A2 Physics-Communication Systems

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A2物理Paper 5提分宝典（一）

你还在因Paper 5第一大题的两页白纸而迷茫吗？

你还在因写了一大段话却没达到得分点上而困扰吗？

参照这份指南，每道类型的题做个两三题，仅需三天，你也可以稳拿13分+！（一般第一题有13分都是死的，另外2分稍微需要思考和运气）

下载链接请戳这里，以下是预览：

A2 Biology-Respiration and Photosynthesis

今天是A2 Biology的前两个章节，呼吸作用和光合作用。它们都是能量的转化过程，有很多可以做对比的地方。因此，我把它俩放到一块儿了。PDF下载链接请戳这里。

以下是预览：

A2 Chemistry-Chemistry of Organic Compounds

Today, we present ‘Chemistry of Organic Compounds’, which collects reactions of organic compounds included in the CAIE 9701 A Level Chemistry syllabus. It does not include aromatic compounds, however, which has been published as a separate issue.

To download the PDF version, please click here.

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This work is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License.

本文作者/Authour(s)：肖博文，石天熠

小技巧：通过不饱和度（DU）方便转换结构式和分子式（一）

CAIE IAL化学有一类题目是结构式（structural formula）和分子式（molecular formula）的转换。

从结构式（一般是skeletal formula）到分子式的转换，虽然是考察最基础的知识，但是经常会因为粗心大意而写错。最常见的原因就是数漏H原子，尤其是遇到较大的，带环的，有多个带H的基团的分子的时候。

从分子式到结构式的转换，属于分析化学的范畴。给你一个分子式，然后告诉你这个分子和特定的几个试剂的反应情况，或是给你图谱分析。

不饱和度（degree of unsaturation）的使用可以有效地帮助解答这两种题。今天介绍第一种，即从结构式到分子式。

我们知道，hexane的molecular formula是C6H14，H的数量=2*C的数量+2.

你可以试着把它的isomers画出来，比如(CH3)2CHCH2CH2CH3, (CH3)3CCH2CH3, (CH3)2CHCH(CH3)2, 你会发现，hexane和它的isomers，全部都是无环，且全部是单键（即饱和）的结构。

那么如果出现了双键和环呢？

还是6个C，这次我们试着加双键和/或环。

我们发现，每加一个双键或环，H的数量就比原来饱和的状态（即hexane，C6H14）少2个。

我们把双键+环的数量叫做不饱和度（Degree of unsaturation，DU）。

于是在碳氢化合物中，H的数量=2*C的数量+2-2*DU。

有一个特殊情况是苯环。我们熟悉的苯环结构是六边形+圆形（Thiele结构），这使得我们无法直观看出苯环的不饱和度。因此，我们看苯环的Kekulé结构：

可以看到，苯环等同于一个有3个双键和1个环的6碳的碳氢化合物。

6碳的饱和状态是6*2+2=14个H，而苯环的DU=3+1=4，H减少2*4=8个，H的实际数量是14-8=6个。

的确，苯的molecular formula就是C6H6.

但是显然CAIE不会出这么简单的题。O，N，和/或卤素原子必然会出现在这类题型。

怎么办呢？

我们再找规律。

我们知道propane的molecular formula是C3H8，它全部是单键。我们给它加个-OH，变成propanol，molecular formula是C3H8O。H的数量仍然是8，所有的共价键也全部是单键。

那我们在O上做一个双键试试？我们得到了Propanoic acid，它的H数量减少了2个。这说明O是不影响H的数量的，但是O上的双键，和C上的双键一样，都会减少2个H的数量。

类似地，我们来研究N和卤素原子对H数量的影响：

我们发现，1个N原子会使H的数量增加1个。

而1个卤素原子会使H的数量减少一个。

于是我们便知道了所有的规律：

饱和状态下，H的数量=C的数量*2+2。

每个双键和环会减少2个H的数量。

O原子不影响H的数量。N原子会增加1个H的数量，卤素原子会减少1个。

根据这些规律，我们可以列出一个从结构式转换分子式的protocol：

第一步，数C的数量。

第二步，计算饱和状态时对应的H的数量，即2*C的数量+2.

第三步，计算不饱和度（DU）和H的减少量。其中不饱和度=双键的数量+环的数量，H的减少量=2*不饱和度。

第四步，寻找N和卤素原子，计算H的变化量。N增加一个H原子，卤素减少一个H原子。

第五步，计算实际H的数量，即第二步算出的数量–第三步算出的数量+第四部算出的变化量。

第六步，写出molecular formula，注意不要漏掉O，N和卤素原子。

实战演练

第一题

答案:

第一步，数C的数量：六元环6个，加上顶部1个和底部3个，共10个。

第二步，计算饱和状态时对应的H的数量：是2*10+2=22个。

第三步，计算不饱和度（DU）和H的减少量：3个双键，1个环，DU=4，H减少2*4=8个。

第四步，寻找N和卤素原子，计算H的变化量：无N和卤素原子，H的变化量=0.

第五步，计算实际H的数量：22-8+0=14.

第六步，写出molecular formula：C10H14O

第二题

答案:

第一步，数C的数量：6+2+1+(6+1+1)=17个

第二步，计算饱和状态时对应的H的数量：是2*17+2=36个。

第三步，计算不饱和度（DU）和H的减少量：5个双键，1个环，DU=6，H减少2*6=12个。

第四步，寻找N和卤素原子，计算H的变化量：无N和卤素原子，H的变化量=0.

第五步，计算实际H的数量：36-12+0=24.

第六步，写出molecular formula：C10H24O4

第三题

答案：

第一步，数C的数量：（注意环之间的C是有重复的，不要直接6*3+5！）一共24个。

第二步，计算饱和状态时对应的H的数量：是2*24+2=50个。

第三步，计算不饱和度（DU）和H的减少量：5个双键，4个环，DU=9，（或者1个苯环（DU=4），3个环，2个双键，DU=4+3+2=9）H减少2*9=18个。

第四步，寻找N和卤素原子，计算H的变化量：有2个N，无卤素原子，H的变化量=+2.

第五步，计算实际H的数量：50-18+2=34.

第六步，写出molecular formula：C24H34N2O3

注意，严谨地来说，DU是π bonds的数量（双键有一个bond，叁键有两个π bonds）+环（仅桥环；螺环化合物不一样）的数量。因为CAIE目前出现的这类题，仅仅包括了双键和桥环化合物，所以我没深入讨论更多的情况。

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© 2018 by CAIEhelp，a division of Brilas Academy （萤火）；Author of this work: 石天熠

A2 Chemistry-Chemistry of Aromatic Compounds

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本文作者/Authour(s)：石天熠，肖博文

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Chemistry of Organic Compounds

Today, we present ‘Chemistry of Organic Compounds’, which collects reactions of organic compounds included by the CAIE 9701 A Level Chemistry syllabus. It do not include aromatic compounds, however, which has been published as a separate issue.

To download the PDF version, please click here.

Below are pictures for preview.

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This work is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License.

本文作者/Authour(s)：张家栋，石天熠

A2 Physics-Gravitational Field and Circular Motion

 

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本文作者/Authour(s)：张家栋，石天熠

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公式

首先我们来看一下公式。因为LaTeX格式与Word内置的公式格式不兼容，所以用图片的形式发出来。

Definition题

1.radian: the angle subtended at the centre of a circle(1) by an arc of length equal to radius of the circle(1).

2.angular velocity: the angle swept out by the radius per second(1).

3.centripetal force:the net force acting on an object moving in a circle(1), it is always directed towards the centre of circle(1).

4.gravitational field: region of space where mass experience a force(1).

5.point mass:the size of the object is extremely small comparing with their separation.

6.Newton’s law of gravitation: two point masses attract each other with a force that is directly proportional to the product of their masses(1) and inversely proportional to the square of their separation(1).

7.gravitational field strength:the force per unit mass

8.gravitational potential:the work done in bringing unit mass from infinity to that point.

简答题/计算题 

1.Similarity and difference between electric and gravitational field.

Similarity: radial field, force inversely proportional to square of separation(1)

Differences: gravitational field always attractive, electric field could be attractive or repulsive(1).

2. What is meant by field of force: region in space where a particle experiences a force.

3. Why is gravitational potential negative?

Potential at infinity is defined as zero(1), gravitational force is attractive so work is done by the object as object moves(1).

4. A calculation problem associated with a graph, what is the min speed to reach another star?

In this situation, the change in potential should be potential difference to the highest point, rather than the potential difference between start and final point.

5. Start from the moon, why the min speed to reach earth is different from the min speed to reach infinity?

At earth, the potential due to the moon is not zero, it is also attracted by the earth(1), so the min escape speed would be lower(1).

AS Chemistry-Periodicity

Explaining Ionisation Energy Variations

General Idea

• Factors that affect ionisation energy: nuclear charge; shielding/proximity of electrons to the nucleus; the energy of the electron for removal (which is affected by other factors)
• The greater the force between the nucleus and the electrons, the higher the ionisation energy. Less shielding, greater nuclear charge, electrons being at low energy all contribute to greater force and hence ionisation energy.

General Trend Across a Period

• First ionisation energy increase
• Nuclear charge increase/electrons are subject to increasing nuclear charge
• Electrons are added to the same shell/same shielding
• Electrons are closer to the nucleus/atoms get smaller

Sodium and Magnesium

• Magnesium has a higher first ionisation energy
• Magnesium has a greater nuclear charge
• In both atoms, the 3s electrons are in the same orbital/same energy level/same shell

Magnesium and Aluminium

• Magnesium has a higher first ionisation energy
• In Mg the outermost electrons are in 3s, while in Al the outermost electron is in 3p
• 3p electron is at higher energy OR is further away/is more shielded from nucleus

Phosphorus and Sulphur

• Phosphorus has a higher first ionisation energy
• For S one 3p orbital has paired electrons and for P 3p sub-shell is singly filled
• Paired electrons repel (spin-spin repulsion)

Decreased First Ionisation Energy Down a Group

• Increasing distance of outer electrons/valence shell from nucleus
• Increased shielding from inner shells
• Reduced attraction

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本文作者：石天熠

A2 Biology-Homeostasis

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All contents are concordant with mark schemes of CAIE 9700 Int’ A Level Biology past paper 4s from year 2014 to 2017 unless being noted with an asterisk (*).

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本文作者/Authour(s)：石天熠，肖博文

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Homeostasis in Humans in General

Define negative feedback

A process in which a change in some parameter, such as blood glucose level, brings about processes which move its level back towards normal again.

Outline how a negative feedback mechanism works

• A initial stimulus, which is the change in a parameter away from the set-point, is detected by receptors.

• In response, hormones are released and they reach their target organs, or effectors.

• Corrective actions are taken by the effectors, allowing restoration of the ‘set point’.

Explain the principles of homeostasis in humans

Homeostasis is the maintenance of constant internal environment irrespective of changes in external environment, using negative feedback mechanisms. This involves the initial stimulus, which is the change in a parameter detected by receptors, and the action taken by the effectors, allowing restoration of the ‘set point’. There is fluctuation around the norm. Examples of homeostasis include thermoregulation, osmoregulation, and blood glucose regulation.

Examples and importance

Homeostasis is the maintenance of constant internal environment, for example several parameters of blood (e.g. blood glucose level, osmolarity and blood pressure), causing them to fluctuate within narrow ranges around the set point.

Homeostasis of body temperature, water potential (osmolarity) and blood glucose level is critical. Low body temperature leads to slowed metabolism and enzyme activity, while high temperature causes enzymes to be denatured; low water potential causes cells to shrink while high water potential causes cells to burst; low blood glucose level reduces cellular respiration while high blood glucose level leads to lowered blood water potential, which further leads to shrinkage of cells.

Human Kidney Anatomy and Physiology

Structure of A Kidney

• A kidney consists of the cortex on the outer side, the medulla and a pelvis. It is connected to the renal artery and the renal vein.
• Nephrons are the functional units of a kidney. A nephron consists of the renal capsule, the proximal convoluted tubule, the loop of Henle, the distal convoluted tubule and the collecting duct.
• The glomerulus is a network of capillaries surrounded by the renal capsule. The afferent arteriole enters it, and the efferent arteriole transports blood away from it.

Water Potential

Tendency of water molecules to move from one region to another

Ultrafiltration

The diameter of afferent arterioles is greater than that of the efferent arterioles, so this builds up a high blood pressure in the glomerulus and molecules tend to move from the capillary into the Bowman’s capsule.

Molecules first pass through holes in the endothelium, then the basement membrane, and finally gaps between podocytes and enter Bowman’s capsule. The basement membrane is selectively permeable: only molecules with molecular weight less than 69,000 can pass through it.

Features of Epithelial Cells of the Proximal Convoluted Tubule

• Microvilli and folded basal membrane: increase surface area for absorption of Na+/glucose/amino acids
• Many mitochondria: provide energy/ATP for Na+/K+ pumps
• Tight junction between cells: hold adjacent cells together; fluid cannot pass between cells/substance must pass through cells
• Many cotransporter proteins: for glucose reabsorption
• Many aquaporins: for water reabsorption

Mechanism of Reabsorption in the Proximal Convoluted Tubule

• Active transport using ATP  by Na+/K+ pumps pump Na+ ions out of the cells into the blood. This sets up a concentration gradient of Na+. 
• Facilitated diffusion using co-transporter proteins allows the transport of glucose, amino acids and ions from the lumen to the PCT cells against their concentration gradients, along with Na+ ions down its concentration gradient. In addition, osmosis occurs as water moves down a water potential gradient into the PCT cells.

Reabsorption in loop of Henle*

1. Sodium and chloride ions are actively transported out of the cell in the upper (thicker) half of the ascending limb
2. This lowers the water potential of the tissue fluid
3. Water moves down the water potential gradient out of the loop of Henle in the descending loop
4. The loss of water increases the concentration of ions in the loop
5. Na+ and Cl+ ions diffuse out of the loop in the lower (thinner) part of the ascending limb down their concentration gradients
6. This mechanism, known as a counter-current multiplier, enables the maximum concentration of solutes to be built up both inside and outside the tube at the bottom of the loop

Why an increase in the quantity of protein in the diet leads to an increase in the concentration of urea in the urine

1. Proteins are hydrolysed into amino acids in gut

2. Excess amino acids cannot be stored

3. Amino acids are deaminated in liver to produce urea

4. Urea in blood is filtered into nephrons by ultrafiltration

Production of ADH

The hypothalamus has neurosecretory cells, which produce ADH, and osmoreceptors, which detects changes in water potential of the blood.

Osmoreceptors shrink when water potential is low, and this causes ADH to be released from the posterior pituitary.

Role of ADH

• ADH decrease the volume of urine produced by affecting the collecting duct.
• Specifically, It increases the permeability to water of the collecting duct so more water move out of the lumen down the water potential gradient.
• It does so according to a series of molecular mechanisms:
  • First, ADH binds to corresponding receptors on cell surface membranes
  • Binding to receptors activates the second messenger system (using cAMP) which, in turn, activates a series of enzyme controlled reactions by means of phosphorylation. This amplifies the original signal.
  • Finally, phosphorylated vesicles, which contains aquaporins,  move to and fuse with the cell surface membrane, so that additional aquaporins are added to the cell surface membrane, making the membrane more permeable.

Process of deamination of amino acids and formation of urea*

Ammonium ions/ammonia is produced in the liver from excess amino acids, which is toxic to the body if they are allowed to accumulate. Ammonium ions/ammonia combine with carbon dioxide to form urea in urea (ornithine) cycle.

Hormonal Control of Female Reproductive System

Describe the role of hormones in the maintenance of the human menstrual cycle

• FSH and LH, which are released by the anterior pituitary gland, stimulates the development of Graafian follicle.
• The follicle produces oestrogen, whose concentration rises in the first 12 days of menstrual cycle, causing endometrium to thicken.
• Around day 14, the surge in LH and FSH concentration stimulates ovulation and LH stimulate development of corpus luteum.
• Corpus luteum produces progesterone, which causes further development of endometrium.
• If there is no fertilisation, the combined high concentration of progesterone and oestrogen inhibits release of LH and FSH, which is an example of negative feedback.
• Corpus luteum degenerates and concentration of progesterone falls, so endometrium break down and menstruation occurs.

Outline the role of progesterone in the human menstrual cycle

1. It is mostly secreted during the second half of the cycle (i.e. from day 14 onwards)

2. It maintains the thickness of the endometrium in preparation for implantation

4. inhibits GnRH production and hence development of new follicle

Describe the roles of LH in the menstrual cycle

1. It stimulates follicle to secrete oestrogen

3. Surge in LH secretion stimulates ovulation

5. It stimulates development of corpus luteum, which secretes progesterone

Role of oestrogen

1. Thickening of endometrium

2. Development of capillaries in the endometrium

3. Inhibition of FSH production

Pills

• Pills contain synthetic hormones that are equilavent to progesterone and oestrogen. Synthetic hormones are broken down more slowly so their activity last longer.
• Progesterone and oestrogen concontrations reamin high, and this reduce the secretion of FSH and LH from the anterior pituitary, which is a form of negative feedback. The lack of FSH prevents maturation of follicle, and the lack of LH prevents ovulation.
• Furthermore, more cervical mucus would be produced, which is hostile to the sperm, preventing implantation.
• Pills are taken daily for 21 days, then there is a 7-day break that allows menstruation before another 28-day cycle starts.

Blood Glucose Level

Mechanism of Action of Adrenaline

• Adrenaline raises blood glucose level using a series of molecular mechanisms
• Adrenaline first binds to its receptors in cell surface membrane of target cells, for example liver cells
• The binding causes the receptor to change its conformation, which in turn activates G protein.
• G protein activates adenylyl cyclase, which produces cAMP.
• cAMP acts as a second messenger and activates kinases.
• These kinases activates other kinases by means of phosphorylation, and this forms an enzyme cascade, amplifying the original signal.
• Finally, a large amount of glycogen phosphorylase is activated, which breaks down glycogen into glucose, a process known as glycogenolysis.
• Glucose diffuses out of the cells into blood down the concentration gradient and via channel proteins, increasing blood glucose concentration

Response to High Blood Glucose Level

• High blood glucose level triggers a series of physiological reactions to lower blood glucose level back to the normal level, which is an example of negative feedback.
• High blood glucose level is detected by β cells in islets of Langerhans in pancreas.
• In response, these cells produce and secrete more insulin into blood, while production of glucagon in α cells is inhibited.
• Insulin increases glucose absorption in liver by phosphorylating glucose, and increases permeability to glucose in muscle cells by adding GLUT4 transporter proteins to their cell surface membranes.
• Insulin increases rate of respiration of glucose and conversion of glucose into glycogen.

Dip stick

• A dip stick is a pad containing immobilised glucose oxidase and peroxidase and chromogen, which is used to measure glucose concentration.
• A dip stick is lowered into the sample (blood, urine, etc.) and glucose oxidase oxidises glucose into gluconic acid (gluconolactone), producing hydrogen peroxide.
• hydrogen peroxide react with chromogen, a colourless compound (catalysed by peroxidase) to produce a brown colour. The darkness of the brown colour is proportional to the concentration of glucose.

Glucose Biosensor

• The biosensor contains a pad with immobilised glucose oxidase and peroxidase.
• The pad is lowered into the sample (blood, urine, etc.) and glucose oxidase oxidises glucose into gluconic acid (gluconolactone), producing hydrogen peroxide.
• Hydrogen peroxide is broken down by peroxidase, producing oxygen, which generates an electric current that can be detected by the electrodes to give a numerical value of glucose concentration.

Symptoms of Diabetes mellitus

• Feeling thirst
  • Patients have high blood glucose concentration
  • This causes decrease in water potential of blood
  • Which is detected by osmoreceptors in the hypothalamus
• Loss of body mass
  • Insulin function is compromised
  • Less glucose is converted to fat and glycogen, and less glucose is taken up by cells for respiration
  • More fats and proteins are respired

Homeostasis in Plants

Opening and Closing of Stomata

• Stomata open in response to:
  • Increasing light intensity
    • To gain CO2 for ph0tosynthesis
    • To allow oxygen out
    • Allows transpiration to occur, bringing water for photosynthesis
• Stomata close in response to:
  • Darkness
    • CO2 not required as no photosynthesis
  • High temperature/low humidity/water stress
    • Prevent water loss by transpiration
    • Maintain cell turgidity/prevent wilting

Mechanism of Stomata Opening

• Stomata opening are dependent on the turgidity of guard cells, which involves a series of molecular mechanisms.
• There are proton pumps in cell surface membranes of guard cells that pump H+ out of the cells, lowering H+ concentration inside the cell. Therefore, the voltage inside the cell is more negative than the outside, building up a electrochemical gradient.
• K+ channels open and K+ move into the cell down the electrochemical gradient by facilitated diffusion. This is followed by entry of Cl– ions.
• Entry of K+ and Cl– ions lowers the water potential inside the cell, so water move into the cell via aquaporins by osmosis, down the water potential gradient.
• Volume of guard cells increases, and they become turgid. Guard cells have uneven thickness of cell walls so that turgidity would cause them to bent in a way that opens the stomata.

Role of Abscisic Acid (ABA)

Plants secrete abscisic acid at times of water stress. It closes the stomata to reduce water loss.

Abscisic acid binds to receptors on cell surface membrane of guard cells, inhibiting proton pumps. H+ concentration inside the cell increases.

Abscisic acid also stimulates the influx of Ca2+ ions, which act as a second messenger to activate channel protein to open, encouraging efflux of K+ ions.

Water potential of the cells increases, so water leaves cells by osmosis. Volume of the guard cells decrease and cells become flaccid.

Thermoregulation

Principles of Thermoregulation

• A change in blood temperature is detected by the hypothalamus.
• The brain sends impulses to effector, which carries out response (e.g. shivering; vasoconstriction/vasodilation),
• Actions taken by the effectors return blood temperature to normal.
• This is an example of negative feedback mechanism

Process of Thermoregulation

When temperature is low

• Vasoconstriction
  • Arterioles in skin get narrower
  • Less blood flow through surface capillaries
  • So there is less heat loss to surroundings
• Shivering
  • Involuntary muscle contraction
  • Releases heat
• Increasing secretion of adrenaline
  • Increases metabolic rate (i.e. rate of respiration)
  • More heat released

When temperature is high

• Vasodilation
  • Arterioles in skin gets wider
  • More blood flow through surface capillaries
  • So there is more heat loss to surroundings
• Sweating
  • More sweat produced
  • Sweat evaporates
  • Using heat energy (latent heat of vaporisation)