Biology 12 Cell Structure and Function. Typical Animal Cell

Transcription

1 Biology 12 Cell Structure and Function Typical Animal Cell Vacuoles: storage of materials and water Golgi body: a series of stacked disk shaped sacs. Repackaging centre stores, modifies, and packages proteins for export out of cell. Lysosome: has a single membrane, pinched off from Golgi body, contains hydrolytic enzymes for intracellular digestion and cell destruction (suicide bags). Nucleus contains chromosomes coiled together as chromatin Nuclear pore: allows materials into nucleus, allows mrna to leave nucleus Free floating ribosomes: site of protein synthesis Nucleolus: where rrna is produced Ribosomes: site of protein synthesis. Nuclear membrane (envelope): double layer protective membrane Cytoplasm: mainly water Rough endoplasmic reticulum: contains ribosomes on surface. Cell membrane: selectively permeable Smooth endoplasmic reticulum: no ribosomes on surface, where steroid hormones and some carbohydrates are synthesized. Mitochondrion (pl. mitochondria): Double layer membrane, site of many of the chemical reactions involved in energy (ATP) production. Glycogen - stored glucose for energy. Glucose is converted to ATP mainly in the mitochondria. Glycogen found in large quantities in muscle and liver cells

2 Organelles: Specialized structures within cells such as the nucleus, mitochondrion, ER etc. These are usually made of plasma membranes, exception - ribosomes do not have a membrane. Nucleus: Largest organelle in the cell, enclosed by a double layer membrane. Control center of cell - controls metabolic functioning of cell & determines cell s characteristics. Contains chromatin - DNA and proteins - during cell division, chromatin condenses to form individual chromosomes. The two most important chemicals in the nucleus are DNA and RNA. Nucleolus: Dark circular body in the nucleus - specialized area of chromatin which produces rrna (ribosomal RNA) which is a component of ribosomes. Therefore, nucleolus helps produce ribosomes. Nuclear Pores: Openings in nuclear envelope (membrane) to allow large molecules to pass from nucleoplasm to the cytoplasm. Chromatin: The genetic material of the cell made of DNA and proteins. During cell division, chromatin condenses to form the chromosomes. Human body cells have 46 chromosomes. Human sex cells have 23 chromosomes.

3 Fluid Mosaic Model of Membrane Components of Cell Membrane: 1. Phospholipids - two layers of phospholids form a cell membrane. Arrangement of phospholipids gives the membrane its fluid properties - membrane flexible Arrangement of phospholipids gives membrane its selectively permeable properties. Polar heads prevent charged particles (H +, Na +, Cl -, amino acids, glucose) from diffusing through phospholipid bilayer. Allow non polar molecules (O 2, CO 2) and small polar (non charged) molecules like H 2O to diffuse through phospholipid bilayer. 2. Embedded proteins-wholly or partially embedded within phospholipid bilayer, give structure. Functions of embedded proteins: a) Carrier proteins - form protein pores, these allow small charged ions (H +, Na +, Cl - ) and large polar molecules (glucose, amino acids) to diffuse in/out of cell. Act as pumps actively transporting molecules in/out of cell. (ions, glucose, amino acids) b) Cell recognition proteins - with glycolipids and glycoproteins, give cell its distinctive antigens or surface features. 3. Glycolipids - carbohydrate attached to phospholipid. Glycoproteins - carbohydrate attached to embedded protein Both serve in cell recognition (antigens) and receptor (docking) sites on cell membrane. (e.g. for protein hormones) These 2 components are found on cell membrane only, not on other membranes inside cell. 4. Cholesterol - help maintain structure and fluidity of membrane.

4 Ribosomes: Made up of ribosomal RNA (rrna) and protein, are not enclosed in a membrane. Site of protein synthesis. Found attached to rough endoplasmic reticulum or free floating in cytoplasm. Attached ribosomes - produce proteins for export (exocytosis) out of cell Free ribosomes - produce proteins to be used inside cell Polysomes a group of ribosomes that translates (reads) a mrna during protein synthesis. Endoplasmic Reticulum: Forms a membranous system of tubular canals that begins at the nuclear membrane and branches throughout cytoplasm Rough ER - have ribosomes attached to surface acts as transport of polypeptides (made at ribosomes) through cell. Rough ER - where protein folding occurs Smooth ER - does not have ribosomes attached - acts as transport (like rough ER) Smooth ER - contains enzymes to detoxify drugs and alcohol (liver cells) and synthesize lipids like steroid hormones (Cells of ovary and testes produce steroid sex hormones) Detoxify = inactivate potentially harmful drugs (including alcohol) by converting them to water soluble compounds that can be eliminated from the body in urine. Golgi apparatus (Golgi Body) Concentrically folded membrane, not continuous with nuclear membrane. Receives transport vesicles from ER. Functions as: 1. Sorting and repackaging centre - package protein into secretory vesicles so protein can be secreted out of cell through cell membrane (exocytosis) or so protein incorporated into cell membrane. 2. Modification of polypeptides (e.g. adding glycogen/sugar chains to protein). 3. Produce lysosomes (contain proteins which are hydrolytic enzymes).

5 Vacuoles: Membrane bound often fluid filled cavities which act as storage sites in cell. Vesicles: Small vacuoles that can be made at ER, Golgi apparatus or from the folding of the cell membrane as in endocytosis. Types of vesicles 1. Transport vesicles - made at ER and transport polypeptides to Golgi apparatus. 2. Secretory vesicles - made at Golgi apparatus and transport to cell membrane for secretion out of cell by exocytosis. Lysosomes: Special type of vesicle made at Golgi apparatus. Contains hydrolytic enzymes for digestion within cell. Functions: 1. Attach to vacuoles and release hydrolytic enzymes to digest contents of vacuole. 2. Suicide bags - release enzymes to destroy cell (cell suicide). Example of lysosome function When white blood cells (macrophages) engulf bacteria or cellular debris these materials are incorporated into a vacuole. Lysosomes attach to these endocytic vacuoles and release hydrolytic enzymes which digest materials in vacuole, therefore, digesting engulfed materials. Intracellular Transport Summary of relationships between organelles: Flagella a whip like tail used for motility (movement) human sperm cell structures - the tail is also called a flagella Cilia tiny hairs that project from the cell membrane used mainly as sweepers in the human body found in the trachea and bronchi (air ways) to sweep out debris before it can reach the lungs also found in the Fallopian tubes in female to sweep egg towards the uterus

6 Mitochondria: (singular: mitochondrion) Sometimes referred to as the powerhouse of the cell. Folded inner membrane increases surface area for chemical reactions to take place. Organelle where aerobic cellular respiration occurs. Most of the ATP - energy currency of cells - is produced here Micrograph of Mitochondrion Cytoskeleton a cellular scafold or skeleton contained within the cytoplasm - made out of protein maintains cell shape, protects the cell, enables cellular movement and plays important roles in intracellular transport (the movement of vesicles and organelles) Cell size: To increase the surface area to volume ratio - many tissues and cells (and components of cells) are highly folded. Example - inner membrane of mitochondria, lining of intestine and stomach, alveoli. When cells grow too big their surface area to volume ratio decreases, therefore, cells divide before they become to big to supply nutrients by diffusion.

7 Movement of Substances in and out of Cell: a. Diffusion - the movement of molecules from an area of high concentration to an area of low concentration until the molecules are equally distributed. b. Osmosis - the diffusion of water, the movement of water molecules from an area of high concentration to an area of low concentration across a differentially permeable membrane. Factors that affect the rate of diffusion: 1. Increased temperature increases kinetic molecular motion and so increases rate of diffusion. 2. Smaller molecules will diffuse more quickly. Cells membranes are permeable to small molecules only e.g. unit molecules 3. Increasing the concentration gradient will increase rate of diffusion. e.g. mountain climbing at extreme altitudes - decreased O 2 concentration (PO 2) in air as increase altitude results in decreased concentration gradient and less diffusion from alveoli into blood stream. Osmotic Pressure - pressure generated by osmotic flow of water. e.g. keeps plants from wilting. Tonicity - refers to the concentration of solutes in a solution. a) Hypertonic - a solution that has a greater concentration of solute and a lesser concentration of solvent (water) as compared to a hypotonic solution. b) Hypotonic - a solution that has a lesser concentration of solute and a greater concentration of solvent (water) as compared to a hypertonic solution. c) Isotonic - concentration of solute is same in 2 or more solutions e.g. inside and outside of cell.

8 Effects of Solution on Animal Cells: An animal cell placed in a 1. Hypertonic solution will shrink because water diffuses out of the cell. This is called crenation. Example: A cell placed in a salt solution shrinks, so eating salty foods leads to a sensation of thirst An animal cell placed in a 2. Hypotonic solution will swell because water diffuses into cell. This is called lysis because the functioning of cell will be disrupted (cell may even burst). Example Normal Red Blood Cells Placed in a salt solution (Hypertonic solution) Placed in water (Hypotonic solution)

9 Transport of Substances across the Cell Membrane 1. Passive Transport - no energy (ATP) required diffusion with concentration gradient. a) Diffusion (osmosis) - non polar substances (O2, CO2 and alcohol) and water (small) diffuse through phospholipid bilayer from areas of high concentration to low concentration. b) Facilitated transport - diffusion of large polar molecules (glucose, amino acids and fatty acids) and ions through carrier protein (protein pore or channel) 2. Active Transport - energy (ATP) required, may move substances against concentration gradient (low to high concentration). Types of Active Transport: a) Na+/K+ pump - requires ATP and carrier protein (pump) Exocytosis and endocytosis - uses the cell membrane, requires ATP, and may move substances against the concentration gradient. b) Endocytosis - brings substances into cell, uses cell membrane to form vesicle, and requires ATP At 1 cell membrane (phospholipid bilayer) begins to deform (wrap around substance to be engulfed At 2 a vacuole forms around substances engulfed At 3 notice the vacuole membrane (phospholipids bilayer) was originally part of the cell membrane.

10 There are 2 types of endocytosis phagocytosis and pinocytosis I. Phagocytosis - called cell eating, it is the process of engulfing large materials like bacteria and cellular debris (parts of destroyed cells) II. Pinocytosis - called cell drinking, it is the process of engulfing large molecules like protein. Notice the difference between the two types of endocytosis is in the size of the material being engulfed. Even though the protein engulfed during pinocytosis ( cell drinking ) is smaller than the bacteria and cellular debris engulfed during phagocytosis ( cell eating ), protein is still a large molecule. c) Exocytosis - substances secreted from cell when vesicle fuses with inner side of cell membrane, requires ATP Summary of exocytosis and endocytosis see diagram below. Notice the role of the cell membrane in both endocytosis and exocytosis. Both of these processes require ATP and both are considered active transport.