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Human Biology Enabling Course –

Module 2

Cellular Level of Organisation

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May 2011

Endeavour College of Natural Health Enabling Course: Human Biology Module 2

Contents

Module 2 – Cellular Level of Organisation

1 Introduction

1.1 Background

2 Anatomy and physiology of the human cell

2.1 The plasma membrane

2.2 Plasma membrane proteins

2.3 Diffusion

2.4 Cytoplasm

2.5 Cytoskeleton

2.6 Endoplasmic reticulum (ER)

2.7 Ribosomes

2.8 The Golgi complex

2.9 Lysosomes

2.10 Mitochondria

2.11 Nucleus

2.12 Cell Division

2.13 Mitosis

2.14 Prophase

2.15 Metaphase

2.16 Anaphase

2.17 Telophase and Cytokinesis

2.18 Meiosis

2.19 Activity

3 References


Module 2 – Cellular Level of Organisation

1 Introduction

1.1 Background

As briefly mentioned in module 1, there are various levels of organisation that make up a

human body. The first level is the chemical level and contains the smallest units known as

atoms. Over millennia atoms formed molecules through a multitude of chemical reactions

and eventually gave rise to the living cell.

The human body is made up of trillions of cells. There are many different types of cells, for

example, liver cells, brain cells and muscle cells. Every cell is programmed by human DNA

to carry out a specific function (physiology) such as the liver cell’s role in detoxifying

substances taken into the body in the form of liquids, food or airborne pollutants. When

viewed under the microscope cells vary in shape and size (anatomy) but they all essentially

share common features and it is these similarities that this module will focus on using a

generic model/picture of a cell as a reference point such as Figure 1.

Figure 1. Human cell


2 Anatomy and physiology of the human cell

2.1 The plasma membrane

Figure 2. Plasma Membrane

As mentioned in Module 1 (cellular level) each cell is a complex factory of important parts

that work integrally to maintain the life and function of the cell. To contain and protect the

inner components of the cell a protective membrane formed known as the plasma

membrane Figure 2. The term plasma refers to the contents of the cell.

The plasma membrane is made up of a phospholipid (phospho = containing phosphorus;

lipid = fat) double layer known as a bilayer. Facing the extracellular (outside) and

intracellular (inside) environments are phosphorous polar (charged) heads (see Figures 2 &

3.) that attract water and known to be hydrophilic (hydro = water; philic = attract).

Between the layers of polar heads are the lipid tails (non-polar) that repel water making them

hydrophobic (phobic = repel). Together this arrangement makes up the cell (plasma)

membrane (Figure 3). It is important to note that plasma membrane and cell membrane are

terms used interchangeably.

Figure 3. (from http://www.prism.gatech.edu)


The plasma membrane separates the contents of the cell (intracellular) from the outside

(extracellular) environment. It functions like a gatekeeper allowing some substances such as

nutrients like glucose to pass into the cell for metabolism while keeping other harmful

substances out. This protective role can break down in the face of some viruses and bacteria

that have developed ways to trick the cell into allowing them to pass.

Figure 4. Cell membrane proteins

2.2 Plasma membrane proteins

The plasma membrane is littered with various shaped bodies some of which penetrate

through the double layer to the inside of the cell (Figures 2 & 4). These structures are protein

molecules that may function as locks and gatekeepers.

This arrangement of surface proteins provide the mechanism of entry for life giving

substances into the cell needed for metabolism and the exit points for waste products that

are produced as a result of metabolism. It’s essentially the same as when you eat food and

your digestive system extracts the nutrients required for health and expels the waste in the

form of urine or faeces.


2.3 Diffusion

Some substances such as oxygen and carbon dioxide enter and exit the cell membrane with

ease a process referred to as simple diffusion (see Figure 5). Diffusion occurs when a

substance moves from an area of high concentration to an area of lower concentration

known as the concentration gradient.

In the example of oxygen and carbon dioxide, the concentration of oxygen is greater on the

outside of the cell as fresh oxygen arrives from the lungs, while on the inside of the cell the

process of metabolism has created a higher concentration of carbon dioxide (waste product).

Therefore according to simple diffusion laws oxygen will naturally flow into the cell where it’s

concentration is lower and carbon dioxide will flow toward the outside of the cell where it’s

concentration is lower. This process ensures that body cells always have a fresh supply of

life giving oxygen and also ensures that waste products are removed from the cell to

maintain homeostasis.

Figure 5. Simple Diffusion


Figure 6 shows that larger substances such as glucose require a protein receptor to facilitate

passage into the cell through a process called facilitated diffusion.

Figure 6. Facilitated Diffusion

Visit the links below to view an animation of receptor proteins at work.

http://telstar.ote.cmu.edu/biology/MembranePage/index2.html

http://www.lionden.com/cell_animations.htm

Other forms of entry into the cell require energy to pump a substance against its

concentration gradient known as active transport as demonstrated in Figure 7.

Figure 7. Active Transport

http://telstar.ote.cmu.edu/biology/MembranePage/index2.html

http://www.lionden.com/cell_animations.htm


Visit the links below to view an animation of passive (simple) and facilitated diffusion.

http://www.youtube.com/watch?v=s0p1ztrbXPY

http://www.youtube.com/watch?v=PkmF7yoWiXU&feature=related

The various functions of membrane proteins will be covered in more detail in classes but it is

important that you view the animation links provided to gain a conceptual overview of the

important role these proteins play in the life and maintenance of human body cells.

2.4 Cytoplasm

Cytoplasm is the name given that collectively refers to the watery electrolyte fluid

(intracellular fluid) and the small organs of the cell known as organelles. Every cell is a little

factory powerhouse with many organelles performing unique functions that contribute to

cellular metabolism. The tasks carried out number in their thousands and are performed in

an orderly, integrated way that is governed by the cells genetic material/code located on the

DNA contained in the nucleus. One function that is performed by every cell in the body is

that of catabolising (breaking down) glucose to provide enough energy for cellular activity.

Figure 8. Cell and its cytoplasm

http://www.youtube.com/watch?v=s0p1ztrbXPY

http://www.youtube.com/watch?v=PkmF7yoWiXU&feature=related


Refer to Figure 8 (above) when you read through the following list of organelles to help you

connect the structure with its function.

2.5 Cytoskeleton

The cytoskeleton comprises proteins called microfilaments, intermediate filaments and

microtubules forming a network of interconnecting structures acting as a framework for the

integrity of the cell, essentially forming the cell’s skeleton.

2.6 Endoplasmic reticulum (ER)

Functionally the ER is involved in synthesising (making), transporting and storing newly

made molecules/substances that are involved in cellular metabolism, for example, protein.

Figure 8 shows two parts to the ER, a rough section called the rough endoplasmic reticulum

(RER) studded with ribosomes (see explanation below) and a smooth section (SER) which

in the picture is coloured pink sitting in front of the RER.

2.7 Ribosomes

Ribosomes are tiny spheres made of protein and ribonucleic acid (RNA). Ribosomes either

occur freely within the cell or cluster with part of the endoplasmic reticulum known as the

rough endoplasmic reticulum (RER) where they are involved in manufacturing proteins

functionally called protein synthesis.

Figure 9. Ribosomes located on RER


2.8 The Golgi complex

The main function of the Golgi complex (Figure 10) is to process, sort and deliver newly

manufactured proteins (from the RER) and lipids to the cell membrane, to lysosomes (for

digestion) and to secretory vesicles that enable contents within the cell to be released

outside the cell, for example, certain glandular secretions. Note in Figure 10 the green

spheres entering the golgi complex, they are newly created proteins from the RER.

Figure 10. Golgi Complex


2.9 Lysosomes

Figure 11. Lysosome

Lysosomes are membrane enclosed spheres or vesicles that contain powerful digestive

enzymes. They function to digest foreign substances and worn out organelles. They play an

important role in the homeostasis of the cell.

2.10 Mitochondria

Mitochondria (Figure 12) are the energy powerhouses of the cell. These organelles utilise

the energy molecules consumed from food such as glucose and fat and produce energy in

the form of ATP (adenosine triphosphate) for cellular metabolism.

Figure 12. Mitochondria


2.11 Nucleus

Figure 13. Nucleus

When viewing a cell under the microscope such as the bottom picture in Figure 13, the

nucleus is usually the most prominent feature. Most body cells have a single nucleus

whereas muscle cells, for example, have several. Some have no nucleus such as red blood

cells. The main function of the nucleus is to house the genetic code found on the DNA.

View the link below for a tour of the cell.

http://people.eku.edu/ritchisong/301notes1.htm

2.12 Cell Division

Especially important during growth and development is the cell’s ability to divide or make

copies of itself. The human lifespan is a dynamic process involving many changes in the

body over time. Some cells, such as neurons (nervous system cells) last a lifetime, whereas

other cells, such as skin cells are sloughed off daily, making up the majority of the dust in

your home.

There are two types of cell division in the human body known as mitosis, which is replication

of body cells and meiosis, which involves only the production cells known as sperm and

oocytes (gametes).

http://people.eku.edu/ritchisong/301notes1.htm


2.13 Mitosis

Mitosis is somatic (all body cells except gametes) cell division and comprises several steps

or phases. Just prior to the mitotic phase (actual cell division), the cells undergo a growth

and replication phase called interphase. During interphase the cell’s cytoplasm increases

and the DNA made up of 46 chromosomes (23 from mum and 23 from dad) make copies of

themselves (replicate) so they can divide neatly into two new cells. Figure 14 shows the

cycle of cell division including a description of interphase.

Figure 14. Mitotic cycle

Once interphase is complete, the cell enters the mitotic or dividing phase. There are four

stages of mitosis.


2.14 Prophase

Figure 15. Prophase (from http://www.chuck16.wordpress.com)

During prophase chromosomes not normally visible under the microscope become visible as

they condense in preparation for division. The nucleolus and nuclear envelope disappear.

The centrosomes responsible for pulling apart (dividing) the chromosomes move to opposite

ends of the cell.

2.15 Metaphase

Figure 16. (from http://www.staff.jccc.net)

The chromosomes which comprise two identical parts called chromatids (having replicated

during interphase) are held together by a centromere. These line up in the centre of the cell

along what is known as the equatorial plane. Later, when the chromatids are separated, this

separation starts from the centromeres.


2.16 Anaphase

Figure 17. (from http://www.staff.jccc.net)

Anaphase results is the actual splitting and separation of the chromatids at the centromere.

Long protein spindle ‘fingers’ extend form the centrosome and grab hold of the centromeres.

Once this occurs, the protein spindles start pulling on the centromeres, ultimately separating

the pairs of sister chromatids from each other. As soon as the chromatids are separated,

they are no longer called sister chromatids, they are now known as individual chromosomes.

In late anaphase a cleavage furrow is present and is the beginning of complete division of

the cell.

2.17 Telophase and Cytokinesis

Figure 18.

Telophase (from http://www.mstiboldo.blogspot.com) Cytokinesis (from

http://www.celldynamics.org)

Telophase starts when the separated chromosomes have move towards opposite sides of

the cell. A nuclear envelope starts to reform around the chromosomes, and the cleavage

furrow deepens. As telophase continues, the chromosomes start to disappear again.

http://www.mstiboldo.blogspot.com/


The final step in Mitosis is Cytokinesis. This is the process by which the cleavage furrow is

so big that it meets in the middle of the cell. Ultimately this will pinch off the two newly

created cells from one another, each with their own nucleus and their own organelles.

Visit http://www.youtube.com/watch?v=D1_-mQS_FZ0&feature=related to view mitosis.

2.18 Meiosis

Meiosis occurs in the production of gametes (sperm and oocytes) only. Like mitosis, meiosis

also involves the stages of prophase, metaphase, anaphase and telophase, however the

original cell undergoes two rounds of division, therefore, meiosis consists of prophase 1 & 2,

metaphase 1 & 2 etc. The critical event that occurs during meiosis that accounts for the

individual differences in humans is that during prophase 1, chromosomes cross over (see

Figure 19), which results in a sharing of genes and creation of new combinations of

characteristics in DNA within the gametes/cells.

http://www.youtube.com/watch?v=D1_-mQS_FZ0&feature=related


Figure 19. Meiosis

Visit http://www.youtube.com/watch?v=D1_-mQS_FZ0 to view of meiosis.

Cell division will be covered in greater detail in classes, but it is important to note the key

differences between mitosis and meiosis.

http://www.youtube.com/watch?v=D1_-mQS_FZ0


2.19 Activity

View the links below to explore different varieties of cell shapes and sizes

http://www.lennartnilsson.com/human_body.html

http://www.cellsalive.com/

3 References

Tortora, G.J., Derrickson, B., 2012. Principles of Anatomy and Physiology, 13th edn, John

Wiley & Sons, Inc, USA.

Winston, R., 2004. Human, DK Publishing, London.

http://www.lennartnilsson.com/human_body.html

http://www.cellsalive.com/

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