Cell Structure and Function: Study Notes for Anatomy & Physiology

Cell Structure and Function

Introduction to Cells

Cells are the fundamental units of life, forming the basis of structure and function in all living organisms. Understanding cell structure and function is essential for studying anatomy and physiology.

• Cell: The smallest living unit; most are microscopic.

• Discovery: Robert Hooke (mid-1600s) observed cork and coined the term "cell".

Cell Theory

The cell theory is a foundational concept in biology, describing the properties of cells.

• All living things are made of cells.

• Cells are the smallest units of structure and function in organisms.

• All cells arise from preexisting cells. (Principle established by Rudolf Virchow)

• This theory replaced the idea of spontaneous generation.

Cell Size and Surface Area-to-Volume Ratio

Cells maintain a large surface area-to-volume ratio to efficiently exchange materials with their environment.

• Surface Area: The area available for exchange of substances.

• Volume: The space inside the cell.

• Ratio: As cells grow, volume increases faster than surface area, limiting cell size.

Example Calculation:

Cell Size	Surface Area	Volume	Surface Area/Volume
2 cm	24 cm2	8 cm3	3:1
1 cm	48 cm2	8 cm3	6:1

Types of Cells

Prokaryotic Cells

Prokaryotic cells are the simplest and most ancient cell type, found in Bacteria and Archaea.

• No membrane-bound nucleus. DNA is concentrated in a region called the nucleoid.

• Organelles are not membrane-bound.

• Structures: Plasma membrane, cell wall, ribosomes, flagellum, basal body.

Eukaryotic Cells

Eukaryotic cells are more complex and include fungi, protists, plants, and animals.

• Nucleus bound by a membrane.

• Possess many organelles.

• Examples: Animal cells, plant cells.

Representative Animal and Plant Cells

• Animal Cell: Contains nucleus, ribosomes, Golgi apparatus, mitochondria, plasma membrane, vesicles, lysosomes, cytoskeleton.

• Plant Cell: Contains nucleus, nucleolus, ribosomes, smooth and rough endoplasmic reticulum, Golgi apparatus, mitochondria, chloroplasts, cell wall, vacuole, vesicles.

Cellular Organelles

Overview of Organelles

Organelles are specialized structures within cells that perform distinct functions.

• Membranous organelles: Derived from membranes (e.g., nucleus, endoplasmic reticulum, Golgi apparatus, lysosomes, vacuoles).

• Bacteria-like organelles: Derived from symbiotic bacteria (e.g., mitochondria, chloroplasts).

Plasma Membrane

The plasma membrane surrounds the cell, controlling the movement of substances in and out.

• Structure: Double layer of phospholipids and proteins.

• Phospholipids: Have hydrophilic heads and hydrophobic tails.

• Fluid Mosaic Model: Describes the dynamic nature of the lipid bilayer with embedded proteins.

Membrane Proteins

• Channels/Transporters: Move molecules in one direction.

• Receptors: Recognize specific chemicals.

• Glycoproteins: Identify cell type.

• Enzymes: Catalyze production of substances.

Cell Walls

• Found in: Plants, fungi, many protists.

• Plant cell walls: Mostly cellulose.

• Fungal cell walls: Contain chitin.

Cytoplasm and Cytoskeleton

The cytoplasm is a viscous fluid containing organelles, filaments, and fibers. The cytoskeleton provides structural support and facilitates movement.

• Cytosol: Fluid portion of cytoplasm.

• Cytoskeleton: Made of microfilaments (actin), microtubules, and intermediate filaments.

• Functions: Mechanical support, anchoring organelles, moving substances.

Cilia and Flagella

• Cilia: Short, used to move substances outside human cells.

• Flagella: Whip-like extensions, found on sperm cells.

• Structure: Bundles of microtubules covered by plasma membrane; basal bodies similar to centrioles.

Centrioles

• Structure: Pairs of microtubular structures.

• Function: Play a role in cell division.

Nucleus

• Control center of the cell.

• Structure: Double membrane (nuclear envelope) with pores.

• Contains: Chromosomes (DNA and proteins), nucleolus.

Chromosomes and Chromatin

• Chromosomes: DNA and proteins, visible during cell division.

• Chromatin: Loosely packed DNA and proteins, present during interphase.

Nucleolus

• Function: Directs synthesis of RNA, forms ribosomes.

• Most cells have 2 or more nucleoli.

Endoplasmic Reticulum (ER)

• Network of interconnected membranes.

• Rough ER: Ribosomes attached; manufactures and modifies proteins.

• Smooth ER: No ribosomes; synthesizes lipids and carbohydrates.

Golgi Apparatus

• Function: Synthesis of plant cell wall, packaging and shipping station.

• Process: Molecules arrive in vesicles, are modified, and pinched off in new vesicles for secretion or use within the cell.

Lysosomes

• Contain digestive enzymes.

• Functions: Aid in cell renewal, break down old cell parts, digest invaders.

Vacuoles

• Membrane-bound storage sacs.

• More common in plants.

• Contents: Water, food, wastes.

Bacteria-Like Organelles

• Mitochondria: Release energy (cellular respiration), have their own DNA, double membrane.

• Chloroplasts: Store energy (photosynthesis), derived from photosynthetic bacteria.

• Endosymbiotic Theory: Modern cells evolved from symbiotic relationships with bacteria.

Photosynthesis and Cellular Respiration

• Photosynthesis: Occurs in chloroplasts, converts solar energy to glucose.

• Cellular Respiration: Occurs in mitochondria, breaks down glucose and fatty acids to release energy as ATP.

Equation for Photosynthesis:

$6CO_2 + 6H_2O + \text{light energy} \rightarrow C_6H_{12}O_6 + 6O_2$

Equation for Cellular Respiration:

$C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{ATP}$

Molecule Movement and Cell Transport

Types of Transport

• Passive Transport: No energy required; moves substances down their gradient.

• Active Transport: Requires energy; moves substances against their gradient.

• Endocytosis: Cell engulfs large materials.

• Exocytosis: Cell expels materials.

Passive Transport

• Diffusion: Molecules move to equalize concentration.

• Osmosis: Special form of diffusion; movement of water from lower to higher solute concentration.

• Facilitated Diffusion: Transport proteins help molecules cross the membrane; no energy required.

Solution Differences and Cell Response

Solution Type	Solute Concentration	Water Movement	Cell Effect
Hypotonic	Lower outside	Water flows into cell	Cell swells/lyses
Isotonic	Equal inside and outside	No net movement	Cell remains normal
Hypertonic	Higher outside	Water flows out of cell	Cell shrinks/crenates

Active Transport

• Requires energy (ATP).

• Example: Sodium-potassium pump.

Equation for Sodium-Potassium Pump:

$3Na^+_{\text{out}} + 2K^+_{\text{in}} + \text{ATP} \rightarrow 3Na^+_{\text{in}} + 2K^+_{\text{out}} + \text{ADP} + \text{P}_i$

Endocytosis and Exocytosis

• Endocytosis: Cell engulfs particles, organisms, or large molecules; forms vesicles.

• Types: Phagocytosis (cell eating), Pinocytosis (cell drinking), receptor-mediated endocytosis.

• Exocytosis: Vesicle fuses with plasma membrane to expel contents.

Summary Table: Eukaryotic Cell Structures and Functions

Organelle	Structure	Function
Nucleus	Double membrane, pores	Control center, contains DNA
Ribosomes	RNA and protein	Protein synthesis
Endoplasmic Reticulum	Membranous network	Rough: protein synthesis; Smooth: lipid synthesis
Golgi Apparatus	Stacked membranes	Modifies, packages, ships molecules
Lysosomes	Membrane-bound vesicles	Digestion, cell renewal
Mitochondria	Double membrane, own DNA	Energy production (ATP)
Chloroplasts	Double membrane, own DNA	Photosynthesis
Vacuoles	Membrane-bound sacs	Storage
Cytoskeleton	Filaments, tubules	Support, movement

Additional info: Some explanations and table entries were expanded for clarity and completeness.