Cell Membrane IB Bio 1 (Quizlet Import)

Phospholipids

They are a class of lipids that are a major component of all cell membranes. They're synthesized by the Smooth ER.

The Phospholipid Bilayer

It's fluid/flexible, which allows for the formation of vesicles and materials taken in by endocytosis and released by exocytosis

Hydrophilic

Water loving; the phosphate-glycerol head of a phospholipid attracts water molecules

Hydrophobic

Water fearing; the fatty acid tails repel water molecules

Amphipathic

A single molecule that contains both hydrophobic and hydrophilic parts

Why phospholipids form bilayers in water

It's because the hydrophilic head associates with the water inside/outside the cell and the hydrophobic tails face each other to stay away from the water.

Evidence from Electron Microscopy that Led to the Davson-Danielli Model

Charles Ernest Overton (1899): biological membranes are made of lipids and semi-permeable

Evert Gorter and Francois Grendel (1925): lipid bilayer made of phospholipids with hydrophilic heads and hydrophobic tails

Electron Microscope (1930's): greater understanding of cells made it possible to see the structures

Observations and Conclusions drawn by Davson and Danielli

The cell membrane includes globular protein molecules attached to each side of the lipid bilayer

Freeze-etched Electron Micrograph Images

Involves the rapid freezing of cells, then breaking them at the weakest points. There are globular structures scattered in the membrane called integral proteins.

Improvements in Techniques

Proteins were found to be: varied in size, and hydrophobic on at least part of their surface

Cell Fusion Experiments

Fusing two cells tagged with different fluorescent proteins and scientists watched as the two populations mixed

The Sandwich Model (Davson-Danielli)

1. The cell membrane proteins are all peripheral to the lipid bilayer

2. The cell membrane proteins are hydrophilic and relatively uniform in shape

3. The cell membrane proteins form a rigid, non-moving layer on either side of the lipid bilayer

The Fluid Mosaic Model (Singer-Nicolson)

1. The cell membrane proteins can span through the lipid bilayer

2. Membrane proteins aren't uniform in shape and some are hydrophobic

3. Membrane proteins can move within the bilayer; they're not locked in place

Osmosis

Movement of water by diffusion across a membrane without using energy

Isotonic

Equal amounts of the solution within and outside the cell. No net movement

Hypotonic

The solution has more water than the cell, so the water moves into the cell

Hypertonic

The solution has more solutes than the cell, so water leaves the cell

Water always moves from...

...a hypotonic to a hypertonic solution

Facilitated Diffusion

Same as regular diffusion, but the molecules move through a channel protein in the membrane for small ions.

Active Transport

Movement of substances from areas of low concentration to high concentration through protein pumps, "against the gradient". It requires energy.

Steps of a Protein Pump

1. Molecule enters the pump

2. Energy (ATP) used to change the shape of the protein

3. Molecule passes to the opposite side of the membrane

It works like a clothespin

Bulk Transport

Transport of many molecules into (endo) or out of (exo) the cell at once

Endocytosis

Process where cells take in molecules or smaller cells by engulfing them

Ex: White blood cells engulf bacteria during an infection

Exocytosis

Contents of secretory vesicles go into the extracellular space

Ex: Secretion of saliva from salivary glands in the mouth

Integral Proteins

1. Embedded

2. Span entire phospholipid bilayer

3. Hydrophobic amino acids

Peripheral Proteins

Loosely attached to the outside of the membrane

Cholesterol

It's a lipid. Its structure consists of: a hydroxl group linked at one end, four linked hydrocarbon rings, a hydrocarbon tail linked at the other end

Cholesterol is Amphipathic

It has a hydrophilic hydroxl group. It has hydrophobic hydrocarbon rings and tail.

Placement of Cholesterol

Polar hydroxl group puts the hydrogen bonds with the phospholipid heads. Nonpolar hydrocarbon rings and tail are put near the hydrocarbon chains of the phospholipid tails.

Placement of Cholesterol pt. 2

The placement allows the hydrophilic hydroxl group to be exposed to water. The hydrocarbon rings and tail are in the hydrophobic core of the membrane

Viscosity

Really thick, difficult to permeate

Cell Membrane at Low Temperatures

It has a high viscosity, it's densely packed, rigid (might break), and is not permeable.

Cell Membrane at High Temperatures

It has lower viscosity, it's less densely packed, won't hold shape, and is too permeable.