Understanding Cells and Tissues: A Comprehensive Overview
Chapter 3: Exploring the Cell
What is a Cell?
The cell is the fundamental structural and functional unit of the body. It is the smallest living unit.
Three Main Regions of a Cell
- Cell Membrane: The outer boundary of the cell.
- Cytoplasm: Includes the cytosol (watery fluid) and organelles.
- Nucleus: The control center of the cell.
Structure of the Cell Membrane
The cell membrane is a phospholipid bilayer, composed of two layers of phospholipids. Embedded within this bilayer are proteins and cholesterol. Carbohydrates also contribute to the cell membrane’s structure.
Significant Characteristic of the Cell Membrane: Semi-Permeability
The cell membrane is semi-permeable, meaning it allows some substances to pass through while restricting others.
How is Semi-Permeability Accomplished?
- The phospholipid bilayer plays a crucial role.
- The phospholipid “heads” are hydrophilic (water-loving), while the “tails” are hydrophobic (water-fearing).
- This arrangement creates an oily, non-water-soluble environment in the middle of the bilayer.
- Consequently, most water-soluble molecules cannot pass through easily.
Substances That Can and Cannot Pass Through the Cell Membrane
Can Pass Through: Water, oxygen, carbon dioxide, lipid-soluble molecules.
Cannot Pass Through (Without Help): Ions (charged molecules), carbohydrates such as glucose, proteins.
Functions of Proteins in the Cell Membrane
- Form receptors on the cell surface.
- Transport ions or molecules across the cell membrane.
- Form selective channels (pores) that allow specific ions to enter or leave the cell.
- Form anchors to support the cell (cytoskeleton).
- Form glycoproteins (with a carbohydrate) to mark cells as “self.”
Organelles: The Functional Units of a Cell
Organelles are specialized structures within a cell, each performing a specific function.
Endoplasmic Reticulum (ER)
A network of tubes and channels for carrying substances. There are two types:
- Rough ER: Contains ribosomes and is involved in protein synthesis.
- Smooth ER: Functions in lipid synthesis.
The ER delivers its products to the Golgi apparatus for further processing.
Ribosomes: The Sites of Protein Synthesis
Ribosomes are organelles responsible for protein synthesis. They are found on the rough ER and freely in the cytosol.
Golgi Apparatus: The Central Receiving and Processing Center
The Golgi apparatus receives, modifies, and delivers proteins. These proteins may be delivered to other parts of the same cell or released into the extracellular space for use by other cells.
Vesicles: Cellular Transport Bubbles
Vesicles are bubble-like structures that transport substances within cells. They are formed by a membrane folding in on itself, creating a sac. The ER, Golgi apparatus, and cell membrane can all form vesicles for transport.
Mitochondria: The Powerhouses of the Cell
Mitochondria are organelles responsible for generating energy in the form of ATP, which the body uses for various functions.
Lysosomes: The Cellular Garbage Disposals
Lysosomes break down bacteria and worn-out cell components.
Microfilaments and Microtubules: The Cytoskeleton
These rod-like proteins form the cytoskeleton, providing structural support for the cell.
Cilia and Flagella: Motile Extensions
Cilia: Finger-like extensions that beat in waves to move substances across the cell surface.
Flagella: Tail-like extensions that propel cells, such as sperm cells.
The Cell Nucleus: The Control Center
The nucleus controls all cell activities and contains DNA.
Three Parts of the Nucleus
- Nuclear Envelope: The membrane surrounding the nucleus.
- Nucleolus: The site of ribosome formation.
- Chromatin: Contains chromosomes, where DNA is stored.
Passive and Active Transport
Passive Transport: Does not require energy from the cell.
Active Transport: Requires energy from the cell.
Diffusion: Movement Down the Concentration Gradient
Diffusion is a type of passive transport where particles move from areas of high concentration to areas of low concentration.
Facilitated Diffusion: Diffusion with Assistance
Similar to diffusion, but utilizes transmembrane protein channels.
Osmosis: Diffusion of Water
Osmosis is the diffusion of water across a semi-permeable membrane.
Hypertonic and Hypotonic Solutions
Hypertonic Solution: Has a higher solute concentration than body fluids.
Hypotonic Solution: Has a lower solute concentration than body fluids.
Effects of Hypertonic and Hypotonic Solutions on Red Blood Cells
In a hypertonic solution, red blood cells shrink. In a hypotonic solution, they swell.
Active Transport: Movement Against the Concentration Gradient
Active transport requires ATP and moves particles from areas of low concentration to areas of high concentration.
Chapter 5: Tissues of the Body
Four Types of Tissue
- Epithelial Tissue
- Connective Tissue
- Muscle Tissue
- Nervous Tissue
General Characteristics of Epithelial Tissue
- One free surface
- Anchored to connective tissue by a basement membrane
- Lack blood vessels
- Readily divide
- Tightly packed
- Classified by number of layers and cell shape
Simple Squamous Epithelium
Single layer of thin, flattened cells. Found in air sacs of lungs and capillary walls.
Microvilli: Increasing Surface Area
Microvilli are fingerlike extensions of the cell membrane that increase surface area for absorption.
Goblet Cells: Secreting Mucus
Goblet cells secrete mucus.
Pseudostratified Columnar Epithelium
Appears stratified but is actually a single layer of cells.
Stratified Squamous Epithelium
Several layers of cells with flattened outermost cells. Found in the skin, oral cavity, esophagus, vagina, and anal canal.
Transitional Epithelium
Cells change shape in response to tension. Found in the urinary bladder, ureters, and urethra.
General Characteristics of Connective Tissue
- Cells surrounded by an extracellular matrix
- Varying degrees of vascularity
- May be rigid or flexible
Important Fiber Types in Connective Tissue Matrix
Collagen and elastic fibers.
Categories of Connective Tissue
- Connective Tissue Proper
- Loose Connective Tissue
- Adipose Tissue
- Dense Connective Tissue
- Specialized Connective Tissue
- Cartilage
- Blood
- Bone
General Characteristics of Muscle Tissue
- Specialized for movement
- Contracts in response to stimuli
Three Types of Muscle Tissue
- Skeletal Muscle: Voluntary, striated, multiple nuclei per cell.
- Smooth Muscle: Involuntary, non-striated, one nucleus per cell.
- Cardiac Muscle: Involuntary, striated, one nucleus per cell.
Nervous Tissue
Located in the brain, spinal cord, and peripheral nerves.
Two Categories of Cells in Nervous Tissue
Neurons and neuroglia.