This is a legacy topic. View the most up to date content by selecting the exam board in the dropdown to the left.
Specifically, this topic is about the epithelial cells lining the small intestine. These cells are animal cells, and their function is to absorb nutrients as part of digestion. This is what epithelial cells look like under a light microscope:
The core components of cells are the outer membrane, the cytoplasm (substance inside which contains all other stuff) and the nucleus (contains DNA). All the other stuff is made up of various components with specific functions – these are called organelles. The ones you must know about are:
1. Plasma membrane, including cell-surface membrane
7. Endoplasmic reticulum
8. Golgi apparatus.
A bit about each…
Plasma membrane = thin boundary between cell and environment
It is made of a phospholipid bilayer, and its function is to control what passes through the cell. Membranes are also found in other organelles such as the nucleus and mitochondria.
Usually it is the large rounded organelle in a cell. It has a double membrane with many pores through which materials can pass. Each cell normally has one nucleus. The main functions are cell division, replication and protein synthesis.
Mitochondrion (mitochondria, plural)
This is basically the easily identifiable sausage-like organelle with the cool inner membrane that forms the cristae. It is the site of aerobic respiration, where most ATP is made. Look:
There are two different kinds of endoplasmic reticulum – rough and smooth endoplasmic reticulum, hence their short names rough ER and smooth ER. The roughness and smoothness business is down to ribosomes attached to the rough ER, but not to smooth ER.
Rough ER – transport system: collects, stores, packages and transports the proteins made on the ribosomes
Smooth ER – synthesis of lipids and some steroids; detoxification e.g. alcohol breakdown.
what does it looook like??? Well, imagine this is a bit like the inner membrane of the mitochondria, but more tightly packed.
Microvilli are really just protrusions of the plasma membrane. They’re like fingers coming out of a hand. The reason they are so relevant to epithelial cells lining the small intestine is that microvilli increase the surface area onto which nutrients can be absorbed. This is a key concept.
These are small vesicles of membrane that contain enzymes which take part in digestion. They look like tiny balls.
My personal favourite ? Ribosomes are made of a small subunit and a large subunit. They are found on the rough ER and free within the cytoplasm, and they are the site of translation where the genetic code is used to build protein. Under the microscope within a cell, they appear as mere dots. But remember, awesome comes in small quantities!
It is a stack of flattened membrane discs which receives packages of protein from the rough ER, and also is involved in synthesising chemicals before they are secreted from the cell.
You will need to know about the difference between transmission and scanning electron microscopes – TEM and SEM. Both (as the name suggests) use a beam of electrons, rather than light, to produce an image of the sample. TEM uses electrons which pass through the sample, so the resulting micrograph (image) shows everything within the sample in black and white, for example organelles in a cell. SEM uses electrons which scan the sample in 3D, resulting in a coloured micrograph with 3D detail, but no components from within the sample.
When talking about microscopes, differentiating between resolution and magnification is important. In principle, it’s not hard to understand. Imagine zooming in a photo to try to see a detail. That is magnification. Now imagine the photo has a low resolution, and if you magnify it, you can only see annoying pixels. If the image had a high resolution, you would be able to see the detail clearly after zooming in. So magnifying is zooming in, while resolution is the focus power. You will need to be able to calculate actual sizes and magnifications of various drawings. The equation for that is:
Image size on paper = Magnification x Actual size
The theory is all good, but in practice, someone’s gotta mash those cells, separate organelles and what not. The technique used is called cell fractionation and it involves ultracentrifugation. These are the steps:
1. Take some tissue such as liver, and blend it in a blender; this is called homogenisation. This must be done in a special solution – an isotonic buffer. Isotonic means of the same water potential as the sample. This ensures enzyme reactions are minimised, organelles don’t get distorted by water gain or loss, and changes in pH are resisted. Learn this (examiners have the hots for isotonic buffers).
2. The resulting mixture is filtered to remove any debris, and then spun in an ultracentrifuge very fast. The increased gravitational field produced will separate the organelles according to their density and even shape.
3. The big ol’ nuclei are the first to separate. The other organelles, still in the liquid at the top called supernatant, are poured into a different test tube and centrifuged further.
4. The organelles are separated in the order: mitochondria and lysosomes, rough ER, plasma membranes, smooth ER, ribosomes.