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Cell Structure

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Prokaryotic cells


Prokaryotes do not have a nucleus like eukaryotes do. Their DNA is not membrane-bound, just free in the cytoplasm. The extra features of prokaryotic cells vs. eukaryotic cells you must learn are:


-the cytoplasm overall does not contain membrane-bound organelles such as mitochondria and endoplasmic reticulum


-prokaryotic ribosomes are smaller than their eukaryotic counterparts; due to their size (and the centrifugation level they separate from the cell at) they are termed 70S ribosomes; the bigger eukaryotic ribosomes are 80S


-as previously covered, and their primary defining element, they lack a nucleus; instead, their DNA is a single circular molecule freely present in the cytoplasm and not associated with any proteins such as histones in eukaryotes; however, the general area where the genetic material hangs out is termed a nucleoid


-they have a cell wall which contains a special glycoprotein called murein (also known as peptidoglycan)



Some prokaryotes also go further to have some specialised parts, some seen in the diagram:


-one or more plasmids which are also circular DNA loops but much smaller; these can be exchanged between cells or even between different species as they can carry genes for antibiotic resistance


-a capsule made of polysaccharides as their outermost layer (on top of the cell wall on top of the plasma membrane)


-one or more flagella which aid in locomotion


Eukaryotic cells


The core components of cells are the outer membrane, the cytoplasm (substance inside which contains all other stuff) and the nucleus (contains DNA). All the other stuff is made up of various components with specific functions – these are called organelles.



The ones you must know about are:


1. Cell-surface membrane
2. Nucleus
3. Microtubules
4. Mitochondria
5. Chloroplasts (plants and algae)
6. Golgi apparatus and Golgi vesicles
7. Lysosomes
8. Ribosomes
9. Rough endoplasmic reticulum and smooth endoplasmic reticulum
10. Cell wall (plants, algae, fungi)
11. Cell vacuole (plants)


In complex multicellular organisms, eukaryotic cells are specialised and therefore organised accordingly into tissues, organs and systems. For example, a small intestine epithelial cell which absorbs nutrients from food is part of the epithelium tissue of the small intestine organ of the digestive system.


So, what are we waiting for? Let’s delve right into these organelles.


1. Cell-surface membrane

Plasma membrane = thin boundary between cell and environment

It is made of a phospholipid bilayer, and its function is to control what passes through the cell.

Membranes are also found in other organelles such as the nucleus and mitochondria.


Armed with our knowledge of lipids, as well as carbohydrates and proteins, we can explore the structure of plasma membranes, specifically in the context of the fluid-mosaic model. Phospholipids have a hydrophilic (water loving) head, and hydrophobic (water repelling) tails. This results in the formation of a phospholipid bilayer (double layer), which forms the basis for the plasma membrane.



The name of fluid-mosaic model comes from:


Fluid = the arrangement of proteins contained in the membrane is always changing
Mosaic = the proteins present are spread around in a mosaic-like fashion.



It’s pretty isn’t it? The proteins are crucial to cell communication as well as the selective permeability of the membrane. The glycoprotein (sugars/carbohydrates attached to a protein) side chains act as receptors. Lipid soluble stuff such as vitamins A, D and K, as well as oxygen and carbon dioxide, can pass freely though the membrane.


It’s also important to understand the role of microvilli. These are elongations of plasma membrane which increase the surface area available for reaction or absorption.


2. Nucleus

Usually it is the large rounded organelle in a cell. It has a double membrane with many pores through which materials can pass. Each cell normally has one nucleus. The main functions are cell division, replication and protein synthesis. These can be achieved due to the presence of chromosomes in the nucleus, which are made of linear DNA tightly wound up with the help of special proteins such as histones, as well as one or more nucleoli.


DNA and chromosomes may seem like completely separate things. Well, they’re not. In fact, all chromosomes are individual DNA molecules coiled and twisted around, because DNA is huge. At least in eukaryotes it is. That’s one of the first differences between eukaryotes and prokaryotes in their DNA – prokaryotes have less DNA.


Eukaryotic DNA is stored within the nucleus of each cell (apart from cells without one, e.g. red blood cells). Because of its sheer size, it must be organised well. Proteins called histones help do just that:



The nucleolus is a part of the nucleus that has many special functions including creating ribosomes.

3. Microtubules

The centriole is a tubey spaghetti thing that aids in cell division when the duplicated chromosomes need to move into their subsequent new offspring cells from the parent cell (during mitosis).



They’re made of a special protein called tubulin because they’re tubeeeeeeeeeeeees. Why didn’t they call it spaghettulin? I guess spaghetti aren’t hollow but…


Microtubules also play a key part in how cell organelles are moved and placed within the cell. Two associated centrioles form the centrosome. The microtubules are also where spindle fibres extend from when they separate chromosomes during cell division.


4. Mitochondria (mitochondrion, singular)

This is basically the easily identifiable sausage-like organelle with the cool inner membrane that forms the cristae. It is the site of aerobic respiration, where most ATP is made. Look:

Mitochondria has its own DNA because back in ancient times it used to be its own simple organism before it got engulfed and started chilling with a bigger cell and then all the multicellular business happened (or was that before?) and then BAM! complex beings and stuff. ATP synthase is the enzyme that catalyses the formation of ATP, while the cristae ensure a large surface area for all these energy-producing reactions to occur.


It’s the powerhouse of the cell and mutations in its DNA can cause severe illness. So much so, that for the inheritable variety, a genetic engineering intervention has been introduced. It was covered in the news as the “three-parent” affair and involves replacing the biological mother’s faulty egg mitochondria with a donor egg with healthy mitochondria.


The main chromosomal DNA in the child’s nucleus is of the biological parents, but because the mitochondrial DNA is sourced elsewhere, it became exaggeratedly dubbed the three-parent technique. The first child to be born of this technique was a boy. As it happens, mitochondrial DNA is passed on the maternal line (that’s how genetic connections can be traced back to “mitochondrial Eve”) so the boy won’t have children with the “foreign” DNA, just in case anything might’ve gone wrong.


5. Chloroplasts (plants and algae)

Chloroplasts contain all the substances and machinery necessary for photosynthesis. Without photosynthesis, you and I would not be here right now. Impressive. Take a closer look at this intricate organelle, the chloroplast, to whom we owe our lives:



This gooey mess whose constituent components have slightly uncommon and incredibly hard to memorise names is a chloroplast. Let’s crunch them one by one.


Thylakoid, a disc-shaped organelle which contains chlorophyll (the lovely green pigment) rhymes with Kayla+Droid. Trust me, once you get the hang of this little word you will love it. Chlorophyll is involved in capturing sunlight (and the light dependent reaction). Multiple thylakoids stack together like towers within the chloroplast. A tower is called a granum, pl. grana. This arrangement enhances the surface area available.


The stroma is the fluid-filled space which is the site of the light-independent reaction.


The outer membrane and inner membrane are selectively permeable to allow O2, CO2, glucose and certain ions through.


Features which make chloroplasts well adapted to serve their function:


1. Chloroplasts are relatively flat and so ease the diffusion of molecules coming in and going out. This is achieved by a shorter diffusion pathway.
2. Plenty of available surface area for the reaction between chlorophyll and light to take place.
Now that wasn’t so bad!


6. Golgi apparatus and Golgi vesicles

The apparatus is a stack of flattened membrane discs which receives packages of protein from the rough ER, and is involved in synthesising chemicals before they are secreted from the cell. There are 3 types of vesicle the Golgi apparatus produces:



a) exocytotic vesicles which are routed to leave the cell; upon packaging their content, they bud off and approach the plasma membrane where they fuse and release the content into the extracellular space; an example of this is antibody release

b) secretory vesicles also head out of the cell; the difference here is that they stand by until a signal arrives for them to move towards the membrane and release their content e.g. neurones secreting neurotransmitters

c) lysosomal vesicles contain proteins and ribosomes headed for lysosomes which degrade them; read on to learn more about the lysosome


7. Lysosomes

These are small vesicles of membrane that contain enzymes which take part in digestion. They look like tiny balls. Its enzymes are thus called lysozymes for example acid hydrolases or proteases which break down its waste products. The cargo includes both digestive enzymes and membrane proteins.



8. Ribosomes

My personal favourite ? Ribosomes are made of a small subunit and a large subunit. They are found on the rough ER and free within the cytoplasm, and they are the site of translation where the genetic code is used to build protein. Under the microscope within a cell, they appear as mere dots. But remember, awesome comes in small packages!



9. Rough endoplasmic reticulum and smooth endoplasmic reticulum

There are two different kinds of endoplasmic reticulum – rough and smooth endoplasmic reticulum, hence their short names rough ER and smooth ER. The roughness and smoothness business is down to ribosomes attached to the rough ER, but not to smooth ER.


Rough ERtransport system: collects, stores, packages and transports the proteins made on the ribosomes


Smooth ER – synthesis of lipids and some steroids; detoxification e.g. alcohol breakdown.

What does it look like? Well, imagine this is a bit like the inner membrane of the mitochondria, but more tightly packed.



10. Cell wall (plants, algae, fungi)

A leaf cell is surrounded by its cell wall which is made of cellulose.

The role of the cell wall is multi-fold:


1. Provides the plant with strength

2. Prevents the cell from bursting due to water flooding in by exerting pressure against the water flow

3. Gives tissues mechanical strength e.g. plants that rise high above the ground

4. Maintains the cell’s specific shape.



Fungi are actually more closely related to us than plants. However, they also sport a cell wall, indeed so do bacteria and they’re not even eukaryotic! Generally, cell walls provide structural support, act as defence, and can have varied other functions depending on species.



Fungal cells are organised as protoplasm, often with multiple nuclei per cell. Protoplasm describes the contents of the cell delimited by the cell membrane.



In plant cells, the presence of a cell wall for each cell mean that neighbouring cells have essentially two sturdy cell walls between them. However, they must still communicate freely and be able to exchange crucial molecules such as nutrients and water. This is enabled by small pores going through cell walls termed plasmodesmata.



11. Cell vacuole (plants)

The vacuole is a very large vesicle which contains a solution of various organic and inorganic compounds. The size of the vacuole changes upon cell requirements, as the compounds within are used up or stored. Some of the functions of the vacuole, depending on cell type, include:


-storing small molecules and water

-isolating waste products or harmful substances

-maintaining cell rigidity by keeping in check hydrostatic pressure








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