Brief Biology Revision

Brief Biology revision for HL, not all topics covered.

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1. Statistical Analysis

Some Statistics

  • Error bars represent graphically the variability of data.

  • Standard Deviation summarises the spread of values around the mean.
  • 68% of normally distributed data falls within one SD.
  • 95% of normally distributed data falls within two SDs.
  • A small SD indicates a smaller spread of data and better reliability.

  • Correlation does not indicate a causal relationship.

Can you...

  • Calculate the mean?
  • Calculate the standard deviation using a GDC?
  • Deduce a significant difference based on the t-test?
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2. Cells

1. Cell Theory

  • Living organisms are composed of cells
  • Cells are the smallest unit of life
  • Cells come from pre-existing cells
  • Unicellular organisms carry out all functions of life
  • These include: Growth, Reproduction, Homeostasis, Nutrition, Response, Metabolism
  • The Surface Area:Volume ratio limits cell size
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2. Cells

2. Membrane

  • Cell membranes consist of a phospholipid bilayer
  • Phospholipids have hydrophobic tails and hydrophilic heads
  • This causes the heads to face the water and the tails to turn away from the water
  • This double layered structure makes the membrane stable and flexible
  • The phospholipids are in a fluid state, so the cell can easily change shape

  • Diffusion is the passive movement of particles from a region of high concentration to a region of low concentration

  • Osmosis is the passive movement of water molecules, across a partially permeable membrane, from a region of lower solute concentration to a region of higher solute concentration
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2. Cells

2. Membrane

  • Membranes are semi-permeable; allowing certain molecules through but not others.
  • Molecules move through passive transport that does not require any input of energy.
  • It can either be done by simple diffusion or facilitated diffusion.
  • Molecules will go from a region of high concentration to a region of low concentration.
  • Simple diffusion involves the diffusion of molecules through the phospholipid bilayer. 
  • Facilitated diffusion involves the use of channel proteins embedded in the membrane.
  • The cell membrane is hydrophobic inside so hydrophobic molecules will pass through by simple diffusion whereas hydrophilic molecules and charged particles will use facilitated diffusion.
  • Water moves through by osmosis which is also by passive transport. 
  • Active transport is the movement of substances through the membrane using ATP.
  • An advantage is that substances can be moved against the concentration gradient.
  • This is possible because the cell membrane has protein pumps embedded it which are used in active transport to move substances across by using ATP.
  • Each protein pump only transports certain substances so the cell can control what comes in and what goes out.
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2. Cells

3. Cell Division

  • 1. Interphase – G1: Cell grows. S: Genome is replicated. G2: Replicated genome is separated.
  • 2. Mitosis – Prophase, Metaphase, Anaphase, Telophase. Two daughter genomes are separated.
  • 3. Cytokinesis – Cytoplasm divides to create two daughter cells. Animal cells are pinched into two.

Interphase is an active period in the life of a cell during which many metabolic reactions occur such as protein synthesis, DNA replication and an increase in the number of mitochondria and/or chloroplast.

Tumours are formed as the result of uncontrolled cell division. This can happen in any organ or tissue.

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2. Cells

4. Mitosis

  • Prophase: Spindle microtubules grow and extend from each pole to the equator. Chromosomes supercoil and the nuclear envelope breaks down.
  • Metaphase: Chromatids move to the equator and the spindle microtubules from each pole attach to each centromere on opposite sides.
  • Anaphase: Spindle microtubules pull the sister chromatids apart splitting the centromeres. This splits the sister chromatids into chromosomes. Each identical chromosome is pulled to opposite poles.
  • Telophase: Spindle microtubules break down and the chromosomes uncoil. The nuclear membrane reforms. The cell then divides by cytokinesis to form two daughter cells with identical genetic nuclei.

Growth, embryonic development, tissue repair and asexual reproduction involve mitosis.

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3. Chemistry of Life

1. Common Biochemical Elements

  • Carbon, hydrogen, oxygen and nitrogen are the most frequently occurring chemical elements in living things.
  • A variety of other elements are needed by living organisms, including sulphur, calcium, phosphorus, iron and sodium.
  • Sulphur: Needed for the synthesis of two amino acids.
  • Calcium: Acts as a messenger by binding to calmodulin and a few other proteins which regulate transcription and other processes in the cell.
  • Phosphorus: Is part of DNA molecules and is also part of the phosphate groups in ATP.
  • Iron: Is needed for the synthesis of cytochromes which are proteins used during electron transport for aerobic cell respiration.
  • Sodium: When it enters the cytoplasm, it raises the solute concentration which causes water to enter by osmosis.
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3. Chemistry of Life

2. Lipids and Carbohydrates

  • Lipids can be used for energy storage in the form of fat in humans and oil in plants.
  • Lipids can be used as heat insulation as fat under the skin reduces heat loss. 
  • Lipids allow buoyancy as they are less dense than water and so animals can float in water. 
  • Carbohydrates and lipids can both be used as energy storage.
  • Carbohydrates are usually used for short term storage whereas lipids are used for long term storage.
  • Carbohydrates are soluble in water.
  • This makes carbohydrates easy to transport around the body.
  • Carbohydrates are easier and quicker to digest so their energy can be used fast.
  • Because lipids are insoluble, they do not have an effect on osmosis.
  • They also contain more energy per gram than carbohydrates which make lipids a lighter store compared to a store of carbohydrates equivalent in energy. 
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3. Chemistry of Life

3. DNA

  • A nucleotide is made of a sugar (deoxyribose), a base and a phosphate group.
  • DNA replication is semi-conservative as the DNA molecules produced are formed from an old and a new strand.
  • The first stage of DNA replication is the unwinding the DNA double helix.
  • Next, the two strands are separated by breaking the hydrogen bonds between the bases.
  • This is done by the enzyme helicase.
  • Each separated strand now is a template for the new strands.
  • There are many free nucleotides around the replication fork.
  • The free nucleotides form hydrogen bonds with their complimentary base pairs on the template strand.
  • Adenine will pair up with thymine and guanine will pair up with cytosine.
  • DNA polymerase is the enzyme responsible for this.
  • The new DNA strands then rewind to form a double helix.
  • The replication process has produced a new DNA molecule which is identical to the initial one.
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3. Chemistry of Life

4. Enzymes

  • Enzymes: Globular proteins which act as catalysts of chemical reactions. 
  • Active site: Region on the surface of an enzyme to which substrates bind and which catalyses a chemical reaction involving the substrates.
  • The active site of an enzyme is very specific to its substrates as it has a very precise shape.
  • This results in enzymes being able to catalyse only certain reactions as only a small number of substrates fit in the active site.
  • This is called enzyme-substrate specificity.
  • The enzyme-substrate complex can be compared to a lock and key, where the enzyme is the lock and the substrate is the key.
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3. Chemistry of Life

5. What affects enzymes?

  • Enzyme activity increases with temperature and usually doubles with every 10 degrees rise.
  • This is due to the molecules moving faster and colliding more often together.
  • However, at a certain point the temperature gets too high and the enzymes denature and stop functioning.
  • This is due to the resultant vibrations, destroying its structure by breaking the enzyme's bonds.
  • Enzymes usually have an optimum pH at which they work most efficiently.
  • As the pH diverges from the optimum, enzyme activity decreases.
  • Both acid and alkali environments can denature enzymes.
  • Enzyme activity increases with an increase in substrate concentration as there are more random collisions between the substrate and the active site.
  • However, at some point, all the active sites are taken up and so increasing the substrate concentration will have no more effect on enzyme activity.
  • Denaturation is changing the structure of an enzyme (or other protein) so it can no longer carry out its function.
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3. Chemistry of Life

6. Induced Fit Model and Activation Energy

  • Initially the substrate does not fit perfectly into the active site of the enzyme.
  • When the substrate binds to the active site, its shape changes and  then perfectly fits the substrate.
  • This weakens the bonds in the substrate and reduces the activation energy.
  • This model is a more precise version of "lock and key" because some enzymes bind to different substrates.
  • Reactants of a chemical reaction need to gain energy before they undergo the reaction.
  • This energy is called the activation energy and is needed to break bonds.
  • At a later stage in the reaction energy will be released as new bonds form.
  • The majority of biological reactions are exothermic; the reaction releases energy. 
  • Enzymes make it easier for reactions to occur by decreasing the activation energy.
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3. Chemistry of Life

7. Enzyme Inhibitors

  • Enzyme inhibitors are substances which inhibit enzyme activity. 
  • An example of a competitive inhibitor is malonate.
  • Malonate is structurally similar to the substrate succinate.
  • Succinate is found in the Krebs cycle of aerobic respiration.
  • Malonate can compete with succinate for the active site and prevent succinate from binding. 
  • An example of a non-competitive inhibitor is ATP. '
  • When ATP binds to an allosteric site on the enzyme phosphofructokinase it changes the enzyme conformation and lowers the rate of reaction so that less ATP is produced.
  • Metabolic pathways are made up of many chemical reactions and these reactions are catalysed by enzymes.
  • Often, the product of the last reaction in the pathway inhibits the enzyme that catalyses the first reaction of the pathway.
  • This is called end-product inhibition and it involves non-competitive inhibitors. 
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4. Ecology and Evolution

1. Definitions

Species: a group of organisms that can interbreed and produce fertile offspring.

Habitat: the environment in which a species normally lives or the location of a living organism.

Population: a group of organisms of the same species who live in the same area at the same time.

Community: a group of populations living and interacting with each other in an area.

Ecosystem: a community and its abiotic environment.

Ecology: the study of relationships between living organisms and between organisms and their environment.

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4. Ecology and Evolution

2. Trophic Levels

  • Consumer: an organism that ingests other organic matter that is living or recently killed.
  • Detritivore: an organism that ingests non-living organic matter.
  • Saprotroph: an organism that lives on or in non-living organic matter, secreting digestive enzymes into it and absorbing the products of digestion.
  • Trophic level: the position of an organism in a food chain. 
  • Producers, primary consumers, secondary consumers and tertiary consumers are examples of trophic levels. 
  • Plants or any other photosynthetic organisms are the producers. 
  • Primary consumers are the species that eat the producers. 
  • Secondary consumers are the species that eat the primary consumers and tertiary consumers in turn eat the secondary consumers. 
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4. Ecology and Evolution

4. Nutrient Flow in an Ecosystem

  • Energy is not recycled. 
  • It is constantly being supplied to ecosystems through light energy and then flows through the trophic levels. 
  • As it flows through the trophic levels energy is lost in feces, tissue loss and death. 
  • However the energy is then lost from the ecosystem as the remaining energy is lost through cell respiration in the form of heat. 
  • As a result, energy needs to be constantly supplied to the ecosystems. 
  • Nutrients on the other hand are different as they constantly have to be recycled. 
  • Carbon, nitrogen and phosphorus are all examples of nutrients. T
  • here is only a limited supply of these as they are not resupplied to the ecosystems like energy. 
  • Therefore, they have to be recycled over and over.
  • Saprotrophic bacteria and fungi (decomposers) recycle nutrients.
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4. Ecology and Evolution

5. Greenhouse Gases

  • The earths mean average temperature is regulated by a steady equilibrium  between the energy  from the sun and the energy reflected  back into space.
  • The incoming radiation is short wave ultraviolet and visible radiation. 
  • Some of the radiation will be absorbed by the atmosphere and some of it will be reflected back from the earths surface into space. 
  • The radiation that is reflected back into space is infrared radiation which has a longer wavelength. 
  • Greenhouse gases such as carbon dioxide, methane, and oxides of nitrogen tend to absorb some of the reflected infrared radiation and re-reflect it back. 
  • This is what causes the greenhouse effect and it results in an increase in average mean temperature on earth. 
  • It is a natural phenomenon. 
  • However, in the past century, there has been an increase of the greenhouse effect leading to higher than normal average temperatures. 
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4. Ecology and Evolution

6. Precautionary Principle

  • The precautionary principle says that if the effects of a human-induced change would be catastrophic, those responsible for the change must prove that it will not do harm.
  • This is the reverse of the normal situation, where concerned people about would have to prove that it will do harm in order to stop the changes going ahead.
  • There is strong evidence that shows that greenhouse gases are causing global warming. 
  • If nothing is done, and the greenhouse gases are in fact causing the enhanced green house effect, it will probably be too late and result in catastrophic consequences. 
  • So even though there is no proof for global warming, the strong evidence for its link with an increase in green house gases is something that can't be ignored. 
  • Anyone supporting that we can continue to emit greenhouse gases should have to provide evidence that it will not cause further damages.
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4. Ecology and Evolution

3. Energy Flow in an Ecosystem

  • Light is the initial energy source for almost all communities.
  • Producers receive their energy from light energy (the sun) by photosynthesis. 
  • After this, the energy in organic matter flows from producers to primary consumers to secondary consumers to tertiary consumers. 
  • However, between these trophic levels, energy is always lost. 
  • All of the trophic levels lose energy as heat through cell respiration.
  • Also, not all organic matter is digested and so there is loss of energy through faeces. 
  • This energy then passes on to the detritivores and saprotrophs. 
  • Another energy loss occurs through tissue loss and death which can happen at any trophic level. 
  • This energy would be passed on to detritivores and saprotrophs as they digest these. 
  • Detritivores and saprotrophs in turn lose energy as heat through cell respiration. 
  • Energy transformations are never 100% efficient.
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4. Ecology and Evolution

7. Global Warming and the Arctic

  • The arctic ice cap may disappear as glaciers start to melt and break up into icebergs. 
  • Permafrost will melt during the summer season which will increase the rate of decomposition of trapped organic matters. 
  • This in turn will increase the release of carbon dioxide which will increase the greenhouse effect even further.  
  • Species adapted to temperature conditions will migrate north which will alter food chains and have consequences on the animals in the higher trophic levels. 
  • Marine species in the arctic water may become extinct as these are very sensitive to temperature changes within the sea water.
  • Polar bears may face extinction as they lose their ice habitat and therefore can no longer feed or breed as they normally would.
  • Pests and diseases may become quite common with rises in temperature. 
  • As the ice melts, sea levels will rise and flood low lying areas of land.
  • Extreme weather events such as storms might become common and have disastrous effects on certain species.
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4. Ecology and Evolution

8. Population/Time Graphs

  • Exponential phase: Rapid increase in population growth. 
  • Natality rate exceeds mortality rate because there are abundant resources available and diseases and predators are rare. 
  • Transitional phase: Natality rate begins to fall/Mortality rate begins to rise.
  • Abundance of resources starts to decline, and predators and diseases increases.
  • However, population is still growing as natality rate still exceeds mortality rate. 
  • Plateau: Population size is constant and no more growth is occurring. 
  • Natality rate is equal to mortality rate and caused by scarce resources as well as increases in predators, diseases and parasites.  
  • If natality rate starts to drop then mortality rate will drop too as more resources become available and vice versa.  
  • This keeps the population number relatively stable. 
  • If a population is limited by a shortage of resources then we say that it has reached the carrying capacity of the environment. 
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4. Ecology and Evolution

9. Evolution

  • Evolution is the cumulative change in the heritable characteristics of a population.
  • Fossils, selective breeding and homologous structures have provided scientists with evidence that support the theory of evolution. 
  • Fossils are not identical but had similarities with existing organisms. 
  • This suggested that organisms changed over time. 
  • Selective breeding of domesticated animals also provides evidence as these animals have similar characteristics to the wild ones and can still breed with them. 
  • As selected wild individuals with desirable characteristics were bred, this resulted in a more desirable species from a human point of view. 
  • This suggests that not only have these animals evolved but also that they can evolve rapidly. 
  • Many bones in the limbs are common to a number of species and therefore suggests that these have evolved from one common ancestor.
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4. Ecology and Evolution

10. Natural Selection Examples

  • Antibiotic resistance in bacteria results from the transfer of a gene that gives resistance to a specific antibiotic usually by means of a plasmid to a bacterium. 
  • Some bacteria will then have this gene and become resistant to the specific antibiotic while others will lack the gene and will die if exposed to the antibiotic. 
  • Over time, the non-resistant ones will all die off as doctors vaccinate patients, but the resistant ones will survive. 
  • Eventually, the resistant ones will be the only ones left as a result of natural selection and so a new antibiotic must be created. 
  • This has to be done on a regular basis as the bacteria keep evolving.
  • The Peppered Moth is another example of evolution in response to environmental change. 


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4. Ecology and Evolution

11. Species and Groups

  • Species are a group of organisms with similar characteristics which can interbreed and produce fertile offspring whereas a genus is a group of similar species.
  • Species need an international name and so biologists name them using the binomial system of nomenclature. 
  • Each species is given two names. 
  • The first is the genus name and is given an upper case first letter. 
  • The second is the species name and is given a lower case first letter. 
  • If the name is printed, italics are used. 
  • If on the other hand the name is hand-written, it is underlined.
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Additional Notes: Data Preparation

D    Data Preparation

  • Small sample group; harder to see outliers.
  • Referring to graph 2, ibuprofen took one week to work for immunity response.
  • None of the drugs were completely effective.
  • Ibuprofen was the most effective because it had the biggest effect on the macrophage, lesser nasal congestion and comparatively high anti-rhino antibody production.
  • The placebo had a big effect on anti-rhino antibody production but it didn't produce as many macrophages. Therefore, fewer b-cells were cloned resulting in fewer antibodies being produced.
  • The more monocytes there are in the blood, the better the body will be at fighting off infection because it will result in more macrophages.
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Additional Notes: Polypeptides, Enzymes and Genes

  • Polypeptides, enzymes and genes are linked by translation of DNA.
  • A polypeptide is formed by amino acids liking together through peptide bonds.
  • There are 20 different amino acids so a wide range of polypeptides are possible.
  • The information required for making polypeptides is stored in genes.
  • The information is in a coded form by the use of triplets of bases which form codons.
  • The sequence of bases in a gene codes for the sequence of amino acids in a polypeptide.
  • The information in the genes is decoded during transcription and translation which leads to protein synthesis.
  • During translation, each tRNA activating enzyme recognises a specific tRNA molecule.
  • The tRNA molecule is made up of double stranded sections and loops.
  • At the 3' end of the tRNA there is the nucleotide sequence CCA to which the amino acid attaches to.
  • The different chemical properties and three dimensional structure of each tRNA allows the tRNA-activating enzymes to recognise their specific tRNA
  • Each tRNA enzyme binds a specific amino acid to the tRNA molecule
  • The tRNA-activating enzyme will bind the amino acid to the tRNA with the matching anticodon
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Additional Notes: Overpopulation

  • Populations tend to produce more offspring than the environment can support.
  • One consequence of overpopulation is the resulting struggle for survival as not all offspring will survive.
  • There will be a lack of habitats and food to support the whole population, resulting in some members having to survive in their realized niche as opposed to their fundamental niche.
  • Darwin’s theory of natural selection suggested that the survivors form the new breeding population and produce offspring.
  • This increases the frequency of advantageous alleles and is an example of evolution because there has been a change in heritable characteristics.
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Additional Notes: Properties of Water

  • The properties of water include heat capacity, boiling and freezing points and the cooling effect of evaporation.
  • Water has a large heat capacity which means that a lot of energy is needed to increase its temperature.
  • This is due to the strength of the hydrogen bonds which are not easily broken.
  • Its stable temperature is beneficial for aquatic animals as they use water as a habitat.
  • Water has a high boiling and freezing point because of its strong hydrogen bonds.
  • If water boiled at lower temperatures, many living organisms would not survive.
  • Water gets less dense as it freezes, which means that ice forms at the surface of water first.
  • This is beneficial to organisms as ice will  form at the surface of lakes or seas and insulates the water underneath, maintaining a possible habitat for organisms to live in.
  • Water can evaporate at temperatures below the boiling point in order to create a cooling effect, such as when we sweat to maintain our body temperature.
  • Cohesion is the effect of hydrogen bonds holding the water molecules together
  • Long columns of water can be sucked up from roots to leaves without the columns breaking.
  • The solvent properties of water mean that many different substances can dissolve in it because of its polarity.
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Additional Notes: Lactose-Free Milk and Muscle Con

  • Lactose is the sugar found in milk.
  • It can be broken down by the enzyme lactase into glucose and galactose.
  • People with lactose intolerance are unable to break down lactose because they don’t have the enzyme lactase.
  • These people have to drink lactose-free milk, which can be made by adding the enzyme lactase to the milk.
  • Another method involves immobilizing the enzyme on a porous surface.
  • The milk is then flowed through the surface with the immobilized lactase, and removes the lactose.
  • Muscles contain units called sarcomeres.
  • Sarcomeres contain actin and myosin filaments.
  • Actin fibres are thin and myosin fibres are thick.
  • Action potential causes Ca2+ to be released from endoplasmic reticulum.
  • Sliding of actin and myosin shortens the sarcomere.
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Additional Notes: Questions

Where are hormones absorbed?

In endocrine glands.

Where is CO2 absorbed?

In respiring tissues.

K+ ions in the soil move into a plant mostly by which transport method?

 K+ ions are mostly taken by the root via active transport.

What is the importance of the vascular bundle?

  • Consists of xylem and phloem which are found in the veins of the leaf.
  • The xylem consists of xylem vessels which are long and tubular and transports water into the leaf to replace water lost through transpiration.
  • The phloem is made of living cells with pores in between them. 
  • Phloem transports the products of photosynthesis out of the leaf.


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5. Human Physiology

1. Digestion

  • Food needs to be broken down and reassembled.
  • Large food molecules need to be broken down into smaller ones. 
  • Enzymes break down large food molecules into smaller ones.
  • Speed up the process of digestion by lowering the activation energy for the reaction.
  • Works at body temperature. 
  • Stomach: 
  • Secretes HCL which kills bacteria and pepsin for protein digestion.
  • HCL provides optimum pH for pepsin.
  • Small intestine:
  • Intestinal wall secretes enzymes.
  • Receives enzymes from the pancreas and has villi for absorption of food particles.
  • Large intestine: 
  • Moves material that has not been digested along.
  • Absorbs water and produces faeces.
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5. Human Physiology

2. Structure of Villi

  • Many villi increase the surface area for absorption.
  • Epithelium is only one cell layer thick and so food is quickly absorbed.
  • Microvilli on the villi increase the surface area for absorption further.
  • Protein channels and pumps are present in the microvilli for rapid absorption.
  • The mitochondria in the epithelium provide ATP needed for active transport. 
  • Blood capillaries are very close to the epithelium so diffusion distance is small.
  • The lacteal takes away fats after absorption. 
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5. Human Physiology

3. The Heart

  • The coronary arteries supply heart muscle with oxygen and nutrients.
  • Atria collect blood from veins.
  • Atria contract, atrioventricular valves open.
  • Blood is pumped into ventricles.
  • Ventricle contracts, atrioventricular valves close and semilunar valves open.
  • Blood is pumped into arteries, semilunar valves close. 
  • Heart muscle can contract by itself (myogenic muscle contraction).
  • Pacemaker initiates contractions.
  • One nerve carries messages from the brain to the pacemaker to speed up the beating of the heart.
  • One nerve carries messages from the brain to the pacemaker to slow down the beating of the heart.
  • Adrenaline signals the pacemaker to increase the beating of the heart. 
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5. Human Physiology

4. Arteries, Veins and Capillaries

  • Arteries: 
  • Thick outer layer of longitudinal collagen and elastic fibres prevents leaks and bulges. 
  • Thick wall withstands high pressure.
  • Thick layers of circular elastic fibres and muscle fibres to pump blood. 
  • Narrow lumen to maintain high pressure.
  • Veins: 
  • Thin layer with few circular elastic fibres and muscle fibres as blood does not flow in pulses.
  • Thin walls so that nearby muscles can help push blood towards the heart.
  • Thin outer layer of longitudinal collagen and elastic fibers as pressure is low.
  • Wide lumen to accomodate the slow flowing blood. 
  • Capillaries:
  • Wall is one cell layer thick so distance for diffusion is small. 
  • Pores allow plasma to leak out and form tissue fluid. Phagocytes can also pass through pores.
  • Very narrow lumen so that many can fit in a small space. 
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5. Human Physiology

5. Phagocytes, Antibodies and Lymphocytes

  • Phagocytes are found in the blood and ingest pathogens. 
  • They do so by recognising pathogens and engulfing them by endocytosis. 
  • Enzymes called lysosomes then digest the pathogens. 
  • Phagocytes can ingest pathogens in the blood but also within body tissue by passing through the pores of capillaries and into the tissues.
  • Antibodies are proteins that defend against pathogens by binding to antigens on the surface of these pathogens and stimulating their destruction. 
  • Antigens are foreign substances which stimulate the production of antibodies. 
  • Antibodies usually only bind to one specific antigen.
  • Each lymphocyte makes one type of antibody.
  • Antibodies are found on the surface of the lymphocyte.
  • Pathogen have antigens on their surface which bind to the antibodies.
  • Lymphocyte becomes active and makes clones of itself. 
  • The clones make more of the specific antibody.
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5. Human Physiology

6. The Nervous System

  • The nervous system consists of the central nervous system (CNS) and peripheral nerves.
  • It is composed of cells called neurons which carry rapid electrical impulses.
  • Nerve impulses are conducted from receptors to the CNS by sensory neurons, within the CNS by relay neurons, and from the CNS to effectors by motor neurons.
  • Resting potential: the electrical potential across the plasma membrane of a cell that is not conducting an impulse. 
  • Action potential: the reversal and restoration of the electrical potential across the plasma membrane of a cell, as an electrical impulse passes along it (depolarization and repolarization).
  • Resting potential rises above threshold level and voltage gated sodium channels open.
  • Sodium ions flow into the cell and more sodium channels open.
  • Inside of cell develops a net positive charge resulting in depolarization.
  • Voltage gated potassium channels open and potassium ions flow out of the cell.
  • Cell develops a net negative charge compared to the outside and results in repolarization. 
  • Concentration gradients restored by sodium-potassium pumps and resting potential is restored.
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5. Human Physiology

7. Homeostasis

  • Homeostasis involves maintaining the internal environment between limits.
  • This includes blood pH, carbon dioxide concentration, blood glucose concentration, body temperature and water balance.
  • Negative feedback is used to do so. 
  • Any change from a set point results in an opposite change. 
  • Homeostasis is controlled by the hypothalamus and pituitary gland in the brain.
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6. Plant Science

1. Rate of Transpiration

Light - The rate of transpiration is much greater when light is available as the stomata close in the dark.

Humidity - Water diffuses out of the leaf, down its concentration gradient, from a high concentration gradient inside the leaf to a lower concentration gradient in the air. Humidity is the water vapour in the air, therefore a rise in humidity means a larger concentration of water vapour in the air and results in a decrease in transpiration rate. 

Temperature - As temperature rises, so does the rate of transpiration.  A rise in temperature leads to an increase in the evaporation rate thereby increasing transpiration rate. It also increases the rate of diffusion between air spaces inside the leaf and the air outside. 

Wind - Wind increases the transpiration rate by removing the humidity around the leaf produced by transpiration.

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6. Plant Science

2. Adaptations of Xerophytes

1) Reduced surface area of the plant - reduced leaves such as spines in cacti (modified leaves)

2) Thick waxy cuticle covering the epidermis

3) Reduced numbers of stomata 

4) Water storage tissues in roots, leaves and stems

5) CAM physiology - Stomata open during the evening/night instead of during the day (when the temperature is at its highest) as the transpiration rate will be lower during cooler hours.

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6. Plant Science

3. Key Plant Features

Phloem tissue - Transports sugars and amino acids from sources to sinks. This transport is known as active translocation and requires energy.

Upper epidermis - Consists of a single layer of cells found on the upper surface of the leaf. It is covered by a thick waxy cuticle. The main function is water conservation. It prevents loss of water from the upper surface where light intensity and heat are the greatest. 

Palisade mesophyll - Consists of tightly packed cylindrical cells. This tissue contains many chloroplasts as it is the main photosynthetic tissue. It is found on the upper half of the leaf where the light intensity is the greatest. 

Spongy mesophyll - Made up of loosely packed cells. This tissue is found in the lower half of the leaf and has few chloroplasts. It provides gas exchange and therefore needs to be close to the stomata. 

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6. Plant Science

4. Additional Facts

Guard cells can regulate transpiration by opening and closing stomata.

The plant hormone abscisic acid causes the closing of stomata.

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