Prelim. Biology- Module 1 (Chapter 3): CELLS AS THE BASIS OF LIFE

Chapter 3: Cell Function

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

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3.1 Movement of materials in/out of cells

Substances required by cells for their functioning need to move into the internal environment of cells and waste substances and cellular products need to pass out of cells into the external environment. These substances move through the semi-permeable membrane.  The substances needed by cells are gases (oxygen and carbon dioxide), nutrients (sugars, amino acids, glycerol and fatty acids) and water, the main solvent in cells. Mineral salts dissolved in water are also required. The substances that must leave the cell are wastes such as urea, uric acid and excess carbon dioxide. They could also be products secreted by cells that may be needed to coat the outside of cells, or may pass to other cells.

In both plant and animal cells, the cell membrane is in direct contact with the cytoplasm inside the cell.  The membrane controls the passage of water and other molecules into our out of cells.  In contrast, a cellulose cell wall in plant cells is permeable- meaning it is a non-selective boundary that allows water and most molecules to pass freely inwards or outwards.

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Movement of molecules

The permeability of a cell membrane to a molecule depends on the molecules: size, electrical charge and lipid insolubility.  Small molecules can move across membranes quickly, whilst larger molecules have a more difficulty moving across membranes unaided. -Water insoluble (hydrophillic) molecules have difficult penetrating a membrane, whereas lipid soluble molecules do not. This due to the membrane's structure. The lipid tails impede the progress of hydrophillic molecules and enhance the movement of lipid-soluble molecules. -Electrically charged molecules, are not very soluble in lipids and therefore have low membrane permeability. Neutral molecules are soluble in lipids and have a high permeability. Water is a polar molecule and is not lipid-soluble, membranes have pores that make them high permeable. Water moves through hydrophillic pores in the membrane called aquaporins.  -Molecules that have low permeability rely on carrier proteins to transport them across membranes in the cell. 

Diffusion

 The movement of materials into and out of cells takes place either passively or actively. Passive movement requires no energy input and includes the processes of diffusion and osmosis. Diffusion is the net movement of any molecule from a region of high concentration to a region of low concentration of that substance, until equilibrium is reached. Equilibrium is reached where there is no net movement of molecules in either direction. Does not require energy input.

Diffusion cont.

Movement from a high concentration to a low concentration is described as movement along a concentration gradient (a gradient is a slope).  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 speed up or slow down, depending on the temperature (as heat increases the kinetic energy of particles)

Diffusion across a cell membrane

Small uncharged particles such as carbon dioxide 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. Oxygen is continually used in the process of cellular respiration, keeping its concentration low inside the cell.    Relatively large molecules (such as glucose and amino acids) and charged particles, do not readily pass through the membrane. They require proteins called carrier proteins and channel proteins in the cell membrane to assist them in diffusion into the cell. This process is called facilitated diffusion.Relatively large molecules (such as glucose and amino acids) and charged particles, do not readily pass through the membrane. They require proteins called carrier proteins and channel proteins in the cell membrane to assist them in diffusion into the cell. This process is called facilitated diffusion.

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Facilitated Diffusion

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Caption: : Carrier proteins

Osmosis

Osmosis is a special type of diffusion. It is the net movement of solvent molecules from a region of high solvent concentration to a region of low solvent concentration through a semi-permeable membrane.  When water is the solvent, just like the diffusion, the movement of water occurs along the concentration gradient and does not require energy.

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Osmosis cont.

Water is very important to living things. It is the medium in which many of the biochemical reactions in cells occur. Water helps keep cells in shape, it forms the fluid that bathes tissues and it also transports materials in solution. A solution is formed when a solute (such as salt or sugar) dissolves in a solvent. The amount of solute dissolved in a given quantity of solvent determines the concentration of the solution.  - A concentration solution contains a large amount of solute in relation to the amount of water, so the water is said to be in low concentration. A dilute concentration contains a small amount of solute in relation to the amount of water, and the water is said to be in high concentration.

Osmosis cont.

Osmosis is the process by which water moves through the cell membrane.  - When water is highly concentrated outside the cell (low solute concentration) than it is inside the cell, water will move by osmosis through the cell membrane and the cell may swell up. - Alternatively, if the concentration of water is lower outside the cell than inside, water will move out of the cell by osmosis and the cell may shrink. Osmotic pressure: the pressure created by water across a semi-permeable membrane due to osmosis. The more water that moves across the membrane, the higher the osmotic pressure created.

- If the fluids inside and outside the cell are of equal solute concentration, the external solution is said to be isotonic to the cell contents.  -If cells are surrounded by a solution that contains a lower solute concentration than their cytoplasm, the external solution is said to be hypotonic. -If cells are surrounded by a solution that contains a higher solute concentration than their cytoplasm, the external solution is said to be hypertonic.

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Osmosis in animal cells

Hypertonic solutions; poses a problem because water moving into animals cells by osmosis can cause the cell to swell and eventually burst the cell membrane, killing the organism. Cells in most animals are not directly exposed to the external environment and are bathed in an isotonic extracellular fluid. Water diffuses equally in both directions, resulting in no net movement of water in and out of cells.  The water concentration in animal cells needs to be kept constant to coordinate biochemical reactions.

Osmosis in plant cells

Plant cells do not burst when soaked in fresh water (hypotonic solutions). - Plant cells usually have large, fluid-filled vacuoles and firm, semipermeable cell walls that surround the cell membrane. They contain cell sap that has a high concentration of solute (therefore a low concentration of water) - When a hypotonic solution surrounds a plant cell, water molecules move by osmosis into the vacuole, causing the vacoule to swell and pushes the cell membrane against the cell wall. Their tough cell wall prevents the cell from bursting. - When the cell wall stretches as much as possible, the cell is turgid. In this state, the osmotic pressure inside the cell is equal to the opposing pressure exerted by the cell wall.

Osmotic pressure and a plant

Plant Osmosis Diagram (binary/octet-stream)

Active transport

Active transport is the movement of molecules from a region of high concentration to a region of low concentration, and requires the input of energy. Active transport requires a carrier protein that spans the membrane to actively move chemicals from a region of low to high concentration, utilizing cellular energy

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Summary of the processes

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Transport across cell membranes

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Endocytosis

When a large molecule has to be moved into a cell, the cell membrane can change its shape to surround the particle and engulf it by the process of endocytosis. If a solid is engulfed, the process is termed phagocytosis (cell eating). If a fluid is engulfed, the process is termed pinocytosis (cell drinking).    

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Exocytosis

Cells also produce waste products that need to be removed out of the cell.  Exocytosis is the process by which these substances are transported to the external environment 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.

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Factors affecting material exchange

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. Hydrophillic, charged ions such as sodium (Na) and potassium (K) cannot cross their hydrophobic center of the membrane.  Physical factors- 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. Very large molecules that need to be transported into our out of the cell are moved by the processes of endocytosis or exocytosis.  Concentration gradient- The relative concentration of the substance on either side of the membrane affects the rate of diffusion of that substance. If high, the substance will diffuse rapidly. If low, the substance will diffuse slowly. Cells need to maintain a high concentration gradient. Surface-area-to-volume ratio- The smaller the cell, the higher the SA:V and the more efficient the exchange of substances into and out of the cell. A high SA:V allows the most efficient movement of substances. Large cells have a low SA:V, and small cells have a large SA:V. Long, flat cells have a higher SA:V than a spherical cell with the same volume. SA is calculated by finding the total area of the surface of the shape.  

3.2 Cell requirements

Cells need to obtain nutrients in the form of organic substances such as glucose, amino acids, fatty acids and glycerol, nucleotides and vitamins. Organic substances are synthesized by living things and contain carbon and hydrogen. Inorganic substances; gases, minerals, phosphates, sodium ions and water. They do not contain carbon and hydrogen.

The substances needed by living cells for their functioning, are used in two main ways: 1. As essential building blocks from which cells and living tissues are made. 2. As a source of stored energy for the cell. Organic nutrients are the main supply of stored energy in living things, but they are also used in the structure of cells. Inorganic nutrients are essential as structural parts of cells and tissue

Inorganic nutrients

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Organic compounds

Every living cell requires large organic molecules as part of their structure and to maintain the biochemical process involved in effective functioning. These large organic molecules are called biomacromolecules. The four main types are based on their chemical composition and structure: complex carbohydrates, lipids, proteins and nucleic acids. Carbohydrates: a group of organic molecules made up of carbon (C), hydrogen (H) and oxygen (O) atoms in the ratio 1:2:1  They are classified as monosaccharides (simple sugars), disaccharides and polysaccharides depending on how many monomers linked  

Classification and uses of carbohydrates

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Lipids

Lipids contain many carbon (C) and hydrogen (H) atoms with a few oxygen (O) atoms.  The fats and oils in the cells of organisms are typically composed of triglceride molecules. Lipids are relatively insoluble in water and has an oily, wax consistency. Lipids have three important functions: 1. Energy storage  2. Structural component of membranes 3. Essential structural parts of hormones, which are chemical messengers produced by cells.

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Proteins

Proteins are made up of the elements carbon, hydrogen, oxygen, nitrogen and sometimes sulfur. These elements combine to form amino acids, which are the building blocks of proteins. Proteins are made up of one or more of these polypeptide chains twisted together into a particular shape. The DNA in the nucleus of the cell controls the sequence and arrangement of the amino acids. Proteins: 1. Form structural components in cells/tissue 2. Are an important structural component of cell membranes. 3. Some proteins have a functional role, such as enzymes, which control the metabolism. 4. Proteins occur suspended in the protoplasm of cells or combine with other macromolecules to form an important structural part of all membranes within the cell. Plants synthesize their own amino acids, animal cells produce most of the amino acids.   

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Nucleic acids

Nucleic acids are very large biomacromolecules that contain the elements carbon, hydrogen, oxygen, nitrogen and phosphorus.  Two types of nucleic acids: DNA (deoxyribonucleic acid) and RNA (ribonucleic acid) -DNA is a double stranded molecule that stores the information that controls the cell. It is the main chemical making up the nucleus. Small amounts are found in mitochondria and chloroplasts. -RNA is found in small amounts in the nucleus and in larger amounts in the cytoplasm. The building blocks of DNA and RNA are called nucleotides. Each nucleotides consists of a sugar, a sub-unit called a base, and phosphate.  DNA nucleotides contain the four bases adenine, guanine, cytosine, thymine.  DNA nucleotides are required by cells to make DNA during cell replication. RNA nucleotides contain the four bases adenine, guanine, cytosine and uracil. They also contain the sugar ribose. RNA nucleotides are required for cells to make ribosomes and to make RNA so that cells can make proteins.   

Nucleic acids demonstrated:

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3.3 Enzymes

Enzymes are protein molecules that control all metabolic reactions in living cells. Without enzymes, the reactions that occur in cells would be so slow as to hardly proceed at all. Enzymes act as biological catalysts, controlling the rate of each step of the complex chemical reactions that take place in cells. Catalysts are chemical substances that can accelerate (speed up) chemical reactions, are unchanged at the end of the reaction, and can be reused many times. Metabolism is the sum of all chemical reactions occurring within a living organism. A specific enzyme catalyses each of these reactions. The minimum amount of energy required to start the reaction is called the activation energy (the energy required to break bonds in reactants). Enzymes speed up reactions by lowering the activation energy required for the reaction. They do this by combining with the reactants and holding them in way that makes the reaction more likely to occur.