Cell Structure and Function

Concepts of the Cell Theory

  • A cell is the basic structural and functional unit of living organisms.

  • The activity of an organism depends on the collective activities of its cells.

  • Continuity of life has a cellular basis.

Chemical Components of Cells

  • Most cells are composed of the following four elements:

    • Carbon

    • Hydrogen

    • Oxygen

    • Nitrogen

Anatomy of the Cell

  • Cells are not all the same; however, they share general structures.

  • All cells have three main regions:

    • Nucleus

    • Cytoplasm

    • Plasma membrane

The Nucleus

  • The control center of the cell.

  • Contains genetic material (DNA).

  • Three regions of the nucleus include:

    • Nuclear envelope (membrane) - barrier of the nucleus consisting of a double membrane with nuclear pores that allow for exchange of material with the rest of the cell.

    • Nucleolus - contains one or more nucleoli, sites of ribosome assembly where ribosomes migrate into the cytoplasm through nuclear pores.

    • Chromatin - composed of DNA and protein, exists when the cell is not dividing, scattered throughout the nucleus, and condenses to form chromosomes when the cell divides.

Plasma Membrane

  • Serves as a barrier for cell contents.

  • Composed of a double phospholipid layer with:

    • Hydrophilic heads (polar)

    • Hydrophobic tails (nonpolar)

  • Contains proteins, cholesterol, and glycoproteins.

  • Functions dynamically in separating intracellular fluid (ICF) from extracellular fluid (ECF).

    • Interstitial fluid (IF) is the ECF that surrounds cells.

Membrane Lipids

  • Composed as follows:

    • 75% phospholipids (forming the lipid bilayer):

    • Phosphate heads: polar and hydrophilic

    • Fatty acid tails: nonpolar and hydrophobic (Review Fig. 2.16b)

    • 5% glycolipids - lipids with polar sugar groups on the outer membrane surface.

    • 20% cholesterol - increases membrane stability and fluidity.

Membrane Proteins

Integral Proteins

  • Firmly inserted into the membrane (most are transmembrane).

  • Functions include:

    • Transport proteins (channels and carriers)

    • Enzymes

    • Receptors

Peripheral Proteins

  • Loosely attached to integral proteins.

  • Include filaments on the intracellular surface and glycoproteins on the extracellular surface.

  • Functions include:

    • Enzymatic activity,

    • Motor proteins,

    • Cell-to-cell links,

    • Support on intracellular surface,

    • Part of glycocalyx.

Functions of Membrane Proteins

  1. Transport - Facilitates movement across the membrane.

  2. Receptors for signal transduction - Bind with external signals leading to changes in the cell.

  3. Attachment to cytoskeleton and extracellular matrix (ECM) - Maintains cell shape and structural integrity, allowing communication between cells.

  4. Enzymatic Activity - Acts catalytically in metabolic processes.

  5. Intercellular Joining - Helps in forming junctions between cells.

  6. Cell-Cell Recognition - Glycoproteins serve as identification tags recognized by other cells.

Plasma Membrane Junctions

  • Types of membrane junctions:

    1. Tight Junctions - Impermeable junctions binding cells into leakproof sheets.

    2. Desmosomes - Anchoring junctions that prevent cells from being pulled apart.

    3. Gap Junctions - Allow for communication between cells.

Membrane Transport

  • Plasma membranes are selectively permeable.

  • Some molecules can easily pass while others cannot.

Types of Membrane Transport

  • Passive Processes: No cellular energy (ATP) required; substances move down their concentration gradient.

  • Active Processes: Energy (ATP) required; occurs only in living cell membranes.

Passive Processes

Types of Passive Processes

  1. Simple Diffusion - Nonpolar lipid-soluble substances diffuse directly through the phospholipid bilayer from higher concentrations to lower concentrations.

  2. Facilitated Diffusion - Lipophobic molecules like glucose and ions use carrier proteins or channel proteins to pass through the plasma membrane.

  3. Osmosis - Movement of solvent (water) across a selectively permeable membrane.

  4. Filtration - Water and solutes are forced through a membrane by hydrostatic pressure; solute-containing fluid is pushed from high pressure to lower pressure.

  5. Dialysis - Artificial filtration, often in kidney failure.

Osmosis

  • Water concentration is determined by solute concentration, as solute particles displace water molecules.

  • Osmolarity: The measure of total concentration of solute particles.

  • Osmosis continues until equilibrium is reached across the membrane.

Tonicity

  • Tonicity refers to the ability of a solution to cause a cell to shrink or swell based on solute concentration.

    • Isotonic: Solution with the same solute concentration as that of the cytosol.

    • Hypertonic: Solution with a greater solute concentration than that of the cytosol; cells lose water by osmosis and shrink.

    • Hypotonic: Solution with a lesser solute concentration than that of the cytosol; cells take on water by osmosis until they become bloated and may burst.

Summary of Passive Processes

  • Type | Energy Source | Example

  • Simple diffusion | Kinetic energy | Movement of O2 through phospholipid bilayer

  • Facilitated diffusion | Kinetic energy | Movement of glucose into cells

  • Osmosis | Kinetic energy | Movement of H2O through phospholipid bilayer or AQPs.

  • Filtration | Kinetic energy | Movement of filtrate through kidney filters

  • Dialysis | Kinetic energy | Use of artificial filters in kidney failure

Active Processes

  • Substances that cannot pass by diffusion are transported using ATP.

  • Two common forms of active transport include:

    1. Active transport (solute pumping) - e.g., movement of sodium (Na⁺) and potassium (K⁺) ions.

    2. Vesicular transport, including:

    • Exocytosis - Moves materials out of the cell using a membranous vesicle.

    • Endocytosis, which includes:

      • Phagocytosis - “cell eating” for solids (e.g., bacteria).

      • Pinocytosis - “cell drinking” for liquids (e.g., hormones).

Cellular Components – Cytoplasm

  • The material outside the nucleus and inside the plasma membrane, serving as a site of most cellular activities.

  • Contains three major elements:

    1. Cytosol - Fluid that suspends other elements (pH 7).

    2. Organelles - Metabolic machinery performing functions for the cell (often referred to as “little organs”).

    3. Inclusions - Chemical substances such as stored nutrients or cell products.

Cytoplasmic Organelles

  1. Mitochondria - “Powerhouses” of the cell, carrying out reactions to break down food using oxygen to provide ATP for cellular energy.

  2. Ribosomes - Comprised of protein and RNA, sites of protein synthesis found freely in cytoplasm or as part of rough endoplasmic reticulum.

  3. Endoplasmic Reticulum (ER) - Fluid-filled tubules for transporting substances:

    • Rough ER - Studded with ribosomes, synthesizes proteins.

    • Smooth ER - Functions in lipid metabolism and detoxification of drugs and pesticides.

  4. Golgi Apparatus - Modifies and packages proteins, producing secretory vesicles and cell membrane components.

  5. Lysosomes - Contain enzymes produced by ribosomes, digesting worn-out or nonusable materials within the cell.

  6. Peroxisomes - Membranous sacs of oxidase enzymes that detoxify harmful substances and breakdown free radicals.

  7. Cytoskeleton - Network of protein structures throughout the cytoplasm providing internal framework:

    • Microfilaments (largest)

    • Intermediate filaments

    • Microtubules (smallest)

  8. Centrioles - Rod-shaped bodies made of microtubules directing the formation of the mitotic spindle during cell division.

Cellular Projections

  • Not found in all cells:

    1. Cilia - Move materials across the cell surface, located in the respiratory system to move mucus & foreign particles.

    2. Flagella - Propel the cell; the only flagellated cell in the human body is sperm.

    3. Microvilli - Tiny, fingerlike extensions of the plasma membrane, increasing surface area for absorption in the small intestine.

Cell Life Cycle

  • Cells have two major periods:

    1. Interphase - Cell grows and carries on metabolic processes.

    2. Cell Division - Cell replicates itself, producing more cells for growth and repair processes.

DNA Replication

  • Genetic material is duplicated to prepare for cell division towards the end of interphase.

  • DNA uncoils; each side serves as a template for synthesis of new strands.

Events of Cell Division

  1. Mitosis - Division of the nucleus resulting in two daughter nuclei.

  2. Cytokinesis - Division of the cytoplasm, forming two daughter cells.

Stages of Mitosis

  • Prophase:

    • Centrioles migrate to the poles directing assembly of the mitotic spindle fibers.

    • DNA appears as double-stranded chromosomes attached at the centromere.

    • Nuclear envelope breaks down.

  • Metaphase:

    • Chromosomes are aligned in the center of the cell on the metaphase plate.

  • Anaphase:

    • Chromosomes are pulled apart to opposite ends of the cell.

    • Cleavage furrow appears.

  • Telophase:

    • Chromosomes uncoil into chromatin.

    • Nuclear envelope reforms.

    • Spindles break down.

Protein Synthesis

  • Gene: A DNA segment carrying the blueprint for building one protein.

  • Proteins serve multiple functions within cells:

    • Building materials

    • Biological catalysts (enzymes)

  • Role of RNA in Protein Synthesis:

    • Transfer RNA (tRNA): Transfers appropriate amino acids to the ribosome.

    • Ribosomal RNA (rRNA): Forms the ribosomes where proteins are synthesized.

    • Messenger RNA (mRNA): Carries instructions from nucleus to ribosomes.

Transcription and Translation

  • Transcription: Transfer of information from DNA to mRNA; three-base sequences on mRNA are called codons (A to T & C to G).

  • Translation: Base sequences of nucleic acid are translated to an amino acid sequence, forming proteins.