Cells as the Basis of Life - Detailed Notes

Cell Structure

• Inquiry Question: What distinguishes one cell from another?
• Investigate different cellular structures, including:
  • Examining prokaryotic and eukaryotic cells
  • Describing technologies used to determine cell structure and function
  • Drawing scaled diagrams of cells
  • Comparing and contrasting cell organelles and arrangements
  • Modelling the fluid mosaic model of the cell membrane

Prokaryotes vs. Eukaryotes

• Cells can be unicellular (single-celled) or multicellular (multi-celled).
• Prokaryotic Cell
  • Features: capsule, pilus, cell wall, plasma membrane, flagellum, nucleoid (DNA), cytoplasm
• Eukaryotic Cell
  • Features: endoplasmic reticulum (smooth & rough), ribosomes, plasma membrane, nucleus, cytosol, nucleolus, Golgi body, lysosome, cytoskeleton, mitochondrion, peroxisome

Prokaryotes

• 'Pro' means before, 'karyon' means 'nucleus.'
• Unicellular organisms, including Archaea and bacteria.
• Very small and found everywhere.

Eukaryotes

• 'Eu' means true, 'karyon' means 'nucleus.'
• Larger and more complex.
• Multicellular organisms, including plants and animals.
• Divided into protists, fungi, plants, and animals.

Prokaryotes vs. Eukaryotes - Key Differences

• Prokaryotes:
  • No membrane-bound organelles
  • No nucleus
  • DNA is circular and contained in plasmids
  • Smaller (cell size 0.1-5 μm)
  • Example: Cyanobacteria
• Eukaryotes:
  • Membrane-bound organelles
  • Nucleus present
  • DNA is linear and found in the nucleus
  • Larger (cell size 10-100 μm, up to 10 times larger than prokaryotes)
  • Example: Humans

Endosymbiosis Theory

• Explains how eukaryotic cells could have evolved from prokaryotic cells.
• States that large prokaryotic cells engulfed smaller bacteria, which lived as symbionts.
• Mitochondria and chloroplasts contain their own DNA, arranged similarly to prokaryotes.

Parts of a Cell

• Organelles and Structures:
  • Nucleus: contains nuclear pore, nuclear envelope, nucleolus, chromatin
  • Mitochondrion
  • Ribosomes
  • Lysosome (primarily in animal cells, lytic vacuoles in plants)
  • Golgi apparatus
  • Smooth endoplasmic reticulum
  • Rough endoplasmic reticulum
  • Cytoplasm
  • Plasma membrane (same as cell membrane)
  • Vacuole
  • Cell wall (plant cells only)
  • Chloroplast (plant cells only)
  • Centrioles (animal cells only)
  • Microtubules
  • Secretory vesicle

Organelles - Animal, Plant, or Both

• Nucleus: Both
• Vacuole: Both (larger in plants, smaller/temporary in animals)
• Chloroplast: Plants only
• Mitochondria: Both
• Cell wall: Plants only
• Cell membrane: Both

Nucleus

• Large organelle surrounded by a double-layered membrane.
• Contains most of the genetic material (DNA and proteins).
• Control center of the cell, coordinating all cell activities.
• Contains the nucleolus, which makes ribosomes.

Ribosome

• Composed of two units.
• Tiny, only visible with an electron microscope.
• Composed of proteins and ribosomal RNA (rRNA).
• Sites of protein synthesis.
• Not a membrane-bound organelle.

Endoplasmic Reticulum (ER)

• Network of intracellular membranous sacs and tubules.
• Links with the cell membrane and other membranous organelles.
• Rough ER: has ribosomes attached, actively produces and exports proteins.
• Smooth ER: no ribosomes, contains enzymes involved in phospholipid synthesis.

Golgi Apparatus

• Also known as the Golgi body.
• Stack of flattened, smooth membrane sacs.
• Responsible for bundling and packaging macromolecules like proteins and lipids synthesized within the cell.

Lysosome

• Specialized for digestion and waste removal.
• Sphere-shaped structure that ingests bacteria, broken cell parts, and other unwanted materials.

Mitochondria

• Membrane-bound organelles.
• Responsible for cellular respiration.
• Highly folded inner membrane increases the surface area for chemical reactions.

Chloroplasts

• Membrane-bound organelles involved in photosynthesis.
• Contain the green pigment chlorophyll.
• Thylakoids inside are disc-shaped.
• Trap light energy in photosynthesis.

Vacuole

• Membrane-bound storage compartments of a cell.
• Animal cells have many small, temporary vacuoles.
• Plant cells contain a single, large, permanent vacuole.
• Vacuoles in plants provide structural support.

Cell Wall

• Rigid structure surrounding the cell membrane of plant cells, fungal cells, and some prokaryotic cells.
• Provides support for the cell.

Cell Membrane

• Semi-permeable barrier that controls the exchange of materials into and out of the cell.

Biological Drawings

• Use a pencil and ruler.
• Make it large.
• No sketching or shading.
• 2D only.
• Label written clearly.
• Title.
• Magnification.
• A scale.

Biological Drawings - Scale

• Example: Length of a red blood cell = 8μm; the length of your drawing = 4cm
• Actual length = 8μm
• Length of drawing = 4cm
• Scale = $\frac{4 \text{ cm}}{8 \mu \text{m}} = \frac{1 \text{ cm}}{2 \mu \text{m}}$
• Therefore, 1cm in your drawing represents 2μm.

Technologies - Microscopes

• Magnification: how many times larger the object becomes.
• Resolution: degree of detail.
• Light Microscope: can see the cell wall, cell membrane, cytoplasm.

Technologies - Microscopes

• Electron Microscopes: able to see more detail and therefore can see structures like the chloroplast, mitochondria, rough endoplasmic reticulum, nucleus (in more detail), Golgi body etc.

Cell Membrane - Phospholipid Bilayer

• external environment
• internal environment
• Components: cholesterol, phospholipid, protein

Cell Membrane - Fluid Mosaic Model

• The membrane is selectively permeable – allows only certain molecules or ions into or out of the cell.
• It allows the concentration of substances inside cells to remain constant compared to outside.
• It is called the fluid mosaic model as the proteins floating on and in it create a ‘lipid sea.’
• It has the ability to flow and change shape.
• Protein molecules are embedded in various patterns like a mosaic.
• Some proteins can move and others are fixed.
• Each phospholipid has a head and a tail.
• One end is hydrophilic (water-liking).
• The other is hydrophobic (water-hating).

Transport Mechanisms

• Passive Transport: Simple diffusion, channel-mediated, carrier-mediated.
• Active Transport: Requires energy
• Factors: electrochemical gradient, membrane potential (negative or positive inside), concentration gradient

Cell Function

• Inquiry Question: How do cells coordinate their activities within their internal environment and the external environment?
• Investigate the way in which materials can move into and out of cells, including:
  • 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:
  • 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

Movement in and Out of Cells

• For substances to move in and out of cells it must pass through the cell membrane, which is semi-permeable.
• It is important for materials to be exchanged for cell functioning and for cells to communicate.

Factors Affecting Permeability

• Size, shape, and the make-up of the molecule can affect its permeability.
• Examples:
  • Small, uncharged molecules (oxygen, carbon dioxide): permeable
  • Lipid-soluble, non-polar molecules (alcohol, chloroform, steroids): permeable
  • Small, polar molecules (water, urea): permeable or semipermeable
  • Small ions (potassium ion $(K^+)$, sodium ion $(Na^+)$, chloride ion $(Cl^-)$) impermeable (ion passes through protein channels)
  • Large, polar, water-soluble molecules (amino acid, glucose): impermeable (molecule passes through protein channels)

Active and Passive Transport

• Active Transport:
  • Requires energy.
  • Generally against a concentration gradient.
  • E.g., the sodium and potassium pump.
• Passive Transport:
  • No energy required.

Diffusion

• Passive transport where particles move from an area of high concentration to an area of low concentration until equilibrium is reached.

Facilitated Diffusion

• Required for large particles to pass through the cell membrane (e.g., glucose and amino acids).
• These particles do not easily pass through the cell membrane and require carrier proteins and channel proteins to assist their movement.

Osmosis

• Passive transport where water molecules move from an area of high concentration to an area of low concentration until equilibrium is reached.
• (Water is the substance that is moving)

Osmosis - Tonicity

• Hypertonic – movement of water out of the cell. Cell becomes flaccid.
• Hypotonic – movement of water into the cell. Cell becomes turgid (full of water).
• Isotonic – equilibrium (equal).

Endocytosis

• 'Endo' = inside & 'cyto' = cell
• Endocytosis is where large particles move into the cell with the help of the cell membrane (active transport).
• Three types of endocytosis:
  1. Phagocytosis – particle is engulfed.
  2. Pinocytosis – liquid is engulfed.
  3. Receptor-mediated endocytosis

Exocytosis

• 'Exo' = outside & 'cyto' = cell
• Exocytosis is where materials are transported from the inside to the outside of the cell in membrane-bound vesicles that fuse with the plasma membrane.

Surface Area to Volume Ratio

• The smaller the cell, the larger the SA:V, which means the faster substances can reach the middle of the cell. This is better than a small SA:V.
• As cell size increases, the SA:V decreases.
• Smaller cells can exchange matter with their environment more efficiently.

Surface Area to Volume Ratio – Calculation Example

• Calculating SA:V
  1. Calculate the surface area.
  2. Calculate volume.
  3. Calculate surface area divided by volume.
  4. This will give you your ratio.
• Example; a 5cm cube:
  1. $SA = (5 \times 5) \times 6 = 150 \text{cm}^2$
  2. $V = 5 \times 5 \times 5 = 125 \text{cm}^3$
  3. $SA:V = \frac{150}{125} = 1.2$

Materials Being Exchanged

• Chemical factors:
  • Uncharged molecules can easily pass through the membrane.
  • Water, sodium, and potassium cannot pass through. Water passes through the membrane through aquaporins.
• Physical factors:
  • Size and shape affect the ability of molecules to move through the membrane.
  • Small molecules can diffuse easily, and large molecules, like glucose, use carrier proteins to assist their movement (endocytosis and exocytosis).

Cell Requirements

Organic: Includes carbohydrates, lipids, proteins, nucleic acids.
Inorganic: Includes water, oxygen, carbon dioxide, nitrogen.
*Cells require organic and inorganic molecules to carry out activities.
* Organic = containing carbon attached to hydrogen.
* Inorganic = carbon (if present) is not attached to hydrogen’s.

Biochemical Processes - Photosynthesis

• Plants use light energy and trap it in chlorophyll inside the chloroplast.
• This is how plants make their own food (sugar – glucose).

Biochemical Processes - Photosynthesis Stages

• Occurs in 2 stages:
  1. Light-dependent stage:
     • Chlorophyll captures sunlight (solar energy) and uses it to produce ATP (adenosine triphosphate).
     • During this stage, water is split into hydrogen ions and oxygen gas.
     • This stage occurs in the thylakoid membranes of the chloroplast.
  2. Light-independent stage:
     • Also known as the dark reactions, produces glucose, water, and adenosine diphosphate (ADP).
     • These reactions do not require solar energy.
     • ATP made in the first stage is used to power the dark reactions.
     • This occurs in the stroma of the chloroplast.

Biochemical Processes - Cellular Respiration

• Organisms break down glucose as a source of energy to drive cellular respiration.
• Word equation:
  • glucose + oxygen $\rightarrow$ carbon dioxide + water + energy (ATP)
• Balanced equation: $C<em>6H</em>{12}O<em>6 + 6O</em>2 + ADP + P \rightarrow 6CO<em>2 + 6H</em>2O + 36ATP$

Biochemical Processes - Cellular Respiration Stages

• Glycolysis is the first step to breaking down glucose in the cytosol; this produces two ATP molecules.
• The second stage occurs in the mitochondria and releases 34 ATP molecules.

Waste Products and Removal

• Nitrogen-containing waste (e.g., urea, uric acid etc.):
  • Produced by the breaking down of proteins.
  • Usually in solution or as a solid (uric acid).
  • Urea in solution can pass through pores in cells.
• Carbon dioxide:
  • Produced during aerobic respiration.
  • Dissolves and diffuses out of cells.
• Oxygen:
  • In plant cells carrying out photosynthesis in surplus of respiration requirements.
  • Diffuses out.
• Water:
  • Produced through respiration and surplus intake.
  • Passes readily through protein channels.

Enzymes

• Enzymes are protein molecules that control all metabolic reactions in living cells.
• They control the rate of reactions (a biological catalyst) – either speed up or slow down.
• There can be up to 1000 reactions, each of these requires a specific enzyme (enzyme specificity).
• Enzymes do not get used up in the reaction and can be reused.
• Enzymes have an active site – this is the area the substrate (the molecule the enzyme acts on) will bind to.
• One enzyme will only catalyse one type of reaction (enzyme specificity).

Enzyme Models

• Two accepted models for Enzymes:
  1. Lock and Key model
  2. Induced fit model

Factors Affecting Enzyme Activity

• Enzymes require specific conditions in order to function at optimal efficiency. Changes to these conditions will mean they work slower or not at all.
• Factors affecting enzyme activity:
  • Temperature
  • pH
  • Substrate concentration

Enzymes - Temperature

• Enzymes within cells function best at the body temperature of the organism they are found in (up to 40 degrees).
• E.g., humans body temperature is 37 degrees Celsius; this is why it is dangerous for us to fluctuate away from this.
• High temperatures cause the enzyme to change shape and denature (therefore the enzyme can no longer bind to the substrate).
• At low temperatures, the enzyme activity slows.

Enzymes - pH

• pH is a measure of acidity or alkalinity (acidic or basic).
• Where an enzyme functions will determine its optimal pH (e.g., pepsin, an enzyme in the stomach, has a pH of 2).
• When outside its optimal range, the activity decreases, and at extremes, it may denature.

Enzymes - Substrate Concentration

• The higher the concentration of the substrate, the greater the rate of enzyme reactions.
• This is until all the available enzymes are used up (saturation point).