Cell Structure 2.docx

Cell Membrane Structure (6) The cell membrane is a fluid phospholipid bilayer coated and embedded with protein. Protein gives elasticity and lipid allows fat-soluble molecules to enter. There are temporary pores throughout the membrane. Function: - Holds in cell contents - thus giving shape, suppor...

Cell Membrane Structure (6) The cell membrane is a fluid phospholipid bilayer coated and embedded with protein. Protein gives elasticity and lipid allows fat-soluble molecules to enter. There are temporary pores throughout the membrane. Function: - Holds in cell contents - thus giving shape, support (provided by proteins) and protection. - Controls entry and exit of molecules. It is a semi-permeable barrier i.e., can let small molecules e.g., water (by osmosis), oxygen and carbon dioxide (by diffusion) through but not large molecules e.g., salt, sugar, protein. Proteins assist in the active transport of materials across the membrane (energy needed). Thus, the cell can control the amount of water and salt conc. (osmoregulation). Phospholipids affect the fluidity and permeability of membrane Cytoplasm Structure - Contains the entire contents of the cell inside the cell membrane, except the nucleus. - Consists of cytosol, a gel-like substance where the organelles are suspended. - Contains all the substances needed by the cell to carry out its functions. Function: - The medium in which most chemical reactions of the cell take place - The first stage of respiration, where glucose is split in a process called glycolysis with the release of a small amount of energy, takes place here. Nucleus Structure (7) - Enclosed by a double membrane. - Contains chromatin (genetic material) - becomes arranged into chromosomes during cell division. These are made of protein and DNA. Genes are located along the chromosome. Contains one or more nucleoli. -Nuclear pores allow passage of mRNA, rRNA, nucleotides. - Nucleoplasm = a liquid in nucleus surrounding nucleolus and chromatin Function: - To control all cell activities (by making enzymes). - Contains genetic material. - Involved in cell division. Nuclear Pores Hundreds of gaps in the nuclear envelope which allow substances to enter and leave the nucleus. Proteins, amino acids, and nucleotides bases enter the nucleus, while mRNA, rRNA and the protein subunits that make ribosomes leave through these pores. Nucleolus Darker staining areas within the nucleus. These provide the individual components. rRNA and proteins that combine in the cytoplasm to form ribosomes, which are the structures used by the cell to build proteins. Chromosomes Structure (8) - Chromosomes are made of Deoxyribonucleic Acid (DNA) and proteins. - These macromolecules form the classic double helix, which is like a twister ladder where alternate deoxyribose sugar and phosphate molecules form the sides and base pairs, held together by hydrogen bonds to form the rungs. - In their spread-out form, Chromosomes are referred to as Chromatin which is a combination of DNA and special proteins called histones. Function: - Chromosomes hold the genetic code for making proteins. - Only about 2% of chromosomes are coding structures(genes) that make proteins. The other 98% are non-coding DNA. - Prior to cell division, the DNA coils around the histones many times in succession. Each coiling makes the chromosomes shorter and thicker until they are clearly visible. - Each chromosome consists of two roughly symmetrical strands called chromatids joined by a structure called the centromere. When the coiling is complete, each chromatid is essentially identical to its partner and as soon as they are separated, they are referred to as chromosomes. Mitochondrian Structure (9) - Consists of a smooth outer membrane and a folded inner membrane with a cavity between them. - Inner membrane is folded into structures called cristae. - In the centre of the mitochondrion is a jelly-like substance called the matrix in which are found ribosomes and mitochondrial DNA. Function: - The ‘powerhouses’ of the cell, producing almost all the energy required by the cell. - The second stage of respiration, The Krebs cycle takes place in the matrix. - The final stage of respiration, the electron transport chain, takes place on the very large surface area of the cristae under the influence of an enzyme called ATP synthase. Ribosomes Found in large numbers in the liver. Structure - Small granular structures made of two sub-units. - Made of RNA (ribonucleic acid) and protein. - Found free in cytoplasm or attached to folded membranes Function - Protein synthesis - Free ribosomes make protein used by cell and those on tubes make proteins for export. Plant Cells Chloroplasts Structure (10) - double membrane - contain chlorophyll and DNA - self-duplicating Both mitochondria and chloroplasts have a double membrane and DNA. Having DNA supports the theory that chloroplasts and mitochondria were once independent prokaryotic organisms that lived symbiotically inside large eukaryotic cells. Function - used to make food(carbohydrates) by photosynthesis - light phase in grana and dark phase in stroma Cell wall Secreted by cell membrane. Structure - Made of cellulose. - Adjacent cells are cemented together by the middle lamella of pectin. Function - Gives strength and protection to the cell. - Controls cell growth and shape. - Prevents osmotic bursting of cell membrane. - Fully permeable to gases and water. Vacuoles Usually only one in plants. Very large & permanent. Small, temporary, and more in animals because they excrete their waste (often called vesicles). Structure - Fluid-filled spaces surrounded by a membrane Function - Temporary storage of food (sugars, amino acids, fats), water, salts (help in osmoregulation), pigments, tannins, gases (O2 & CO2) and excretory products. - The cell sap makes the cells turgid.