Biology Unit 2 Notes

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2.3.1

OrganelleStructure and function

nucleus
enclosed in double membrane with pores
contains chromosomes with genes made of DNA to control protein synthesis

ribosomes
made of RNA and protein
free in cytoplasm or attached to RER
site of protein synthesis

rough endoplasmic reticulum
interconnected sacs with ribosomes attached
transport proteins to other parts of cell

smooth endoplasmic reticulum

synthesis of lipids and steroids

mitochondria
double membrane – inner folded into cristae
site of later stages of aerobic respiration

centrioles
one pair found in animal cells
made of protein microtubules
involved in spindle formation and cellular transport

lysosomes
digestive enzymes wrapped in membrane
breakdown of unwanted structures or old cells

nucleolus
dense body in nucleus
synthesis of ribosomes

2.3.2

Proteins synthesised on the ribosomes of the RER are moved to other parts of the cell through the cavities of the endoplasmic reticulum.

The Golgi apparatus is a stack of membrane-bound sacs formed from fused vesicles from the ER.

Proteins are modified here and packaged in vesicles. Some eg enzymes and hormones are released from the cell.

See Figure 3.9 on page 101.

2.3.3

The cells described above, with membrane-bound organelles are eukaryotic.

Organisms with eukaryotic cells are classified into 4 kingdoms: Animals, Plants, Fungi and Protoctists.

The 5th kingdom is the Prokaryotes, with prokaryotic cells which: are smaller than eukaryotic cells
have no membrane-bound organelles
have no nucleus
have circular DNA, not associated with protein
have small rings of DNA, called plasmids
always have a cell wall

To compare prokaryotic & eukaryotic cells, see Figures 3.4 and 3.8 on pages 98 & 100.

2.3.4
Mitosis is a type of cell division, which retains the full or diploid number (2n) of chromosomes.

In humans, a cell with 46 chromosomes divides to form 2 identical daughter cells, each with 46 chromosomes.

Before nuclear division, a copy of each chromosome is made by semi-conservative replication of the DNA. Each double helix is called a chromatid.

These stages are part of the cell cycle:

interphase
G1 (first gap phase)
synthesis of cellular proteins and organelles

S (synthesis phase)

replication of DNA

G2 (second gap phase)

synthesis of spindle proteins
division
mitosis (nuclear division)
separation of the 2 DNA helices making up the chromosome

cytoplasmic division
cleavage of a single cell into two daughter cells

2.3.5

Mitosis, with identical daughter cells, ensures genetic stability - important for:

growth: development from a single cell to a multicellular organism repair: regeneration of lost or damaged parts or replacement of old or damaged cells asexual reproduction eg budding in Hydra, vegetative reproduction in plants

2.3.6

Cell division is a continuous process, but 4 stages of mitosis (nuclear division) can be described:

prophase
chromosomes condense (get shorter and thicker)
microtubules are organised into a spindle by the centrioles nuclear membrane breaks down

metaphase
the centromeres of the chromosomes attach to the spindle at the equator

anaphase
centromeres split
spindle fibres pull chromatids to opposite poles
spindle breaks down

telophase
chromosomes unravel
two nuclear envelopes form

Make sure you are familiar with the details of the core practical in which you observed the stages of mitosis.

2.3.7

The sex cells or gametes are adapted for sexual reproduction.

OVUM
large cell, incapable of independent movement
wafted along oviducts by cilia and muscular contractions of the tubes cytoplasm contains protein and lipid food reserves
surrounded by a jelly-like coat – the zona pellucida – which hardens...
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