Cell Anatomy

The cell body needs multiple organelles to survive and function, just as we need our specialized organs to function, digest food and regenerate.


The outsides:

Plasma membrane
Double membrane bound structure that serves as the cell's 'skin'. Much like our own skin, it is a living boundary between the cell and it's surroundings.

It controls what goes into the cell and what exits. It is a phospholipid bi-layer with embedded proteins.

Cell wall
A cell wall is found only in plants. It's made of cellulose and is found in addition to the cell membrane. It's permeable and protective and has fibrils lying at right angles to each other for added strength.


Secondary cell wall
A secondary cell wall forms inside the primary cell wall in plants. Secondary cell walls are made of lignin which makes them even stronger than a primary cell wall.


The insides:

Cytoplasm
The cytoplasm is the 'goo' that holds all the organelles and cushions them. It's defined as a 'semifluid medium that contains the organelles'.


The cell`s brains:

Nuclear envelope
Double membrane that surrounds the nucleus. It separates the nucleus from the cytoplasm and helps the nucleus keep its shape. It provides chromatin attachment sites and helps funnel substances to or from the nuclear pores.

Nucleus
Stores genetic information for the eukaryotic cell. The nucleus controls structure and function.

Chromatin
Chromatin looks grainy on micrograph. In reality, it is a threadlike material. Just before cell division, chromatin coils into rodlike structures called chromosomes. Chromatin is found in the nucleoplasm.

Chromosomes:
Chromosomes are coiled chromatin. Chromosomes are rodlike structures that are seen in the nucleus during cell division. Chromosomes contain the gereditary units or, genes.

Nucleolus
Smaller, darker body seen in the nucleus. The nucleolus contains the chromatin needed to produce rRNA (Ribosomal RNA)

Nuclear pores:

Nuclear pores are what control what enters and exits the nucleus. The permit passage of proteins into the nucleus and the exit of ribosomal subunits from the nucleus. A complex of 8 proteins is associated with each nuclear pore.


Organelles

Ribosomes
Ribosomes are composed of 2 subunits and are found either free floating in the cytoplasm or attached to the endoplasmic reticulm. They are the site of protein synthesis in the cytoplasm.

Polyribosomes
When several ribosomes are arranged in a functional group to create a protein, this functional group is called a polyribosome.

Endoplasmic reticulum
Membranous system of canals surrounding the nucleus. It`s continuous with the nuclear envelope. ER branches off into the cytoplasm.

It is involved in the synthesis and modification of macromolecules.

Two kinds of Endoplasmic Reticulum:
1. Smooth ER
Smooth er has no attached ribosomes. It specializes in hormone production and is involved in
detoxification in the liver.

2. Rough ER
Rough Endoplasmic reticulum is ER studded with ribosomes. It specializes in protein synthesis.


Peroxisomes
Small vacuoles attached to smooth ER. Peroxisomes contain enzymes capable of detoxifying drugs.

Golgi apparatus
The golgi apparatus is a stack of 6 or more flattened saccules. Its inner face is directed at the nucleus. This is so it can recieve proteins.

The outer face of the golgi apparatus is directed towards the plasma membrane.

The golgi apparatus recieves proteins from the nucleus. Once recieved, the golgi apparatus modifies the protein with a carbohydrate chain or a phosphate group, packages the modified protein up into a tidy secritory vesicle and sends it off to the plasma membrane where it is discharged.

The golgi apparatus also forms lysosomes.

Vesicle
a small vacuole. A vacuole is a large membrane enclosed sac.

Lysosome
A lysosome is a vesicle filled with hydrolytic enzymes for digestion. It is formed in the golgi apparatus.
A lysosome can break down macromolecules into smaller components that the cell can use or it can even digest parts of the cell or, the cell itself- for cell death or cell rejuvination.


The outsides and attachments

Cytoskeleton
The cytoskeleton is actually found within the cytoplasm. It is made of several types of fiberous protein elements . It maintains the cell`s shape and allows the organelles to move.

Microtubules
Small cylinders made of a globular protein called tubulin.

Centrioles
Found in animal cells, centrioles lie at right angles to each other. Each animal cell has two centrioles. They are short cylinders made up of a pattern of 9+0 microtubule triplets ( a donut of 9 sets of triplets)

Cells that have Cilia and flagella, the centrioles are believed to give rise to basal bodies that direct the organization of microtubules in these tructures.

Cilia and Flagella
Cilia and flagella are hairlike extensions that give locomotive cells their movement capabilities. They move either in a an undulating pattern or like oars to move the cell. Sperm move with the aid of flagella.

They are membrane bound cylinders with a matrix of 9+2 pattern of microtubules. 9+2 microtubule doublets are arranged in a ring around 2 microtubule doublets.

A Basal Body lies in the cytoplasm at the base of each cilium or flagellum.

Cell size

Why are cells so small?



Simply put, it's to be efficient.


Since nutrients enter through the surface of the cell, it makes sense that cells are small for efficiency's sake. A smaller cell has more area to absorb nutrients and more area to expell waste, in relation to it's inside area.

As a shape gets bigger, the volume gets larger in relation to its surface.

The example from my notes says:

"As cells get larger, the amount of surface area in relation to volume decreases.

...a cube with 1mm sides has a surface area of 6mm2 and a volume of 1mm3. This gives a surface area to volume of 6:1.

A larger cube with sides of 2mm has a surface area of 8mm2 and a volume of 24mm2. This gives a surface area to volume ratio of 3:1."

So, it would make sense that cells are small.

Other reasons I have picked up around the web and paraphrased here (but are for fun- these aren't being taught in my class (yet)):


1. Many processes in the cytoplasm (cell inner 'filling' for lack of a better term) are achieved by diffusion. If a cell was large, it would take a longer time for nutrients and processes to make their way around the cell. This goes back to the efficiency reason for cell size.

2. If you’re painting a huge picture the smaller your brush is the more detail you will have. This makes a lot of sense! it's a pretty cool reason, actually!

3. This probably reiterates what I said above but I hear that we have billions of small cells because our cells need to be specialized into liver cells, blood cells, brain cells etc.


next up: Organelles!

Positive and negative feedback

Since I always find learning easier when I teach things to other people, here's my attempt at a school blog!

For now, the topics will mostly be biology 12. My curriculum uses Inquiry into life by Sylvia S Mader, Eighth edition.

Biology 12 is all about memorization as the theories themselves aren't too hard to understand. The memorization comes in with terminology. What seems like endless cell organelles and functions!! Everything is unfamiliar since it's new, it's a challenge to memorize and of course, I've been out of school for so long tbat the memorization is that much more difficult. So, here goes!

Disclaimer: I am not a biology expert and this is simply my own take on the subject and this blog is mostly for me and my own learning. If you happen to find the blog, Well, use at your own risk!!



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Positive feedback


The first assignment I have is to learn what Positive and Negative feeback are and learn how to diagram feedback loops.

Positive feedback defined by my handout is "A mechanism that brings about an ever greater change IN THE SAME DIRECTION".

There are many examples around on how this works but I'd like to understand the initial definition myself before I get into that. I'm convinced that I can't find a better explanation within my own head but I'll try.

my definition:

Positive feedback is when a stimulation causes a reaction. This reaction causes more stimulus and in return, causes another reaction to occur on top of the original reaction. It builds and builds and builds....


like a fever. a fever can cause metabolic changes to occur. These metabolic changes push the fever even higher. A situation like this is where positive feedback loops can be harmful.



Positive feedback can be a GOOD thing though, like in blood clotting to heal a wound or in the body's digestion of protein.


Childbirth is another example of positive feedback. The baby's head presses against the cervix causing the pituitary glad to release oxytocin. Oxytocin causes the uterus to contract again, pressing the baby's head against the cervix again, stimulating it.... this continues until birth occurs.

Positive feedback helps the body cope with intermittent events.


Negative feedback.


Self regulatory mechanism that is activated by an imbalance and results in a fluctuation above and below a mean.


So... much... scientific jumble! (to me, anyhow)

Negative feedback is the body's way of correcting a [low] imbalance by activating it's regulatory system. Like, when blood sugars are too high. The pancreas releases insulin to correct it.

sometimes, it can be harmful- like if you are diabetic and your pancreas doesn't stop releasing insulin. Now the blood sugar is too low.

Basically, any time the body feels that a necessary resource is too low, the negative feedback response kicks in and a hormone is released to bring the body's levels back to an even keel.

Summary notes:

positive feedback helps the body deal with intermittent events like injury, sickness or birth.
Negative feedback is used for maintenance of the body on an ongoing basis.