Transcript Presentation

Cell
Structure and
Function
Part 3
The Cell Membrane
Movement Through the Membrane
1
Membrane Structure
• All living organisms are made of _______.
• All the parts of a cell are contained inside the
cell membrane (__________ membrane), which
provides protection and support for the cell.
• Because the cell membrane is extremely thin, it
took a long time for people to find ways to see
how it was put together.
• Membranes are made mainly of __________
and a few _______. The lipids form a double
layer called a lipid bilayer.
2
Head
Tails
H20
H20
H20
H20
H20
• A typical lipid has two main
parts: A hydrophilic
(“______________”) head,
and a hydrophobic
(“________________”) tail.
• When lipids are in water,
the tails all gather together
away from the water.
• Under the right conditions
they form a double layer
(the lipid ______) with the
heads on the outside and
the tails on the inside.
H20
H20
H20
H20
3
Other molecules, such as proteins and cholesterol can
be embedded in the lipid bilayer. All the lipids, etc.,
can move within the layer. For these reasons we call
this the “_____________ model” of a membrane.
4
Movement across membranes
• The cell membrane, then, is not a solid wall.
Some molecules can move into or out of the
cell.
• There are several ways this can happen:
– General movement (by ___________)
– Selective movement (by ___________ or active
transport)
• The molecules may be water, salts, sugars,
other nutrients, or even larger molecules.
5
Definitions
_________ - a liquid (often water) that other molecules are
dissolved in.
_______ - the substance that is dissolved in the solvent, such
as salt dissolved in water.
__________ - a solute dissolved in a solvent.
_________________ - a measure of how much solute is
dissolved in the solvent (Ex: 5% salt, 8 g/ml salt, etc.)
___________ - if a substance can cross a membrane we say
the membrane is permeable to that substance.
Semi-permeable (or _____________ permeable) - Most
biological membranes are selective; only certain molecules
can pass through.
6
Diffusion
• Diffusion is the process by which molecules move by their
kinetic energy from an area of ____ concentration to an
area of ____ concentration. The process continues until the
concentration is equal everywhere.
• Diffusion Demo
• If we poured a bit of syrup into a glass of water, the sugar
molecules of the syrup would eventually be spread evenly
throughout the water. If someone spilled perfume in one
corner of the room, after a bit we will be able to smell it in
other parts of the room, even if there is no breeze.
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Osmosis
• Cell membranes are _________ to water. Water
can diffuse in or out of a cell.
• Most _____ molecules (such as salt) are too large
to go through the selectively permeable cell
membrane. If there is a difference in solute
concentration inside a cell compared to outside,
the solute can’t diffuse to equalize the
concentration in each are.
• What CAN happen, though, is that ________ (the
solvent in this case) can move in or out of the cell
until the concentration is the same in each place.
8
Osmosis
• The movement of water through a semipermeable
membrane, from an area of _____ solute (i.e.
sugar) concentration to an area of ______ solute
concentration is called _________.
• Remember, low solute concentration means high
water (solvent) concentration, and vice versa.
Water
Sugar
5% Sugar
Comparing two sugar solutions
5% Sugar: High solvent
concentration, low solute
concentration.
20% Sugar: Low solvent
concentration, high solute
concentration, relative to the 5%
sugar solution.
20% Sugar
9
Osmosis in cells _______
• There are three situations that can occur when we talk about
the concentration of a solute inside and outside a cell.
• In the first case, the concentration outside can equal the
concentration inside the cell. We say that the solute
concentration is isotonic (iso = ________, tonic = __________)
in relation to the cell.
• Water will diffuse in and out of the cell equally, so there will be
no real change.
water
water
Isotonic
5% Sugar in cell
5% Sugar outside cell
Effect?
Water flows, but no
net change.
What happens to the cell?
10
Osmosis in cells - hypertonic
• In the second case, the concentration outside the cell is
_______ than the concentration inside the cell. We say that the
solute concentration is ___________ (hyper = more, tonic =
pressure) in relation to the cell.
• Since the solute can’t pass through the membrane, water will
mainly diffuse _____ of the cell until the concentration
equalizes.
water
Hypertonic
Effect
5% Sugar in cell
Water flows both
25% Sugar outside cell ways, but mainly out.
What happens to the cell?
11
Osmosis in cells - hypotonic
• In the third case, the concentration outside the cell is _____
than the concentration inside the cell. We say that the solute
concentration is _____________ (hypo = less, tonic =
pressure) in relation to the cell.
• Since the solute can’t pass through the membrane, water will
mainly diffuse ____________ of the cell until the concentration
equalizes.
water
Hypotonic
25% Sugar in cell
5% Sugar outside cell
Effect
Water flows both
ways, but mainly in.
What happens to the cell?
12
Active Transport
• Osmosis is a passive process. The cell doesn’t use
any energy to move water. It is driven by diffusion of
water due to differences in solute concentrations.
The larger the difference, the faster it happens.
• Obviously, other molecules DO get into the cell,
sometimes moving to areas of higher concentration.
This process is called _______________, and does
require energy. It is also one of the functions of some
of the membrane proteins.
• We will talk more about active transport later.
13