Cell

Biochemistry

E. coli is a Gram-negative, facultatively anaerobic, rod-shaped bacterium of the genus Escherichia that is commonly found in

the lower intestine of warm-blooded organisms (endotherms)

•  

The defining membrane-bound structure that sets eukaryotic cells apart from prokaryotic cells is the nucleus, enclosed by the nuclear envelope, which contains the genetic material. Most eukaryotic cells also contain other membrane bound organelles such as 

mitochondria or the Golgi apparatus.

Hierarchy in cell structure

• Cell organelle to building blocks

Supramolecular complex

MacromoleculesMonomeric

Units

Chromosome DNA Nucleotides

Plasma membrane Protein Amino Acid

Cell Wall Cellulose Sugars

Fig: The Structural Hierarchy

Functions

• Cell wall:• Support (grow tall)• Protection• allows H2O, O2, CO2 to diffuse in & out of cell

• Cell membrane :• Support• Protection• Controls movement of materials in/out of cell• Barrier between cell and its environment (selective

barrier)• Maintains homeostasis

• Nucleus:• Controls cell activities• Contains the hereditary material of the cell

• Nucleolus:• Make ribosomes

• Endoplasmic Reticulum:• Carries materials through cell• Aids in making proteins

• Golgi apparatus :• Have a cis & trans face• Modify proteins made by the cells• Package & export proteins

• Mitochondria :• Breaks down sugar (glucose) molecules to

release energy• Site of aerobic cellular respiration

• Lysosomes :• Breaks down larger food molecules into smaller

molecules• Digests old cell parts

• Peroxisomes : • contain  enzymes involved in a variety of biochemical

pathways• contain the enzyme catalase, which decomposes hydrogen

peroxide either by converting it to water or by using it to oxidize another organic compound  

• involved in the catabolism of very long chain fatty acids, branched chain fatty acids, D-amino acids, and polyamines, and biosynthesis of plasmalogens, i.e. ether phospholipids critical for the normal function of mammalian brains and lungs.

• Cytoskeleton :• Strengthen cell & maintains the shape• Moves organelles within the cell

• Cytoplasm (Cytosol) :• Supports and protects cell organelles

Others

• Vacuole :• Store food, water, metabolic & toxic wastes• Store large amounts of food or sugars in plants

• Ribosomes :• Synthesizes proteins

• Nuclear membrane :• Controls movement of materials in/out of

nucleus

Differential centrifugation – to separate cell

organelles

Density centrifugation – separation of cell organelles according to density with highest density at the bottom

• Biological Membrane

The cell membrane is selectively permeable and able to regulate what enters and exits the cell, thus facilitating the 

transport of materials needed for survival.

Composition :

• Lipids • The cell membrane consists of 3 classes of amphipathic

 lipids: phospholipids, glycolipids, and sterols. • Carbohydrates• Plasma membranes also contain carbohydrates,

predominantly glycoproteins, but with some glycolipids (cerebrosides and gangliosides). 

• Proteins• The cell membrane has large content of proteins, typically

around 50% of membrane volume.• Integral proteins, Peripheral proteins and lipid anchored

proteins

Fluid Mosaic Model

•  The plasma membrane that surrounds these cells has two layers (a bilayer) of phospholipids.

• The Fluid Mosaic Model states that membranes are composed of a Phospholipid Bilayer with various protein molecules floating around within it. The 'Fluid' part represents how some parts of the membrane can move around freely, if they are not attached to other parts of the cell. The 'mosaic' part illustrates the 'patchwork' of proteins that is found in the Phospholipid Bilayer.

Modes of Movements of Molecules through membranes

• Diffusion• Diffusion refers to the process by which molecules

intermingle as a result of their kinetic energy of random motion (Simple and facilitated )

• Mediated transports• Mediated transport refers to transport mediated by a 

membrane transport protein. The types are:• Facilitated diffusion and active transport (There are 2

types of active transport: primary and secondary-uniport, symport, and antiport )

• Energetic transport systems : Active transport

Ionophore• An ionophore is a lipid-soluble molecule usually synthesized by 

microorganisms to transport ions across the lipid bilayer of the cell membrane.

• Two types are:• Carrier ionophore - bind to a particular ion, shielding its charge from the surrounding

environment, and thus facilitating its crossing of the hydrophobic interior of the lipid membrane

• Channel ionophore - introduce a hydrophilic pore into the membrane, allowing ions to

pass through while avoiding contact with the membrane'shydrophobic interior

eg : Ionomycin (Ca2+) Salinomycin (K+) Monensin (Na+, H+)

Diseases due to abnormality of membrane fluidity and loss of membrane transport

• Spur cell anaemia • Spur cells, or acanthocytes:• are large erythrocytes covered with spike like projections that vary in

width, length, and distribution  • encountered in acquired or inherited disorders • characterized by diminished deformability, which is responsible for their

entrapment and destruction in the spleen. • Seen in severe liver damage (alcoholic liver cirrhosis) or associated

with acanthocytosis (associated with neuromuscular disorder) • Most acanthocytic disorders are associated with acquired abnormalities

of the outer leaflet of the lipid bilayer. However, some rare conditions have normal lipids and abnormal membrane proteins

• In severe liver disease, free cholesterol in red blood cells equilibrates with abnormal lipoproteins containing a high free cholesterol-to-phospholipid ratio, resulting in the preferential expansion of the outer leaflet and the development of the spur cell shape

• Result : alteration of the red blood cell shape and a decrease of the cell’s fluidity.

Loss of glucose transport from intestine

• Na-K pump : primary active transport (cell and blood)

• Na-Glucose Co-transport : secondary active transport (lumen to cell)

• Facilitated diffusion : to blood

• Conditions :• Diarrhoea, Drugs like used in type 2 diabetes

that inhibits co-transport or diseased condition

Fructose malabsorption

• Fructose malabsorption, formerly named "dietary fructose intolerance" (DFI), is a digestive disorder[1] in which absorption of fructose is impaired by deficient fructose carriers in the small intestine's enterocytes. This results in an increased concentration of fructose in the entire intestine.

• Fructose is naturally present in fruits, some vegetables and honey. It is also used as a sweetener in some soft drinks and fruit drinks. When the body doesn’t absorb fructose properly, it is called fructose malabsorption or fructose intolerance. This is a common condition and different from hereditary fructose intolerance, which is a rare, genetic and sometimes fatal disorder affecting the liver.

• Causes of Fructose Intolerance• Fructose malabsorption occurs when the body is

not able to break down fructose during the digestive process. When undigested fructose reaches the intestines, it reacts with naturally occurring bacteria and generates carbon dioxide and hydrogen gases, which can cause bloating, abdominal pain, HEARTBURN , diarrhea and gas.

Hartnup Disease