CELL BIOLOGY

Cell Biology is a foundation for all applied life sciences -whether agriculture, horticulture, animal science or human health sciences.

What is a Cell? -

The word cell is derived from the Latin “cella” which means “small room”. Cells are the units from which all living organisms are built. Some organisms (e.g. bacteria) have only one cell in the entire organism. Others are multicellular. A human body can contain an estimated 100,000 billion cells.

 Each cell is a self-contained and partially self-sufficient compartment designed to carry out a limited series of functions. While the structure and function of cells is extremely variable, their basic structure is similar. All cells are bound by an outer membrane and contain cytoplasm and DNA. 

Lesson Structure

There are 10 lessons in this course:

1. Introduction to Cells and Their Structure
   • The cell defined
   • History of cell biology
   • Prokaryotic and eukaryotic cells
   • Cell shape and size
   • Cell structure
   • Cell Components
   • Animal cells
   • Plant cells
   • Human cells
2. Cell Chemistry
   • Cell chemical composition
   • Carbohydrates
   • Lipids
   • Nucleic acids and proteins
   • Enzymes
   • Cell membranes
   • Golgi apparatus.
3. DNA, Chromosomes and Genes
   • DNA defined
   • Chromosomes
   • Genes
   • DNA replication
   • Telomeres and telomerase
   • Genetics
   • Genetic inheritance
   • Phenotype and genotype
   • Mutation
4. Cell Division: Meiosis and Mitosis
   • Mitosis
   • Meiosis
5. Cell Membranes
   • Membranes
   • Structure of cell membranes
   • Movement of molecules through cell membranes
   • Endocytosis
   • Osmosis and filtration
   • Hydrostatic pressure
   • Active transport
   • Electro-chemical gradient
   • Nutrient and waste exchange in animal cells
   • Mediated and non-mediated transport.
6. Protein Structure and Function
   • Protein structure
   • Fibrous proteins
   • Globular proteins
   • Protein organisation
   • Primary to quaternary structure
   • Protein function.
7. Protein Synthesis
   • Overview
   • Function of RNA
   • Transcription and translation
   • Initiation
   • Elongation
   • Termination.
8. Food, Energy, Catalysis and Biosynthesis
   • Sources of energy
   • Metabolism within the cell
   • Catabolic metabolism
   • Anabolic metabolism
   • ATP movement
   • Krebs cycle
   • Production and storage of energy
   • Energy production pathways from different foods
   • Biosynthesis of cell molecules
   • Mitochondria
   • Chloroplasts.
9. Intracellular Compartments, Transport and Cell Communication
   • Cell communication
   • Endocrine signalling
   • Paracrine signalling
   • Autocrine signalling
   • Cytoskeleton
   • Actin filaments
   • Intermediate filaments
   • Microtubules.
10. The Cell Cycle and Tissue Formation
    • The cell cycle
    • Phases of the cell cycle
    • Cell cycle regulation
    • Cell death
    • Cells to bodies
    • Stem cell
    • Tissue types including connective, epithelial, nerve; blood.

Aims

• Review the fundamental structure of cells
• Discuss the scope and nature of cell biology.
• Describe chemicals that make up a cell and the biochemical processes involving those chemicals.
• Describe storage of genetic information in cells and how information is moved between generations.
• Describe the main ideas that underpin molecular biology.
• Discuss membrane structure and transport across cell membranes.
• Discuss protein structure and function.
• Describe and discuss protein synthesis.
• Describe the significant processes involved in transfer and storage of energy in a cell.
• Describe the significant processes that occur in cell communication and intracellular transport
• Describe the life cycle of cells and how they combine to create different types of tissues.

MEMBRANES
Membranes include any barrier surrounding and containing cells or cellular organelles. They are selectively permeable, allowing the transport of molecules and ions within and between cells. This facilitates the separation and regulation of metabolic processes. Plants, fungi and bacteria also have a cell wall outside the cell membrane.  The cell membrane is a plasma membrane.  Plasma membranes

Membranes are composed principally of lipids, particularly phospholipids (figure 1), which are simply lipids that contain phosphorus. Lipids usually compose about 50% of the membrane mass, although this varies according to the type of membrane. Proteins are the other major constituent of membranes, generally constituting around 25%, and up to 75% of the membrane mass.

Structure of Cell Membranes
The structure of membranes varies from organism to organism. Animal membranes consist primarily of two layers of phospholipids, collectively called the Phospholipid Bilayer. The bilayer consists of two rows of hydrophilic (water attracting) phospholipid ‘heads’ on the outer surfaces of the membrane, and the hydrophobic (water repelling) fatty acid tail facing inwards.  This bilayer is stable and spontaneously forms a barrier between two aqueous environments

There are four major kinds of phospholipid in mammalian membranes and in addition to this the membranes contain glycolipids and cholesterol. An important property of lipid bilayers is that the individual lipid and protein molecules are free to rotate, and to move laterally. This has given rise to the Fluid Mosaic Model.  This is the current model for understanding the structure of cell membranes.  It is important to understand that the lipids and proteins inserted into the liquid bilayer are free to move on the 2D plane, thus the name fluid mosaic. 

Membrane proteins are inserted into the Phospholipid Bilayer, and have the role of effecting specific functions.

Proteins may be in the lipid bilayer in different ways:

• Transmembrane or integral proteins expand though the entire membrane
• Peripheral proteins which are which are indirectly associated with the cell and may be bound to the cell by either linkage with a fatty acid chain, or an oligosaccharide or through protein-protein interactions

Carbohydrates within the Cell Membrane

The Glycocalyx is a carbohydrate coating over the cell surface in bacteria, the blood and in the digestive system. These carbohydrates act as markers for recognition between cells, as well protection for the cell, cell adhesion, regulation of inflammation and various roles within reproduction.

• Bacterial cell walls are primarily a peptidoglycan which is a polymer of amino acids and sugar (i.e. Polysaccharide chains cross linked with a peptide).  This layer forms outside the cell membrane, providing structural strength and protection against the cells own osmotic pressure.
• Plant cell walls are fibrous polysaccharides such as cellulose, embedded in a gel like matrix of proteins and polysaccharides. This creates rigid walls which nevertheless have a capacity to expand fast to ingest water.  They consist primarily of pectin, cellulose and hemicellulose.
• Animal cells are surrounded by a matrix of secreted polysaccharides and proteins. Receptors on the cells surface hold this extracellular matrix anchoring the matrix to the cytoskeleton.
• Cholesterol plays an important role in determining membrane fluidity.  They slot into the layer with their polar hydroxyl head lining up with the phospholipid heads.  As cholesterol has a rigid tail made up of hydrocarbon rings, the interaction of this rigid tail with the phospholipid tails reduces the mobility making the membrane stiffer.  Conversely, at lower temperatures it has the opposite effect maintaining the fluidity of the membrane.

MOVEMENT OF MOLECULES THROUGH CELL MEMBRANES
Control of the internal composition of a cell is in part maintained by the selective permeability of the plasma membrane.  Small uncharged molecules (both polar and nonpolar) are free to diffuse through the phospholipid bilayers.  Examples of these include 02, CO2¬, H20.  However larger uncharged polar molecules and ions may not.  Examples of these include amino acids, Na+, Cl-, and glucose.

To get around this, the membrane contains proteins which act as transporters.  There are two types of protein transporters.  They are: channel proteins and carrier proteins.

Channel Proteins:  Essentially these are open pores in the membrane which allow any molecule of appropriate size to pass through such as an Ion channel.   Pores such as this may be selectively opened and closed in response to stimuli allowing the cell to control movement of molecules.

Carrier Proteins:  These proteins work very differently to channel proteins.  They actively bind to a specific molecules and transport them across the membrane by the protein itself changing its conformational structure that opens a channel through which the molecule can be transported.

Passive diffusion is where small hydrophobic molecules cross plasma membranes by diffusing through a phospholipid layer. In contrast, facilitated diffusion refers to molecules that are carried through a membrane by a carrier or channel protein. This process can allow a charged or polar molecule to move through a membrane without reacting with the membrane.

Ion channels allow selected ions to move across a plasma membrane rapidly. (e.g. transmission of electric signals through nerve and muscle cells).

ATP hydrolysis creates energy which can be harnessed to move molecules against their electro-chemical gradients. Ion gradients can frequently be harnessed as an energy source to move other molecules

Endocytosis
Endocytosis involves ingesting material outside of a cell (extracellular material) into vesicles formed by a plasma membrane. Phagocytosis is where cells “eat” other cells or large particles (eg. Bacteria cell debris). Other forms of endocytosis include receptor-mediated endocytosis and protein trafficking.

OSMOSIS AND FILTRATION
Cells are enclosed by a special tissue or skin called a semi-permeable membrane. The word permeable means porous or penetrable. Semi-permeable means that this membrane only allows certain things to pass through it, such as items dissolved in water. The membrane around cells allows water and other items in solution (i.e. dissolved in water) to pass into or out of the cell.

Osmosis is the process whereby water can pass through a semi-permeable membrane from a weak solution to a stronger solution.  For example, A and B are two liquids separated by a semi-permeable membrane such as a cell wall. Liquid A is a solution of sugar and contains dissolved sugar particles (or molecules as very small particles are called). Liquid B is pure water, which is called distilled water, and this does not contain any dissolved matter at all.

The movement which takes place is of water from Solution B through the semi-permeable membrane to Solution A. This causes Solution A to increase in volume and Solution B to decrease in volume. The pressure exerted by this movement is called osmotic pressure. As Solution B gets less and Solution A increases, this osmotic pressure will increase until water starts being forced back through the membrane from Solution A to Solution B. This counter pressure is called the filtration pressure.

The movement with osmosis is always of water, from a weak to a stronger solution. If Solution A was a strong sugar solution and Solution B was a weak sugar solution, the water movement would go on until both solutions were of equal strength.  At this point, movement would stop.

THE OBJECT OF OSMOSIS IS TO HAVE SOLUTIONS OF EQUAL IONIC STRENGTH ON EACH SIDE OF THE SEMI-PERMEABLE MEMBRANE.

Hydrostatic Pressure
This is the movement of very small particles of matter, or molecules, across a semi-permeable membrane because the pressure on one side of the membrane is much greater than the pressure on the other side of the membrane. A good example of this force is the blood pressure of the body which is caused by the heart pumping blood around the system. This causes the pressure inside the capillaries to be higher than that in the surrounding fluid. This higher pressure forces particles through the capillary walls.

Active Transport
This is the movement of particles or molecules across a semi-permeable membrane against the force exerted by osmotic pressure or hydrostatic pressure. This movement requires energy before it can take place as it overcomes existing forces.

 
Phagocytosis
In Greek, "phagos" means eating. Phagocytosis is the action of a cell when it reaches out, engulfs a molecule into a cell and digests it.  One of these molecules can be bacteria.

Pinocytosis
With pinocytosis, the molecule attaches itself to the cell wall. It is then drawn into the body of the cell although it is still surrounded by a part of the cell wall. The gap is repaired by the cell wall growing together.