- 92 -


The Law

Belief - IV

Our Cellular Biology - III

We will revert back to the other constituents of cytoplasm. 

We have already understood that proteins are made from chains of smaller amino acid molecules and they serve structural and catalytic functions. Proteins like enzymes convert some cellular molecules like lipids, carbohydrates, nuceic acids into other forms that may help the cell meet its energy requirement. discharge wastes and build structures. Carbohydrate molecules store energy and provide energy for immediate demands. Lipids are involved in energy storage, as well as relaying signals within cells and from the blood streams into the cell's interior.

eukaryote cell

prokaryote cell

Cells are of two types - prokaryote and eukaryote. If the DNA within a cell is not separated from the cytoplasm, it is a prokaryote cell. The prokaryote cells live as a single cell. If the DNA is partitioned off, from the cytoplasm, in a membrane bound nucleus, such a cell is called eukaryote. The eukaryote cells can be both single cell or multiple cell entities.
Human beings, animals, plants are made from multiple cell eukaryote cells. 

An eukaryote cell has other membrane bound organelles within it, besides the membrane bound nucleus containing the DNA. A prokaryote cell has no other organelle other than the DNA.

We will look at the other organelles within the cytoplasm. Besides the nucleus, the other organelles are mitochondrin, chloroplasts, the endoplasmic reticulum, the Golgi apparatus, and lysosomes. Each of these organelles has a specific function critical to the survival of the cell. Most of the organelles are separated from the cytoplasm by their own individual membranes. The membranes of these organelles are also made of lipid bilayers, similar to but not identical to the cell membrane. This partitioning of the organelles permits different types of bio-chemical reactions to take place in different organelles. Although each organelle performs a specific function in a cell, all of the cell's  organelles work together like a team to meet the overall needs of the cell.

Of all the organelles, the nucleus is the most critical. This is where the cell's DNAs reside. Recall that the DNA contains the recipe for building proteins. The membrane that surrounds the nucleus - nuclear envelope - partitions the DNA from cell's protein synthesis machinery, which is located in the cytoplasm. Tiny pores in the nuclear envelope, called nuclear pores, selectively permit certain macro molecules to enter and leave the nucleus - including the RNA which carries the information from DNA to protein manufacturing machine in the cytoplasm.

The mitochondrion and the chloroplast are enclosed by the double membranes. Mitochondrion is an organelle found in large numbers in most cells, in which the biochemical processes of respiration and energy production occur. As mentioned, it has a double membrane. Mitochondrion uses oxygen to produce energy, which cells then utilise to drive many processes. Mitochondria are the energy factories of the body. Mitochondria are found in plants, fungi, animals, human bodies. Chloroplasts are found in plant cells and some algae. They convert solar energy into energy-storing sugars such as glucose. Chloroplasts also produce oxygen, which make them essential for all life.

Mitochondrion

Within eukaryote cells, mitochondria function somewhat like batteries, as they convert energy from one form to another. Accordingly cells with higher energy needs meet that requirement by increasing the number of mitochondria they contain. For example the muscle cells of people who exercise regularly possess more mitochondria than muscle cells of people who lead a sedentary life.

Note: Prokaryote cells, on the other hand, do not contain mitochondria. So they must rely on their immediate environment to obtain energy.

Cells need to manage a wide range of processes and functions in their tiny package - moving, growing, discharging waste, and so on. These function require energy. Humans make their energy from fuels and foods. Cells make their energy from food molecules and sunlight. Cellular nutrients come in many forms, including sugar and fats. In order to provide a cell with energy, these molecules have to pass across the cell membrane - which is not an easy, but not an impossible,  task as the membrane acts as a barrier. As discussed earlier, various proteins that are embedded in the membrane permit specific molecules into the cell, although they may require some energy input to accomplish the task. Complex organic food molecules such as sugars, fats, and proteins are rich sources of energy for cells because much of the energy used to form these molecules is literally stored within the chemical bonds that hold them together. Cells do not release the energy stored within their nutrient molecules at one go but do so through a series of oxidation reactions, thereby releasing the energy in steps. In this case the nutrient molecules donate electrons. During the oxidation process the nutrient molecules gradually lose their energy, some of which is stored in the acceptor molecule, which the acceptor molecule stores for

Note: Oxidation describes a type of chemical reaction in which electrons are transferred by a donor molecule to an acceptor molecule, changing the ccomposition and energy content of both the donor and acceptor molecules.


later use. Eventually when the carbon atoms in the nutrient molecules are completely oxidised, they are released as waste in the form of carbon dioxide.
Cells do not use the energy from oxidation reactions immediately. Instead they convert they convert the energy to small energy rich molecules (such as ATP). The energy stored in these energy molecules is used throughout the cell for meeting energy needs of all processes and function.  

Note: Adenosine triphosphate (ATP) is considered by biologists to be the energy currency of life. It is the high-energy molecule that stores the energy we need to do just about everything we do. It is present in the cytoplasm and nucleoplasm of every cell, and essentially all the physiological mechanisms that require energy for operation obtain it directly from the stored ATP. 

When energy is abundant, eukaryotic cells make larger, energy rich molecules to store their excess energy. The resulting sugars and fats - polysaccharides and lipids - are then held in reservoirs within the cells. Excess glucose is also converted to glycogen and stored in that form. A cell can rapidly mobilise Glycogen whenever it needs energy quickly. Under normal circumstances, human store sufficient glycogen to meet a day;s requirement. One gram of fat contains nearly six times that is contained in one gram of glycogen, but the energy from fat is less readily available than that from glycogen. Both forms are important as cells need both quick-fire energy and long term energy. Fats are stored in droplets in the cytoplasm. Humans generally store enough fats to supply their cells with several weeks' worth of energy.

Note: Glycogen is a polysaccharide that is formed from excess glucose in the body. Any excess glucose will be stored as glycogen in the liver and muscle cells for future use in the event that energy needs increase dramatically.

This conversion of glucose to glycogen is hormonally controlled. Specifically, insulin, which is released from the pancreas, will control the conversion of glucose into glycogen to lower blood sugar. The reverse process is also hormonally controlled. Whenever the body needs more sugar, glucagon, which is also produced in the pancreas, will control the conversion of stored glycogen into usable glucose for ATP needs.

Namaste

پھر ملیں گے

Prabir