WJEC Biology for AS: Student Bk

The structure of biological membranes is described on p52. Link Key Term Organelle: a specialised structure with a specific function inside a cell. Going further The three principles of cell theory are: • All living organisms are composed of one or more cells. • The cell is the basic unit of life. • Cells can only arise from pre- existing cells. How Science Works The concept of cells dates back to the 1660s and has developed as microscope technology and the chemistry of stains have advanced. Cells and their organisation Cell structure All cells are surrounded by a membrane made of phospholipids and proteins. Biological membranes are so thin that their structure cannot be distinguished in the light microscope, and in the electron microscope, they appear as a single line. Eukaryotic cells contain membrane-bound organelles , which are enclosed areas in the cytoplasm. The advantage is that potentially harmful chemicals, such as enzymes, are isolated and molecules with particular functions, such as chlorophyll, can be concentrated in one area. Membranes provide a large surface area for the attachment of enzymes involved in metabolic processes, and they provide a transport system inside the cell. When looking at images of cells, it is important to understand the units of measurement. The standard unit of measurement is the metre (m). Biology works in the range of nanometres (nm), when considering molecules, to kilometres (km), when considering ecosystems. But for cells and organelles, micrometres, µm, are the most convenient unit. Generalised structure of an animal cell Generalised structure of a plant cell YOU SHOULD KNOW › › › ››› The generalised structure of plant and animal cells ››› The structure and functions of organelles ››› How to recognise and identify organelles from electron micrographs ››› The relationship between a cell's function and its organelles ››› That organelles are often interrelated through their functions Fig 19 vesicle Golgi body mitochondrion lysosome rough endoplasmic reticulum with ribosomes plasma or cell membrane ribosomes centriole nuclear envelope chromatin nucleolus nuclear pore 30 µm Fig 20a cell wall cell membrane chloroplast mitochondrion cytoplasm Golgi body nucleolus nucleus ribosomes plasmodesma nuclear pore rough endoplasmic reticulum vacuole tonoplast vesicle 30 µm Cell structure and organisation 31