PRELIMINARY HSC BIOLOGY NOTES
Cells as the Basis of Life
Table of Contents
Overview of Week 1’s Inquiry Question
Learning Objective #1 – What is a cell? Prokaryotic vs Eukaryotic Cells
Learning Objective #2 – Organelles in different types of cells
Learning Objective #3 – Comparing cell organelles by structure and function
Learning Objective #4 – Historical development of the cell theory
Learning Objective #5 – Constructing scaled diagrams for cells
Learning Objective #6 – Importance of the fluid mosaic model
HSC Biology Syllabus Lecture Video – Cells as the Basis of Life
Week 1 Homework Set (Essential for Band 5)
Curveball Questions (Moving from Band 5 to Band 6!)
Solutions to Week 1 Homework Set
Before we hop on the materialistic train and start digging into the content, please give me a minute to walk you through what you should keep in mind as the ‘major highlights’ for this week’s material.
The inquiry (overarching) question for this week deals with differentiating two types of cell structure, prokaryotic and eukaryotic cells.
For those two types of cells, we are required to explore their unique components. This examining their organelles (the ‘organs’ of a cell).
After the overview of eukaryotic and prokaryotic cells & their organelles, we will travel back in time to have a look at how we came about to understand cells’ structure that we do today.
This will be followed by you learning how to draw a cell to scale (a scaled diagram)!
Lastly, we will end this week’s note with understanding the structure and function of the cell membrane. The cell membrane encloses the cytoplasm (fluid) of the cell which occupies the organelles.
Learning Objective: Investigate different cellular structures, examining a variety of prokaryotic and eukaryotic cells
What is a cell?
Before we start exploring the cellular structures of prokaryotic and eukaryotic cells, we need to first understand what cells are. So, please bare with me 🙂
Cells are the most basic or fundamental living unit for life. Different cells specialise in different functions. Our organs are made up of many millions of cells, usually a combination of cells with different functions (specialisations).
Within cells, there are organelles. You can think of organelles are the ‘organs’ (parts) of your cells.
The image below is of an ANIMAL cell and its organelles.
As you can see from the diagram, the cytoplasm is a fluid medium that occupies many organelles in your cell. The cell membrane encloses the cytoplasm of a cell.
The nucleus is basically the ‘brain’ of the cell. It stores all the DNA and transmits appropriate information for the appropriate cell activities to be executed. This includes controlling the process of protein-synthesis (the making of proteins) which makes up an organism’s functions, appearance and behaviour!
Remember, an organism is made up of many cells. Each cell has a nucleus and many organelles. We will explore how protein-synthesis works in the later weeks!
Some of organ shown in the above animal cell diagram are the mitochondria and vacuole. In case you are wondering, each organelle is made up of atoms.
Cells make up organs and an organism’s tissues.
Generally, multicellular organisms (organisms made up of more than one cell such as humans) have eukaryotic cells and unicellular organisms have prokaryotic cells.
Eukaryotic cells are the more evolved version of prokaryotic cells. We will discuss evolution in the later weeks.
Both eukaryotic cells and prokaryotic cells have a cell membrane. However, prokaryotic cells lack a true nucleus that isolated genetic material (DNA/RNA) from its organelles.
Also prokaryotic cells DO NOT have membrane-bounded organelles (i.e. organelles do not have a membrane on its outer layer).
The above animal cell diagram is a eukaryotic cell. This is because all eukaryotic cells have membrane- bounded organelles (e.g. mitochondria) and it has a true nucleus that isolates its DNA material from the rest of the organelles in the cytoplasm!
We will NOW explore the difference in cellular structures between prokaryotic and eukaryotic cells!
Cellular structure differences between prokaryotic cells and eukaryotic cells!
We have mentioned the difference between the nucleus between prokaryotic and eukaryotic cells.
We have mentioned that prokaryotic cells do not have a true nucleus isolating genetic material from its cell organelles whereas eukaryotic cells have a true nucleus.
We have also touched on the fact that, unlike prokaryotic cells, eukaryotic cells have membrane-bounded organelles.
The table below highlights some other cellular structure differences between prokaryotic and eukaryotic cells.
Now, let’s move on to explore the structure and functions of these cell organelles that were mentioned!
It is important to realise that there are more than just organelles occupying in the cytoplasm. There are other substances such as amino acids, dissolved mineral ions, sugars, enzymes (proteins) that are essential for cell activities in the cytoplasm of the million of cells that makes up a multicellular organism!
Let’s move back in time to see how we got to understand the structure and function of cell organelles!
Learning Objective: Investigate different technologies used to determine cell’s structure and function
Many scientists advanced our knowledge of the Cell Theory. During their work, many forms of technologies were used.
The cell theory has three components:
- Cells are the smallest organic building blocks of life
- All living organisms are composed of cells
- All cells are derived (‘born’) from pre-existing cells through cell division
Historical development of the cell theory (1480s – 1940s)
- Leonardo Da Vinci used primitive magnifying glass (simple microscope) to observe tiny objectives.
- Han Janssen reduced the magnifying glass lenses to suit the building of microscopes.
- Robert Hooke used the compound microscope and observed a section of a cork. He discovered small compartments which he called cells.
- Anton Leeuwenheok used microscope to find microbes in water molecules
- Henri Dutrochet proposed that all living things are composed of cells
- Robert Brown used staining techniques, discovering and naming the nucleus of plant cells.
- Schleiden and Schwann proposed that cells are the smallest building blocks of life.
- Rudolph Virchow proposed that all cells come from pre-existing cells through cell division
- Walther Flemming used staining techniques to observe cell division
- Ernst Ruska constructed the world’s first electron microscope
NOTE: Staining techniques is a form of technology.
Going further into microscopes
From Simple to Compound microscopes:
The compound microscope had two lenses – objective and eyepiece lens.
This produced greater degree of magnification compared to the single sense magnifying glass (simple microscope)
Due to the introduction of compound microscope, there is enough magnification power to observe cell structures such as cells that cannot be done using a simple microscope (used by Leonardo)
Introduction and use of the electron microscope:
Electron microscope employs beams of electrons to either analyse the surface of a specimen, providing information regarding the surface landscape and chemical composition.
It can also be used to observe the complex, internal structure of cell specimens and their organelles. Cell structure if often considered to be related to cell function.
For example, the discovery of the structural similarities between the endoplasmic reticulum and peroxisomes organelles led to the understanding of how these organelles allowed proteins to travel between these organelle and were responsible for peroxisome biogenesis disorder (disease).
Furthermore, electron microscopy can be used to locate and track molecules and compounds in the body to further our understanding on chemical reaction pathways.
By doing so, more information is revealed about how molecules and compounds react with cells and its organelles & vice versa.
Overall, the electron microscope provide scientists with greater understanding of the structure and function of cells.
Learning Objective: Draw scaled diagrams of a variety of cells
Cells are measured in micrometres (µm). One micrometre equates to 10-6 metres.
Typically, a prokaryotic cell is 1-10 micrometres in diameter.
For an eukaryotic cell, it is approximately 10-100 micrometres in diamater (on average).
When preparing a scaled diagram (a diagram with 1cm = 1µm) to convey the diameter of any cell in HSC Biology, it usually comes with a microscope exercise.
So, let’s explore how we can draw a scaled diagram for a red blood cell using what we see under a microscope!
Experimental procedure: Drawing a scaled diagram for a red blood cell!
In order to draw the scaled diagram, we need to know the diameter of a red blood cell!
Here is the procedure to find out the diameter of a red blood cell (or any other cell).
1. Set up your light (compound) microscope CORRECTLY
2. Place and centre your mini grid on the microscope stage
3. Set your microscope to low magnification power and observe the mini grid for diameter of your field of view. Your total magnification power would be the magnification power, your objective lens multiplied by the magnification of your eyepiece lens
4. Change to high power and calculate your new field of view’s diameter.
- For example, if you total magnification power was 100x at low power (objective lens magn. power times eyepiece magn. power) and your total magnification power at high power was 400x, you need to divide the diameter of your field of view measured at low power by four (in this case) to obtain the new field of view diameter at high power (the size of field of view you are actually viewing the RBC at)
5. Remove mini grid from the microscope stage and replace it with your specimen slide.
6. Estimate the amount of red blood cells that can fit across your field of view’s centre
7. Calculate the diameter of red blood cells by dividing your field of view at high power by the amount of red blood cells
8. Draw the red blood cell to scale
Remember to include a scale in your diagram!
Example 1cm = 1µm
Learning Objective: Model the structure and function of the fluid mosaic model of the cell membrane
The fluid mosaic model is a way for us to see how cell membranes are structured and function.
The cell membrane comprises a phospholipid bilayer with many compounds (‘molecules’) such as proteins, glycoproteins, glycolipids and lipoproteins fitted on or sitting inside the bilayer.
Due to this array of protein and lipid compounds, it gives the membrane a mosaic appearance. The ‘fluid’ component of the model’s name is derived from how the phospholipid molecules and embedded proteins can shift positions on the cell membrane. It is fluid and not static! For each phospholipid, there is a hydrophobic tail and a hydrophilic head. Bare with me with these two new ‘hydro’ (water) terms.
The phospholipid bilayer has a unique structure such that the hydrophobic tails are touching each other while their hydrophilic heads are point in opposite directions. One exposed into the extracellular (outside cell) environment and the other in the intracellular (inside cell) environment.
The hydrophobic end does not dissolve in water and repels with water (e.g. oil is a hydrophobic substance). On the other hand, the hydrophilic head dissolves and mixes well with water.
The combination of having these two hydro properties is why phospholipids are called amphipathic molecules.
So, why should we care about the fluid mosaic model?
The fluid mosaic model helps account for how the cell membrane is structured and functions.
The cell membrane is important because it acts as a physical separation barrier between the intracellular and extracellular environments with selective permeability property to regulate the movement of substances across the membrane (aka in and out of the cell).
These substances include dissolved ions, nitrogenous wastes, organic molecules, gases such as oxygen and carbon dioxide.
The cell membrane also provides and maintain the cell’s shape (cell shape varies depending on cell type – e.g. bone cells, plant cells and red blood cells)
Cell membrane’s selective permeability property comes from its amphipathic nature of the phospholipids. This selective permeability facilitates and restricts various substances in and out of the cell.
The hydrophobic ends hinders highly soluble (high polar) molecules from entering the cell while facilitating non-polar molecules to diffuse through the bilayer (in and out of the cell). Why and How?
Way to think about it
Polar molecules are soluble in polar solvents (e.g. water). Non-polar molecules are soluble in non-polar solvents (e.g. oil). ‘Like dissolves like’.
You can imagine the hydrophilic ends as ‘polar solvents’ and hydrophobic ends as ‘non-polar solvents. Hence, the hydrophilic ends hinder the movement of non-polar molecules passing through the cell membrane and hydrophobic ends hinder the movement of polar molecules passing through the cell membrane. The amphipathic property gives the cell membrane its selective permeability.
Proteins supply energy in the form of ATP to assist the movement of substances across the phospholipid bilayer (cell membrane). For example, proteins supplying energy to move polar molecules through the hydrophobic layer. Without energy, polar molecules will struggle to move through the hydrophobic layer. As this transportation process requires energy, it is called active transport. More on this type of transport later.
However, substances can also move through the membrane via passive transport (no ATP or energy required. One example of passive transport is diffusion – more on this later).
So why is it important to understand how the cell membrane is structured and functions?
Well, substances need to move pass the cell membrane to keep cells alive.
For example, the mitochondria is an organelle inside the cell.
The mitochondria breaks down glucose into ATP via cellular respiration. However, glucose is not made inside the cell. It is obtained from the food that organisms (e.g. humans) consume. Once it is consumed, it is broken down and is dissolved in our blood and travels in our blood vessels. Cells surrounding the vessels will take the glucose! How?
This is done by moving the glucose dissolved in the blood stream and into the cell (across the cell membrane!). Glucose is just one example.
Hence, by understanding how our cell membrane is structured (hydrophobic ends and hydrophilic head) and functions, we can understand how cells may be capable uptake different substances.
This is particularly useful in the field of medicinal chemistry where chemists need to design structure of new medicine compounds so that it can pass through the cell membrane.
Movement of substances across cell membranes
(In and out of cells)
But, first, some DEFINITIONS!
Solvent – the medium in which solutes are dissolved in (e.g. water)
Solute – the substances that are dissolved in the solvent (e.g. sugar)When you throw a teaspoon of sugar in a cup of water, the sugar is the solute and the water is the solvent. Together, the sugar water is called a solution.
Concentration gradient – The process whereby solutes move from an area of low solute concentration to high solute concentration (more solute for a fixed amount of solvent)
Diffusion – Molecules will move from area of high solute concentration to low solute concentration (moving along the concentration gradient)
Osmosis – Water will move from an area of low solute concentration to high solute concentration (due to the attraction to solute charges as water is polar. This means it is attracted to both positive and negative ions of the solute)
Active transport – Moving molecules against the concentration gradient. Moving substances from low solute concentration to high solute concentration (against the concentration gradient)
The movement of substances in and out of the cell membranes can exist in two forms:
• Active Transport
• Passive Transport
It is important to note that you CANNOT actively transport water.
The movement of water can ONLY be done via passive transport (osmosis).
The movement of water will change the solute concentration and thereby the concentration gradient.
The change in concentration gradient will affect the direction and extent of diffusion.
No energy is required for diffusion and osmosis.
For active transport, you do not need to worry about concentration gradient as it uses energy to move substances against the concentration gradient!
Imagining diffusion and osmosis
(Types of Passive Transport)
Suppose we have a fish tank that is separated by a piece of glass in the middle. Both side of the tank has same amount of water (volume wise). We drilled a really tiny hole in the glass, allowing water to pass through but not big enough for the sugar molecules (solute) to pass through. We throw 100g of sugar (solute) on the left side of the fish tank and 200g of sugar on the right side of the tank.
Since the volume of water is the same on both sides of the tank, the right side of teh tank has higher solute (sugar) concentrastion. Therefore, osmosis will cause water to move to the right. The water level on teh left side of the fish tank will decrease over time until the solute concentration is equal on both sidfes of the tank.
Diffusion can be imagined when you put a drop of purple (or any other colour) dye in a glass of water. You will see that the dye moelcules will spread in all directions in the glass from initialy entering the water as one drop in one area. This is because the surrounding water has lower solute concentration (no dye molecules). So the drop of dye (containing many dye molecules) will spread itself out in the water of the beaker to even out fhe solute concentration (moving along the concentration gradient).
Diving deeper into the fluid mosaic model.. (Contains content BEYOND the syllabus)
The model helps us:
- Understand the cell membrane’s structure.
- Understand the dynamic capacity of cell membrane pertaining to allowing substances to be transported across the cell membrane (in and out cells).
The model places great importance of the compounds embedded in the phospholipid bilayer such as the various types of protein complexes and lipid rafts (e.g. cholesterol).
What makes the model useful is not more than just a visual representation of cell membranes, but, also demonstrating the dynamic capacities of cell membranes. For example, the role of the cytoskeletal gates in controlling lateral diffusion.
Lateral diffusion is the movement or shift of a phospholipid molecule with another (nearby) phospholipid on the same side of the bilayer membrane. As a result of lateral diffusion, the two phospholipids switches positions on the cell membrane. Lateral movement occurs approximately 10 million times per second.
Phospholipids can ALSO perform a ‘flip-flop’ event with a phospholipid that is on the SAME longitude (up & down) on the cell membrane but facing on the opposite side. Flip-flop activities are slow (occurs approximately once per month) if they are uncatalysed.
However, in the presence of the catalyst flippase, the rate of phospholipids performing flip-flop activities can increase into the thousands-per-month range.
Generally speaking, there are two categories of protein in the phospholipid bilayer. These are integral and peripheral protein groups.
- Integral proteins are embedded in the hydrophobic compartment of the bilayer with sections of its protruding on the hydrophilic surface.
- Peripheral proteins DO NOT have sections situated in the hydrophobic area of the bilayer. They are only located on the hydrophilic section of the membrane.
Glycolipids and Glycoproteins
Glycolipids and glycoproteins allow cell-cell recognition. It is not important to understand the specifics of the cell-cell recognition process right now.
Rather, have an idea that this process allows the body to recognise whether or not a cell is foreign (through detecting the antigen on the foreign cells – more about antigens in Year 12 Biology). For now, just understand that foreign cells (e.g. bacteria) have antigens.
By recognising the presence of antigens, it allows white blood cells in the body to initiate a response to attack foreign cells such as bacteria which has the antigen. While this is an awesome defence mechanism enabled by Glycolipids and glycoproteins, viruses such as HIV take advantage of glycoproteins to successfully bond with it and trick its way into entering the cell (across the cell membrane!)
Cell-Cell recognition and antigens will be covered in Year 12 Biology.
Verifying the fluid mosaic model through experiments
The movement of phospholipid and proteins on the cell membrane is supported by experiments. Scientists use mark the proteins of a mouse and human cell membrane to differentiate the two species’ proteins (so that they florescent in different colours).
The two cells were forced into one, followed by one hour of observation. After one hour, the proteins of the two cells were scattered uniformly throughout the fused cell. This scattering is evidence of lateral diffusion of the phospholipids and the protein complexes on organisms’ cell membranes.
Cholesterol (makes up the lipid rafts) stabilises the proteins on the cell membrane. This stability is important because the cell membrane shifts regularly to regulate the movement of substances in and out of the cell, lipid rafts help protein to anchor themselves on the cell membrane. Proteins are important in the process of actively transporting substances across the cell membrane. Recall from earlier that this is important to cells’ and therefore the organism’s survival!
Cholesterol also helps maintain the fluidity of the membrane by lowering the solubility of polar molecules that are attempting to pass the cell membrane.
It is able to do this because of its structure – possessing a polar and non-polar component. The non-polar component lowers the ability of highly soluble polar molecules thus hindering them as they attempt to pass through the cell membrane (through the hydrophobic layer).
HSC Biology Syllabus Lecture Video – Cells as the Basis of Life
[Video will be uploaded HERE soon!]
Week 1 Homework Set
(Essential for Band 5)
Question 1: Describe the fluid mosaic model in terms of its features [6 marks]
Question 2: Discuss the safety hazards of using a microscope to determine the size of red blood cells and the safety precautions you would take [4 marks]
Question 3: Outline the independent and dependent variable for the experiment in determining the size of the red blood cells [2 marks]
Question 4: Explain the concept of using a control and controlled variable(s) in an experiment [4 marks]
Question 5: Using a table, distinguish between prokaryotic and eukaryotic cells [8 marks]
Question 6: Outline the structure and function of 8 organelles in an animal cell [8 marks]
Question 7: Describe the historical development of humanity’s knowledge of cells through time [6 marks]
Question 8: Write out the procedure for the experiment in determining the size of a red blood cell (RBC). Draw a scaled diagram for a RBC [6 marks]
Question 9: Draw a diagram of the fluid mosaic model of a cell membrane [4 marks]
Question 10: Describe five technologies used to determine cell structure and function. HINT: one of technology can be the one used to the verify the fluid mosaic model [5 marks]
Question 11: In addition to the lack of a true nucleus and cytoskeleton, prokaryotic cells also lack organelles. True/False
(Moving from Band 5 to Band 6!)
Curveball Question 1: Describe the importance of the fluid mosaic model [6 marks]
Curveball Question 2: Explain lateral diffusion and its importance for cell membranes’ function [3 marks]
Curveball Question 3: Explain how would you calculate the size of a white blood cell found within a blood specimen that contains lots of red blood cells [4 marks]
Curveball Question 4: Active transport such as actively transporting water requires energy. True or False?
Curveball Question 5: Justify the purpose of classifying organisms into eukaryotes and prokaryote organisms [2 marks]
Curveball Question 6: Provide two reasons to why it is important cell to use active transport in addition to passive transport? [4 marks]
Research Task: Research the differences between a plant an animal cell. See below for questions.
- Outline 6 common features between an animal and plant cell. [6 marks]
- Distinguish an animal and plant cell using 6 features [6 marks]
Solutions to Week 1 Homework Set
Solutions to each week’s homework set will be uploaded one week subsequent to the homework set’s upload date.
Have a go at the homework set. Come back here next week to check uploaded solutions! <3