Cell Membranes and Transport: Part 1, Chapter 6

What are the three things with which cells must constantly interact and communicate?

• With other cells that live in the same tissue or nearby tissues

• With the chemical and physical environment that surrounds the cell

• With the chemical and physical environment outside of the body (for multicellular organisms)

What is Intracellular Fluid?

The water-based chemical environment inside of a cell

What is Extracellular fluid?

The water-based chemical environment outside of a cell

What are the chemical properties of membrane/phospholipid bilayer?

Phospholipid Bilayer = Two layers of phospholipids

• Fatty acid tails clump together in the middle to separate from water (hydrophobic)

• Allows the head groups to face the water on either side of the bilayer (hydrophilic)

What is the definition of permeability?

The scientific term for a molecule’s ability to pass through a membrane

Which molecules are naturally permeable through the plasma membrane, and which are not?

Permeable: Non-polar molecules

Non-Permeable: Polar molecules + Electrolytes

What are the five different groups of membrane proteins? What life functions do they perform for the cell?

• Receptors → Receptor proteins bind to a signal in the ECF and then transmits a signal to the ICF

• Enzymes → Speeds up one or more chemical reactions

• Cell Identity Marker → Identifies a cell to other cells nearby

• Cell Adhesion Molecules → Proteins that attach to each other in order to stick cells together

• Transporters → Moves hydrophilic molecules into and out of the cell

What is diffusion and why do molecules diffuse?

Diffusion: The process where molecules move from areas of higher concentration to areas of lower concentration

• Happens because of the second law of thermodynamics: Systems tend to move toward a state of equilibrium

What is a concentration gradient?

The difference in the concentration of a solute in a solution between two regions.

How do molecules move in a concentration gradient?

Passive Transport → Traveling down a gradient, uses no energy

• Diffusion → Gases and small solutes can slip through membrane using no energy

• Facilitated Diffusion → Large/Polar molecules require transport proteins to cross the membrane

Active Transport → Moving up a gradient, uses ATP (ex. sodium-potassium pump)

How is osmosis a special case of diffusion for water?

It is a case where only the water (solvent) is moving across the membrane rather than the entire solution. Continues until concentration is equalized on both sides

How do the chemical properties of membrane/phospholipid bilayer allow it to function as a boundary between the cell and its environment?

The hydrophobic fatty layer core forms the boundary between the ICF and the ECF

• Hydrophilic molecules cannot move through the membrane’s hydrophobic core

Why are Receptor proteins needed in the Plasma Membrane?

Most chemical signals are hydrophilic, so the receptor protein needs to bind to the signal to tell the ICF to let it in (hydrophilic chemical signals cannot enter the membrane)

Why are Enzyme proteins needed in the Plasma Membrane?

Small molecules must be chemically modified or broken down before they enter or leave the cell, and enzymes increase the speed of those required chemical reactions

Why are Cell Identity Marker proteins needed in the Plasma Membrane?

• Some cells change their structure and function based on what kinds of cells they are surrounded by

• Immune cells use the markers to identify which cells are a part of the body and which aren’t

Why are Cell Adhesion Marker proteins needed in the Plasma Membrane?

• Most cells in multicellular organisms will stick together to form tissues, and those tissues stick together to form organs

• Cells that live at the surface of the body must stick together to create a closed barrier between the body and the environment

Why are Transporter proteins needed in the Plasma Membrane?

• Channel Proteins → Channels open a water-filled hole in the membrane that molecules can move through by diffusion

• Carrier Proteins → Bind to molecules and physically move them through the membrane

What life functions do Receptors and Enzymes perform for the cell?

• Receptors → Responsiveness

• Enzymes → Metabolism

Why can hydrophilic molecules not move through the plasma membrane?

The hydrophobic core of the membrane prevents the movement of hydrophilic molecules and electrolytes through the cell membrane