Unit 14: Cell biology

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Cells

We are made up of billions of cells that carry out hundreds of fundamentally different roles in the body. Cells are microscopic – we only know so much about them because of the invention of microscopes, which have the ability to magnify these small units of life. In this topic guide you will become familiar with the common features that cells share and also the differences between cells. On successful completion of this topic you will: •• understand the structural features of eukaryotic and prokaryotic cells (LO1). To achieve a Pass in this unit you will need to show that you can: •• discuss the structural features of prokaryotic and eukaryotic cells (1.1) •• explain the role of subcellular organelles in the cell (1.2).

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Unit 14: Cell biology



1 Introduction to cells All organisms, except viruses, are classified as eukaryotic or prokaryotic based on their cellular structure. Prokaryotic organisms do not have a true nucleus and the genetic material is not enclosed by a membrane. Eukaryotic cells, on the other hand, are larger with a true nucleus and other organelles; DNA is enclosed inside the membrane-bound nucleus.

Microscopes Light microscopes Cells are too small to be seen with the naked eye so, in order to see them in detail, we must magnify them. Light microscopes pass light from a bulb on the base through a condenser lens and then through the specimen. It then continues through the objective lens to the eyepiece.

Key term Resolution: The ability to distinguish between two separate points.

There are a number of objective lenses to enable specimens to be viewed at different magnifications. Magnification can be calculated by multiplying the objective magnification by the eyepiece magnification. Light microscopes are capable of magnification up to ×1 500; however, the resolution is low on a light microscope. See the Case study on page 4 for an example of where microscopes are used in hospitals.

Electron microscopes Electron microscopes have a much higher resolution than light microscopes and therefore can distinguish between smaller structures more effectively. They use a beam of electrons, which has a shorter wavelength than light. Magnets are used instead of lenses to focus the beam of electrons onto the specimen; however, this is not visible to the human eye. A black and white image called an electron micrograph is produced on a screen or photographic paper. Colour micrographs can be produced by adding colour using computer software. However, micrographs are always initially black and white. We can work out the actual size of the organelles present in the specimen because of the relationship between the magnification, the actual size and the image size.

actual size =

image size magnification

By measuring the image size of the organelle of interest on the micrograph in mm and dividing it by the magnification, you will get an answer in mm. This can be converted to µm by multiplying the answer by 1000, because 1 mm = 1000 µm. See Figure 14.1.1 to compare both types of microscope.

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Unit 14: Cell biology Figure 14.1.1: Diagram showing the difference between the two microscopes. Notice a light source is used for the light microscope and electrons are used in the electron microscope.

Light microscope Lamp

Glass lens

Transmission electron microscope Illumination

Condenser lens

Electrons

Electromagnetic lens

Specimen Glass lens

Objective lens

First image

Glass lens

Ocular

Projector lens

Electromagnetic lens Electromagnetic lens

Final image Fluorescent screen

Eye

Eye

Specimen fixation Fixation is important when preparing samples for microscopic examination. It prevents decay and preserves biological samples. Fixation stops enzyme activity and maintains the structure.

Specimen staining Biological specimens are usually colourless and therefore need to be stained in order to view them under the microscope. We use coloured chemical stains that bind to chemicals inside the specimen to accentuate the features to be observed. There are basic dyes (a common example is methylene blue) that bind to negatively charged molecules and there are acidic dyes that possess negatively charged functional groups so they will bind to positively charged structures. Haematoxylin and eosin stain (HE stain) is a popular staining method in use on biological samples. It is the most widely used stain in medical diagnosis, for example biopsies in suspected cancer patients. Hemalum is formed from aluminium ions and oxidised haematoxylin – this is added to the sample and it stains the nuclei of cells blue. Another stain is added called eosin Y – this colours eosinophilic structures in various shades of red and pink.

Specimen sectioning To allow the light or electron beam from the microscope to penetrate the sample, it needs to be in thinly cut sections. When cutting a specimen it can become distorted. In order to prevent this, specimens are embedded in wax or resin. Once it has solidified it is safe to cut the specimen, which will keep its shape.

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Unit 14: Cell biology

Take it further Find out more about histology techniques by visiting this link: http://histologylab.ccnmtl.columbia.edu/histological_techniques/.

Case study Cytology screening takes place in hospitals. One example is a cervical smear test. This is where a screener will use microscopes to observe cervical smears. They are looking for any abnormal looking cells – cells that have enlarged nuclei or are different in shape. If any abnormalities are identified then a follow-up appointment will be made.



2 Structure of cells Structure of prokaryotic cells Organisms such as bacteria are classed as prokaryotic cells – they are small cells (usually 0.1–10 µm in size). Some have an obvious structure known as a flagellum to help the bacteria move.

Key terms Prokaryotic cell: A cell with no true nucleus. Nucleoid: Genetic material found in the cytoplasm of a prokaryotic cell.

Bacterial cells have very strong cell walls and beneath this structure is a plasma membrane to control the contents of the cell. Some bacteria also have an extra sticky layer called a capsule, which helps the bacteria survive attacks from foreign bodies like those produced by our immune system. The genetic material is called the nucleoid, is described as circular and can be found in the cytoplasm. There are also smaller pieces of DNA called plasmids that exist in the cytoplasm. Plasmids often contain antibiotic resistance genes, which are useful for genetic engineering. Other organelles present are ribosomes for protein synthesis and mesosomes where cellular respiration occurs. See Figure 14.1.2 for the structure of a prokaryotic cell.

Figure 14.1.2: Structure of a prokaryotic cell.

Ribosomes Capsule Pili Cell wall

Plasmid

Plasma membrane

Nucleoid (circular DNA)

Bacterial flagellum

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Unit 14: Cell biology

Structure of eukaryotic cells Key term Eukaryotic cell: A cell with a true nucleus.

All animals, plants, fungi and protoctists are made of eukaryotic cells. When you use an electron microscope you will see they have a very complex internal structure, consisting of very small structures known as organelles enclosed by the cell membrane. The cytoskeleton provides cells with a stable shape. It is made from a network of protein fibres that work together to cause movement around the cell. Microtubules with microtubule motors are smaller fibres used to move smaller molecules and organelles through the cell. See Figure 14.1.3 for the structure of a eukaryotic cell.

Nucleus In eukaryotic cells there is usually one nucleus. This nucleus is the largest organelle and is approximately 10 µm in size. The nucleus is surrounded by a double membrane known as the nuclear envelope that contains nuclear pores large enough for molecules to pass through. Inside the nucleus is the DNA, where the instructions needed for the cell to produce its proteins are housed. This DNA is typically divided into linear segments known as chromosomes. Also found inside the nucleus is a dense structure called the nucleolus, which makes RNA and ribosomes.

Mitochondria There are lots of very important organelles known as mitochondria. They are sausage shaped and are usually 1–10 µm in size. They have a double membrane, which is highly folded, forming projections called cristae to increase the surface area for respiration. Inside this membrane is the central part known as the matrix. This is where aerobic respiration takes place – ATP (adenosine triphosphate) is produced and can be used to provide the energy needed for cell reactions to take place.

Ribosomes Ribosomes exist in large numbers and are approximately 20 nm in size. They are free in the cytoplasm and are also attached to the endoplasmic reticulum. Ribosomes play a vital role in protein synthesis, with mRNA and tRNA.

Endoplasmic reticulum Endoplasmic reticulum (ER) is a network of structures that continue from the nuclear membrane. There are two types, rough ER and smooth ER. Rough ER is studded with ribosomes and its job is to transport proteins that have been synthesised on the ribosomes. Smooth ER is not studded with ribosomes and is involved with making lipids.

Golgi body There are stacks of flattened membrane-bound sacs found in the cytoplasm called Golgi bodies. They receive materials from the ER and their role is to modify and package chemicals, for example proteins, ready for secretion. Another important function of the Golgi body is to make lysosomes.

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Unit 14: Cell biology Lysosomes Lysosomes are single membrane-bound organelles, usually spherical in shape. They are usually around 100 nm in size and are free in the cytoplasm. These structures contain lytic enzymes and the function of these small organelles is to digest surplus intracellular material no longer needed by the cell.

Peroxisomes Peroxisomes are small, single membrane-bound organelles that are also synthesised by the ER. Their function is to break down fatty acid molecules into smaller fatty acid molecules.

Cilia/flagella In eukaryotic cells, cilia and flagella are microscopic structures that extend from the cell surface, though they are not present on all cells. They are identical in ultrastructure but differ in size and function. If cilia are present there are usually hundreds of them, which may be motile or immotile. If flagella are present there are usually only a small number. Motile cilia move in a wave-like pattern to facilitate the movement of materials, and some also receive signals from the environment. Flagella are larger structures that facilitate cell motility (e.g. the sperm tail).

Cell wall Plant cells contain structures like the cell wall that are not present in animals. Cell walls are made of cellulose, providing them with the strength, rigidity and stability to keep the cell’s shape.

Chloroplasts Chloroplasts are specialised organelles where photosynthesis takes place, allowing plants to produce carbohydrates from carbon dioxide and water using energy from sunlight. They are bound by a double membrane and contain many chlorophyll molecules, which provide a large surface area to absorb sunlight. Figure 14.1.3: Structure of a eukaryotic cell.

Secretion vesicle

Cytosol Golgi complex

Centrosome

Smooth endoplasmic reticulum

Endocytic vesicle Endoplasmic reticulum and ribosomes Mitochondrion Plasma membrane

Nucleoplasm and chromatin Actin

Nuclear membrane and nuclear pores Lysosome

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Unit 14: Cell biology

Link Cellular organelles are also discussed in Unit 7: Molecular biology and genetics.

Activity 1 If a mitochondrion measured 88 mm on a micrograph and was magnified by 10 000×, what would the actual size of the mitochondrion be? 2 A ribosome had an actual size of 3.5 µm and the image size was 140 mm. Calculate the magnification used to make this organelle visible. Remember to convert the actual size to mm. 3 Explain the role of each organelle present in the cell. 4 Explain how each organelle works with other organelles to carry out the work they need to do.

Portfolio activity (1.2) You can generate evidence for your portfolio by creating a table of structures and functions. •• Produce a table to compare eukaryotes and prokaryotes. •• Produce a table of structures and functions present in eukaryotes and prokaryotes (see example below). Organelle Nucleus

Structure

Function

Double membrane-bound, 10 µm, consists of a nucleolus…

Checklist In this topic you should now be familiar with the following ideas about cells:

Take it further Find out more about the different types of electron microscope including the Transmission Electron Microscope (TEM) and the Scanning Electron Microscope (SEM). Compare the advantages and disadvantages of using each.

 cells are classified into two types depending on whether a nucleus is present: eukaryotic (meaning ‘true nucleus’) and prokaryotic (meaning ‘before nucleus’)  prokaryotic cells contain circular DNA in the cytoplasm  eukaryotic cells have a nucleus which houses linear DNA  organelles are smaller structures inside cells that can be seen using microscopes  the nucleus, mitochondria, ribosomes, endoplasmic reticulum, Golgi bodies, lysosomes, peroxisomes, cilia and flagella are all common organelles found in animal cells  plant cells have cell walls and chloroplasts.

Further reading Alberts, B. et al. (2009) Essential Cell Biology, Garland, Abingdon, UK

Acknowledgements The publisher would like to thank the following for their kind permission to reproduce their photographs: Corbis: Steve Gschmeissner/Science Photo Library All other images © Pearson Education We are grateful to the following for permission to reproduce copyright material: Diagram showing the difference between the two microscopes. Copyright 2008 from Molecular Biology of the Cell, Fifth Edition by Alberts et al. Reproduced by permission of Garland Science/ Taylor & Francis LLC. In some instances we have been unable to trace the owners of copyright material, and we would appreciate any information that would enable us to do so.

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