Cell wall and Cell Membrane Structure and Function

Introduction–
Prokaryotic and Plant cells are surrounded by a strong cell wall. Paper, textiles, fibers (cotton, flax, hemp), charcoal, lumber, and other wood products are obtained from the cell wall.
Structure of Cell Wall: –
Cell wall contains 3 types of layers, middle lamella, primary wall and secondary wall. Sometimes also define tertiary cell wall.
1. Middle lamella:

• It is the first layer formed during cell division from phragmoplast. The outermost layer of the cell.
• It is shared by adjacent cells.
• It is composed of pectin and proteins.

2. Primary Wall:

• It is composed of pectin, cellulose, hemicellulose, and protein.
• It is formed after middle lamella
• All plant cells have a middle lamella and primary wall.

3. Secondary Wall:

• It is formed after cell growth stopped.
• It is extremely rigid and often layered.
• It is made of cellulose, hemicellulose, and lignin.

4. Tertiary  Wall:

•  It is the innermost layer of the cell wall it

Ultrastructure of cell wall: – 
Cell wall contains Fibrils and Matrix
(1) Fibrils-

• Approximately 3000 glucose molecules made a cellulose molecule.
• 100 cellulose molecule make a micelle.
• 20 micelle make a micro-fibril
• 250 micro-fibril make a macro-fibril of cellulose of 250 Ǻ diameter.
• Fibrils found transverse to an axis in primary wall and parallel in secondary wall.

(2) Matrix-

• It is basal part of the cell wall.
• It contains hemicellulose, pectin, glycoprotein, lipids, and water.
• It finds between spaces of fibrils.

Functions of Cell Wall: –

1. Maintaining and determining cell shape.
2. Prevents the cell membrane rupture due to water pressure (turgor pressure).
3. Provide mechanical support and allows the plants to grow tall.
4.  Biochemical activity in wall contributes to cell-cell communication.
5.  Provide mechanical protection from insects and pathogens.

Plasma membrane–
Structure of Plasma Membrane-

• The major role of lipids is structure and protein have a functional role.
• We can distinguish two cells by their protein content, not by lipid.

Following theories and model were given to explain the structure of plasma membrane-

• Bimolecular lipid leaflets model by Gortner & Grandel-

E. Gorter and F. Grendel (1925) Plasma membrane made micelle of phospholipid in which the polar hydrophilic groups of the molecules being situated on the outside and non-polar hydrophobic ends being situated toward inside. Each unit has 40 to 70 Ǻ diameter.

• Sandwich model by James Danille & Hugh Davson-

According to this hypothesis plasma membrane is made up of protein-lipid and protein like a sandwich. Protein layer is polar hydrophilic being situated outward and two layers of phospholipid present between layers of protein.

• Unit membrane model by Robertson-

The membrane of the cell and their organelles made up of the plasma membrane and similar in structure. Two layers of protein situated outward and inward with a thickness of 20-25Ǻ each. Lipid layer is present between protein layers have 30-35Ǻ thickness. Total thickness of membrane is 70-80Ǻ

• Fluid mosaic model by S. Jonathan Singer and Garth Nicolson-

This model presents cellular membrane as dynamic structures in which the membrane is made up of a bilayer of phospholipids. Extrinsic protein found outside of bilayer and intrinsic protein anchored in the bilayer.

THE CHEMICAL COMPOSITION OF MEMBRANE:

• The component of the plasma membrane is lipids (50%), proteins (50%) and carbohydrates (present or absent).

A) Membrane lipids:
Membrane lipids are an amphipathic molecule. If amphipathic molecule dissolves in water it forms either bilayer liposome or single layers micelles.
There are three types of membrane lipids-
I) Phosphoglyceride(phospholipid): 
Phosphoglyceride + choline- Phosphatidylcholine
Phosphoglyceride + Ethanolamine – Phosphatidylethanolamine
Phosphatidylethanolamine is major phospholipid in bacteria
Phosphoglyceride + Serine – Phosphatidylserine
When RBC complete their lifespan they destroyed in spleen only when Phosphatidyl serine binds to the outer surface of RBC that is a signal of cell apoptosis.
Phosphoglyceride + Inositol – Phosphatidylinositol
At physiologic pH, the head of Phosphatidylserine & Phosphatidylinositol have negative charge and Phosphatidylcholine & Phosphatidylethanolamine are neutral
II) Sphingolipids:

1.  Derivatives of sphingosine (an amino acid with long hydrocarbon chain)
2. When a fatty acid is attached in amide linkage to the ­­–NH2 on C-2 it’s called ceramide that is structural parents of all sphingolipids.
3. Sphingomyelins contain phosphocholine or phosphoethanolamine. It found insulated the axon of some neurons.
4. Cerebroside have a single sugar linked to ceramide.
5. Gangliosides have multiple sugar linked to ceramide.

III) Cholesterol:

1. All carbon of cholesterol comes from acetyl Co-A
2. Cholesterol provides flexibility, rigidity, and fluidity. Cholesterol acts as a temperature fluidity buffer.
3.  At high temperature it do not allow extensive movement of lipid molecules and at low temperature do not allow strong hydrophobic interaction.  All the biological fatty acids is membrane is in the cis double bond from.
4. Prokaryote and plant do not contains cholesterol in cell membrane present only in animals.
5. Plants contain stigmasterol, sitosterol, compasterol instead of cholesterol

Membrane Lipid Asymmetry-

• Phosphatidylserine (PS)-  net negative charge at physiologic pH, almost present inner leaflet, cytoplasmic, the appearance of PS on the outer surface of lymphocytes marks the cell for destruction by macrophages. Its appearance on the outer surface of platelets leads to blood coagulation.
• Phosphatidylcholine (PC)- neutral at physiologic pH found on both sides, esp. outer leaflet, exoplasmic
• Phosphatidylethanolamine (PE) neutral at physiologic pH, found on both sides but concentrated in the inner leaflet. Important in membrane budding and fusion.
• Phosphatidylinositol (PI) net negative charge at physiologic pH almost all cytoplasmic
• Sphingomyelin mostly found at outer leaflet,  exoplasmic
• Cholesterol equal on outer or inner  leaflets

LIPID RAFT
It is a cluster of sphingomyelin cholesterol and protein present in gram-negative bacteria and helps the cell locomotion and attachments.

B) Membrane Carbohydrate:

• Membrane carbohydrate is covalently linked to protein to form   glycoprotein and covalently linked to lipid to from glycolipids.
• The carbohydrate on plasma membrane of RBC determine blood group.

• The membrane of chloroplast contain glycolipids.
• Glycolipids are present on outside of membrane it helps in glycosylation
• There are two type of glycosylation O linked glycosylation occurs in Golgi complex and N-linked glycosylation occurs in Endoplasmic Reticulum.

C) Membrane Protein:

1. Integral Protein- penetrate lipid bilayer, transmembrane protein, contain a-helix or b-sheet structures, amphipathic in nature, example Glycophorin A
2. Peripheral Protein- located outside of lipid bilayer, either cytoplasmic, non-covalently bonded to polar head of lipid or an integral protein.

3. Lipid Anchored protein- located outside either extracellular or cytoplasmic. Covalently linked to lipid. LAP can be reversibly attached and detached from membrane.

Four different types of lipid anchoring motifs have been found-

1. Amide-linked myristoyl anchor
2. Thioester linked fatty acid anchor
3. Thioester linked prenyl anchor
4. Amide-linked glycosyl Phosphatidyl inositol anchor.

• Amide-linked myristoyl anchor- In myristoyl anchor 14 carbon chain myristic acid may be linked via an amide bond to the α amino group of N-terminal glycine residue of selected proteins. Its reaction is called N-myristoylation.
• Thioester linked fatty acyl anchor- Protein anchored to the membrane via fatty acyl thioester include G-protein coupled receptor surface glycoprotein of several viruses and transferrin receptor protein. Myristate, palmate, stearate and oleate can be thioester-linked to cysteine residues in proteins. G-protein coupled receptors are anchored to the lipid bilayer in this way.
• Glycosylphosphatidylinositol anchors (GPI anchored) – The carboxyl terminal amino acid of a protein with an ethanolamine group linked to an oligosaccharide. The oligosaccharide is linked to the inositol group of a phosphatidylinositol. Found only in eukaryotes  Ethanolamine, galactose, mannose, inositol are anchored or bound with glycerol diphosphate

Membrane study techniques-

1. Freeze fracturing
2. Fluorescence Recovery After photobleaching (FRAP)

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Reference –

Wikipedia
Cell Biology, Gerald Karp
Principle of Biochemistry, Lehninger
Plant Biochemistry, Hans-Waltet Heldt