3.15 describe a DNA molecule as two strands coiled to form a double helix, the strands being linked by a series of paired bases; adenine (A) with thymine (T), and cytosine (C) with guanine (G)

a molecule of DNA is made up of two strands that are coiled together and held in place by chemicals known as bases. the four bases are as follows... adenine, thymine, cytosine and guanine. The bases are paired together. Adenine ALWAYS pairs with thymine and cytosine ALWAYS pairs with guanine. This ensures that you will always have an equal amount of adenine to thymine and cytosine to guanine. This diagram may help...


NOTE: A good way to remember which base pairs with which is with the rhyme 'Apples on a Tree, Cars in a Garage' (Apple = Adenine, Tree = Thymine. Cars = Cytosine, Garage = Guanine)

3.14 understand that a gene is a section of a molecule of DNA and that a gene codes for a specific protein

A gene is just a section of DNA, and each separate gene codes for a particular protein.

3.13 understand hat the nucleus of a cell contains chromosomes on which genes are located

Okay so basically every cell contains a nucleus, every nucleus contains chromosomes (humans are diploid and have 46 chromosomes, except for sex cells which are haploid and have 23). Each chromosome is made up of a long coil of DNA. DNA can be sectioned, a small section of DNA is known as a gene.

3.12 understand the roles of oestrogen and testosterone in the development of secondary sexual characteristics

Oestrogen
Stimulates the secondary sexual characteristics in women. For example, hair on underarms and around pubic area, a womens hips may widen, the development of breasts, the start of menstruation.

Testosterone

Stimulates the secondary sexual characteristics in men. For example,
 they will grow extra air on their face and body, their muscles will develop, they will begin to produce sperm, their voice will deepen and their penis and testes will enlarge.

3.11 understand how the developing embryo is protected by amniotic fluid

Once an ovum has been fertilized, it will develop into an embryo which will implant into the uterus. It is at this point that the amnion membrane forms. This is full of amniotic fluid and surrounds the embryo, protecting it against any knocks and bumps :)

3.10 describe the role of the placenta in the nutrition of the developing embryo

When an ovum is fertilized, it develops into an embryo and implants into the uterus. Once the embryo has implanted a placenta will develop. This lets the blood of the mother and baby get very close (BUT NOT TOUCH!) to allow the exchange of food, oxygen and waste.

NOTE: the blood of ma and ba to not touch. this is because they may have different blood types.

3.9 understand the role of oestrogen and progesterone in the menstrual cycle

Oestrogen causes the lining of the uterus to thicken and grow. It also stimulates the release of an ovum (well, it stimulates the release of LH which stimulates the release of an ovum).

Progesterone maintains the lining of the uterus. The lining breaks down when the level of progesterone falls.

3.7 understand that plants can reproduce asexually by natural methods (illustrated by runners) and by artificial methods (illustrated by cuttings)

Asexual reproduction produces clones.

Natural methods

Runners grow new plants from their stems. this is the process in a strawberry plant (for example)...

- the parent plant grows runners (fast growing shoots that grow sideways just above the ground)

- these runners will grow roots into the soil at various distances and new plants will begin to grow

NOTE: examples of runners include the strawberry plant and the spider plant.

NOTE NOTE: There is no genetic variation between the new plants and the parent plant as the new plants are clones of the parent plant

Artificial methods

Humans can use artificial methods to clone plants. The process is as follows...

- Take a cutting from the parent plant

- Plant the cutting

That's it :) the plants that grow from the cuttings will be genetically identical copes of the parent plant. this method is used often as it is quick and cheap, whilst waiting for the plant to naturally clone can take a long time.

3.6 understand how germinating seeds utilise food reserves until the seedling can carry out photosynthesis

Inside a developed seed, there is a food store. When the seed starts to germinate, it gets the glucose needed for respiration from its food store - this means it can respire, meaning it has enough energy to grow. A plant will begin to photosynthesise once its leaves will begin to grow, this is when it stops using its food store.

3.5 understand the conditions needed for seed germination

All germination is is a fancy word for when the seed begins to grow. Different seeds need different conditions for germination (depending on their characteristics) but the main are as follows...

- Water. This is needed to activate the enzymes that break down the food reserves in the seed (so it can start to eat, basically)
- Oxygen. this is needed for respiration which will provide the energy needed for growth
- Optimum temperature. This changes for each seed depending on the best temperature for their enzymes to work at.

NOTE: Germination will only occur once all of these conditions are present.

3.4 understand that the growth of the pollen of the pollen tube followed by fertilised leads to seed and fruit formation

If a grain of pollen lands on the stigma of a plant, a pollen tube will grow (out of the pollen grain), down through the style to the ovary and into the ovule. A nucleus from the male gamete will then move down the pollen tube and will fuse with the nucleus of the female gamete in the ovule, this is fertilization. A fertilized female gamete forms a seed and the ovary develops into a fruit around the seed (kind of like a protective layer).

Base notes: CGP

3.3 describe the structures of an insect-pollinates and wind-pollinated flower and explain how each is adapted for pollination

Okay so first it is best to know what each of the plant does, I have put this in red so if you know it just skip past...

The anther - contains pollen grains (these produce the male gametes/sex cells)
the filament - holds up the anther so they are easily reached by insects brushing against them
The stigma - the sticky bit where the pollen grains attach to
the style - this supports the stigma
the ovary - the ovary contains the female gametes, eggs (inside ovules)
Insect pollinated

Brightly coloured petals to attract insects
Scented flowers and nectaries, again to attract insects
The pollen grains are big and sticky, to easily stick to insects as they fly from plant to plant
A sticky stigma so that any pollen picked up from other plants will stick to the stigma.



Wind pollinated
 

In wind pollinated plants, there are no/small/dull petals as no insects need to be attracted
no scents or nectaries
small light pollen grains (and lots of them) so they can be easily carried by the wind
Long filaments that hang the anthers outside the flower so that lots of pollen gets blown away by the wind (hence wind-pollination)
A big feathery stigma to catch pollen carried past by the wind.

image credit: sacsplash.org // Wikipedia

sorry for delays!

As I have been back at school I have not been able to keep up with the blog as much as I was over Easter break.. I leave school this Friday for study leave and will be finishing all blogs next week (iGCSE Chemistry, iGCSE Physics and iGCSE Biology), sorry for any inconvenience, thank you for sticking with me! If you have any questions about any posts do not hesitate to comment and I will reply as soon as possibly, hope any exams you have already done went smoothly (for those doing Cambridge iGCSE english... what was the obsession with the bees?), good luck for any doing art and textiles exams over the next few days also. I will endeavour to finish the blogs as soon as possible next week so I have all the rest of study leave to answer any questions or anything you have. Good luck once more, I know you will all do amazing :)

Millie

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