Module 1_ Cells as the Basis of Life PDF

Module 1: Cells as the Basis of Life Cell Organelles WHAT IS A CELL - The smallest structural and functional unit of a living organism. WHAT IS AN ORGANELLE - A membrane-bound compartment within the cell, held in the cytoplasm. ORGANELLES THAT MAKE AND PROCESS CELL SU...

Module 1: Cells as the Basis of Life Cell Organelles WHAT IS A CELL - The smallest structural and functional unit of a living organism. WHAT IS AN ORGANELLE - A membrane-bound compartment within the cell, held in the cytoplasm. ORGANELLES THAT MAKE AND PROCESS CELL SUBSTANCES ORGANELLES FUNCTION NUCLEUS - holds the cell's genetic information. - It holds chromosomes (containing DNA) - The control centre of the cell as it provides instructions on what the cell needs to do and coordinates the biomechanical activities. - The nuclear membrane has small gaps called nuclear pores. - The nucleus contains the nucleolus which is where the ribosomes are made and is made of proteins and ribonucleic acid (RNA). DNA - Stands for deoxyribonucleic acid - Is the main form of genetic material found in a cell, containing instructions for cellular activity. GENES - Consists of DNA that contains the ‘code’ for making proteins. RIBOSOMES - They are made of ribosomal RNA and protein. - Sometimes not considered organelles as they are not membrane-bound. - Ribosomes make proteins - They are connected to the endoplasmic reticulum unless they are floating around the cytoplasm. ENDOPLASMIC RETICULUM (ER) - The ER is connected to the nucleus in some parts. - Rough ER - A network of flattened, connected - Smooth ER membranes. - It creates a ‘tunnel’ in which substances can move in. The rough ER - Has ribosomes attached to it - Modifies and processes the proteins made by the ribosomes. The smooth ER - Makes lipids (fat) GOLGI BODY - Flat membrane sacs stacked on top of each other. - Its role is to package and process substances the cell has made. Eg, proteins made in the rough ER can be sent to the Golgi body. - After processing, the membrane pinches off around the substance, forming a bubble or a vesicle. - This vesicle transports the substance to specific parts of the cell or out of the cell (excreted). It transports substances to wherever they are needed. LYSOSOMES - They are a vesicle made by the Golgi body. - Membrane-bound sacs that contain digestive enzymes (breaking down enzymes) ENZYMES - Proteins that act as catalysts (speed up biochemical reactions) ORGANELLES INVOLVED IN ENERGY TRANSFORMATION ORGANELLES FUNCTION CHLOROPLASTS - Disk-shaped organelles - Only found in plants - Have their own DNA - Have a green pigment called Chlorophyll  - They use the sun's energy (light) to perform photosynthesis, producing a chemical energy called glucose. MITOCHONDRIA - Found in both plant and animal cells. - Have their own DNA - They perform chemical respiration (combine oxygen and glucose to create ATP). - The number of mitochondria depends on how much energy the cell needs. The cells can not use the energy developed by the chloroplasts, so the mitochondria use respiration to transform the energy into ATP energy, an energy the cell can use. ORGANELLES INVOLVED IN CELL STRUCTURE AND STORAGE ORGANELLES FUNCTION CELL MEMBRANE - It separates the cell contents from the outside environment (holding the organelles together and protecting them) - Its semi-permeable so that it can control what goes in and out of the cell. CELL WALL - Cell wall is found in plant cells, fungal cells and some prokaryotic cells. - It is an external structure that surrounds the cell membrane. - Provides the cell with structural strength and protection. - The cell wall is permeable allowing most molecules through to the cell membrane. CYTOPLASM - Gives the cell shape - Suspends all of the organelles in the cell - Watery, gel-like fluid inside the cell membrane CYTOSOL - The liquid part of the cytoplasm.  - The cytosol allows organelles to have different compositions within the cell. - It also allows enzymes and reactants to be in high concentration so that processes within the organelles are efficient. - It also allows processes that require different environments to happen at the same time. - Makes the cell less vulnerable to external changes as the cytosol will get affected. CYTOSKELETON - The organelles are held together by a network of microtubules, microfilaments and intermediate filaments called the cytoskeleton. - It is a protein web. - Its main role is to hold the organelles in place. CENTRIOLES - Small cylinder structures are made up of microtubules (wound up together). - They divide the chromosomes apart (cell division). PILLI AND FLAGELLA - Flagella are the longer version of Pili - They are used to help the cell move around. - They are made of microtubules. - Flagella are like hair-like appendages. They act as a tail. VACUOLE - A membrane-bound vesicle (bubble), containing fluid. - The fluid is made of water or other dissolved substances, eg, sugars and salts - In plants, the fluid is called ‘sap’. - Its role is to store substances until the cell wants to use them. - Its size varies, but it is found in most cells - They are larger and permanent in plants, giving the plant cells structural support (or turgor  pressure, meaning the water pushes OUT on the cell walls helping the cell to keep its structure). - They are smaller and temporary in animals. The Fluid Mosaic Model Model for the cell membrane It is used to explain the structure and function of all cell membranes. STRUCTURE OF THE CELL MEMBRANE - The cell membrane is made up of 3 main things; phospholipids, cholesterol and proteins. - Cell membranes are phospholipid bilayers (2 layers of phospholipid molecules) - Phospholipid molecules consist of a phosphate head and 2 lipid tails. - The lipid tails hate water (hydrophobic) - The phosphate head loves water (hydrophilic) - The phosphate heads face the outside of the cell membrane as they like water - The lipid tails face inside (away from water). - Within the phospholipid bilayer, there are carbohydrates, cholesterol and proteins.  - The carbohydrates are called glycoproteins if they are attached to proteins and Glycolipids if they are attached to lipids (the lipid tails). - Carbohydrates don’t attach to the phosphates. - Cholesterol is found in between the phosphate lipid tails. The cholesterol regulates the fluidity and permeability of the cell membrane by causing the phospholipids to stay closer together. - Some proteins are a permanent part of the cell membrane (integral proteins). These are lodged inside the membrane. Their role is to transport large molecules across the cell membrane. - Some proteins are temporary (peripheral proteins). They are attached to the outside of the membrane loosely. Their role is to communicate and to assist in some transport. - The glycoproteins play a crucial role in cell recognition. FUNCTION OF THE CELL MEMBRANE - The cell membrane has 3 main functions; FUNCTIONS SEPARATES - The cell membrane separates the contents of the cell from the external environment (most important) - The intracellular fluid is the fluid found inside the cell (cytoplasm) - The extracellular fluid is the fluid found outside the cell membrane. REGULATES - Controls what substances can enter and leave the cell (regulating the contents of the cell). - This is called semi-permeable. (allows certain molecules to pass through via passive transport or active transport). COMMUNICATES - The cell membrane communicates and recognises cells. - Eg, the immune system needs to recognise whether the cell belongs to the organism or whether it is a pathogen (an invader). - The parts of the cell membrane responsible for cell recognition are glycoproteins, glycolipids and integral proteins.  The fluid mosaic model of the cell membrane is special because it is the perfect balance of fluidity, allowing things to move through the membrane but not being too fluid to allow things to easily leave the cell. Cell membranes can break and reassemble during cell division. Prokaryotes and Eukaryotes EUKARYOTES Eukaryotic cells have a nucleus and membrane-bound organelles (eg. Mitochondria) - They have a cell membrane - Eukaryotic cells have the DNA in the nucleus and have much more DNA than prokaryotic - The DNA inside eukaryotic cells does not connect with itself so it is therefore straight. - The DNA forms tightly bound and organised chromosomes. - Animals, plants, fungi and protists are eukaryotes. (Protists have flagella) - Found in multicellular organisms and rarely unicellular organisms. - Eukaryotes are 10-100 microns large. - Eukarotyes reproduce sexually and take a share of the (parents') genomes. PROKARYOTES Prokaryotic cells do not have a nucleus or any membrane-bound organelles. - They DO have ribosomes (technically not a membrane-bound organelle) - They have a cell wall made from peptidoglycan. - They also have a capsule outside the cell wall (for protection) - The DNA is found in the nucleoid which is found in the cytoplasm and is small and circular. - The DNA forms loose, single-loop strands of DNA. - Bacteria and Archaea are Prokaryotes - Found in only unicellular organisms. (the whole organism is made of one cell(eg. yeast)). - They use flagella and pili to move around. - Prokaryotes are 1-10 microns large. - Prokaryotes reproduce asexually and become a clone of the parent cell. - Prokaryotes have a higher metabolic rate, increased growth rate and a shorter generational time. SIMILARITIES - They both have a cell membrane, proteins and ribosomes. - They both have DNA - They both have a cytoplasm (in the eukaryotes everything is the cytoplasm except the nucleus and in the prokaryotes everything is the cytoplasm).  THE EVOLUTION OF EUKARYOTIC CELLS - It is thought that all eukaryotes are the result of endosymbiosis. The endosymbiotic theory describes how large host cells and bacteria could become dependent on each other resulting in a permanent relationship. This is where the host bacteria holds a guest bacteria inside it (thought to be an organelle). Both of these bacteria benefit from living and working together. COMPARING PROKARYOTIC AND EUKARYOTIC CELLS (can be used in exams to show differences) Drawing cells - To title the diagram, include the name of what you drew and the magnification (eg, A human cheek cell viewed under 400x magnification) - We measure cells using micrometres (um) (1 millimetre contains 1000 micrometres) - To show the size of the cell, every biological drawing needs to either have the magnification written or the scale bar. - MAGNIFICATION - The eyepiece is usually 10x magnification and the objective lens is usually 100x to find the magnification, you multiply these together. (You multiply the ocular lens (10x) with the objective lens (4x, 10x, 100x) to find total magnification). - To find the magnified size you divide the diameter of the field of view (what you see in the microscope, by the number of cells that fit across it.)  SCALE BAR - Shows the ratio between the drawn size and the actual size of the object. - - - The diameter of the cell drawn is 10cm but we need the same units. - To do this, we know that 10 cm has 100 mm therefore 100 x 1000 (micrometres) is the micrometres measurement of 10,000 UM. - The actual cell size is 2 UM. The actual length represented by the scale bar (a) should be ¼ or ⅓ of the actual size of the cell (A). - In this case, the scale bar should be 0.5 micrometres (um) - To find the length of the scale bar we rearrange the equation.  - - This equals 25,000 um therefore equally 25 mm and 2.5 cm - This is the length of the scale bar. - Microscopes THE LIGHT MICROSCOPE (OPTICAL) - A light microscope uses visible light and 2 lenses to make specimens and samples put on the stage, look bigger. - The light shines through the specimen, travelling through the objective and ocular lenses, and then the light is bent to create a magnified image.  THE FLUORESCENCE MICROSCOPE - Similar to a light microscope - The sample is coated in a fluorescent substance that attaches to parts of the specimen. - The specimen is lit with light causing the fluorescent coating to emit light. - The fluorescent light is then directed through filters that separate the emitted light from normal light. THE CONFOCAL MICROSCOPE - A highly focused laser passes through the specimen. - An image is taken of the small part of the specimen the laser is pointed at. - Move the laser and a different part of the specimen is focused. - Collect all the images taken and a 3D image of the specimen can be created. THE ELECTRON MICROSCOPE - Uses a beam of electrons and electromagnets to make the specimen look bigger. - An electron gun shoots a beam of electrons towards the specimen (in a vacuum so the electrons don't bounce off anything except the specimen). - The electron beam is controlled by electromagnets ensuring it stays straight - When the electrons hit the specimen the beam scatters. - The way the electrons scatter depends on the structure of the specimen. - The scattering is detected by a machine that measures the ricochet and then the computer creates an image (an electron micrograph). PREPARING THE SPECIMENS - Treat the specimen with chemicals that increase structural strength (so the electron bean doesn’t blow the specimen up due to its heat) - Remove water from the specimen (so the water doesn’t evaporate in the vacuum) DIFFERENCE BETWEEN THE TWO ELECTRON MICROSCOPES The TEM (transmission) microscope shoots one broad beam at the specimen. Place the specimen in resin. (provides a look at the inside of the specimen). The SEM (scanning) shoots one fine beam that scans the specimen. You have to coat the specimen in gold, causing the electrons to bounce off the surface of the specimen. (provides a look on the outside of the specimen). DIFFERENCE BETWEEN LIGHT MICROSCOPES AND ELECTRON MICROSCOPES (important) Cell Function MATERIALS MOVING IN AND OUT OF CELLS SUBSTANCES NEEDED BY THE CELLS SUBSTANCES THAT MUST LEAVE THE CELLS - Gases (oxygen and carbon dioxide) - Urea and Uric acid - Nutrients (sugars) - Carbon dioxide - Water - Mineral salts The permeability of a molecule through the cell membrane depends on; SIZE OF MOLECULE LIPID SOLUBILITY - Lipid-soluble molecules can easily move through the phospholipid bilayer. - This is because hydrophilic substances can only dissolve in hydrophilic substances and hydrophobic substances can only dissolve in hydrophobic substances. ELECTRICAL CHARGE - The cell membrane is permeable to neutral (uncharged molecules).  - The cell is impermeable to charged molecules (ions). - In summary, the cell membrane is permeable to molecules that are small or uncharged and impermeable to molecules that are large or charged ions. MOLECULES CAN MOVE IN AND OUT OF CELLS VIA: - Passive transport - Active transport TRANSPORT AROUND THE CELL MEMBRANE PASSIVE TRANSPORT The movement of materials across the cell membrane from high to low concentration and without the expenditure of energy (ATP) until an equilibrium is reached. - Passive transport is slow and does not let the cell control the nutrients it needs. Concentration gradient - gradual change in concentration from one region to another. TYPES OF PASSIVE TRANSPORT DIFFUSION - The movement of any molecule from high concentration to low  concentration until equilibrium is reached. - Eg, oxygen is continually used in cellular respiration, keeping its concentration low inside the cell, therefore the oxygen moves from high concentration to low concentration inside the cell. - SIMPLE DIFFUSION - The diffusion of substances directly through the phospholipid bilayer. - This can happen when the cell membrane is permeable to the material. - If the membrane is impermeable to the material, it will not diffuse. - FACILITATED DIFFUSION - Used when a material is impermeable to the cell membrane. - The diffusion of substances through the cell membrane via channel and carrier proteins. - The channel protein provides a tunnel through which certain materials can diffuse. (Each tunnel has a unique diameter and amino acid lining, so only selected molecules can fit through). - Carrier proteins bind to material and change their shape to allow diffusion. OSMOSIS - The movement of water from an area of high water concentration to an area of low water concentration across a semi-permeable membrane. - Water is not lipid soluble and, therefore needs protein channels to move through the cell membrane as the bilayer is hydrophobic. - These channels are called aquaporins. - OSMOTIC PRESSURE - A physical force drawing water to one side of the membrane, created by water moving across the semipermeable cell membrane.  - ISOTONIC - Equal solute concentration inside and outside the cell (iso = same). (The rate at which water enters the cell is the same as the rate at which water leaves the cell). - HYPOTONIC - The solute concentration outside the cell is lower than inside the cell (hypo = lower) (A solution with a lower solute concentration and therefore a higher water concentration). - HYPERTONIC - The solute concentration outside the cell is higher than inside the cell (hyper = higher)(A higher solute concentration with therefore a low water concentration). THE RATE OF DIFFUSION DEPENDS ON; - The concentration gradient, the greater the concentration difference, the faster the diffusion will occur. - Temperature and heat increase the rate of diffusion.  ACTIVE TRANSPORT The movement of materials across the cell membrane that DOES require the expenditure of energy (ATP or adenosine triphosphate). - Energy is used to bind specific materials to the carrier protein and open and close the protein. - Active transport is used to move materials from low to high concentration (so against the concentration gradient). - Allows the cell to control what it wants and doesn’t want in the cell. TYPES OF ACTIVE TRANSPORT BULK TRANSPORT Bulk transport is for when materials are too big or there is too much of it to move. - The two types of bulk transport include endocytosis and exocytosis. - They both require energy - The formation of vesicles inside the cell membrane. ENDOCYTOSIS - The process of transporting materials INTO the cell. - The cell membrane engulfs the external substance. - The cell membrane then pinches off and forms a vesicle inside the cell.  - PINOCYTOSIS - This is where the cell membrane engulfs a liquid - PHAGOCYTOSIS - This is where the cell membrane engulfs a solid. - RECEPTOR-MEDIATED - Where the protein receptors trigger endocytosis of a very specific molecule. EXOCYTOSIS - The process of transporting materials OUT of the cell. (usually wastes). - Opposite of endocytosis - The vesicle with the substance in it, comes up to the cell membrane. - The cell membrane joins with the vesicle and the phospholipids rearrange to join with the vesicle, fusing together. - The vesicle contents are then released into the external environment. The rate of Exchange Concentration gradients The steepness of the concentration gradient is based on the difference in concentration between the high and the low areas.  - The steeper the concentration gradient, the FASTER the material will move towards the area of low concentration in passive transport. - and the SLOWER the material will move towards the high concentration in active transport. - The rate of passive transport can not be changed to suit the cell. - The rate of active transport can be changed depending on the level of energy used. SURFACE AREA TO VOLUME RATIO - If the SA: V is too small, then the rate of chemical exchange (diffusion), is decreased and the cell will die. This is because the area of the surface of the cell affects the rate at which nutrients can enter and wastes can leave the cell. - As the cell grows, its nutrient requirements increase, but the rate of nutrient intake decreases. The cells need to maintain a large SA: V ratio to ensure efficiency. - A smaller cell size allows a faster movement of substances between the centre and the surface of the cell. This therefore means the cell can perform at an optimal level of functioning. - If the cell is too big, it will not be able to get the nutrients it needs due to its smaller SA: V ratio and it will not be able to get rid of the waste produced quickly enough, the cell will die. - If cells need a higher SA: V ratio, the shape of the cell can flatten. This means the cell's surface area increases but the volume stays the same. Therefore making the SA: V ratio larger. - If the cell has organelles that can’t be flattened, then the cell membrane extends. It does this by folding in on itself, increasing the surface area of the cell membrane. The higher the SA: V, the faster the rate of exchange. CALCULATING SURFACE AREA TO VOLUME RATIO As the size of an object decreases, the surface area to volume ratio increases. Wastes and Nutrients CELL REQUIREMENTS HOW DO ORGANISMS GET NUTRIENTS? - There are two main ways organisms can make their own energy. These are autotrophs and heterotrophs. - Cells use chemical energy from the breakdown of food to make ATP. (energy). This is done through cellular respiration. AUTOTROPHS HETEROTROPHS - Organisms which can make their - Organisms which can not make their own food. own food. - Examples of these are plants and - Examples of these are animals and algae. fungi - They use external energy such as - These organisms get their energy sunlight to turn inorganic from consuming other organisms. compounds such as soil, into food. An example of this process is photosynthesis. THE NUTRIENTS THAT CELLS NEED - These go into two categories; organic and inorganic compounds. ORGANIC COMPOUNDS INORGANIC COMPOUNDS - Compounds which contain - Compounds that don’t contain carbon and hydrogen. both carbon and hydrogen. - Organic compounds are only made by living organisms. CARBOHYDRATES WATER - The main function of - Makes up 70-90% of the carbohydrates is to be an easily organism. accessible energy source and in - Water is in all reactions either plants, to provide some structure. being the reactant or where the reaction is happening. LIPIDS OXYGEN  - They are used for long energy - Oxygen is needed for cellular sources and as a structural role respiration. in cell membranes. PROTEINS CARBON DIOXIDE - They provide structure to the cell - The main source of carbon membrane and tissue atoms for almost all organic - They provide growth, repair and compounds and is needed for maintenance photosynthesis - They form enzymes. NUCLEIC ACIDS MINERALS - Made of chains of nucleotides - A mixed bag of other inorganic which are either DNA or RNA. compounds cells need to survive - Carries genetic information (iron). (DNA) - Helping the cell make proteins (RNA) VITAMINS - (Won’t be tested on) Waste removal THE MAIN TYPES OF CELLULAR WASTE CARBON DIOXIDE - When carbs or lipids are broken down during cellular respiration, carbon dioxide is produced. - Carbon dioxide needs to be removed because it makes the cells very acidic due to it reacting with water and making an acid. - Carbon dioxide is removed by simple diffusion. NITROGENOUS WASTES - Waste products containing nitrogen (eg. ammonia, uric acid and urea).  - They are formed when unwanted proteins and nucleic acids are broken down. - It needs to be removed because it is very basic (opposite of acidity). - Too much base and too much acidity can destroy cellular structure and enzymes. WATER - Excess water needs to be removed as it takes up too much space and causes the cell to pop. - Water is removed by osmosis. HOW WASTES ARE REMOVED - Waste removal happens at two levels, the cellular level and the body system level. - Cellular level refers to the movement of wastes from the cytoplasm to the external environment. - They can be moved by passive or active transport. - Body system levels refer to the removal of waste by plants. - Waste is excreted as gases or through old leaves and excreted into the soil. - In animals, the circulatory system transports wastes to the excretory system including the lungs, liver and kidneys. Photosynthesis - Photosynthesis happens in the chloroplasts. - (Chloroplast structure below) - THE TWO STAGES OF PHOTOSYNTHESIS LIGHT-DEPENDENT REACTIONS LIGHT INDEPENDENT REACTIONS  - Occurs in the thylakoids - They don’t use sunlight and use - Chlorophyll captures light energy ATP energy instead. from the sun and uses the light - Occurs in the stroma. energy to break down water into - ATP energy is used to combine oxygen and hydrogen. The water is carbon dioxide with hydrogen, which removed from the cell and the produces glucose (energy). hydrogen is moved to the stroma. - FACTORS AFFECTING THE RATE OF PHOTOSYNTHESIS - The factors include light intensity, carbon dioxide levels and temperature. LIGHT INTENSITY - Affects the rate of photosynthesis as the more light available, the faster photosynthesis can happen. - The more energy provided, the faster the process can happen until it reaches a certain point where the chloroplasts can not work faster and the rate plateaus. CARBON DIOXIDE LEVELS - Affects the rate of photosynthesis as the more carbon dioxide available, the faster photosynthesis can happen. - Similarly to the light intensity, the chloroplasts can only go so fast so therefore plateaus. (The graph looks the same as the light intensity graph). TEMPERATURE - Affects the enzymes which catalyse photosynthesis.  A combination of all three factors affects the rate of photosynthesis. - The limiting factor is the factor that prevents the chloroplasts from achieving the maximum rate of photosynthesis (eg, at midnight, the light intensity is low.) Respiration ATP - Adenosine triphosphate (ATP) is the energy currency of life. - Chemical energy is stored in the bonds of molecules and when the bond is broken, energy is released and when the bond is fixed, energy is used. - To get energy, the cell breaks off a phosphate. RESPIRATION Cellular respiration is the process by which organic compounds (carbs, lipids and proteins) are broken down to produce ATP energy. - Glucose (a carbohydrate) is the cell's favourite to break down. There are two ways the cell can break down the organic compounds depending on how much oxygen is available. AEROBIC RESPIRATION (lots of - This happens in the mitochondria oxygen) and the cytosol. - In respiration, glucose is broken down through a series of chemical reactions, each producing energy. - This energy is then used to repair the ATP from ADP so the cell can reuse it. ANAEROBIC RESPIRATION (not - This happens only in the cytosol. enough oxygen) (won’t get tested on much) - RELATIONSHIP BETWEEN PHOTOSYNTHESIS AND RESPIRATION -  - The oxygen needed for cellular respiration comes from the byproduct of photosynthesis. - The carbon dioxide produced as a result of cellular respiration is used as a reactant in photosynthesis. This relationship allows all nutrients to be recycled. DIFFERENCE BETWEEN PHOTOSYNTHESIS AND RESPIRATION - (very important) Enzymes - Enzymes are biological catalysts (they speed up reactions occurring inside cells). - They can not be changed in the reaction. - They are made of protein. - Enzymes bond to substrates (a substrate is a molecule that bonds with an enzyme). - The enzymes either break down a molecule into smaller molecules or synthesise and make the molecule into a larger molecule.  - - Each enzyme only catalyses one reaction. - Enzymes speed up reactions. - Anything with the ending ‘ase’ is an enzyme. HOW DO ENZYMES WORK - There are two models to describe how enzymes work; the lock and key model and the induced fit model. THE LOCK AND KEY MODEL - The lock and key model is old. - The active site fits perfectly with the substrate. THE INDUCED FIT MODEL Same as the lock and key model except; - The active site does not fit perfectly with the substrate. - The enzyme is exactly the same as before the reaction. -  - ENVIRONMENTAL FACTORS WHICH AFFECT ENZYMES - Enzymes are very sensitive meaning they work at maximum efficiency in only their optimal conditions. - Enzymes are proteins. The three main factors affecting enzyme activity include; - Temperature - pH - Substrate concentration TEMPERATURE - As the temperature increases, the movement of molecules speeds up causing more collisions (active sites), with the enzymes and the substrates. - When the optimum temperature is reached, the enzyme activity drops. This is because high temperatures break (denature) the enzyme, destroying the enzyme's shape, therefore limiting the ability for it to fit the substrate. pH - Any variation above or below the optimum pH reduces the enzyme’s activity. - The optimum pH varies between the different enzymes. SUBSTRATE - A more concentrated CONCENTRATION solution has more substrate molecules meaning the more likely a collision with the active site will occur. - When the graph plateaus, it is called the saturation point. - The saturation point occurs when all the active sites are full.