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Biology Unit 2…read more

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Cells and organelles
(Part 1)
Cells can be eukaryotic or prokaryotic cells. Organelles found in eukaryotic cells:
Organelle Description Function
Nucleus A large organelle surrounded by a nuclear Chromatin is made from proteins and DNA. The pores
envelope (double membrane) which contains many allow substances to move between the nucleus and the
pores. The nucleus contains chromatin and cytoplasm. The nucleolus make ribosomes
Lysome A round organelle, surrounded by a membrane, Contains digestive enzymes. These are kept separate
with no clear internal structure from the cytoplasm by the surrounding membrane, but
can be used to digest invading cells or break down worn
out components in cell
Vesicle A small fluid filled sac in the cytoplasm, Transports substances in and out of cell & between
surrounded by a membrane organelles. Some formed by Golgi apparatus or the
endoplasmic recitlium but others formed at cell
Ribosome A very small organelle that floats free in the The site where proteins are made
cytoplasm or is attached to the rough ER
Endoplasmic rectilium (ER) There are 2 types of ER- the smooth (is a system It processes and packages new lipids and proteins. The
of membranes enclosed in fluid filled space)and rough ER folds and processes proteins that have been
rough (similar but covered in ribosomes made at the ribosomes
Centrion Hollow cylinders containing a ring of microtubules Involved with the separation of chromosome during
(tiny protein cylinders) cell division
Golgi apparatus A group of fluid filled flattened sacs. Vesicles It processes and packages new lipids and proteins and
are often seen at the edges of sacs makes lysosomes
Mitochondrion Usually oval shaped, have double membrane, have The site of aerobic respiration where ATP is produced
inner folds called cristae and has matrix- has and are found in large numbers in cells that are active
enzymes that's involved for respiration and need a lot of energy…read more

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Cells and organelles
(Part 2)
Differences between prokaryotic and eukaryotic cells:
· Proteins are made at the ribosomes
· The ribosomes in the rough ER makes proteins that are excreted or attached to the cell
membrane. The free ribosomes in the cytoplasm make proteins that stay in the ribosome
· New proteins produced at the rER are folded and processed and they're transported from
the ER to the Golgi apparatus
· Then they are transported from the ER to the Golgi apparatus in vesicles
· At the Golgi apparatus, the proteins may undergo further processing e.g. sugar chains are
trimmed or more are added
· The proteins enter more vesicles to be transported around the cell
Eukaryotes Prokaryotes
Larger cells (2-200µm diameter) Extremely small cells (less than 2.0µm diameter)
DNA is linear DNA is circular
Nucleus present- DNA is inside nucleus No nucleus- DNA free in cytoplasm
No cell wall (in animals), cellulose cell wall (in Cell wall made of a polysaccharide, but not
plants) or chitin cell wall (in fungi) cellulose or chitin
Many organelles, mitochondria present Few organelles, no mitochondria
Large ribosomes Small ribosomes
Example: human liver cell Example: E.coli bacterium…read more

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Cell organisation
Similar cells are grouped together into tissues examples:
· Squameous epithelium tissue is a single layer of flat cells lining a surface. Is found in
many places including the alveoli in the lungs
· Xylem tissue is a plant tissue with 2 jobs- it transports water around the plant, and it
supports the plant. It contains xylem vessels cells and parenchyma cells
The leaf is an example of a plant organ and it's made up of the following tissues:
1. Lower epidermis- contains stomata (holes) to let air in and out for gas exchange
2. Spongy mesophyll- full of spaces to let gas circulate
3. Palisade mesophyll- most photosynthesis occurs here
4. Xylem- carries water to the leaf
5. Phloem- carries sugars away from the leaf
6. Upper epidermis- covered in a waterproof waxy cuticle to reduce water loss
The lungs is an example of an animal organ and it's made up of the following tissues:
1. Squameous epithelium tissue- surrounds the alveoli (where gas exchange occurs)
2. Fibrous connective tissue- helps to force air back out of the lungs when exhaling
3. Blood vessels- capillaries surrounding the alveoli
The respectiory system is made up of all the organs, tissues and cells involved in
breathing- the lungs, trachea, larynx, nose, mouth and diaphragm are all part of the
respiectory system…read more

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The cell cycle and mitosis
The cell cycle starts when a cell is produced by cell division and ends with the cell dividing to produce 2 identical
cells. It consists of mitosis and interphases- which has G 1, S & G2
Mitosis is needed for growth of multicellular organisms and for repairing damaged tissues
Some organisms reproduce asexually using mitisos to produce genetically identical to the original parent
Interphase is when the cell prepares itself to divide- the cell's DNA is unravelled and replicated to double the
genetic content the organelles are also replicated so the ATP content is increased (ATP is needed for cell
division). It consists of 4 stages:
Prophase- The chromosomes condense and the centrioles move to opposite ends of the cell, forming a network
of protein fibres across called the spindle. The nuclear envelope breaks down and the chromosomes lie free
in the cytoplasm
Metaphase- the chromosomes (each with 2 chromatids) line up along the middle of the cell and become
attached to the spindle by their centrometer
Anaphase-the centrometers divide, separating each pair of sister chromatids. The spindle fibres contract,
pulling chromatids to opposite ends of the cell, centromeres first
Telophase- the chromatids reach opposite poles of spindle, uncoil & become long & thin- becoming
chromosomes. A nuclear enevelope forms around each group of chromosomes so there are 2 nuclei and
cytoplasm divides, so there are 2 daughter cells that are genetically identical to the orgional cell.
Root tips can be used to observe mitosis:
1. Cut the tip from the growing root (about 5mm long)
2. place the root tip on a watch glass and add a few drops of HCL acid
3. add a stain e.g. Acetic orcien, toludine blue so that the chromosome becomes darker and easier to see
4. warm the watch glass by passing it slowly through the Bunsen flame
5. place root tip under microscope slide and use mounted needle to break it open and spread the cells out thinly
6. add a few more drops of stain, place cover slip warm the slide for a few sec to intensity the stain and look.…read more

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Production of gametes
DNA is passed to new offspring by gametes and are found in all organisms that reproduce sexually, the
male gametes are produced in pollen grains and female gametes are contained in ovules and each
gamete has 23 chromosomes
The sperm cell has mitochondria provides energy for swimming, flagellum allows sperm to swim and
acrosome has digestive enzymes to break down the egg cell's zona pellucida
Egg cell has follicle cells to form protective coating and zona pecullida is a protective layer than sperm
have to penetrate.
Stages of meiosis:
1. The DNA replicates so there are 2 identical copies of each chromosome, called chromatids
2. The DNA condenses to form double-armed chromosome, made from 2 sister chromosomes
3. The chromosomes arrange themselves into homologus pairs- pairs of matching chromosome
4. First division- the homologous pairs are separated, halved the chromosome number
5. Second division- the pairs of sister chromatids are separated
6. 4 new gametes that are genetically different from each other are produced
Meiosis produces cells that are genetically different:
Crossing over chromatids:
1. The homologous pairs of chromosomes come together and pair up
2. 2 of the chromatids in each homologous pair twist around each other
3. The twist breaks off their original chromatid and rejoin onto the other chromatid, recombining
their genetic material
4. The chromatids still have the same genes but they have different combinations of the alleles
5. This means that each of the 4 new cells formed form meiosis contains chromatids with different
Independent assortsoment of chromosomes:
1. The 4 daughter cells formed from meiosis have different combination of chromosomes
2. All your cells have a combination of chromosomes ­ half from mum (maternal), half from dad
3. When the gametes are produced, different combinations of those maternal and paternal
chromosomes go into each cell
4. This is called independent assortment (separation) of the chromosomes…read more

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is there a mark scheme?

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