(2) Membrane Structure and Function

Plasma membrane

Semi-permeable outer cell membrane made of a phospholipid bilayer with embedded proteins.

Bilayer of Phospholipids

A structure featuring hydrophilic heads and hydrophobic tails oriented towards aqueous environments.

Fluid Mosaic Model

• Plasma membrane is made up of a bilayer of phospholipids with embedded proteins, carbohydrates, glycolipids, and glycoproteins, and, in animal cells, cholesterol.

Passive Transport

Movement across a membrane that does not require energy, relying on concentration gradients.

Active Transport

Movement of molecules against a concentration gradient, requiring energy such as ATP.

Simple Diffusion

Under passive transport:
Movement of molecules from high to low concentration without the need for energy.

Facilitated Diffusion

Under passive transport:

Passive movement of molecules across the cell membrane aided by transport proteins.

Sodium-Potassium Pump

A vital protein that exchanges sodium (Na+) ions out of the cell for potassium (K+) ions into the cell, utilizing ATP.

Endocytosis

The active transport process of large particles into a cell.

Exocytosis

The process of expelling materials from the cell into the extracellular fluid.

Osmosis

Under passive transport:

The diffusion of water across a semi-permeable membrane from low to high concentration of solute.

Tonicity

A measure of the ability of an extracellular solution to influence the movement of water into or out of a cell.

Isotonic

Extracellular fluid that has the same concentration of solutes as the cell interior.

Hypotonic

Extracellular fluid with lower solute concentration than the cell interior, causing water influx and potentially cell swelling.

Hypertonic

Extracellular fluid with a higher solute concentration than the cell interior, causing water to exit the cell and potentially cell shrinkage.

Phagocytosis

A type of endocytosis where a cell takes in large particles.

Pinocytosis

A form of endocytosis where small molecules dissolved in extracellular fluid are absorbed by the cell.

Receptor-mediated Endocytosis

Endocytosis that uses receptor proteins to selectively take in specific substances.

Purposes of plasma membrane

• Transport of molecules into and out of the cell.

• Facilitate enzymatic activity.

• Signal transduction between cells.

• Cell to cell recognition and communication.

• Intercellular joining.

• Serving as an attachment point for the cytoskeleton and extracellular matrix.

components that make up plasma membrane

phospholipids, cholesterol, membrane carbohydrates, and membrane proteins

What makes the membrane less fluid?

when phospholipids are saturated and/or long

What makes the membrane less fluid?

when phospholipids are unsaturated and/or short

What cells only have cholesterol

animal cells

Integral Proteins

serve as channels or pumps to move materials into or out of the cell.

Peripheral Proteins

• on the exterior or interior surfaces of membranes, attached either to integral proteins or to phospholipid molecules.

  • Provide structural support, facilitate intracellular signaling, and carry out enzymatic activities

Lipid-anchored proteins

• proteins that are covalently attached to lipid molecules embedded within biological membranes

  • carry out essential functions such as signal transduction, cell recognition, and enzymatic activity

What do carbohydrates do?

they bound either to proteins (forming glycoproteins) or to lipids (forming glycolipids).

glycoproteins

mediate cell-to-cell recognition, cell adhesion, and signaling

glycolipids

act as surface markers for cell-to-cell recognition, help stabilize the cell membrane, and serve as receptors for signaling molecules

Cholesterol

• keeps cell membrane stable by preventing it from becoming either too rigid or too fluid as temperatures change

• It breaks up hydrophobic interactions between fatty acids

What does cholesterol do at high temp?

decreases membrane fluidity

What does cholesterol do at low temp?

increases membrane fluidity

What must exist for cells to utilize passive transport?

an existing gradient

What is required for active transport?

ATP and a protein pump

Osmosis

movement of water across cell membrane from low solute concentration to high solute concentration

Tonicity

focused on the solutes that direct the flow of water across the membrane

isotonic

extracellular fluid has the same concentration of non-permeating solutes as the cell interior

Hypotonic

extracellular fluid has a lower concentration of non-permeating solutes than the cell interior

effect of being hypotonic

Water rushes into the cell.

Animal cells can swell and burst, while plant cells build up turgor pressure

Hypertonic

extracellular fluid has a higher concentration of non-permeating solutes than the cell interior.

effect of being hypertonic

Water flows out of the cell. Animal cells shrink

Carrier proteins

Under passive transport - Facilitated Diffusion:

Transmembrane proteins which bind to a solute and undergo a change in shape to translocate the solute across the membrane

Channel proteins

Under passive transport - Facilitated Diffusion:

Transmembrane proteins which form a channel in the membrane to allow rapid diffusion of particles.  

Gated-ion channels

Under passive transport - Facilitated Diffusion:

allow for the flow of ions to occur through the channel. 

Ligand-Gated Ion Channels

Under passive transport - Facilitated Diffusion - Gated-ion channels:

use small molecules to trigger the channel to open and allow for ions to pass.

Voltage-Gated Ion Channels

Under passive transport - Facilitated Diffusion - Gated-ion channels:

open in response to a change in charge across the membrane.

Mechanically Gated Ion Channels

Under passive transport - Facilitated Diffusion - Gated-ion channels:

open in response to a physical stimulus.

Light-Gated Ion Channels

Under passive transport - Facilitated Diffusion - Gated-ion channels:

open in response to light wavelengths

Sodium-Potassium Pump

Under active transport:

protein that exchanges 3 Na⁺ ions (moves out of the cell) with two K⁺ ions (moves into the cell).

Protein Pump (specific definition, not the general one)

Under active transport:

proteins that generate a proton gradient (H⁺) across a cell membrane.  A proton gradient is also known as a pH difference.

Secondary Active Transport

Under active transport:

a cellular process that moves a molecule against its electrochemical gradient by coupling its movement to the downhill flow of an ion

Endocytosis

active transport of large particles into a cell

Phagocytosis

under endocytosis:

cell takes in large particles

pinocytosis

under endocytosis:

small molecules dissolved in extracellular fluid are brought into the cell

Receptor-mediated Endocytosis

under endocytosis:

employs receptor proteins in the plasma membrane that have a specific binding affinity for certain substances

Exocytosis

expels material from the cell into the extracellular fluid.

What does increased temperature do in simple diffusion?

diffuses particles much faster

What does decreased temperature do in simple diffusion?

diffuses particles much slower