Cell Structure

Epidermis light microscope image

Prokaryotes and Eukaryotes

Although all cells share some similarities, not all cells are the same. They differ in their structure, function and organisation.

Some organisms consist of only one cell that has to perform all functions. Other organisms contain many different cells, each with a specific structure and function, organised to work together to ensure effective functioning.

Multicellular organisms are made of many cells working together

Cell theory

The cell theory is an important theory in the field of biology. The cell theory states that:

all organisms are made up of cells
new cells are produced from existing cells
the cell is the smallest organisational unit of a living thing

Prokaryotic bacteria cells

Prokaryotic cells

Prokaryotes are unicellular organisms which do not have a membrane-bound nucleus, mitochondria, or any membrane-bound organelles (organelles enclosed within membranes). There are two types of prokaryotes: archaea and bacteria.

With the work coming from the Greek words pro (before) and karyon (nucleus), prokaryotes vary in diameter from 0.1-5.0 micrometres. The four main features of all prokaryotes is the cell membrane, the cytoplasm, ribosomes and genetic material. Prokaryotes do not have any membrane bound organelles, so there is no nucleus enclosing the genetic information. Most of the genetic material forms a large loop called the chromosome, with the rest in small circular rings called plasmids.

Some prokaryotes have extra structures such as a cell wall, pili, flagella, and a capsule. Pili are hair-like structure on the surface that help cells stick to nearby substances. Flagella are whip-like tails that provide the cell with movement. The cell wall protects the cell and provides extra structure. The capsule is a layer composed of complex carbohydrates outside the cell wall and protects the cell.

Most prokaryotic cells are unicellular. Some bacterial species clump together as a colony but they are still individual organisms.

Prokaryotes can be divided into two groups: bacteria and archaea. These cells are similar in that they are small in shape and size but are different in terms of their genetic information and proteins. Bacteria can be found in many different environments and can be beneficial or harmful to other organisms. Archaea are often found in harsh environments such as very hot environments like hot springs.

Assorted prokaryotic cell shapes

a. cocci b. bacilli c. spirilli

Eukaryotic plant cells

Eukaryotic cells

Labelled Eukaryotic animal cell

Eukaryotes are organisms which have a nucleus and other organelles which are enclosed within membranes. The nuclear envelope encloses the nucleus. Eukaryotes may be unicellular or multicellular. Animal, plant and fungi cells are all eukaryotic.

'Eukaryotic' is derived from the Greek eu (true) and karyon (nucleus). These cells range in size from 10-100 micrometres and are much more complex than prokaryotic cells due to the way they package their organelles with membranes.

Eukaryotes are characterised by a membrane bound nucleus which contains the genetic material of the cell. Each organelle has a specific function within the cell. Together these organelles carry out all of the biochemical processes and reactions, such as cellular respiration and photosynthesis, that are required for cell function.

Eukaryotic organisms can be unicellular or multicellular.

Comparison of plant and animal cells

Eukaryotic cells can either be animal or plant cells

Prokaryotic cells are comparatively more simple than eukaryotic cells

Cell types - summary

There are two different kinds of cells - prokaryotic and eukaryotic.

Prokaryotic cells are relatively small and primative. They do not possess membrane-bound structures. This means they lack sophisticated internal detail. Bacterial cell walls are typically composed of a carbohydrate/protein material called peptidoglycan (also known as murein).

While prokaryotic cells do not feature a membrane-bound nucleus, their cells do not contain a single, coiled chromosome that contains all of the deoxyribonucleic acid (DNA) (genes) necessary to control and direct all the activities of the cell. There are also specialised regions within prokaryotic cells where cellular respiration can occur.

Prokaryotes are represented by two domains: Bacteria (bacteria and blue-green algae) and Archaea (which includes extremophiles). Blue-green algae are photosynthetic bacteria.

Eukaryotic cells are relatively larger and more complex than prokaryotic cells. They possess membrane-bound organelles such as a nucelus, mitochondria and lysosomes.

Eukaryotic organisms (Domain Eukarya) include the kingdoms:

Protista
Fungi
Plantae
Animalia.

Note: viruses are non-cellular parasitic agents of disease. They are composed of a core of ribonucleic acid (RNA) or DNA surrounded by a protein coat. Prions are also non-ceullar agents of disease, but they are composed only of protein.

Bacteria cells stained with Methylene blue

Bacterial gram stain

A comparison of the sizes of common objects and their visibility using the naked eye, a compound light microscope and an electron microscope (scanning and transmission)

Hooke's microscope

Hooke's cork cells

Microscope technologies

As cells are very small, scientists invented the first microscope in order to see greater detail. As a result, Robert Hooke was able to view and describe the structures inside a sample or cork that he named 'cells' as they looked like little prison cells.

The streak method of agar plating

Cell culturing

Cell culturing is the process scientists use to grow cells under controlled conditions in vitro meaning 'in glass'. Each cell culturing method is specialised, and will depend on the organism you are trying to grow.

To grow cells we must replicate their natural growth conditions. Firstly, an appropriate medium must be chosen. Most cells like to have an adherent surface to grow on (e.g. agar - a jelly like medium that can be infused with various nutrients), but some can be grown free-floating in medium. The addition of a food source (carbohydrates), amino acids, vitamins, minerals, gases (CO2 and O2) and growth factors are important. It is also essential that the culture conditions are regulated, so a pH buffer is added, and temperature kept constant.

Bacterial growth on nutrient agar

Cell fractionation process

Cell Fractionation

Cell fractionation is a process used by scientists to separate out different cell components so that they can be analysed individually. Cell component function is preserved, allowing us to conduct experiments which test what the component does, as well as its structure. An example of a fractionation technique is differential centrifugation, which separates organelles by their density.

Cell fractionation process in detail

The magnification power of the naked eye, the light microscope, and the electron microscope

Amoeba cells light micrograph

Light Microscopy

Light microscopy is the passing of visible light through a sample, with the use of lenses to magnify the image. It can be used to successfully image objects 1 mm to 0.2 micrometres in size.

To calculate the total magnification when using a light microscope, the magnification of the eyepiece is multiplied by the objective lens being used. Typically, the eyepiece has a magnification of 10x.

In microscopy, resolution is also important in order to see the detail of structures. Resolution is the ability to distinguish between two separate objects. It is the smallest distance between two objects where each can be observed as separate. The a light microscope, the maximum resolution is 200nm. This means that the best light microscopes will only be able to distinguish two separate structures if the distance between them is 200nm or more. If the distance is less, two distinct object will blur together.

Both living and non-living specimens can be viewed using a light microscope.

Plant vascular tissue

Typical light microscope

The path of light in a light microscope

Rat kidney epithelial cell

Fluorescence Microscopy

Fluorescence microscopes are able to detect fluorescence or phosphorescence, often producing images which can identify certain aspects of cells. For example, mammalian cells can be stained with fluorescent compounds which bind to nucleic acids, enabling us to identify where the nucleus of the cell is. Fluorescent microscopy is useful to identify cells which express fluorescent reporter proteins, a common technique in bacterial culturing.

The fluorescence microscope is similar to the light microscope. Structures beyond the limit of a light microscopes resolution are able to be seen using this microscope. The sample to be viewed is labelled with a fluorescent substance that will attach to specific structures. The sample is illuminated with a high-intensity source of light the causes the fluorescent substance to emit light. This fluorescent light is directed through filters that separate it from surrounding light and the viewer is able to see only those areas of the sample that are fluorescing.

Macrophage fluorescence micrograph

Typical fluorescence microscope

The path of light in a fluorescence microscope

Virus' electron micrograph

Electron Microscopy

Electron microscopes use a beam of electrons to illuminate objects. As electrons have significantly smaller wavelengths compared to visible light photons, electron microscopy can image objects on a much smaller scale, anywhere from 1mm to 0.nm. The interaction between the electrons and the object forms a viewable image on a screen.

The use of the electrons provides much greater magnification. Electron microscopes also have a much high resolution than light microscopes as electrons have a smaller wavelength than light. This allows a greater observation of structures within cells, such as organelles, which might be difficult to distinguish under a light microscope. The electron microscope reveals structures at not only the cellular level but also at the subcellular levels. Materials that were formerly believed to have little or no structure have been shown to have elaborate internal organisation.

There are two main types of electron microscopes: the scanning electron microscope (SEM) and the transmission electron microscope (TEM).

The TEM transmits electrons through the specimen. The TEM produces a 2D image and is the most common form of electron microscope. The TEM can magnify up to 1 500 000 times and has a resolution of about 3nm.
The SEM bombards solid specimens with a beam of electrons which causes secondary electrons to be emitted from the surface layers of specimen. The SEM has poorer resolution (about 10nm) than a TEM, but gives excellent 3D images of surfaces.

A drawback of electron microscopy, however, is that the images must be produced in a vacuum. This means that cells must be dead to be imaged, so we can't see cellular processes in action and the structure of the cell might be slightly altered from natural conditions. It is also significantly more expensive than light microscopy. The preparation of the specimen is very complicated and comes with the risk of tampering with the specimen.

Transmission micrograph

Gram positive and negative bacilli bacteria

Staining

Staining methods can be used to enhance the appearance of cell structures under the microscope. A very common staining technique is Gram staining, which colours bacterial cell membranes either pink or purple, and can therefore help in identifying the type of cell.

Gram staining technique steps

Antibody HDAC4 is stained vs unstained

Immunohistochemistry

All cells have different protein markers, antigens, coating their exterior. Antigens act as tissue identifiers. For each antigen, there exists a specifically shaped complementary antibody which binds to it.

Immunohistochemistry is a technique which takes advantage of these different antigens/antibody pairings by conjugating marker molecules to antibodies, then introducing them into a tissue culture. The antibodies will bind to their complementary tissue antigens, and these can be imaged due to the presence of the enzyme attachment. This technique allows us to identify different biological tissues, and can identify the location of specific proteins within a tissue.

Technique for preparing a wet mount slide

The light microscope - summary

Most cells are so small that they can only be seen with a microscope. The lght microscope (also called the compound microscope) uses light and a system of lenses to magnify the image. One lens is called the objective lens and the other is the eyepiece or ocular lens. The total magnification of a microscope is calculated by multiplying the magnifying power of the ocular lens by the magnifying power of the objective lens.

Beneath the microscope stage is a mirror or build in light source, a condenser lens system to concentrate the light, and an iris diaphragm mechanism to regulate the amount of light passing through the object.

Plant vs animal cell organelles

Cell organelles

All eukaryotic cells contain membrane-bound internal structures called organelles, each with a specific structure and function. Different organelles share the common feature of having internal structures that are enclosed by their own membrane. Each organelle has a different structure and function, they all work together to contribute to the effective functioning of a cell as a unit.

Endosymbiosis is believed to be the origin of eukaryotic cells

Cell organelles

Organelles are distinct structure within cells that perform specific functions. Some organelles are visible using the light microscope, while others are not. The details of many cell organelles are only visible when using the electron microscope.

Endosymbiotic theory:

Mitochondria and chloroplasts are organelles that display some unusual features, including their own ribosomes and DNA in the form of a single circular chromosome. They also feature a double membrane and the ability to replicate independently of the cells that contain them. Scientists believe these prokaryotic-like features suggest they originated around 1.5 billion years ago as free-living bacteria that were engulfed by other free-living bacteria in a relationship that offered mutual advantage. This is called the endosymbiotic theory.

Size comparison of common microscopic molecules

Organelle size

One of the differences between organelles is their size. Some organelles, such as the nucleus, vacuoles and chloroplasts, can be seen with a light microscope.

Comparison of organelles visible to types of microscope

The arrangement of organelles also varies depending on whether the cell is a plant or animal. Plant and animal cells contain many of the same organelles. The plant cell, however, possesses a cell wall, chloroplasts and a large vacuole that the animal cell does not.

Advances in biology mean that we are able to look at smaller and smaller biological objects. Centimetres, millimetres and even micrometres are often too large to measure some objects.

Scientific diagram of plant cell

Drawing cell diagrams

When drawing different types of cells, it is useful to draw these cells to scale. This allows a more accurate representation of the size of these cells. Diagrams that are drawn to scale will always contain a 'scale bar' which indicates the scale to which the diagram is drawn. The scale to be used can be calculated by dividing the actual size of the cell by the length of the diagram to be drawn.

Eukaryotic animal cell organelles

Cell size can be calculated from a microscope using this equation

Investigating cells - summary

Cytology is the study of cells. Cytologists use a variety of tools and techniques to study cells. The main tool used by cytologists is the microscope. There are many different types of microscopes but the two main ones are the light microscope and the electron microscopes.

Cell size:

Cells vary greatly in size. Most cells are only visible under a light microscope, and their size is usually measured in micrometres (μm). There are 1000micrometres in 1 millimetre (mm). Although most cells are microscopic, there are some exceptions - the egg cell of some bird species can be many centimetres (cm) in diameter. The thickness of cell membranes also differs between cells, and can be between 0.004 and 0.1μm thick.

Low-power view of corn stem (x30)

A high-power view of a corn stem (x600)

Biological drawings - summary

The following guidelines should help you to make simple and effective scaled diagrams of biological structures.

Drawings should be:
- made in grey lead pencil
- large
- fully labelled with the name of the specimen, the type of preparation and the magnification
- given a size perspective so that comparisons can be made between specimen sizes - draw each specimen in relation to the size of the field of view observed.
Lines to labels should be ruled - they should not have 'arrowheads' and should not cross over.
Drawings of low-power images should not show the detail of cells, just the 'area of cell types'
Drawings of images made under high power should show the detail of a few cells only of each type

The fluid mosaic model of the cell membrane

The Fluid Mosaic Model

The Fluid Mosaic Model is used to explain the structure & function of all cell membranes. The main function of the cell membrane is to separate the cytoplasm from the external environment and in eukaryotes to create organelles. Furthermore the membrane regulates the movement of substances.

The phospholipid bilayer

The Lipid Bilayer

The current theory of how the cell membrane functions is called the 'fluid mosaic model'. This model describes the cell membrane as a two-dimensional liquid, which restricts diffusion of molecules across it's lipid bilayer. The lipid bi-layer is formed as the hydrophilic 'heads' face outward, and the hydrophobic 'tails' bind together inwardly to form a structure with two layers.

Components of the cellular membrane, including proteins, cholesterol and carbohydrates, and are incorporated into the phospholipid bilayer. They are not static, but move around the membrane as required, depending on the cellular environment. They perform important functions such as channeling specific molecules across the membrane, maintaining structural integrity, and allowing for cell recognition.

Cellular membranes (also known as plasma membranes) are composed of a phospholipid bi-layer, which forms a selectively permeable protective layer around cellular contents. The fluid mosaic model describes the cell membrane as a double layer of lipids with the ability to flow and change shape, like a 2D fluid. Specialised protein molecules are embedded in the lipid in various patterns. Some of these proteins can move sideways, but others are fixed in position.

The fluid part of the membrane is composed of 2 layers of phospholipids. Each phospholipid in these layers can be represented by a head and two tails. A phosphate group on the head makes this end hydrophilic (water loving) and a fatty acid tail which is hydrophobic (water hating).

When these molecules form a bilayer, the hydrophilic heads are positioned facing outwards towards the interstitial fluid (fluid between cells) as well as inwards towards the cytoplasm. This leaves the water hating tails facing inwards. This structure is not rigid and thus is fluid.

The phospholipid bilayer with embedded proteins

The structures embedded in the fluid mosaic model

Embedded structures

A type of lipid called cholesterol is interspersed among the phospholipid layer in plants, which makes the membrane more flexible. Membrane flexibility in plants is increased by a different lipid - phytosterol. Cell membranes are able to break and reassemble themselves during processes such as cell division. This means that the cells can change shape and grow.

This structure forms the basis of the cell membrane and all other membranes within cells, such as those around organelles. Proteins are then interspersed throughout this structure.

Protein molecules are scattered throughout, and suspended in, the lipid bilayer. Some proteins penetrate all the way through the bilayer, forming channels that allow some materials to cross the membrane. Other proteins may be partly embedded in the membrane. It seems that some proteins are fixed in place, while others travel about freely. The proteins are described as 'floating' in the lipid bilayer.

Some proteins function as pores or form active carrier systems or channels for transport, while other proteins have carbohydrates attached for cell recognition. These proteins enable cell-to-cell interaction and the exchange of substances between the cell and the environment.

Transport proteins act like passageways that allow specific substances to move across the membrane. Membrane proteins are also involved in cellular communication. Receptor proteins are different in different types of cells. They cause the cells to respond only to certain signals from substances such as hormones that bind to them, giving them specific functions. Recognition proteins are made up of a protein molecule with a carbohydrate molecule attached. These proteins identify the cell and are called antigens. They allow the immune system to distinguish between foreign particles and the body's own cells. This ensures that the immune system will destroy only foreign particles that display 'non-self' markers.

The cell membrane controls the exchange of material between the inside and outside of the cell. It is selectively permeable, meaning that it allows only certain molecules or ions into or out of the cell. The membrane controls movement of ions and organic molecules in and out of the cell, performing an important role in maintaining cell structure and function. It is also responsible for cell adhesion (the binding of cells to their environments), and cells signaling (communication between cells).

Biological membranes are composed of a phospholipid bilayer with large protein molecules embedded in the bilayer. These proteins provide channels for the passive and active movement of certain molecules across the cell membrane

Short carbohydrate molecules attached to the outside of the membrane are involved in cell adhesion and cell recognition

Cellular membranes - summary

The internal environment of cells is the intracellular fluid - the medium inside cells. The external environment of cells is the extracellular fluid - the watery medium surrounding cells.

Cell membranes:

The cell membrane (also known as the plasma membrane) controls entry ande exit of substances into and out of cells. It controls which substances leave and ter, when and how much. It responds to intrsuctions from the nucelus. It can detect and respond to external stimuli.

The cell membrane is described as being semipermeable because it is permeable to some substances but not others.

The composition of the cell membrane is basically the same as that of all membranes within cells (nclduing the membranes of the nuclear envelope, mitochondria, Golgi apparatus, endoplasmic reticulum, vacuoles, lysosomes and chloroplasts).

The cell membrane consists of a double layer of special lipid molecules called phospholipids. This is called the phospholipid bilayer. The bilayer has protein molecules scattered through it in a random arrangement. The total structure is fluid. This means that the molecules can move around relative to each other. The structure of the cell membrane is commonly described using the fluid mosaic model.

In summary, the cell membrane is flexible, semipermeable barrier between the intracellular and extracellular environments.

Cell structure

Inquiry question: What distinguishes one cell from another?

Investigate different cellular structures, including but not limited to:
- Examining a variety of prokaryotic and eukaryotic cells
- Describe a range of technologies that are used to determine a cell's structure and function
Investigate a variety of prokaryotic and eukaryotic cell structures, including but not limited to:
- Drawing scaled diagrams of a variety of cells
- Comparing and contrasting different cell organelles and arrangements
- Modelling the structure and function of the fluid mosaic model of the cell membrane

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