chapter 7 (cell membrane structure and function)

What is the Fluid Mosaic Model?

The model describing the cell membrane structure (proposed by Singer and Nicolson, 1972), which is composed of a fluid phospholipid bilayer and a mosaic of various proteins

Why is the membrane described as a mosaic?

Because it is studded with a variety of proteins on the surface and embedded within the bilayer

Why is the membrane described as fluid?

Because the lipids move laterally and the structure behaves like a Soap Bubble due to the latest membrane flip-flopping cholesterol content and unsaturated phospholipids

What is the function of the cell membrane?

It is the chemically porous and semi-permeable border of the cell

What is the function of cell membrane proteins?

They are integral, include the transmembrane (proteins), and are peripheral; they carry out three types of functions

What is the cell membrane often thought of as the "brain" of the cell?

Because it is a semi-permeable barrier that decides what can pass in and out of the cell; it's the master regulator

How does the cell membrane know what to let in or out?

It uses the permeability of the lipid bilayer (small, non-polar molecules slip through easily) and the help of transport proteins

What part of the lipid bilayer is hydrophobic and what part is hydrophilic?

The interior is hydrophobic and the exterior (faces the water outside and inside the cell) is hydrophilic

How can the membrane expand or contract?

Through Endocytosis or Exocytosis

What are the two types of Transmembrane Proteins that permit the movement of substances?

Channel Proteins and Carrier Proteins

How do Channel Proteins work?

They are hydrophilic channels or pathways (like aquaporins for water) that can accommodate **polar

water-soluble** molecules; they allow passive transport (diffusion)

How do Carrier Proteins work?

They change shape and thus shuttle substances across the membrane; they may or may not require ATP (for active transport)

What is Facilitated Transport?

The process where both channel and carrier proteins facilitate movement across the membrane

What are the three mechanisms that drive substance movement across the cell membrane?

Diffusion, Active Transport and Electrical Charges

What is Diffusion?

A passive physical process where a substance moves down its own concentration gradient (from high to low concentration) until equilibrium is reached

How is diffusion studied in a lab?

Using a transport protein to carry a chemical across a membrane

What is Osmosis?

The diffusion of water; it is the movement of water across a semi-permeable membrane from an area of lower solute concentration to an area of higher solute concentration

What is the difference between an Isotonic, Hypotonic, and Hypertonic solution?

An Isotonic solution has an equal solute concentration compared to the cell; a Hypotonic solution has a lower solute concentration; a Hypertonic solution has a higher solute concentration

What happens to red blood cells (RBCs) in an Isotonic solution?

They have an equal water exchange and are in a stable state

What happens to RBCs in a Hypotonic solution?

The cells take up water and burst (this is called Hemolysis)

What happens to RBCs in a Hypertonic solution?

The cells shrivel and die (this is called Crenation)

What happens to plant cells in an Isotonic solution?

The cells are in a flaccid (limp) state with equal water exchange

What happens to plant cells in a Hypotonic solution?

The cells swell and become turgid (firm), which is the desired condition for plants

What happens to plant cells in a Hypertonic solution?

The cell pulls away from the cell wall and shrivels (this is called Plasmolysis); the cell eventually dies if this condition continues

What is Active Transport?

A process that requires energy in the form of ATP to move substances against the concentration gradient (from low to high concentration)

What is an example of active transport in animals?

The **Sodium-Potassium ($Na^+

K^+$) Pump**

How does the $Na^+

K^+$ Pump work?

It moves 3 $Na^+$ out of the cell and 2 $K^+$ into the cell; the pump is a carrier protein

What is the net result of the $Na^+

K^+$ Pump's action?

A net negative charge forms inside the cell membrane (3 positive charges out, 2 positive charges in) and a positive charge outside the membrane

What is Electrical Charge difference or gradient across the membrane?

It is measured as voltage and is the **electrical

membrane potential** that enables charged ions to move across the cell membrane

How can positive ions move in the body?

Positive ions like $Na^+$, $Ca$, $Mg$, and $H$ can move easily into the cell, while negative ions (like $Cl^-$) can move out of the cell

What is another name for the Electrical Charge gradient in the body?

It is often referred to as an Electrochemical Force or Gradient

What is an example of active transport in plants?

The Proton pump ($H^+$ pump)

How does the Proton pump work?

It pumps $H^+$ ions out of the cell; these accumulate outside of the cell, then when coupled with the **$H^+$

Sucrose Cotransporter**, they help to *store sucrose* in the plant cell during winter months

What is WaterFall analogy in the context of the proton pump?

The accumulation of $H^+$ ions outside the cell creates a potential energy gradient that can be harnessed to help with other processes, like sucrose transport

What are the three methods for Bulk Transport of larger substances across cell membranes?

Movement, Fluid Cell Membranes, Endocytosis, or Secretion

What is Endocytosis?

Movement of substances into the cell (e.g., phagocytosis or pinocytosis)

What is Exocytosis?

Secretion or movement of substances out of the cell (e.g., Insulin)

What are the three types of Endocytosis?

Phagocytosis (cellular eating), Pinocytosis (cellular drinking), and Receptor Mediated Endocytosis (involving ligands, targets like cholesterol, and receptor proteins)

What is an example of Receptor Mediated Endocytosis failure?

Hypercholesterolemia (results in a lack of $\text{LDL}$ receptors)