1. Cell Structure and Function
Part 3
The Cell Membrane
Movement Through the Membrane

2. 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.

3. Head
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.
Tails
H20
H20
H20
H20
H20
H20
H20
H20
H20

4. 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.

5. 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.

6. 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.

7. 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.

8. 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.

9. 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.
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.
Water
Sugar
5% Sugar
20% Sugar

10. 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.
Isotonic
5% Sugar in cell
5% Sugar outside cell
Effect?
Water flows, but no net change.
water
water
What happens to the cell?

11. 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.
Hypertonic
5% Sugar in cell
25% Sugar outside cell
Effect
Water flows both ways, but mainly out.
water
What happens to the cell?

12. 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.
Hypotonic
25% Sugar in cell
5% Sugar outside cell
Effect
Water flows both ways, but mainly in.
water
What happens to the cell?

13. 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.