Introduction to College Biology — Cell Structure and the Characteristics of Life

What Is Biology?

Definition of biology:
→ The study of the structure, function, growth, evolution, and distribution of living things

Seven characteristics of life:
1. Cellular organization: cells are the basic structural and functional unit
2. Metabolism: acquiring and using energy through chemical reactions
3. Growth: growing and developing
4. Responsiveness: reacting to environmental stimuli
5. Homeostasis: keeping the internal environment stable
6. Reproduction: producing offspring
7. Evolution: changing across generations

Biology's hierarchical organization:
atom → molecule → cell organelle → cell → tissue → organ
→ organ system → organism → population → community → ecosystem → biosphere

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Prokaryotic vs. Eukaryotic Cells

Prokaryotic cells:

• No nuclear envelope (DNA sits directly in the cytoplasm)
• No membrane-bound organelles
• Very small (1–10 μm)
• Bacteria, Archaea
• Structure: cell wall, plasma membrane, ribosomes, nucleoid
• Some have flagella, a capsule

Eukaryotic cells:

• A nucleus enclosed by a double membrane
• Various membrane-bound organelles
• Size: 10–100 μm
• Plants, animals, fungi, protists
• Animal cells: have centrioles and lysosomes
• Plant cells: have a cell wall, chloroplasts, and a vacuole

Endosymbiotic theory:

• Mitochondria and chloroplasts are traces of prokaryotic cells engulfed by, and living symbiotically within, a larger cell
• Evidence: their own DNA, a double membrane, independent division

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Cell Organelles

Nucleus:

• Double nuclear envelope (with nuclear pores)
• Stores DNA (chromosomes) and genetic information
• Nucleolus: synthesizes rRNA

Mitochondria:

• Produce ATP (cellular energy)
• Double membrane: outer membrane + inner membrane (cristae)
• The main site of aerobic respiration
• Carry their own DNA (semi-autonomous organelle)

Chloroplast — plants only:

• Photosynthesis (light energy → chemical energy)
• Thylakoid + stroma
• Carries its own DNA

Ribosome:

• Protein synthesis (translating mRNA)
• No membrane; present in all cells
• Bound to the ER or free-floating

Endoplasmic reticulum:

• Rough ER: ribosome-studded, protein synthesis and processing
• Smooth ER: lipid synthesis, detoxification

Golgi apparatus:

• Modifies, packages, and sorts proteins and lipids
• Transports secretory vesicles to the plasma membrane or outside the cell

Lysosome — animal cells:

• Stores digestive enzymes
• Intracellular digestion, breaking down worn-out organelles

Vacuole — plant cells:

• Central vacuole: stores water, ions, pigments
• Maintains turgor pressure

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Plasma Membrane Structure

Fluid mosaic model (Singer & Nicolson):

• Phospholipid bilayer
  • hydrophilic heads: face the water
  • hydrophobic tails: face inward
• Proteins: integral (span the membrane) + peripheral
• Fluidity: phospholipids and proteins can move laterally

Cholesterol (animals):

• Regulates membrane fluidity
• At high temperature: restrains fluidity
• At low temperature: preserves fluidity

Functions of the plasma membrane:

• Selective permeability (semipermeable membrane)
• Regulates the movement of substances into and out of the cell
• Signal reception (receptor proteins)
• Cell recognition (glycoproteins, glycolipids)

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Membrane Transport

Passive transport (no energy required):

• Simple diffusion: from higher to lower concentration (O₂, CO₂, lipid-soluble substances)
• Facilitated diffusion: uses membrane proteins (channels/carriers) (glucose, ions)
• Osmosis: water crossing a semipermeable membrane
  • hypotonic solution → water flows into the cell → swelling
  • hypertonic solution → water flows out of the cell → shrinking

Active transport (energy required):

• Movement against the concentration gradient
• Consumes ATP
• Example: the Na⁺-K⁺ pump (3 Na⁺ out, 2 K⁺ in)

Bulk transport:

• Endocytosis: taking in external material
  • phagocytosis: engulfing large particles
  • pinocytosis: fluids and small molecules
• Exocytosis: ER → Golgi apparatus → plasma membrane

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Cell Division

The cell cycle:

• G1 phase (growth): cell growth, protein synthesis
• S phase (synthesis): DNA replication
• G2 phase (growth): preparation for division
• M phase (mitosis): the division itself

Stages of mitosis:

1. Prophase: chromosomes condense, spindle fibers form
2. Prometaphase: the nuclear envelope breaks down, spindle fibers attach
3. Metaphase: chromosomes align at the metaphase plate
4. Anaphase: sister chromatids move to opposite poles
5. Telophase: the nuclear envelope re-forms
6. Cytokinesis: two daughter cells

Meiosis:

• Forms gametes (eggs, sperm)
• One round of DNA replication, two rounds of division
• Result: chromosome number is halved (2n → n)
• Genetic diversity: crossing over, independent assortment

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Frequently Asked Questions

Q. What is the difference between plant and animal cells? A. Plant cells have a cell wall (cellulose), chloroplasts, and a central vacuole, which animal cells lack. Conversely, animal cells have centrioles and lysosomes, which plant cells lack. This difference underlies many of the functional distinctions between the two cell types.

Q. What is the relationship between osmotic pressure and the cell? A. When a cell sits in a hypotonic solution (low salt concentration), water flows in and the cell swells; in a hypertonic solution (high salt concentration), water flows out and the cell shrinks. Plant cells don’t burst in a hypotonic solution because their cell wall lets them maintain turgor pressure instead — which is why a plant can stand upright.