Cell Organelles

Essay by dx91High School, 11th grade February 2008

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Notes from ‘O’ Level BiologyCells Summary1.Cells contain living matter called protoplasm2.Protoplasm consists of 3 partsa)Nucleus – controls the activities of the cell and is responsible for cell reproductionb)Cytoplasm – where most of the cell activities take placec)Cell membrane – controls substances entering/leaving the cell3.Cells differ in size and shape according to their function4.Cells group to form tissues, tissues group to form organs, organs function in organ systems and various organ systems that make up an organism.

5. Diffusion occurs when molecules of a substance move from a region of higher concentration to a region of lower concentration of the substance (down a concentration gradient)6. Osmosis is a special kind of diffusion involving movement of water molecules across the cell membrane to where they are in lower concentration. It supplies the plant with water7. Active transport occurs only in living cells taking up a substance against a concentration gradient8.

The ratio of surface area to volume determines the rate at which substances will move into a cell.

Membrane systems and organellesNucleus•Consists of nucleoplasm bound by two membranes known as the nuclear envelope, which has perforations called nuclear pores•Chromatin, found in the nucleoplasm, condenses to form chromosomes during cell division. The denser areas of chromatin that are more darkly stained are called heterochromatin, and the remaining areas of chromatin are called euchromatin•Nucleolus (within the nucleus) is a spherical body rich in ribosomal RNA•Functions of the nucleus:-To control all cellular activity-To undergo nuclear division for cell replication to occur-Contain the DNA, essential for inheritance-Produce ribosome’s (nucleolus)-Produce RNAEndoplasmic Reticulum (ER)•Series of membranes creating channels within the cytoplasm•2 types of ER: rough ER and smooth ER•Rough ER is studded with tiny granules easily seen with an electron microscope. These are called ribosomes, and they play an important role in...