Cells: Structure, Function, and Processes

Overview and Objectives

This study guide covers the fundamental aspects of cells as relevant to Anatomy & Physiology. Topics include the characteristics of a typical cell, plasma membrane structure and function, organelles, nucleus, transport processes, osmotic pressure, metabolism, and cell division.

• Describe the characteristics of a typical cell.

• Explain the structure and function of the plasma membrane.

• Identify cell organelles and discuss their functions.

• Describe the organization and function of the nucleus.

• Discuss passive and active transport processes.

• Explain the effects of osmotic pressure on the cell.

• Describe how cell metabolism maintains homeostasis.

• Compare cell mitosis and cell meiosis.

Characteristics of a Typical Cell

Definition and Diversity

Cells are the basic structural and functional units of life. The human body contains many different types of cells, each specialized for particular functions.

• Cell Theory: All living organisms are composed of cells. The activity of an organism depends on the collective activities of its cells.

• Cell Diversity: Human cells vary in size, shape, and subcellular components, reflecting their specialized functions (e.g., muscle cells, nerve cells, fat cells, blood cells).

• Common Features: Despite diversity, most cells share three basic parts: plasma membrane, cytoplasm, and nucleus.

Example: Red blood cells transport oxygen, neurons transmit information, and muscle cells contract to produce movement.

Cellular Environment

Cells exist in a complex environment with various fluids and materials inside and outside the cell.

• Intracellular Fluid (ICF): Fluid within cells, also called cytosol.

• Extracellular Fluid (ECF): Fluid outside cells, including:

  • Interstitial Fluid: Surrounds and bathes cells.

  • Blood Plasma: Fluid component of blood.

  • Cerebrospinal Fluid: Surrounds nervous system organs.

• Cellular Secretions: Substances like saliva, mucus, and gastric fluids.

• Extracellular Matrix: Non-cellular material that provides structural support and acts as a "glue" to hold cells together.

Structure and Function of the Plasma Membrane

Plasma Membrane Overview

The plasma membrane, also known as the cell membrane, is a selectively permeable barrier that separates the intracellular fluid (ICF) from the extracellular fluid (ECF). It regulates the entry and exit of substances, maintaining the internal environment of the cell.

• Phospholipid Bilayer: The fundamental structure of the membrane, composed of two layers of phospholipids with hydrophilic (water-loving) heads facing outward and hydrophobic (water-fearing) tails facing inward.

• Membrane Proteins: Integral and peripheral proteins serve as channels, carriers, receptors, enzymes, and anchors for cell junctions.

• Carbohydrates: Attached to proteins and lipids on the extracellular surface, forming the glycocalyx, which is important for cell recognition and signaling.

• Cell Junctions: Specialized structures that connect adjacent cells (tight junctions, desmosomes, gap junctions).

Functions of the Plasma Membrane

• Selective Permeability: Allows some substances to pass while restricting others.

• Communication: Contains receptors for signaling molecules (e.g., hormones).

• Structural Support: Anchors the cytoskeleton and helps maintain cell shape.

• Cell Adhesion: Facilitates the formation of tissues and communication between cells.

Cell Organelles and Their Functions

Cytoplasm and Organelles

The cytoplasm is the cellular material between the plasma membrane and the nucleus. It consists of cytosol, inclusions, and organelles.

• Cytosol: Gel-like solution containing water, ions, proteins, and nutrients.

• Inclusions: Non-living substances such as glycogen granules, lipid droplets, and pigments.

• Organelles: Specialized structures performing specific cellular functions. They can be membranous (e.g., mitochondria, endoplasmic reticulum) or non-membranous (e.g., ribosomes, centrioles).

Major Organelles and Their Functions

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

• Mitochondria: Site of ATP (energy) production through cellular respiration.

• Ribosomes: Sites of protein synthesis; can be free or attached to rough endoplasmic reticulum (RER).

• Endoplasmic Reticulum (ER):

  • Rough ER: Studded with ribosomes; synthesizes proteins.

  • Smooth ER: Lacks ribosomes; synthesizes lipids and detoxifies chemicals.

• Golgi Apparatus: Modifies, sorts, and packages proteins and lipids for secretion or delivery to other organelles.

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

• Peroxisomes: Break down fatty acids and neutralize toxic compounds.

• Cytoskeleton: Network of protein filaments (microfilaments, intermediate filaments, microtubules) that provide structural support and facilitate movement.

• Centrioles: Organize microtubules during cell division and form the bases of cilia and flagella.

• Cilia and Flagella: Extensions that aid in movement of substances across the cell surface (cilia) or propel the cell itself (flagella).

• Microvilli: Fingerlike projections that increase surface area for absorption (e.g., in intestinal cells).

Organization and Function of the Nucleus

Nucleus Structure and Role

The nucleus is the largest organelle and serves as the control center of the cell, housing most of the cell's genetic material.

• Nuclear Envelope: Double membrane that encloses the nucleus and contains nuclear pores for material exchange.

• Nucleolus: Site of ribosome synthesis.

• Chromatin: DNA and associated proteins; condenses to form chromosomes during cell division.

• Cell Types: Most cells are uninucleate (one nucleus), but some are multinucleate (e.g., skeletal muscle) or anucleate (e.g., mature red blood cells).

Passive and Active Transport Processes

Overview of Membrane Transport

Cells must exchange materials with their environment. The plasma membrane is selectively permeable, allowing some substances to cross more easily than others.

• Passive Transport: Does not require energy (ATP). Substances move down their concentration gradient.

• Active Transport: Requires energy (ATP). Substances move against their concentration gradient.

Passive Transport Mechanisms

• Simple Diffusion: Movement of small, nonpolar molecules (e.g., O2, CO2) directly through the lipid bilayer.

• Facilitated Diffusion: Movement of larger or polar molecules via membrane proteins (channels or carriers).

• Osmosis: Diffusion of water across a selectively permeable membrane through aquaporins.

Factors Affecting Diffusion Rate:

• Concentration gradient (greater difference = faster diffusion)

• Molecule size (smaller = faster)

• Temperature (higher = faster)

Active Transport Mechanisms

• Primary Active Transport: Direct use of ATP to move substances (e.g., Na+/K+ pump).

• Secondary Active Transport: Uses energy stored in concentration gradients created by primary active transport (e.g., symporters and antiporters).

• Vesicular Transport: Movement of large particles via vesicles (endocytosis and exocytosis).

Na+/K+ Pump Equation:

Vesicular Transport Types

• Endocytosis: Uptake of materials into the cell (phagocytosis, pinocytosis, receptor-mediated endocytosis).

• Exocytosis: Release of materials from the cell (e.g., hormones, neurotransmitters).

Osmotic Pressure and Its Effects on Cells

Osmosis and Tonicity

Osmosis is the movement of water across a membrane in response to solute concentration differences. Tonicity describes how a solution affects cell volume.

Cell Metabolism and Homeostasis

Metabolic Processes

Cell metabolism includes all chemical reactions that occur within a cell, enabling it to maintain homeostasis, grow, and respond to environmental changes.

• Anabolism: Synthesis of complex molecules from simpler ones (requires energy).

• Catabolism: Breakdown of complex molecules into simpler ones (releases energy).

• Homeostasis: The ability of cells to maintain a stable internal environment despite external changes.

Example: Bone marrow increases cell division (hyperplasia) to meet increased demand for red blood cells; muscle atrophy occurs when cell metabolism slows due to disease or disuse.

Cell Division: Mitosis and Meiosis

Cell Cycle and Mitosis

The cell cycle is the series of events that cells go through as they grow and divide. It consists of interphase (growth and DNA replication) and the mitotic phase (division).

• Interphase: Cell grows, performs normal functions, and duplicates DNA.

• Mitotic Phase: Includes mitosis (division of the nucleus) and cytokinesis (division of the cytoplasm).

• Mitosis Stages: Prophase, Metaphase, Anaphase, Telophase.

• Purpose: Produces two genetically identical daughter cells for growth and repair.

Meiosis

• Definition: Special type of cell division that produces gametes (sperm and egg cells) with half the number of chromosomes.

• Purpose: Ensures genetic diversity and maintains chromosome number across generations.

Genetic Information and Protein Synthesis

• DNA: Double helix molecule containing genetic instructions for protein synthesis.

• Gene: Segment of DNA that codes for a specific polypeptide.

• Transcription: DNA code is copied into messenger RNA (mRNA).

• Translation: mRNA is decoded to assemble amino acids into a polypeptide chain.

Base Pairing: Adenine (A) pairs with Thymine (T), Cytosine (C) pairs with Guanine (G).

Summary Table: Key Cell Structures and Functions

Additional info: Some content was expanded for clarity and completeness, including definitions, examples, and summary tables to ensure the notes are self-contained and suitable for exam preparation.