The Cell and its Function
Eukaryotic cells are about 15 times wider than a typical prokaryote and can be as much as 1000 times greater in volume.
Organisation of the Cell.
The nucleus and the cytoplasm are separated by the nuclear membrane, The different substances that make up the cell are collectively called the protoplasm.
The protoplasm is composed of: Water: 70 to 85%
Electrolytes Na,K, Cl, HCO3, Ca, Mg, PO4
Proteins - 10 to 20% - Structural proteins and Globular proteins (enzymes)
Lipids - 2% - Phospholipids, cholesterol, triglycerides, and neutral fats.
Carbohydrates - 1% total body, but 3% of Muscles and 6% of Liver cells. Stored as glycogen.
Physical Structure of the Cell
Highly organised - organelles = cell membrane, nuclear membrane, endoplasmic reticulum (ER), Golgi Apperatus, mitochondria, lysosomes and centrioles.
The cell and its organelles are surrounded by membranes composed of lipid and proteins. Cell membrane, nuclear membrane, membrane of the ER, mitochondria, lysosomes, and Golgi Apparatus. Protien molecules penetrate the membrane providing pathways to allow movement through the membrane.
The cell membrane is a lipid bilayer with inserted proteins. The lipid layer is composed almost entirely of phospholipids (hydrophillic and hydrophobic) and cholesterol. The lipid bilayer is highly permeable to lipid soluble substances, such as oxygen, carbon dioxide and alcohol, BUT ACTS AS A BARRIER TO water soluble substances such as ions and glucose.
There are two types of membrane proteins:
the integral proteins that protrude through the membrane; provide structural channels (pores) through which water soluble ions can diffuse. Carrier proteins transport substances - sometimes against the natural gradient for diffusion. The peripheral proteins attached to an integral protein may act as an enzyme to catalyse a reaction.
and
the Peripheral Proteins that attach to the inner surface of the membrane and do not penetrate.
Membrane carbohydrates occur in combination with proteins and lipids in the form of glycoproteins and glycolipids. The glyco- part protrudes outside the cell. Proteoglycans - carbohydrate bound by protein cores, form the cell glycocalyx.
The carbohydrates on the outersurface of the cell have multiple functions:
- Negatively charged - repel other negatively charged molecules
- The glycocalyx of cells may attach to other cells, thus the cells attach to each other
- CH's act as receptors for binding hormones
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| Proteoglycans |
Lipids are made in the ER wall. Proteins are manufactured in the ribosomes on the outersurface of the ER - mRNA to synthesize proteins that then enter the Golgi apparatus for further modification.
The Golgi apparatus functions in association with the ER similar membranes to smooth ER, prominent in secretory cells. Small transport ER vesicles pinch off from the ER and then fuse with the Golgi apparatus. The substances are then processed in the Golgi apparatus to form lysosomes, secretory vesicles, and other cytoplasmic components.
Lysosomes provide intracellular digestive system. Digest food, damaged cells and bacteria. The membrane surrounding the lysosomes prevent the digestive enzymes from damaging the cell components. When the lysosome membrane is damaged - the cell organelles that come into contact with the lysosome are split especially the highly diffusible amino acids and glucose.
Mitochondria release energy in the cell. Energy is provided from the chemical reaction of oxygen with the 3 different types of food.: Glucose derived from carbohydrates, fatty acid derived from fats, and amino acids derived from proteins. After entering the cell, food is split into smaller molecules that, in turn enter the mitochondria, where other enzymes remove the carbon dioxide and the hydrogen ions in a process called the citric acid cycle.
An oxidative enzyme system, which is also in the mitochondria, causes progressive oxidation of hydrogen atoms. End products water and CO2. The energy liberated is used to generate ATP.
Mitochondria are self replicating and can form as many mitochondria as required when more ATP is required.
An oxidative enzyme system, which is also in the mitochondria, causes progressive oxidation of hydrogen atoms. End products water and CO2. The energy liberated is used to generate ATP.
Mitochondria are self replicating and can form as many mitochondria as required when more ATP is required.
A simplified view of the process
- The citric acid cycle begins with the transfer of a two-carbon acetyl group from acetyl-CoA to the four-carbon acceptor compound (oxaloacetate) to form a six-carbon compound (citrate).
- The citrate then goes through a series of chemical transformations, losing two carboxyl groups as CO2. The carbons lost as CO2 originate from what was oxaloacetate, not directly from acetyl-CoA. The carbons donated by acetyl-CoA become part of the oxaloacetate carbon backbone after the first turn of the citric acid cycle. Loss of the acetyl-CoA-donated carbons as CO2 requires several turns of the citric acid cycle. However, because of the role of the citric acid cycle in anabolism, they may not be lost, since many TCA cycle intermediates are also used as precursors for the biosynthesis of other molecules.[3]
- Most of the energy made available by the oxidative steps of the cycle is transferred as energy-rich electrons to NAD+, forming NADH. For each acetyl group that enters the citric acid cycle, three molecules of NADH are produced.
- Electrons are also transferred to the electron acceptor Q, forming QH2.
- At the end of each cycle, the four-carbon oxaloacetate has been regenerated, and the cycle continues.
Major metabolic pathways converging on the TCA cycle
Several catabolic pathways converge on the TCA cycle. Reactions that form intermediates of the TCA cycle in order to replenish them (especially during the scarcity of the intermediates) are called anaplerotic reactions.
The citric acid cycle is the third step in carbohydrate catabolism (the breakdown of sugars). Glycolysis breaks glucose (a six-carbon-molecule) down into pyruvate (a three-carbon molecule). In eukaryotes, pyruvate moves into the mitochondria. It is converted into acetyl-CoA by decarboxylation and enters the citric acid cycle.
In protein catabolism, proteins are broken down by proteases into their constituent amino acids. The carbon backbone of these amino acids can become a source of energy by being converted to acetyl-CoA and entering into the citric acid cycle.
In fat catabolism, triglycerides are hydrolyzed to break them into fatty acids and glycerol. In the liver the glycerol can be converted into glucose via dihydroxyacetone phosphate and glyceraldehyde-3-phosphate by way of gluconeogenesis. In many tissues, especially heart tissue, fatty acids are broken down through a process known as beta oxidation, which results in acetyl-CoA, which can be used in the citric acid cycle. Beta oxidation of fatty acids with an odd number of methylene groups produces propionyl CoA, which is then converted into succinyl-CoA and fed into the citric acid cycle.[12]
The total energy gained from the complete breakdown of one molecule of glucose by glycolysis, the citric acid cycle, and oxidative phosphorylation equals about 30 ATP molecules, in eukaryotes. The citric acid cycle is called an amphibolic pathway because it participates in both catabolism and anabolism.
There are many cytoplasmic structures and organelles Tubular structures called microtubules transport substances from one part of the cell to the other.
Cells secrete special substances, like digestive enzymes. Formed by the ER-Golgi apparatus, and are released through secretory vesicles. The secretory vesicles store the enzymes, and are expelled by the cell membrane.
Cells secrete special substances, like digestive enzymes. Formed by the ER-Golgi apparatus, and are released through secretory vesicles. The secretory vesicles store the enzymes, and are expelled by the cell membrane.
The Nucleus acts as a control centre of the cell and contains large amounts of DNA, also called genes. They control reproduction and determine the proteins of the cells. Reproduction - process of mitosis in which 2 daughter cells are formed, each received one of the two sets of genes.
The nuclear membrane is actually two sets of membranes:
The outer membrane is continuous with the ER, and
the space in between the two nuclear membranes is also continuous with the compartment in the ER.
Both layer are penetrated by thousands of nuclear pores, about 100nm in diameter.
The nucleoli - does not have a membrane and contain large amounts of RNA - and proteins like those of the ER. The nucleoli become enlarged when the cell is synthesizing proteins. Ribosomal RNA is stored in the nucleolus and transported through the nuclear membrane pores to the cytoplasm.
The outer membrane is continuous with the ER, and
the space in between the two nuclear membranes is also continuous with the compartment in the ER.
Both layer are penetrated by thousands of nuclear pores, about 100nm in diameter.
The nucleoli - does not have a membrane and contain large amounts of RNA - and proteins like those of the ER. The nucleoli become enlarged when the cell is synthesizing proteins. Ribosomal RNA is stored in the nucleolus and transported through the nuclear membrane pores to the cytoplasm.
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