Unit 1 – Cell Size, Structure & Function

Cell Theory

• Three tenets first formulated in the 1800 s underpin all modern biology.
– All organisms are composed of cells.
– The cell is the basic unit of structure and organisation in organisms.
– All cells come from pre-existing cells.

Classification of Cells: Prokaryotic vs Eukaryotic

• Cells are divided into two fundamental groups—prokaryotes and eukaryotes.
• Size range
– Prokaryotes: 0.5{-}10\,\mu m (relatively small)
– Eukaryotes: 30{-}150\,\mu m (relatively large)
• Prokaryotic characteristics
– No membrane-bound nucleus; DNA is a single circular chromosome located in a nucleoid region.
– No membrane-bound organelles.
– Usually unicellular; includes Bacteria and Archaea.
– Often possess cell wall, capsule, pili and one or two flagella.
• Eukaryotic characteristics
– DNA enclosed by a double-membrane nucleus with nuclear pores.
– Possess numerous membrane-bound organelles (mitochondria, ER, Golgi, etc.).
– Greater internal complexity; can be unicellular (protists) or multicellular (fungi, plants, animals).
– Generally larger and metabolically more versatile.

Cell Size and Surface Area-to-Volume Ratio (SA:V)

• A cell’s ability to exchange materials with its environment is limited by its SA:V ratio.
• Mathematical relationships for a cube-shaped cell of side length n:
– SA = 6n^2
– V = n^3
– \frac{SA}{V} = \frac{6}{n}
→ As n increases, \frac{SA}{V} decreases.
• Lower size limit (≈ 10\,\mu m): minimum space needed to house essential components.
• Upper size limit (≈ 100\,\mu m): beyond this, volume (metabolic demand) increases faster than surface area (supply).
• Consequences
– When needs outweigh supply, the cell must stop growing, divide, or die.
– Adaptations to increase SA:V include elongation, flattening, and surface projections (root hairs, intestinal villi).

Cellular Structures and Organelles

Non-membrane Structures (present in all or most cells)

• Plasma (cell) membrane – phospholipid bilayer that is selectively permeable.
• Cytosol – aqueous, jelly-like matrix; site of many metabolic reactions.
• Cytoskeleton – protein filaments & microtubules that maintain shape, anchor/move organelles, form spindle fibres.
• Cell wall – semi-rigid outer layer (composition varies: cellulose in plants, chitin in fungi, peptidoglycan in bacteria, silica/cellulose in some protists).

Membrane-Bound Organelles (eukaryotes)

• Nucleus
– Double membrane with pores; stores DNA; nucleolus synthesises rRNA & ribosome subunits.
– Functions: DNA replication control, transcription regulation, genetic storage.
• Ribosomes (NOT membrane bound)
– rRNA + \text{protein}; translate mRNA into polypeptides; free or bound to rough ER.
• Endoplasmic Reticulum (ER)
– Rough ER (ribosome-studded): synthesises & transports proteins.
– Smooth ER: synthesises & transports lipids.
• Golgi Apparatus
– Stack of flattened sacs; modifies, tags, and packages proteins/lipids into vesicles for secretion or intracellular use.
• Secretory export pathway
– Ribosomes → Rough ER → Golgi apparatus → Vesicle → Plasma membrane (exocytosis).
• Lysosomes
– Golgi-derived vesicles containing hydrolytic enzymes; digest macromolecules, organelles, pathogens; trigger apoptosis.
• Vacuoles
– Membrane sacs for storage; large permanent central vacuole in plants (turgor maintenance); small, temporary in animals.
• Mitochondria
– Double membrane; inner folds (cristae) + matrix; site of aerobic respiration & ATP synthesis.
– Highly active cells (e.g. cardiac muscle, liver) possess many mitochondria.
• Chloroplasts (plants, some protists)
– Double membrane + internal thylakoid/grana system with chlorophyll; site of photosynthesis.
• Centrioles (animal cells)
– Perpendicular rod-like microtubule structures; organise spindle fibres during mitosis/meiosis.
• Cilia & Flagella
– Motile membrane extensions with microtubule cores; movement & fluid propulsion (cilia numerous/short, flagella few/long).
• Pili (prokaryotes)
– Surface hairs that connect bacteria for DNA exchange (conjugation).

Cellular Respiration (Aerobic)

• Purpose: convert chemical energy in glucose into usable ATP; if halted, cell starves of energy and dies.
• Overall equation
C6H{12}O6 + 6\,O2 \rightarrow 6\,CO2 + 6\,H2O + 30{-}32\,ATP
• Three stages & ATP yield
– Glycolysis (cytosol): splits 6-C glucose into two 3-C pyruvate; \text{net }2\,ATP.
– Krebs (citric-acid) cycle (mitochondrial matrix): produces loaded coenzymes; \text{net }2\,ATP.
– Electron Transport Chain (inner membrane/cristae): uses electrons to pump H^+, driving ATP synthase; 26{-}28\,ATP.
• Total aerobic yield ≈ 30{-}32\,ATP per glucose.

Plasma Membrane Structure (Fluid Mosaic Model)

• Semi-permeable phospholipid bilayer with dynamic “sea” of lipids, proteins, and carbohydrates.

Lipids

• Phospholipids are amphipathic: hydrophilic phosphate head + hydrophobic fatty-acid tails; spontaneously form bilayers in water.
• Cholesterol (animal membranes)
– Intercalates among tails; modulates fluidity.
– High T°: prevents excessive fluidity; low T°: prevents crystallisation.

Proteins

• Integral proteins – embedded; include transmembrane channels, carriers, pumps.
• Anchor proteins – covalently bound to a fatty acid inside bilayer.
• Peripheral proteins – loosely bound on membrane surface.
• Six functional categories (TRACIE mnemonic):
– Transport (channels & carriers).
– Reception (signal binding).
– Anchorage (cytoskeleton–ECM linkage).
– Cell identity (glycoprotein antigens).
– Intracellular joining (junctions).
– Enzymatic activity (ATP synthesis, etc.).

Carbohydrates

• Extracellular short chains attach to lipids (glycolipids) or proteins (glycoproteins).
• Roles: cell recognition, signalling, tissue formation, blood-group antigens (A, B, AB, O).

Membrane Function and Transport

• Key functions: boundary, selective transport, cell identity, signal reception, site of enzyme pathways.

Passive Transport (no ATP)

Simple Diffusion

• Movement of small non-polar gases (O2, CO2, N_2) or very small polar molecules (water, urea) directly through bilayer.
• Proceeds down concentration gradient until equilibrium; rate increases with larger gradient, greater surface area, shorter distance, and thinner barriers.

Facilitated Diffusion

• Large polar molecules (glucose) or ions (K$^+$, Na$^+$, Cl$^-) require protein channels or carriers.
• Still passive (down gradient) but faster and selective.

Osmosis

• Net diffusion of free water molecules from lower solute concentration (less negative water potential \Psi) to higher solute concentration (more negative \Psi) across a semi-permeable membrane.
• Water potential relation: \Psi = \Psis + \Psip (solute + pressure).
• Tonicity definitions
– Hypotonic (less negative \Psi): cell gains water → animal cell lysis; plant cell turgid.
– Isotonic: no net movement → animal cell normal; plant cell flaccid.
– Hypertonic (more negative \Psi): cell loses water → animal cell shrivels; plant cell plasmolyses (membrane retracts from wall).

Active Transport (requires ATP)

Protein Pumps

• Move substances against gradients.
• Example: Na$^+$/K$^+$-ATPase pumps 3 Na$^+$ out & 2 K$^+$ in per ATP, maintaining membrane potential.
• Proton (H$^+$) pumps eject H$^+$, generating electrochemical gradients used for co-transport.

Coupled (Co-) Transport

• Return diffusion of H$^+$ (or Na$^+$) down gradient drives uptake of another molecule (e.g. sucrose) via the same carrier; simultaneous passive + active process.

Bulk Transport (Cytosis)

• Endocytosis – membrane invaginates forming vesicle.
– Phagocytosis: uptake of solids (e.g. bacteria by macrophages).
– Pinocytosis: uptake of liquid droplets.
• Exocytosis – vesicle fuses with plasma membrane releasing contents (hormone secretion, waste removal).
• Requires cytoskeletal rearrangement & ATP.

Differences Between Plant and Animal Cells

• Cell wall of cellulose present only in plants.
• Chloroplasts present in plants/protists; absent in animals.
• Vacuoles: large permanent central vacuole in plants vs small temporary vesicles in animals.
• Centrioles organise spindle in animals; absent from most plant cells (spindle forms from other microtubule-organising centres).

Sample Numerical References & Equations

• SA/V for cube sizes in transcript table (side length in \mu m):
– 1\,\mu m → SA/V = 6:1; 2\,\mu m → 3:1; 3\,\mu m → 2:1; 6\,\mu m → 1:1.
• ATP yield summary: Glycolysis 2; Krebs 2; ETC 26{-}28; total 30{-}32.
• Water potential gradient drives osmosis from \Psi = 0 (pure water) toward \Psi = -600\,\text{kPa}$$ (hypertonic salt solution example).

Connections & Implications

• SA:V concept underlies need for organelles (compartmentalisation) and specialised exchange surfaces (lung alveoli, intestinal villi).
• Fluid mosaic membrane is central to homeostasis, signalling, immunity, and bioenergetics (respiration, photosynthesis).
• Malfunction in transport proteins leads to diseases (e.g. cystic fibrosis – chloride channel defect).
• Understanding transport mechanisms informs biotechnology (drug delivery, synthetic membranes) and medicine (IV fluid tonicity, dialysis).