CELLS: STRUCTURE AND FUNCTION
Cellular processes

You can click to go straight to any one of the following topics:

Compounds in cells which includes Organic compounds, Structures of important subunits, Inorganic compounds.

Enzymes: Structure and function including Lock-and-key model, Effects of pH and temperature.

Cellular Biochemistry including Photosynthesis and Respiration.

Cellular energy including ATP.

Cell membranes as barriers. including Features of cell membranes, Comparison of diffusion, osmosis and active transport, Packaging for safety, Endocytosis, Exocytosis, Rough Endoplasmic Reticulum, Golgi bodies, Lysosomes.

COMPOUNDS FOUND IN CELLS

Organic Compounds

These are complex molecules containing carbon.
They usually contain hydrogen and oxygen too.
They may contain other elements such as phosphorus, nitrogen or sulphur.
Sometimes metals are associated with organic molecules to assist in their biological activity.
Organic molecules range in size form relatively small to very large.
Many organic compounds need special methods of entry/exit if they are to cross cell membranes.

**Characteristics of Organic Compounds**
	CARBOHYDRATES	PROTEINS	LIPIDS	NUCLEIC ACIDS
Elements present	C, H, O	C, H, O, N	C, H, O	C, H, O, N, P
Sub Units	sugar (monosaccharides)	amino acids	glycerol & fatty acids, steroids	nucleotides
Examples	Glucose, Sucrose, Glycogen, Starch, Cellulose, Chitin	Enzymes, Keratin, Haemoglobin, Insulin, Antibodies	fats, oils, cholesterol, oestrogen, testosterone	DNA, mRNA, tRNA, rRNA

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Structures of important molecular subunits

Students need to be able to recognise the basic structures of a few biological molecules.
You should also understand how these molecules combine together to form the common macromolecules.

Structures of Biological Molecules
Monosaccharide (glucose)
Amino acid (general)
Triglyceride
Nucleotide

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Inorganic Compounds

These compounds are generally not based on carbon, although some have carbon in them.
They are generally smaller molecules than are organic compounds.
They are often able to cross most cell membranes easily.
Examples are water, carbon dioxide, mineral ions.

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ENZYMES

Always made of protein.
Made on the ribosomes.
Organic catalysts. (Speed up reactions which would otherwise be too slow to sustain life)
Lower the activation energy required to allow the chemical reaction to proceed. (The technical explanation for catalysts.)
Specific for a single substrate or for a group of similar substrates.
Names often end in ?????ase. (The ????? is the substance the enzyme acts upon. eg protease, lipase, ribonuclease )

Lock and key model of enzyme-substrate formation

This is one explanation for the mechanism of action of enzymes.

Because enzymes are large, complex, 3-dimensional substances, it is likely that each one is folded and twisted in its own unique way when it is active.

It can be shown that proteins, and therefore enzymes, are held in their correct 3-D structure by a combination of a few strong chemical bonds and very many weak chemical bonds.

So the 3-D structure of the active site of an enzyme can be thought of as a complex lock into which only one, or at most a few similar, key(s) can possibly fit. This 'key' is the substrate for that enzyme.

Once the substrate has entered the active site (the key is in the lock), an enzyme-substrate complex has formed. Within this, minor rearrangements of the enzyme's weak bonds can occur, twisting the substrate into a position where the chemical reaction can procede much more rapidly than it otherwise would have done.

Once the chemical reaction is complete, the products fall away from the enzyme, leaving the enzyme free to repeat the process with another substrate molecule.

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Effect of temperature and pH on enzyme activity.

Because enzymes are proteins, they are sensitive to changes in their immediate environment.
(Think of what would happen to egg white under the following conditions - it's a good model for enzymes, although egg white is not itself an enzyme.)
Changes of pH (acidity or alkalinity) can alter the protein's 3-D structure, so it loses activity.
(Try putting egg white into either vinegar, an acid, or a carb soda (sodium bicarbonate) solution, an alkali.)
Changes of temperature can also alter a protein's 3-D structure and decrease its activity.
(Can you think how to test this with egg white?)
A protein which has been altered by conditions outside its normal range, is described as denatured. This does not mean that the protein's amino acids have changed, it just means that the protein's unique folding is no longer present, so the biological activity is lost. (You can still eat denatured protein and derive the nutritional benefit of the amino acids after digestion, which is just as well, since most of us prefer our steak and eggs cooked, not raw!)
Other things can denature proteins, such as high salt concentrations or physical damage as in whipping egg whites, but these are uncommon in living things.
Each enzyme has its own particular optimal conditions of pH and temperature.
The optimal conditions of an enzyme usually reflects the conditions in which it is normally found.
Examples: pepsin in the stomach works best at pH 2-3 and 37^oC, some plant enzymes work best in temperatures of summer sunny days - up to 50^oC.

You should make sure that you can explain what is occuring in graphs such as the following:

enzyme/temp

enzyme/temp

enzyme/pH

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CELLULAR BIOCHEMISTRY

Biochemistry is the study of the chemical relationships in living things.

It is a complex but fascinating branch of biology, and the backbone of the work around the world to understand and use the biochemistry of many organisms, including humans, for the benefit of all.

Much of the work in Biochemistry Departments in Universities and Research Institutions now revolves around genetic engineering and the possibilities the techniques associated with it have opened up.

In VCE Biology you need to know some fundamental Biochemistry to explain the basic relationships needed for cellular life. This is the 'tip of the Biochemistry iceberg'!

Photosynthesis

The biological significance of photosynthesis is that it is a means of trapping energy from the sun and storing that energy in a form accessible to cells.
Without photosynthesis, there could be no life as we know it on the Earth.
Only occurs in the chloroplasts of autotrophs. (Why?)
Needs the green pigment chlorophyll to be present.
Only occurs when light levels are adequate.
Biochemically, it can be described as two sets of reactions; the light reactions and the dark reactions.
The light reactions begin the pathway of photosynthesis, but can only occur in adequate light and it needs chlorophyll present.These reactions occur in the chloroplasts' grana.
The dark reactions, also called the Calvin cycle, take the products of the light reactions and complete the pathway of photosynthesis. This does not need light. These reactions occur in the chloroplasts' stroma.
Many individual steps are involved in the pathway of photosynthesis, each step needing its own enzyme.

For the CAT 1 VCE Biology exam, you should be able to write out the balanced equation for the overall process of photosynthesis:

photosynthesis

The glucose produced in photosynthesis does not remain as glucose for very long. Depending on the needs of the plant at the time, it will suffer one or more of the following fates:

Be metabolised via respiration to provide cellular energy.
Be converted to sucrose (a disaccharide) for transport via the phloem to another part of the leaf or the plant.
Be converted to starch (a polysaccharide) at a storage site. This can be reclaimed for energy generation later if necessary.
Be converted to cellulose (another polysaccharide) for new cell walls in dividing cells.
Be converted to other types of molecules, such as amino acids, lipids or nucleic acids, as needed by the plant's cells.

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Respiration

Cellular respiration is fundamental to the continuation of all life.
Cellular respiration involves the sequential breakdown of glucose molecules (which contain 6 carbon atoms), with the accompanying release of the energy trapped in the chemical bonds of the glucose.
Cellular respiration releases energy packaged in the bonds of the cell's energy carrier, ATP.
Respiration involves many individual reactions, each with its own enzyme.
The first sequence of reactions in respiration occur in the cell's cytoplasm. This is called glycolysis.
Glycolysis needs no oxygen, but it releases just 2 ATP molecules and converts glucose from a 6-carbon compound to two pyruvate molecules, each with 3 carbons.
If the cell has no oxygen, respiration stops at glycolysis. This is anaerobic respiration. But as pyruvate build up is toxic, cells dispose of that safely.
Plant and yeast cells respiring anaerobically convert their pyruvate to ethanol (with 2 carbons) and CO₂. This is called fermentation.
Animal cells respiring anaerobically convert their pyruvate to lactate (with 3 carbons). This has no special name, it is called anaerobic respiration by biochemists.
In the presence of oxygen, aerobic respiration occurs in plant and animal cells.
The enzymes for aerobic respiration are located in the mitochondria, so the first step in the process is the entry of the pyruvate from glycolysis into these mitochondria.
In a complex sequence of reactions, known as the Krebs cycle, or the TCA cycle, the pyruvate molecules are completely broken down to CO₂. Water is also produced, and the energy released is trapped in 36 ATP molecules.
Water and Carbon Dioxide can leave the cell as wastes. ATP is used wherever the cell's metabolism requires energy.
Not all of the energy released in respiration is trapped in ATP, there is also a great deal of energy released as heat. This can be used to maintain body temperature if necessary, or simply be lost to the environment.

For the CAT 1 VCE Biology exam, you should be able to write out the following equations for glycolysis, fermentation, and aerobic respiration:

cellular respiration

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CELLULAR ENERGY

Green plants (autotrophs) trap solar energy and store it in chemical bonds in the process of photosynthesis.
Plants and the animals that eat them (heterotrophs) release the energy in the process of respiration.
Much of the energy released in respiration is itself trapped in a useful form for the cell, ATP.
Some of the energy released in respiration can be used for generating heat (in homeotherms) or in a few organisms, for generating light (for example fireflies).

ATP

ATP

Adenosine Triphosphate is the major energy carrier of the cell.
It is always involved in linked reactions in cells.
In catabolic reactions, complex molecules are broken down, releasing their chemical bond energy. ATP is produced from ADP and inorganic Phosphate (P_i) to trap this released energy.
Anabolic reactions are those in which new complex molecules are formed from simpler precursors. This requires new chemical bonds to be formed. The energy for these bonds comes from ATP which is itself broken down to ADP and P_i.
ATP is also used in specialised processes such as the contraction of muscles to give movement.

The cycling of energy within a cell is sometimes shown in a diagram: energy cycle

energy cycle

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CELL MEMBRANES AS BARRIERS

Features of Cell Membranes

All cells are enclosed by a membrane.
The cell membrane acts as a barrier between the intracellular (within the cell) and intercellular (between the cells) fluid.
Cell membranes are impermeable to most water soluble compounds.
Most intracellular organelles are surrounded by membranes.
Cell membranes consist mainly of proteins and phospholipids with small amounts of cholesterol. There are often carbohydrates on the cell's outer surface.
The phospholipids form a bilayer, with the hydrophillic (water attracting) phosphate groups on the outside and the lipid chains forming a hydrophobic (water repelling) lattice in the middle.
The proteins embedded in the bilayer have two main functions: receptors for messages such as the large protein hormones, or channels for transport of substances such as amino acids, glucose and certain small hormone molecules.
The presence of the lipids allows the passage of certain small lipid soluble substances directly through the memebrane. Examples; certain anaesthetic gasses, alcohol.
Cholesterol aids the stability of the cell membrane and decreases its permeability to small water soluble molecules.
Carbohydrates on the cell membrane's outer surface aid cell recognition which is important in cell-cell communication and defense mechanisms.
Wanted substances need to move in to cells, wastes and substances specifically manufactured for export need to move out. This is also true for the cell's membrane bound organelles.
Transport across membranes may be by diffusion, osmosis or active transport.

Students should be familiar with the structure of membranes, and be able to correctly label the parts.

Here is a small membrane diagram, you can click on the picture to see the large version of it.

Membrane diagram - small

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Comparison of diffusion, osmosis and active transport.

	DIFFUSION	OSMOSIS	ACTIVE TRANSPORT
Energy requirements	ATP not required	ATP not required	ATP required
Distance of travel	Solutes and gases- long distances	Solvent (water)- short distances	Short distances
Speed of process	Rapid in gases, slow in solution	Slow process	Rapid process
Definition	Transport of gases or dissolved substances (solutes) in solution from a region of high concentration to a region of lower concentration of the transported substance.	Transport of a solvent (water) through a semipermeable membrane from a solution of low solute concentration to a solution of higher solute concentration.	Transport of a substance from lower to higher concentration of that substance, using energy from the cell, through a living cell membrane.
Examples	Uptake of oxygen by a cell performing aerobic respiration. Oxygen continues to diffuse into the cell as O₂ is removed by respiration.	The movement of water into cells of a marine (salt water adapted) animal placed in fresh water. May eventually cause cells to swell and burst.	Transport of glucose against its concentration gradient from the gut lumen into the cells lining the intestine.

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Packaging for safety.

Many substances which enter cells or which are made in cells either for export or as a metabolic waste, are potentially toxic.

Cells have evolved mechanisms to protect themselves from these threats.

These mechanisms include:

Endocytosis

Movement into a cell
The membrane surrounds large particles.
The particles are 'engulfed', but safely surrounded by membrane.
The particles are now in vacuoles which may fuse with lysosomes (see below).

Exocytosis

Expulsion of large particles from the cell.
Particles expelled are frequently enzymes or hormones.
Prior to expulsion, particles are safely surrounded by membrane.
Vacoules formed by Golgi bodies move to cell membrane, fuse with it, and liberate their contents to the extracellular environment.

Rough endoplasmic reticulum

Involved in the transport and synthesis of export proteins for the Golgi bodies.
Contains enzymes for additional processing and folding of proteins.
Retains the proteins within an enclosed network of membranes to prevent them from becoming active in, and damaging to, the cytoplasmic contents.
Abundant in pancreatic cells. (In various types of pancreatic cells there is much production of either the hormone Insulin or of digestive enzymes.)

Golgi bodies

Package molecules (usually proteins) in vesicles for export from the cell.
Well developed in secretory cells.
Presence of much rough endoplasmic reticulum and Golgi bodies is a clue as to the function of the cell.

Lysosomes

Membrane-bound vesicles.
Contain enzymes which breakdown debris and foreign microorgansims.
Many of these in a cell would suggest a role as scavenger. Example, certain white blood cells.
Single celled organisms use these to deliver digestive enzymes to small food particles captured by endocytosis.

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Last update : 4 March 97