Cell Structure PDF

# The structure of the cell ## The Smooth Endoplasmic Reticulum (SER) * Notice that the SER does not have ribosomes on its surface. * Also notice how the cisternae (flattened sacs) inside the SER are more tubular than the cisternae of the RER. ## The Cytoplasm * Cytoplasm is the liquid part of t...

# The structure of the cell ## The Smooth Endoplasmic Reticulum (SER) * Notice that the SER does not have ribosomes on its surface. * Also notice how the cisternae (flattened sacs) inside the SER are more tubular than the cisternae of the RER. ## The Cytoplasm * Cytoplasm is the liquid part of the cell. It is rather like wallpaper paste in texture. * It is mainly water, but contains all kinds of dissolved nutrients, materials and enzymes. ## The Epithelial cell * This is what the epithelial cell looks like when you view it using an electron microscope. ### Diagram of the Epithelial cell | | | |---|---|---| | **microvilli border** | ** plasma membrane ** | **tight junction** | | **nucleus** | **mitochondria** | ## Microvilli * These pictures show you what microvilli look like. * The top picture shows how tiny they are. 1nm is 10-9m, i.e. one thousand millionth of a millimetre. A microvillus is about 100nm wide. So how many microvilli would fit side-by-side to spread across 1mm of your ruler? (1mm is 10-3m, or one thousandth of a metre) * Microvilli are finger-like extensions of the plasma membrane. They make extensions of the cell, rather like the bristles on a hairbrush. They greatly increase the surface area for absorption of digested food materials. ## The Nucleus * This diagram shows the nucleus as seen under an electron microscope. * Notice that it has a double membrane around it, called a nuclear envelope. * There are gaps in the nuclear envelope, called nuclear pores. * You cannot see the chromosomes inside the nucleus unless the cell is dividing. But the nucleus has a speckled appearance - this is because the DNA is ‘unwound’ and not formed into chromosomes most of the time. The DNA, and the proteins that are associated with it, form a substance called chromatin, and this gives the nucleus a speckled appearance. * This is surrounded by a double membrane called the nuclear envelope. * The nucleus carries the cell’s genetic information in the form of DNA. The DNA is arranged into chromosomes. ## Ribosomes * Ribosomes are small structures made of RNA and protein. They don’t have a membrane around them. Some of them are attached to the RER, but others are free in the cell cytoplasm. * Ribosomes make proteins, such as enzymes. ### Diagram of Ribosomes <start_of_image> magnification: x10,000 | | | |---|---|---| | **microvilli** | **ribosomes** | **rough endoplasmic reticulum (RER)** | | **cell nucleus** | **plasma membrane** | ## The Structure of the Cell Membrane * The plasma membrane is made of a double layer (bilayer) of phospholipids, with some proteins embedded in it. You will learn more about this later. The plasma membrane controls what enters and leaves the cell. We say it is selectively permeable. ## The Rough Endoplasmic Reticulum (RER) * On the right is an electron microscope picture of the RER. Notice that the ribosomes are only on the outside. The other pictures are 3D drawings of the RER. * This a membrane-bound structure that forms cavities in the cell, rather like a multi-storey car park. On the outside, the RER has many dark structures attached to it, called ribosomes. This is why it is called rough endoplasmic reticulum. * The ribosomes make proteins which pass into the cavities of the RER. The RER then transports the proteins through the cell. * This is a membrane-bound structure that forms cavities in the cell, rather like a multi-storey car park. On the outside, the RER has many dark structures attached to it, called ribosomes. This is why it is called rough endoplasmic reticulum. * The ribosomes make proteins which pass into the cavities of the RER. The RER then transports the proteins through the cell. ## The Golgi apparatus * This is a series of membrane-bound cavities, rather like the RER, except that it has no ribosomes attached, and the cavities are more tubular. * It synthesises, stores and transports lipids (such as steroid hormones). It also synthesises, stores and transports carbohydrates. ### Diagram of the Golgi Apparatus | | | |---|---|---| | **microvilli** | **ribosomes** | **rough endoplasmic reticulum (RER)** | | **cell nucleus** | **plasma membrane** | **smooth endoplasmic reticulum** | ### Diagram of the Golgi Apparatus | | | |---|---|---| | **cristae** | **matrix** | **inner membrane** | | **outer membrane** | **intermembrane space** | **matrix** | | **cristae** | **inner membrane** | **outer membrane** | ## Mitochondria * Mitochondria have a smooth outside membrane and an inner, folded membrane. The folds inside the mitochondrion are called cristae. The organelle is bean-shaped. * Mitochondria contain the enzymes needed for aerobic respiration. They release energy from glucose and supply the cell with energy for processes such as active transport. * An epithelial cell from the small intestine would contain lots of mitochondria - many more than shown here. * These are membrane-bound ‘packets’ of substances. Some are vesicles of substances produced by the Golgi body. * Some lysosomes contain powerful enzymes that can break down worn-out organelles, or can break down substances taken into the cell. ## A ribosome * A ribosome is actually made of two subunits, but you don’t need to learn any detail about it until semester 2. * The black dots in this electron micrograph are ribosomes. ### Diagram of a Ribosome | | | |---|---|---| | **light subunit** | **heavy subunit** | **small subunit** | | **large subunit** | | | ## The Golgi apparatus * Notice that the Golgi body is made of stacks of membrane. In the middle of the membranes are spaces called cisternae. * Here, proteins from the RER are modified and packaged into vesicles for secretion. * The Golgi body is another organelle made of membranes. In 3D it looks like a stack of pancakes. Inside the membranes are spaces called cisternae. * The Golgi body receives proteins from the RER and modifies them, e.g. adding sugars, to make molecules such as mucus. ### Diagram of the Golgi Apparatus | | | |---|---|---| | **vesicles breaking off the Golgi body** | **microvilli** | **ribosomes** | | **rough endoplasmic reticulum (RER)** | **cell nucleus** | **plasma membrane** | | **smooth endoplasmic reticulum** | | | * The modified proteins, eg mucus, are packaged into membrane-bound vesicles. These are moved towards the cell membrane and the contents are secreted from the cell.