IB BIO SL MIDTERM 2025

Osmosis Definition

the passive movement of water molecules across a selectively permeable membrane, from a region of low concentration to high concentration, no energy required

Osmosis in cells (Hypertonic solution)

• solution has a higher solute concentration than cell

• water moves out

• plant = plasmolysis

• animal = shrinks

Osmosis in cells (Hypotonic solution)

• solution has a lower solute concentration than cell

• water moves in

• plant = turgid

• animal = burst (lysis)

Osmosis in cells (Isotonic solution)

• same solute concentration in cell and solution

• no net movement of water

How Osmosis Happens

Aquaporins - water can pass through phospholipid bilayer (slow), but mostly passes through protein channels (aquaporins) (fast)

Real-World Examples

plant cells taking up water from soil

Polarity of Water

• Polar molecule

• oxygen carries a partial negative charge

• hydrogen carries a partial positive charge

• this allows water molecules to form hydrogen bonds

hydrogen bonding

• form between + hydrogen of one water molecule and - oxygen of another

• individually weak, but collectively strong

cohesion

• attraction between water molecules

• causes by hydrogen bonding

• results in: high surface tension (water droplets)

• important for: transport of water in plans (xylem)

adhesion

• attraction between water and other substances

• water stick to polar/charged surfaces

• important for: water moving along xylem walls in plants (capillary action)

thermal properties

• high specific heat capacity - water has to absorb lots of heat before its temperature increases

  • results in: stable temperatures in organisms

• high latent heat of vaporization - large amount of energy needed to evaporate water

  • used in: sweating to cool organisms

solvent properties

• excellent solvent

• dissolves - ionic substances, and polar molecules

• forms hydration shells around ions

• important for: transport of substances in blood and cytoplasm, and biochemical reactions

water as a medium

• most biochemical reactions occur in aqueous environments

• allows molecules to move and collide for reactions

What is DNA

• deoxyribonucleic acid

• stores genetic information

• found in: nucleus, mitochondria/chloroplasts, nucleoid region

Nucleotides

• building blocks of DNA/RNA

• made up of

  • phosphate group

  • deoxyribose sugar

  • nitrogenous base

Nitrogenous bases

• adenine

• guanine

• cytosine

• thymine → uracil (RNA)

sugar-phosphate backbone

• nucleotides join by phosphodiester bonds (between phosphate of one nucleotide and the sugar of the next)

• strong covalent bonds

double helix

• 2 polynucleotide strands

• twist to form a double helix

• backbone is on the outside

• bases face inwards

complementary base pairing

• A→T(2 hydrogen bonds)

• C→G(3 hydrogen bonds)

antiparallel structure

• 2 strands run in opposite directions

• strand 1 - 5→3

• strand 2 - 3→5

stability of DNA

• hydrogen bonds allow strands to separate

• phosphodiester bonds give strength and stability

• double helix protects base sequence

Prokaryotic cells

• no nucleus

• DNA is free in cytoplasm

• ex. Bacteria

Eukaryotic cells

• has nucleus

• DNA enclosed in a nuclear envelope

• ex. animal, plant, fungus

ribosomes

pro - 70s

euk - 80s

DNA and genetic material

prokaryotes - single circular chromosome

• in nucleoid region

• may have plasmids

• no histone proteins

eukaryotes - multiple linear chromosomes

• inside nucleus

• associated with histones

organelles

prokaryotes - not membrane bound

eukaryotes - membrane bound

metabolism and respiration

pro - on cell membrane

euk - in mitochondria

cell division

pro - binary fission

euk - mitosis (meiosis)

Paramecium

• unicellular eukaryote

• found in freshwater

• protists

• all life functions in one cell

nutrition

• heterotrophic

• feeds on bacteria and small organisms

• food enters in oral groove

• digestion occurs in food vacuoles

movement

• uses cilia

• allows: movement through water

• directing food towards oral groove

reproduction

• asexual reproduction (binary fission)

• sexual reproduction (conjugation)

  • involves genetic exchange

  • results in genetic variation

Polysaccharides

• long chains of monosaccharides (simple sugars) linked by glycosidic bonds

• they are carbohydrates used for energy storage/structural purposes

• they are macromolecules, insoluble in water, often branched or unbranched

formation

• condensation reactions

• longer chains = polysaccharides

• broken down my hydrolysis (addition of water) to release monosaccharides for energy

main polysaccharides

• starch (m = glucose) - sometimes branched - energy storage (plants) - ex. potatoes

• glycogen (glucose) - highly branched - energy storage (animals) - liver

• cellulose (glucose) - unbranched - structural support (plant walls)

• chitin (N-acetylglycosamine) - structural support in fungi cell walls

more branched =

faster energy release

humans cannot digest

cellulose

functions

energy storage - starch + glycogen

• branched allows for rapid hydrolysis to glucose

structural support - cellulose + chitin

• straight chains form strong fibers

amylose vs. amylopectine

amylose = unbranched = slow energy release

amylopectine = branched = rapid energy release

both are forms of startch

Condensation reaction

• two molecules join together

• one water molecule is released

• covalent bond is formed

• build polymers from monomers

in carbohydrates

• forming a disaccharide

• two monosaccharides join together

• forms glucosidic bonds

• water is released

condensation chart

carbohydrate - monosaccharides - glycosidic bond

proteins - amino acids - peptide bond

lipids - fatty acids + glycerol - ester bond

nucleic acids - nucleotides - phosphodiester bond

Fatty acids

• long hydrocarbon chains with a carboxyl group at one end

• key components of lipids

• used for energy storage, insulation, and membrane stucture

types

saturated - no carbon-carbon double bonds

• carbon atoms are fully saturated with hydrogen

• straight chains - pack tightly

• solid at room temp

unsaturated - contains one or more C=C double bonds

• fewer hydrogen atoms

• bent chains

• liquid at room temp

unsaturated types

monounsaturated - one carbon=carbon double bond

• cause a single bend

polyunsaturated - two or more double bonds

• multiple bends

• increase membrane fluidity

cis vs trans

cis - hydrogen atoms on same side of double bond

• creates a kink

• most natural unsaturated fats

trans - hydrogen atoms on opposite sides

• chain is straighter

• often artificially produced

• linked to health risks

structure - feature

saturated/straight - pack tightly - solid fats

unsaturated/kinked - pack loosely - liquid oils

more double bonds - greater membrane fluidity

cis vs trans - kinked vs straight structure

peptide bond

• covalent bond that links 2 amino acids

• forms between - carboxyl group (COOH) and the amino group (NH2)

• formed by condensation reaction

• releases one water molecule

picture

formation

• 2 amino acids align

• OH is removed from carboxyl group

• H is removed from amino group

• OH + H = H2O

• remaining atoms form peptide bond

dipeptides, polypeptides, proteins

dipeptide - 2 amino acids

polypeptides - many amino acids

protein - one or more polypeptide chains folded into a functional shape

importance

allows formation of - enzymes

• hormones

• structural proteins

sequence of amino acids determines protein structure and function

why membrane transport needed

• membranes are selectively permeable

• transport allows cells to obtain nutrients, remove waste, maintain internal conditions (homeostasis)

diffusion

• movement from high to low concentration

• no energy needed

• occurs for - small, non polar molecules (oxygen)

facilitated diffusion

• passive

• uses channel or carrier proteins

• for - ions, polar molecules (glucose)

• moved down the concentration gradient

osmosis

• diffusion of water

• through partially permeable membrane

• from high water potential to lower water potential

active transport

• moves substances against concentration gradient

• uses carrier proteins

• requires ATP

endocytosis + exocytosis

endo - substances enter cell in vesicles

• required ATP

exo - vesicle fuses with membrane to release substances

• used for secretion

structure - function

hydrophobic bilayer - blocks ions/polar molecules

channel proteins - allows ions to pass

carrier proteins - change shape during transport

ATP - provides energy for active transport

fluid mosaic model

fluid - phospholipids and proteins can move sideways

mosaic - proteins are embedded throughout the membrane

phospholipid bilayer

• hydrophilic (polar) phosphate head

• hydrophobic (non-polar) fatty acid tails

• heads face aqueous environments

• tails face inwards

membrane proteins

integral - spans the bilayer, inside

• functions: transport (channel, carrier) , receptors, enzymes

peripheral - attached to membrane surface, outside

• functions: structural support, cell signaling

cholesterol

• located between phospholipids

• regulated membrane fluidity

  • not become too rigid at low temps

  • prevents excessive fluidity at high temps

carbohydrates

• glycoproteins (protein + carbohydrate)

• glycolipids (lipid + carbohydrate)

functions: cell recognition, cell adhesion, receptor sites

• found on external surface only

membrane fluidity factors

unsaturated fatty acids → increase fluidity

shorter fatty acid chains → increase fluidity

higher temperature → increase fluidity

cholesterol → stabilizes fluidity

phospholipid

made up of - glycerol

• 2 fatty acid chains

• 1 phosphate group

nucleus

• contains genetic material

• controls gene expression

• controls cell activities (protein synthesis)

nuclear envelope

• double membrane

• continuous with the rough ER

• separates nuclear contents from cytoplasm

• function: protects DNA

  • controls movement of substances in and out

Nuclear Pores

• protein-lined openings in nuclear envelope

• allows selective transport of: mRNA

  • Ribosomal subunits

  • proteins

• active transport is often involved

chromatin

DNA + histone proteins

• condenses into chromosomes during cell division

nucleolus

• not membrane bound

• dense region in nucleus

• function: synthesizes rRNA

  • assembles ribosomal units

large nucleolus = high protein synthesis

structure-function

double membrane - protects DNA
nuclear pores - regulate gene expression

chromatin organization - controls transcription

nucleolus - ribosome production

stem cells

• undifferentiated cell that

  • can divide by mitosis

  • can differentiate into one or more specialized cell types

potency

range of cell types a stem cell can differentiate into

types of stem cells

• totipotent

• pluripotent

• multipotent

totipotent stem cells

• can differentiate into all cell types

• can form a whole organism

• ex. zygote

pluripotent stem cells

• can differentiate into almost all cell types

• cannot form a whole organism

• found in: embryonic stem cells

multipotent stem cells

• can differentiate into a limited range of cell types

• found in adult tissues

• ex. bone marrow stem cells → blood cells

Embryonic vs adult stem cells

F - E - A

potency - pluripotent - multipotent

source - embryo - adult tissue

SA:Vol important?

• exchange of substances occurs across the cell surface

• metabolic reactions occur inside the cell volume

• high SA:Vol allows faster and more efficient diffusion

small or large?

smaller cells have a higher SA:Vol ratio

• allows uptake of nutrients

• efficient removal of waste

• fast cell division and metabolism

what happens as cells get larger

• volume increases faster than surface area

• SA:Vol decreases

• diffusion becomes less efficient

• cells will need to divide

activation energy

• the minimum energy required to start a chemical reaction

  • break existing bonds

  • allow new bonds

• energy difference between reactants and the peak (higher peak = higher activation energy)

role of enzymes

• lower activation energy

• do not change: overall energy released, products of the reaction

• increase reaction rate

how they lower activation energy

• form enzyme-substrate complexes

• correctly orient substrates

• strain bonds

• provide an alternative reaction pathway

why it matters in cells

• many metabolic reactions have high activation energy

• without enzymes, reactions would be to slow at body temp

• enzymes allow reactions to occur at 37 degrees

enzyme temp effects

low - reaction rate is slow

increasing - kinetic energy is increasing

• reaction rate increases

optimum temp - humans = 37c

high temp - active site changes shape

• enzyme becomes denatured

effect of substrate concentration on enzymes

low - less substrate molecules available

• rate of reaction is low

increasing - more frequent collisions

• rate of reaction increases

high - all enzyme active sites are occupied

• rate reaches a max and stops increasing

anaerobic respiration

• breakdown of glucose without oxygen to release energy in form of ATP

• releases less energy than aerobic respiration because glucose is not fully broken down

in animals

• glucose → lactic acid + ATP

• occurs in muscle cells

• happens during vigorous exercise

• effect: causes muscle fatigue, leads to oxygen dept

in yeast and plants

• glucose → ethanol + carbon dioxide + ATP

• called fermentation

• carbon dioxide causes bread to rise (waste product)

oxygen dept

• the extra oxygen required after exercise to break down lactic acid

• transported to liver

compare aerobic and anaerobic

F - A - AN

oxygen - required - not required

ATP yield - high - low

Products - CO2 and H2O - lactic acid/ethanol

location - mitochondria - cytoplasm

Taq Polymerase

• a heat-stable DNA polymerase enzyme used in the polymerase chain reaction

why is it needed in PCR

• high temps (95c) to separate DNA strands

• lower temps for primer binding and DNA synthesis

• Taq does not denature at high temps (found in hot water springs)

role of taq

• extends DNA strands by adding free nucleotides to primer and synthesizing new DNA strands

• optimally works at 72c

advantages

• does not need to be replaced after each cycle

• allows rapid/efficient DNA amplification

• makes PCR practical

DNA profiling

• a technique used to identify individuals by analyzing specific regions of DNA that vary between people

steps

• extract DNA from cells

• apply STR’s using PCR (taq)

• separate DNA fragments by size using gel electrophoresis

• visualize bands to produce a DNA profile

• compare profiles

Gel electrophoresis

• dna is negatively charged

• moved towards the positive electrode

• smaller fragments move faster and further

• patter of bands = DNA profile