PART 1: CELL STRUCTURE AND MICROSCOPY

1. Introduction to cytology
2. The cell theory.
3. Cell classification and microscopy.
4. Differences between light and electron microscope.

PART 2: ULTRASTRUCTURE.

1. General cell composition.
2. Cell organelles.
3. The cell membrane.
4. (YOU CAN WATCH IT AS A YOUTUBE MUSIC VIDEO)

PART 3: CELL CYCLE.

1. Description of the cell cycle.
2. Mitosis
3. Meiosis.

PART 4: QUESTIONS.

1. Essay type.
2. MCQ.

ULTRASTRUCTURE

In this section, we shall study the details on most of the cell organelles. The cell being the basic functional unit of the living organism, we should be able to locate local parts of the cell (organelles) that are used for the different characteristics observed in living things. These characteristics are Movement, Respiration, Growth, Reproduction, Excretion, Excitability and Nutrition. Approximately 90% of the cell is fluid (cytoplasm) and in this fluid cytoplasm is found all the different molecules and organelles of the cell. For the molecules that make up the cell;

• 50% protein.
• 15% nucleic acid (RNA and DNA).
• 15% carbohydrates.
• 10% lipids
• 10% other molecules.

The cell cytoplasm can be split or study in two ways: as cytosol and as organelles. The different cell organelles include;

• THE NUCLEUS: This organelle is separated from the cell by a nuclear membrane. This membrane has pores (holes) that nucleic acids, mostly RNA can pass out into the cytoplasm for protein synthesis. The nucleus houses an even smaller nucleus called the nucleolus. Within these, is found the cell genetic material. It is however important to note that this organelle is found only in eukaryotes. Prokaryotic cells do not have a nucleus but have their genetic material swimming freely in their cytoplasm.

• THE ENDOPLASMIC RECTICULUM: This is a membranous structure generally seen around the nucleus like a network of threads or labyrinth. There exist fundamentally two types of endoplasmic reticulum. (We will refer to them as ER for simplicity) The two types of ER are the Rough ER and the Smooth ER. The rough ER is so because of ribosomes lining its membranous exterior (like carefully placed watchtowers at the beach). (See figure above). The rough ER is responsible for the production of proteins, though technically, it’s the ribosomes on the rough ER that produce the proteins. On the other hand, the smooth ER is just exactly like the rough ER but without the ribosomes. It is generally a little further from the nucleus. The smooth ER is responsible for the synthesis of lipids.

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• RIBOSOMES: These are the structure in charge of protein synthesis in the cell. There exist two principal types according to the two types of cells. Remember that prokaryotic cells do not have membrane bound organelles. Hence, they do not have an ER. Their ribosomes are therefore free in the cytoplasm. They are referred to as free ribosomes or 70S ribosomes. They possess a large 50S subunit and a small 30S subunit. Ribosomes found on the ER are called bound ribosomes or 80S ribosomes. They consist of a large 60S subunit and a small 40S subunit.

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• MITOCHONDRIA: Mitochondria (Singular: mitochondrion) are the zone for tissue respiration. All aerobic cell have mitochondria. It is a double membrane bound organelle. It has its own genetic material (DNA) and synthesizes its own proteins. It is relatively autonomous within the cell. In prokaryotes, the mitochondria are replaced by Mesosomes. Mitochondria are found in all eukaryotic cells, though the number may vary with the type of cell.

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• CHLOROPLAST: this is the site for photosynthesis in plants and plant-like cells/organisms. This suggest that they are absent in animal and animal-like cells. True. Within the chloroplast is a stack of structures like grains of rice. This stack is called the granum (plural: grana) while the rice-like structures that are stacked are called Thylakoids. It is in the membranes of these thylakoids that photosynthesis takes place. (More will be discussed on this subject matter under nutrition in Plants).

The chloroplast and mitochondria both have their own genetic material. It is suggested by the theory of endosymbiosis that, these were whole and independent organisms that found refuge within cells during a phase of extinction.

• GOLGI APPARATUS or GOLGI BODY: This structure or organelle serves in the transport of proteins and lipids produced in the cell to any area of need within the cell.

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• LYSOSOMES: These can be seen as the digestive system of the cell. It contains lysozymes which are enzymes used for breaking own organic matter within the cell. It is thus used to digest food substances engulfed by the cell. At some point, this organelle breaks open and the lysozymes the host cell. This organelle is thence also referred to as the suicide bag.

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• THE CELL MEMBRANE: This part of the cell delimits the cell from its environment. The cell membrane is selective to what goes in and out of the cell and has as major function the control of all inputs and exits of the cell.

The figure presented above is a single membrane, not a double membrane. The membrane is described using the fluid mosaic model proposed by Singer and Nicholson in 1972. According to this model the cell membrane;

• Is a phospholipid bilayer: this entails the cell membrane is made up of phospholipids as shown in the figure above (blue head and yellow tail). The blue heads are known as hydrophilic heads while the yellow tails are hydrophobic tails. The hydrophilic heads are phosphates while the hydrophobic tails are lipids. This repeats itself twice as shown in the figure to form the phospholipid bilayer.
• Has peripheral proteins: peripheral proteins are proteins that are found at the periphery. Either at the top or bottom of the cell membrane.
• Has trans-membrane proteins: trans-membrane proteins run from the exterior end of the membrane into the interior without breaking. Most of them are transfer proteins like sodium-potassium ATP pump (Na⁺/K⁺ ATP pump).

Both trans-membrane and peripheral proteins could (but must not) possess sugar molecules attached to them, making them glycoproteins.

• Has cholesterol sandwiched into the lipids: the presence of cholesterol in between the lipid functions give the cell membrane its fluidity. The more the cholesterol, the more fluid the cell membrane and the lesser the cholesterol the more rigid the cell membrane.

Thanks for reading.

Notes by Buo Genesis Kelly for the Alpha Study Group.

You can watch our Learner music video (LMV) on this topic via the following link.