Exchange And Transport Systems: - Size And Surface Area.

- Size And Surface Area - 

Every organism's needs to be able to exchange substances or molecules with its environment in order to sustain life.

For example, Humans exchanges oxygen and carbon dioxide with its environment.

Cells need to take in oxygen (for aerobic respiration) and nutrients. // Also need to excrete (get rid of) waste products i.e. carbon dioxide and urea // heat needs to be exchanged so that the organism can maintain a constant temperature.


Smaller animals have a higher surface area : volume ratios

A mouse has a much larger surface area : volume ratio compared to an elephant.

However they are unable to perform exchange via their surface.

Although some mammals or other organisms have a large surface area to volume ratio, they are multi-cellular which means that they have a large diffusion distance and high demand as well as this they have a specialised exchange and transport system. They also have impermeable surface - this is to prevent pathogens entering and reducing water loss.

The exchange system would mean that the increases rate of diffusion of nutrients in and wastes out and the transport system had delivered nutrients and remove waste from all cells.


Animals with a high surface area : volume ratio tend to lose more water as it evapourates from their surface // small animals living in the desert would have kidney structure adapted to producing less urine to compensate for the loss of water.

High metabolic rates // small animals in the cold regions would have to eat large amount of high energy foods to maintain the amount of energy they use.

Large animals are adapted to extreme hot conditions // elephants have large flat ears to increase their surface area allowing them to lose more heat // hippos have behavioral adaptations to spend much of their day in water which helps them lose heat.

Exchange And Transport Systems: - Exchange Of Gases In The Lungs

- Exchange Of Gases In The Lung - 

Role of the alveoli in gas exchange 

Epithelial cells // alveolus is a network of pulmonary capillaries // narrow // red blood cells are flatterned against the thin capillary walls in order to squeeze through.

Diffusion of gases between the alveoli and the blood will be very rapid.


Red blood cells slow down passing through pulmonary capillaries allowing more time for diffusion.

Distance between the alveolar air and red blood cells is reduced as red bloods are flattened against the walls of the capillaries.

Alveoli and capillaries are very thin = short diffusion pathways // made of specialised squamous cells

Exchange And Transport Systems: - Mechanisms Of Breathing.

- Mechanisms Of Breathing - 

There is two types of ventilation 

Inspiration = Active process - requires energy 

External intercostal muscle - Contracts 

Internal intercostal muscle - Relax 

Ribs - Upwards and outwards 

Thoracic cavity - Increases in volume 

Diaphragm - Contract 

Expiration = Passive process - does not require 

External intercostal muscle - Relaxes 

Internal intercostal muscle - Contracts

Ribs - Downwards and inwards 

Thoracic cavity - Decreases in volume 

Diaphragm - Relaxes 

Movement of the intercostal muscles = antagonistic - opposing. 

Exchange And Transport Systems: - Structure Of The Human Gas-Exchange System

- Structure Of The Human Gas-Exchange System - 

Mammalian lung:

Lungs are site of gas exchange in mammals 

They are in the body as the air is not dense to support and protect 

Body as whole would otherwise lose a great deal of water and dry out. 

Lungs are specialised for gas exchange:

When you breath in air this travels down the Trachea // windpipe.

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Bronchi/Bronchus is the split of the trachea at the end of the wind pipe. 

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Bronchioles is the further branching of the bronchi into smaller tubes. 

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Alveoli are at the end of the bronchioles which are 'air sacs'. 

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The rib-cage, intercostal muscles and diaphragm all work together to move air in and out. 

Exchange And Transport Systems: - Limiting Water Loss

- Limiting Water Loss - 

Limiting water loss in insects 

Insects have evolved the following adaptations:

- Small surface area to volume ratio = minimise area over which water is loss 

- Waterproofing covering = outer skeleton of chitin is covered with waterproof cuticle.

- Spiracles = opening of the tracheae at the body surface can be closed due to water loss occurs when the body is asleep. 

Limiting water loss in plants 

Terresteral plants have waterproof covering 

Some have restricted supply of water = limiting water loss through transpiration // xerophytes

Modifications of a plant is made by:

- Thick cuticles 

- Rolling up of leaves 

- Hairy leaves 

- Stomata in pits and grooves 

- Reducing surface area to volume ratio of the leaves

Exchange And Transport Systems: - Gas Exchange In Plants

- Gas Exchange in plants - 

Gas exchange is able to occur in plants due to their adaptations.

These adaptations would include - 

Thin = Short diffusion pathway// Carbon dioxide and oxygen can pass in and out of the plant cell. 

Large surface area = this would allow for an effective gas exchange because its able to occur over a larger area. 

Air spaces between the spongy mesophyll = allows a diffusion gradient to be maintained 

Guard cells = controls the amount of Carbon dioxide and oxygen diffuses in and out of the cell. 

For a most gaseous exchange to occur:
- There are many small pores // stomata // no cell is far from a stoma // short diffusion path.
- Numerous interconnecting air-spaces occur throughout the mesophyll
- Large surface area of mesophyll cells // rapid diffusion

Stomata:

Each stoma is surrounded by pair of special guard cells which open and closes the stomata pores // controls the rate of gaseous exchange // plants evolved to balance the conflict between gas exchange and controlling the water loss in the plant.

Exchange And Transport Systems: - Gas Exchange In Fish

- Gas Exchange in fish -

Fish have many different adaptation that give would give the fish large surface area for gas exchange. 

In a fish there are rows of gill filaments which are stacked like pages of a book. 

On the gill filaments there are gill lamellae which has a network of capillaries on them. This provides a larger surface area for gas exchange. 

counter-current flow 

The water enters the mouth of the fish and leaves through the gills. 

As the water passes through the gills the water passes over the filaments and over the lamellae. 

Water and blood flow over and through the lamellae in the opposite direction. // Parallel. 

When the blood first comes close to the water, water is fully saturated with oxygen and the blood has small amounts. 

This creates a steep concentration gradient // oxygen diffuses out of the water and into the blood. 

As the blood is absorbing more oxygen as it moves along the lamellae // blood reaches the end of the lamella 80% saturated with oxygen. The blood is highly saturated than it was at the beginning of the lamellae.

The concentration has continued to be maintained so it can continue to absorb oxygen from the water. 

Concurrent flow 

The water enters the mouth of the fish and leaves through the gills. 

As the water passes through the gills the water passes over the filaments and over the lamellae. 

The blood flows in same direction as the water does.

Water is slightly less saturated as it proceeds to move along the lamellae // water has still highly saturated with oxygen compared to the blood, diffusion still occurs until water and the blood have reached equal saturation. 

The steep concentration gradient continues to decrease as the diffusion of oxygen continues.

Exchange And Transport Systems: - Gas Exchange In Insects

- Gas Exchange in insects -

Insects do not have a transport system so gases would need to transported directly into the respiring tissues. 

Insects use tracheae to exchange gases 

Insects have microscopic air-filled pipes = Tracheae

The air moves into the tracheae through the pores on the surface = Spiracles 

On the insect there are spiracles which are placed along side the body // These spiracles are openings of small tubes running into the insects body. 

Oxygen moves down a concentration gradient towards the cells.

The tracheae branches off into tracheoles //
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Adaptations =
- Thin = quicker rate of diffusion
- Permeable walls = diffusion occurs down a gradient (active transport not needed to pass the molecules)
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Oxygen diffuses into the respiring cells.

Water at the end of the tracheoles allows for a concentration gradient to be maintained // the

Carbon dioxide from the respiring diffuse into the water (higher concentration of CO2 // lower concentration of 02 in the respiring cells).

Rhythmic abdominal movement = moves the air in and out of the spiracles.

Nucleic Acids: - Inorganic Ions

- Inorganic Ions -

Inorganic ions are found in organisms in solution in the cytoplasm of cells and in body fluids, including parts of larger molecules.

They may be in areas that range from very high to very low concentration.

Functions of the ions can vary, however the specific function of a particular ion is based on the relation to it's properties.

Nucleic Acids: - Water

- Water - 

Dipolar water molecule: water is made up of two atoms of hydrogen and one of oxygen. The molecule has no overall charge although oxygen has a slightly negative charge and hydrogen has a slightly positive charge.

Water and hydrogen bonding:

A positive pole of one water molecule would attract the negative pole of another water molecule, the attraction force between the opposite charges is a hydrogen bond. Each bond is weak, however together they form important forces that cause water molecules to stick together, giving water unusual properties.

Specific heat capacity:

Water molecules stick together, it takes more energy -heat- to separate them then if they did not bond to one another. Resulting in the boiling point being high (100 degrees celsius)

Without hydrogen bonding water would be a gas at temperatures on earth.

Water acts as a buffer against sudden temperatures variations - aquatic environment is a temperature stable one.

Organisms are mostly water, protects them from sudden temperature change especially in terrestrial environment.

Latent heat of vaporisation of water:

Hydrogen bonding = lots of heat energy required to evaporate 1 gram of water. - latent heat of vaporisation.

Evaporation of water in mammals is therefore a very effective means of cooling, body heat is used to evaporate the water.

Cohesion and surface tension:

Tendency of molecules sticking together = cohesion

• Hydrogen bonding = large cohesive forces, allowing it to be pulled through a tube - xylem vessel
• Water molecules meet air, tend to be pulled back into the body of water than escaping from it = surface tension.
• Water acts like a skin, strong enough to support small organisms - pond skaters

Importance of water to living organisms:

Water in metabolism -

• Used to breakdown complex molecules - hydrolysis
• Joins molecules together through the use of condensation reaction
• Chemical reactions take place in aqueous medium
• Water = major raw material in photosynthesis

Water as a solvent -

• Readily dissolves other substances - gases ie, oxygen + carbon dioxide
• Waste - ammonia + urea
• Inorganic ions + small hydrophilic molecules - amino acids, monosaccharides, + ATP
• Enzymes - reactions which takes place in a solution.

Important features of water:

• Evaporation cools and allows organisms to control their temperature.
• Not easily compressed = providing support - hydrostatic skeleton of animals// earthworm + turgor pressure in herbaceous plants
• Transparent = aquatic plants can photosynthesise // light penetrate jelly like fluid that fills the eye and reach the retina.

Nucleic Acids: - ATP

- ATP - 

ATP - Adenosine triphosphate is a nucleotide and has three phosphate groups and is key in storing energy. The bonds between the phosphate groups are unstable therefore have a low activation energy - can be easily broken. They release a considerable amount of energy. In living cells it is only the terminal phosphate that is removed

Structure of ATP:

• Adenine - nitrogen containing organic base.
• Ribose - sugar molecule with 5 carbon ring structure - backbone
• Phosphates - chain of three phosphate groups

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Synthesis of ATP: ATP to ADP is a reversible reaction therefore energy can be used to add an inorganic phosphate to ADP to re-form ATP. This reaction is catalysed by enzyme ATP synthase. The reaction is a condensation reaction.

The synthesis ATP from ADP involves addition of phosphate molecule to ADP.

• In chlorophyll containing plant cells during photosynthesis (photophosphorylation)
• In plant and animal cells during respiration (oxidative phosphorylation)
• In plant and animal cells when phosphate groups are transferred from donor molecules to ADP (substrate level phosphorylation)

Roles of ATP:

• ATP serves as an immediate energy source of a cell
• Cells do not require large quantities of ATP
• ATP is rapidly re-formed from ADP and inorganic phosphate

ATP is required as energy-requiring processes in cells:

• Metabolic processes
• Movement
• Active transport
• Secretion
• Activation of molecules

Nucleic Acids: - DNA Replicationn

- DNA Replication - 

DNA replication: cell division occurs in 2 main stages:

• Nuclear division: nucleus divides
• Cytokinesis - the whole cell divides

Semi-conservative replication ~ this ensures the genetic continuity between generations.

The enzyme DNA helicase breaks the hydrogen bonds between complementary bases in polynucleotides strands, causing the double helix to unwind/unravel.

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The enzyme DNA polymerase joins adjacent nucleotide's in a condensation reaction.
  

The exposed nucleotide's act as a template to which the complementary free nucleotide's bid by specific base pairing

2 identical molecules of DNA are formed. This is due to each molecule retaining half of the original DNA material. Watson and Crick model of DNA replication suggested that the original DNA molecule splits into 2 separate strands therefore each of the new molecules would have one strand of the original material. This would be the semi-conservative model.  The conservative model suggests that the original molecule remained intact and the other molecule formed would be made of entirely new material. 

Nucleic Acids: - Structure of RNA and DNA

- Structure of DNA and RNA -

Deoxyribonucleic acid and ribonucleic acid are important information-carrying molecules. In all living cells, DNA holds genetic information and RNA transfers genetic information from DNA to the ribosomes.

DNA molecule is a double helix with two polynucleotides chains held together by hydrogen bonds between specific complementary base pairs.

• Adenine is complementary to thymine
• Guanine is complementary to cytosine

Ribosomes are formed from RNA and protein.

Both DNA and RNA are polymers of nucleotides. Each nucleotides is formed from

• Pentose sugar
• Phosphate group
• Nitrogen-containing organic base

Nitrogen containing bases are:

• Adenine - A
• Thymine - T
• Guanine - G
• Cytosine - C
• Uracil - U ( RNA molecules only // replaces Thymine (T))

Pentose sugar, phosphate group and organic base are joined as result of condensation reactions, forming a mononucleotide.

2 mononucleotides may in turn be joined by the condensation reaction between the deoxyribose sugar of one mononucleotide and the phosphate group of another. The bond formed between them is called a phosphodiester bond.

RNA structure:

• RNA molecules is a relatively short polynucleotide chain
• The pentose sugar is ribose and the organic bases are adenine, guanine, cytosine, and uracil.

DNA structure // Double helix structure (DNA) ~

• The pentose sugar is deoxyribose and the organic bases are adenine, thymine, guanine and cytosine.
• Alternating phosphate and deoxyribose molecules make up the ‘uprights’ and pairs of organic bases comprise the ‘rungs’. The complementary base pairing ensures a standard ‘rung length’.

Stability of DNA ~

The phosphodiester backbone protects the more chemically reactive organic bases.

Hydrogen bonds link the organic base pairs forming bridges between the phosphodiester uprights

The higher the proportion of G-C pairings, with 3 hydrogen bonds, A-T pairings have 2 hydrogen bonds.

Function of DNA ~

- DNA is responsible for passing genetic information from cell to cell.

- Adaptations.

- Large molecule - carries an immense amount of genetic information.

- Base pairing leads to DNA being able to replicate and transfer into as mRNA.

- The base pairs are found in the helical cylinders therefore they are protected from outside chemical and physical forces.

- Mutations in DNA due to the stability of the molecule. // whether or not a mutation would occur.

- Separate strands are held by hydrogen bonds - easy to separate.

- The simplicity of DNA led to many scientists doubt that it carried the genetic code.

Biological Molecules: - Enzyme Inhibition

- Enzyme Inhibition - 

Enzyme inhibition: enzyme inhibitors are substances that directly or indirectly interfere with the function of the active site of an enzyme, reducing its activity.

Competitive inhibitors: inhibitors have a molecular shape which would be similar to the substrate, which allows them to occupy the active site instead of the substrate, competing with the substrate molecule.

If the substrate concentration is increased, the effect of the inhibitor rescued.

The inhibitors is not permanently bound to the active site thus when it leaves, another molecule can take it place.

The concentration of the inhibitor determines how long it will take for all substrate molecules to occupy the active site.

- Non competitive inhibitors -

They alter the shape of the enzyme and active site which means that substrate molecules can no longer occupy it and so the enzyme cannot function. Because the inhibitor and the substrate are not competing for the same site, an increase in substrate concentration does not decrease the effect of the inhibitor.

Biological Molecules: - Factors Affecting Enzyme Action

- Factors Affecting Enzyme Action - 

There are many factors that effect enzyme action, this includes:

1. Temperature.  
2. pH levels.
3. Concentration. 

However before delving into detail about how they effect the enzyme action. First, we have to see how enzyme-catalysed reactions are measured. 

To measure the progress of an enzyme-catalysed reaction the time course is measured. 

2 changes that are frequently measured:

> Formation of the products of the reaction. 

> Disappearance of substrate.

Measuring rate of change 

The change in the rate of reaction can be measured at any point on the curve of a graph. 

The gradient is equal to the gradient of the tangent to the curve at the point the tangent is the point at which a straight line touches the curve but without cutting across it. 

You can draw the tangent to the curve at the point shown. Using this line you can find the gradient 

     a 

 = ㄧ     

     b

Effects of different factors on the rate of enzyme action it is important to stress the fundamental experimental technique of changing only a single variable in each experiment. 

Rate of an enzyme reaction all the other variables must be kept constant when investigating the effects of a named variable. 


Another thing to remember is that the active site and substrate is not the same as a lock and key.  

Temperature ● pH ● Concentration 

Factors effecting enzyme action. 

Temperature: 

When there a temperature increase, there is an increase in kinetic energy of molecule. 

The molecules move around rapidly colliding with each other more often. 

Enzyme-catalysed reaction = enzyme and substrates molecules come together more often in a given time. 

More effective collisions = More enzyme-substrate complexes being formed. 

At first the substrate does not fit as easily into the changed active site = slowing rate of reaction. 

Human enzymes begin at temperature of 45℃. 

Around 60℃ enzyme is so disrupted its stops working together = the enzyme denatures. 

Denaturation is a permanent change and enzyme does not function again. 

Many enzymes have an optimum temperature of around 40℃ in the human body. 

Optimum temperature = the most/best favourable point or degree or condition at which an organism can work at which would obtain the best results. 

pH: 

pH of a solution is the measure of hydrogen ion concentration. 

The pH of a solution is calculated using : 

pH = -log₁₀[H⁺] 

H⁺ concentration of 1 x 10⁻⁹ therefore has a pH of 9 

An increase or decrease in pH reduces rate of enzyme action. If the pH level is extreme the enzyme becomes denatured. 

The pH level affects enzymes works in two ways:

1. pH alters the charge on the amino acids that make up the active site of the enzyme = substrates can no longer attach to the active site. In other words, enzyme-substrate complex cannot be formed. 
2. pH may cause the bonds maintaining the enzyme's tertiary structure to break = active site changes. 

Concentration: 

The active site can be reused to repeat the procedure on another substrate molecule once it is free. This means that enzymes are not used up during the chemical reactions, resulting in them working efficiently at very low concentrations.

1. Low enzyme concentration: there are too few enzyme molecules to find an active site at one time.
2. Intermediate enzymes concentration: all the substrate molecules can occupy an active site as the same time. The rate of reaction had doubled to it's maximum as a result.
3. High enzyme concentration: adding more enzyme molecules has no effect as there are already enough active sites to accommodate all the substrate molecules.