biology cells

nucleus structure

Bounded by a double membrane (the nuclear envelope) which is a continuous with another organelle called endoplasmic reticulum. The nucleolus (small spherical body found within the nucleus) manufactures ribosomal RNA which is used to produce ribosomes. The nucleus is the largest organelle. The nucleus contains many large pores which allow the passage of large molecules, e.g. RNA and enzymes, between it and the cytoplasm. The nucleus also contains a cytoplasm-like material called nucleoplasm which contains the chromosomes. Chromosomes contain the hereditary material, the deoxyribonucleic acid or DNA, attached to proteins called histones. The chromosomes are only visible as the familiar separate bodies when the cell is undergoing division. Between divisions, the chromosomal material forms a diffuse granular mass called chromatin. Some chromatin contains densely packed DNA, and stains more darkly. This is called heterochromatin. More lightly staining area of the material are called euchromatin. When a cell divides, the DNA coils upon itself, shortening and thickening the chromosomes so that they can be seen through the light microscope.

centrioles

Centrioles are small hollow cylindrical organelles, present in pairs in animal cells, fungi and some algae. Each centriole is made up of nine triplets of microtubules and is arranged at 90o to the other, forming the centrosome. During cell division the centrioles replicate themselves and migrate to opposite poles of the cell. They are thought to have a role in the formation of spindle fibres, which are also made of microtubules. During the division of the nucleus, the chromosomes attach to the spindle fibres and are separated into the two daughter nuclei

chloroplasts function

Chloroplasts are the site of photosynthesis and are located in photosynthesising cells, in particular cells in the palisade layer of leaves.

chloroplasts structure

Chloroplasts belong to a higher group of organelles known as plastids. They are large organelles and are bounded by a double membrane known as the chloroplast envelope with the inner membrane folding into a series of lamellae. Within the chloroplast envelope is a region called the stroma. This is a colourless, gelatinous matrix which embeds structures like stacks of coins called grana. Each granum is comprised of a series of flattened sacs called thylakoids. Within these thylakoids are located photosynthetic pigments e.g. chlorophyll. Chloroplasts usually have one or more starch grains and smaller lipid droplets produced through photosynthesis.

cilia and flagella

Cilia and flagella are organelles belonging to some animal cells and a few types of plant cells. They are thin cytoplasmic threads projecting from the surface of the cell, also containing microtubules. They are similar in structure, but flagella are longer and fewer in number than cilia. Cilia are present in large numbers on the surface of some cells, such as the epithelia lining the trachea and the inside of oviducts. Their function is to beat backwards and forwards in one direction, pushing extracellular fluids along. The longer flagella normally move the whole cell or organism, as in the case of the tail of the sperm cell, which is a single flagellum. Cilia and flagella both contain a characteristic arrangement of nine outer pairs of microtubules and two central ones, all extending the length of the organelle. This is called the (9+2) arrangement.

lysosomes function

Lysosomes act to digest material which the cell consumes. This material is broken down within the lysosome and the useful chemicals are absorbed into the cytoplasm. Lysosomes digest parts of the cell, such as worn out organelles. This is known as autophagy. After the death of a cell the lysosomes are responsible for its breakdown, known as autolysis. Lysosomes can also release their contents (enzymes) to the outside of the cell through a process called exocytosis. These enzymes can then breakdown other cells.

cytoskeleton

One of the most recent discoveries regarding cell structure is an extensive 3D network of fibrous protein structures present throughput the cytoplasm. This network is called the cytoskeleton and it is made up of 3 types of fibres: microtubules, microfilaments and intermediate fibres. Microtubules are tiny tubes made of protein called tubulin. The walls of the microtubules are formed by the globular tubulin molecules arranged in a helical pattern. Microtubules are responsible for producing movements of the cell or of organelles.

vacuole

Plant cells have a large vacuole within the cytoplasm. The vacuole is important in the storage of ions and water, and plays an important part in the development of turgor for support. The tonoplast is the membrane that surrounds the vacuole in a plant cell.

ribosomes function

Site of protein synthesis, they help to move along messenger RNA (mRNA).

mitochondria function

Site of respiration where ATP is produced. For every glucose molecule broken down 38 molecules of ATP are produced. ATP in turn then releases energy. The number of mitochondria found within a cell will vary according to its metabolic activity. Highly active cells (e.g. muscle cells) will therefore contain more mitochondria.

ribosomes structure

Small granules found in all cells. Can exist in groups known as polyribosomes, can be associated with the rough endoplasmic reticulum or can occur freely within the cytoplasm. Ribosomes are made of one large unit and one small unit and comprise of RNA known as ribosomal RNA (rRNA), and protein.

Golgi apparatus function

The Golgi apparatus is a centre of manufacturing, warehousing, sorting and shipping. Here the products of the ER are passed from stack to stack where there are modified, e.g. protein may be altered by adding carbohydrate to form glycoproteins, and stored ready for their destination. Once the protein is modified, vesicles transport the protein either within the cell or fuse with the cell-surface membrane to release their contents outside the cell.

ER function

The RER provides the 'scaffolding' for the ribosomes to make the protein and the ER then operates as a distribution network for the proteins. The ER is joined with the nuclear envelope, facilitating the transport of the RNA (which carries the DNA code) from the nucleus to the ribosomes (the sites of protein synthesis). RER is particularly common in cells whose function is to secrete protein. The SER has many roles involving the metabolism of lipids, including the synthesis of cholesterol.

ER structure

The endoplasmic reticulum (ER) is an elaborate system of membranes found throughout the cell and is an extension of the out nuclear membrane. The membranes form a series of flattened sheets called cisternae. Where the membranes are lined with ribosomes they are called rough endoplasmic reticulum (RER). Where the membranes lack ribosomes they are called smooth endoplasmic reticulum (SER).

cell wall

The main component is cellulose which is laid down as microfibrils. Each microfibril consists of many cellulose molecules cross linked to each other. The primary cell wall is made of many microfibrils orientated in different and random directions. The relatively lose arrangement allows the cell wall to expand as the cell grows. When the cell reaches full size additional layers of cellulose may be deposited to form the secondary cell wall in a lattice type arrangement which gives the strength necessary in cell wall function. Cell walls of adjacent cells are linked together by the middle lamella which is made of pectin. Calcium pectate forms a gel or cement that acts as an adhesive and holds neighbouring cells together. The function of the cell wall is to provide support. They restrict the outwards expansion of the protoplast as the cell takes in water, thus providing the support associated with turgor pressure. Fully permeable

mitochondria structure

These are large organelles bounded by a double membrane (mitochondrial envelope), the outer of which controls the entry and exit of chemicals. The inner membrane is folded inwards, giving rise to extensions called cristae. They function to increase the surface area, therefore increasing the number of enzymes that can be embedded within the membrane allowing more respiratory processes to take place. The surface of these cristae has stalked particles along its length. The remainder of the mitochondrion is the matrix - semi rigid material containing protein, lipids and traces of DNA.

lysosomes structure

Tiny spherical bodies containing enzymes, mostly hydrolytic enzymes (breaking-down enzymes). The membrane of lysosomes and its contents are made by the RER and transferred to the Golgi apparatus for further processing. Some lysosomes will be formed from the budding of vesicles from the maturing face of the Golgi apparatus.

nucleus function

To control the cell's activity like a control centre, retaining the organism's hereditary material (the chromosomes). The nucleus also carries the instructions for the synthesis of proteins, ribosomes and RNA and is involved in cell division.

Golgi apparatus structure

Transitional/transport vesicles leave the endoplasmic reticulum and travel to the Golgi apparatus forming the bottom layer of a series of flattened sacs called cisternae. Two poles exist within the Golgi apparatus - the convex forming face (usually the side nearest the nucleus) and the concave maturing face (the side furthest away from the nucleus).