through directly
Amphipathic – both
hydrophobic and hydrophilic
parts
Membrane Behavior
• Fluid Mosaic Model
•
•
•
•
The cell membrane is “fluid”
Thousands of phospholipids move side-to-side (laterally) in the membrane
Proteins also are fluid in membrane, but move more slowly due to big size
Note: fluid here does not mean liquid – fluid in this context means the membrane is flexible
• “Mosaic” part – there are many different types of molecules embedded in the
membrane
• Phospholipids, proteins, glycoproteins, glycolipids, cholesterols, etc.
• Video: Visualizing Membrane Fluidity
Selective = choose carefully
Permeable = allow to pass
Barrier = obstacle to prevent
movement
Putting it together
The cell membrane is a selectively permeable, hydrophobic barrier
• Some substances cross more easily than others
• Phospholipid tails keep hydrophilic, polar molecules from passing directly
through
• So how do polar molecules, like water, enter or leave the cell?
• Membrane proteins provide passage for polar molecules to go through
the hydrophobic barrier
• Recall that proteins are highly specific about what molecules they will interact
with.
Copyright Pearson Prentice Hall
Why Are Cells So Small?
Surface area = 6s2
Volume = s3
• A cell needs to be able to get enough nutrients in and waste out so that the cell
can function.
• But as a cell increases in size, the volume (inside the cell) increases more rapidly
than the surface area (membrane area).
• To get around this problem, it is advantageous for cells to maintain a high surface area
to volume ratio, by staying small in size.
• High surface area, low volume
Why Are Cells So Small?
s
Surface area = 6s2
Volume = s3
Cell Membrane Structure
• To summarize, cells can maintain high
surface area to volume in 2 ways:
• 1: Cells stay small in size (as seen on last
page)
• 2: Membranes can achieve high surface
area by folding in on themselves
• Example - The inner membrane of the
mitochondria is folded in order to increase
its surface area
•Allows for surface area to grow without
much change in cell volume