Biology

Diffusion is the gradual movement of particles from places of high concentration to places of low concentration. This means that they move from where there are lots of particles in one place to where there are hardly any particles.

It happens in both solutions and gases, because the particles in these substances are free to move about randomly. Particles in solids do not diffuse because they are held rigidly in place.

Eg when you spray perfume, the smell does not just stay in one place, it moves about to fill up the whole room. This is down to diffusion.

The bigger the difference in concentration, the faster the diffusion rate.

KEY: Diffusion is when particles S---P----R-------E----------A--------------D

Cell membranes can hold the cell together but also let things in and out. Dissolved substances can move in and out of cells by diffusion. Only very small molecules can diffuse through cell membranes, things like oxygen, glucose, amino acids and water.

Big molecules like starch and proteins can't fit through the membrane.

Just like diffusion in the air, particles flow from where there's a high concentration (outside of the cell) to where there's a low concentration (inside the cell).

KEY: Cell Membranes are like the bouncers. They're tough enough to protect the cell and if your names not on the list, you're not getting in.

Most cells vary from the generic structure of a cell because they are adapted to do a specific job.

Eg 1) Palisade leaf cells are adapted for photosynthesis because they have many chloroplasts at the top of the cell, close to the sunlight. They are tall and have a lot of surface area exposed down the side for absorbing CO2 from the air. Because they're thin you can pack loads of them at the top of the leaf.

2) Red blood cells are adapted to carry oxygen because they have a concave shape with a big surface area to absorb more oxygen. They can also pass smoothly through capillaries. They're packed with haemoglobin, the pigment that absorbs oxygen, and have no nucleus to leave even more room for haemoglobin.

3) Sperm cells have a long tail and streamlined head so they can swim to the egg. There are a lot of mitochondria in the cell to provide the energy needed

Similar cells are organised into tissues. Tissues are a group of similar cells which work together to carry out a particular function. In mammals, examples of tissues include:

Muscular tissue, which contracts and moves whatever its attached to

Glandular tissue, which makes and secretes chemicals like enzymes and hormones

Epithelial tissue, which covers some parts of the body

Tissues are organised into organs

Organs are organised into organ systems,which work together to perform a particular function.

Organ systems include:

Digestive system (digesting food)