Cell Function

Diffusion across the cell membrane

Diffusion and Osmosis

The plasma membrane which surrounds the cell is selectively permeable. This means that it only allows certain substances to move in and out, through a number of different mechanisms. This permeability ensures that the cell only collects molecules that it requires (such as sugar (glucose), fats (lipids), and amino acids (make up proteins), and excludes other materials (such as wastes).

It also allows the cell to excrete unwanted molecules, such as metabolic wastes, so that the cell can maintain a constant internal environment. There are a number of mechanisms used to pass materials through the cell membrane, including diffusion, osmosis, active transport, endocytosis, and exocytosis.

Diffusion down the concentration gradient

Simple Diffusion

Diffusion is the movement of molecules from an area of high concentration to an area of low concentration, until an equilibrium is reached. It does not require and energy input.

Diffusion is the movement of any molecule from a region of high concentration to a region of low concentration of that substance until an equilibrium is reached. Equilibrium is reached when there is no net movement of molecules in either direction. When a molecule moves from high concentration to low concentration, it is called moving down the concentration gradient.

The rate of diffusion changes depending on the concentration gradient. If there is a greater difference in the concentration of substances, the gradient will be steeper and diffusion will occur faster. Diffusion can also be sped up with a change in temperature. Heat increases the rate of diffusion because the kinetic energy of the particles increases.

In biological terms, this refers to diffusion across a semi-permeable membrane. Molecules which are needed for cell function, such as water and ions, use diffusion to move in and out of cells.

Diffusion is a passive transport mechanism, meaning that no input of energy is required for the molecules to move through the membrane. The movement of materials into and out of cells takes place either passively or actively. Diffusion can also be facilitated by the cell. This means a mechanism, such as an ion channel (protein), exists across the membrane to flow certain molecules through.

Facilitate diffusion across a cell membrane

Facilitated Diffusion

Larger molecules and charged particles do not readily pass through the phospholipid bilayer. They require certain proteins called carrier proteins and channel proteins in the cell membrane to assist them in diffusing into the cell. This process is called facilitated diffusion.

Small molecules such as carbon dioxide and oxygen move easily through the cell membrane by simple diffusion. These particles pass between the phospholipid molecules from a region of high concentration to a region of low concentration. The concentration gradient is usually maintained for the diffusion of oxygen molecules because oxygen is removed for use in the cell through cellular respiration, meaning that oxygen concentration is constantly low.

Small ions such as sodium diffuse rapidly through the cell membrane (left and middle in image above), from a high ion concentration to a low ion concentration, via narrow passageways called channel proteins. These channel proteins are specific for particular ions.

Carrier proteins bind to molecules on one side of the membrane (right in image above). They then change shape and release the substance on the other side. The direction of movement, whether into or out of the cell, depends on the direction of the concentration gradient.

Protein channels vs carrier proteins

Blood cells in solutions of various concentrations

Osmosis

Osmosis is a subset of diffusion, referring solely to the passive movement of water across a membrane, to equalise the concentration of a solute on both sides. Osmosis is technically the movement of any solvent, but as water is the most common solvent in biology we almost exclusively refer to water when we talk about osmosis. Osmosis works like diffusion in that the movement occurs along the concentration gradient.

Water helps keep cells in shape, as it forms the fluid that bathes tissues and also transport materials in solution. A solution is formed when a solute dissolves in a solvent.

Diffusion and osmosis are used by the cell to regulate their internal conditions, ensuring optimal cell function. If the fluids inside and outside the cell are equal, the external solution is said to be isotonic to the cell contents. When cells are surrounded by a solution that contains lower solute concentration than their cytoplasm, the external solution is said to be hypotonic to the cell contents. The reverse applies if the cells are surrounded by a solution of higher solute concentration: the external solution is hypertonic to the cells and net movement of water molecules will be out of the cells.

Aquaporins are protein channels that transport water

Osmosis in cells does not happen through the membrane as the lipid bilayer rejects the water inside near the fatty acid tails. Instead the water moves through special protein channels called aquaporins.

In cells of unicellular eukaryotes that are surrounded only by a cell membrane. Hypotonic solutions, such as fresh water, pose a problem because water moving into animal cells by osmosis can cause the cell to swell and eventually burst the cell membrane, killing the organisms.

Diffusion across a semi-permeable membrane where the solute is too large to cross the membrane

Comparison of active vs passive transport across the cell membrane

Active Transport

Active transport occurs in cells when molecules are either too large to move across a membrane on their own, or when molecules are trying to move against a concentration gradient (i.e. from areas of low concentration to high concentration).

Active transport is important for moving molecules, such as glucose, in and out of the cell. It is also essential for creation of membrane potentials; which cells use to drive production of ATP in the mitochondria.

Exocytosis and Endocytosis involve the use of vesicles

Exocytosis and Endocytosis

Sometimes particles are too large to move through the cell membrane by either diffusion or active transport. In these circumstance the cell membrane can change its shape to surround the particle and engulf it in a process called endocytosis.

Endocytosis is a type of active transport used to bring external materials, such as proteins, into the cytoplasm of a cell. It does so by engulfing them in part of the outer membrane, forming a vesicle inside of the cell cytoplasm. If a solid particle is engulfed, the process is termed phagocytosis. If a liquid is engulfed then the process is called pinocytosis.

Exocytosis is the opposite of endocytosis, a mechanism for cells to actively transport large or charged molecules to the extra-cellular space. It is also used as a method to add proteins, lipids, and other molecules to the cell membrane. These substances are contained within vesicles inside the cell. Cells also produce waste products that need to be moved out of the cell. During exocytosis, a membrane-bound vesicle moves to the cell membrane, fuses with it and then releases its contents to the exterior of the cell. The vesicle membrane becomes part of the cell membrane.

Active transport energy in the form of ATP

Active transport

Active transport is the movement of molecules from a region of low concentration to a region of high concentration, and requires the input of energy. This movement goes against the concentration gradient and involves movement across a cell membrane that has receptors for the molecules.

Diffusion and osmosis both rely on a concentration gradient to direct the passive flow of substances from regions of high concentration to regions of low concentration. Sometimes in living things, a chemical may need to be moved against the concentration gradient, such as when kidney cells reabsorb glucose and amino acids so they are not lost in urine. Active transport requires a carrier protein that spans the membrane to actively move chemicals from a low to a high concentration, utilizing cellular energy.

Movement of solutes against the concentration gradient by active transport

The speed of solute movement can be affected by a variety of factors

Rate of Material Exchange

The characteristics of the cell membrane and the physical and chemical properties of the material will determine whether it can move through the cell membrane.

Whether or not material can be exchanged across a membrane, and the type of transport used, is determined by the physical and chemical properties of a molecule, and how they interact with the cell membrane.

Solutes of different types diffuse at different speeds

Chemical factors

The chemical properties of a substance affect its transport across cell membranes. Many uncharged molecules, such as ethanol, can easily penetrate the cell membrane because they can dissolve in the phospholipid bilayer. Hydrophilic, charged ions such as sodium and potassium cannot cross the hydrophobic centre of the membrane. Channel proteins specific for each ion allow their movement through the cell membrane.

The solubility of a molecule will also affect how it moves across a membrane. Solubility refers to whether a substance is polar or non-polar. Non-polar substances move more easily through the cell membrane than polar molecules, and therefore will be diffused at a faster rate. Water is not lipid-soluble and therefore cannot move through the hydrophobic 'tails' in the cell membrane. Water moves through the membrane through special protein channels called aquaporins.

Large molecules are unable to pass through membranes of a limited size

Physical factors

The physical properties such as size and shape of the molecules affect the movement of substances across the cell membrane. Small molecules are able to diffuse easily between the phospholipids.

The mass of the molecule will affect transport, as large molecules will generally be unable to pass through the selectively permeable membrane without the aid of a transporter protein. This helps maintain the integrity of the membrane, and ensures that only required molecules are able to enter a cell.

Glucose and amino acids are examples of large molecules that use carrier proteins to move through the cell membrane. Very large molecules that need to be transported into or out of the cell are moved by the process of endocytosis or exocytosis.

The size and polarity of molecules will affect the speed of their diffusion

The rate of diffusion increases as concentration increases

Concentration gradient

The concentration of a solute will also effect the rate at which it diffuses. The greater a difference in concentration across a membrane, the more rapidly it will diffuse. As the system approaches equilibrium, this rate will slow.

The relative concentration of the substance on either side of the membrane affects the rate of diffusion of that substance. If the concentration gradient is high, then the substance will diffuse rapidly. As the concentration gradient decreases, the rate of diffusion will be slower. In order to maintain a rapid rate of diffusion, cells need to maintain a high concentration gradient. When the concentration reaches equilibrium, there will be no net movement across the cell membrane.

Diffusion of different solutes may occur simultaneously in cells across different areas of the membrane

Diffusion of solutes occurs until an equilibrium is reached for all solutes

If the size of an object increases without a change to surface area, then the SA:V ratio decreases

Surface Area: Volume ratio

The process of diffusion for exchanging nutrients and wastes can be effected by a number of factors, such as the ratio of the cell surface area to volume ratio. The term 'surface-area-to-volume ratio' refers to the ratio between the external membrane surface of a cell and the size of its contents. The larger a cell gets, the smaller it's surface-area-to-volume ratio becomes, because more cytoplasm is being contained in a relatively smaller coating of membrane.

As the surface-area-to-volume ratio decreases, the efficiency of diffusion also decreases, as the cell contents are less available to the site of exchange. This decreases the efficiency of the cell, as it will not be able to excrete waste and receive nutrients at a rapid enough pace, as these molecules are travelling longer distances to the membrane. This is why cells are restricted in their size, and large organisms such as humans require the coordination of many cells to perform large-scale functions.

The shape of cells also makes a difference to the SA:V. Spherical cells have a relatively small SA:V compared with cells of other shapes. Long, flat cells have a higher SA:V than a spherical cell with the same volume.

Cells often have specific features that ensure they have the highest SA:V possible. An example is the root hair cells that cover root tips of most plants. The long, thin extensions of the single cells that form root hairs increase the surface area through which water and mineral salts can be absorbed.

Root hair cells have a projection that increases their SA:V ratio, allowing for more efficient diffusion of water into the plant

Concentration gradients, size, and charge play a role in the mechanism of transport

Movement across the cell membrane - summary

The exchange of materials between cells and their external environment occurs through the processes of:

Diffusion
Osmosis
Facilitated diffusion
Active transport

The process by which substances move across the cell membrane depends on several factors - these include the lipid nature of the cell membrane and the size and polarity of molecules. Hydrophilic substances dissolve in water and do not readily pass across phospholipid membranes. Hydrophobic substances do not readily dissolve in water - such molecules can dissolve in the phospholipid membrane.

Lipid-soluble molecules (non-polar) such as chloroform and alcohol dissolve in the lipid bilayer and pass through.
Water-soluble molecules tend to be repelled by the phospholipid bilayer. However, very small molecules such as water and urea are small enough to pass directly between phospholipid molecules.
Other small uncharged molecules such as oxygen and carbon dioxide also pass directly across the cell membrane between the phospholipid molecules.
Large water-soluble molecules (polar) such as simple sugars and amino acids cannot pass directly across the cell membrane. The passage of these molecules depends on transport channels that span the cell membrane.

Some substances enter and leave cells by other means. When a section of the cell membrane wraps around a substance for import into the cell, pinching off to form a vesicle inside the cytoplasm, the process is called endocytosis. Pinocytosis refers to a similar process related to the import of liquid droplets. Exocytosis is the opposite of endocytosis and involves vesicles, such as those associated with the Golgi apparatus, merging with the cell membrane to facilitate the export of substances.

Cellular structures are constructed of monomer sub-units that bunch together as polymers to form the complex structures

Cell Requirements and Waste

In order to maintain life, cells require certain fundamental molecules. The types of molecules required vary depending on the types of cells and organisms. Essentially, all cells require and energy source and a carbon source, in order to drive all cell processes.

Cells need to obtain nutrients in the form of organic substances such as glucose, nucleotides and vitamins. Cells also need inorganic nutrients such as gases, minerals, and water. The substances needed by living cells are used either as a source of energy or as a building block for other cells and tissues.

Diffusion of minerals in a root hair cell

Inorganic nutrients

Inorganic nutrients are part of the non-living world and do not contain carbon and hydrogen in long chains.

Water is an inorganic nutrient that makes up the cytoplasm in cells. It is used as a transport medium through which important molecules are called in solution.
Mineral salts are dissolved as ions in the cytoplasm and vacuoles. These assist in chemical reactions and are used to make many macromolecules and body tissues (e.g. calcium is used in bones). Sodium and chloride ions assist in water balance in cells and with the transmission of impulses between nerve cells.
Gases such as oxygen and carbon dioxide are dissolved in cytoplasm and are used in chloroplasts and mitochondria as part of photosynthesis and cellular respiration.

Apart from an energy source and a carbon source, cells also require other molecules to aid in metabolism processes. Animal cells require O2 gas in order to conduct respiration and plant cells require CO2 for photosynthesis. Ions are also essential for cell functioning, performing important roles in protein function as cofactors in creating cellular conditions for respiration and in electrical communication, particularly for neurons. There are also a number of vitamins essential for cell structure and function, such as Vitamin C for collagen synthesis.

Blood vessels in the lungs use diffusion to remove CO2 and take in O2

Diffusion occurs in the digestive system to absorb nutrients from food

The four organic molecule types

Organic Nutrients

Every living cell requires large organic molecules as part of their structure and to maintain the biochemical processes involved in effective functioning. These large organic molecules are called biomacromolecules. there are four main types based on their chemical composition and structure: complex carbohydrates, lipids, proteins and nucleic acids.

Plants and other organisms that carry out photosynthesis absorb inorganic nutrients from the soil and the air and use these to make their own organic nutrients. In contract, organisms that do not carry out photosynthesis need to ingest food to obtain obtain organic nutrients for their cells.

Carbohydrates are a group of organic molecules made of carbon, hydrogen and oxygen. These are classified as simple or complex sugars.
- Glucose is a monosaccharide that is used as 'quick energy'.
- Starch is a polysaccharide and is used to store energy in plant cells.
- Cellulose is a polysaccharide and is used to create support and strength in cell walls.
Lipids contain many carbon and hydrogen atoms with a few oxygen atoms. The fats and oils in the cells of organisms are made of lipids. Lipids have three main functions in cells:
- energy storage
- structural component of cell membrane
- structural component of hormones (chemical messengers)
Proteins are made up of carbon, hydrogen, oxygen, nitrogen and sometimes sulfur. These elements combine to form amino acids, which are the building blocks of proteins. Plant cells can synthesise their own amino acids whilst animals cells produce most of theirs. Nine of the amino acids are called 'essential' because they cannot be synthesised by cells in the body and must be sourced through food. Proteins have many roles in cells:
- structural component of cells and tissues
- structural component of cell membrane
- as enzymes and hormones
Nucleic acids are very large biomacromolecules that contain carbon, hydrogen, oxygen, nitrogen and phosphorus. There are two types of nucleic acids: DNA (deoxyribonucleic acid) and RNA (ribonucleic acid).
- DNA stores the information that controls the cell.
- RNA assists in the manufacture of proteins

The central dogma

Phagocytosis takes in solid materials and exocytosis removes waste from a cell

Removal of cellular products and wastes

The products of cellular reactions that are not required are called wastes. They need to be transported to other cells or removed from the organism entirely. Removal from the cell will usually occur via diffusion through the cell membrane.

Oxygen and carbon dioxide gases are removed from the cell through simple diffusion. Any excess water that is not required is removed via osmosis. Waste chemicals that dissolve in water are removed at the same time.

Oily, fat-soluble substances are attached to small electrically charged molecules by the cell to make them water-soluble so they can be removed when the water diffuses out.

Electron micrograph of exocytosis

There are a number of organelles which are responsible for removing wastes from cells. It is essential that wastes are removed for continued, optimal function of cells.

Vacuoles are membrane bound enclosed compartments, containing dissolved inorganic and organic molecules, enzymes, and in some cases solids. They isolate material that may be harmful to the cell, aid in regulating the pH of the cell by taking protons, contain waste products, and assist in transport of substances from the cell through exocytosis.

Lysosomes are vesicles which dispose of cell wastes. They contain hydrolytic enzymes, functioning best at acidic pH ranges (4.5-5.0), which break down biomolecules. Lysosomes digest unwanted cell material to aid in maintaining cellular function, as well as breaking down pathogens entering the cell.

Proteins and other substances luke mucus, are produced by an animal cell. These cellular products are packaged into vesicles and leave the cell by exocytosis. Wastes that leave the cell are said to be excreted from the cell. Useful products that leave a cell to be used elsewhere are said to be secreted.

The interaction of cellular respiration and photosynthesis

Cellular Respiration and Photosynthesis

Cell biology is dependent on a number of biochemical processes, which power cells, recycle waste, and produce essential molecules for structure and function. The biochemical processes that occur in a cell are called 'metabolism'.

Metabolism is the sum of all chemical processes which take place within an organism.

Metabolism therefore includes all anabolic (biochemical processes which synthesis molecules) and catabolic (biochemical processes which breakdown molecules) processes that occur within a cell.

Different cells have different biochemical pathways, each contributing to how an organism will function. This is important especially in multicellular organisms, such as humans, as we require specialised cells to perform specialised tasks to maintain health. For example, our liver cells, hepatocytes, are responsible for storing excess glucose, contributing to the function of the liver. Cells also have specialised organelles to perform biochemical functions.

The process of respiration in the presence of oxygen

Cellular respiration

One essential biochemical process required for all cells is the ability to produce energy. This is essential so that the cell can breakdown or build-up molecules that it needs to survive. The process of producing energy is called respiration. Contrary to how we commonly use the word respire, in terms of cell biology it does not mean breathing, but production of energy. There are two types of respiration, aerobic, meaning in the presence of oxygen, and anaerobic, meaning without oxygen. Whether or not a cell will use aerobic or anaerobic respiration will depend on the type of cell (e.g. whether it is a bacterium found in soil, or a human brain cell), and potentially the availability of oxygen (some human cells can perform anaerobic respiration when there is limited oxygen available, for example during exercise).

Respiration always requires a food source, glucose, which is broken down to produce by-products, and energy. The energy released comes from the breaking of chemical bonds during the process.

Aerobic respiration:

Glucose + Oxygen → Carbon dioxide + Water + Energy

C₆H₁₂O₂ + 6O₂ → 2CO₂ + 6H₂O + ATP

Aerobic respiration happens consistently in animal and plant cells, which is why they need a constant supply of oxygen to stay alive. Many of the steps involved require the mitochondria (the powerhouse of the cell). The process is complex and requires many steps.

The process of respiration without oxygen present

Anaerobic respiration is the production of energy without oxygen. Different cells have different pathways for this, but some of the common ways cells perform anaerobic respiration are:

Anaerobic respiration in human cells:

Glucose → Lactic acid + Energy

C₆H₁₂O₆ → 2C₃H₆O₃ + ATP

We can see that human cells are able to use glucose to produce lactic acid and energy, when the cells are starved of oxygen. This is why when you run, you can get cramps or pain in the leg muscles because there is a build-up of lactic acid affecting your muscle cells.

Microbes, such as yeast, are also able to respire without oxygen. This is how we make alcohol, through the process of fermentation.

Anaerobic respiration in yeast cells:

Glucose → Ethanol + Carbon dioxide + Energy

C₆H₁₂O₆ → 2C₂H₅OH + CO₂ + ATP

Process of photosynthesis in relation to the plant

Photosynthesis

Photosynthesis is a process used by autotrophs, 'self-feeding' organisms, to produce their own glucose. This can then be used in respiration to produce energy. Non-photosynthetic organisms, heterotrophs, rely upon consumption of external sources of glucose (for example, plants) in order to obtain molecules essential for respiration. The process of photosynthesis converts light energy into chemical energy, and stores this energy in the bonds of glucose molecules. In this process, carbon dioxide and water are converted to glucose and oxygen in the presence of light.

Process of photosynthesis inside the chloroplast

Photosynthesis:

Carbon dioxide + water → Glucose + oxygen

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

Chloroplasts are the specialised organelles responsible for performing photosynthesis in plants and algae. They obtain their green colour from chlorophyll, a green pigment which is excited by certain wavelengths of light. This is how the light energy is converted into chemical energy. Once glucose has been produced, it is moved around the organisms to cells which need it to produce energy. Excess glucose is stored as saccharise structures in plants (for example, fruit and vegetables).

Enzymes lower the activation energy of reactions

Enzymes

Enzymes are proteins found in all cells which are responsible for speeding up, or catalysing, chemical reactions happening in organisms. For example, when cells are breaking down glucose for energy, the molecule might eventually break down of its own accord, but that would take years. So to speed up the breaking down of chemical bonds, we have enzymes, allowing glucose digestion to happen in a matter of seconds, and a quick release of energy for cells to use. Enzymes are essential in all areas of life, and without them organisms would just be blobs of chemicals slowly reacting over millions of years.

Enzymes are highly specific biological catalysts which increase the rate of metabolic reactions by lowering the 'activation energy'.

Enzymes are able to catalyse reactions by lowering the activation energy, the energy needed to begin a reaction between two molecules. Less energy is needed to breakdown or synthesise a molecule when an enzyme is present to facilitate a reaction.

Proteins have a complex structure

Chemical Structure of Enzymes

Enzymes are composed of protein molecules that are often highly folded to create a particular chemical 'shape'. The surface of the enzyme with a specific shape is called the active site. It is here that the reactants, also called substrates, in a chemical reaction temporarily bind. When this occurs, a substrate-enzyme complex is formed. The products of the reaction are then released room the active site but the enzyme remains unchanged, meaning it can repeat the process again.

The way in which enzymes interact with biomolecules to speed up chemical reactions is with their active site. The active site is an area of the enzyme which contains a specific sequence of amino acids with reactive side chains, that can chemically interact with a substance. The biomolecules, called substrates, bind to the active site and undergo a chemical reaction. The shape and reactivity of enzyme active sites makes them very specific, and they are usually only able to act on one type of substance.

Enzymes are made of amino acids, which are bonded in a specific linear order, and then folded to form proteins. Protein folding occurs on four levels to influence the final shape. All of these interactions are chemical bonds which can be effected by the environment in which the protein is situated.

Primary structure: sequence of a chain of amino acids held together by peptide bonds.
Secondary structure: hydrogen bonding of the peptide backbone causes the amino acids to fold in a repeating pattern, either an alpha helix or a beta pleated sheet.
Tertiary structure: three-dimensional pattern of a protein due to side chain interactions.
Quaternary structure: protein consisting of more than one polypeptide linked together.

Amino acids join together to make polypeptides which in turn join together to make proteins

The lock and key model of enzyme function

Models of Enzyme Function

There are two main theories about how enzymes and substrates interact to catalyse reactions.

Firstly, the lock and key model describes the enzyme as a key and the substrate as the lock. When the key interacts with the lock, the lock changes shape, but the key remains unchanged for continued use. This is analogous with a substrate interacting with an enzyme, forming an enzyme-substrate complex, and the separating, the substrate changed into products, and the enzyme conserved. In this model, it is thought that the active site is rigid and the small substrate molecule is shaped to fit the active site exactly.

The induced fit model is another theory about how enzymes and substrates interact, elaborating upon the lock and key model. The enzyme's active site is not perfectly shaped to accommodate the substrate, but changes shape to bind the substrate and act on it. This model is based on the idea that proteins are not rigid and so the active site is able to adjust to fit the substrate.

The induced fit model of enzyme function

The lock and key model of enzyme function

A denatured enzyme will have an altered shape that will prevent the creation of the substrate-enzyme complex

Effect of Environment on Enzymes

Enzymes are highly specific, and due to their chemical nature, they are significantly affected by changes to their environment. There are a number of scenarios in which enzyme activity is effected by the environment, including: changing pH, changing temperature and changing substrate concentration. When factors such as these are altered, there may be changes either to the enzyme's active site, or to the rate of substrate-enzyme interactions. When the shape of the protein is significantly altered, it is called denaturation (loss of quaternary, tertiary, and secondary folding structures). Most importantly, this changes the shape of the active site, so the enzyme can no longer act on its substrate. This denaturation can happen at increased temperatures, or at pH outside of the optimal ranges. It occurs because enzymes are made of amino acids, whose chemical bonds can be changed depending on acidity, alkalinity, or temperature of their environment.

pH is a measure of the amount of hydrogen ions (H+) in solution or the acidity/alkalinity of a substance. Solutions are acidic if the pH value is <7, and alkaline if >7. These charged ions in solution have an effect upon the chemical structure of enzymes, as they can interfere with the bonds which determine the shape of the protein. At pH ranges both too acidic and too alkaline, enzymes will denature. Each

Enzymes have an optimal pH range

Enzymes have an optimal temperature range

Temperature is another environmental factor which effects the rate of enzyme activity. At low temperatures increasing towards the optimal temperature, the rate of reaction is limited due to the energy in the system. This simply means that the substrate and enzyme are not encountering each other as quickly because they move slower at cooler temperatures. Past the optimal temperature, the enzymes will begin to denature as the heat is altering the chemicals bonds within the protein. This changes the shape of the active site, so that the substrate can no longer interact with it, and the rate of activity quickly drops off. In most living things, enzymes function normally at temperatures up to 40 degrees Celsius. Some organisms that live in hostile environments have optimal temperatures that are much lower or higher than this. In humans, the optimal temperature for the functioning of enzymes in 37 degrees Celsius.

The amount of substrate available to react with an enzyme will also affect the rate of reaction. As substrate concentration increases, the rate of reaction will increase, as more chemical reactions are happening. This will occur until the point at which all enzymes are engaged in an enzyme-substrate complex, at which point the reaction will plateau. The reason for this is that a set amount of enzyme can only perform so many reactions at a certain rate, so no matter how much more substrate you add, the enzymes rate of reaction will not increase.

Enzymes have an optimal substrate concentration

Cell Function

Inquiry question: How do cells coordinate activities within their internal environment and the external environment?

Investigate the way in which materials can move into and out of cells, including but not limited to:
- Conducting a practical investigation modelling diffusion and osmosis
- Examining the roles of active transport, endocytosis and exocytosis
- Relating the exchange of materials across membranes to the surface-area-to-volume ratio, concentration gradients and characteristics of the materials being exchanged
Investigate cell requirements, including but not limited to:
- Suitable forms of energy, including light energy and chemical energy in complex molecules
- Matter, including gases, simple nutrients and ions
- Removal of wastes
Investigate the biochemical processes of photosynthesis, cell respiration and the removal of cellular products and wastes in eukaryotic cells
Conduct a practical investigation to model the action of enzymes in cells
Investigate the effects of the environment on enzyme activity through the collection of primary or secondary data

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