Biology Topic 3 Summary

Prokaryotic cells do not have a nucleus or any other membrane-bound cell organelles. Examples are Bacteria and Cyanobacteria.

• Infolding of cell surface membrane - photosynthetic membrane (site of respiration)
• Plasmid - small circle DNA
• Capsule - slimy layer on surface for protection and to prevent dehydration
• Pili - thin, protein tubes allowing bacteria to adhere to surfaces
• Flagellum - hollow cylindrical thread-like structure that helps move the cell
• Cell Wall made of murein 
• Their DNA is not associated with any proteins and lies free within the cytoplasm.
• Ribosomes are present

Eukaryotic cells contain discrete membrane-bound organelles such as, nuclei, mitochondria and chloroplasts.

• Nucleus - contains chromosomes and a nucleolus. The DNA in chromosomes contain genes that control the synthesis of proteins.
• Rough Endoplasmic Reticulum - A system of interconnected membrane-bound flattened sacs. Ribosomes are attached to the outer surface, hence making it a rough ER. The ribomsomes are in control of synthesising protein.
• Smooth Endoplasmic Reticulum - like rER, but doesn't have any attached ribosomes. SER is involved with making lipids & steroids. Found near the muscles and liver.
• Ribosomes - made of RNA & protein. Found in the cytoplasm or attached to the endoplasmic reticulum.
• Golgi Body - Stacks of parallel flattened-bound sacs formed by the fusion of vesicles coming from the rER. Modifies protein and packages them in vesicles for transport. 
• Lysosome - spherical sacs containing digestive enzymes and bound by a single membrane. Involved in the breakdown of unwanted structures within cells and the descrution of old cells that are to be replaced.

• Cell surface membrane - phospholipid bilayer containing proteins and other molecules froming a partially permeable barrier
• Mitochondria - Site of aerobic respiration (the production of A.T.P) Outer and inner membrane (double membrane). Containing their own genetic material. Inner membrane folds to form cristae, which is surrounded by a fluid matrix.
• Chloroplast - found in plant cells only and are enclosed by a double membrane. Stroma contains a series of membrane-bound flattended sacs called thylakoid membranes. Contains starch grains (which act as a carbohydrate store), lipid stores and DNA, RNA and ribosomes - all involved in nucleic acid and protein synthesis. 

The Differences between Prokaryotic Cells and Eukaryotic Cells.

Prokaryotic cells are much smaller; their cell wall is made of murein whereas eukaryotic cells are made of cellulose (in plant cells only). Prokaryotic cells have no membrane-bound organelles, whereas eukaryotic cells do such as, choloroplasts (in plant cells only), nucleus, mitochondria, lysosomes and golgi body. Genetic material in prokaryotic cells is present, but in a single-circular DNA form, but no nucleus present, but in eukaroytic cells a nucleus enclosed by a double membrane is present with many linear DNA molecules forming chromosomes.

Nucleus: 

• Transcription of DNA to mRNA.
• mRNA leaves the nucleus.

Cytoplasm:

The golgi body appears as a stack of parallel flattened membrane pockets that are called cisternae, which are formed by vesicles coming from the rough endoplasmic reticulum and fuse into eachother. 

• The ribosomes from the rER synthesise the protein
• Once they are made they are sent up the lumen of the ER to be pinched off from its own membrane to form a vescile containing the protein.
• This is then to fuse with other vesicles to form the flattened sacs of the golgi apparatus.
• When the protein is secreted into the body it will undergo modification
• Once modified it is ready to be enclosed to form a lysosome
• The vesicle fuses with the cell surface membrane releasing the protein by the process of exocytosis

Gametes are highly speciailiesd, with structures and functions that are different from other body cells.

Ovum:

• large cell / surface area (to increase the chance of fertilisation - a bigger target) that is in capable of independent movement
• It is wafted along by ciliated cells lining the tubes and by muscular contractions of the tubes
• The cytoplasm of the ovum contains proteins and lipid food reserves for a developing embryo
• Surrounding the cell is a jelly-like coating called the zona pellucida
• The presence of the zona pellucide is to prevent polyspermy
• The presence of cortical granules is to produce the 'cortical reaction' is to also prevent polyspermy
• The nucleus is a haploid to restore the diploid number of chromones 
• Presence of nutrtients / food store / oill droplets - energy supplement / building blocks for growth

Sperm: 

• Much smaller than an ovum and can move by itself
• Tail to help move the sperm to targetted location - powered by energy released by the mitochondria
• Membrane receptors to trigger acrosome reaction
• Acrosomes contain hydrollytic enzymes (acrosin) to break down the follicle cells / zona pellucida
• Nucleus - haploid to restore the diplod number of chromosomes after fertilisation

• Sperm reaches the ovum
• chemicals released from cells surrounding ovum trigggering the acrosome reaction
• the acrosome swells fusing with sperm cell surface membrane
• digestive enzymes in the acrosome are released
• the enzymes digest through follicle cells and the zona pellucida
• the sperm fuses with the ovum membrane
• enzymes released from the lysosomes in the ovum (cortical reaction) thicken the jelly-like layer preventing polyspermy. 
• The sperm nucleus enters the ovum 
• nuclei of ovum and sperm fuse
• meosis is completed

The fusion of the sperm and the ovum membrane causes three important things to occur:

1. A temporary change in the electrical charge on the inside of the ovum membrane from negative to positive - neglecting any other sperm from entering.

2. The cortical reaction - where by cortical granules pass out of the ovum through the ovum membrane by exocytosis into the zona pellucida. This causes the zona to turn into a tough fertilisaion membrane. 

3. Completion of meiosis with meiosis II giving the female haploid nucleus and a second polar body.

1 and 2 prevent polyspermy: the result of multiple sets of chromosomes into the ovum.

Yes