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Cell Structure

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Cell Structure

Cells are the basic units of all life. They make up every organ of every animal, plant, fungus, and bacteria. Cells in a body are like the building blocks of a house. They can be divided into two main categories: prokaryotes and eukaryotes.

Prokaryotic and eukaryotic cell structures

The definition of prokaryote roughly translates from Greek as: 'without kernel' meaning 'without nucleus'. Hence, prokaryotes never have a nucleus. Prokaryotes are usually unicellular, which means that bacteria, for example, are only made up of one single cell. There are, however, exceptions to that rule where the organism is unicellular but has a nucleus, so it's a eukaryote. Yeast is one example.

On the other hand, eukaryote in Greek translates to “true nucleus”. This means that all eukaryotes have a nucleus. Except for yeast, eukaryotes are multicellular as they can be made up of millions of cells. Humans, for example, are eukaryotes, and so are plants and animals. In terms of cell structure, eukaryotes and prokaryotes share some traits but are different in others. The following table shows the similarities and differences while also giving us a general overview of the cell structures we will be discussing in this article.

Table 1. Features of prokaryotic and eukaryotic cells.

Prokaryotic cells

Eukaryotic cells
Size
1-2 μm
Up to 100 μm
Compartmentalization
No
Membranes that separate different organelles of the cell
DNA
Circular, in the cytoplasm, no histones
Linear, in the nucleus, packed with histones
Cell membrane
Lipid bilayer
Lipid bilayer
Cell wall
Yes
Yes
Nucleus
No
Yes
Endoplasmic reticulum
No
Yes
Golgi apparatus
No
Yes
Lysosomes & Peroxisomes
No
Yes
Mitochondria
No
Yes
Vacuole
No
Some
Yes
Yes
Plastids
No
Yes
Plasmids
Yes
No
Flagella
Some
Some
Yes
Yes

Cell Structure cell structure of a prokarytoic cells example StudySmarterFigure 1. An example of prokaryotic cells. Source: StudySmarter Originals.

Figure 2

Cell organelles

There are many cell organelles, the presence of the organelles will depend on whether an organism is prokaryotic or eukaryotic, cell's type and function.

Cell membrane

Both eukaryotic and prokaryotic cells contain cell membranes that are made up of a phospholipid bilayer (as seen below). The phospholipids (red in the figure) are made up of heads and tails. Heads are hydrophilic (water-loving) and face into the extracellular medium, while the tails are hydrophobic (do not like water) and face inwards.

The cell membrane separates the cellular contents from the surrounding medium. The cell membrane is a single membrane.

Figure 3

If there are two lipid bilayers on the membrane, we call this a double membrane (Figure 4).

Most organelles have single membranes, except the nucleus and the mitochondria, which have double membranes. In addition, cell membranes have different proteins and sugar-bound proteins (glycoproteins) embedded in the phospholipid bilayer. These membrane-bound proteins have different functions, for example, facilitating communication with other cells (cell signalling) or allowing specific substances to enter or leave the cell.

Cell signalling: Transport of information from the cell's surface to the nucleus. This allows communication between the cells and the cell and its environment.

Figure 4

Regardless of the structural differences, these membranes provide compartmentalization, separating the individual contents that these membranes surround. One good way to understand compartmentalization is to imagine walls of a house that separate the interior of the house from the external environment.

Cytosol (matrix)

The cytosol is a jelly-like liquid within the cell and supports the function of all the cells' organelles. When you refer to the whole contents of the cell, including the organelles, you would call it the cytoplasm. The cytosol consists of water and molecules such as ions, proteins, and enzymes (proteins that catalyse a chemical reaction). Various processes take place in the cytosol, such as the translation of RNA into proteins, also known as protein synthesis.

Flagellum

Though flagella can both be found in prokaryotic and eukaryotic cells, they have a different molecular build. They are, however, used for the same purpose: motility.

Figure 5

Flagella in eukaryotes are made up of microtubules that have tubulin - a structural protein. These types of flagella will use ATP to move forwards and backwards in a sweeping/whip-like motion. They can be easily confused with cilia as they resemble them in structure and motion. An example of the flagellum is one on the sperm cell.

Flagella in prokaryotes, also often called "the hook" is enclosed by the cell's membrane, it contains protein flagellin. Different from the eukaryotic flagellum, the movement of this type of flagellum is more like a propeller - it will move in clockwise and anti-clockwise motions. In addition, the ATP is not used for the motion; the motion is generated with a proton-motive (movement of protons down the electrochemical gradient) force or the difference in ion gradients.

Ribosomes

Ribosomes are small protein-RNA complexes. You can either find them in the cytosol, mitochondria or membrane-bound (rough endoplasmic reticulum). Their main function is to produce proteins during translation. The ribosomes of prokaryotes and eukaryotes have different sizes, with prokaryotes having smaller 70S ribosomes and eukaryotes having 80S.

Figure 6

70S and 80S refer to the ribosome sedimentation coefficient, an indicator of the sizes of ribosomes.

Eukaryotic cell structure

Eukaryotic cell structure is much more complex than prokaryotic. Prokaryotes are also single-celled, so they can not "create" specialized structures. For example, in the human body, eukaryotic cells form tissues, organs and organ systems (e.g. cardiovascular system).

Here are some structures unique to eukaryotic cells.

Nucleus and nucleolus

Figure 7

The nucleus contains most of a cell's genetic material and has its own double membrane called the nuclear membrane. The nuclear membrane is covered in ribosomes and has nuclear pores throughout. The biggest part of the eukaryotic cell's genetic material is stored in the nucleus (different in prokaryotic cells) as chromatin. Chromatin is a structure where special proteins called histones package the long DNA strands to fit inside the nucleus. Inside the nucleus is another structure called the nucleolus that synthesizes rRNA and assembles ribosomal subunits, which are both needed for protein synthesis.

Mitochondria

Mitochondria are often referred to as energy-producing cell's powerhouses and for a good reason - they make ATP which is essential for the cell to carry out its functions.

Figure 8

They are also one of the few cell organelles that have their own genetic material, mitochondrial DNA. Chloroplasts in plants are another example of an organelle with its own DNA.

Mitochondria have a double membrane just like the nucleus, but without any pores or ribosomes attached. Mitochondria produce a molecule called ATP which is the organism’s energy source. ATP is essential for all organ systems to function. For example, all our muscle movements require ATP.

Endoplasmic reticulum (ER)

There are two types of endoplasmic reticulum - the rough endoplasmic reticulum (RER) and smooth endoplasmic reticulum (SER).

Figure 9

The RER is a channel system that is directly connected to the nucleus. It is responsible for the synthesis of all proteins as well as the packaging of these proteins into vesicles that are then transported to the Golgi apparatus for further processing. For proteins to be synthesised, ribosomes are needed. These are directly attached to the RER, giving it a rough appearance.

In contrast, the SER synthesises different fats and stores calcium. The SER does not have any ribosomes and therefore has a smoother appearance.

Golgi apparatus

The Golgi apparatus is a vesicle system that bends around the RER on one side (also known as the cis side), the other side (trans side) faces towards the inside of the cell membrane. The Golgi apparatus receives the vesicles from the ER, processes the proteins and packages the processed proteins to be transported out of the cell for other uses. Furthermore, it synthesises lysosomes by loading them with enzymes. In plants, the Golgi apparatus also synthesises cellulose cell walls.

Figure 10

Lysosome

Lysosomes are membrane-bound organelles that are packed with specific digestive enzymes called lysozymes. Lysosomes break down all unwanted macromolecules (i.e. big molecules made up of a lot of parts) they are then recycled into new molecules. For example, a large protein would be broken down into its amino acids, and those can later be reassembled into a new protein.

Cytoskeleton

The cytoskeleton is like the bones of cells. It gives the cell its shape and keeps it from folding in on itself. All cells have a cytoskeleton, which is made up of different protein filaments: big microtubules, intermediate filaments, and actin filaments which are the smallest part of the cytoskeleton. The cytoskeleton is found in the cytoplasm near the cell membrane of a cell.

Plant cell structure

Plant cells are eukaryotic cells just like animal cells, but plant cells have specific organelles that are not found in animal cells. Plant cells, however, still have a nucleus, mitochondria, a cell membrane, Golgi apparatus, endoplasmic reticulum, ribosomes, cytosol, lysosomes and a cytoskeleton. They also have a central vacuole, chloroplasts, and a cell wall.

Figure 11

Vacuole

Vacuoles are large, permanent vacuoles mostly found in plant cells. A vacuole of a plant is a compartment that is filled with isotonic cell sap. It stores fluid that maintains turgor pressure and contains enzymes that digest chloroplasts in mesophyll cells.

Animal cells also have vacuoles but they are much smaller and have a different function - they help sequester waste material.

Chloroplasts

Chloroplasts are organelles present in leaf mesophyll cells. Like mitochondria, they have their own DNA, termed chloroplast DNA. Chloroplasts are where photosynthesis takes place within the cell. They contain chlorophyll, which is

a pigment responsible for the green colour that is typically associated with leaves.

Figure 12

There is a whole article dedicated to the humble chloroplast, go have a look!

Cell wall

The cell wall surrounds the cell membrane and, in plants, is made of a very sturdy material called cellulose. It protects the cells from bursting at high water potentials, makes it more rigid and gives plant cells a distinctive shape.

It is important to note that many prokaryotes also have a cell wall; however, the prokaryotic cell wall is made of a different substance called peptidoglycan (murein). And so do fungi! But theirs is made of chitin.

Prokaryotic cell structure

Prokaryotes are much simpler in structure and function than eukaryotes. Here are some of the features of these types of cells.

Plasmids

Plasmids are DNA rings that are commonly found in prokaryotic cells. In bacteria, these rings of DNA are separate from the rest of the chromosomal DNA. They can be transferred into other bacteria to share genetic information. Plasmids are often where the genetic advantages of bacteria originate, such as antibiotic resistance.

Antibiotic resistance means that the bacteria will be resistant to the antibiotics. Even if one bacterium with this genetical advantage survives, it will divide at a high speed. This is why it is essential for people taking antibiotics to finish their course and also only take antibiotics when required.

Vaccines are another good way to lower the risk of antibiotic resistance in the population. If a lower number of people are infected, a lower number will need to take antibiotics to combat the disease and thus a decreased use of antibiotics!

Capsule

A capsule is usually found in bacteria. Its sticky outer layer prevents the cell from drying out and helps bacteria, for example, stick together and stick to surfaces. It is made up of polysaccharides (sugars).

Cell Structure - Key takeaways

  • Cells are the smallest unit of life; they have a specific structure made up of a membrane, cytoplasm and different organelles.
  • Eukaryotic cells have a nucleus.
  • Prokaryotic cells have circular DNA that is in the cytoplasm. They do not have a nucleus.
  • Plant cells and some prokaryotes have a cell wall.
  • Both eukaryotic and prokaryotic cells can have a flagellum.

Frequently Asked Questions about Cell Structure

Cell structure includes all the structures that make up a cell: the cell surface membrane and sometimes cell wall, the organelles and the cytoplasm. Different cell types have different structures: Prokaryotes vary from eukaryotes. Plant cells have different structures than animal cells. And specified cells may have more or fewer organelles depending on the function of the cell. 

Though energy itself cannot be produced, energy-rich molecules can. This is the case with ATP, and it is mainly produced in the mitochondria. The process is called aerobic respiration. 

Mitochondria, Golgi apparatus, nucleus, chloroplasts (only plant cells), lysosome, peroxisome and vacuoles.

The cell membrane is made of a phospholipid bilayer, Carbohydrates and Proteins. It closes off the cell to the extracellular space. It also transports material in and out of the cell. Receptor proteins in the cell membrane are needed for communication between cells. 

Mitochondria, Endoplasmic Reticulum, Golgi apparatus, Cytoskeleton, Plasma membrane and Ribosomes are found in both plant and animal cells. Vacuoles can both be present in animal cells and plant cells. However, they are much smaller in animal cells and can be more than one, whereas a plant cell usually only has one big vacuole. Lysosomes and Flagella are usually not found in Plant cells.

Final Cell Structure Quiz

Question

What is the defining difference between eukaryotic and prokaryotic cells?

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Answer

Prokaryotic cells do not have a nucleus, whereas eukaryotic cells have a nucleus.

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Question

Which structures within the eukaryotic cell contain a double membrane?

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Answer

Nucleus, mitochondria

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Question

What gives the rough endoplasmic reticulum its rough appearance?


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Answer

Membrane-bound ribosomes

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Question

Where is genetic material found in prokaryotes?


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Answer

Within the cytosol as a circular DNA

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Question

What is the main function of mitochondria?


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Answer

Production of ATP

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Question

What organelles are mostly found in plant cells and not in animal cells?


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Answer

Vacuole and cell wall

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Question

How are antibiotic-resistant bacteria often created?


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Answer

Transmission of plasmids

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Question

Which organelles have their own DNA?


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Answer

Mitochondria and chloroplasts

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Question

Which organelle is responsible for breaking down molecules?


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Answer

 Lysosomes 

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Question

What is the sedimentation coefficient of prokaryotes and eukaryotes?


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Answer

Eucaryotes: 80S


Prokaryotes: 70S

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Question

What main types of eukaryotic cells are there?

 

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Answer

 Animal, Plant, Funghi, Protist

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Question

What organelle gives eukaryotes their name?


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Answer

The nucleus

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Question

What organelles do plant cells have that animal cells do not?

Chloroplasts, cell wall, center vacuole

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Answer

Chloroplasts, cell wall, center vacuole

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Question

What organelle is responsible for photosynthesis?


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Answer

Chloroplasts

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Question

What organelles can produce energy sources?


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Answer

Chloroplasts and mitochondria 

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Question

What is the main function of animal vacuoles?


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Answer

 Storage of substances 

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Question

What are the functions of plant vacuoles?


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Answer

Regulate PH, control size, control turgor 

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Question

Which cell can be bigger, animal or plant?


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Answer

Plant cells can be up to 100 micrometers big, animal cells however can only be up to 30 micrometers.

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Question


What is the cell wall in plants made of?


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Answer

Cellulose, a polysaccharide

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Question

What feature do plant cells and prokaryotic cells share?


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Answer

They both have cell wall

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What plant cells are similar to animal skin cells?


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Answer

Epidermal cells in plants have a similar function to skin cells in animals.

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What type of specialised animal cells are there?


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Answer

Skin cells, muscle cells, nerve cells, fat cells, blood cells and the two different reproductive cells

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What do many specialised cells together form?

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Answer

Specialised cells together form tissues, such as muscle tissue.

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Question

What are plant stem cells also called?


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Answer

Meristematic cells

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Question

What gives a plant cell its rectangular shape?

 

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Answer

The stable cell wall 

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Question

What do eukaryotes and prokaryotes have in common in comparison to viruses? 


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Answer

They are living cells.

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Question

Are there unicellular eukaryotes?


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Answer

Yes, they are called protists.

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Besides bacteria, what is the second type of prokaryotes?



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Answer

Archaea

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What cells can viruses infect?


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Answer

Eukaryotes and prokaryotes

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Question

How big are viruses? 


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Answer

20-400nm 

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Why are viruses not alive?


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Answer

They can’t reproduce without a host cell as they do not have ribosomes. 

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Question

How do bacteria multiply mathematically?


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Answer

Exponentially 

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Question

How are bacteria studied?


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Answer

They are put on to a medium with nutrients to multiply and are then often viewed under a microscope. 

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Question

What is the difference between gram-positive and gram-negative bacteria?


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Answer

Gram-positive bacteria have a thicker cell wall.

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What is the most common type of bacteria reproduction?


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Answer

Binary fission. 

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Question


How many orders of magnitude are bacteria bigger than viruses?


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Answer

Bacteria are measured in micrometers whereas viruses are measured in nanometers. These differ by a factor of 1000. 1 micrometers is a thousand nanometers. 

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Question


Which cells can be viewed through a light microscope?


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Answer

Eukaryotic and prokaryotic cells but not viruses. 

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Question


Which types of genetic material can viruses have?


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Answer

DNA or RNA 

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Question

What device is needed to view viruses?


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Answer

Electron microscope 

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What gives bacteria genetic advantages?


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Answer

Plasmids, which are transferred from one cell to another via pili.

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How can cell mass be measured?

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Answer

By weight or by density

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What is cell confluency

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Answer

The percentage of cells in a defined area

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Why do we measure cell confluency?

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Answer

To determine the rate of cell growth

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How is cell confleuncy measured?

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Visually by counting or with specific software

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What do we need to measure cell size?

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Answer

Graticule and stage micrometer

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Question

How do you calibrate a graticule?

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Answer

  1. The stage micrometer is put on the stage where a slide would go 
  2. This calibration only works for one lens/ magnification
    Line up scale on the graticule and stage micrometer

  3. The length of the devision needs to be calculated based on the micrometer

  4. The units are compared e.g. 10 units on the micrometer are equivalent to 50 units on the graticule 

  5. If one unit on the micrometer is 10µm then  one unit on the graticule is 2µm 
    when changing the magnification of the lense it needs to be divided by how many times the new magnification is greater so the measurement on the graticule needs to be divided by 10 if the calibration was done at 40x but is now at 400x

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Question

What is red blood cell volume measurement also called?


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Answer

Haematocrit

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Question

How red blood cell volume measurement done manually?

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Answer

When doing it manually a drop of blood with a known mass is taken and put swiped across a slide in a film. The slide is placed under a microscope and the red blood cells are counted.

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Question

What is the typical haematocrit?

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Answer

35-50%

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Question

Identify three types of microscopes used in laboratories.

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Answer

Light microscope 

Scanning electron microscope 

Transmission electron microscope

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