AS Biology. Foundation. Cell membranes and Transport 1

The Cell
AS Biology. Foundation. Cell membranes and Transport 2

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All living things are surrounded by a membrane.
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A cell membrane is also known as plasma membrane.
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Controls exchange of materials such as nutrients and waste between cells and their environment.
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Has other important functions for example to enable cells to receive hormones.
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To understand the function of anything in biology, you must study the structure first!
AS Biology. Foundation. Cell membranes and Transport 3

Cell Membranes from Opposing
Neurons (TEM x436,740).
Nerve cell
Cell membrane
{
Gap between cells
} cell membrane
7nm wide
Nerve cell
AS Biology. Foundation. Cell membranes and Transport 4

Cell membranes are made of
PHOSPHOLIPIDs
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HYDROPHILIC heads
(water liking)
-Attracted to the water called POLAR
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HYDROPHOBIC tails
(water fearing)
-Not attracted to the water called NON-POLAR
AS Biology. Foundation. Cell membranes and Transport
A Phospholipid
5

A phospholipid
HYDROPHILIC HEAD
At the other end of the phospholipid is a phosphate group and several double bonded oxygens. The atoms at this end of the molecule are not shared equally.
This end of the molecule has a charge and is attracted to water. It is POLAR
HYDROPHOBIC TAILS
The two long chains coming off of the bottom of this molecule are made up of carbon and hydrogen. Because both of these elements share their electrons evenly these chains have no charge. They are NON POLAR. Molecules with no charge are not attracted to water; as a result water molecules tend to push them out of the way as they are attracted to each other. This causes molecules with no charge not to dissolve in water.
AS Biology. Foundation. Cell membranes and Transport
3D model of a
Phospholipid
6

Phospholipids can form:
BILAYERS
-2 layers of phospholipids with hydrophobic tails protected inside by the hydrophilic heads.
The PHOSPHOLIPID
BILAYER is the basic structure of membranes.
AS Biology. Foundation. Cell membranes and Transport 7

Fluid mosaic model
Cell membranes also contain proteins within the phospholipid bilayer.
This ‘model’ for the structure of the membrane is called the:
FLUID MOSAIC MODEL
FLUID- because individual phospholipids and proteins can move around freely within the layer, like it’s a liquid.
MOSAIC- because of the pattern produced by the scattered protein molecules when the membrane is viewed from above.
AS Biology. Foundation. Cell membranes and Transport 8

Cell Membranes from Opposing
Neurons (TEM x436,740).
AS Biology. Foundation. Cell membranes and Transport
} Phospholipid Bilayer
7nm wide
9

Movement of selected molecules across the cell membrane
AS Biology. Foundation. Cell membranes and Transport 10

1. Diffusion
2. Osmosis
3. Facilitated Diffusion
4. Active Transport
AS Biology. Foundation. Cell membranes and Transport 11

1.
Diffusion -passive transport - no energy expended
2. Osmosis - Passive transport of water across membrane
3.
4.
Facilitated Diffusion - Use of proteins to carry polar molecules or ions across
Active Transport- requires energy to transport molecules against a concentration gradient – energy is in the form of ATP
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Movement of molecules from an area of high concentration to an area of low concentration.
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Movement from one side of a membrane to another, unfacilitated
AS Biology. Foundation. Cell membranes and Transport 13

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Solutions are made of solute and a solvent
Solvent - the liquid into which the solute is poured and dissolved. We will use water as our solvent today.
Solute - substance that is dissolved or put into the solvent. Salt and sucrose are solutes.
AS Biology. Foundation. Cell membranes and Transport 15

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Hypotonic Solution - One solution has a lower concentration of solute than another.
Hypertonic Solution - one solution has a higher concentration of solute than another.
Isotonic Solution - both solutions have same concentrations of solute.
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Plant and Animal Cells put into various solutions
AS Biology. Foundation. Cell membranes and Transport 18

Types of Transport
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We are using dialysis tubing as the cell membrane - It is selectively permeable
The solute is either the eosin starch solution or the sugar solution
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What is the solvent?
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Solute is eosin-starch-chloride solution
Solvent is Water
Indicator for presence of starch is IKI
Starch is made of amylose and amylopectin – amylopectin is insoluble
Iodine is not very soluble in water, but with KI it forms a I (triiodine ion) which is soluble.
3
-
I
3
combines with the amylose and the starch molecule turns blue-black.
AS Biology. Foundation. Cell membranes and Transport 21

AS Biology. Foundation. Cell
Elmhurst College Website 22

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Indicator for presence of chloride ions is silver nitrate, AgNO
3

A white precipitate, AgCl, forms if chloride is present.
AS Biology. Foundation. Cell membranes and Transport 23

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Indicator for the presence of sulfate ions is Barium chloride, BaCl

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Barium sulfate, BaSO
4 precipitate
- forms a white
BaCl + NaSO
4
----> BaSO
4
+ NaCl
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Fern gametophytes are used to see result of putting live cells in solutions of varying tonicity
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We will make an osmometer to see osmosis
Sugar solution in a dialysis tube is used to simulate a cell membrane.
Various concentrations of solute may be used around the room.
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Moving amoeba
AS Biology. Foundation. Cell membranes and Transport 27