CELL STRUCTURE & FUNCTION

CELL STRUCTURE & FUNCTION

CELL TYPES Living cells can be classified into 2 different types on the basis of their internal structure: 4. Prokaryotic Cells 5. Eukaryotic Cells 1. Prokaryotic Cells Are the smallest of living cells. Examples include bacteria, microbes, cyanobacteria or blue-green algae.

Prokaryotic Cells Continued.. Most are very small, with diameters from 0.5-1.0micrometers (1µm = 0.001 mm). Prokaryotes are never truly multicellular but some form colonies of cells. Prokaryotic Cells: vi. Have a cell membrane & cytosol vii. Lack membrane bound organelles viii. Have no nuclear membrane to house their DNA

Prokaryotic Cells Continued..

2. Eukaryotic Cells Are larger than prokaryotic cells, ranging in size from 10µm - 100µm. Animals, fungi, plants & protists (protists are unicellular or very simple multicellular life forms) have eukaryotic cells. Eukaryotic cells are often specialized & organized into complex life forms.

Eukaryotic Cells Continued Eukaryotic Cells: iii. Contain membrane bound organelles suspended in a fluid & semi-fluid matrix known as cytosol. (Membrane, organelles & cytosol together is known as the cytoplasm Have a distinct double layered nuclear membrane DNA is contained within the nucleus

Eukaryotic Cells Continued

VIRUSES Viruses are non-living infectious agents which share some characteristics with living things. Viruses consist of a nucleic acid core (DNA or RNA) with a coat of protein, known as a capsid. Some viruses are also enveloped in a modified membrane from a host cell. Outside the cells of living organisms viruses are inert they do nothing.

Viruses Continued.. Inside a host cell, the virus nucleic acid is activated. Viruses reproduce inside the cells of other organisms, using the host cells energy sources & raw materials to produce more viruses. Ultimately the host cell dies releasing more viruses to infect other cells.

CELL SPECIALIZATION Eukaryotic cells vary a great deal in structure. Variations in structure are related to function. Cells vary in the number of each organelle present, their shape, size, & life cycle.

Cell Specialization Continued.

CELLULAR ORGANELLES A light microscope allows examination of cells. Not all organelles are visible with a light microscope for the following reasons: v. Many organelles are too small to be seen at a magnification of 400x vii. The image is 2D. Any focused view through a light microscope shows only a plane in the 3D structure. viii. Many organelles are transparent. Staining with various chemicals can highlight particular organelles. Staining, however kills cells, removing any chance of observing movement.

Cellular Organelles Continued SEM & TEM allow the fine detail of organelles to be seen. Below are diagrams of generalized animal & plant cells. A generalized cell shows the organelles you are likely to find in a particular cell type, but does not show the number of these organelles, nor does it accurately show their relative sizes. Size & Number - related to cell specialization.

Cellular Organelles Continued Generalized Plant Cell

Cellular Organelles Continued. Generalized Animal Cell

Cellular Organelles Continued.. Differences Between Animal & Plant Cells

THE CELL MEMBRANE All cells (eukaryotic & prokaryotic) are surrounded by a cellular membrane. Is ultra-thin with average thickness of less than 0.01µm (0.00001mm). Cell membranes form the boundary of cells. They maintain the internal environment of the cell by controlling the movement of substances into & out of the cell.

The Cell Membrane Continued. Cell membranes are composed of a phospholipid bilayer, with proteins embedded throughout the phospholid. The phospholipid & protein components of the membrane allow the passage of different molecules. Carbohydrate chains are involved in cell recognition.

The Cell Membranes Continued

The Cell Membrane Continued. Cell membrane are selectively (semi-/partially/ differentially) permeable, allowing some substances to pass between the internal & external environments, but not others. Several factors are important in determining the right of way : 1. The Size of the Particles Small molecules pass through the cell membrane more readily than large molecules. Large charged molecules are the exception to this rule & may pass through easily.

The Cell Membrane Continued. 2. The Solubility of Particles in Water The liquids that bathe cells are usually solutions of molecules & ions in water. Substances which don t dissolve in water can t pass through the cell membrane. These are termed hydrophobic. Those which dissolve readily in water are termed hydrophilic. 3. Conditions Inside or Outside the Cell If [ ] of particles is greater on the inside of the cell compared with the outside, particles tend to leave the cell.

The Cell Membrane Continued 4. Structure of the Cell Membrane There are spaces between the molecules that make up the cell membrane which allow the passage of certain substances. These spaces may be too small for large molecules to pass through, but large enough for smaller molecules. The rates & degrees at which certain substances penetrate the cell membrane varies.

INTERNAL & EXTERNAL ENVIRONMENTS OF CELLS All cells need to exchange materials with their environments to grow, reproduce & survive. This exchange can only occur through the cell s membrane. SURFACE AREA TO VOLUME RATIO Area refers to the coverage of a surface. - one unit of measurement of area = 1 square centimeter (cm2)

Surface Area to Volume Ratio Continued.. Volume refers to the amount of space taken up by an object. - usually expressed as cubic centimeters (cm3) SA:V ratio of an object identifies how many units of external surface area are available to supply each unit of internal volume. The larger a cell becomes, the smaller its surface area to its volume ratio.

Surface Area to Volume Ratio Continued.. This cube has a side length of 1cm. Each face has an area of 1cm2. The total Surface Area is 6cm2. Its Volume is L x W x D = 1 x 1 x 1 = 1cm3. The ratio of SA:V is 6:1 This cube has a side length of 2cm. Each face has an area of 4cm2.The total Surface Area is 24cm2. Its Volume is L x W x D = 2 x 2 x 2= 8cm3. The ratio of SA:V is 24:8 or 3:1

Surface Area to Volume Ratio Continued.. When a cell grows its volume increases at a greater rate than its surface area. If cells become too large they are unable to carry out sufficient exchange of materials with the environment. Prokaryotic cells, which lack storage organelles & specialized organelles for cellular processes, must be even smaller to allow exchange of sufficient materials for their life processes.

MOVEMENT OF MATERIALS THROUGH CELL MEMBRANES 1. Simple Diffusion Is defined as: The net movement of molecules from a region where they are at relatively high concentration to a region where they are at a lower concentration.

Simple Diffusion Continued The difference in concentration between the 2 regions is known as the concentration [ ] gradient. Diffusion occurs when ever a concentration gradient exists & will proceed until all molecules are evenly distributed. When this happens equilibrium has been achieved. It occurs mainly in gases & liquids

Simple Diffusion Continued Why Does Diffusion Occur? Diffusion occurs because: v. There is a great deal of space between the molecules of all substances. vii. All molecules are in a state of constant random motion BROWIAN MOTION so they constantly collide & intermingle.

Simple Diffusion Continued Factors Which Affect the Rate of Diffusion 1. HEAT Warmer molecules diffuse faster than cooler ones. Increase in temperature causes the molecules to move & collide at faster rates. This increases rate of dispersal & equilibrium is reached much faster. 2. SIZE OF MOLECULES Small molecules diffuse faster than large molecules. 3. CONCENTRATION OF SUBSTANCES More concentrated substances diffuse faster than less concentrated substances.

Simple Diffusion Continued Processes Which Rely on Diffusion Excretion in some simple animals Gas exchange Digestion in some simple plants & animals A Living Organism Absorption of mineral salts by plant roots from soil Photosynthesis in green plants

Simple Diffusion Continued Diffusion requires no energy to proceed, so is referred to as a passive process. It relies on substances following their concentration gradients. The steeper the gradient, the faster the rate of diffusion. Two other forms of diffusion exist: vi. Facilitated Diffusion vii. Osmosis Like simple diffusion, these are also passive processes, requiring no energy.

i. Facilitated Diffusion Sometimes the process of diffusion requires the assistance of other molecules, such as proteins, to be successfully carried out. This is known as facilitated diffusion. Facilitated diffusion is often used when larger molecules, such as glucose, need to move across the cell membrane. 2 types of proteins are involved in facilitated diffusion: viii. Channel Proteins ix. Carrier Proteins

Facilitated Diffusion Continued.. i. Channel Proteins Facilitated diffusion occurs when channel proteins open & close areas the cell membrane to allow molecules or ions to pass through. The channels allow some substances to pass through but not others. The channels select which molecules the cell does or does not need.

Facilitated Diffusion Continued. Facilitated diffusion through a channel protein in the cell membrane of a cell. A concentration gradient is required.

Facilitated Diffusion Continued.. i. Carrier Proteins Carrier proteins are embedded in the cell membrane. Carrier proteins bind to molecules or ions on one side of the membrane, change shape & release the particular molecule or ion on the other side of the membrane.

Facilitated Diffusion Continued.. The Carrier protein picks up the particles on one side of the membrane, changes shape & releases them on the other side. Again a concentration gradient is required.

ii. Osmosis Describes the movement of water across a membrane. Is defined as: The net movement of water molecules, across a differentially permeable membrane, from an area in which water is in high concentration (low solute concentration) to an area where water is in a low concentration (high solute concentration).

Osmosis Continued. The [ ] gradient results from the substances dissolved in water, known as the solute, not the water itself. For Example: When a substance such as Sucrose (solute), is dissolved in water (solvent), the [ ] of water to decrease. This causes water to move across the membrane from an area of low solute [ ] (high water [ ]) to an area of high solute [ ] (low water [ ]).

Osmosis Continued

Osmosis Its Affect on Plant & Animal Cells

2. Active Transport Is defined as: The net movement of dissolved substances into or out of cells against a concentration gradient, that is, from an area of low concentration to an area of higher concentration. This requires energy to proceed. ATP (adenosine triphosphate) provides the energy required. Active transport involves the use of carrier proteins, which along with ATP, move molecules or ions into & out of cells against their [ ] gradients.

Active Transport Continued. Active transport enables cells to maintain stable internal conditions, in spite of extreme variations in external surroundings. Active transport via a carrier protein in cell membrane. Energy is transferred to the carrier protein. This enables the protein to move the particles against a [ ] gradient

LEVELS OF ORGANIZATION Unicellular organisms carry out all the metabolic processes necessary for life. They are cells that are capable of independent existence. Multicellular organisms have millions of cells that depend on each other for survival. During their development, groups of cells become specialized to perform a particular function, that serve the whole organism.

Levels of Organization Continued. Specialized cells have fewer functions that those found in unicellular organisms, but their functions are very highly developed. Multicellular organisms have different levels to the organism that interact to ensure proper functioning for the whole organism. Levels of Life: Cells Organ Systems tissues Organs Organism

Levels of Organization Continued.

Levels of Organization Continued..