air resistance - decreases the velocity of an object

torque - changes the rotation of an object

forces act on objects

forces cause changes

diameter of a hydrogen atom, H 1.0 × 10-10 m

width of a hydrogen molecule, H2 2.9 × 10-10 m

length of an H-H bond 0.74 × 10-10 m

Nucleus and shells

An atom consists of:

a positively charged nucleus

surrounded by negatively charged electrons

all the atoms of a given element have the same atomic number

the atomic number of each element is unique - no two elements have the same atomic number

Most cells are so small that you can only see them with a microscope.

Cells have different components and each performs its own function within the cell.

Inside cells are various structures that are specialised to carry out a particular function. Both animal and plant cells have these components:

Cell membrane – this surrounds the cell and allows nutrients to enter and waste to leave it.

Nucleus – this controls what happens in the cell. It contains DNA, the genetic information that cells need to grow and reproduce.

Cytoplasm – this is a jelly-like substance in which chemical reactions happen.

Mitochondria – these are the powerhouse of the cell. They are structures where respiration takes place.

Cell wall - this is an outer structure that surrounds the cell and gives it support.

Vacuole - this is a space within the cytoplasm of plant cells that contains sap.

Chloroplasts - these contain chlorophyll and are the site of photosynthesis

Red blood cells are some of the smallest cells in the human body. These have a diameter of 0.008 mm, meaning a line of 125 red blood cells is only 1 mm long.

The ovum (or egg cell) is one of the largest cells in the human body. It has a diameter of roughly 0.1 mm, so you can see them without a microscope. A line of 10 egg cells is 1 mm long.