Chapter 6: A Tour of the Cell

Fundamentals of Cell Structure

Cells are the basic units of life, and their structure is closely related to their function. Understanding the components and organization of cells is essential for studying biology.

• Prokaryotic Cells: Cells without a nucleus or membrane-bound organelles. Examples: Bacteria.

• Eukaryotic Cells: Cells with a nucleus and membrane-bound organelles. Examples: Plants, Animals, Fungi, Protists.

• Organelles: Specialized structures within cells that perform distinct functions (e.g., mitochondria, chloroplasts, endoplasmic reticulum).

• Cytoplasm: The region between the cell membrane and the nucleus, containing organelles and cytosol.

• Nucleoid: Region in prokaryotes where DNA is located.

• Cell Membrane: The boundary of the cell, controlling movement of substances in and out.

Surface Area to Volume Ratio

The surface area to volume ratio is important for cell efficiency. As cells grow, their volume increases faster than their surface area, limiting the rate of exchange with the environment.

• Smaller cells have a higher surface area to volume ratio, allowing for more efficient transport of materials.

Similarities of Cells

• All cells contain DNA, cytoplasm, cytosol, and ribosomes.

Prokaryotes vs. Eukaryotes

Prokaryotes and eukaryotes differ in their structure and complexity.

• Prokaryotes: No nucleus, no membrane-bound organelles, smaller size.

• Eukaryotes: Nucleus present, membrane-bound organelles, larger size.

Key Organelles and Their Functions

• Nucleus: Contains genetic material (DNA); controls cell activities.

• 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; break down waste materials and cellular debris.

• Vacuoles: Storage organelles; large central vacuole in plants stores water and maintains turgor pressure.

• Mitochondria: Site of cellular respiration; produces ATP (energy).

• Chloroplasts: Site of photosynthesis in plant cells.

• Cell Wall: Provides structure and protection in plant, fungal, and some prokaryotic cells.

• Plasmodesmata: Channels between plant cells for transport and communication.

• Cytoskeleton: Network of microtubules, microfilaments, and intermediate filaments; provides structural support and facilitates movement.

• Flagella & Cilia: Structures for cell movement; flagella are long and whip-like, cilia are short and hair-like.

Chapter 7: Membrane Structure and Function

Fluid Mosaic Model

The cell membrane is described by the fluid mosaic model, which depicts the membrane as a flexible layer made of lipid molecules interspersed with proteins.

• Phospholipid Bilayer: Double layer of phospholipids with hydrophilic heads and hydrophobic tails.

• Amphipathic: Molecules with both hydrophilic and hydrophobic regions (e.g., phospholipids).

• Membrane Proteins: Integral and peripheral proteins perform various functions such as transport, signaling, and structural support.

• Glycoproteins: Proteins with carbohydrate chains attached; involved in cell recognition.

• Selective Permeability: The membrane allows certain substances to pass while blocking others.

Membrane Proteins and Their Functions

• Transport Proteins: Move substances across the membrane.

• Channel Proteins: Provide passageways for molecules.

• Carrier Proteins: Bind and transport specific molecules.

• Receptor Proteins: Receive signals from outside the cell.

• Enzymatic Proteins: Catalyze chemical reactions.

• Cell Recognition Proteins: Help cells identify each other.

Role of Carbohydrates in Membranes

• Carbohydrates are attached to proteins and lipids on the extracellular surface of the membrane.

• They play a role in cell recognition and adhesion.

Membrane Transport Mechanisms

Cells transport substances across their membranes using various mechanisms.

• Passive Transport: Does not require energy; substances move down their concentration gradient.

  • Diffusion: Movement of molecules from high to low concentration.

  • Facilitated Diffusion: Movement through transport proteins.

  • Osmosis: Diffusion of water across a membrane.

• Active Transport: Requires energy (ATP); moves substances against their concentration gradient.

  • Examples: Sodium/potassium pump, proton pump.

• Bulk Transport: Movement of large molecules via endocytosis (into cell) and exocytosis (out of cell).

Key Equations

• Electrochemical Gradient: The combined effect of concentration and electrical gradients on ion movement.

Osmosis and Tonicity

• Osmosis: Movement of water across a selectively permeable membrane.

• Tonicity: The ability of a solution to cause a cell to gain or lose water.

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

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

  • Hypertonic: Water leaves the cell; cell shrivels.

• Plasmolysis: In plant cells, loss of water causes the cell membrane to pull away from the cell wall.

• Turgor Pressure: Pressure of the cell contents against the cell wall; maintains plant rigidity.

Chapter 12: The Cell Cycle

Overview of the Cell Cycle

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 mitotic phase (division).

• Interphase: G1 (growth), S (DNA synthesis), G2 (preparation for division).

• Mitotic Phase: Mitosis (nuclear division) and cytokinesis (cytoplasm division).

Cell Division: Mitosis and Binary Fission

• Mitosis: Division of a eukaryotic cell's nucleus into two genetically identical daughter nuclei.

• Binary Fission: A form of asexual reproduction in prokaryotes; cell splits into two identical cells.

Phases of Mitosis

• Prophase: Chromosomes condense, spindle forms.

• Metaphase: Chromosomes align at the cell's equator.

• Anaphase: Sister chromatids separate and move to opposite poles.

• Telophase: Nuclear envelopes reform, chromosomes decondense.

• Cytokinesis: Division of the cytoplasm, resulting in two daughter cells.

Mitotic Spindle

• Composed of microtubules; responsible for separating chromosomes during mitosis.

Regulation of the Cell Cycle

• The cell cycle is regulated by internal and external controls (e.g., cyclins, checkpoints).

• Loss of control can lead to uncontrolled cell division and cancer.

Cell Signaling

• Cells communicate via signaling pathways to coordinate activities such as growth, division, and response to stimuli.

Summary Table: Comparison of Prokaryotic and Eukaryotic Cells

Summary Table: Types of Membrane Transport

Additional info: Some explanations and examples have been expanded for clarity and completeness.