Cell organelles structure v function

  • Created by: Amh
  • Created on: 07-10-14 11:44

Cell surface

The Cell Surface • All cells are surrounded by a plasma membrane • Single membrane - that has two layers (phospholipid bilayers) • Membrane is selectively permeable; because we don't want leakage of key proteins and carbs ect. whilst keeping important molecules in Thickness of lipid layer is about 5nm ( so ions can't pass)>v The membrane is semi permeable, only very small molecules that have no charge or polarity (it serves as a barrier to most water-permeable molecules) Larger molecules must leave via protein gates.  Phospholipid bilayer is made up of a row of hydrophillic heads attached to 2 hydrophic fatty acid chains.  There is also protein in the membrane, some act as receptors, and and some act as protein gates allowing molecules in and out The phospholipid bilayer is amphipathic - has bot hydrophillic and phobic properties (http://antranik.org/wp-content/uploads/2012/03/phospholipid-bilayer-phosphate-is-hydrophilic-the-lipid-is-hydrophobic.gif?c3f22f)

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Why is a plasma membrane effective

It is an Amphipathic phospholipid bilayer. 

Hydrophillic heads dissolve readily in water because they contain charged atoms or polar groups (chemical groups who have an uneven distribution of positive and negative charges) these charged atoms can foorm electrostatic attractions or hydrogen bonds with water molecules (with is charged) 

The hydrophobicc chains are insoluble and therefore do not form favourable bonds with water. Meaning the hydrophobicc components cluster together in hopes to keep away from water whereas the hydrophillic heads want to to be near the water due to electrostatic attraction.

Arrangement is energetically favourable

This also makes the bilayer self sealing as molecules try to seal breaks to arrangement is still energentically favourable 

The bilayer has fluidity. The level of the fluidity is dependent on the hydrophobic tails e.g. by chain length and the number of bonds.

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Cell wall in plants

The cell wall in plants surrounds the plasma membrane.

This double membrane system means it is even more tricky to get molecules into and out of cells 

It is made up of a complex carbohydrate called cellulose.

Cellulose must be digested by cellulase 

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Bacteria cell walls

Can be classified into gram positive and gram negative. Gram positive cell walls are 90% peptidglycan and gram negative are only 10% 

This cell wall can be divided into gelatenous polysaccharide layers such as; capsules, glycocalyx, and slime layer.

Making it even tougher to get through into cell

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Cell appendages

Plasma memebranes and cells walls can have different appendages such as:


Function of these appendages:

Motility - flagella and cillia both move the cell
To move extra cellular fluid - the cillia (mucus, repiratory tract)
To increase surface are - villi/microvilli - such as in the mitochondria
(micro)villi are needed to increase SA:V ratio because when you increase the size or shape 
of cell it doesn't do this.

Cubodial cells

Width (μm) 10 20 40

SA (μm2) 600 2,400 9,600

V (μm3) 1,000 8,000 64,000

SA:V 0.600 0.300 0.150

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The function of the nucleus depends on whether cell is specialised or ready to reproduce

In a cell thats going to proliferate the nucleus forms a nucleolus  a transient structure that holds the chromosomes which are chromatids that condense in the preliminary stages of proliferation. Chromatins are DNA + histones. The nucleolus is where in interphase chromosome migrate to  (an area in nucleus). In the nucleolus different chromosomes carrying genes for rRNA cluster. And ribsomal RNA is turned into ribosomes.

In a cell that is not dividing, the nucleus contains the DNA needed for protein synthesis. It also houses the firt stage of protein synthesis - transcription (DNA to RNA). 

The nucleus is double membrane bound with small pores that allow RNA to leave (and not much else). This extra membrane is to keep the DNA safe. There is also the Nuclear lamina - a network of protein filaments that form a thin layer underlying the inner membrane

There are also Centrioles in the nucleus, which are microtubules used in mitosis and meiosis to pull the chromotids apart. 

The double membrane of the nucleus is called the nuclear envelope and it communicates with the cytosol. The outerm membrane of nucleus is continous with the membrane of the Endoplasmic Reticulum.

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Double membrane bound organelle. The inner and outer membrane are seperated by aqueos intermembrane space. The inner membrane is convuluted into cristae to increase the surface area to volume ratio. Inside the inner membrane is the matrix.

Mitochondrion has it's own DNA. Mitochondria has a maternal inheritance because paternal DNA is fragile. 

Mitochondrion contains series of oxidative enzymes

• Catalyse aerobic catabolism of fuels (carbohydrates, fatty acids and amino acids / proteins)

• Harness energy as ATP

 Oxidative metabolism of glucoseC6H12O6 + 6O2 = 6CO2 + 6H2O + ATP

 Mitochondria harness energy from C-based fuels by oxidative phosphorylation

• Generate and metabolise reactive oxygen species (ROS) and free radicals

• Mediate apoptosis

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Synthetic organelles

• Chloroplasts – photosynthesis

• Ribosomes – translation of proteins

• Endoplasmic reticulum –protein and lipid synthesis

• Golgi apparatus –protein processing, sorting & secretion

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Only found in photosynthetic cells -  plants algae, purple bacteria

• Catalyse anabolic (rather than catabolic) metabolism

• Harness light energy in photosynthesis

Energy + 6CO2 + 6H2O = C6H12O6 + 6O2

Bound by double membrane, with it's own DNA like mitochondria - suggesting a prokaryotic descent 

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In nucleus (nucleolus), DNA is transcribed into RNA:

• Messenger (m)RNA

• Ribosomal (r)RNA

• Transfer (t)RNAs

• mRNA code is translated into amino-acid sequence (peptide / protein) by ribosomes

Ribosomes have 2 subunits: small and large. Small sub unit in humans is 40S and large is 60S total is 80S.

• Each subunitis a complex of:

• rRNA

• proteins

rRNA acts as an enzyme


• Several ribosomes translating the same mRNA template

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endoplasmic reticulum

Continuous, highly convoluted membrane system –nucleus to plasma membrane. 

Smooth and rough ER are the same organalle, all ER is smooth till a ribosome attaches.

• Smooth ER – site of lipid synthesis

• Rough ER – site of protein synthesis (translation)

• Synthesis generally  occurs in lumen of the cisternae

It's distribution in cells is usually minimal, bar where it has an important role e.g. adrenal gland

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Golgi body

Series of specialised, stacked cisternae through which proteins (and lipids) are processed prior to:

• insertion into plama membrane

• secretion

• Molecules enter on cis face and exit from trans face of GA – move via vesicular trafficking

Enzymes in Golgi can add carbohydrate:

• carbohydrate + lipid = glycolipid

• carbohydrate + protein = glycoprotein

• Golgi Apparatus “sorts” molecules for final destination

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• “Small”, spherical, sealed, single membrane (lipid bilayer)  contains ICF(http://upload.wikimedia.org/wikipedia/commons/thumb/0/01/Liposome_scheme-en.svg/250px-Liposome_scheme-en.svg.png)Vesicular Trafficing

• Vesicle buds off from one membrane (cisterna)

• Vesicle passes through the cytoplasm and fuses with another membrane • Traffics lumenal and membrane content of vesicle

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Endosome: In biology, an endosome is a membrane-bounded compartment inside eukaryotic cells. It is a compartment of the endocytic membrane transport pathway from the plasma membrane to the lysosome

• Incoming vesicle formed by endocytosis

• Buds off from plasma membrane

• Pinocytosis

• Phagocytosis

• 3 types: early, recycling and late  endosome (EE, RE & LE)


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What is a Lysosome?

• Late endosome fuses with vesicle containing “lysozymes” (acid hydrolyases)

• At low pH, hydrolyse Chemical bonds to degrade contents of lysosome

membrane-bound cell organellefound in animal cells (they are absent in red blood cells). They are structurally and chemically spherical vesicles containinghydrolitic enzymes, which are capable of breaking down virtually all kinds of biomolecules, including proteinsnucleic acids,carbohydrateslipids, and cellular debris.

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Mitochondrion variations

Some cells like hepatocytes have c.2,000 mitochondria per cell (accounting or 20% of cell volume) this is because the liver cell has many metabolic responsibilities 

Not all cells have the same number  (or type) of mitochondria

• Unicellular organisms usually have a single mitochondrion

• Erythrocytes and some unicellular eukaryotes* have zero mitochondria

*(microsporidians, metamonads & archamoebae

Mature “lens fibre” cells (long, thin cells which comprise the lens) of the eye have:

• No nuclei

• No mitochondria

• No endoplasmic reticulum

So light can pass without being refracted

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