THE CELL MEMBRANE

Mr. ChapmanBiology 30

Needs of the Cell Cells require protection, nutrients,

water, elimination of waste, and response to stimuli.

The cell membrane performs all these functions for the cell.

The cell membrane acts as a barrier between the outside environment and the cell.

Home0stasis – Maintaining a Balance Cells must keep the proper

concentration of nutrients and water and eliminate wastes.

The plasma membrane is selectively permeable – it will allow some things to pass through, while blocking other things.

Cell Membrane Components The cell membrane consists of a double

layer of phospholipids. This is also called a phospholipid bilayer.

Remember that a phospholipid consists of a phosphate group, glycerol, and two fatty acid chains.

Phospholipids

Remember that hydrophilic means water loving, and hydrophobic means water hating.

Cell Membrane Orientation The cell membrane is oriented in such a

way that the two layers of phospholipids sandwich themselves.

The heads will touch the outside environment as well as the cytoplasm inside the cell. The fatty acid chains are attracted to each other.

Cell Membrane Orientation The heads of the phospholipids are like

the bread of the sandwich and the chains are the fillings in the sandwich.

Cell Membrane Function The cell membrane is responsible for

allowing substances into and out of the cell.

This is important because the cell needs to maintain equilibrium.

The cell will need to take in nutrients and expel waste, or excess amount of substances.

Components of the Cell Membrane Have Specific

Functions

Components of the Cell Membrane Have Specific

Functions1. Cholesterol – helps to strengthen the

membrane.2. Proteins – help transport some

molecules across the membrane, or act as receptors.

3. Carbohydrates – act as simple ID tags on the surface of the cells, to identify cell type.

4. Phospholipids – make up the majority of the cell membrane.

Fluid Mosaic Model Scientists have developed the fluid mosaic

model, which describes the cell membrane in two different ways:1. The cell membrane is flexible not rigid.

Phospholipids can move from side to side and pass each other. The membrane acts as a fluid.

2. The numerous proteins and molecules that line the membrane and are embedded make the membrane colourful and display a mosaic of shapes and textures.

Transport Across the Membrane The cell membrane is selectively

permeable, meaning that it allows some substances to cross but not others. The membrane is also called semi-permeable.

Being selectively permeable allows the cell to maintain homeostasis, which is the ability to maintain equilibrium.

Cells need to have specific conditions to carry out certain functions; therefore they must be able to control what comes in and out of them.

Transport Across the Membrane Molecules cross the membrane in several ways, which

will be the topic of the next section.

Some of these methods require the cell to expend energy; others do not.

Several factors affect whether energy is required to move a particle. These include:

1. Size2. Polarity3. Concentration inside and outside of the cell

In Order to Finish: There is more to learn about the cell membrane,

but I would like you to complete this task on your own.

Assignment:

Read Section 3.3 of the textbook. Some of this you will find to be review of what we just talked about, and some will be new.

Complete Section 3.3 of the Study Guide. This time, I am going to take this in and grade your responses, so make sure to be thorough and do a good job. Cell membranes are very important to understand.