2.4 Cell organelles

Cytoplasm (ESG4Z)

The cytoplasm is the jelly-like substance that fills the cell. It consists of up to \(\text{90}\%\) water. It also contains dissolved nutrients and waste products. Its main function is to hold together the organelles which make up the cytoplasm. It also nourishes the cell by supplying it with salts and sugars and provides a medium for metabolic reactions to occur.

All the contents of prokaryotic cells are contained within the cytoplasm. In eukaryotic cells, all the organelles are contained within the cytoplasm except the nucleolus which is contained within the nucleus.

Functions of the cytoplasm

• The cytoplasm provides mechanical support to the cell by exerting pressure against the cell's membrane which helps keep the shape of the cell. This pressure is known as turgor pressure.
• It is the site of most cellular activities including metabolism, cell division and protein synthesis.
• The cytoplasm contains ribosomes which assist in the synthesis of protein.
• The cytoplasm acts a storage area for small carbohydrate, lipid and protein molecules.
• The cytoplasm suspends and can transport organelles around the cell.

Nucleus (ESG52)

The nucleus is the largest organelle in the cell and contains all the cell's genetic information in the form of DNA. The presence of a nucleus is the primary factor that distinguishes eukaryotes from prokaryotes. The structure of the nucleus is described below:

Nuclear envelope: two lipid membranes that are studded with special proteins that separates the nucleus and its contents from the cytoplasm.

Nuclear pores: tiny holes called nuclear pores are found in the nuclear envelope and help to regulate the exchange of materials (such as RNA and proteins) between the nucleus and the cytoplasm.

Chromatin: thin long strands of DNA and protein.

Nucleolus: the nucleolus makes RNA another type of nucleic acid.

Schematic Diagram	Micrograph

Functions of the nucleus

• The main function of the cell nucleus is to control gene expression and facilitate the replication of DNA during the cell cycle (which you will learn about in the next chapter).
• The nucleus controls the metabolic functions of the cell by producing mRNA which encodes for enzymes e.g. insulin.
• The nucleus controls the structure of the cell by transcribing DNA which encodes for structural proteins such as actin and keratin.
• The nucleus is the site of ribosomal RNA (rRNA) synthesis, which is important for the construction of ribosomes. Ribosomes are the site of protein translation (synthesis of proteins from amino acids).
• Characteristics are transmitted from parent to offspring through genetic material contained in the nucleus.

Mitochondria (ESG53)

A mitochondrion is a membrane bound organelle found in eukaryotic cells. This organelle generates the cell's supply of chemical energy by releasing energy stored in molecules from food and using it to produce ATP (adenosine triphosphate). ATP is a special type of "energy carrying" molecule.

Structure and function of the mitochondrion

Mitochondria contain two phospholipid bilayers: there is an outer membrane, and an inner membrane. The inner membrane contains many folds called cristae which contain specialised membrane proteins that enable the mitochondria to synthesise ATP. Inside the inner membrane is a jelly-like matrix. Listed from the outermost layer to the innermost compartment, the compartments of the mitochondrion, are:

• Outer mitochondrial membrane
• Intermembrane space
• Inner mitochondrial membrane
• Cristae (folds of the inner membrane)
• matrix (jelly-like substance within the inner membrane)

Schematic Diagram	Micrograph

The table below relates each structure to its function.

Endoplasmic reticulum (ESG54)

The endoplasmic reticulum (ER) is an organelle found in eukaryotic cells only. The ER has a double membrane consisting of a network of hollow tubes, flattened sheets, and round sacs. These flattened, hollow folds and sacs are called cisternae. The ER is located in the cytoplasm and is connected to the nuclear envelope. There are two types of endoplasmic reticulum: smooth and rough ER.

Smooth ER: does not have any ribosomes attached. It is involved in the synthesis of lipids, including oils, phospholipids and steroids. It is also responsible for metabolism of carbohydrates, regulation of calcium concentration and detoxification of drugs.

Rough ER: is covered with ribosomes giving the endoplasmic reticulum its rough appearance. It is responsible for protein synthesis and plays a role in membrane production. The folds present in the membrane increase the surface area allowing more ribosomes to be present on the ER, thereby allowing greater protein production.

Schematic Diagram	Micrograph
Smooth endoplasmic reticulum
Rough endoplasmic reticulum

Ribosomes (ESG55)

Ribosomes are composed of RNA and protein. They occur in the cytoplasm and are the sites where protein synthesis occurs. Ribosomes may occur singly in the cytoplasm or in groups or may be attached to the endoplasmic reticulum thus forming the rough endoplasmic reticulum. Ribosomes are important for protein production. Together with a structure known as messenger RNA (a type of nucleic acid) ribosomes form a structure known as a polyribosome which is important in protein synthesis.

Diagram: Free Ribosome	Diagram: Polyribosome

Golgi body (ESG56)

The Golgi body is found near the nucleus and endoplasmic reticulum. The Golgi body consists of a stack of flat membrane-bound sacs called cisternae. The cisternae within the Golgi body consist of enzymes which modify the packaged products of the Golgi body (proteins).

Schematic Diagram	Micrograph

Functions of the Golgi body

It is important for proteins to be transported from where they are synthesised to where they are required in the cell. The organelle responsible for this is the Golgi Body. The Golgi body is the sorting organelle of the cell.

Proteins are transported from the rough endoplasmic reticulum (RER) to the Golgi. In the Golgi, proteins are modified and packaged into vesicle. The Golgi body therefore receives proteins made in one location in the cell and transfers these to another location within the cell where they are required. For this reason the Golgi body can be considered to be the 'post office' of the cell.

Vesicles and lysosomes (ESG57)

Vesicles are small, membrane-bound spherical sacs which facilitate the metabolism, transport and storage of molecules. Many vesicles are made in the Golgi body and the endoplasmic reticulum, or are made from parts of the cell membrane. Vesicles can be classified according to their contents and function. Transport vesicles transport molecules within the cell.

Lysosomes are formed by the Golgi body and contain powerful digestive enzymes that can potentially digest the cell. Lysosomes are formed by the Golgi body or the endoplasmic reticulum. These powerful enzymes can digest cell structures and food molecules such as carbohydrates and proteins. Lysosomes are abundant in animal cells that ingest food through food vacuoles. When a cell dies, the lysosome releases its enzymes and digests the cell.

Vacuoles (ESG58)

Vacuoles are membrane-bound, fluid-filled organelles that occur in the cytoplasm of most plant cells, but are very small or completely absent from animal cells. Plant cells generally have one large vacuole that takes up most of the cell's volume. A selectively permeable membrane called the tonoplast, surround the vacuole. The vacuole contains cell sap which is a liquid consisting of water, mineral salts, sugars and amino acids.

Functions of the vacuole

• The vacuole plays an important role in digestion and excretion of cellular waste and storage of water and organic and inorganic substances.

• The vacuole takes in and releases water by osmosis in response to changes in the cytoplasm, as well as in the environment around the cell.

• The vacuole is also responsible for maintaining the shape of plant cells. When the cell is full of water, the vacuole exerts pressure outwards, pushing the cell membrane against the cell wall. This pressure is called turgor pressure.

• If there is not sufficient water, pressure exerted by the vacuole is reduced and the cells become flaccid causing the plant to wilt.

Centrioles (ESG59)

Animal cells contain a special organelle called a centriole. The centriole is a cylindrical tube-like structure that is composed of 9 microtubules arranged in a very particular pattern. Two centrioles arranged perpendicular to each other are referred to as a centrosome. The centrosome plays a very important role in cell division. The centrioles are responsible for organising the microtubules that position the chromosomes in the correct location during cell division. You will learn more about their function in the following chapter on Cell Division.

Plastids (ESG5B)

Plastids are organelles found only in plants. There are three different types:

1. Leucoplasts: White plastids found in roots.
2. Chloroplasts: Green-coloured plastids found in plants and algae.
3. Chromoplasts: Contain red, orange or yellow pigments and are common in ripening fruit, flowers or autumn leaves.

The colour of plant flowers such as an orchid is controlled by a specialised organelle in a cell known as the chromoplast.

Chloroplast

The chloroplast is a double-membraned organelle. Within the double membrane is a gel-like substance called stroma. Stroma contains enzymes for photosynthesis. Suspended in the stroma are stack-like structures called grana (singular = granum). Each granum is a stack of thylakoid discs. The chlorophyll molecules (green pigments) are found on the surface of the thylakoid discs. Chlorophyll absorbs energy from the sun in order for photosynthesis to take place in the chloroplasts. The grana are connected by lamellae (intergrana). The lamellae keep the stacks apart from each other.

The structure of the chloroplast is neatly adapted to its function of trapping and storing energy in plants. For example, chloroplasts contain a high density of thylakoid discs and numerous grana to allow for increased surface area for the absorption of sunlight, thus producing a high quantity of food for the plant. Additionally, the lamellae keeping the thylakoids apart maximise chloroplast efficiency, thus allowing as much light as possible to be absorbed in the smallest surface area.

Schematic Diagram	Micrograph

The differences between plant and animal cells (ESG5C)

Now that we have looked at the basic structures and functions of the organelles in a cell, you would have noticed that there are key differences between plant and animal cells. The table below summarises these differences.

Examining plant cells under the microscope

Aim

To study the microscopic structures of plant cells.

Apparatus

• onion
• blade
• slides and coverslips
• brushes
• compound microscope
• tissue paper
• forceps
• dropper
• iodine solution
• watchglass
• petri dish containing water

Method

1. Peel off the outer most layer of an onion carefully, using a pair of forceps.
2. Place the peeled layer in a watchglass containing water. Make certain that the onion peel does not roll or fold.
3. Using a scalpel or a thin blade, cut a square piece of the onion peel (about \(\text{1}\) \(\text{cm$^{2}$}\)).
4. Remove the thin transparent skin from the inside curve of a small piece of raw onion and place it on a drop of iodine solution on a clean slide.
5. Cover the peel with a coverslip ensuring that no bubbles are formed.
6. Using a piece of tissue paper wipe off any excess iodine solution remaining on the slide.
7. Observe the onion skin under low power of the microscope and then under high power.
8. Draw a neat diagram of 5-10 cells of the typical cells you can see.

Examining animal cells under the microscope

Aim

To study the microscopic structures of human cheek cells under a compound microscope.

Apparatus

• clean ear bud
• clean slide
• methylene blue
• dropper
• water
• tissue paper
• forceps
• microscope

Method

1. Place a drop of water on a clean glass slide.
2. Using a clean ear bud, wipe the inside of your cheek. The ear bud will collect a moist film.
3. Spread the moist film on a drop of water on a clean glass slide, creating a small smear on the slide.
4. Use a coverslip to cover the slide gently.
5. Place one or two drops of stain on the side of the cover slip.
6. Use a piece of tissue to remove the excess dye.
7. Observe the cheek cells under low power magnification and then under high power magnification.

Questions

1. What are the shapes of epidermal cells of the onion peel and the human cheek cells?
2. Why is iodine used to stain the onion peel?
3. What is the difference between the arrangement of cells in onion cells and in human cheek cells?
4. Why is a cell considered the structural and functional unit of living things?