CELL SURFACE AND THE EXTRACELLULAR MATRIX (1.3)

What are the functions and activities of cell membrane?

1. Cell Adhesion

2. Signal Transduction

3. Vacuole Formation

What is the extracellular environment of cell membrane?

1. Extracellular Matrix

2. Adhesion Molecules

3. Signaling Complexes

The outer boundary of the cell that separates it from the world

• is a thin fragile structure about 5-10 nm thick

Plasma membrane

• not detectable with light microscope need electron microscope

• All membranes examined closely (plasma, nuclear or cytoplasmic) from plants, animals or microorganisms have the same ultra structure

What are the functions of the plasma membrane?

1. Compartmentalization

2. Scaffold for biochemical activities

3. Selectively permeable barrier

4. Transporting solutes

5. Responding to external signals

6. Intercellular interaction

7. Energy transduction

Membranes form continuous sheets that enclose intracellular compartments (acid hydrolyses within the vacuole)

Compartmentalization

Membranes provide a framework that organizes enzymes for effective interaction (carbon fixation, cellular respiration)

Scaffold for biochemical activities

Membranes allow regulated exchange of substances between compartments (H2O)

Selectively permeable barrier

Membrane proteins facilitate the movement of substance between compartments (H+)

Transporting solutes

Membrane receptors transduce signals from outside the cell in response to specific ligands (Hormone)

Responding to external signals

Membranes mediate recognition and interaction between adjacent cells (plasmodesmata)

Intercellular interaction

• plasmodesmata are microscopic channels that connect the cytoplasm of adjacent plant cells, allowing the direct communication and the transport of molecules like water and nutrients. They act as bridges, creating a continuous network within the plant’s tissues

Membranes transduce photosynthetic energy, convert chemical energy to ATP, and store energy

Energy transduction

Why were membranes found to be mostly composed of lipids?

Membranes were found to be mostly composed of lipids because their dissolving power matched that of oil

• the lipid bilayer accounted for the 2:1 ratio of lipid to cell surface area

What is the most energetically favored orientation for polar head groups?

The most energetically favored orientation for polar head groups is facing the aqueous compartments outside of the bilayer

Lipid solubility isn’t everything hence what is it about lower surface tension and why does it relate to fluid mosaic model?

• surface tension of cell membranes was much lower than what would be expected from pure lipid structure. This suggested something else was on the surface which was protein.

• Fluid mosaic model: this model proposes that proteins are not just on the surface but also embedded within the fluid-like bilayer like a mosaic (for selective transport and cell signaling)

Protein is present in the form of?

• proteins, both as individual molecules and complexes, are found within a fluid lipid bilayer

• These proteins penetrate the bilayer and extend into the surrounding aqueous environment

What does lipid bilayer fluidity contribute too?

Due to lipid bilayer fluidity membranes are dynamic structures in which the components are mobile and capable of coming together for transient interactions

Membranes are lipid-protein assemblies held together by?

Noncovalent bonds

Is a structural backbone and barrier to prevent random movements of materials into and out of the cell

Lipid bilayer

• a unique complement of membrane proteins that contributes to the specialized activities of that cell type

Why does the ratio of lipid protein varies?

The ratio of lipid to protein varies depending on the:

• type of cellular membranes

• The type of organisms

• The type of cell

Describe some of the important roles of membranes in the life of a eukaryotic cell

• boundary and protection

• Transport

• Signaling

• Metabolism

What do you think might be the effect of a membrane that was incapable of performing one or another of these roles in the life of a eukaryotic cell

• loss of homeostasis

• Energy failure

• Communication breakdown

• has a very high ratio of protein/lipid

• Contains the protein carriers of the electron-transport chain, and relative to other membranes, lipid is diminished

Inner mitochondrial membrane

• Has a low ratio of protein/lipid

• Acts as electrical insulation for the nerve cell, best carried out by a thick lipid layer of high electrical resistance with a minimal content of protein

Myelin sheath

Membrane lipids are amphipathic with three main types:

• Phosphoglycerides

• Sphingolipids

• Cholesterol

Are diacylglycerides with small functional head groups linked to the glycerol backbone by phosphate ester bonds

Phosphoglycerides

Are ceramides formed by the attachment of sphingosine to fatty acids

Sphingolipids

Is a smaller and less amphipathic lipid that is only found in animals

Cholesterol

Lipids with a phosphate group are?

Phospholipids

Phospholipids built on a glycerol backbone are called?

Phosphoglycerides

Membrane glycerides are diglycerides;

Two hydroxyl groups of glycerol are esterified to fatty acids; the third is esterified to a hydrophilic phosphate group

Most Phosphoglycerides have a small hydrophilic group linked to phosphate:

Choline, ethanolamine, serine or inositol

• this group, together with the negatively charged phosphate, forms a highly water- soluble domain, called the head group

Membrane lipids: Phosphoglycerides: Fatty acrylic chains are?

Hydrophobic, unbranched hydrocarbons approximately 16 to 22 carbons in length

In phospholipids, a fatty acid may be fully saturated and?

• saturated, monounsaturated, or polyunsaturated

Phosphoglycerides often contain?

One unsaturated and one saturated fatty acyl chains

• with fatty acid chains at one end of the molecule and a polar head groups at the other end, all of the Phosphoglycerides exhibit a distinct amphipathic character

Are derivatives of sphingosine, an amino alcohol that contains a long hydrocarbon chain

Sphingolipids

What does Sphingolipids consist of?

Sphingolipids consist of sphingosine linked to fatty acid by its amino group, called a ceramide

In Sphingolipids, if the substitution is phosphorylcholine the molecule is?

Sphingomyelin

In Sphingolipids, if the substitution is carbohydrate, the molecule is a?

Glycolipid

In Sphingolipids, If the carbohydrate is a simple sugar, the glycolipid is a?

Cerebroside

In Sphingolipids, If the carbohydrate is a small cluster of sugars that includes sialic acid, the glycolipid is a?

Ganglioside

The nervous system is rich in?

Glycolipids

Myelin sheaths contain a high content of?

Galactocerebroside

What happens to humans who are unable to synthesize ganglioside?

• humans unable to synthesize ganglioside suffer from a serious neurological disease characterized by severe seizures and blindness

• Mice lacking the enzyme that makes this glycolipid exhibit severe muscular tremors and eventual severe muscular tremors and eventual paralysis

What are the roles of glycolipids?

• play a role in infectious diseases; toxins that cause cholera and botulism both enter their target cell by first binding to cell-surface gangliosides, as does the influenza virus

Is a smaller and less amphipathic lipid that is only found in animals

Cholesterol

What makes up to 50% of animal membrane lipids

A sterol

In cholesterol the -OH group is oriented?

Oriented toward membrane surface

In cholesterol the carbon rings are?

Are flat and rigid; interfere with movement of phospholipid fatty acid tails

How does cholesterol moderate the properties of the membrane?

Interacts with the fatty acids tails of phospholipids to moderate the properties of the membrane

1. Cholesterol functions to immobilize the outer surface of the membrane, reducing fluidity

2. ?

2. It makes the membrane less permeable to very small water-soluble molecules that would otherwise freely cross

3. It functions to separate phospholipid tails and so prevent crystallization of the membrane

4. It helps secure peripheral proteins by forming high density lipid rafts capable of anchoring the protein

Cholesterol

Campesterol

B- Sitosterol