3.4 Eukaryotic Cell Structure

The nucleus contains the organism's hereditary material and controls the cell's activities.

It is usually spherical and between 10 and 20 micrometers in diameter,

The Nuclear Envelope is the double membrane that surrounds the nucleus. The outer membrane is continous with the endoplasmic reticulum of the cell and often has ribosomes on its surface. It controls what goes in and out of the nucleus and contains the reactions taking place within it. 

Nuclear Pores allow large molecules,such as messenger RNA, out of the nucleus. There are typically 3000 pores in each nucleu,each 40-100 nm in diameter.

Nucleoplasm is the granular jelly like materia; that makesup the bulk of the nucleus.

Chromosomes consists of protein bound linear DNA.

The Nucleolus is a small spherical region within the nucleoplasm. It manufactures ribosomal RNA and assembles ribosomes. There may be more than one nucleolus in each nucleus.

The functions of the nucleus are to:

- act as a control centre of the cell through the production of mRNA and tRNA

- retain the genetic material of the cell in the form of DNA and chromosomes

- manufacture ribosomal RNA and ribosomes

Mitochondria are mde up of the following structures:

A Double Membrane surround the organelle and controls the entry and exit of materials. The inner of the two membranes is folded to form extensions known as cristae.

Cristae, in some species will span the whole length of the mitochondrion. They provide a large surface area for enzymes and other proteins involved in respiration to attach to.

The Matrix makes up the remainder of the mitochondrion. It contains proteins, lipids, ribosomes and DNA that allows the mitochondria to control the production of some of their own proteins. Many enzymes involved in respiration are found in the matrix.

Mitochondria are the sites of the aerobic stages of respiration. This is the Krebs cycle of the oxidative phosphorylation pathway. They are therefore responsible for the production of ATP. Because of this, the number and size of the mitochondria and the number of their cristae are high in cells that have a high level of metabolic activity and require a plentiful supply of ATP.

Chloroplasts are the organelles that carrel out photosynthesis.

The Chloroplast Envelope is a double plasma membrane that surrounds the organelle. It is highly selective in what it allows to enter and exit the chloroplast.

The Grana are stacks of up to 100 disc like structures called Thylakoids. Within the thylakoids is the pigment Chlorophyll. Some thylakoids have tubular extensions that join with the thylakoids in the adjacent grana. This is where the first stage of photosynthesis takes place.

The Stroma is a fluid-filled matrix where the second stage of photosynthesis takes place. It contains a number of other structures such as starch grains.

Chloroplasts are adapted to their function of harvesting sunlight and carrying out photosynthesis in the following ways:

The granal membranes provide a large surface area for the attachment of chlorophyll, electron carriers and enzymes that carry out the first stage of photosynthesis. These chemicals are attached to the membrane in a highly ordered fashion.

The fluid filled stroma possesses all the enzymes needed to make sugars in the second stage of photosynthesis.

Chloroplasts contain both DNA and ribosomes so they can quickly and easily manufacture some of the proteins needed for photosynthesis.

The endoplasmic reticulum (ER) is an elaborate, three-dimensional system of sheet-like membranes, spreading through the cytoplasm of the cells. It is continuous with the outer nuclear membrane. They enclose a network of tubules are flattened sacs called cisternae. There are two types of ER:

-          Rough endoplasmic reticulum (RER) has ribosomes present on the outer surfaces of the membranes. Its functions are to provide a large surface area for the synthesis of proteins and glycoproteins. And to provide a pathway for the transport of materials, especially proteins, throughout the cell.

-          Smooth endoplasmic reticulum (SER) lacks ribosomes on its surface and is often more tubular in appearance. Its functions are to synthesis, store and transport lipids and carbohydrates.

So, cells that manufacture and store large quantities of carbohydrates, proteins and lipids have a very extensive ER. Such cells include liver and secretory cells, for example the epithelial cells that line the intestines.

The Golgi apparatus occurs in almost all eukaryotic cells and is similar to the SER in structure except that it is more compact. It consists of a stack of membranes that make up flattened sacs, or cisternae, with small rounded hollow structures called vesicles.

The proteins and lipids produced by the ER are passed through the Golgi apparatus in strict sequence. The Golgi modifies these proteins, often adding non-protein components, such as carbohydrate to them. It also allows them to be accurately sorted and sent to correct destinations by ‘labelling’ them. Once sorted, the modified proteins and lipids are transported in Golgi vesicles which are regularly pinched off from the ends of the Golgi cisternae. These vesicles may move to the cell surface, where they fuse with the membrane and release their contents to the outside.   

The functions of the Golgi apparatus are to:

-          Add carbohydrate to proteins to form glycoproteins

-          Produce secretory enzymes, such are those secreted by the pancreas

-          Secrete carbohydrates, such as those used in making cells walls in plants

-          Transport, modify and store lipids

-          Form lysosomes

The Golgi apparatus is especially well developed in secretory cells, such as the epithelial cells that line the walls of the intestines.

Lysosomes are formed when the vesicles produced by the Golgi apparatus contain enzymes such as proteases and lipases. They also contain lysozymes, enzymes that hydrolyse the cell walls of certain bacteria. As many as 50 such enzymes may be contained in a single lysosome. Up to 1.0 micrometres in diameter, lysosomes isolate these enzymes from the rest of the cell before releasing them, either to the outside or into a phagocytic vesicle within the cell.

The functions of the lysosomes are to:

Hydrolyse material ingested by phagocytic cells, such as white blood cells and bacteria.

To release enzymes to the outside of the cell in order to destroy material around the cell.

Digest worn out organelles so that the useful chemicals they are made of can eb re-used.

Completely break down the cells after they have died.

Ribosomes are small cytoplasmic granules found in all cells. They may occur in the cytoplasm or be associated with the RER. There ate two types, depending on the cells in which they are found.

80S – found in eukaryotic cells, is around 25nm in diameter

70S – found in prokaryotic cells, mitochondria and chloroplasts, is slightly smaller

Ribosomes have two subunits, one large and one small – each of which contains ribosomal RNA and protein. Despite their small size, they occur ion such vast numbers that they can account for up to 25% of the dry mass of a cell. Ribosomes are the site of protein synthesis.

Characteristic of all plant cells, the cell wall consists of microfibrils of the polysaccharide cellulose, embedded in a matrix. Cellulose microfibrils have considerable strength and so contribute to the overall strength of the cell wall. T hey have the following features:

They consist of a number of polysaccharides such as cellulose.

There is a thin layer, called the middle lamella, which marks the boundary between adjacent cell walls and cements adjacent cells together.

The functions of the cellulose cell wall are:

To provide mechanical strength in order to prevent the cell bursting under pressure created by the osmotic entry of water.

To give mechanical strength to the plant as a whole.

To allow water to pass along it and so contribute to the movement of water through the plant.

The cell walls of algae are made up of either cellulose or glycoproteins, or a mixture of both.

The cell walls of fungi do not contain cellulose but comprise a mixture of a nitrogen containing polysaccharide called chitin, a polysaccharide called glycan and glycoproteins.

A fluid filled sac bounded by a single membrane may be termed a vacuole. Within mature plant cells there is usually one large central vacuole. The single membrane around it is called the tonoplast. A plant vacuole contains a solution of mineral salts, sugars, amino acids, wastes and sometimes pigments.

Plant vacuoles serve a variety of functions:

They support herbaceous plants, and herbaceous parts of woody plants by making the cells turgid.

The sugars and amino acids act as a temporary food store.

The pigments may colour petals to attract pollinating insects.