Reading 4 - Human Physiology

Chapter 4: The Cell Membrane (Part 1)

What is the cell / plasma membrane?

Protective barrier of the cell that separates the inner contents of a cell from its exterior environment, regulating which material can pass in or out

What is the structure of the cell membrane composed of?

What do each of the parts do?

Composed of a lipid bilayer interspersed with various molecules; primarily back-to-back phospholipids in the bilayer, with cholesterol and various other proteins embedded in the membrane

Phospholipids — compose the bilayer

Cholesterol — contribute to the fluidity of the membrane

Proteins — various functions

What are the two parts of a phospholipid?

What are the characteristics of these two parts?

1. Phosphate group “head”

   • Negatively charged, polar, hydrophilic

2. Lipid “tail” of two fatty acid chains

   • Uncharged, nonpolar, hydrophobic

What are the two types of fatty acid chains that can be located in the lipid “tail”?

Saturated fatty acids and Unsaturated fatty acids

What is the most important trait of a phospholipid for composing the cell membrane?

What does it mean?

Amphipathic — molecule exhibits a difference in polarity between its two ends, resulting in a difference in water solubility

How are the layers of the cell membrane organized?

Lipid tails face the lipid tails of the other layer

Phospholipid heads face outwards, one to the interior and one to the exterior of the cell

Why do the phospholipid heads / phosphate groups face the interior and exterior of the cell?

What are some examples of fluids found in the exterior of the cell?

Polar and hydrophilic

• Attracted to water in the Cytosol / interior

• Attracted to the fluid in the exterior

Interstitial fluid and blood plasma

What is Plasma?

Liquid part of the extracellular fluid that is found in the blood

What is the relative fluidity of the cell membrane, its lipids, and its proteins?

Cell membrane — remains fluid

Lipids and Proteins — not rigidly locked in place

What are the two types of proteins commonly associated with the cell membrane?

Are they connected to membrane?

Integral Proteins — membrane-associated protein that spans the entire width of the lipid bilayer and is physically connected to the membrane

Peripheral Proteins — membrane-associated protein that does NOT span the entire width of the lipid bilayer, but is attached peripherally to integral proteins, membrane lipids, or other components of the membrane; NOT strongly bound to the membrane

What types of Integral proteins have contact with both the ECF and ICF?

What are they called?

Channel Proteins — has an inner pore which allows the passage of one or more substances

Carrier Proteins — requires binding of a solute that elicits a conformational change to allow solute movement across the cell membrane

Called Transmembrane Proteins

Why are Transmembrane Proteins important for the cell membrane?

Illustrate specificity in ensuring the proteins transport only one type / family of molecules

Can a Peripheral Protein be attached to something besides the cell membrane?

Yes; can be attached to the internal or external surface of an integral protein, or be a part of the cell’s cytoskeleton

What kinds of integral proteins are located only on one side of the membrane?

Enzymes — most often a protein that facilitates a chemical reaction

Receptors — protein molecule that contains a binding site for another specific molecule (called a ligand)

Glycoproteins and Glycolipids

What is the function of a Receptor in regards to the cell membrane?

What is a Ligand?

Selectively binds to a specific molecule (ligand) outside of cell to incite a chemical reaction inside the cell

Ligand — molecule that binds with specificity to a specific receptor molecule

What is an example of integral proteins that can serve as both a Receptor and an Ion Channel?

Receptors on nerve cells that bind neurotransmitters, like Dopamine

What are Glycoproteins and Glycolipids?

Glycoproteins — protein that has one or more carbohydrates attached

Glycolipids — membrane carbohydrate with a lipid attached that is important in cell recognition and holding cells together

What is the Glycocalyx?

How is it formed?

What can the extensions from the Glycocalyx be used for by the cell?

Coating of sugar molecules surrounding the cell membrane

Formed by Glycoproteins and Glycolipids that extend into the extracellular matrix from membrane proteins and lipids

Used for cell recognition and holding cells together

What are the various roles of the Glycocalyx?

Why is it important for all the cells in our bodies?

How can this cause some problems?

• Can have molecules that allow the cell to bind to another cell

• Can contain receptors for hormones

• Can have enzymes that break down nutrients

Gives each cell the “identity” of belonging to the person’s body due to being products of the body’s genetic makeup

• Keeps immune cells from attacking our own body cells

Can be the reason that organ donations are rejected by the body

What are the two types of lipids that are used to help form the plasma membrane?

1. Phospholipids

2. Cholesterol

   • Associated with the fatty acid tails of the phospholipids

   • Helps the membrane retain fluidity

How does Cholesterol help the Plasma Membrane retain fluidity?

Prevents phospholipids from packing close together

Why is the cell membrane’s structure so important?

Cells must have the ability to exchange materials with the extracellular fluid

Where does material exchange occur in a cell?

How is this made possible?

Across the plasma membrane

Made possible via the plasma membrane’s structure, which allows for some substances to be transported easily while others require use of a transmembrane protein

Can all materials be freely and easily transported across the plasma membrane?

No; some require the use of a transmembrane protein

What happens when molecules that are always moving collide?

They generate energy and move from a higher state of energy to a lower state of energy

What are the three types of driving forces that cause solutes to move?

1. Chemical

2. Electrical

3. Electrochemical

What is the Chemical driving force in a cell based on?

The concentration / chemical gradient

What is a Concentration Gradient?

How do molecules behave according to the concentration gradient?

Concentration Gradient — difference in concentration of a substance across the cell membrane

Molecules will spread / diffuse from the greater conc. to the lesser conc. until they reach equilibrium

• Said to move “down” the concentration gradient

What is the Electrical driving force in a cell based on?

The electrical gradient

What is the electrical gradient?

Is it relevant to all solutes?

Electrical Gradient — difference in electrical charge across the cell membrane

No; only relevant to solutes that are charged

Under normal conditions, what is the charged state of the ECF and ICF?

ECF — net positive charge

ICF — net negative charge

What is included in the Electrochemical driving force?

The chemical and electrical gradients