S-Cool Revision Summary

S-Cool Revision Summary

The cell is the basic unit of an organism and consists of a jelly-like material surrounded by a cell membrane.

It can be seen with a light microscope (LM) but many of the structures within a cell - organelles - can only be seen clearly with an electron microscope (EM). That is partly because an EM has a greater magnifying power (ability to enlarge something).

There are 2 basic cell types:

Prokaryotic: bacteria and cyanobacteria (which used to be called blue-green algae).

Eukaryotic: all other cells, such as protoctista, fungi, plant and animal cells.

Much of what you will need to know applies to the structure of eukaryotic cells. They are characterised by having membrane-bound organelles.

Cytoplasm refers to the jelly-like material with organelles in it.

If the organelles were removed, the soluble part that would be left is called the cytosol. It consists mainly of water with dissolved substances such as amino acids in it.

Also present in the cytosol are larger proteins and enzymes used in reactions within the cell. Running through the cytosol is endoplasmic reticulum (ER), a system of flattened cavities lined by a thin membrane. It is the site of the synthesis of many substances in the cell and so provides a compartmentalised area in which this takes place. The cavities also function as a transporting system whereby substances can move through them from one part of the cell to another.

There are 2 types of ER:

Rough (RER): looks rough on the surface because it is studded with very small organelles called ribosomes. Ribosomes are made of RNA and protein and are the site of protein synthesis

Smooth (SER): obviously looks as though it has a smooth surface. It is where lipids and steroids are made so you would expect there to be a lot of SER in liver cells where lipid is metabolised.

The Golgi apparatus is a series of flattened layers of plate-like membranes.

The proteins that are made by the RER for export from the cell are pinched off at the end of the cavity of the RER, so that a layer of membrane surrounds them. The whole structure is called a vesicle. This vesicle will move through the cytosol and fuse with the membrane of the Golgi apparatus.

In the cavity of the Golgi apparatus, the vessel proteins are modified for export - for example, by having a carbohydrate added to the protein. At the end of a Golgi cavity, the secretory product is pinched off so that the vesicle containing the substance can move through the cytosol to the cell surface membrane.

The vesicle will fuse with this membrane and so release the secretory product. If the vesicle contains digestive enzymes, it is called a lysosome. Lysosomes may be used inside the cell during endocytosis, or to break-down old, redundant organelles.

A typical cell may contain 1,000 mitochondria, though some will contain many more. Generally, they are sausage-shaped organelles whose walls consist of 2 membranes.

The inner membrane is folded inwards to form projections called cristae. Inside this is the matrix.

Most of the reactions for aerobic respiration take place in the mitochondria so it is an incredibly important organelle.

These are only found in plant cells. Chloroplasts like the mitochondria - have an envelope of two membranes making up the outer "wall".

They have pairs of membranes called thylakoids arranged in stacks, each stack being called a granum. Connecting different grana together are inter-granal thylakoids. Surrounding the internal membranes, inside the envelope is the stroma.

The reactions of photosynthesis take place in the membranes and stroma of the chloroplast.

The nucleus is separated from the surrounding cytoplasm by the double membrane around it, the nuclear envelope. This regulates the flow of substances into and out of the nucleus.

Vacuole: fluid-filled space in the cytoplasm surrounded by a membrane called the tonoplast, containg a solution of sugars and salts called the cell sap.

Microtubules: hollow rod-like structures with walls of tubulin protein. Provide the structural support of cells and can aid transport through the cell.

Microfilaments: rod-like structures made of contractile protein. Again, like microtubules, provide support and aid movement.

Centrioles: a pair of short hollow cylinders, usually found near the nucleus of an animal cell. They are involved in the formation of spindle fibres used in mitosis.

Cilia: hollow tubes extending outside some cells. They move fluid, which is outside the cell - for example, ciliated cells lining the respiratory tract move mucus, away from the lungs.

Flagella: similar to cilia, though longer. Used in the movement of the whole cell. The only structure like this in humans is the tails of the sperm.

Much of the membrane is made up of a 'sea' of phospholipids with protein molecules 'floating' in between the phospholipids. Some of these proteins span the whole width of the membrane.

Because the membrane is fluid, and because of the mosaic arrangement of the protein molecules, the structure of the membrane is called the fluid mosaic model.

The phospholipids are arranged in two layers (a bilayer). The phosphate heads are polar molecules and so are water-soluble. The lipid tails are non-polar and therefore are not water-soluble.

Functions of a membrane it's:

  1. Selectively permeable barrier.
  2. Structural, keeping the cell contents together
  3. Allows communication with other cells
  4. Allows recognition of other external substances
  5. Allows mobility in some organisms, e.g. amoeba
  6. The site of various chemical reactions.

It is important that the cell is supplied with all the substances it needs (e.g. oxygen) and that waste substances (e.g. carbon dioxide), or substances for export, leave the cell. There are various processes by which this can happen...

Diffusion

This is the process that is used in oxygen entering a cell, and carbon dioxide leaving.

These molecules will move from where they are at a high concentration to where they are at a lower concentration. i.e. they diffuse down a concentration gradient.

Fick's Law

Fick's law is used to measure the rate of diffusion.

Osmosis

This is a special case of diffusion in which we are concerned only with the movement of water.

Water potential

This is a measure of the tendency of water molecules to move from one place to another.

The movement of water molecules from a region of higher water potential to a region of lower water potential through a semi-permeable membrane.

Solute potential and pressure potential

The water potential of a cell is dependent upon the combination of its solute and pressure potentials.

Osmosis in animal and plant cells

If the water potential surrounding an animal cell is higher than that of the cell, it will gain water, swell and burst. If the surrounding solution's water potential is lower than that of the cell, it will loose water and shrivel up. This is why it is so important to maintain a constant water potential inside the bodies of animals.

In animal Cells:

Water potential = Solute potential

In Plant Cells:

Water potential = Solute potential + Pressure potential

If charged particles or large molecules are to move across the membrane, another process needs to be found, as they are less soluble (or even insoluble) in lipid. They move through protein-lined pores.

Channel proteins: These line a water-filled pore in the membrane so water-soluble molecules can easily pass through. Different channels allow different substances to pass through (the channels are selective). Some channels are gated (they will only open when appropriately stimulated).

Carrier proteins: In this case, the substance actually combines with a protein and is carried from one side of the membrane to the other. (The exact details of this process remain unclear.) These proteins are specific for a particular substance.

In both these cases, substances are moving down the concentration gradient so no energy is required.

Sometimes substances need to be moved from where they are at a lower concentration to where they are at a higher concentration - against the concentration gradient. This allows cells to take up essential molecules even when they are at a low concentration outside.

Because molecules are moved against the concentration gradient, it requires energy. It is thought that active transport uses carrier proteins similar to those involved in facilitated diffusion.

If very large molecules or groups of molecules need to enter or exit a cell, they do so using vesicles.

The material to be transported out of the cell is surrounded by membrane. The vesicle will fuse with the cell surface membrane and the contents leave. This is called exocytosis .

Materials entering the cell can do so when the plasma membrane invaginates to surround the material. The membrane seals off to form a vesicle, which can then move into the cell. This is endocytosis.

If the material is fluid, minute vesicles are formed. This type of endocytosis is called pinocytosis.

If the material is relatively large, and is digested by enzymes after fusion of the vesicle with a lysosome, it is called phagocytosis. This occurs in white blood cells that ingest bacteria and other foreign bodies.

A tissue is defined as a collection of cells, together with any extracellular secretion, that is specialised to perform one or more particular function. Tissues may contain only one type of cell, or several types.

Examples:

Epithelial tissues are animal tissues and form sheets covering surfaces. Two tissues that you need to know about are squamous and ciliated epithelia. Both are one cell thick and so are called simple epithelia. The cells rest on a basement membrane which, is a network of collagen and glycoproteins that is secreted by cells underneath the epithelial tissue.

Squamous epithelia: In this tissue, the cells are of one type and are smooth, flat and very thin. They are packed closely together like tiles on a roof and provide a low friction surface over which fluids can move. It is found lining the cheeks, inside blood vessels, lining the chambers of the heart and forms the alveoli in the lungs.

Ciliated epithelia: This tissue is made up of cells with cilia and so is often found in areas where it is needed to transport something - for example, lining the oviducts and bronchioles of the lungs. Sometimes the cells are shaped like cubes and the tissue is called cuboidal ciliated epithelia. If the cells are tall and narrow, it is referred to as columnar ciliated epithelia.

Xylem and phloem: These two plant tissues differ from the above examples in that they are made up of more than one cell type. Xylem has the dual function of support of the plant and transport of water and dissolved mineral salts. It is made up of vessel elements, tracheids, fibres and parenchyma cells. Phloem tissue is responsible for translocation which is the transport of soluble organic substances - for example, sugar.

Palisade mesophyll: This tissue is found in the leaf and is made up of one type of cell. The cells are tall and thin and are tightly packed together. Their function is to harness the light energy required for photosynthesis and so each cell is packed with chloroplasts.

Organs: An organ is part of the body which, forms a structurally and functionally separate unit and is made up of more than one type of tissue. Examples of plant organs are leaves, roots and stems. Examples of animal organs are the liver, brain, heart and kidney. Organs may be organised into groups with particular functions and are then called systems - for example, the digestive system.