Eukaryotic Cells

A typical eukaryotic cell consists of a cell surface membrane, and cytoplasm within which a nucleus and membrane systems/organelles are embedded. 

These include:

• Nucleus 
• Mitochondria
• Endoplasmic reticulum (ER)
• Ribosomes
• Golgi apparatus
• Vesicles
• Lysosomes
• Microtubules 
• Cell Wall
• Plasmodesmata 
• Chlorplasts

This is the largest organelle, usually about 10-25 μm.

Most of the DNA is found within the nucleus of a cell- DNA carries the information for protein synthesis. This DNA is attached to a histone protein coat  (protects and supports DNA) which makes up the chromosomes.

Between divisons, chromosomes stretch out into a tangled mass called chromatin.

• Heterochromatin is more dense and the inactive form of chromatin. It contains densley packed DNA and is the darker stained areas
• Euchromatin is less dense and the active for of chromatin (lighter areas)

The nucleus has a double membrane with pores that allow molecules to move in and out (mainly large molecules). The outer boundary is the nuclear envelope or membrane, which forms the RER

The nucleolus is the darkest stained region of the nucleus and is the location for RNA=>DNA that codes for ribosomal RNA (rRNA). rRNA is used to make ribosomes- this is vital for PROTEIN SYNTHESIS. (ie the nucleolus is also the site of rRNA synthesis). Once produced, ribosomes are transported out of the nucleolus, through the nucleus and into the cytoplasm.

Mitochondria are bean shaped organelles that occur in ALL living plant and animal cells.

They range in size from 0.5-1 μm thick to several um long.

Each mitochondion is enclosed with a double membrane (both phospholipid membranes);

• An outer membrane that surrounds the entire organelle, known as the envelope.
• The inner membrane is highly folded to form cristae. The purpose of cristae is to increase the internal surface area for reactions and it also increases the number of enzymes that can be embedded within the membrane. It is at the cristae that energy is transferred to a molecule called adenosine triphosphate (ATP)

Mitochondria are the site of respiration which produces ATP=>the energy currency of the cell. Its energy is used to:

• Contract muscles
• Build up large molecules from smaller units 
• Power active transport

Mitochondria contain a fluid inside the inner membrane and this fluid-filled space is called the matrix. It contains free enzymes that catalyse reactions in the early stages of respiration.

Small loops of DNA are found in the matrix.

The number of mitochondria present in a cell depends on its energy demands. Eg muscle cells contain thousands

The Endoplasmic Reticulum (ER) is made up of folded membranes forming sheets, tubes or flattened sacs in the cytoplasm. It originates from the outer membrane of the nucleus and often remains attached to it. 3d membrane system encloses sacs called cisternae.

There are two types of Endoplasmic Reticulum: 

• Rough Endoplasmic Reticulum (RER)
• Smooth Endoplasmic Reticulum (SER)

• The RER has numerous ribosomes on its surace, giving it a grainy appearance. This is where it gets the name rough endoplasmic reticulum. RER is the main site of protein synthesis due to its ribosomes that are STUDDED ONTO THE SURFACE
• Once the primary structure of the protein is synthesised on the RER, the secondary and tertiary structures develop within the cisternae.
• Also the site of protein transport
  
• The rough endoplasmic reticulum is composed of interconnected cisternae
• Particularly common in cells that have a protein-secreting function.

RER provides 'scaffolding' for the ribosomes to make proteins and the ER then operates as a distribution network for the proteins.

RER is joines with the nuclear envelope, facilitating transport of mRNA (messenger RNA) 

This gives more surface area for proteins to be synthesised, stored and transported

Smooth Endoplasmic Reticulum does NOT have attached ribosomes and is NOT connected to the nucleus.

The SER is normally smaller/less extensive in comparison to the RER, and it is also more tubular. 

 The SER has many roles including

• Synthesis of lipids (Eg phospholipids)
• Detoxification of drugs and poisons 
• Carbohydrate metabolism 
• Involvement in lipid metabolism 

• Very small and dense organelles, about 20nm in diameter.
• Made in the nucleolus 
• Each ribosome is formed of a large and small sub-unit, and is made of protein and ribosomal RNA (rRNA) 
• They carry out PROTEIN SYNTHESIS
• Can be found  attached to the ER; these produce proteins for use OUTSIDE OF THE CELL
• Can also be found free in the cytoplasm; these tend to produce proteins for use INSIDE OF THE CELL
• Ribosomes occure in both prokarytoic and eukaryotic cells but the ribosomes found in prokaryotes are much smaller
• Several ribosomes may occur along the same lenghth of RNA => POLYSOMES/POLYRIBOSOMES. This creates 'hot spots' for protein synthesis

The Golgi apparatus/body consists of a stack of sac-like flattened membranes surrounding spaces called CISTERNAE.

They are present in ALL cells but is most prominent in the most metabolically active cells.

They characteristically have a number of small VESICLES both entering and leaving the system.

The Golgi body takes enzymes and other proteins that have been synthesised in the ER and package them into these membrane bound vesicles.

Its appearace is constantly changing as material comes in one side from the ER and is lost  from the other as completed vesicles 'bud' off. These vesicles transport materials to other parts of the cell; or fuse with cell membrane to release their contents outside the cell.

The main function of the Golgi apparatus is to chemically modify/ package proteins that are synthesised at ribosomes. (Think of an airport).

• The Golgi Apparatus modifies proteins (Eg it can add CHO and form a glycoprotein)
• Once it has modiefied proteins it packages them into the membrane bound vesicles;
• Proteins can be secreted from the cell if needed elswhere (exocytosis) or;
• The Golgi sends them to different places within the cell (Eg for storage)
• It manufactures lysosomes (therefore contains enzymes)
  

SUMMARY: Material enters through the cis face, is modified and packaged in the lumen then leaves through the trans face in membrane bound vesicles.

-Vesicles containing proteins are pinched off the RER and coalesce with the 'forming'/convex edge of the system.

-Within the main body of the Golgi, proteins can be modified;

• carbohydrates added to form glycoprotein
• lipid added to form lipoprotein
• may have prosthetic groups or cofactors added
• different polypeptides can be joined to form proteins with a quartenary structure
• can be labelled, packaged or sorted (for export) 

-Once protein is modified, vesicles bud off maturing face and they transport portein either within the cell or fuse with the cell surface membrane to release contents outside of the cell via exocystosis.

• These are small spherical vesicles, around 0.2-0.5 μm
• They are formed in the Golgi apparatus 
• Lysosomes contain a concentrated mixture of hydrolytic digestive enzymes
• They have relatively thick membranes to ensure the hydrolytic enzymes are not accidentally released inside the cell as this would result in apoptosis (self-digestion of the cell)
• They fuse with other vesicles in the cell that contain surplus or old organelles that need to be destroyed or digested 
• To breakdown old/unwanted material there are two main steps;
• They fuse with phagocyted (engulfed) material enclosed in a phagosome- the membrane bound vesicle in the phagocyte 
• Once fused they then release their enzymes and digest the whole cell (autolysis)

• These are hollow cylinders, about 25nm in length 
• Made of a protein called tubulin
• Microtubules form the cytoskeleton.
• The cytoskelton is the network of fibres that [maintains cell shape], [keeps organelles anchored in place] and also [provides a network aiding the movement of structures within the cell]
• Microtubules are often arranged into structures called centrioles; these are found in pairs in animal cells, fungi and some algae,
• These pairs are arranged at right angles to each other, each composed of nine triplets of microtubules 
• Centrioles are involved in the assembly of spindle fibres- formed of microtubules and important in the movement of chromosomes during mitosis/meiosis 
• Centrioles are also important constituents of cilia and flagella

In addition to all previous organelles listed, plant cells can also have;

• Plant Cell Wall
• Chloroplasts
• Plasmodesmata 
• Secondary Cell Wall
• Middle Lamella

• Located immediately outside the plasma membrane
• Made of the carbohdrate cellulose
• Around 1 μm thick 
• The cellulose is laid down in microfibrils embedded in a matrix made up of several polysaccharides
• Each microfibril consists of many cellulose molecules cross-linked to each other 
• The spaces between the microfibrils are usually wide enough for most molecules to pass through 
• The rigid nature of the cell wall allows it to support the cell
• The cell wall has a more fixed box-like structure
• It allows internal pressures to build up as a result of osmatic entry of water- this makes the cell turgid

• Plasmodesmata are thin strands of cytoplasm which extend between neighbouring plants cells 
• The plasmodesmata allow materials to be transported from one cell to another by providing 'gaps' (small pores) in the cell walls of adjacent cells 
• As the cell membranes pass through the pores, the neighbouring cells are joined both physically [the cytoplasm is continuous between adjacent cells] and metabollically 
• As the plasmodemata provides a connection between adjacent plant cells, it enables cell material to be transferred across 
• Example- the cytoplasm and RER can connect between neighbouring plant cells

• Chloroplasts are large organelles found in ONLY in the green parts of plants, mainly the leaves and young stem 
• The function of the chloroplasts is to make sugars in photosynthesis 
• They are bounded by two bilayered phospholipid membranes (they have a double membrane)
• The envelope (outer membrane) surrounds the organelle 
• The inner membrane is highly folded and creates thylakoids. In between these is a fluid called stroma.
• At intervals, the thylakoids are arranged into stacked structures called grana. (sing=granum)
• Between grana, the membranes are less concentrated and referred to as   inter-grana
• Structures called lamellae project from the grana and connect them 
• Their function is to ensure that the grana are kept seperate to maximise the surface area which maximised the efficiency of the chloroplasts

• The thylakoids contain the light absorbing pigment chlorophyll to capture light energy 
• The arrangement of thylakoids into grana/distribution of the grana maximises apsorbtion of light energy 
• In the stroma, enzymes catalyse reactions that use ATP and hydrogen ions produced in the grana to convert carbon dioxide into sugars 
• Chloroplasts usually have one or more starch grains and smaller lipid droples, both of which are products of photosynthesis 
• Starch grains are large and clear whereas lipid droplets are dark little circles 
• Chloroplasts are the sites of photosynthesis, therefore located in photosynthesising cells 

• When the cell reaches full size, additional layers of cellulose can be deposited to form the secondary cell wall
• Each layer of cellulose has the microfibrils orientated in the same direction
• However additional layers are orientated in different directions to other layers
• This lattice type arrangement gives the great strength necessary in cell wall function- ie to provide the cell with support 

• Between neighbouring cell walls is the middle lamella
• Largely made of polysaccharides called pectin
• A sticky material- described as a gel or 'cement'- formed by calcium pectate acts as an adhesive;
• the role of the middle lamella.is to stick adjacent cells together

PROKARYOTIC CELLS

Size: Usually < 5μm

Site of DNA: DNA free in cytoplasm

DNA organisation: Circular (arranged as a loop) and without associated protein

Ribosomes: Small - 20nm 

Internal structure: No complex organelles

Cell walls: Complex- made of peptidoglycan (a glycoprotein)

Plasmids: Usually present 

Microtubules: Not present 

EUKARYOTIC CELLS

Size: 10-200μm

Site of DNA: DNA inside 

DNA organisation: DNA linear and in chromosomes. Chromosomes contain both helically                                               arranged  DNA and packaging protein (histones)

Ribosomes: Large - 25 nm

Internal structure: Complex membrane-bound organelles including nucleus, mitochondria etc

Cell walls: Cellulose cell wall in plants, chitin cell wall in fungi, no cell well in animals

Plasmids: Not present 

Microtubules: Spindle fibres and other microtubules present 

Nucleus:

• Double membrane with pores that allow molecules to move in and out (mainly large molecules)
• Stores genetic marerial call chromatin 
• Two main types of chromatin- euchromatin is less dense and the acitve form of chromatin (lighter areas), heterochromatin is more dense and the inactive form of chromatin (darker areas)
• Central part called nucleolus is the site of rRNA synthesis 
• Outer membrane of the nucleus forms the RER

Mitochondria:

• Double membrane (envelope)
• Inner membrane folds to form cristae; the purpose of cristae is to increase the surface area of reactions
• Outer membrane surrounds the organelle 
• Mitochondria carries out respiration which produces ATP- the energy currency of the cell 

Mitochondria (cont):

• Has a matrix (fluid inside the inner membrane) which contains enzymes for helping out some stages of respiration 
• Small loops of DNA found in the matrix

Rough ER:

• Main site of protein synthesis due to its ribosomes that are studded onto the surface 
• Site of protein transport
• Composed of interconnected cisternae 

Smooth ER:

• Involved in lipid synthesis 
• no ribosomes on surface
• normally smaller/less extensive than the RER
• Not connected to the nucleus 

Golgi apparatus:

• modifies proteins- eg. it can add CHO to form a glycoprotein 
• once it has modified proteins it packages them 
• proteins can be secreted from the cell by exocytosis if needed elsewhere
• Golgi sends proteins to different places within the cell eg. for storage 
• manufactures lysosomes containing digestive enzymes 
• material eneter through the cis face, modified and packaged in the lumen then leaves through the trans face in membrane bound vesicles 

Lysosomes:

• produced by the golgi 
• contain powerful hydrolytic enzymes (lysozymes) as lysosomes are needed to destroy old or unwanted material 
• to break down the old material, there are two main steps; they fuse with phagocyted material (engulfed material). Once fused they release they enzymes and digest the whole cell (autolysis)

Ribosomes:

• made in nucleolus 
• made up of protein and rRNA
• can be found in free in the cytoplasm- these tend to produce proteins used inside of the cell 
• most are attached to the RER- these tend to produce proteins for use outside of the cell
• each ribosome is formed from a larger and smaller sub-unit 
• ribosomes are larger in eukaryotic cells than in prokaryotic cells 

Microtubules:

• unbranches tubes that provide a cytoskeleton for the cell 
• can transport materials within the cell
• found in the spindle during cell division 
• major components of cilia and flagella 

Chloroplasts:

• only found in plants, specifically only found in the green parts of plants 
• its function is to make sugars in photosynthesis 
• photosynthesis occurs in chloroplasts
• has double membrane 
• no foldings or inner membrane 
• has internal system of membranes calles thylakoids that are arranged into stacks called grana 
• thylakoids contain the light absorbing pigment chlorophyll 
• chloroplasts have stroma, a liquid that contains enzymes for photosynthesis 
• starch grains are larger and clearer, lipid droplets are dark little circles 

Plasmodesmata:

• found in plant cells only 
• provides a connection between adjacent plant cells so that cell material can transfer across 
• eg. cytoplasm and RER can connect between neighbouring plant cells