Cell Structure and Function

Chapter 7

Part 2

The Cell Membrane

The cell membrane is called selectively permeable because it regulates what enters and leaves the cell.

A double layer of phospholipids makes up the membrane (phospholipid bilayer).

Phospholipids are arranged with the polar head (hydrophilic) facing out in the bilayer, and the non-polar tails (hydrophobic) facing the interior of the bilayer.

Membrane Proteins

Various proteins are embedded in the lipid bilayer. Some membrane protein functions:

• Marker proteins – identifies the cell type

• Receptor proteins – binds to substances outside the cell and allow chemical messages to be transmitted across the cell membrane

• Enzymes – assists in chemical reactions

• Transport proteins – helps moves substances across the cell membrane

7.3 Cell Transport

Passive Transport – movement across a membrane without using energy. • Diffusion is the simplest type of passive

transport. Particles will move down the

concentration gradient• Movement from an area of high

concentration to an area of low concentration is known as diffusion.

Equilibrium will be reached when the concentration of a substance is equal throughout the space.

• Small, uncharged molecules can easily pass through the cell membrane.

• However, some molecules cannot easily diffuse across the cell membrane.

• Facilitated diffusion is diffusion using transport proteins without using energy.

Facilitated Diffusion

Diffusion Demonstration

What molecules will move through the cell membrane?

Osmosis The diffusion of water through a

selectively permeable membrane. Water will move down the

concentration gradient from where water is in high concentration to where water is in low concentration.

Water moves in and out of the cell at the same rate• Equal water concentration inside and

outside the cell

• Cell will not change size

• ISOTONIC SOLUTION

Direction of Water Movement

Water moves out of the cell• Lower water concentration outside the cell

• Cell shrinks

• HYPERTONIC SOLUTION

Water moves into the cell• Lower water concentration inside the cell

• Cell swells

• HYPOTONIC SOLUTION

Practice Problem

A cell contains 5% NaCl. The cell’s membrane is permeable to water, but not NaCl. • If this cell is placed in a solution containing

10% NaCl, what will happen to the cell? Why?

• What type of solution is this cell in?

More Practice ProblemsThe cell membrane of a cell is permeable to water but not to NaCl. The cell

contains 2% NaCl. You place this cell in 3 different solutions.

Beaker 1 contains a solution that is 5% NaCl. Beaker 2 contains a solution that is 1% NaCl. Beaker 3 contains a solution that is 2% NaCl.

Answer the following questions:

A. What will happen to the cell in beaker 1? What direction does the water move? What type of a solution is it in?

B. What will happen to the cell in beaker 2? What direction does the water move? What type of solution is it in?

C. What will happen to the cell in beaker 3? What direction does the water move? What type of solution is it in?

Active Transport

Cells must move some substances against their concentration gradient• from an area of low concentration to an area of high

concentration

• usually requires a transport protein pump

Or move molecules that are too large to be transported through the membrane

This REQUIRES ENERGY from ATP!

Bulk Transport (Movement in Vesicles)

Move substances such as proteins and polysaccharides, which are too large to be transported by carrier proteins

Endocytosis – vesicle pinches in from the cell membrane surrounding materials to be brought into the cell.

Exocytosis – vesicle moves to the outside of the cell, fuses with the cell membrane and releases it’s contents.

7.4 Homeostasis and Cells

Unicellular organisms carry out all functions necessary for life.

Our cells, however, cannot live on their own.• They must become specialized for a certain

job, and communicate with each other to maintain homeostasis.

Cell Specialization Different cells have different jobs Examples:

• Muscle cells – movement

• Nerve cells – send/receive messages

• Red Blood cells – carry gasses to and from cells

red blood cell nerve cell

Levels of Organization Cells → Tissues → Organs → Organ

Systems