Cell Structure and Mitosis

Today, as the title implies, I'm going to be focusing on cells. Cells are the building blocks that make up tissues, which then make up organs, which then make up systems of the body (digestive system, respiratory system etc.) which then combine to form the organism, which is a word not to be confused with a word that looks somewhat similar but without the "ni" in the middle.

Much of the stuff I'm going to talk about is probably stuff that you've covered in years 8-10, so I'm just going to gloss over it. If you want more details on anything, feel free to post down in the comments!

Components of Cells

Most cells are specialised for certain purposes in the body, and hence they all have slightly different structures. There are, however, many components that most cells have in common. Let's have a look at them:

• The cell membrane, or plasma membrane, separates the cell from neighbouring cells and the external environment. It is made up of a double layer of lipid molecules as well as various proteins associated with the lipids. The cell membrane allows some substances to get through but not others.
• The cytoplasm is the fluid within the cell. It is 75% to 90% water. Inorganic materials and most carbohydrates are dissolved in the cytoplasm, while other organic compounds such as proteins and lipids are simply suspended in the cytoplasm. The term protoplasm refers to the nucleus and cytoplasm together (i.e. all of the contents of the cell).
• The cell contains a variety of small structures called organelles, which carry out various functions. (This is where I get annoyed at Blogger's apparent lack of an indented list feature.)
• One of these organelles is called the nucleus. Nearly all cells contain one, though some contain more, and some don't have a nucleus at all. It is the largest organelle, is normally oval or spherical in shape, and is surrounded by a nuclear membrane. This nuclear membrane is actually a double membrane- two membranes separated by a space. The membrane contains many gaps, or nuclear pores, which allow substances to pass through. The nucleus is often thought of as the "control centre" for the cell as it contains DNA (deoxyribonucleic acid), which determine which proteins and enzymes a cell can make, which then determine which chemical reactions can take place in the cell.
• The nucleus contains an area called the nucleolus, which mainly consists of RNA (ribonucleic acid), which plays a role in synthesising proteins.
• The nucleoplasm is the fluid inside the nucleus. Suspended in the nucleoplasm are chromatin threads (long strands of DNA) and the nucleolus.
• Ribosomes are very small, spherical organelles that can be found either floating around in the cytoplasm or attached to membranes within the cells. At the ribosomes, amino acids combine into proteins according to instructions in the DNA.
• The endoplasmic reticulum are pairs of parallel membranes that extend through the cytoplasm and connect to the cell membrane and nuclear membrane. There are two types: rough (granular) endoplasmic reticulum and smooth (agranular) endoplasmic reticulum. The main difference is that the former has ribosomes attached to it, while the latter does not. The membranes of the endoplasmic reticulum is thought to provide a surface where chemical reactions can take place, while the channels are for storing or transporting molecules.
• The Golgi apparatus, a series of flattened, membranous bags, is like the "post office" of the cell, as it is involved in packaging up proteins and so forth to prepare them for secretion from the cell. At the Golgi apparatus, molecules such as sugar, sulfate or phosphate are added, before the edges of the Golgi apparatus pinch off to form a vesicle (a liquid-filled sac) containing the proteins, which then travel through the cytoplasm to the cell membrane, where they then leave the cell.
• Lysosomes are small membrane-bounded spheres formed by the Golgi apparatus. Lysosomes contain digestive enzymes which break down material transported into the cell via vesicles as well as worn-out organelles.
• Mitochondria (singular mitochondrion) are the energy centres of the cell. They have two membranes, which contain enzymes which are involved in energy-releasing cellular reactions. The inner membrane has a series of folds (which the outer membrane lacks), increasing the surface area for chemical reactions.
• The centrioles are a pair of cylindrical structures, which are made up of rings of very fine tubules. The centrioles are located at right angles to each other. They are located near the nucleus and are involved in cellular reproduction.
• Cilia are small hair-like projections that move the whole cell, or move substances over the cell. They can be found in the respiratory tract (see my post on the respiratory system) for more details. Flagella are longer and there are only one or two per cell. In humans, only sperm cells have flagella.
• The cytoskeleton gives the cell its shape and assists the cell in moving. The cytoskeleton consists of microtubules and microfilaments. The former keeps organelles in place or moves them around the cell, while the later moves materials around the cytoplasm or moves the entire cell.
• Some cells also contain other chemical substances which aren't considered to be part of the structure of the cell. For example, red blood cells contain haemoglobin. These extra chemical substances are known as inclusions.

As well as permanent components of the cell, there are also many substances that move in and out of cells. For example, oxygen is taken into the cell to be used in cellular respiration, while carbon dioxide, a product of cellular respiration, is released from the cell. Which brings me to my next point...

Methods of Passing Through a Cell Membrane

• First up are our good ol' friends diffusion and osmosis. You can read more about them in a previous post of mine. The main idea is that the particles of a substance will move from an area of higher concentration to an area of lower concentration in order to even out the concentrations. Thus if the concentration of the given substance in the cell is lower, stuff will move into the cell; if the concentration in the cell is higher, stuff will move out of the cell.
• Active transport uses the energy of the cell in order to force particles to move from an area of lower concentration to an area of higher concentration (i.e. the opposite of diffusion).
• Endocytosis ("endo" = inwards) is yet another method of absorbing substances. The outer membrane folds around the substance in question, leaving the substance floating around the cytoplasm in a liquid-filled sac called a vesicle. (Don't worry, I'll provide diagrams in a sec.) There are two forms- if the substance is solid, it's called phagocytosis (cell-eating), whereas if the substance is liquid, it's called pinocytosis (cell-drinking).
• Exocytosis ("exo" = outwards) is the opposite of endocytosis- that is to say, the vesicles are pushed out of the cell, rather than taken into the cell.

And here's a very crude Paint diagram of what endocytosis looks like! (Or what I think it looks like...)

Basically, if you haven't already guessed, the outline is the cell membrane while the purple is the cytoplasm and all the other crap in the cell (protoplasm?). It basically folds around the little red dot, which is the particle to be "swallowed up" by the cell. Eventually the cell membrane joins up again, leaving a little bubble (called a vesicle) in the cell which contains the substance in question.

Cell Reproduction via Mitosis

The last bit I need to cover in this post is how cells reproduce via mitosis. There are at least two different ways that cells reproduce (the other being meiosis), but I'm only going to cover mitosis for now.

Cells reproduce for a variety of reasons- they reproduce to replace damaged or worn out cells as well as to help the body grow. While cells reproduce, their DNA (containing genetic information) needs to be passed along. During mitosis, this is achieved by replicating the DNA before the division process begins.

Mitosis is usually broken down into four stages (five if you include interphase), though in reality it is quite a fluid process- the cell doesn't just do one stage at a time because it looks nice in human bio books! Let's have a look at the four (five?) stages:

1. Interphase is the period between divisions. While the cell is not dividing, the DNA is in the form of long strands known as chromatin. During this phase, the DNA molecules form exact copies of themselves. This results in twice the amount of DNA, which is great, because this means that at the end you'll end up with two cells, each with the normal amount of DNA!
2. Prophase is the first actual stage of mitosis. The two pairs of centrioles become visible and move to opposite ends of the cell. Microtubules begin to radiate from them, eventually forming a framework of fibres called a spindle. Meanwhile, the nuclear membrane breaks down and the nucleus disappears. The chromatin threads become tightly coiled into roughly X-shaped chromosomes (well they're roughly X-shaped in diagrams anyway), made up of two chromatids which are joined at a point called the centromere. The two chromatids are identical DNA molecules (remember how the DNA replicated itself during interphase?). At the end of prophase, the chromatid pairs begin to move towards the centre of the cell.
3. Metaphase is the second stage of mitosis. During this stage, the chromosomes line up on the equator of the spindle. The centromere of each chromosome is attached to a spindle fibre.
4. Anaphase is the step where the chromatids actually begin to separate off into different parts of the cell, probably because of some kind of pull from the spindle fibres. Apparently the chromatids get called chromosomes once they get pulled apart (!).
5. Telophase is the final step- the two sets of chromosomes group together at opposite ends of the cell, nuclear membranes form around each group, a nucleolus appears in each new nucleus, the spindle fibres disappear, and the chromosomes uncoil to become chromatin threads again. The centrioles then duplicate, ready for the next division. The cytoplasm also begins to divide during telophase: a furrow appears between the two nuclei, which gradually deepens until the cell is split in two.

The original cell is called the parent cell; the two cells formed at the end are known as daughter cells. (Why not sons? I don't know.) The genetic information in the daughter cells is the same as that in the parent cells. However, different genes may be activated depending on whether or not the cell needs to specialise (e.g. stomach cells need to secrete digestive enzymes, nerve cells need to transmit information, and so on). This is called differentiation, and is not to be confused with finding the gradients of curves in maths.

The Structure of DNA

This is a topic I'll cover in more depth in my next post, but here's just a quick overview:

• Each DNA molecule consists of two strands, twisted into a double helix.
• The strands contain alternating sugars and phosphates.
• The strands are linked by pairs of nitrogen bases
• A sugar, phosphate and nitrogen base combined is called a nucleotide (see my post on organic compounds related to human bio)
• There are four kinds of nitrogen bases: adenine, thymine, cytosine and guanine
• The order of the bases is called the genetic code. Each gene consists of up to 1000 pairs of bases

Replication of DNA

This won't be covered in the next post, but understanding this will help you understand protein synthesis (which I am going to cover in the next post). So listen up!

As I've mentioned before, the DNA molecules undergo replication during interphase. During replication, the two strands separate. Nucleotides then come in and pair up with the nucleotides of the two strands, resulting in two full strands of DNA.

But wait! you might say. What is there to stop a different combination of nucleotides from pairing up with the two lonely strands of DNA?

The answer to that is quite simple: each nucleotide only has one other kind of nucleotide that it can pair up with. Adenine can only pair up with thymine, cytosine can only pair up with guanine, and so on.

As an example- let's just say our original strand of DNA has 5 pairs of bases like this:

A - T

G - C

G - C

T - A

C - G

When the DNA splits, you'll get these two strands:

A -

G -

G - 

T - 

C -

and

 - T

 - C

 - C

 - A

 - G

Given the rules that I just told you about which bases can pair with which, work out which nucleotides will then bond with each strand of DNA. You'll find that you'll end up with two exactly identical strands of DNA.

Now onto the next post- cellular respiration and protein synthesis!