Biology: Cells

• Cells are the building blocks of life.
• takes in raw materials => processes materials, make new molecules => molecule used by cell or transported (eg insulin transported to liver and muscles)
• Use two types of microscopes to see cells => light microscopes(1000 x, colour images)/electron microscopes(black and white images, can be artificially colourized, 200000 x)

Protoplasm:

-About 70% of protoplasm is water. 

-It is made of the nucleus, cell surface membrane and the cytoplasm

Nucleus:

• It contains chromatin
• It controls cell activities 
• Essential for cell division

Chromatin

• A network of long thread-like stuctures found within the nucleus.
• Made up of proteins and deoxyribonucleic acid (DNA)
• When the cell is dividing, chromatin becomes highly coiled structures called chromosomes

Cell surface membrane:

-Also called plasma membrane

-partially permeable 

-A thin layer surrounding the cell
-Controls substances entering or leaving the cell

Cytoplasm:

• Contains enzymes and specialized structures called organelles. 
• In gel or sol state because of jelly-like consistency 
• Gel state (a semi-solid state) in the outer region, sol state (liquid state) in the inner regions

Centrioles:

-Cannot be seen under light microscope

-Responsible for cell division

-Found only in animal cells

Cell wall:

• Made of cellulose
• Protects cell from injury and gives the plant a fixed shape
• Fully permeable
• Absent in plant cells

Rough endoplasmic reticulum (RER):

• Ribosomes attached to its surface
• Ribosomes that are attached to the RER make proteins that are transported out of the cell.
• Ribosomes that lie freely in the cytoplasm make proteins that are used inside the cell.
• RER transports proteins made by ribosomes to Golgi apparatus for secretion out of cell

Smooth endoplasmic reticulum (SER):

• does not have ribosomes attached to its surface
• synthesises substances such as fats and steriods (sex hormones in mammals)
• more tubular than RER

Golgi apparatus:

• stores and modifies substances sent by ER
• packages these substances in vesicles for secretion out of cell
• shaped like a disc
• consits of a stack of flattened spaces by membranes
• vesicles fuses with one side of Golgi apparatus and pinched off from the other side

Mitochondria:

• Small sausaged-shaped organelles
• Aerobic respiration occurs in mitochondria
• Food substances are oxidised to release energy.

Physics: Density

Density: mass per unit volume.

p = m/v

Where:

• p = density (kg/m3 )
• m = mass of object in kg
• v = volume of object in  m3

SI unit of density = kg/m3

 Substances that float on water has lower densities than water.