Passive Transport Across Cell Membranes

• Two main types of transport across a cell membrane: Passive and Active

Passive Transport

• Movement of materials across the cell membrane without energy expenditure.
• Substances move across the membrane spontaneously.
• Passive transport occurs due to a difference in concentration.
• Concentration gradient: Gradual change in concentration from one region to another (high to low).
• Materials move spontaneously from high to low concentration, down their concentration gradient.
• Movement continues until everything is balanced.
• Diffusion: Movement of any molecule from high to low concentration.

Simple Diffusion

• Substances diffuse directly through the phospholipid bilayer.
• Permeable materials will diffuse through the membrane (high to low), e.g., O2, CO2, urea, alcohols, steroids, and lipids.
• Oxygen readily diffuses across capillaries to body cells due to respiration.
• Impermeable materials won't diffuse across the membrane, regardless of the concentration gradient.

Facilitated Diffusion

• Substances diffuse through the cell membrane via channel and carrier proteins.
• Channel proteins provide a 'tunnel' for diffusion; each channel has a unique diameter and amino acid lining.
• Channel proteins only allow particular molecules to fit through.
• Carrier proteins bind to materials, change shape to allow diffusion, and revert to their original shape afterward.

Osmosis

• Water moves from high to low water concentration across a semipermeable membrane.
• Solution concentration refers to the amount of solute, not water.
• Semipermeable membrane allows water movement but not solute (e.g., sugar).
• Water moves from high to low concentration, regardless of how "unbalanced" it looks; amount of water molecules compared to concentration molecules matters.

Isotonic Solutions

• Solutions with equal solute concentrations.
• A cell in an isotonic solution is balanced.
• Rate of water leaving the cell = rate of water entering; concentrations are balanced.

Hypotonic Solution

• Solution with lower solute concentration (high water concentration).
• Water moves into the cell to achieve balance, e.g., fresh water cell.
• An animal cell will continue to absorb water until it bursts (lysed).
• A plant cell swells, but the cell wall prevents bursting (turgid).

Hypertonic Solution

• Solution with higher solute concentration (low water concentration).
• Water moves out of the cell to achieve balance, e.g., cell in saltwater (ocean).
• Water moves out of the cell to balance the concentration gradient.
• Animal cells shrivel, and plant cells become flaccid or plasmolysed.

Osmosis vs. Diffusion

• Osmosis involves water movement across a semipermeable membrane from high to low concentration.
• Diffusion involves any material moving from high to low concentration, not necessarily across a semipermeable membrane.

Cell Requirements: Inorganic

• Water (H2O) acts as an essential solvent and transport medium.
• Cellular reactions occur in water.
• Oxygen (O2) is needed for cellular energy (respiration) and is gained from air or dissolved in water.
• Carbon Dioxide (CO2) is a carbon source for organic molecules (photosynthesis) and is taken in as a gas.
• Nitrogen (N2) forms part of amino acids (proteins) and is absorbed by plants from the air.
• Minerals are essential for enzymes and vitamins (calcium, sodium/potassium, magnesium, iron).

Cell Requirements: Organic

• Carbohydrates provide energy and structural support (sugars and starches).
• Lipids (Fats) store energy and form cell membranes.
• Proteins are composed of amino acids and have functions, including enzymes, hormones, antibodies, and transport.
• Nucleic Acids (DNA/RNA) are responsible for genetic information, with DNA storing the genetic code and RNA helping make proteins.
• Autotrophs (Producers) like plants make their own food via photosynthesis.
• Heterotrophs (Consumers) obtain food by eating other organisms (animals and fungi).

Cell Requirements: Waste Removal

• Autotrophs and heterotrophs produce waste products from metabolism, known as excretions.
• Carbon dioxide is produced from cellular respiration.
• Nitrogenous wastes result from the breakdown of proteins and nucleic acids.

Photosynthesis and Chloroplasts

• Photosynthesis is the process by which plants use sunlight to make organic compounds (food).
• Chloroplasts are the site of photosynthesis, converting light energy into food (glucose) and releasing oxygen.
• Leaves and stems are green due to the chlorophyll pigment, which absorbs light energy.

Stages of Photosynthesis

• Light-dependent reactions happen in the thylakoids, where chlorophyll captures light energy, breaking down water into oxygen and hydrogen, also making ATP
• Light-independent reactions occur in the stroma, use ATP energy to combine carbon dioxide with hydrogen, producing glucose.

Factors Affecting the Rate of Photosynthesis

• Light intensity: More sunlight means faster photosynthesis.
• CO2 levels: More carbon dioxide means faster photosynthesis.
• Temperature affects the enzymes catalyzing photosynthesis.
• A combination of factors (light intensity, CO2 levels, and temperature) affects the rate.

Respiration

• ATP (adenosine triphosphate) is the 'energy currency' of life.
• Every living cell uses ATP to grow, repair, and survive.
• Chemical energy is stored in molecular bonds.
• Breaking a bond releases energy (forms ADP).
• Making a bond adds energy back for storage, converting ADP to ATP.

Cellular Respiration

• Organic compounds are broken down to produce ATP Energy.
• Carbohydrates, lipids, and proteins are the fuel sources.
• Aerobic respiration (with oxygen) occurs in the cytosol and mitochondria
• Anaerobic respiration (without oxygen) occurs in the cytosol.

Enzymes

• Enzymes are proteins acting as biological catalysts, speeding up chemical reactions.
• Molecules upon which enzymes act are called substrates.
• Enzymes convert substrates into different molecules called products.
• When an active site binds to a substrate, it forms an enzyme-substrate complex.

Enzyme Models

• Lock and Key Model: Active site and specific substrate fit together like a lock and key, highly specific, no rxn occurs if there is no fit.
• Induced Fit Model: The active site changes shape slightly to accommodate the substrate.
• More accurate than the lock and key model, active sites capable of changing shape.
• Shape of the active site is affected by pH and temperature, and they only work at/in specific temperature and pH.
• Increasing temperature to 40°C causes a permanent change to the active site.
• Enzymes stop working when heated and become denatured.

Accuracy, Validity and Reliability

• Reliability: Consistent results.
• Validity: Fair testing ensuring the independent variable is valid
• Control: Element that remains unchanged or unaffected by variables.
• Accuracy: Closest to true value using accurate measurements and equipment.

Prokaryotes

• Organisms made of prokaryotic cells (archaea and bacteria).
• Prokaryotic cells lack a nucleus or membrane-bound organelles (except ribosomes).
• Ribosomes float in the cytoplasm.
• Most prokaryotic cells have a cell wall made of peptidoglycan.
• Most have a capsule outside the cell wall.
• DNA is not enclosed in a nucleus and floats in the cytoplasm.
• DNA is small and circular.
• Unicellular
• Flagella and pili aid in movement.

Eukaryotes

• Organisms made of eukaryotic cells (animals, plants, fungi, and protists).
• Eukaryotic cells have a nucleus (still a membrane bound organelle) and membrane-bound organelles.
• Mitochondria and endoplasmic reticulum are present.
• Eukaryotic DNA is enclosed in the nucleus.
• DNA is straight and does not reconnect with itself.
• Eukaryotic cells have more DNA than prokaryotic cells.
• Generally multicellular, forming a single organism.
• Plant cells have extra organelles: vacuoles, cell wall, and chloroplast.
• Protists may have flagella but also possess a nucleus and other membrane-bound organelles.

Eukaryotes vs. Prokaryotes

• Eukaryotes have a nucleus and membrane-bound organelles, while prokaryotes do not.
• Eukaryotic DNA is linear and located in the nucleus, while prokaryotic DNA is circular and in the cytoplasm.
• Eukaryotes have a large amount of DNA, while prokaryotes have a small amount.
• Eukaryotes include animals, plants, fungi, and protists, while prokaryotes include bacteria and archaea.
• Eukaryotes are generally multicellular, while prokaryotes are unicellular.
• Eukaryotes have large cells, while prokaryotes have very small cells.
• Both eukaryotes and prokaryotes have ribosomes, though they have different locations.