2.5 Membrane Permeability - AP Biology
Summary
TLDRIn this AP Biology lesson, Mr. Poser delves into the concept of membrane permeability, a vital aspect of cell structure and function. He explains how the phospholipid bilayer's amphipathic natureβfeaturing hydrophilic heads and hydrophobic tailsβgives the plasma membrane its selective permeability. While small nonpolar molecules can diffuse freely, larger polar molecules require assistance from transport proteins. The teacher also clarifies the difference between cell walls, composed of cellulose and providing structural support in plant cells, and cell membranes. The lesson highlights the importance of membrane proteins in facilitating the transport of essential molecules like glucose and sodium.
Takeaways
- 𧬠The plasma membrane is crucial for maintaining a cell's internal environment by selectively allowing substances to pass through.
- π¬ The cell membrane's structure, being a fluid mosaic, primarily composed of phospholipid bilayers, plays a key role in its function.
- π§ Phospholipids are amphipathic molecules with a polar phosphate head and nonpolar fatty acid tails, which arrange to face water-loving and water-repelling sides outward and inward, respectively.
- π« The cell membrane is selectively permeable, meaning it allows certain substances to pass through more easily than others based on its structure.
- β Small nonpolar molecules like gases (CO2, O2) can diffuse freely across the membrane due to their compatibility with the hydrophobic interior.
- β Large polar molecules and charged molecules cannot easily pass through the membrane's hydrophobic core and require transport proteins.
- π οΈ Transport proteins, such as channel and carrier proteins, facilitate the movement of larger or charged molecules across the membrane.
- π§ Water, a polar molecule, can cross the membrane to some extent, but more efficiently through aquaporins, specialized channel proteins.
- πΏ Plant cells have an additional protective layer, the cell wall, made of cellulose and other carbohydrates, providing structural support and a barrier for some substances.
- π It's important to distinguish between the cell wall and the cell membrane, as they serve different functions and are composed of different materials.
Q & A
What is the primary function of the plasma membrane?
-The plasma membrane separates the cell from its surroundings and allows cells to maintain a separate and stable internal environment.
What is the structure of the cell membrane described as?
-The structure of the cell membrane is described as a fluid mosaic, with the phospholipid bilayer making up most of it.
What is an amphipathic molecule and how does it relate to the cell membrane?
-An amphipathic molecule is one that has both polar and nonpolar regions. In the context of the cell membrane, the phospholipid, which is amphipathic, has a polar phosphate head and two nonpolar fatty acid tails.
How does the arrangement of the phospholipid bilayer contribute to the selective permeability of the membrane?
-The arrangement of the phospholipid bilayer, with the polar heads facing outward and the nonpolar tails facing inward, contributes to selective permeability by allowing small nonpolar molecules to pass through easily while hindering the passage of large polar molecules.
What is meant by the term 'selectively permeable' in relation to cell membranes?
-The term 'selectively permeable' means that the cell membrane allows some substances to pass through more easily than others, based on the structure of the membrane.
How do small nonpolar molecules like carbon dioxide and oxygen cross the plasma membrane?
-Small nonpolar molecules like carbon dioxide and oxygen can cross the plasma membrane by simple diffusion, moving from an area of high concentration to an area of low concentration.
What role do membrane proteins play in the transport of large polar molecules and charged molecules?
-Membrane proteins, such as channel and transport proteins, are essential for the transport of large polar molecules and charged molecules across the plasma membrane, as these types of molecules cannot pass through the hydrophobic fatty acid tails by themselves.
How does the cell membrane facilitate the passage of water molecules?
-Water molecules can pass through the plasma membrane in small amounts, but primarily they move through special channel proteins called aquaporins, which facilitate the passage of water from areas of high concentration to areas of low concentration.
What is the difference between a cell wall and a cell membrane?
-A cell wall, found in plant cells, is made of cellulose and provides structural support and a barrier for some substances. The cell membrane, on the other hand, is a selectively permeable barrier made mostly of phospholipids and is found in all cells, including plant cells.
Why are proteins an important component of the plasma membrane despite the membrane being mostly phospholipid bilayer?
-Proteins are an important component of the plasma membrane because they play a crucial role in the transport of essential molecules, such as glucose and sodium, which are necessary for cell function and cannot pass through the phospholipid bilayer by themselves.
Outlines
π¬ Introduction to Membrane Permeability
The video begins with an introduction to the topic of membrane permeability by a biology teacher, Mr. Poser. He explains that the plasma membrane is crucial for separating the cell from its surroundings and maintaining a stable internal environment. The structure of the cell membrane, which is a fluid mosaic primarily composed of phospholipid bilayers, is discussed. Phospholipids are amphipathic molecules with a polar phosphate head and nonpolar fatty acid tails. The arrangement of these bilayers results in a hydrophobic interior and a hydrophilic exterior, which influences the membrane's selective permeability. The teacher elaborates on how different molecules can cross the membrane through various mechanisms such as active transport, carrier proteins, and channel proteins. However, not all substances can pass through easily, emphasizing the membrane's selective nature. The hydrophobic core of the bilayer composed of fatty acid tails prevents the passage of large polar molecules like glucose without assistance from membrane proteins.
πΏ Clarification on Cell Walls and Membranes
In the second paragraph, the teacher addresses a common misconception between cell walls and cell membranes. He explains that while plant cells have both a cell wall and a plasma membrane, the cell wall is distinct, being composed of cellulose fibers and other carbohydrates and proteins. The cell wall provides structural support and acts as a barrier for some substances. The plasma membrane, which is beneath the cell wall, is responsible for selective permeability and is composed mainly of the phospholipid bilayer. The video concludes with the teacher mentioning the importance of aquaporins, which are channel proteins that facilitate the passage of water molecules across the membrane. The teacher then signs off, indicating the end of the video.
Mindmap
Keywords
π‘Membrane Permeability
π‘Phospholipid Bilayer
π‘Amphipathic Molecule
π‘Selective Permeability
π‘Carrier Proteins
π‘Channel Proteins
π‘Simple Diffusion
π‘Osmosis
π‘Cell Wall
π‘Cellulose
Highlights
Starting topic 2.5 on membrane permeability, continuing the unit on cell structure and function.
The plasma membrane's role in separating the cell from its surroundings and maintaining a stable internal environment.
Review of the cell membrane structure as a fluid mosaic, primarily composed of phospholipid bilayers.
Explanation of phospholipids as amphipathic molecules with a polar phosphate head and nonpolar fatty acid tails.
Arrangement of phospholipid bilayers with polar heads facing outward and nonpolar tails facing inward.
Selective permeability of membranes allowing some substances to pass through more easily than others.
The structure of the membrane contributing to its selective permeability, with fatty acid tails keeping out polar molecules.
Small nonpolar molecules like gases can move freely across the membrane through simple diffusion.
Large polar molecules and charged molecules cannot pass through the fatty acid tails without assistance.
Mention of the importance of membrane proteins in assisting the passage of essential molecules like glucose and sodium.
Small polar and uncharged molecules, such as water, can pass through the membrane in small amounts.
Introduction of aquaporins, special channel proteins that allow water molecules to pass through the membrane.
Clarification on the difference between a cell wall and a cell membrane, with plant cells having both.
Cell walls are made of cellulose and provide a structural boundary and barrier for some substances.
The plasma membrane's location beneath the cell wall and its role in cell function.
Transcripts
hi everybody your human anatomy i mean
ap biology teacher here
mr poser uh today we are starting uh
topic 2.5 on membrane permeability and
thus continuing our unit
on our structure and function of the
cell uh
by discussing what's able to go through
a membrane and why
so as i put in this bullet point up here
the plasma membrane separates the cell
from its surroundings
and allows cells to maintain a separate
and stable internal environment now how
is it able to do that
um in our last video topic 2.4 we talked
about the structure of the cell membrane
how it's a fluid mosaic
but what most makes up most of the fluid
mosaic is what's called the phospholipid
bilayer
so we took a look at the phospholipid
itself which is called an amphipathic
molecule meaning that it has one
polar side to it called a phosphate head
and it's got two nonpolar fatty acid
tails so it's got
a polar side and a nonpolar side um and
the way that the bilayers arrange is
that the phosphate head the polar side
of the
hydrophilic water loving sides are to
the outside
and then the hot water heating the
hydrophobic parts are to the inside and
thus it
it'll make uh polar molecules pretty
it'll give it a hard time trying to
cross the membrane
but a lot of different molecules can
cross the membrane through a lot of
different
ways say like active transport you can
use carrier proteins channel proteins
you can use pumps different pump
proteins
so there's lots of different ways for
molecules to cross the plasma membrane
but it's not going to allow everything
through super easily
and that's where we get to this term and
membranes are what we call selectively
permeable they allow some substances
across more easily than others so if
you're being selective
you're choosing right you're choosing
one thing or the other and permeable
means that is something able to go
through or not so if you're impermeable
or if a layer is impermeable
then nothing can go through it but if
it's permeable something can go through
it
membranes are selective about what they
let through and what they don't
and the structure of it really lends
itself to
its selective permeability structure
meets function once again um so as i
just said yeah the membrane selective
permeability is a
result of its structure fatty acid tails
in the middle keep out
polar molecules okay so as i was just
saying the hydrophilic parts are on the
outside but the hydrophobic parts are
oriented towards
the inside so polar molecules like water
are going to have a hard time crossing
they can in small amounts but large
polar molecules like say
glucose or something like that is not
going to be able to just pass through
the bilayer
by itself it's going to need some help
from one of those proteins
but small nonpolar molecules can move
freely across the membrane so if we have
two
gases over here let's say we got carbon
dioxide and we've got oxygen
these are both very small molecules are
only two or three atoms each
and they're nonpolar there's no one side
of that molecule that has a positive
charge and
or a negative charge they're the charge
or the electronegativity it's called of
both of those molecules is equal
so they're both nonpolar and they're
able to pass through the bilayer
by themselves through simple diffusion
um just
moving from a high concentration to a
low concentration it's not a big deal
for those small nonpolar molecules but
if we take a look at large
polar molecules like say glucose and
charged molecules
they absolutely cannot pass through the
fatty acid tails
by themselves okay they're okay
interacting with the hydrophilic sides
on the outside of the membrane but they
cannot get through
the hydrophobic regions one second
all right i will call her back in just a
minute
um but yeah so large polar molecules and
charged molecules cannot pass through so
i put up some x's there
um all right so these types of molecules
what they have to do to cross the
membrane
is they have to move through channel and
transport proteins remember proteins are
a really really big part
of the plasma membrane despite you know
it's mostly the phospholipid bilayer
there's molecules that really help other
things like glucose and sodium pass
through because glucose and sodium are
both essential for cell function
so yeah without those membrane proteins
nothing that
nothing like that would be able to get
across all right um
small polar and uncharged molecules can
pass through
small amounts so water obviously is
going to be a really big topic in the
motion of water from highly low
concentration called osmosis
it's going to be a topic that we're
going to discuss a lot coming up
here soon so water can get through the
bilayer in
some amounts okay kind of it's really
hard
for it to you know since this is
hydrophobic after all it's kind of hard
for water molecules to pass through
um the bilayers particularly these
uh fatty acid tails that are nonpolar um
but most of the time i'm going to add
something in here
water can move through aquaporins
and aquaporins are special channel
proteins that are
made for water molecules to pass through
so
if you know the cell needs more water it
can open aquaporins and water could
really flow through
from the inside to the outside um one
last thing i'd like to note here
is that a lot of students
confuse a cell wall with a cell membrane
so we're going to talk about that real
quick cell walls of plant cells are made
of cellulose which is a polysaccharide
hopefully we talked about that in the
first unit
and those are fibers embedded in other
carbohydrates and proteins
cell walls provide a structural boundary
for and a barrier for some substances
so if we take a look at this diagram
here
we have the cell wall that is made up of
lots of these different
cellulose fibers along with some
proteins like pectin
what it's able to do is provide like a
kind of structural barrier
to the outside and then right beneath it
is the plasma membrane
all right so plant cells have both a
cell wall
and a plasma membrane um
yeah i believe that's what i wanted to
talk about for this video so
let me know if you have any questions i
gotta take that phone call
um and we'll see you later bye
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