cell organelles

  • Created by: Huzaima1
  • Created on: 30-12-18 15:14


- specialised structures
- characteristic shapes
-perform specific functions in cellular growth, maintenance and reproduction
- each type of organelle has its own set of enzymes that carry out specific reactions, and serve as a functional component for specific biochemical processes

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  • the centrosome is located near the nucleus and consists of two components: a pair of centrioles and pericentriolar material.
  • The two centrioles are cylindrical structures, each composed of nine clusters of three microtubules (triplets) arranged in a circular pattern.The long axis of one of the centrioles is at a right angle to the long axis of the other.
  • The pericentriolar material, surrounding the centrioles, contains hundreds of ring-shaped complexes composed of the protein tubulin - tubulin complexes are the organizing centres for growth of the mitotic spindle, which plays a critical role in cell division, and for microtubule formation in non-dividing cells.
  • During cell division, centrosomes replicate so that succeeding generations of cells have the capacity for cell division.

function of the centrosomes

1. the pericentriolar material of the centrosome contains tubulins that build microtubules in non dividing cells
2. the pericentriolar material of the centrosome forms the mitotic spindle during cell division

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cilia, flagella, cilia and smoking application

- cilia = numerous, short hair-like projections which extend from the surface of the cell. Cilia move fluids along a cell's surface. Each cilium contains a core of 20 microtubules surrounded by plasma membrane. E.g. cilia are in the respiratory tract to help sweep foreign particles trapped in mucus away from the lungs - In cystic fibrosis, the extremely thick mucous secretions produced interfere with ciliary action and the normal functions of the respiratory tract.
- flagella = similar in structure to cilia, but typically much longer. They move an entire cell. A flagellum generates forward motion by rapidly wiggling in a wave like pattern
- main component of both = microtubules
- the movement of cilia is paralyzed by nicotine in cigarette smoke - smokers cough often to remove foreign particles from their airways. Cells that line the uterine (fallopian) tubes also have cilia that sweep oocytes (egg cells) toward the uterus, and females who smoke have an increased risk of ectopic (outside the uterus) pregnancy

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Endoplasmic Reticulum

- a network of membranes in the form of flattened sacs or tubules; extends from the nuclear envelope, to which it is connected and projects throughout the cytoplasm
- two distinct forms of ER which differ in structure and function
- RER = folded into a series of flattened sacs, studded with ribosomes on the outer surface. Proteins synthesised by these ribosomes enter spaces with the ER for processing and sorting. RER synthesises glycoproteins and phospholipids that are transferred into cellular organelles, inserted into the plasma membrane, or secreted during exocytosis
- SER = extends from the RER to form a network of membrane tubules, but does not have ribosomes on the outer surface therefore does not synthesise proteins. It synthesises fatty acids and steroids e.g. estrogens and testosterone; inactivates or detoxifies lipid soluble drugs and other potentially harmful substances; removes the phosphate group from glucose-6-phosphate allowing the free glucose to enter the bloodstream; and stores and releases calcium ions that trigger contraction in muscle cells (from sarcoplasmic reticulum which isth a form of SER)
- SER APPLICATION: SER detoxifies drugs. Individuals who repeatedly take drugs, such as the sedative phenobarbital, develop changes in the smooth ER in their liver cells - prolonged administration results in increased tolerance to the drug; the same dose no longer produces the same degree of sedation. With repeated exposure to the drug, the amount of smooth ER and its enzymes increases to protect the cell from its toxic effects. As the amount of smooth ER increases, higher and higher dosages of the drug are needed to achieve the original effect. This could result in an increased possibility of overdose and increased drug dependence.

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- site of protein synthesis
- contain a large amount of RNA
- consists of two subunits; one large and one small made separately in the nucleolus where one made exit and assemble in the cytoplasm
- ribosomes attached to the RER synthesise proteins destined for specific organelles, for insertion in the plasma membrane or secretion from the cell
-whereas other free ribosomes synthesise proteins used in the cytosol

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Golgi complex

- consists of 3-20 cisternae - small, flattened membranous sacs with bulging edged
- the cisternae are often curved
- Golgi complexes are more extensive in cells which secrete proteins
- the cisternae at the opposite ends of the Golgi complex differ from each other in size, shape and enzymatic activity
- the convex ENTRY (CIS) face is a cisterna that faces the rough ER. the concave EXIT (TRANS) face is a cisterna that faces the plasma membrane. sacs between the entry and exit faces are called MEDIAL CISTERNAE. transport vesicles from the ER merge to form the entry face.
- different enzymes in the entry, medial and exit cisternae of the Golgi complex permit each of these areas to modify, sort and package proteins into vesicles for transport to different destinations; the entry face receives and modifies proteins produced by the RER. the medial cisternae add carbohydrates to the proteins to form glycoproteins, and lipids to proteins to form lipoproteins. the exit face modifies the molecules further and then sorts and packages them for transport to their destinations.
- proteins arriving at, passing through, and exiting the Golgi complex do so through maturation of the cisternae and exchanges that occur via transfer vesicles

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transfer vesicles

1 - proteins synthesised by ribosomes on the rough ER are surrounded by a piece of the ER membrane, which eventually buds from the membrane surface to form transport vesicles
2 - transport vesicles move toward the entry face of the golgi complex
3 - fusion of several transport vesicles creates the entry face of the golgi complex and releases proteins into its lumen [space]
4 - the proteins move from the entry face into one or more medial cisternae. Enzymes in the medial cisternae modify proteins to form glycoproteins, glycolipids and lipoproteins. Transfer vesicles that bud from the edges of the cisternae move specific enzymes back toward the entry face and move some partially modified proteins towards the exit face.
5 - the products of the medial cisternae move into the lumen of the exit face
6 - within the exit face cisterna, the products are further modified and are sorted and packaged
7 - some of the processed proteins leave the exit face and are stored in secretory vesicles. these vesicles deliver the proteins to the plasma membrane, where they are discharged by exocytosis into the extracellular fluid. For example, certain pancreatic cells release the hormone insulin in this way.
8 - other processed proteins leave the exit face in membrane vesicles that deliver their contents to the plasma membrane for incorporation into the membrane. In doing so, the Golgi complex adds new segments of plasma membrane as existing segments are lost and modifies the number and distribution of membrane molecules
9 - finally, some processed proteins leave the exit face in transport vesicles that will carry the proteins to another cellular destination. For instance, transport vesicles carry digestive enzymes to lysosomes

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- membrane-enclosed vesicles that form from the golgi complex
- contain as many as 60 kinds of powerful digestive and hydrolytic enzymes that can break down molecules once lysosomes fuse with vesicles formed during endocytosis
- lysosomal enzymes work best at acidic pH conditions; the lysosomal membrane includes active transport pumps that import hydrogen ions
- therefore, the lysosomal interior has a pH of 5, 100 x more acidic than the pH of the cytosol (pH 7)
- lysosomal enzymes also help recycle worn out cell structures; a lysosome can engulf another organelle, digest it and return the digested components to the cytosol for reuse so organelles are continually replaced
- the process by which entire worn out organelles are disgested is called autophagy; where the organelle to be digested is enclosed by a membrane derived from the ER to create a vesicle called an autophagosome. The vesicle then fuses with the lysosome. In this way, e.g. the liver cell recycles half its cytoplasmic contents every week
- autophagy is required for renewal, cellular differentiation, control of growth and tissue remodelling
- lysosomal enzymes may also destroy the entire cell that contains them;known as autolysis, which occurs in some pathological conditions and is also responsible for the tissue deterioration that occurs immediately after death.
- some lysosomal enzymes also act in extracellular digestion (but most act within a cell) e.g. fertilisation

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application of lysosome disfunction

- example of a disorder caused due to a faulty or the absence of lysosomal enzymes is Tay Sachs disease
- affects most children of Ashkenazi (eastern European Jewish) descent
- inherited condition characterised by the absence of a single lysosomal enzyme Hex A
- This enzyme normally beaks down a membrane glycolipid called ganglioside GM2 that is especially prevalent in nerve cells
- As the excess ganglioside GM2 accumulates, the nerve cells function less efficiently
- children with Tay Sachs disease typically experience seizures and muscle rigidity
- they gradually become blind, demented, and uncoordinated - usually die before age 5
- tests can now reveal whether an adult is a carrier of the defective gene

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- similar in structure to lysosomes but smaller
- also called microbodies
- contain several oxidases, enzymes that can oxidise (remove hydrogen atoms from) various organic substances
- e.g. amino acids and fatty acids are oxidised in peroxisomes as part of normal metabolism
- in addition, enzymes in peroxisomes oxidise toxic substances such as alcohol
- therefore, they are abundant in liver cells, where detoxification of alcohol and other damaging substances occur
- a by product of the oxidation reaction is hydrogen peroxide and associated free radicals such as superoxide
- However, peroxisomes also contains enzymes that destroy superoxide and the enzyme catalase - which decomposes hydrogen peroxide, protecting other parts of the cell from the toxic effects of hydrogen peroxide
- without peroxisomes, by products of metabolism could accumulate inside a cell and result in cellular death
- peroxisomes could self- replicate; new peroxisomes may form from pre-existing ones by enlarging and dividing
- application: peroxisomal disorders form a heterogenous disease group, with different degrees of severity. They are all metabolic diseases that share dysfunction of peroxisomes - they all lead to brain disorders and respiratory infections. e.g:
- Zellweger syndrome - usually fatal within the first year of life
- NALD: usually fatal within the first 10 years
- RCDP: In its most severe form is fatal within the first year or 2 of life

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- tiny barrel shaped structures consisting of four stacked rings of proteins around a central core
- degrade 'free' cytosolic proteins (unneeded, damaged or faulty)
- such protein destruction plays a part in negative feedback by halting a pathway once the appropriate response has been achieved; e.g. in metabolic pathways once proteins have accomplished their function, they need to be degraded
- located both in the cytosol and in the nucleus
- contain enzyme proteases: break down proteins into peptides
-other enzymes then break down the peptide into amino acids, which can then be recycled into new proteins
- Application: some diseases could result from failure of proteasomes to degrade abnormal proteins; e.g. misfolded proteins accumulate in the brain cells of Alzheimer's disease and Parkinson's disease. Ongoing research being undertaken into discovering why the proteasomes fail to clear these abnormal proteins

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- site of aerobic respiration, power house of the cell [produces ATP/energy]
- amount of mitochondria present in the cell depends on its activity
- e.g. liver, muscle and kidney cells - active cells that use ATP at a high rate so have large numbers of mitochondria
-consists of:
- outer mitochondrial membrane = contains transport proteins for shuttling pyruvate into mitochondria
- inner mitochondrial membrane with small fluid-filled space between them = contains ETC and ATP synthase for oxidative phosphorylation
- intermembrane space = small space for the quick accumulation of protons
- matrix = has appropriate enzymes and a suitable pH for the Krebs cycle
- cristae = highly folded so as to increase SA to vol ratio for chemical reactions - ETC occurs here
- mitochondria also play an important early role in apoptosis: in response to stimuli such as large numbers of destructive free radicals, DNA damage, growth factor deprivation, or lack of oxygen and nutrients, certain chemicals are released from mitochondria following the formation of a pore in the outer mitochondrial membrane. One of the chemical released into the cytosol of the cell is cytochrome c, where in the cytosol, along with other substances, initiates the cascade of activation of protein-digesting enzymes that bring about apoptosis
- self replicate
- ribosomes are also present inside to synthesise proteins needed for mitochondrial functions
- other functions: cell signalling, cell differentiation, cell death and cell growth

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1- controls cellular structure
2 - directs cellular activities
3 - produces ribosomes in nucleoli

- spherical or oval shaped, surrounded by a perforated double membrane called the nuclear envelope
- many nuclear pores extend through the envelope: control the movement of substances between the nucleus and the cytoplasm
- in centre, nucleoli are present - clusters of proteins, DNA and RNA responsible for rRNA synthesis, assembly of rRNA and proteins into ribosomal subunits - prominent in muscle and liver cells/cells that synthesise large amounts of protein; they disperse and disappear during cell division and reorganise once new cells are formed
- only responsible for transcription
- contain genes: control cellular structure and direct cellular activities, genes are arranged along chromosomes; somatic cells = 46 chromosomes/ 23 pairs
- each chromosome is a long molecule of DNA that is coiled together with several proteins - this complex of DNA, proteins and some RNA is called chromatin
- the total genetic information carried in a cell or an organism is known as its genome
- Chromatin consists of double stranded (helix) DNA wrapped around histone proteins - helps the coiling and folding of DNA
- before cell division, the DNA replicates and the loops condense even more, forming a pair of chromatids
- during cell division, a pair of chromatids consitutes a chromosome
- involved in translation: producing mRNA

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how does it all fit together

- macromolecules = proteins, carbs and fats
- cytoskeleton = support/structure
- smooth ER = makes fats/releases glucose
- rough ER = makes proteins
- golgi body = processing/packaging
- nucleus = transcription
- ribosomes = translation (synthesis of proteins)

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