Cell Structure and Function

Overview of the Cell

The cell is the basic structural and functional unit of all living organisms. It contains various organelles that perform specialized functions necessary for life.

• Plasma Membrane: Separates intracellular fluid from extracellular fluid; composed of phospholipids, cholesterol, glycolipids, and embedded proteins.

• Cytoplasm: Material between the plasma membrane and the nucleus, containing cytosol and organelles.

• Nucleus: Contains genetic material (DNA) and controls cellular activities.

Plasma Membrane

Structure and Composition

The plasma membrane is a selectively permeable barrier that regulates the movement of substances into and out of the cell.

• Phospholipid Bilayer: Forms the basic structure, with hydrophilic heads facing outward and hydrophobic tails inward.

• Cholesterol: Stabilizes membrane fluidity.

• Proteins: Integral and peripheral proteins serve various functions.

Functions of Membrane Proteins

• Transport: Channels and carriers move substances across the membrane.

• Enzymatic Activity: Catalyze reactions at the membrane surface.

• Receptors: Bind signaling molecules and initiate cellular responses.

• Cell Recognition: Glycoproteins serve as identification tags.

• Intercellular Joining: Proteins help cells adhere to one another.

• Attachment: Anchor the membrane to the cytoskeleton and extracellular matrix.

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: Direct movement of small, nonpolar molecules (e.g., O2, CO2).

• Facilitated Diffusion: Movement of larger or polar molecules via protein channels or carriers.

Example: Movement of glucose into cells via facilitated diffusion.

Osmosis

Osmosis is the diffusion of water across a selectively permeable membrane.

• Occurs when the concentration of solute molecules is different on either side of the membrane.

• Water moves from an area of lower solute concentration to higher solute concentration.

Equation:

Where: J = flux D = diffusion coefficient dC/dx = concentration gradient

Osmolarity and Tonicity

Osmolarity refers to the concentration of solute particles in a solution, while tonicity describes the effect of a solution on cell volume.

• Isotonic: No net movement of water; cell volume remains constant.

• Hypotonic: Water enters the cell; cell may swell and burst.

• Hypertonic: Water leaves the cell; cell shrinks.

Active Transport

Active transport moves substances against their concentration gradient using energy (ATP).

• Requires carrier proteins (pumps).

• Example: Sodium-potassium pump (Na+/K+ ATPase).

Equation:

Vesicular Transport

Vesicular transport moves large particles and macromolecules across the plasma membrane.

• Endocytosis: Uptake of materials into the cell.

• Exocytosis: Release of materials out of the cell.

• Phagocytosis: Engulfment of large particles.

• Vesicular trafficking: Movement of vesicles within the cell.

Cytoplasmic Organelles

Major Organelles and Their Functions

• Mitochondria: Generate ATP by aerobic cellular respiration.

• Ribosomes: Synthesize proteins; composed of rRNA and protein.

• Endoplasmic Reticulum (ER): Network of membranes; rough ER synthesizes proteins, smooth ER synthesizes lipids.

• Golgi Apparatus: Modifies, concentrates, and packages proteins and lipids.

• Lysosomes: Contain digestive enzymes; break down waste materials.

• Peroxisomes: Contain oxidases and catalases; detoxify harmful substances.

• Cytoskeleton: Provides structural support; includes microtubules, microfilaments, and intermediate filaments.

Nucleus

The nucleus is the control center of the cell, containing genetic material and directing cellular activities.

• Nuclear Envelope: Double membrane surrounding the nucleus.

• Nucleolus: Site of ribosome synthesis.

• Chromatin: DNA and associated proteins.

Cytoskeleton

Components and Functions

• Microtubules: Hollow tubes made of tubulin; determine cell shape and distribution of organelles; organized by centrosomes.

• Microfilaments: Strands of actin; involved in cell movement and shape.

• Intermediate Filaments: Tough, insoluble protein fibers; provide mechanical strength.

Summary Table: Effects of Osmosis on Cells

Additional info:

• Some details, such as the specific chemical composition of organelles and the full mechanism of membrane transport, have been expanded for academic completeness.

• Images referenced in the original notes have been described and their content incorporated into the explanations above.