Cell Structure and Function

Parts of a Cell

The cell is the basic structural and functional unit of life. Each cell contains specialized structures that perform distinct functions necessary for survival.

• Plasma (Cell) Membrane: A thin boundary separating the cell's interior from its external environment. It regulates the movement of substances in and out of the cell.

• Cytoplasm: The jelly-like substance between the plasma membrane and the nucleus, containing organelles and cytosol.

• Nucleus: Contains genetic material (DNA) and controls cellular activities, including growth and reproduction.

• Ribosomes: Sites of protein synthesis.

• Rough Endoplasmic Reticulum (RER): Studded with ribosomes; synthesizes and transports proteins.

• Smooth Endoplasmic Reticulum (SER): Synthesizes lipids and detoxifies chemicals.

• Golgi Apparatus: Modifies, sorts, and packages proteins for secretion or use within the cell.

• Mitochondria: The "powerhouse" of the cell; produces ATP (energy).

• Lysosomes: Contain digestive enzymes to break down toxins and waste.

Example: Muscle cells have more mitochondria to meet high energy demands.

DNA Structure and Function

DNA Basics

DNA (deoxyribonucleic acid) is the hereditary material in almost all living organisms. It encodes genetic instructions for development and function.

• Structure: Double helix composed of nucleotides (A, G, C, T).

• Monomer: Nucleotide (phosphate, sugar, nitrogenous base).

• Base Pairing: A-T, G-C.

• Polymer: DNA is a polymer of nucleotides.

• Central Dogma: DNA → mRNA (transcription) → Protein (translation).

• Genetic Code: Sequence of bases determines amino acid sequence in proteins.

Example: The gene for hemoglobin determines the sequence of amino acids in the hemoglobin protein.

Cell Membrane Structure

Schematic Structure of a Phospholipid Molecule

Phospholipids are the main component of cell membranes, forming a bilayer that separates the cell from its environment.

• Hydrophilic Head: Water-loving, faces outward toward water.

• Hydrophobic Tails: Water-fearing, face inward away from water.

• Bilayer Arrangement: Two layers of phospholipids with tails facing each other.

Mosaic Model

The fluid mosaic model describes the structure of cell membranes as a mosaic of components (phospholipids, proteins, carbohydrates) that gives the membrane a fluid character.

• Phospholipid Bilayer: Provides fluidity and flexibility.

• Proteins: Embedded within or attached to the bilayer, serving as channels, carriers, receptors, or enzymes.

• Carbohydrates: Attached to proteins or lipids, involved in cell recognition.

Components of the Cytoskeleton

The cytoskeleton provides structural support, maintains cell shape, and assists in cell movement.

• Microvilli: Increase surface area for absorption.

• Microtubules: Support cell shape, help position organelles, and are involved in cell division.

• Intermediate Filaments: Provide mechanical strength.

Membrane Transport Mechanisms

Diffusion

Diffusion is the passive movement of molecules from an area of higher concentration to an area of lower concentration.

• Simple Diffusion: Molecules move directly through the phospholipid bilayer (e.g., O2, CO2).

• Facilitated Diffusion: Molecules move via specific transport proteins (channels or carriers).

• Direction: Always down the concentration gradient.

Example: Glucose enters cells via facilitated diffusion using a carrier protein.

Mechanisms of Transport

Cells use various mechanisms to move substances across membranes.

• Primary Active Transport: Uses ATP to move substances against their concentration gradient (e.g., Na+/K+ pump).

• Secondary Active Transport: Uses energy from the gradient of another molecule (e.g., symporters, antiporters).

• Direction: Moves substances against (active) or with (passive) their concentration gradient.

Osmosis

Osmosis is the passive movement of water molecules across a semipermeable membrane from an area of higher water concentration to lower water concentration.

• Passive Process: Does not require energy.

• Membrane: Allows water to pass but restricts solutes.

Solution Types

Cells can be affected by the type of solution they are placed in:

Bulk Transport

• Exocytosis: Vesicles fuse with the plasma membrane to release contents outside the cell (e.g., waste, hormones).

• Endocytosis: Plasma membrane engulfs material, bringing it into the cell.

• Phagocytosis: "Cell eating"; cell engulfs large particles (e.g., bacteria).

• Pinocytosis: "Cell drinking"; cell engulfs extracellular fluid and solutes.

Genetic Code and Protein Synthesis

Genetic Code

The genetic code is the set of rules by which information encoded in DNA is translated into proteins.

• Codon: Sequence of three nucleotides that codes for a specific amino acid.

• Gene: Segment of DNA that codes for a protein.

Transcription

Transcription is the process by which a gene's DNA sequence is copied to make an mRNA molecule.

• Location: Nucleus

• Product: mRNA (messenger RNA)

Translation

Translation is the process by which mRNA is decoded to build a protein, with the help of ribosomes and tRNA.

• Location: Cytoplasm (ribosome)

• Product: Polypeptide (protein)

Types of RNA

• mRNA (messenger RNA): Carries genetic instructions from DNA to ribosomes.

• tRNA (transfer RNA): Brings amino acids to the ribosome during translation.

• rRNA (ribosomal RNA): Forms the core of ribosomes, where proteins are synthesized.

Cell Cycle and Division

Phases of the Cell Cycle

The cell cycle is the series of events that cells go through as they grow and divide.

• G1 Phase (Gap 1): Cell grows and makes proteins and organelles.

• S Phase (Synthesis): DNA is replicated; each chromosome now has two identical sister chromatids.

• G2 Phase (Gap 2): Cell grows more and checks DNA for errors before division.

• M Phase (Mitosis): Cell divides its copied DNA into two new nuclei.

DNA Replication (During S Phase)

1. Unzipping: DNA double helix is separated by enzymes.

2. Copying: DNA polymerase adds new nucleotides to each strand.

3. Result: Two identical DNA molecules, each with one old and one new strand (semi-conservative replication).

Chromatin, Chromosome, Chromatid

• Chromatin: Loose, stringy form of DNA inside the nucleus (not dividing).

• Chromosome: Tightly packed DNA, visible during cell division.

• Chromatid: One half of a duplicated chromosome.

Mitosis

Mitosis is the process by which a cell divides its nucleus and contents to produce two identical daughter cells.

1. Interphase: G1, S, G2 phases (cell prepares for division).

2. Prophase: Chromatin condenses into chromosomes.

3. Metaphase: Chromosomes align at the cell's equator.

4. Anaphase: Sister chromatids are pulled apart to opposite poles.

5. Telophase: Nuclear membranes reform around each set of chromosomes.

6. Cytokinesis: Cytoplasm divides, resulting in two new cells.

Example: Skin cells divide by mitosis to replace dead or damaged cells.

Summary Table: Solution Types and Effects on Cells

Additional info: Some explanations and examples have been expanded for clarity and completeness based on standard Anatomy & Physiology curriculum.