3.2 understand that fertilisation involves the fusion of a male and female gamete to produce a zygote that undergoes cell division and developed into an embryo

When fertilisation occurs, a male gamete fuses with a female gamete to form a zygote (aka a fertilised egg). This zygote will then undergo cell division by mitosis (reproducing genetically identical cells), this develops into an embryo (effectively a ball of cells).

The embryo contains chromosomes from mum and dad and therefore receives characteristics from both parents. The process of fertilisation is random resulting in genetic variation.

NOTE: Gametes are haploid cells meaning they contain half the number of chromosomes (23) instead of the full 46 like in every other cell. This means that the zygote ends up with a full set of chromosomes (46) as the male gamete (of 23 chromosomes) will fuse with the female gamete (of 23 chromosomes) when fertilisation occurs.

3.1 understand the difference between sexual and asexual reproduction

Asexual reproduction (e.g. flowering plants) involves one parent and mitosis (cell division where the cell reproduces itself by splitting to form two genetically identical cells with identical sets of chromosomes), sexual reproduction (e.g. humans) involves two parents and meiosis (cell division producing 4 haploid cells that are not identical).

Definitions for exam...

Asexual reproduction involves only one parent. The offspring have identical genes to the parent, so there is no variation between parent and offspring.

Sexual reproduction involves the fusion of male and female gametes. Because there are two parents, the offspring contain a mixture of their parent's genes.

definition source: CGP

2.90 understand the sources, roles and effects of the following hormones: ADH, adrenaline, insulin, testosterone, progesterone and oestrogen

ADH
Source: Pituitary gland
Role(s): ADH controls water content
Effect(s): Increases/decreases the permeability of the kidney tubules (in the nephrons) to allow more/less water to be reabsorbed into the bloodstream

Adrenaline
Source: Adrenal gland
Role(s): Hypes your body up, makes you ready for action
Effect(s): Increases your heart rate, increasing your blood flow to muscles (as faster heart rate means blood is pumped quicker) and increases your blood sugar levels

Insulin
Source: Pancreas
Role(s): helps to control blood sugar levels
Effect(s): Stimulates the liver to turn glucose into glycogen for storage

Testosterone
Source: Testes
Role(s): Testosterone is the main male sex hormone
Effect(s): Promotes male secondary sexual characteristics (post 3.12)

Progesterone
Source: Ovaries
Role(s): helps in pregnancy
Effect(s): maintains the lining of the uterus (so the fertilised egg can implant itself there)

Oestrogen
Source: Ovaries
Role(s): oestrogen is the main female sex hormone
Effect(s): Controls the menstrual cycle, promotes female secondary sexual characteristics (post 3.12)

2.89 describe the role of the skin in temperature regulation, with reference to sweating, vasoconstriction and vasodilation

When you are cold (and need to warm up)
- Blood vessels near the surface of the skin constrict (tighten) so that less heat can be transferred from the blood to the surroundings (this is vasoconstriction)
- You shiver to generate heat in your muscles (exercise will do the same thing, you just heat up)
- Very little sweat is produced (as sweat can cool you down)
- Hairs (on your arms, for example) stand on end (perpendicular to your skin) to trap a layer of air which will act as an insulator to keep you warm

When you are too hot (and need to cool down)
- Blood vessels near the surface of the skin dilate (widen) so that more blood can flow near the surface, allowing for more heat to radiate into the surroundings (this is vasodilation)
- You produce lots of sweat as, when it evaporates, it transfers heat from your skin to your surroundings (this cools you down)
- Your hairs lie flat to avoid any insulating layers of air being trapped

2.88 understand the function of the eye in focusing nearing distant objects, and in responding to changes in light intensity

By changing the shape of the lens the eye can focus on things near/far (this is known as accommodation). By changing the shape of the pupil the eye can adjust to different lighting intensities (e.g. see in bright light as well dim light).

Near objects
The ciliary muscle will contract (slackening the suspensory ligaments), this makes the lens appear more 'fat' (basically, its a little more curved).

Distant objects
The ciliary muscles relax and the suspensory ligaments contract, this makes the lens flatter

Light intensity
Bright light - triggers a reflex that makes the circular muscles contract causing radial muscles to relax, making the pupil smaller (allowing less light to enter so you don't get blinded). Light receptors detect bright light and send a message along a sensory neurone to an unconscious part of the brain, the message is passed onto a relay neurone that relays the message to a motor neurone which tells the circular muscle (the effector) in the iris to contract.

Dim light - this triggers a reflex that makes the radial muscles contract and circular muscles relax, this makes the pupil bigger allowing more light to enter. This works as light receptors detect a dim light and send a message to the sensory neurone to an unconscious part of the brain, this message is then passed to a relay neurone which relays it to a motor neurone which tells the redial muscles (the effector) to contract.

image source: midlandstech.edu

2.87 describe the structure and function of the eye as a receptor

The eye detects when light is strong/little and enlarges/shrinks the pupil when necessary (shrinking in bright light and enlarging in dim light)

Conjunctiva - this lubricates and protects the surface of the eye
Cornea - bends (refracts) light into the eye (it is transparent and has no blood vessels supplying it with oxygen so oxygen diffuses in from the outer surface)
Iris - this controls the diameter of the pupil, therefore it controls how much light enters the eye (big pupil = lots of light, little pupil = little light)
Lens - focuses the light onto the retina
Retina - the light-sensitive part of the eye. It is covered with  light receptors known as rods and cones (rods are more sensitive in dim light but cannot see colour, cones are sensitive to colours but are not very good in dim light, cones are found all over the retina but mostly at the fovea)
Optic nerve - this carries impulses to the brain

image credit: BBC

notes credit: CGP

2.86 describe the structure and functioning of a simple reflex arc illustrated by the withdrawal of a finger from a hot object.

- the pain receptor is stimulated by the hot pan - electrical impulses travel along the sensory neurone to the central nervous system - in the central nervous system the electrical impulses are passed onto a relay neurone - relay neurones pass the message to a motor neurone (via a synapse) - the electrical impulse will then travel from the motor neurone to the muscle - the muscle contracts and your hand moves away from the hot pan NOTE: the gap inbetween neurones is known as the 'synapse' and messages are passed across these synapses using chemicals Photo credit: Xth notes

2.85 understand that stimulation of receptors in the sense organs send electrical impulses along nerves into and out of the central nervous system, resulting in rapid responses

When receptor in a sense organ detect a stimulus, electrical impulses are sent along sensory neurones (nerves) to the central nervous system (either to your spinal chord or an unconscious part of the brain). In the central nervous system, the sensory neurone passes the message onto the relay neurone - the gap between is called a synapse (the message is passed across the synapse with the help of chemicals). Next, relay neurones pass the message on to a motor neurone. The message will pass from a motor neurone to the effector. Lastly, the effector reacts (for example, a muscle will contract). This happens super duper quickly as they are automatic responses as you don't think about them (as they do not pass through the conscious part of your brain).

In other words, this is the reflex arc...

2.84 understand that the central nervous system consists of the brain and spinal chord and is linked to sense organs by nerves

The nervous system is made of all of the neurones in your body (including sensory, relay and motor neurones). However, the central nervous system consists of only the brain and the spinal chord. The central nervous system (CNS) is linked to sense organs (eyes, skin, nose, tongue, ears) by sensory neurones (these are nerves).

2.81 describe the geotropic responses of roots and stems

Okay so i've included a little about what auxins are which I have put in red but if you don't need that just skip past the red...

Auxins are the plant hormones which control growth at the tips of roots and shoots of plants - incase you were wondering, they move to the tips of the shoots and roots in a solution (they are dissolved in water). They work by diffusing backwards to stimulate the cell elongation process which occurs in the cells just behind the tip of the shoot/root. They are involved in the growth of plants in response to light (phototropism) and gravity (geotropism). NOTE: in shoots, extra auxins promote growth but in roots extra auxins slows down growth.

Negative geotropism - when a shoot is growing sideways, gravity will produce an uneven distribution of auxins (more of the bottom than on the top). This means that the underside of the shoot will grow much faster than the side with light (as it has auxins which stimulate growth). This bends the shoot upwards.

Postive geotropism - likewise to a stem, a root growing sidewise will have more auxins on its lower side as gravity will produce an unequal distribution of auxins. However, in roots, extra auxins will slow down growth. This means that the cells on the top side of the root will elongate faster, meaning the root will bend downward.

2.82 describe positive phototropism of stems

Okay so i've included a little about what auxins are which I have put in red (this is the same as point 2.81, so you can skip it if you have read that post). If you are an auxin wizz, just skip past the red...

Auxins are the plant hormones which control growth at the tips of roots and shoots of plants - they move to the tips of the shoots and roots in a solution (they are dissolved in water). They work by diffusing backwards to stimulate the cell elongation process which occurs in the cells just behind the tip of the shoot/root. They are involved in the growth of plants in response to light (phototropism) and gravity (geotropism).

When the tip of a shoot is exposed to light, the light dissolves the auxins. This means that there are more auxins on the shaded side of the tip (as no sunlight has shone there so the auxins have not been dissolved). This makes the cells grow faster on the shadey side, meaning the shoot will grow towards the light.

2.80 understand that plants respond to stimuli

Firstly, if you don't know what a stimulus is, it may be an idea to have a read of post 2.79 and then come back to here.

Plants respond to many stimuli, here are quite a few, it may be an idea to pick like 3 to remember for the exam as there is no need to learn all of them...

- Climbing plants have a sense of touch, so they can find things to climb and reach sunlight
- Plants can sense gravity, so their roots and shoots grow in the right direction (if they didn't sense gravity it would be chaos, some roots would grow up and some shoots would grow down etc)
- Plants sense the direction of light and grow towards it to maximise light absorption for photosynthesis

Specific examples...
- If cattle overgraze (eat LOADS of the field) they start to eat lots of white clover (as there is no grass left), the white clover will respond by producing toxins to avoid being eaten
- At low temperatures carrots produce antifreeze proteins which bind to ice crystals and lower the temperature that water freezes at, stopping more ice crystals from growing (ngl, this ones pretty cool)

Example credit: CGP

2.79 understand that a coordinated response requires a stimulus, a receptor and an effector

A stimulus is a thing or event that provokes a specific reaction. For example, if you touch a hot object and you react (usually by reflex), the hot object is a stimulus.

Receptors detect stimuli and effectors produce a response.

Receptors are a group of cells that detect an external stimuli, they are situated in the sense organs - the sense organs are the eyes (sight), the nose (smell), the ears (sound), the tongue (taste) and the skin (touch).

Effectors, for example muscle cells and cells found in glands, are cells that coordinate a response to the stimuli. They can react in different ways depending on the stimuli, for example, glands can secrete hormones whereas muscles can contract. They are only found in muscles and glands.

NOTE: receptors and effectors communicate, this is via the hormonal system or nervous system (sometimes it can be both)

2.78 understand that homeostasis is the maintenance of a constant internal environment and that body water content and body temperature are both examples of homeostasis

For your cells to function properly, the conditions in your body need to be kept constant. Homeostasis involves balancing what comes in with what comes out (basically, if you drink 500ml of water, you will wee 500ml of water). Water content and body temperature both need to be kept constant, therefore they are examples of homeostasis.

Definition for exams: Homeostasis is the maintenance of a constant internal environment

2.77 understand that organisms are able to respond to changes in their environment

Plants and animals both increase their chance of survival by responding to changes in their surroundings. For example, plants grow towards the sun to maximise the amount of light they absorb, meaning more photosynthesis. One stimuli animals respond to is a change in temperature. For example, if it is hot you will sweat, if it is cold your hairs stand up (this is to help regulate the body temperature to 37ºC).

2.76 understand that urine contains water, urea and salts

In the beginning of the ultrafiltration/reabsorption process water, urea, salts and glucose are 'squeezed' our of the blood due to high pressure. However, during reabsorption all of the glucose (and a little bit of water and salts) is reabsorbed. Everything that remains in the glomerulus filtrate (water, urea and salts) is combined to form urine.

2.75 describe the role of ADH in regulating the water content of the blood

Osmoregulation determines how much water is reabsorbed into the blood at nephrons. The amount of water reabsorbed is controlled by ADH (anti-diuretic hormone). ADH makes the nephrons more/less permeable to allow more/less water to be reabsorbed back into the blood. This occurs like this...

- The brain is constantly monitoring the level of water in the blood
- The brain instructs the pituitary gland to release ADH into the blood (according to how much water needs to be reabsorbed)

NOTE: the process of osmoregulation is controlled by a mechanism called 'negative feedback'. This basically means if the water gets too high/low a mechanism will be triggered that brings the level down/up (back to normal)

2.74 understand that selective reabsorption of glucose occurs at the proximal convoluted tubule

ONLY glucose is absorbed at the first (proximal) convoluted tubule, this makes for selective reabsorption as only a certain substance(s) (in this case, glucose) can be absorbed, not everything is absorbed.

2.73 understand that water is reabsorbed into the blood from the collecting duct

If you don't know what happens regarding the water leaving the blood, it may be a good idea to go to one of these posts... 2.69 2.71 2.72. If you don't know what a nephron is or how it works, i would recommend checking out 2.71.

The glomerulus filtrate will flow along the nephron (next to the capillary). All the useful stuff (such as the glucose, some of the salt, and some of the water (depending on osmoregulation) will be reabsorbed (by active transport) back into the blood. The glucose is reabsorbed at the first (proximal) collecting tubule, the water is reabsorbed from the collecting duct, sufficient salt is kind of just absorbed throughout.

2.72 describe ultrafiltration in the Bowman's capsule and the composition of the glomerular filtrate

I have kind of answered this in 2.69 and 2.71 but here it is again just for bants

The renal artery will take oxygenated blood from the heart into the glomerulus (this is part of a nephron, in a kidney, go to 2.71 if you are unsure what a nephron is/how it work). This blood will build up in pressure as it is 'squashed' up. Due to the high pressure, all small molecules (such as glucose, water, urea and salts) will pass through a membrane inbetween the glomerulus and the Bowman's capsule, (all the bigger molecules will stay in the blood). The filtered liquid (the small molecules, e.g. water, glucose, urea and salts) is known as the glomerular filtrate.

2.71 describe the structure of a nephron, to include Bowman's capsule and glomerulus, convoluted tubules, loop of Henlé and collecting duct

Each of your two kidneys contains thousands of nephrons to help with osmoregulation and removal of urea (basically, they filter the blood). This is what they look like...

Inbetween the glomerulus and bowman capsule is a membrane that allows small molecules (such as water, glucose, urea and salts) to travel through, this is known as the filtrate and travels along through the orangey tube. As it travels, useful substances (such as glucose, some salt and a certain amount of water (depending on osmoregulation)) will be reabsorbed into the bloodstream (the red tube). All the unwanted stuff (like

urea, water and salt) is collected at the collecting duct. Here it all combines together to make urine which travels down the ureter, into the bladder where it is stored.

image credit: BBC

2.70 describe the structure of the urinary system, including the kidneys, ureters, bladder and urethra

Lets start with a diagram...

- The aorta takes oxygenated blood to the kidney (as the renal artery) and the vena cava takes deoxygenated blood away from the kidney (as the renal vein)

- The kidney is where the urea is turned into urine, there the blood is filtered (and the waste parts are taken out) and where osmoregulation occurs

- The urine travels down the ureter into the bladder

- Urine is stored in the bladder

- The urine travels down the urethra and out of the body

2.69 understand how the kidney carries out its roles of excretion and osmoregulation

Excretion
The kidneys remove urea from the blood. This is done in the nephron....

- Blood (from the renal artery) flows through the glomerulus, increasing pressure as it 'bunches up'.
- This high pressure causes small molecules (water, urea, glucose and salts) too squeeze through the membrane in-between the blood vessels (in the glomerulus) and the Bowman's capsule.
- This membrane acts as a filter so the bigger molecules (proteins and blood cells) do not leave the blood.
- As the filtrate (the small molecules that have been filtered out) flows through the nephron, the useful stuff (such as glucose) is selectively reabsorbed back into the bloodstream.
- The remaining substances are waste as they are of no use to the body, these substances include water, salts and urea.
- These remaining substances combine to form urine, which flows out of the nephron, through the ureter and down to the bladder where it is stored.

Osmoregulation
Osmoregulation is the adjustment of water content of the blood (it is a form of homeostasis). Basically, the kidney makes sure the blood has the right level of water in it. To do this, it has to constantly balance the amount of water that enters and leaves the blood. This is done by adjusting the amount of water the blood reabsorbed when flowing through the nephron. If the person is dehydrates (or very sweaty) the kidneys will reabsorb lots of water (this means less water is lost in urine). Similarly, if someone is very hydrated, the kidney can lower the level of water reabsorbed into the bloodstream, this means the person will lose more water through urine (this is why you wee loads if you drink loads). This is controlled by ADH (produced in the pituitary gland) which changes the permeability of the kidney tubules accordingly (to allow more/less water to be reabsorbed).

There is a little more on how osmoregulation takes place in post 2.75

2.68 recall that the lungs, kidneys and skin are organs of excretion

Excretion (removal of waste products) is carried out by the skin, the lungs and the kidneys.

The skin excretes waste salt etc as sweat (perspiration), the lungs excrete carbon dioxide, the kidneys excrete urea (urea is produces from excess amino acid in the liver).

2.67 understand the origin of carbon dioxide and oxygen as waste products of metabolism and their loss from the stomata of a leaf

During photosynthesis, plants use light energy (from the sun) to combine carbon dioxide and water. This produces glucose and oxygen. The plant can use glucose (in respiration) but cannot use oxygen, therefore, oxygen is a waste product.

During respiration, plants use oxygen and produce carbon dioxide (this time, the oxygen is useful and carbon dioxide is a waste product.

Gases diffuse in and out of a leaf via the stomata when they are open (during daytime/light/when photosynthesis is occurring)...

2.66 understand the general structure of the circulation system to include the blood vessels to and from the heart, the lungs, the liver and the kidneys

The circulation system is responsible for getting blood to cells in order that useful substances (e.g. glucose and oxygen) can be delivered and waste (such as carbon dioxide) can be removed. It's probably best to remember where everything goes then you will just have to recall the diagram in your head should you need to know where anything goes etc...

- The pulmonary vein takes oxygenated blood from the lungs to the left atrium (which pumps it into the left ventricle)

- The oxygenated blood then travels through the aorta from the left atrium (heart) into the brain, and also around the body, including the liver, gut and kidney

- The aorta ‘transforms’ into the hepatic artery when transporting blood to the liver, and the renal artery when transporting blood into the kidneys. (NOTE: when travelling to the gut it 'transforms' into the mesenteric artery, but we dont need to know this at iGCSE level)

- The hepatic portal vein will take blood from the gut to the liver.

- On going back into the heart, the renal vein takes the deoxygenated blood from the kidney, the hepatic vein takes the deoxygenated blood from the liver, these both ‘transform’ into the vena cava. (the vena cava also take deoxygenated blood from the brain)

- The vena cava takes deoxygenated blood into the right atrium (which pumps it into the right ventricle)

- The pulmonary artery takes deoxygenated blood from the right ventricle into the lungs to be oxygenated

THE END :) (the process repeats, it takes about 1 minute for this cycle to be completed)

NOTE: It might be easier to learn this if you know an easy way to remember which veins/arteries go where. Just remember that ‘pulmonary’ is anything to/from the lungs, ‘hepatic’ is anything to/from the liver, ‘renal’ is to/from the kidneys. Aorta is an Artery, Vena cava is a Vein.

Photo credit: BBC

2.65 describe the structures of arteries, veins and capillaries and understand their roles

Arteries

Arteries carry blood away from the heart. Their walls are strong, elastic and thick as the heart pumps blood out at high pressure. The lumen (hole in the middle) is quite small. Arteries also contain thick muscle layers to make the arteries strong and retain their shape.

NOTE: the largest artery is the aorta (taking oxygenated blood from the left ventricle to the rest of the body)

Capillaries

Capillaries branch off arteries and are involved in the exchange of materials (such as food, oxygen and waste) at tissue cells etc. They carry blood really really close to every cell in order for the cell to exchange substances with the blood (by diffusion). In order to make substance exchange quick and easy, capillaries are only 1 cell think and have permeable walls. Permeable walls are needed so substances can diffuse in and out, and having walls 1 cell think is useful as it will increase the rate of diffusion by decreasing the distance over which diffusion occurs.

Capillaries supply cells with food and oxygen and they remove away all waste from cells, such as carbon dioxide.

Veins

Arteries branch into capillaries, and capillaries eventually join together forming veins. Veins take blood to the heart, this means the blood is at not as high pressure than in arteries, therefore the walls of veins do not have to be as thick. Despite the low pressure the blood is under, veins have larger lumen (than arteries) to help encourage blood flow. Veins also have valves to stop the blood flowing backwards (as the blood is under low pressure it is not really forces through the veins, it kind of needs a little encouragement).

NOTE: the largest vein is the vena cava (takes deoxygenated blood from the body into the right atrium)

2.64 explain how the heart rate changes during exercise and under the influence of adrenaline

Exercise
When you exercise your muscles need more energy, therefore, you respire more. In order to get more oxygen to your muscles quicker (and the carbon dioxide away faster) your heart rate increases. This increases the rate of blood flow which will increase the amount of oxygen/carbon dioxide going in/out of your muscle cells. It's done like this...

- Exercise will inevitably increase the amount of carbon dioxide in the blood (as oxygen is being converted into carbon dioxide quicker)
- This high level of carbon dioxide is detected by receptors in the aorta artery (and also in the carotid artery, in your neck)
- The receptors in these arteries send signals to the brain
- The brain then sends signals to the heart, causing more forceful and frequent contractions (basically, making it pump more)

Adrenaline
Adrenaline is released by the adrenaline glands when an organism is threatened.

This adrenaline will bind to a specific receptors in the heart, causing the heart (cardiac) muscles to contract more frequently and more forcefully. This will increase the amount of oxygen supplied to the tissues.

2.63 describe the structure of the heart and how is functions

Okay, so there are a few different parts to the heart. Lets start with a diagram...



- The vena cava pumps deoxygenated blood (blood that has been around the body) into the right atrium.

- The right atrium receives the deoxygenated blood and it moved into the right ventricle through the tricuspid valve. NOTE: the valves are there so the blood cannot flow backers, it has to flow forwards.

- The right ventricle pumps the deoxygenated blood into the lungs (so it can be oxygenated again) through the pulmonary artery. This is helped with a series of contractions (the heart pumping).

- The pulmonary vein pumps oxygenated blood (from the lungs) into the left atrium.

- The left atrium receives the deoxygenated blood and it moved into the left ventricle through the bicuspid valve.

- The left ventricle pumps the oxygenated blood through the aorta around the body, helped with a series of contractions (the heart pumping).

NOTE: the left ventricle has a thicker muscle wall than the right ventricle as it has to pump blood around the body, whereas the right ventricle only has to pump blood to the lungs (which is not very far away). This means the oxygenated blood (in the left ventricle) is under higher pressure than the deoxygenated blood (in the right ventricle)

Picture credit: bbc

2.62 understand that platelets are involved in blood clotting, which prevents blood loss and the entry of micro-organisms

Platelets are small fragments of bone marrow. When you damage a blood vessel (e.g sever it if you cut yourself) the platelets in your blood will clump together at the damaged area, preventing any more blood from escaping and microorganisms getting into your body via the wound. This is known as blood clotting, and the clump of platelets is known as a blood clot.

NOTE: incase you were wondering, the platelets are held together by a mesh of a protein called fibrin.

2.61 understand that vaccination results in the manufacture of memory cells, which enable future antibody production to the pathogen to occur sooner, faster and in greater quantities

If you get infected with an infection (duh) it can take a while for your phagocytes to produce the antibodies needed to lock onto the antigens. This obviously is bad because your condition could severely worsen etc. To avoid this, you can have a vaccination. All a vaccination is is injecting dead (or inactive) pathogens of that particular infection into your body. Although the pathogens are dead, your lymphocytes will still recognise them as foreign objects and will still produce antigens to deal with them. Some of these lymphocytes will remain in the blood as memory cells, so if another pathogen of the same infection (e.g. if you really do catch the disease), the pathogens will recognise that pathogen and will be quickly able to produce antigens to destroy them in great numbers.

NOTE: If you are unsure of what pathogens do, or the difference between pathogens and antigens, have a look here... 2.60

2.60 describe how the immune system responds to disease using white blood cells, illustrated by phagocytes ingesting pathogens and lymphocytes releasing antibodies specific to the pathogen

Phagocytes
Phagocytes detect all 'foreign' things in your body (things the shouldn't be there, for example pathogens). Then they engulf these pathogens and digest them.

NOTE: its not just pathogens the will 'eat', they will destroy anything that is not meant to be there. They are non-specific.


Lymphocytes
Every type of pathogen has different molecules on its surface (aka antigens). When a lymphocyte comes across an antigen that isn't recognised (a 'foreign' antigen), the lymphocyte will produce antibodies. These antigens 'lock' onto the pathogens, this 'marks' them, so the phagocytes know they are to be destroyed.

Some lymphocytes stay in the blood and become memory cells. These remember the shape of the antigen needed for that pathogen. This means that, should that infection come back, the lymphocytes will be quick to destroy the pathogens as they already know the shape of the antigen that needs to be made - this is why you are immune to most diseases you've already had before, e.g. if you get chicken pox it's very unlikely you will get it again (as the memory cells remember the shape of the antigen so can quickly and efficiently make antibodies).



photos credit: leavingbio.net

2.59 explain how adaptations of red blood cells, including shape, structure and the presence of haemoglobin, make them suitable for the transport of oxygen

Red blood cells have a little dent in the middle (a bi-concave shape). This makes for a large surface area for the absorption and release of oxygen. They dont have a nucleus which frees up space for haemoglobin. In the lungs, haemoglobin reacts with oxygen to become oxyhemoglobin, this reaction reverses when the blood reached tissue cells and the oxygen is released.

2.58 understand the role of plasma in the transport of carbon dioxide, digested food urea, hormones and heat energy

Plasma is like the yellowy liquid part of the blood, it carries everything that needs transporting around your body (including red blood cells, white blood cells and platelets). It also transports...
- digested food (e.g. amino acids and glucose) from the gut to your bodies cells
- Carbon dioxide from our cells to the lungs
- Urea (salt, water amino acids) from the liver (where it's made) to the kidneys
- Hormones
- Heat energy (ensuring you are the same temperature throughout your body)

2.57 describe the composition of the blood: red blood cells, white blood cells, platelets and plasma

The four main components of the blood are plasma, platelets, red blood cells and white blood cells

Note source: CGP