Biology Key Points for O Level

Cell structure and organization
• Chloroplasts o Site of photosynthesis where light energy is absorbed o Consist of grana which are stacked up to increase surface area for absorption of light o Only present in plant cells
• Cell membrane (plasma membrane) o Lipid bilayer that surrounds the cell, interspersed with protein molecule o Presence of temporary pores o Selectively permeable
 Controls movement of substances o Diffusion, osmosis and active transport can therefore take place o Prevent outflowing of cytoplasm o Tonoplast
 Plasma membrane surrounding the vacuole
• Cell wall o Made of cellulose (a polysaccharide), and may be deposited with ligin o Maintain cell turgidity, provide mechanical support, protect from mechanical damage o Freely permeable to water and most solutes o Only present in plant cells
• Cytoplasm o Protoplasm surrounding the nucleus o There are numerous organelles present in the cytoplasm o Organelle
 Specialized membrane bound structures
• Cell vacuole o Fluid-filled space enclosed by a membrane o In animal cells, these are present as numerous small ones, and these are usually not permanent o In plant cells, there is usually a large central vacuole o Cell sap
 Fluid in vacuoles, contains sugars, mineral salts and amino acids o Tonoplast
 Membrane around the vacuole
• Nucleus o Nucleoplasm
 Small spherical mass of denser protoplasm o Nuclear membrane
 Nuclear envelope o Found in the cytoplasm o Control activities of the cell
 Reproduction, repairs and maintenance o Chromatin
 Network of long thread-like structures, containing the heredity materials o Chromatin is made up of proteins and DNA (Deoxyribose Nucleic Acid) o Chromosomes

Condensed and highly coiled chromatin threads during cell division o Nucleoli
 A spherical structure that plays a part in the build up of proteins • Endoplasmic reticulum o A series of flattened sheets o Smooth endoplasmic reticulum
 Sites of synthesis and transport of steroids and lipids o Rough