Biology Unit 2 (AQA) GCSE

Palisade Leaf Cells- for photosynthesis

• packed with chloroplasts-most at the top of the cell so they have good access to sunlight. 
• Tall shape-a lot of exposed surface area down the side for absorbing carbon dioxide.
•  Thin shape- you can pack lots of cells at the top of a leaf. 

Palisade leaf cells-top of leaf-most photosynthesis happens here. 

Guard Cells- to open and close pores

• special kidney shape to open and close the stomata(pores).
• when plant has lots of water-guard cells fill-stomata open-gases exchanged for photosynthesis.
• when plants is lacking water-stomata close-stops water vapour escaping.
• sensitive to light so close at night-saves water

function: to allow gas exchange and control water loss.

Red Blood Cells- carry oxygen 

• Concave shape- bigger surface area for absorbing oxygen also helps them pass smoothly through capillary walls.
• Packed with haemoglobin to absorb the oxygen.
• No nucleus- more space for haemoglobin. 

Red blood cells are an important part of blood-actually counted as a tissue.

Sperm&Egg Cells- for reproduction 

• Egg cell- function: carry female DNA and nourish developing embryo so contains huge food reserves. 
• When sperm and egg fuse, egg cell membrane changed to not allow any more sperm to get in.
• Sperm- function: get male DNA to the female DNA, long tail, streamlined shape to help it swim.
• Lots of Mitochondria in sperm in cell to provide energy needed, andenzymes in head to digest through the cell membrane.

Sperm and Eggs are very important in the reproductive system.

Respiration= many reactions <---all catalysed by enzymes :)

RESPIRATION=the breakdown of glucose to release energy and goes on in every cell in your body. 

glucose+oxygen --->carbon dioxide+water+energy 

this is aerobic respiration, this means with oxygen which produces a lot of energy. 

Energy released by Respiration used to:

• Build up larger molecules from smaller ones eg: proteins from amino acids.
• In animals to allow their muscles to contract for movement.
• In mammals and birds, for keeping body temperature a constant.
• In plants to build sugars, nitrates and other nutrients to amino acids, which are then built into proteins.  

DNA= deoxyribose nucleic acid 

• It contains all the instructions to put an organism together and make it work.
• It is found in the nucleus of a cell in really long molecules called chromosomes.

A gene is a section of DNA...

• it contains the instructions to make specific proteins, this is done by stringing together amino acids in a particular order. 
• only 20 amino acids are used, but they make up thousands of different proteins. 
• DNA also determines what proteins the cell produces eg:keratin, haemoglobin. 
• DNA therefore determines what kind of cell it is eg: red blood cell, skin cell. 

Everybody has unique DNA, except identical twins, uses of DNA testing-

forensic science, DNA fingerprinting and paternity testing.

Mitosis- makes new cells for growth and repair

Body cells= 2x each chromosome- one from mother, one from father (23 pairs)

Body cell divides- makes identical copy of original-same number of chromosomes.

MITOSIS IS WHEN A CELL REPRODUCES ITSELF BY SPLITTING TO FORM TWO IDENTICAL OFFSPRING.

In a cell that is not dividing, the DNA is all spread out in long strings---> If the cell gets a signal to divide, it needs to duplicate its DNA- so there is one copy for each new cell, The DNA is copied to form X-shaped chromosomes. Each arm of the chromosome is the exact replica of the other---> The chromosomes then line up at the centre of the cell and cell fibres pull them apart. The two arms of each chromosome go to opposite halves of the cell.--->Membranes from around each of the sets of chromosomes. These become the nuclei of the two cells.---> Lastly the cytoplasm divides---> You now have two new cells containing exactly the same DNA- they're identical. 

Gametes= half the usual number of chromosomes. 

Meiosis involves TWO DIVISIONS thus produces FOUR GAMETES. 

As with mitosis, before the cell starts to divide, it duplicates its DNA- one arm of each chromosome is an exact copy of the other arm.

In the first division the chromosome pairs line up in the centre of the cell. 

The pairs are then pulled apart, so each new cell will have a mixture of the mother and fathers chromosomes. Mixing up genes in this way creates variation in the offspring. This is a huge advantage of sexual reproduction. 

In the second division the chromosomes line up again in the centre of the cell, then are pulled apart again.

You get four gametes each with only a single set of chromosomes in it.

After two gametes join at fertilisation, the cell grows by repeatedly dividing by mitosis.