cell membrane


The inside of all cells also contain a jelly-like substance called cytosol. Cytosol is composed of water and other molecules, including enzymes, which are proteins that speed up the cells chemical reactions. Everything in the cell sits in the cytosol, like fruit in a jello mold. The term cytoplasm refers to the cytosol and all of the organelles, the specialized compartments of the cell. The cytoplasm does not include the nucleus. As a prokaryotic cell does not have a nucleus, the DNA is in the cytoplasm.

the plasma membrane

The plasma membrane ( Figure 1.1) is made of a double layer of special lipids, known as phospholipids. The phospholipid is a lipid molecule with a hydrophilic ("water-loving") head and two hydrophobic ("water-hating") tails. Because of the hydrophilic and hydrophobic nature of the phospholipid, the molecule must be arranged in a specific pattern as only certain parts of the molecule can physically be in contact with water. Remember that there is water outside the cell, and the cytoplasm inside the cell is mostly water as well. So the phospholipids are arranged in a double layer (a bilayer) to keep the cell separate from its environment. Lipids do not mix with water (recall that oil is a lipid), so the phospholipid bilayer of the cell membrane acts as a barrier, keeping water out of the cell, and keeping the cytoplasm inside the cell. The cell membrane allows the cell to stay structurally intact in its water-based environment. The function of the plasma membrane is to control what goes in and out of the cell. Some molecules can go through the cell membrane to enter and leave the cell, but some cannot. The cell is therefore not completely permeable. "Permeable" means that anything can cross a barrier. An open door is completely permeable to anything that wants to enter or exit through the door. The plasma membrane is semipermeable, meaning that some things can enter the cell, and some things cannot.


the plasma membrane and cytosol

If the outside environment of a cell is water-based, and the inside of the cell is also mostly water, something has to make sure the cell stays intact in this environment. What would happen if a cell dissolved in water, like sugar does? Obviously, the cell could not survive in such an environment. So something must protect the cell and allow it to survive in its water-based environment. All cells have a barrier around them that separates them from the environment and from other cells. This barrier is called the plasma membrane, or cell membrane.

instructional diagrams

No diagram descriptions associated with this lesson


everything in the cell sits in the cytosol.

-->  a. true

b. false

the plasma membrane and the cell membrane are the same thing.

-->  a. true

b. false

the plasma membrane is made of a double layer of ____.

a) cells

-->  b) phospholipids

c) proteins

d) dna

in a prokaryotic cell, the dna is located in the ___.

-->  a) cytoplasm

b) nucleus

c) phospholipid

d) enzyme

the middle of the cell membrane bilayer is an area without ____.

a) proteins

b) enzymes

-->  c) water

d) phospholipids

the cell membrane keeps _ out of the cell, and keeps the _ inside the cell.

a) proteins, water

b) cytosol, cytoplasm

c) cytoplasm, water

-->  d) water, cytoplasm

the phospholipid has a _ head and two _ tails.

a) semipermeable, cytoplasm

b) hydrophobic, hydrophilic

-->  c) hydrophilic, hydrophobic

d) cytosol, cytoplasm

diagram questions

No diagram questions associated with this lesson