Bio - 2.2 Cell structure

• Mitochondria are the powerhouses of the cell and are responsible for producing energy through cellular respiration.
• Ribosomes synthesize proteins important for the functioning of the cell.
• Lysosomes contain digestive enzymes that break down waste products within cells.
• Golgi apparatus modifies and packages proteins produced by ribosomes for transport to other parts of the cell or outside the cell.
• The cell membrane separates the cell's contents from the outside environment and neighbouring cells. It determines which substances go in and out of the cell
• Cytoplasm is the jelly-like watery material inside the cell that fills all the space between the nucleus and cell membrane. It is made up of the cytosol and organelles
• The cytosol is the liquid part of the cytoplasm. It is 75% to 90% water, with a complex mixture of dissolved substances such as salts and carbohydrates. Other compounds, such as proteins and fats,

  do not dissolve but are suspended in the watery fluid.
• The cytosol is where most of the metabolic reactions occur. It also plays a role in controlling the osmotic pressure of the cell and the flow of chemicals into and out of the cell.
• The structures within a cell are called organelles. Different types of organelle are specialised for particular functions. Many of the organelles are formed by the cell’s internal membranes.
• The nucleus is the largest organelle of the cell and is usually oval or spherical in shape.
• A nuclear membrane separates the nucleus from the cytoplasm. The nuclear membrane is actually a double membrane. Numerous gaps, or nuclear pores, in the nuclear membrane allow large molecules, such as messenger RNA, to enter and leave the nucleus.
• Inside the nucleus is the DNA, which contains inherited information. When the cell is not dividing, the DNA is in the form of long threads called chromatin. DNA contains the information that determines the type of proteins a cell can make. In this way the nucleus, with its DNA, controls the structure of the cell and the way it functions.
• Inside the nucleus, the nucleolus plays a part in manufacturing proteins.
• Ribosomes are very small, spherical organelles. At the ribosomes, amino acids are joined together to make proteins. Ribosomes may be either free in the cytoplasm or attached to membranes within the cells such as the endoplasmic reticulum.
• Pairs of parallel membranes extend through the cytoplasm of the cell from the cell membrane to the nuclear membrane. The network of channels formed by the parallel membranes is called the endoplasmic reticulum, or ER. It is thought that the membranes of the endoplasmic reticulum provide a surface for chemical reactions, while the channels are for storing or transporting molecules.
• The endoplasmic reticulum can be classified as rough endoplasmic reticulum, when ribosomes are attached to the outside of some membranes, or smooth endoplasmic reticulum, when there are no ribosomes attached to the outside.
• The Golgi body is a series of flattened membranes stacked

  one upon the other. Usually the Golgi body is positioned near the nucleus. Its function is to modify proteins and to package them for secretion from the cell. Proteins produced at the ribosomes pass through the channels of the endoplasmic reticulum to the Golgi body. At the edges of the membranes of the Golgi body, small sacs of liquid containing proteins are formed. These sacs are surrounded by a membrane and are called vesicles.
• Lysosomes are small spheres, bound by a membrane, that are formed from the Golgi body. They contain digestive enzymes that are able to break down large molecules. When particles, or liquids, are taken into a cell they form vesicles in the cytoplasm. Lysosomes can join with these vesicles, and the digestive enzymes they contain break down the material inside the vesicle. Lysosomes also digest worn-out organelles in a similar way.
• Mitochondria are spherical or sausage-shaped structures that are spread throughout the cytoplasm. Each has a double membrane. The smooth outer membrane surrounds the mitochondrion, while the inner membrane, called the cristae, is arranged into a series of folds that extend into the interior of the organelle.
• Some of the chemical reactions of cellular respiration occur in the mitochondria. The folding of the inner membrane produces a large surface area on which these chemical reactions can take place. Because the reactions of the mitochondria make energy available for the cell’s activities, these organelles are often called the ‘powerhouses’ of the cell.
• mitochondria diagram

  A) outer membrane

  B) fold of inner membrane

  
• golgi body

  A) transport vesicle

  B) newly formed vesicle

  C) secretory vesicle

  D) cell membrane

  E) secreted proteins

  
• Some cells have fine projections that can beat back and forth to move either the whole cell or substances over the surface of the cell. If the projections are short and numerous, resembling tiny hairs, they are called cilia. If they are longer, and there is only one or two of them, they are called flagella. One place in which cilia occur is in the cells lining the trachea, where they move mucus and trapped particles towards the throat. In humans, only one type of cell – the sperm cell – has a

  flagellum; this enables the sperm to swim to the egg.
• The cytoskeleton is a framework of protein fibres that gives the cell its shape and assists cell movement. It consists of:

  • microtubules – hollow rods that keep organelles in place or move them around the cell

  • microfilaments – which move materials around the cytoplasm or move the whole cell.
• Inclusions are chemical substances that are not part of the cell structure but are found in the cytoplasm of the cell. Examples of inclusions include haemoglobin, the red pigment in red blood cells, and the pigment melanin in cells of the skin, hair and iris of the eye.
• cell structure and organelles

  A) vesicle

  B) vesicles

  C) rough ER

  D) smooth ER

  E) plasma/cell membrane

  F) ribosomes

  G) mitochondria/n

  H) golgi body

  I) nucleolus

  J) nucleus

  K) centrioles

  L) cytoskeleton - microtubles

  M) cytoplasm

  N) lysosome

  
• Centrioles – a pair of cylindrical structures usually located near the nucleus; involved in the reproduction of the cell.