Module #3

Intro

Membranes are continuous, unbroken sheets, enclosing compartments

Structure allows fusion without losing continuity

• Cell (plasma) membrane → physically separates cells from external environment

• Nuclear envelope

• Mitochondrial membranes

• Chloroplast membranes

• Lysomal membrane

• Endoplasmic reticulum

In eukaryotic cells, the internal membranes define organelles

Membrane Functions

There are several functions of membranes including:

1. Comparetmentalizion → compartments have different contents and activities

2. Provide a selectively permeable barrier → prevent unrestricted exchange of molecules

3. Transporting solutes → exhange of moecules across the mebrane

4. g

5. Responding to external signals → signal transduction

6. Intercellular interaction

7. Energy transduction →

Cell Membrane Structure

Made of the following components:

1. Lipids

2. Proteins

3. Carbohydrates

Composed of two layers of lipids and the major type → phospholipids

Recall that phospholipids have both hydrophoic, nonpolar regions and hydrophilic, polar regions and are thus amphipathic

• Hydrophilic region → head group

  • Contains glycerol, a phosphate group, and a polar grouo

• Hydrophobic region → fatty acid tail

Phospholipid Arrangement

Phospholipids can arrange themselves spontaneously

• Arrangement depends on the size of the polar head groups

• If the head is large and bulky with one hydrophobic tail that is buried → form spheres call miclelles

• If they contain smaller heads and have two hydrophobic tails → form a bilayer with a closed structure & inner space

If phospholipid are added to a test tube of water, they spontaneously form enclosed bilayers called → liposomes

• Liposomes enclose an inner space → a bilayer make effective cell membranes

Because of the bilayer arrangement the membranes are self-healing → phospholipids spontaneously rearrange

• Due to the tendency of water to excluse nonpolar molecules

Membranes are Dynamic

Membranes are dynamic because they are contunally moving, forming and re-forming → membranes are said to be fluid

• Q: How are embranes fluid?

• A: Lipids can move laterally withing the membrane

  • Movement is affected by nature of the phospholipid tails

THe following features of lipids can affect fluidity:

1. Length of the fatty acid tails

   1. Longer = less fluid

2. Presence of C=C

   1. Fewer = Less Fluid

3. Cholesterol → inserts into the lipid bilayer of animal cell membranes

   1. Can increase or decrease fluidity depending on the temp.

   2. Acts like a buffer for fluidity

Lipid Rafts

Lipids, and other components such as proteins, may be found in discrete regions of the membrane → lipid rafts

• Lipid rafts are specialied microdomains that serve as platforms for various cell processes

Lipids move in the plane of the membrane

Rarely would they move between the layers of the bilayer

Membrane Proteins

1. Transporters that move ions and molecules

2. Recpetors that allow the cell to receive signals

3. Enzymes that catalyze cehmical reactions in the cell

4. Anchors that attach to other proteins

There are two types of membrane proteins

1. Integral → permanently associated with the membrane and can span the entire lipid bilaer (transmembrane)

2. Peripheral → termporarily associated with lipid bilayer or with integra proteins via noncovalent bods

3. Can be either cytoplamsic or extracellular

Membrane Carbs

All face outward to extracelular space

Covalently linked to either:

• Protein → glycoprotein

• Lipid → glycolipid

Fluid Mosaic Model

Core of membrane is the lipid bilayer

The lipid bilayer is a fluid structure that allows molecules to move laterally with the membrane

Contains a mosaic (mixture) of different types of molecules

Membrane Transport

The plasma membrane is selectively permeable

• SOme molecules are more permeable that others → gases (e.g. O2 & CO2) and nonpolar molecules (e.g. lipids) can move across the bilayer

• Small uncharged polar molecules (e.g. H2O) can move across the bilayer

• Macromolecules (e.g.) proteins & carbs) are too large to move across the membrane → need another way

• Movement of substanec into the cells → influx

• Movement of substances out of cells → efflux

• When one exceeds the other → net flux

Diffusion

Diffusion is a spontaneous process

Net movement of molecules from area of high concentration to are of low concentration

• Net movemnt stops when no concentration gradient → random motion in both directions

Passive Transport

Passive transport occurs across a membrane via diffusion → no energy required

• Driven by concentration gradient

Facilitated diffusion → molecules move down the concentration gradient trhough a protein transporter

Two types of membrane transporters:

1. Channel protein

   1. Channel formed by integral membrane proteins → surround aquous pore

   2. Permeable to specific molecules → selective

   3. Some exist in either open or closed confirmation → gated channels

2. Carrier protein

   1. Specific molecules bind to transporter & moves down concentration gradient

   2. Mechanism:

      1. Molecule binds to carrier protein on one side of membrane → this triggers

Osmosis

The diffusion of water across a semi-permeable membrane is known as osmosis

Due to concentration gradient of H2O → moves from a higher [H2O] to lower [H2O]

The osmotic pressure is key → important to consider the tonicity of the solution a cell is surrounded by

• If equal of either side of the membrane → isotonic

Hypertonic Solutions

If a solution is hypertonic relative to cell (higher [solute] than the cell) → water molecules will move out of the cell, throuhg the semipermeable membrane, and into the surrounding solution

As water moves out of the cell → the cell will shrink

• Animal cells will chrivel & die → crenation

• Plants cell will pull away

Hypotonic Solutions

If the solution is hypotonic (solution has less [solute] than the cell) the cell will absorb water

As water moves into the cell - the cell will swell

• Animal cells will swell & eventually burst

  • If red blood cell → hemolysis

• Plant cells will build pressure againt cell wall → turgid (turgor pressure)

Active Transport

Movement of substances against the concentration gradient → active tranport

• Active energy oupled transport → requires energy

• Uses protein carrier → active transport

• Substance

Primary Active Transport

We are going to examine the sodium-potassium pump

→ Na+/K+-ATPase

• Only present in animal cells → moves Na+ ions out of the cell and K+ ions into the cell

• Na+/K+ pump uses energy released from hydrolysis of ATP to move 3 Na+ ions and 2K+ ions against the [] gradient

  • The sodum and potassium ions move in opposite directions

  • Na+/K+-ATPase (is an antiporter)

Chloroplasts

Chloroplasts are where photosynthesis takes place

• Captures the sun’s energy to synthesize simple sugars for use in the plant cell

Surrounded by a double membrane and have an internal membrane-bound compartment → thylakoid

• The thylakoid membrane contains light-colecting molecules called pigments

  • Chlorophyll is the most common → produces green colour seen in many plants

Thylakoids

flattened membrane sacs within chloroplast

Space inside a thylakoids → lumen

Orderly stacks of thylakoids → grana

Space surrounding thylakoids → stroma

Origin of Mitochondria & Chloroplasts

Chloroplasts resemble cyanobacteria (a photosynthetic bacteria) in organization and biochemistry

• Originated as an endosymbiotic relationship between a cyanobaterial cell that had been engulfed by a eukaryotic cell

Mitochondia resemble free-living bacteria in organization and biochemistry

• Like chloroplasts, mitochondria originated as endosymbiotic bacteria, in this case, proteobacteria