The Cell Membrane

Also known as a

“plasma membrane”

Plasma Membrane

• Controls materials entering and leaving the cell

• Helps regulate homeostasis

• Plasma membrane has selective permeability

similar to a window screen

Made of phospholipids with embedded proteins.

Phospholipid

Types of proteins in cell membrane

1. Cell-surface marker: identifies cell type and identifies cells as “self”.

2. Receptor Protein: recognizes and binds to substances outside the cell

3. Enzyme: Assists chemical reactions inside the cell

Example: sucrase and

maltase are embedded

into cells of the

small intestine,

where they digest

disaccharides

4. Transport protein: Helps substances move across cell membrane.

Cellular Transport

Sections 3-4 and 3-5

I. Passive Transport• Materials move across membranes

because of a concentration gradient.

• Does not require energy.

Diffusion is a type of passive transport.

• Molecules move from an area of high concentration to an area of low concentration.

A. Diffusion

• Continues until equilibrium is reached

B. Osmosis

• How is osmosis different from diffusion?

• It only deals with movement of water

• Isotonic: same concentration of dissolved particles inside and outside of cell; cell stays the same size

Three Types of Solutions

• Hypotonic: less dissolved particles outside cell than in

cell; cell gains water and swells

• Hypertonic: more dissolved particles outside cell than in cell; cell loses water and shrinks

• Osmosis in Plant cells

• Practice Problem:

• Sea water has a higher concentration of salt than human body cells have.

• Why might drinking large amounts of sea water be dangerous for humans?

• Facilitated diffusion is passive transport of materials across the plasma membrane with the aid of transport proteins .

• Molecules such as sugars or amino acids enter the cell through facilitated diffusion

• Channel protein shown at right – provides a tubelike opening for small molecules

C. Facilitated Diffusion

• Carrier proteins change shape to let larger molecules into the cell

Cells use energy to transport materials against the concentration gradient

II. Active Transport

• Active transport is powered by chemical energy (ATP).

Active Transport

• Active transport occurs through transport proteins

A cell can import large materials in vessicles during the process of endocytosis.

• Cells use energy to transport material in vessicles.

Endocytosis

Exocytosis

•Exocytosis is the process of expelling material from the cell.