Key Terms:
Cell membrane: a boundary that controls what enters and leaves a cell
Molecules: atoms that are bonded together
Selectively permeable: only some substances can pass through
Permeable: allows all substances to pass through
Impermeable: does not allow any substances to pass through
Pores: holes
Fluid: unfixed shape that can flow
Hydrophobic: water hating
Hydrophilic: water loving
Phospholipid: a lipid attached to a phosphate

Identify that there is movement of molecules into and out of cells

The cell membrane is the passage where substances enter and leave the cells. Molecules can enter the cell because the cell membrane is selectively permeable. The pores allow some substance in and not others.
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Substances that enter cell:
- Gases such as carbon dioxide and oxygen
- Water
- Sugars
- Amino acids
- Glycerol
- Fatty acids
Substances that leave cells:
- Urea
- Uric acid
- Gases such as carbon dioxide
- Mucus
- Hormones

Describe the current model of membrane structure and explain how it accounts for the movement of some substances into and out of cells

Th current model of cell membrane structure is the Fluid Mosaic Model which was proposed by Nicholson and Singer (1976).
Made of mostly lipids and some proteins.

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- Hydrophobic tails are position in the middle of the bi layer
- Hydrophilic heads are on the outsides of the bi layer
- The structure is not rigid but fluid.
- Proteins are found all over the membrane in random places
- Proteins may go all the way through the membrane acting as a channel for molecules to travel through it
- They can be attached to lipids to become a glycolipid
- They can be attached to proteins to become a glycoprotein
- Carbohydrates used are to distinguish the cell as its own or foreign.
- They can be attached to cell membrane near the cytoplasm side as an anchorage
1. The membrane allows lipid based substances to go through, this mean the membrane must be made of lipids.
2. After the membrane has been broken it reseals suggesting that is a fluid structure not hard.
3. The total surface area has been calculated to prove that the cell membrane is a phospholipid bilayer.

Why can substances enter via the membrane?
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The cell membrane is selectively permeable, only molecules with a certain size, lipid solubility and charge can pass through it. Membranes that do not allow any substances to pass through is called impermeable. If a membrane lets all substances to enter or leave it is called permeable. The substances that cannot move through the membrane have to used protein channels.

Perform a first-hand investigation to model the selectively permeable nature of a cell membrane

Aim: To model the selectively permeability of a cell membrane.
- Tea strainer
- Smarties
- Fine grain sugar
- Cup and spoon
1. Mix the smarties and sugar evenly in a cup using a spoon
2. Pour the mixture over a strainer
The sugar travels through the holes in the strainer leaving the smarties behinds.This resembles the cell membrane as small particles could go through and not the large particles


A. Which substances find it hard to move through the cell membrane?
B. Why is the Fluid Mosaic Model called fluid?
C. Name some substances that leave cells.
D. Describe the structure of the cell membrane?
E. What is the evidence that supports Singer and Nicholson's model.
F. Label the diagram below.

For More Information BIOLOGY IN FOCUS:

Movement of molecules pg 108
Fluid Mosaic Model pg 109-111
First Hand Investigation pg 112