Chapter 7 - Membrane Structure and Function

What are membrane molecules held in by?

Weak hydrophobic structures

What is the plasma membrane?

Boundary that separates the living cell from its surroundings

What is the property of the plasma membrane that controls what substances go in and out?

Selective permeability

What is the most abundant lipid in the plasma membrane?

Phospholipids

What does it mean when a molecule is emphipathic?

It has both hydrophobic and hydrophilic properties

What is the fluid mosaic model?

Describes the structure and behavior of the cell membrane as a flexible, two-dimensional liquid-like layer as a patchwork of different molecules

What is the freeze fracture technique?

Splits a membrane along the middle of the phospholipid bilayer

How do the lipids and proteins in the membrane move around?

Drift laterally in the plane of the membrane, RARELY flip flop from one layer to the other

Are lateral movements of phospholipids fast or slow?

Rapid

What is membrane fluidity influenced by?

The components of the membrane

Why are membranes rich in unsaturated fatty acids more fluid than those rich in saturated fatty acids?

Unsaturated fatty acids have more carbon-carbon double bonds, creating structural kinks in their hydrocarbon tails

What effect does that steroid cholesterol have on membrane fluidity in warm temperatures?

At warm temperatures, cholesterol restrains movement of phospholipids

What effect does that steroid cholesterol have on membrane fluidity in cool temperatures?

At cool temperatures, it maintains fluidity by preventing tight packing

The temperature at which a membrane solidifies depends on…

The types of lipids the membrane contains

What do membrane proteins determine?

Most of the membrane’s specific functions

What are integral proteins

Membrane protein that penetrates the hydrophobic core

• Consists of one or more stretches of nonpolar amino acids, often coiled into alpha helices

• amphipathic

C-terminus vs. N-terminus

N-terminus on the outside of the protein, C-terminus on the inside of the protein

What are peripheral proteins?

Membrane proteins that are bound to the surface of the membrane

What are transmembrane proteins?

A type of integral protein that spans the entire membrane

What are some of the functions of membrane proteins?

• Transport molecules in/out of cells

• Enzymatic activity (speeds up reactions)

• Signal transduction, relaying messages

• Cell-cell recognition

• Forms gap or tight junctions

• Attachment to the cytoskeleton and extracellular matrix

• Maintaining cell shape

• Stabilizing the location of certain membrane proteins

What is cell-cell recognition?

Cells recognize each other by binding to surface molecules, often containing carbs on the extracellular side of the plasma membrane

Why is cell-cell recognition important?

• Sorts and organizes cells into tissues and organs during development

• Basis for rejection of foreign cells by the immune system

Glycolipids

Membrane carbohydrates covalently bonded to lipids

Glycoproteins

Membrane carbohydrates covalently bonded to proteins

How is the asymmetrical distribution of proteins, lipids, and associated carbohydrates determined in the plasma membrane?

When the membrane is built by the ER and Golgi apparatus

What kind of molecules can easily pass through the cell membrane? And why?

Hydrohpobic, or nonpolar molecules

• Because its interior is made of nonpolar (hydrophobic) lipid tails

What are transport proteins?

Amphipathic, they allow the passage of hydrophilic substances across the membrane

Aquaporins

Channel pretein that facilitate the passage of water through the plasma membrae

Channel proteins

A hydrophilic channel that certain molecules or ions can use as a tunnel to get through the plasma membrane

What is diffusion

The tendency for molecules to spread out evenly into the available space

• Substances diffuse down their concentration gradient

Passive transport

The movement of molecules or ions across a cell membrane from an area of high concentration to an area of low concentration (down a concentration gradient) without using energy (ATP).

What is osmosis

Diffusion of water across a selectively permeable membrane

How does water diffuse across a membrane

Moves from the region of lower solute concentration to the region of higher solute concentration until the solute concentration is equal on both sides because there is more water in the lower concentration side and it wants to go to the higher concentration side to even the amount of water and solute out

What is tonicity

Ability of a surrounding solution to cause a cell to gain or lose water

What is an isotonic solution?

Solute concentration is the same as that inside the cell, no net movement of water movement across the plasma membrane

What is a hypertonic solution?

Solute concentration is less inside the cell than outside, cell loses water as it is moving outside, this causes the cell to shrink

What is a hypotonic solution?

Solute concentration is more inside the cell than outside, water is moving into cell from the outside and cell is swelling up

What does it mean when a cell is turgid?

In a hypotonic solution, a plant cell that is swollen and firm because it has absorbed water

What does it mean when a cell is flaccid?

In an isotonic solution, a plant cell that lacks internal water pressure, causing the cell membrane to pull away slightly from the rigid cell wall and the plant to droop

What does it mean when a cell turns plasmolysis?

In a hypertonic solution, a plant cell loses water and shrinks, membrane pulls away from the wall

What is osmoregulation?

The control of solute concentrations and water balance

What does the contractile vacuole do for the protist paramecium?

Because it is hypertonic to its pond water environment, it acts as a specialized water pump to control internal fluid levels and prevent the cell from bursting

What is facilitated diffusion?

Transport proteins speed the passive movement of molecules across the plasma membrane

What are ion channels?

Ion activates the channel, open or close in response to a stimulus (gated channels)

Two types of channel proteins

Ion channels and aquaporins

What are carrier proteins?

Binds to specific molecules and changes shape to move them across the cell barrier

What is active transport?

• Moves substances against their concentration gradient, requires energy

• Performed by specific proteins

• Allows cells to maintain concentration gradients

• Sodium-potassium pump

What is the sodium potassium pump?

3 sodiums and pushes it outside the cell, brings 2 potassium from outside the cell and brings it inside the cell

What is the main electrogenic pump of plants, fungi, and bacteria?

A proton pump

What are electrogenic pumps?

A transport protein that generates voltage across a membrane and helps store energy that can be used for cellular work

What is cotransport?

Occurs when active transport of a solute indirectly drives transport of other solutes

What is bulk transport?

Small molecules and water enter or leave the cell through the lipid bilayer via transport proteins, and large molecules cross the membrane in bulk via vesicles

• Requires energy

• Occurs by exocytosis and endocytosis

What is exocytosis?

Transport vesicles migrate to the membrane, fuse with it, and release their contents

• Used for exporting cellular products

What is endocytosis?

When the cell takes in macromolecules by forming vesicles from the plasma membrane → three types: phagocytosis, pinocytosis, receptor-mediated endocytosis

What is phagocytosis?

When a cell engulfs a particle in a vacuole, which then fuses with a lysosome to digest the particle

What is pinocytosis?

When molecules are taken up when extracellular fluid is “gulped” into tiny vesicles

What is receptor-mediated endocytosis?

Binding of ligands to receptors triggers vesicle formation

What is a ligand?

Any molecule that binds specifically to a receptor site of another molecule

What is membrane potential?

The voltage difference across a membrane

How is voltage created?

By differences in the distribution of positive and negative ions across a membrane

What is the electrochemical gradient?

Drives the diffusion of ions across a membrane (Uses two combined forces)

What are the two forces that the electrochemical gradient uses?

• Chemical force, the ions concentration gradient

• Electrical force, the effect of the membrane potential on the ion’s movement