Characteristics of living things
Unicellular and multicellular organism
Much more detail can be seen of the structure of the inside of a cell with a very powerful microscope e.g. electron microscope.
This is called the ultrastructure and the tiny structures within a cell which carry out particular jobs are called organelles.
Pleurococcus: a unicellular Plant
Paramecium: a unicellular animal
The cells of a multicellular organism, all have the same components but they vary in shape, size and function. There are at least 20 different types of cell in the human body each with different job – blood cells, nerve cells and so on.
The structure of a cell is related to its function. Cells with the same structure and function tend to be grouped together and form tissues.
A tissue is a group of cells specialised to perform a specific function. If this did not happen, then every cell would have to be able to perform every function – not very efficient.
Cells become specialised through a process called differentiation, once they have been formed through cell division.
In most multicellular organisms, different tissues are grouped together to form organs and organs are grouped together to form systems, e.g.
Muscle cells Muscle tissue Heart (organ) circulatory system
The human digestive system consists of the alimentary canal, liver, pancreas and gall bladder.
Topic 2 – Absorption and secretion of materials
What you need to know:
The role of the cell wall
The role of the plasma membrane (in relation to diffusion and osmosis)
Absorption and secretion
Absorption: taking substances into cells
Secretion: exporting substances out from cells
Cell boundaries – the cell wall
Only found in plant cells and consists mainly of cellulose fibres along sugars, structural proteins and enzymes.
The cellulose fibres are closely packed together, laid down in layers and run in parallel directions. This creates a strong and rigid but slightly elastic structure.
In a cell wall
Bundles of parallel cellulose fibres laid down in layers.
Many spaces are present between the cellulose fibres of the cell wall. Hydrophilic (water loving)
Functions of the cell: - prevents cell lysis (bursting) - provides structural support
Fluid Mosaic Model
Cell membrane consists of a double layer (bilayer) of constantly moving phospholipid molecules which contain a variety of proteins arranged on a mosaic like pattern.
A bilayer of phospholipids form because the heads of the molecules are hydrophilic (water-loving) and water soluble.
They form hydrogen bonds with water molecules
The tails of the phospholipids are hydrophobic (water-hating) and not water soluble, so they point inwards away from the water on the inside of the bilayer.
The membrane is porous, small molecules like water pass through the bilayer quickly but larger molecules like glucose depend on the protein molecules in the membrane.
They form channels which allow larger molecules to pass through.
The membrane is dynamic as its fluid nature allows some movement of membrane proteins.
This is the movement of molecules or ions across a plasma membrane against a concentration gradient (from low concentration to high concentration)
Active transport works in the opposite direction to diffusion and therefore requires energy in the form of ATP (which is formed during respiration)
Protein molecules in the membrane act as carrier molecules.
Each protein has specific sites that attract and bind