Cell Structure PDF

**Podcast info ** Cell structure Rey points 1 A unicellular organism has one single cell. \[ 2 A multicellular organism is made up of many cells. L 1. The ultrastructure of a cell is its fine structure as revealed at high magnification. 2. The ultrastructure of an animal cell include...

**Podcast info ** Cell structure Rey points 1 A unicellular organism has one single cell. \[ 2 A multicellular organism is made up of many cells. L 1. The ultrastructure of a cell is its fine structure as revealed at high magnification. 2. The ultrastructure of an animal cell includes its cell membrane, nucleus, cytoplasm, mitochondria and ribosomes. \[ 3. The ultrastructure of a plant cell includes its cell wall, cell membrane, nucleus, cytoplasm, sap vacuole, mitochondria and ribosomes. 4. Chloroplasts are present in green plant cells. O 5. The ultrastructure of a fungal cell includes its cell wall, cell membrane, nucleus, cytoplasm, vacuole, mitochondria and ribosomes. 6. The ultrastructure of a bacterial cell includes its cell wall, cell membrane, cytoplasm, plasmids and ribosomes. \_ 7. The nucleus contains genetic material (DNA) and controls cell activities. 8. The cell membrane controls the entry and exit of substances into and out of the cell. 9. Cell organelles, such as nuclei, mitochondria and chloroplasts, are compartments found in the cytoplasm and are the site of chemical reactions.\ Г 10. Bacterial cells do not contain organelles. 11. The cell wall is for support and shape and prevents plant cells from bursting. \[ 12. Plant, fungal and bacterial cell walls have different structures and are composed of different chemicals. 13. The vacuole stores a watery solution of salts and sugars and helps to support cells. L 14. Chloroplasts contain chlorophyll and are the sites of photosynthesis. 15. Mitochondria are the sites of aerobic respiration in cells. 16. Ribosomes are the sites of protein svnthesis in cells. L 17. In bacterial cells, plasmids hold some of the genetic material (DNA) of the cell. Cell structure Function Nucleus Contains genetic information (DNA) in animal, plant and fungal cells and so controls cell activities (see Key Areas 1.4 and 1.5 on pages 16 and 19) Plasmid A small ring of genetic material in a bacterial cell Cell membrane A selectively permeable membrane that controls entry and exit of substances such as 0,, COz, glucose and waste to and from all cells (see Key Area 1.2 on page 5) Cytoplasm Watery, jelly-like material within cells containing organelles that are the sites of various chemical reactions Cell wall The outer layer of plant, fungal and bacterial cells, which helps support the cell Vacuole Membrane-bound sac that stores a solution of water, salts and sugars and helps support plant and fungal cells Chloroplast Makes carbohydrate in green plant cells using light energy in the process of photosynthesis (see Key Area 1.7 on page 27) Mitochondrion Main site of ATP production in aerobic respiration in animal, plant and fungal cells (see Key Area 1.8 on page 32) Ribosome Site of protein synthesis in cells Summary notes Living organisms and cells The bodies of living organisms are made up of cells. Living organisms are either unicellular, with only one cell, or multicellular, with more than one cell. Organisms can be divided into groups. These include animals, plants, fungi and bacteria. Cell types Animal cells are surrounded by a cell membrane and contain a nucleus, ribosomes and mitochondria. Plant cells are usually bigger than other cells and are surrounded by a cellulose cell wall with a membrane inside. They contain a nucleus and cytoplasm with a large central vacuole, ribosomes and mitochondria. Green plant cells also have chloroplasts. Fungal cells are surrounded by a cell wall with a membrane inside and contain a vacuole, a nucleus, ribosomes and mitochondria. Bacterial cells are usually very much smaller than other cells and are surrounded by a cell wall with a membrane inside. Their cytoplasm contains plasmids and ribosomes, but no organelles. Cell structure The fine structure of cells, which can only be seen using a high-magnification microscope, is called ultrastructure. Ultrastructure includes organelles such as nuclei, mitochondria and chloroplasts, which are specialised compartments found in most cells. Other ultrastructure features such as ribosomes and plasmids are not organelles. Some structures, such as ribosomes, are found in cells from all types of organism. Figure 1.1 shows the general structure of different cells. (a) cell membrane cytoplasm nucleus ribosome mitochondrion \(b) cell wall cell membrane ribosome cytoplasm nucleus chloroplast vacuole mitochondrion \(c) cell wall cell membrane nucleus ribosome cytoplasm mitochondrion vacuole (d) cel wall cell membrane chromosome ribosome cvtoplasm plasmid Figure 1.1 General structure of different cells (not to scale): (a) typical animal cell - a human cheek epithelial cell: (b) typical plant cell - a leaf mesophyll cell; (c) typical fungal cell - a yeast cell; (d) typical bacterial cell - a bacillus Functions of cell structures The different structures present in cells have different functions in the life of the cell. The different functions are shown in the table below and some of these are covered in other Key Areas, as indicated. Cell structure Function Nucleus Contains genetic information (DNA) in animal, plant and fungal cells and so controls cell activities (see Key Areas 1.4 and 1.5 on pages 16 and 19) Plasmid A small ring of genetic material in a bacterial cell Cell membrane A selectively permeable membrane that controls entry and exit of substances such as 0,, CO2. glucose and waste to and from all cells (see Key Area 1.2 on page 5) Cytoplasm Watery, jelly-like material within cells containing organelles that are the sites of various chemical reactions Cell wall The outer laver of plant, fungal and bacterial cells, which helps support the cell Vacuole Membrane-bound sac that stores a solution of water, salts and sugars and helps support plant and fungal cells Chloroplast Makes carbohydrate in green plant cells using light energy in the process of photosynthesis (see Key Area 1.7 on page 27) Mitochondrion Main site of ATP production in aerobic respiration in animal, plant and fungal cells (see Key Area 1.8 on page 32) Ribosome Site of protein synthesis in cells (see Key Area 1.5 on Note on cell walls Cell walls are found in plant, fungal and bacterial cells. The walls of cells from these different groups of living organism differ in their structure and in the chemical substances of which they are made. Plant cell walls are composed of cellulose, whereas those of fungi and bacteria are varied in structure and chemical composition. h Pacific Ocean Fungal Bacterial Hints Er tips Remember that the cell walls of different types of organism are similar but have different structures and chemical compositions. Key words Aerobic respiration - release of energy from food by a cell using oxygen Bacterial cell - a tiny individual cell of a bacterium Cell membrane - selectively permeable membrane enclosing the cell cytoplasm and controlling the entry and exit of materials Cell wall - supports and prevents cells from bursting; plant, fungal and bacterial walls have different structures and chemical compositions Cellulose - structural carbohydrate of which plant cell walls are composed Chloroplast - organelle containing chlorophyll; the site of photosynthesis Cytoplasm - jelly-like liquid containing cell organelles and the site of many chemical reactions DNA - deoxyribonucleic acid; substance in chromosomes that carries the genetic code of an organism Fungal cell - individual cell of a fungus Mitochondrion - organelle that is the site of aerobic respiration and All production in cells (pl. mitochondria) Multicellular - having many cells Nucleus - organelle that is the control centre of a cell containing the genetic information of the organism (pl. nuclei) Organelle - membrane-bound compartment with a specific function in animal, plant and fungal cells Photosynthesis - process carried out by green plants to make their own food using light energy Plasmid - circular genetic material present in bacterial cells and used in genetic engineering or modification Protein - substance composed of chains of amino acids and containing the elements carbon, hydrogen, oxygen and nitrogen Ribosome - site of protein synthesis Ultrastructure - fine structure and detail of a cell and its organelles revealed by an electron microscope Unicellular - single-celled Vacuole - membrane-bound sac containing cell sap in plant and fungal cells Key Area 1.2 Transport across cell membranes Rey points 1. The cell membrane is composed of lipid and protein. 2. The cell membrane is selectively permeable. O\ 3 A difference in concentration of a substance is a concentration gradient. \[ 3. Passive transport of a substance does not require additional energy and describes the movement of its molecules down the concentration gradient from a high concentration to a lower concentration. O 4. Diffusion and osmosis are examples of passive transport processes. L 5. Diffusion is the movement of substances from a high concentration to a lower concentration down a concentration gradient. L 6. Examples of substances that enter most cells by diffusion are oxygen, glucose and amino acids. O 7. Examples of substances that leave most cells by diffusion are carbon dioxide and urea. O 8. Diffusion is important to cells because it helps provide the cell with raw materials and helps to remove waste products. \[ 9. Osmosis is the movement of water from a region of high water concentration to a region of lower water concentration through a selectively permeable membrane. O 10. An animal cell placed in a solution with a water concentration higher than that inside the cell will take up water by osmosis and could burst. D 11. An animal cell placed in a solution with a lower water concentration than that inside the cell will lose water by osmosis and could shrink. L 13 A plant cell placed in a solution with a water concentration higher than that inside the cell will take up water by osmosis and become turgid. \[ 14 A plant cell placed in a solution with a lower water concentration than that inside the cell will lose water by osmosis and become plasmolysed. L 1. Plasmolysed is the term used to describe a cell in which the vacuole has shrunk due to water loss, causing the cell membrane to pull away from the cell wall. 2. Turgid is the term used to describe a cell or tissue in which the vacuole has swollen due to water gain and presses the cytoplasm and cell membrane against the cell wall. \[ 3. Active transport is the movement of molecules from a region of low concentration to a region of higher concentration, against the concentration gradient. O 4. Active transport requires additional energy (ATP) to allow membrane proteins to move molecules against the concentration gradient. Summary notes Cell membrane structure Cell membranes are at, or just inside, the boundaries of all cells. Membranes are extremelv thin and are composed of protein and lipid molecules in a layered arrangement containing pores (Figure 1.2). protein molecule double layer of lipid molecules pore Figure 1.2 Structure of the cell membrane Transport into and out of cells The cells of a living organism exchange substances with each other and with their surroundings. This can happen in several ways including diffusion, osmosis and active transport. These terms all refer to the movement of molecules and involve concentration gradients, which are differences in concentration of these molecules between regions. Diffusion and osmosis are passive because they do not require input of additional energy but active transport is active because of the additional energy it requires. Diffusion Diffusion is the passive movement of molecules from a region of high concentration to a region of lower concentration, which continues until they are evenly spread out. In living cells diffusion often occurs through membranes (Figure 1.3). Concentration Diffusion of gradient - high oxygen molecules concentration of oxygen to low No concentration gradient - oxygen molecules evenly spread concentration of oxygen high concentration of oxygen molecules \- membrane low concentration of oxygen molecules Time Figure 1.3 Diffusion of oxygen molecules through a cell membrane A good example of diffusion in action is the exchange of gases in the lungs of a mammal. Oxygen molecules move from a region of high concentration in the lungs to a region of lower concentration in the red blood cells (Figure 1.4). Carbon dioxide moves by diffusion in the opposite direction. Diffusion is also the method of absorption of glucose and other foods through the small intestine of a mammal. 1 Breathes air with high concentration of oxygen molecules into lung spaces 2 Oxygen molecules diffuse from high concentration in lung spaces to lower concentration in the blood 3 Blood transported to cells of body Oxygen molecules diffuse from high concentration in blood to lower concentration in body cells Figure 1.4 Diffusion of oxygen in the body of a mammal Hints b tips ( Glucose and Oxygen enter cells by diffusion where they are involved in Respiration to release Energy. Remember: GORE! Osmosis Osmosis is a special case of diffusion. It is the passive movement of water molecules from a region of high water concentration to a region of lower water concentration. Water movement by osmosis is always through a selectively permeable membrane (Figure 1.5). Concentration gradient - high concentration of water to low concentration of water high concentration of water molecules Movement of No concentration water molecules gradient - water by osmosis molecules evenly spread pore 117889000 -membrane low concentration of water molecules Time Figure 1.5 Osmosis A good example of osmosis in action is the uptake of water into the root cells of plants. When the soil is moist after rain, the soil water concentration is higher than that of plant root cells. Water therefore passes from high concentration in the soil to lower concentration in the root cells by osmosis. There is more about osmosis in plant roots  Active transport Active transport is the movement of molecules from a region of low concentration to a region of higher concentration against a concentration gradient. Active transport requires energy. In some cases it is essential that cells with high concentration of a substance receive more of that substance from areas where the substance is at a lower concentration. A good example of this is found in certain unicellular freshwater plants such as Nitella. The cells of this plant require potassium ions from their environment to survive. However, potassium ion concentrations in freshwater are very low. Nitella cells use energy from their respiration process to take up potassium ions from low concentrations in freshwater to the higher concentrations that must be maintained in their cells. The energy goes to the cell membrane proteins, which carry the ions through the membranes from outside to inside (Figure 1.6). INTINA low concentration of potassium ions outside Nitella cells potassium ion. carrier protein molecule double layer of lipid molecules high concentration of potassium ions inside Nitella cells Figure 1.6 Active transport of potassium ions in Nitella Another example of active transport involves the movement of potassium and sodium ions through the membranes of nerve cells. Active transport in nerve cells To function normally nerve cells need to maintain high concentrations of potassium ions inside and high concentrations of sodium ions outside the cells. To do this, their membranes have special proteins called sodium-potassium pumps. These proteins use energy to pump sodium ions out of the cell and pump potassium ions in (Figure 1.7). The energy source is a substance called ATP, which is formed using energy released during respiration. sodium ion potassium ion outside nerve cell protein carrier in membrane double lipid layer inside nerve cell Figure 1.7 Sodium-potassium pump in nerve cells Animal cells and water The results of placing animal cells in solutions of different water concentration can be shown using red blood cells (Figure 1.8). If animal cells are placed in a solution with a water concentration higher than the cell contents, water is taken in by osmosis and the cells might burst. If placed in a solution of lower water concentration than the cell contents, water will be lost by osmosis and the cell will shrink. normal red blood cell water passes in from high water concentration to low water concentration higher water concentration equal water concentration lower water concentration no net gain or loss of water and cell remains unchanged selectively permeable membrane water passes out from high water concentration to low water concentration Figure 1.8 Effects of water on a red blood cell Plant cells and water The results of placing plant cells in solutions of different water concentration are shown in Figure 1.9. If plants cells are immersed in a solution with a water concentration higher than the cell contents, they will take up water by osmosis and their vacuoles will fill. Further intake of water is prevented because vacuole size is limited by the presence of the cell wall. Plant cells that have full vacuoles are said to be turgid, the normal state for a plant cell. Plant cells immersed in a solution with a lower water concentration than the cell contents lose water by osmosis and their vacuoles shrink and eventually pull the cell membrane and cytoplasm away from the cell walls of the cell. This is not desirable for the plant and may actually kill it. Cells with membranes pulled away from their walls are said to be plasmolysed. membrane pressed to wall vacuole swells normal plant cell fully permeable cell wall water passes in from high water higher water concentration to low water concentration concentration and cell gains turgor (tissue in this state is turgid) equal water concentration lower water selectively permeable concentration membrane no net gain or loss of water and cell remains unchanged vacuole shrinks cell membrane pulls away from cell wall water passes out from high water concentration to low water concentration, vacuole shrinks, cell membrane pulls away from cell wall and cell becomes plasmolysed (tissue in this state is flaccid) Hints b tips Remember that diffusion and osmosis are passive processes - they don\'t need extra energy. Osmosis is a type of diffusion but always involves water passing through a membrane. For your exam you must remember that active transport requires energy. Hints E tips To take up substances against a Concentration gradient, cells use Oxygen for Respiration to release Energy for the process of active transport. Remember: CORE Sessi Questic A Resti 1. Namet 2. Give tv 3. State El 4. State c 5 Give a) b) 6 Give 7 State B 2 Ex Des De Figure 1.9 Effects of water on a plant cell Key words Active transport - transport of molecules against their concentration gradient Concentration gradient - difference in concentration between two solutions, cells or solutions and cells Diffusion - passive movement of molecules from an area of high concentration to an area of lower concentration Lipid - fat or oil with molecules composed of fatty acids and glycerol Osmosis - movement of water molecules from an area of high to an area of lower water concentration through a selectively permeable membrane Passive transport - movement of molecules without the need for additional energy, e.g. diffusion and osmosis Plasmolysed - description of a plant cell in which the vacuole has shrunk and the membrane has pulled away from the wall due to water loss Selectively permeable - refers to a membrane that controls the movement of certain molecules depending on their size Turgid - description of a swollen plant cell with a full vacuole resulting from water intake due to osmosis