# Anatomy and Physiology of Animals/The Cell

!original image by pong cc
by{width="400"}

## Objectives

After completing this section, you should know:

:\*that cells can be of different shapes and sizes

:\*the role and function of the plasma membrane; cytoplasm, ribosomes,
rough endoplasmic reticulum; smooth endoplasmic reticulum, mitochondria,
golgi bodies, lysosomes, centrioles and the nucleus

:\*the structure of the plasma membrane

:\*that substances move across the plasma membrane by passive and active
processes

:\*that passive processes include diffusion, osmosis and facilitated
diffusion and active processes include active transport, pinocytosis,
phagocytosis and exocytosis

:\*what the terms hypotonic, hypertonic isotonic and haemolysis mean

:\*that the nucleus contains the chromosomes formed from DNA

:\*that mitosis is the means by which ordinary cells divide

:\*the main stages of mitosis

:\*that meiosis is the process by which the chromosome number is halved
when ova and sperm are formed

## The Cell

!**Diagram 3.1**: A variety of animal
cells{width="400"}
The cell is the basic building block of living organisms. Bacteria and
the parasite that causes malaria consist of single cells, while plants
and animals are made up of trillions of cells. Most cells are spherical
or cube shaped but some are a range of different shapes (see diagram
3.1).

Most cells are so small that a microscope is needed to see them,
although a few cells, e.g. the ostrich's egg, are so large that they
could make a meal for several people.

A normal cell is about 0.02 of a millimetre (0.02mm) in diameter. (Small
distances like this are normally expressed in micrometres or microns
(μm). Note there are 1000 μms in every mm). 
!**Diagram 3.2**: An animal
cell{width="400"}

When you look at a typical animal cell with a light microscope it seems
quite simple with only a few structures visible (see diagram 3.2).

Three main parts can be seen:

-   an outer cell membrane (plasma membrane),
-   an inner region called the cytoplasm and
-   the nucleus

!**Diagram 3.3**: An animal cell as seen with an electron
microscope{width="400"}

However, when you use an electron microscope to increase the
magnification many thousands of times you see that these seemingly
simple structures are incredibly complex, each with its own specialized
function. For example the plasma membrane is seen to be a double layer
and the cytoplasm contains many special structures called **organelles**
(meaning little organs) which are described below. A drawing of the cell
as seen with an electron microscope is shown in diagram 3.3.


## The Plasma Membrane

!**Diagram 3.4**: The structure of the plasma
membrane{width="400"}

The thin plasma membrane surrounds the cell, separating its contents
from the surroundings and controlling what enters and leaves the cell.
The plasma membrane is composed of two main
molecules,phospholipids(fats) and proteins. The phospholipids are
arranged in a double layer with the large protein molecules dotted about
in the membrane (see diagram 3.4). Some of the protein molecules form
tiny channels in the membrane while others help transport substances
from one side of the membrane to the other. 

### How substances move across the Plasma Membrane

Substances need to pass through the membrane to enter or leave the cell
and they do so in a number of ways. Some of these processes require no
energy i.e. they are passive, while others require energy i.e. they are
**active**.

Passive processes include: a) diffusion and b) osmosis, while active
processes include: c) active transport, d) phagocytosis, e) pinocytosis
and f) exocytosis. These will be described below.

!**Diagram 3.5**: Diffusion in a
liquid{width="400"}
**a) Diffusion**

Although you may not know it, you are already familiar with the process
of diffusion. It is diffusion that causes a smell (expensive perfume or
smelly socks) in one part of the room to gradually move through the room
so it can be smelt on the other side. Diffusion occurs in the air and in
liquids.

Diagram 3.5 shows what happens when a few crystals of a dark purple dye
called potassium permanganate are dropped into a beaker of water. The
dye molecules diffuse into the water moving from high to low
concentrations so they become evenly distributed throughout the beaker.

In the body, diffusion causes molecules that are in a high concentration
on one side of the cell membrane to move across the membrane until they
are present in equal concentrations on both sides. It takes place
because all molecules have an in-built vibration that causes them to
move and collide until they are evenly distributed. It is an absolutely
natural process that requires no added energy.

Small molecules like oxygen, carbon dioxide, water and ammonia as well
as fats, diffuse directly through the double fat layer of the membrane.
The small molecules named above as well as a variety of charged
particles (ions) also diffuse through the protein-lined channels. Larger
molecules like glucose attach to a carrier molecule that aids their
diffusion through the membrane. This is called **facilitated
diffusion**.

In the animal's body diffusion is important for moving oxygen and carbon
dioxide between the lungs and the blood, for moving digested food
molecules from the gut into the blood and for the removal of waste
products from the cell.  !**Diagram 3.6**:
Osmosis{width="400"}
**b) Osmosis**

Although the word may be unfamiliar, you are almost certainly acquainted
with the effects of osmosis. It is osmosis that plumps out dried fruit
when you soak it before making a fruit cake or makes that wizened old
carrot look almost like new when you soak it in water. Osmosis is in
fact the diffusion of water across a membrane that allows water across
but not larger molecules. This kind of membrane is called a
**semi-permeable membrane**.

Take a look at side **A** of diagram 3.6. It shows a container divided
into two parts by an artificial semi-permeable membrane. Water is poured
into one part while a solution containing salt is poured into the other
part. Water can cross the membrane but the salt cannot. The water
crosses the semi-permeable membrane by diffusion until there is an equal
amount of water on both sides of the membrane. The effect of this would
be to make the salt solution more diluted and cause the level of the
liquid in the right-hand side of the container to rise so it looked like
side **B** of diagram 3.6. This movement of water across the
semi-permeable membrane is called osmosis. It is a completely natural
process that requires no outside energy.

Although it would be difficult to do in practice, imagine that you could
now take a plunger and push down on the fluid in the right-hand side of
container **B** so that it flowed back across the semi-permeable
membrane until the level of fluid on both sides was equal again. If you
could measure the pressure required to do this, this would be equal to
the **osmotic pressure** of the salt solution. (This is a rather
advanced concept at this stage but you will meet this term again when
you study fluid balance later in the course). 
!**Diagram 3.7**: Osmosis in red cells placed in a hypotonic
solution{width="400"}

The plasma membrane of cells acts as a semi-permeable membrane. If red
blood cells, for example, are placed in water, the water crosses the
membrane to make the amount of water on both sides of it equal (see
diagram 3.7). This means that the water moves into the cell causing it
to swell. This can occur to such an extent that the cell actually bursts
to release its contents. This bursting of red blood cells is called
**haemolysis**. In a situation such as this when the solution on one
side of a semi-permeable membrane has a lower concentration than that on
the other side, the first solution is said to be **hypotonic** to the
second.  !**Diagram 3.8**: Osmosis in red cells
placed in a hypertonic
solution{width="400"}

Now think what would happen if red blood cells were placed in a salt
solution that has a higher salt concentration than the solution within
the cells (see diagram 3.8). Such a bathing solution is called a
**hypertonic** solution. In this situation the "concentration" of water
within the cells would be higher than that outside the cells. Osmosis
(diffusion of water) would then occur from the inside of the cells to
the outside solution, causing the cells to shrink.

!**Diagram 3.9**: Red cells placed in an isotonic
solution{width="400"}

A solution that contains 0.9% salt has the same concentration as body
fluids and the solution within red cells. Cells placed in such a
solution would neither swell nor shrink (see diagram 3.9). This solution
is called an **isotonic** solution. This strength of salt solution is
often called **normal saline** and is used when replacing an animal's
body fluids or when cells like red blood cells have to be suspended in
fluid.

**Remember** - osmosis is a special kind of diffusion. It is the
diffusion of water molecules across a semi-permeable membrane. It is a
completely passive process and requires no energy.

Sometimes it is difficult to remember which way the water molecules
move. Although it is not strictly true in a biological sense, many
students use the phrase **"SALT SUCKS"** to help them remember which way
water moves across the membrane when there are two solutions of
different salt concentrations on either side.

As we have seen water moves in and out of the cell by osmosis. All water
movement from the intestine into the blood system and between the blood
capillaries and the fluid around the cells (tissue or extra cellular
fluid) takes place by osmosis. Osmosis is also important in the
production of concentrated urine by the kidney.

**c) Active transport**

When a substance is transported from a low concentration to a high
concentration i.e. uphill against the concentration gradient, energy has
to be used. This is called **active transport**.

Active transport is important in maintaining different concentrations of
the ions sodium and potassium on either side of the nerve cell membrane.
It is also important for removing valuable molecules such as glucose,
amino acids and sodium ions from the urine. 
!**Diagram 3.10**:
Phagocytosis{width="400"}

**d) Phagocytosis**

Phagocytosis is sometimes called "cell eating". It is a process that
requires energy and is used by cells to move solid particles like
bacteria across the plasma membrane. Finger-like projections from the
plasma membrane surround the bacteria and engulf them as shown in
diagram 3.10. Once within the cell, enzymes produced by the lysosomes of
the cell (described later) destroy the bacteria.

The destruction of bacteria and other foreign substance by white blood
cells by the process of phagocytosis is a vital part of the defense
mechanisms of the body.

**e) Pinocytosis**

Pinocytosis or "cell drinking" is a very similar process to phagocytosis
but is used by cells to move fluids across the plasma membrane. Most
cells carry out pinocytosis (note the pinocytotic vesicle in diagram
3.3).

**f) Exocytosis**

Exocytosis is the process by means of which substances formed in the
cell are moved through the plasma membrane into the fluid outside the
cell (or extra-cellular fluid). It occurs in all cells but is most
important in secretory cells (e.g. cells that produce digestive enzymes)
and nerve cells. 

## The Cytoplasm

Within the plasma membrane is the **cytoplasm**. It consists of a clear
jelly-like fluid called the a) **cytosol** or **intracellular fluid** in
which b) **cell inclusions**, c) **organelles** and d)
**microfilaments** and **microtubules** are found.

### a) Cytosol

The cytosol consists mainly of water in which various molecules are
dissolved or suspended. These molecules include proteins, fats and
carbohydrates as well as sodium, potassium, calcium and chloride ions.
Many of the reactions that take place in the cell occur in the cytosol.

### b) Cell inclusions

These are large particles of fat, glycogen and melanin that have been
produced by the cell. They are often large enough to be seen with the
light microscope. For example the cells of adipose tissue (as in the
insulating fat layer under the skin) contain fat that takes up most of
the cell.

### c) Organelles

**Organelles** are the "little organs" of the cell - like the heart,
kidney and liver are the organs of the body. They are structures with
characteristic appearances and specific "jobs" in the cell. Most can not
be seen with the light microscope and so it was only when the electron
microscope was developed that they were discovered. The main organelles
in the cell are the **ribosomes, endoplasmic reticulum, mitochondrion,
Golgi complex** and **lysosomes**. A cell containing these organelles as
seen with the electron microscope is shown in diagram 3.3.

#### Ribosomes

!**Diagram 3.11**: Rough endoplasmic
reticulum{width="400"}
**Ribosomes** are tiny spherical organelles that make proteins by
joining amino acids together. Many ribosomes are found free in the
cytosol, while others are attached to the rough endoplasmic reticulum.

#### Endoplasmic reticulum

The **endoplasmic reticulum (ER)** is a network of membranes that form
channels throughout the cytoplasm from the nucleus to the plasma
membrane. Various molecules are made in the ER and transported around
the cell in its channels. There are two types of ER: smooth ER and rough
ER.

:   

    :   **Smooth ER** is where the fats in the cell are made and in some
        cells, where chemicals like alcohol, pesticides and carcinogenic
        molecules are inactivated.

```{=html}
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```

:   

    :   The **Rough ER** has ribosomes attached to its surface. The
        function of the Rough ER is therefore to make proteins that are
        modified stored and transported by the ER (Diagram 3.11).

#### Mitochondria

!**Diagram 3.12**: A
mitochondrion{width="400"}
**Mitochondria** (singular mitochondrion) are oval or rod shaped
organelles scattered throughout the cytoplasm. They consist of two
membranes, the inner one of which is folded to increase its surface
area. (Diagram 3.12)

Mitochondria are the "power stations" of the cell. They make energy by
"burning" food molecules like glucose. This process is called **cellular
respiration**. The reaction requires oxygen and produces carbon dioxide
which is a waste product. The process is very complex and takes place in
a large number of steps but the overall word equation for cellular
respiration is-

:   

    :   **Glucose + oxygen = carbon dioxide + water + energy**
    :   **or C~6~H~12~O~6~ + 6O~2~** = **6CO~2~** + **6H~2~O** +
        **energy**

**Note** that cellular respiration is different from respiration or
breathing. Breathing is the means by which air is drawn into and
expelled from the lungs. Breathing is necessary to supply the cells with
the oxygen required by the mitochondria and to remove the carbon dioxide
produced as a waste product of cellular respiration.

Active cells like muscle, liver, kidney and sperm cells have large
numbers of mitochondria. 

#### Golgi Apparatus

!**Diagram 3.13**: A Golgi
body{width="400"}

The **Golgi bodies** in a cell together make up the **Golgi apparatus**.
Golgi bodies are found near the nucleus and consist of flattened
membranes stacked on top of each other rather like a pile of plates (see
diagram 3.13). The Golgi apparatus modifies and sorts the proteins and
fats made by the ER, then surrounds them in a membrane as **vesicles**
so they can be moved to other parts of the cell.

#### Lysosomes

**Lysosomes** are large vesicles that contain digestive enzymes. These
break down bacteria and other substances that are brought into the cell
by phagocytosis or pinocytosis. They also digest worn-out or damaged
organelles, the components of which can then be recycled by the cell to
make new structures.

### d) Microfilaments And Microtubules

Some cells can move and change shape and organelles and chemicals are
moved around the cell. Threadlike structures called **microfilaments**
and **microtubules** that can contract are responsible for this
movement.

These structures also form the projections from the plasma membrane
known as **flagella** (singular flagellum) as in the sperm tail, and
**cilia** found lining the respiratory tract and used to remove mucus
that has trapped dust particles (see chapter 4).

Microtubules also form the pair of cylindrical structures called
**centrioles** found near the nucleus. These help organise the spindle
used in cell division.

## The Nucleus

!**Diagram 3.14**: A cell with an enlarged
chromosome{width="400"}
!**Diagram 3.15**: A full set of human
chromosomes

The **nucleus** is the largest structure in a cell and can be seen with
the light microscope. It is a spherical or oval body that contains the
**chromosomes**. The nucleus controls the development and activity of
the cell. Most cells contain a nucleus although mature red blood cells
have lost theirs during development and some muscle cells have several
nuclei.

A double membrane similar in structure to the plasma membrane surrounds
the nucleus (now called the nuclear envelope). Pores in this nuclear
membrane allow communication between the nucleus and the cytoplasm.

Within the nucleus one or more spherical bodies of darker material can
be seen, even with the light microscope. These are called **nucleoli**
and are made of RNA. Their role is to make new ribosomes.

### Chromosomes

Inside the nucleus are the chromosomes which:

-   contain DNA;
-   control the activity of the cell;
-   are transmitted from cell to cell when cells divide;
-   are passed to a new individual when sex cells fuse together in
    sexual reproduction.

In cells that are not dividing the chromosomes are very long and thin
and appear as dark grainy material. They become visible just before a
cell divides when they shorten and thicken and can then be counted (see
diagram 3.14).

The number of chromosomes in the cells of different species varies but
is constant in the cells of any one species (e.g. horses have 64
chromosomes, cats have 38 and humans 46). Chromosomes occur in pairs
(i.e. 32 pairs in the horse nucleus and 19 in that of the cat). Members
of each pair are identical in length and shape and if you look carefully
at diagram 3.15, you may be able to see some of the pairs in the human
set of chromosomes. 

## Cell Division

!**Diagram 3.16**: Division by mitosis results in 2 new cells identical
to each other and to parent
cell{width="400"}
!**Diagram 3.17**: Division by meiosis results in 4 new cells that are
genetically different to each
other{width="400"}

Cells divide when an animal grows, when its body repairs an injury and
when it produces sperm and eggs (or ova). There are two types of cell
division: **Mitosis** and **meiosis**.

**Mitosis**. This is the cell division that occurs when an animal grows
and when tissues are repaired or replaced. It produces two new cells
(daughter cells) each with a full set of chromosomes that are identical
to each other and to the parent cell. All the cells of an animal's body
therefore contain identical DNA.

**Meiosis.** This is the cell division that produces the ova and sperm
necessary for sexual reproduction. It only occurs in the ovary and
testis.

The most important function of meiosis it to half the number of
chromosomes so that when the sperm fertilises the ovum the normal number
is regained. Body cells with the full set of chromosomes are called
**diploid** cells, while **gametes** (sperm and ova) with half the
chromosomes are called **haploid** cells.

Meiosis is a more complex process than mitosis as it involves two
divisions one after the other and the four cells produced are all
genetically different from each other and from the parent cell.

This fact that the cells formed by meiosis are all genetically different
from each other and from the parent cell can be seen in litters of
kittens where all the members of the litter are different from each
other as well as being different from the parents although they display
characteristics of both. 

## The Cell As A Factory

To make the function of the parts of the cell easier to understand and
remember you can compare them to a factory. For example:

-   The nucleus (1) is the managing director of the factory consulting
    the blueprint (the chromosomes) (2);
-   The mitochondria (3) supply the power
-   The ribosomes (4) make the products;
-   The chloroplasts of plant cells (5) supply the fuel (food)
-   The Golgi apparatus (6) packages the products ready for dispatch;
-   The ER (7) modifies, stores and transports the products around the
    factory;
-   The plasma membrane is the factory wall and the gates (8);
-   The lysosomes dispose of the waste and worn-out machinery.

The cell compared to a
factory

## Summary

-   Cells consist of three parts: the **plasma membrane, cytoplasm** and
    **nucleus**.
-   Substances pass through the plasma membrane by **diffusion** (gases,
    lipids), **osmosis** (water), **active transport** (glucose, ions),
    **phagocytosis** (particles), **pinocytosis** (fluids) and
    **exocytosis** (particles and fluids).
-   **Osmosis** is the diffusion of **water** through a **semipermeable
    membrane**. Water diffuses from high water \"concentration\" to low
    water \"concentration\".
-   The cytoplasm consists of **cytosol** in which are suspended **cell
    inclusions** and **organelles**.
-   organelles include **ribosomes, endoplasmic reticulum, mitochondria,
    Golgi bodies** and **lysosomes**.
-   The **nucleus** controls the activity of the cell. It contains the
    **chromosomes** that are composed of **DNA**.
-   The cell divides by **mitosis** and **meiosis**

## Worksheets

There are several worksheets you can use to help you understand and
learn about the cell.

Plasma Membrane
Worksheet

Diffusion and Osmosis Worksheet
1

Diffusion and Osmosis Worksheet
2

Cell Division
Worksheet

## Test Yourself

You can then test yourself to see how much you remember.

1\. Complete the table below:

                     \|Requires energy   \|Requires a semi permeable membrane?   \|Is the movement of water molecules only?   \|Molecules move from high to low concentration?   \|Molecules move from low to high concentration?
  ------------------ ------------------- --------------------------------------- -------------------------------------------- -------------------------------------------------- --------------------------------------------------
  Diffusion          ?                   ?                                       ?                                            Yes                                                ?
  Osmosis            ?                   ?                                       ?                                            ?                                                  ?
  Active Transport   ?                   Yes                                     ?                                            ?                                                  Yes

2\. Red blood cells placed in a 5% salt solution would:

:   swell/stay the same/ shrink?

3\. Red blood cells placed in a 0.9% solution of salt would be in a:

:   hypotonic/isotonic/hypertonic solution?

4\. White blood cells remove foreign bodies like bacteria from the body
by engulfing them. This process is known as
..............................

5\. Match the organelle in the left hand column of the table below with
its function in the right hand column.

  Organelle                          Function
  ---------------------------------- ------------------------------------------------
  a\. Nucleus                        1\. Modifies proteins and fats
  b\. Mitochondrion                  2\. Makes, modifies and stores proteins
  c\. Golgi body                     3\. Digests worn out organelles
  d\. Rough endoplasmic reticulum    4\. Makes fats
  e\. Lysosome                       5\. Controls the activity of the cell proteins
  f\. Smooth endoplasmic reticulum   6\. Produces energy

6\. The cell division that causes an organism to grow and repairs
tissues is called:

7\. The cell division that produces sperm and ova is called:

8\. TWO important differences between the two types of cell division
named by you above are:

:   a\.
:   b\.

/Test Yourself Answers/

## Websites

-   <http://www.cellsalive.com/> Cells alive

    :   Cells Alive gives good animations of the animal cell.
-   Cell Wikipedia

    :   Wikipedia is good for almost anything you want to know about
        cells. Just watch as there is much more here than you need to
        know.
-   <http://personal.tmlp.com/Jimr57/textbook/chapter3/chapter3.htm>
    Virtual cell

    :   The Virtual Cell has beautiful pictures of lots of (virtual?)
        cell organelles.
-   <http://www.wisc-online.com/objects/index_tj.asp?objid=AP11403>
    Typical animal cell

    :   Great interactive animal cell.
-   <http://www.wiley.com/college/apcentral/anatomydrill/> Anatomy drill
    and practice

    :   Cell to test yourself on by dragging labels.
-   <http://www.maxanim.com/physiology/index.htm> Max Animations

    :   Great animations here of diffusion, osmosis, facilitated
        diffusion, endo- and exocytosis and the development and action
        of lysosomes. A bit higher level than you need but still not to
        be missed.
-   <http://www.stolaf.edu/people/giannini/flashanimat/transport/diffusion.swf>
    Diffusion

    :   Diffusion animation - good and clear.
-   <http://www.tvdsb.on.ca/westmin/science/sbi3a1/Cells/Osmosis.htm>
    Osmosis

    :   Nice simple osmosis animation.
-   <http://zoology.okstate.edu/zoo_lrc/biol1114/tutorials/Flash/Osmosis_Animation.htm>
    Osmosis

    :   Diffusion and osmosis. Watch what happens to the water and the
        solute molecules.
-   <http://www.wisc-online.com/objects/index_tj.asp?objid=NUR4004>
    Osmotic Pressure

    :   Do an online experiment to illustrate osmosis and osmotic
        pressure.
-   <http://www.stolaf.edu/people/giannini/flashanimat/transport/osmosis.swf>
    Osmosis

    :   Even better osmosis demonstration - you get to add the salt.

## Glossary

-   Link to Glossary

