Biology SL Exam Notes

Exam Details

• Diploma Programme Biology Standard Level Paper 1A and 1B.
• Paper 1A: 30 marks, Paper 1B: 25 marks, Total: 55 marks.
• Calculator is required.

Unit 1 - Chemistry of Life

A11 Water

• Essential properties for life are due to polar covalent bonds (unequal electron sharing between oxygen and hydrogen).
• Cohesive forces attract water molecules to each other.
• Adhesive forces attract water molecules to other polar molecules.
• Water is an excellent solvent for polar molecules, making it the "solvent of life" in cytoplasm, intercellular fluids, and blood.
• Challenges and opportunities as a habitat:
  • Buoyancy: Important for aquatic organisms to remain near the surface.
  • Viscosity: Organisms adapt body shapes and propulsion to overcome water resistance.
  • Thermal conductivity: Organisms in cold water require physiological adaptations or insulation.
  • Specific heat: High specific heat protects aquatic organisms from extreme air temperatures.

A1.1.1 Water's Role in the Origin of Life

• The first cells originated in water, which remains the medium for most life processes.

A1.1.2 Hydrogen Bonds in Water Molecules

• Hydrogen bonds form due to polar covalent O-H bonds, where electrons are shared unequally, causing small positive and negative charges.
• Skill: Ability to draw two or more water molecules and hydrogen bonds between them.

A1.1.3 Cohesion Due to Hydrogen Bonding

• Hydrogen bonding causes cohesion of water molecules, enabling water transport under tension in xylem and surface tension for habitats (e.g., insects and spiders).

A1.1.4 Adhesion Due to Hydrogen Bonding

• Hydrogen bonding causes adhesion to polar or charged materials, impacting capillary action in soil and plant cell walls.

A1.1.5 Solvent Properties of Water

• Hydrogen bonding allows hydrophilic molecules to dissolve in water, making it an excellent medium for metabolism and transport.
• Most enzymes catalyze reactions in aqueous solutions.
• Some molecules function by being hydrophobic and insoluble.

A1.1.6 Physical Properties of Water in Aquatic Habitats

• Physical properties include buoyancy, viscosity, thermal conductivity, and specific heat capacity.
• Contrast water's properties with those of air and illustrate consequences using animals in water versus air/land (e.g., black-throated loon, Gavia arctica, and ringed seal, Pusa hispida).
• Note: Common or scientific names are acceptable in exams (e.g. 'Common Frog' or 'Common tree frog').

B11 - Carbohydrates and Lipids

• Variations in form allow diversity of function.
• Carbohydrates exist as monosaccharides, disaccharides, and polysaccharides.
• Monosaccharides are the smallest carbohydrate molecules and monomers of disaccharides and polysaccharides.
• Glucose is a monosaccharide often used in cell respiration for energy.
• Glucose molecules form energy storage (amylose, amylopectin, glycogen) and structural molecules (cellulose).
• Lipids exist as triglycerides, phospholipids, cholesterol, and steroid hormones.
• Lipids have low solubility in aqueous solutions.
• Amphipathic properties of phospholipids make them ideal for membrane bilayers.

B1.1.1 Chemical Properties of Carbon

• Chemical properties of carbon atom allow for the formation of diverse compounds upon which life is based.
• A covalent bond is formed between two atoms which share a pair of electrons.
• Carbon can form up to four single bonds or a combination of single and double bonds with other carbon atoms or atoms of other non-metallic elements, such as hydrogen and oxygen.
• Examples include Carbon dioxide, water, glucose (single ring), sucrose (double ring), glycogen, starch, and cellulose molecules.
• NOS: Scientific conventions are based on international agreement (SI metric units have prefixes “kilo”, “centi”, “milli”, “micro” and “nano” for example kilogram).

B1.1.2 Macromolecule Formation

• Macromolecules like polysaccharides, polypeptides, and nucleic acids are produced by condensation reactions linking monomers to form a polymer.

B1.1.3 Polymer Digestion

• Polymers are digested into monomers by hydrolysis reactions.
• Water molecules are split to provide -H and -OH groups, incorporated into a specific bond to produce monomers.
• Hydrolysis: Hydro - water, Lysis - splitting.

B1.1.4 Form and Function of Monosaccharides

• Pentoses (e.g., ribose) and hexoses (e.g., glucose) are monosaccharides recognizable by carbon atom number and ring structure.
• Glucose has high solubility, transportability, chemical stability, and high energy yield from oxidation.
• Properties of monosaccharides link to their cellular use in respiration.
• Examples of macromolecules: Carbon dioxide, water, glucose, sucrose, glycogen, starch and cellulose.

Energy Storage

• Carbohydrates and lipids are used for chemical energy storage.
• Carbohydrate polysaccharide molecules (starch and glycogen) are often used for short-term energy storage.
• Lipids in the form of triglycerides are often used for longer-term energy storage.
• Carbohydrates hydrolyze into glucose, a soluble molecule that can be easily transported within cells and between cells.
• One product of triglyceride hydrolysis is fatty acids, which have very low solubility in aqueous solutions and thus are not easily transported.
• Triglycerides can store approximately twice the chemical energy compared to the same mass of carbohydrates.

B1.1.5 Polysaccharides as Energy Storage

• The compact nature of starch in plants and glycogen in animals makes them good storage molecules due to coiling, branching, and relative insolubility (due to large molecular size).
• It is relatively easy to add or remove alpha-glucose monomers (by condensation and hydrolysis) to build or mobilize energy stores.

B1.1.6 Structure of Cellulose

• Alternating orientation of beta-glucose monomers gives straight chains grouped in bundles held by hydrogen bonds.
• These structures form cell walls.

B1.1.7 Glycoproteins Function

• Glycoproteins (carbohydrate and protein component) act as ABO antigens in cell membranes for cell recognition.

B1.1.8 Hydrophobic Properties of Lipids

• Lipids dissolve in non-polar solvents but are only slightly soluble in aqueous solvents.
• Lipids include fats, oils, waxes, and steroids.

B1.1.9 Formation of Triglycerides and Phospholipids

• Triglycerides and phospholipids are formed by condensation reactions that join fatty acids to glycerol, and phosphate to glycerol in phospholipids.
• One glycerol links three fatty acids or two fatty acids and one phosphate group.

B1.1.10 Fatty Acids

• Fatty acids can be saturated (no double C=C bonds), monounsaturated (one C=C bond), or polyunsaturated (two or more C=C bonds).
• The number of double carbon (C=C) bonds affects melting point.
• Different types of fatty acids are found in oils and fats.

B1.1.11 Triglycerides

• Triglycerides are used in adipose tissues for energy storage and thermal insulation.
• The body temperature and habitat of an organism affect the use of triglycerides as thermal insulation.

B1.1.12 Phospholipid Bilayers

• The formation of phospholipid bilayers is a consequence of the hydrophobic fatty acid tails and the hydrophilic phosphate regions.
• "Amphipathic" describes molecules with both hydrophobic and hydrophilic regions.

B1.1.13 Steroids

• Non-polar steroids (e.g., oestradiol and testosterone) can pass through the phospholipid bilayer.
• Steroids consist of a skeleton of carbon atoms making four fused rings.
• Hydrogen atoms and other functional groups are attached to this skeleton.

B12 - Proteins

B1.2.1 Amino Acid Structure

• A generalized amino acid includes an alpha carbon atom attached to an amine group, carboxyl group, R-group, and hydrogen.

B1.2.2 Dipeptide Formation

• Condensation reactions form dipeptides and longer amino acid chains.
• Equation: amino acid 1 + amino acid 2 = dipeptide + water

B1.2.3 Essential Amino Acids

• Essential amino acids must be obtained from food; non-essential amino acids can be made from other amino acids.
• Some amino acids have nonpolar R groups, and some have polar R-groups.
• DNA codes the amino acid sequence in a protein.
• A polypeptide is created when condensation reactions occur between adjoining carboxyl groups and amine groups of adjoining amino acids.

Protein Diversity

• Polypeptides vary in length from a few to thousands of amino acids.
• The sequence of amino acids determines molecular shape and biological function.
• The variety of polypeptide chains is nearly infinite.

Environmental Effects on Proteins

• Temperatures higher than optimum affect protein shape and function.
• Many bonds creating protein shape are weak hydrogen bonds between polar amino acids.
• Higher temperatures increase molecular motion, stressing hydrogen bonds.
• Excessively high temperatures can lead to denaturation, or unfolding, of a protein.
• Breaking hydrogen bonds denatures a protein, leading to decreased activity.
• Non-optimal pH alters positive and negative charges needed for hydrogen bonding.
• Protein in a non-optimal pH environment changes shape, decreasing activity.
• Proteins fold differently in polar (water) versus nonpolar (lipids) solvents.
• Both enzymes and substrates change shape when binding occurs.
• There is an infinite variety of possible peptide chains.
• Genes code for 20 different amino acids.
• Genes can be any length, making peptide chains with any number of amino acids, from a few to thousands.
• Amino acids can be in any order in the polypeptide chain.

Examples

• Insulin (made of 2 polypeptides).
• Amylase (a large single polypeptide).
• Collagen (three polypeptide chains).
• Aquaporin (four polypeptides).
• Changes in pH and temperature can change protein structure.

B1.2.5 Protein Denaturation

• The 3D shape of polypeptides and proteins is held together by bonds, including hydrogen bonds, which can be broken by extremes of pH or high temperatures.
• Changes to polypeptide structure which stop the protein functioning is called "denaturation”.

C11 - Enzymes

• Enzymes are catalysts that increase the reaction rate in cells.
• This makes chemical reactions much faster than they would be without enzymes, enabling digestion of food, respiration, photosynthesis etc.
• Enzymes increase the rates of reactions in cells.

C1.1.1 Enzymes & Molecular Interaction

• Enzymes are globular proteins with an active site that combines with a substrate to catalyze a reaction.
• If the active site of an enzyme is changed, it will not bind to the substrate and increase the reaction rate.
• Extremes of pH and temperature can change the shape of the enzyme and therefore the active site.
• Enzymes are essential to metabolic pathways involving both catabolic and anabolic reactions.
• Enzymes may catalyze intracellular and/or extracellular reactions.

C1.1.2 Metabolism

• Metabolism is the complex network of interdependent and interacting enzyme catalysed chemical reactions occurring in living organisms.
• Enzymes have a specificity and only catalyse one reaction.
• So many different enzymes are required by living organisms for their metabolism and this enzyme specificity allows control of specific reactions in metabolism.

C1.1.3 Anabolic and Catabolic Reactions

• Anabolic reactions include the formation of macromolecules from monomers by condensation reactions including protein synthesis, glycogen formation and photosynthesis.
• Catabolic reactions include hydrolysis of macromolecules into monomers in digestion and oxidation of substrates in respiration.

C1.1.4 Enzyme Structure

• Enzymes are globular proteins with an active site for catalysis, made of a few amino acids.
• Interactions between amino acids within the overall three-dimensional structure of the enzyme ensure that the active site has the necessary properties for catalysis.

C1.1.5 Induced-Fit Binding

• Interactions between substrate and active site allow induced-fit binding where both substrate and enzymes change shape when binding occurs.

C1.1.6 Enzyme-Substrate Complex

• Collisions between substrate molecules and active sites are needed to form an enzyme-substrate complex.
• Sometimes large substrate molecules are immobilized & sometimes enzymes can be immobilized by being embedded in membranes. This can reduce movement.

C1.1.7 Denaturation

• The shape of the active site fits only the substrate molecule giving specificity.
• If the enzyme is denatured the active site changes shape and loses its ability to function.

C1.1.8 Factors Affecting Enzyme Activity

• Effects of temperature, pH and substrate concentration on the rate of enzyme activity are explained with reference to collision theory and denaturation.
• Optimum pH has the highest rate; as the pH becomes more different from the optimum pH the shape of the enzyme's active site changes and activity stops when the enzyme is 'denatured'.
• As the temperature increases the rate of enzyme substrate collisions increases so the reaction rate increases, until the temperature is so high that it breaks some of the bonds holding the 3D shape of the enzyme - at this point the rate of reaction falls rapidly because the enzyme molecules become denatured.
• Application of skills: Interpreting graphs showing the effects of temp and pH on enzyme activity is required, e.g. identifying optimum pH or temperature, or range of temperature/pH within which an enzyme is active.
• NOS: Students should be able to describe the relationship between variables as shown in graphs. They should recognise that generalized sketches of relationships are examples of models in biology. Models in the.

C1.1.9 Measurements

• Interpret graphs axes and curve see diagram.
• Measurements in enzyme-catalysed reactions (mass or volume of product made, or reactant used up)
• Application of skills: Determine reaction rate through experimentation and using secondary data.
• $(Rate = products made / time taken)$

C1.1.10 Activation Energy

• Effect of enzymes on activation energy - enzymes lower activation energy.
• Energy is required to break bonds within the substrate (activation energy).
• There is an energy yield when bonds are made to form the product(s).

Unit 2 - The Cell

A22 - Cell Structure

• Whether unicellular or multicellular, all organisms are composed of cells.
• Features common to all cells include DNA, cytoplasm and a plasma membrane forming an exterior boundary.
• Prokaryotic cells display a simple composition, lacking membrane-bound organelles in their cytoplasm.
• Eukaryotic cells are compartmentalized, with isolated areas carrying out specialized tasks.
• The cytoplasm of eukaryotic cells has many unique organelles working togethers, exhibiting all the life functions of the cell/organisms.
• Variations of the cell structure result in some unique cellular compositions, such as cells with multiple nuclei and cells with no nuclei.
• Evidence indicates that all eukaryotes evolved from a common ancestor.
• Endosymbiosis explains a mechanism for the development of some organelles of eukaryotic cells.
• Changes in gene expression result in differentiation of cells.
• Multicellularity appears to have evolved many times in various ways.

A2.2.1 Cell Theory

• Cells are the basic structural unit of all living organisms.
• NOS: Note: deductive reasoning can be used to generate predictions from theories. A general theory becomes a specific prediction using deductive reasoning , and this can be tested in experiments - it's the Scientific method.
  *Example: General theory: Reasoning using the cell theory, a specific newly discovered organism can be predicted to consist of one or more cells.

A2.2.2 Microscopy Skills

• Application of skills: Students should have experience of making temporary mounts of cells tissues. (Slides with coverslips, and a little water or stain, and a thin layer cells or tissues)
• Using a microscope - you should be able to do:
  • focusing with coarse and fine adjustments
  • measuring sizes using an eyepiece graticule,
  • calculating of the actual size or magnification of a sketch or a cell,
  • producing a scale bar
  • taking photographs.
• NOS: Students should appreciate that measurement using instruments (to get numbers) is quantitative observation.

A2.2.3 Developments in Microscopy

1. Electron Microscopy (EM)

   • Definition: Uses a beam of electrons instead of light to visualize specimens.
   • Types:
   • Transmission Electron Microscopy (TEM): Electrons pass through thin slices.
   • Scanning Electron Microscopy (SEM): Electrons scan the surface to create 3D images.
   • Advantages:
   • Extremely high resolution (can resolve structures as small as 0.1 nm).
   • Allows detailed visualization of ultrastructure, such as organelles, membranes, and viruses.
   • SEM provides 3D images of surfaces.

2. Freeze Fracture (Freeze Etching)

   • Definition: A preparation technique often used with electron microscopy where cells are frozen and fractured along membrane planes.
   • Advantages:
   • Reveals the internal architecture of cell membranes, including embedded proteins.
   • Preserves the native state of membranes with minimal distortion.

3. Cryogenic Electron Microscopy (Cryo-EM)

   • Definition: A form of TEM where samples are flash-frozen in vitreous ice without fixation or staining.
   • Advantages:
   • Preserves native structure of biomolecules in a near-natural hydrated state.
   • Enables visualization of large macromolecular complexes (e.g., ribosomes, viruses, proteins).

4. Fluorescent Stains & Immunofluorescence (Light Microscopy)

   • Definition:
   • Fluorescent stains bind to specific cell components and emit light under UV.
   • Immunofluorescence uses antibodies tagged with fluorescent dyes to target specific proteins.
   • Advantages:
   • Highly specific localization of molecules within cells or tissues.
   • Allows study of dynamic processes in living cells (e.g., protein movement, cell division).

A2.2.4 Structures Common to Cells in All Living Organisms

• Typical cells have
  • DNA as genetic material and
  • a cytoplasm composed mainly of water, which is
  • Enclosed by a plasma membrane composed of lipids.
• Students should understand the reasons for these structures.

A2.2.5 Prokaryote Cell Structure

• Include these cell components:
  • cell wall,
  • plasma membrane,
  • cytoplasm,
  • naked DNA in a loop and
  • 70S ribosomes.
• The type of prokaryotic cell structure required is that of Gram-positive bacteria such as Bacillus and Staphylococcus.
• Students should appreciate that prokaryote cell structure varies.

A2.2.6 Eukaryote Cell Structure

• Students should be familiar with features common to eukaryote cells:
  • a plasma membrane enclosing
  • A compartmentalized cytoplasm with
  • 80S ribosomes;
  • a nucleus with chromosomes made of DNA bound to histones, contained in a double membrane with pores;
  • membrane-bound cytoplasmic organelles including;
    • mitochondria,
    • endoplasmic reticulum,
    • Golgi apparatus and a variety of
    • vesicles or vacuoles including lysosomes; and
    • a cytoskeleton of microtubules and microfilaments.

A2.2.7 Processes of Life in Unicellular Organisms

• Includes:
  • Movement
  • Response to stimuli
  • Metabolism
  • Homeostasis
  • Growth
  • Reproduction
  • Excretion
  • Nutrition

A2.2.8 Differences in Eukaryotic Cell Structure

• Between Animals, Fungi and Plants:
  Include, A2.2.9—Atypical cell structure in eukaryotes.
• Use numbers of nuclei to illustrate one type of atypical cell structure in aseptate fungal hyphae, skeletal muscle, red blood cells and phloem sieve tube elements.

Summary Table of Differences

A2.2.10 Cell Types

• Cell types should be identified as prokaryote, plant or animal cells in light and electron micrographs.
• In electron micrographs, these structures should be identified:
  • nucleoid region,
  • prokaryotic cell wall,
  • nucleus,
  • mitochondrion,
  • chloroplast,
  • sap vacuole,
  • Golgi apparatus,
  • rough and smooth endoplasmic reticulum,
  • chromosomes,
  • ribosomes,
  • Cell wall
  • Plasma membrane
  • Microvilli.

A2.2.11

• Drawing diagrams of organelles and annotating them with their function, based on electron micrographs
• Nucleus
• Mitochondria
• Chloroplast
• Sap vacuole
• Golgi apparatus,
• Rough and smooth endoplasmic reticulum
• Chromosomes
  *
• Also other cell structures in electron micrographs:
• Cell wall,
• plasma membrane,
• secretory vesicles,
• microvilli.

B22 - Organelles and compartmentalization

• There is a structure - function correlation with all cell organelles.
• Each organelle of the cell has a unique structure that allows that organelle to perform its function, for example lysosomes are packages of digestive enzymes surrounded by a double membrane
• The cell membrane around the lysosomes prevents the digestive enzymes from damaging healthy structures in the cell
• The lysosomes can move anywhere in the cell and have a flexible membrane that can fuse with vesicles when necessary
• Mitochondria have a small intermembrane space so that a hydrogen ion concentration gradient can be quickly achieved
• Rough ER is most often seen transporting the proteins produced by its attached ribosomes.
• Compartments are areas of the cell that have been isolated from other parts of the cell so that specialized functions can be carried out within them.
  • This makes cells processes more efficient and stops the enzymes from one cell process interfering with another process, for example lysosomes contain enzymes used to break down substances ingested into the cell and worn out cell components, and by packaging them in a organelle the digestive enzymes are contained and controlled
  • Separating the nucleus from the rest of the cell means that mRNA can be processed before it reaches the ribosomes which translate it.

B2.1.1

• Lipid bilayers naturally form continuous sheets in water, making membranes. The are made of phospholipids and other amphipathic lipids (e.g. cholesterol)
  • Organelles are cellular compartments and include the nucleus, vesicles, ribosomes, mitochondria, chloroplasts and cell membrane
  • The cell wall, cytoskeleton and cytoplasm are not considered to be organelles.
• Clathrins are anchor proteins that function in the process of endocytosis
• The nuclear membrane is a double membrane that has pores that are essential for the overall functions of the cell

B2.2.1

• -Organelles as discrete subunits of cells that are adapted to perform specific functions
• Students should understand that the cell wall, cytoskeleton and cytoplasm are not considered organelles, and that nuclei, vesicles, ribosomes and the plasma membrane are.
• NOS:* Students should recognize that progress in science often follows development of new techniques. For example, study of the function of individual organelles became possible when ultracentrifuges had been invented and methods of using them for cell fractionation had been developed.

B2.2.2

*Advantage of the separation of the nucleus and cytoplasm into separate compartments
*Limit to separation of the activities of gene transcription and translation—post-transcriptional modification of mRNA can happen before the mRNA meets ribosomes in the cytoplasm
*In prokaryotes this is not possible—mRNA may immediately meet ribosomes

B2.2.3

Advantage of compartmentalization in the cytoplasm of cells.
*Include concentration of metabolites and enzymes and the separation of incompatible biochemical processes
*Include lysosomes and phagocytic vacuoles as examples

B21 - Membrane and Transport

• Amphipathic lipids and phospholipids form continuous sheet-like bilayers when in water.
• The bilayer forms because the hydrophilic portion of the phospholipid molecules in attraction to the water, which the hydrophobic portion of the molecules faces inwards away from contact with water.
  • Integral proteins are embedded in one of both lipid bilayers oa membrane.
  • Peripheral proteins are attached to one or other surface of the lipid bilayer.
  • Cholesterol is often present near the phospholipid tails of the cell membrane and has a role in the control of membrane fluidity
  • Glycoproteins and glycolipids have carbohydrates structures attached to them and often have roles in cell adhesion and recognition.
    *- Cell membrane have low permeability to large molecules and hydrophilic molecules
  • Diffusion and osmosis are examples of passive transport.
    Proteins pumps allow the movement of materials across the cell membrane that otherwise would not be able to pass because of their chemical properties
    Aquaporins are important in allowing polar water molecules to pass through cell membrane
    Channel proteins have pores, often with controlling gates, that allow ions and other polar materials to pass through cell membranes.
    Facilitated transport involves carrier proteins or channel proteins that aid in the movement of materials across the cell membrane.

B2.1.2 Membrane Permeability

• The hydrophobic hydrocarbon chains (lipid tails) that form the core of a membrane have low permeability to large molecules and hydrophilic particles, including ions and polar molecules, so they function as effective barriers between aqueous solutions.

B2.1.3 Diffusion of Oxygen and Carbon Dioxide

• Simple diffusion of oxygen and carbon dioxide molecules happens between phospholipids across membranes.

B2.1.4 Integral and Peripheral Membrane Proteins

• Integral and peripheral membrane proteins have diverse structures, locations and functions.
  * Integral proteins are embedded in one or both lipid layers of a membrane.
  * Peripheral proteins are attached to one surface of the bilayer.

B2.1.5 Osmosis and Aquaporins

• Water molecules move across membranes by osmosis. This movement is made faster by the presence of aquaporins - which are protein channels for water
• Osmosis is driven by:
  * the random movement of particles,
  * impermeability of membranes to solutes and
  * differences in solute concentration

B2.1.6 Channel Proteins

• The structure of channel proteins makes membranes selectively permeable by allowing specific ions to diffuse through when channels are open but not when they are closed. This is called facilitated diffusion.

B2.1.7 Pump Proteins

• Pump proteins use energy from adenosine triphosphate (ATP) to transfer specific particles across membranes and can move particles against a concentration gradient. This is called active transport.

B2.1.8 Selective Permeability

• Facilitated diffusion and active transport allow selective permeability in membranes.
• Simple diffusion does not allow selective permeability as it depends only on the size of particles and their hydrophilic or hydrophobic properties.

B.2.1.9 Glycoproteins and Glycolipids

• Glycoproteins and glycolipids are carbohydrate structures linked to proteins or lipids in membranes.
• Carbohydrates are located on the extracellular side of membranes, and has a role in cell adhesion and cell recognition. (E.g. Blood group antigens)

B2.1.10

*A two-dimensional diagram of the fluid mosaic model of membrane structure is required.

• This should include:
  *peripheral proteins
  *integral proteins,
  *glycoproteins,
  *phospholipids and
  *cholesterol.
  *hydrophobic regions
  *hydrophilic regions.

D23 - Water potential

• Water is a polar molecule that must pass through the cell membrane via specialized channels called aquaporins
• The hydrogen bond that water forms with ions and other charged particles contribute to the transport of essential materials into a cell and waste products out of a cell
• Salutation is the interaction of a solvent with a dissolved solute
• Water will move out of a cell when it is placed in a hypertonic environment
• Water will move into a cell when it is placed in a hypotonic environment
• The cell membrane contribute to the control of materials moving into and out of the cell.
• Osmosis requires a selectively permeable membrane to occur
• Water moves from regions of higher water potential to region of lower water potential.
• Turgor pressure (pressure potential) plays a large role in controlling water movement in and out of plant cells.
  *Because animal cells do not have a cell wall, turgor pressure is not a large factor in water movement in and out of animal cells.
  *Human red blood cells and all cells without a cell wall, when placed in a hypotonic solution, will swell and possibly burst.
  *Human red blood cells nad all cells without a cell wall, when placed in a hypertonic solution, will shrink and undergo crenation.
  *Plant cells when placed in an hypotonic solution, will develop high tutor pressure and the cell contents push against the cell wall, which maintains plant shape
• Cells possessing a cell wall will undergo plasmolysis when placed in a hypotonic environment
  D2.3.1
  *Solvation is the interaction between solute particles and water molecules (Also called Hydration).
  *These interactions include: hydrogen bond formation between polar molecules attaction between both positively and negatively charged ions and polar water molecules.
  *D2.3.2 Water moves from less concentrated solutions (hypotonic) to more concentrated solutions (hypertonic). The direction of movement is expressed in terms of solute concentration, not water concentration. Solutions which are “isotonic” have the same solute concentration.
  *D2.3.3_Water moves by osmosis into or out of cells (across membranes)
  *The direction of net movement of water can be predicted
• If the environment of a cell is hypotonic, compared to the cytoplasm, water will move into the cell
• If the surroundings are hypertonic, compared to the cell, water will move out of the cell
• A cell in an isotonic solution is in a dynamic equilibrium, an equal amount of water moves in each direction,there is movement of water across the membrane.

D2.3.4 Plant tissues change in length and mass when bathed in hypotonic solutions and in hypertonic solutions (caused by water movement).
*Application of skills: measure changes in tissue length and mass,

• When cells are placed in an isotonic environment, there is dynamic equilibrium rather than no movement of water.
  *Analyse data to deduce the isotonic solute concentration. (where the mass or length doesn't change)
• Use standard deviation (to show the variability of individual data about the mean of a sample - with five or more repeats)
• Use standard error (to show the variability of several means of data samples - if there are five or more means sampled.)
  *(Students are not required to memorize formulae for calculating these statistics)
  These statistics allow the reliability of length and mass measurements to be compared
  Standard deviation or standard error could be shown graphically as error bars.

D2.3.5 Effects of water movement on cells that lack a cell wall (Animal cells)
Cells that don't have a cell wall swell and burst in a hypotonic medium.
They show shrinkage and crenation (wrinkling) in a hypertonic solution

• Freshwater unicellular organisms need toremove water using contractile vacuoles,to avoid bursting.
• Multicellular organisms maintain isotonic tissue fluid to prevent harmful changes to cells.
  D2.3.6 Effects of water movement on cells with a cell wall (Plant cells, Fungi and Bacteria cells)
  Development of turgor pressure in a hypotonic medium and
  Plasmolysis in a hypertonic medium.
  D2.3.7 Medical application of isotonic solutions
  *Include as exaples:
  *intravenous fluids given as part of medical treatment and
  *bathing of organs ready for transplantation.

B23 - Specialized Cells

B2.3.1 - Following fertilization, stem cells are produced by mitosis and then develop into specialized cells by differentiation. Concentration gradients (of gene transcription factors can switch on genes) leading to specialisation of cells in early embryo development.
Location and function of stem cell niches in adult humans includes The stem cell niche can maintain the cells or promote their proliferation and differentiation
Differences between totipotent, plunpoent and multipotent stem cells
Cells in early-stage animal embryos are totipotent, but soon become pluripotent
Stem cells in adult tissue such as bone narrow are multipotent
There are several adjectives used to describe stems cells- these relate to the number of different types of differentiated cell that they can become. These include:

1. Totipotent can become any type in the adult body and any cell of the extraembryonic membranes example:
2. Pluripotent stem cells. Can make any differentiated dell in the body. examples are: Embryonic stems cells can be isolated from the inner cell masss.
3. Multipotent stem cells- In adult tissue, the same species may be a couple types of realed cells, example stem cells in the bone marrow can produce lymphatic and phagocytes,
   There research lokkming to try and reprogram these multipotent stems cells in hope and provide new treaments to repair damage issues.
   Many different cells exist in an organism, and all have a size and shape that means they can carry out their essential function efficiently
   The unique structures of erythrocytes and kidney cells allow specialized functions
   Lung cell possess properties that bring about efficient oxygen/carbon dioxide exchange
   Striated muscle fiberss have unique features that allow them to bring about movement
   Specialized features of egg and sperm are essential to the production of a zygote.

C1.2.1 ATP distributes energy within cells

C12 - Cellular respiration

• Oxygen is required in aerobic cell respiration, but not for anaerobic cell respiration ATP is known as the energy currency of the cell and i used to distribute energy within cells ATP is a nucleotide with a high-energy phosphate bond ATP is essential for active transport across cell membranes, the synthesis of macromolecules needed by the cell, and all types of movement involving the cell Cell respiration is the metabolic pathway that produces ATP in the cell
  • Lactic acid fermentation is a form of anaerobic cell respiration, it results in fewer ATP molecules than aerobic respiration but the cell is producing some ATP and can therefore sustain life processes for a period of time
    *movement of the whole cell or cell components such as chromosomes.
    *ATP (adenosine triphosphate) is a nucleotide. (It is made of; phosphate, ribose - a pentose sugar, adenine - a nucleotide base)
• The properties of ATP that make it suitable for use as the energy currency within cells.
• It is stable in aqueous solutions
  It has three phosphate molelcules