Cells Study Guide

Composite Cell Structure

Basic Structure: A cell consists of three main parts:

  • Nucleus: Houses genetic material (DNA) that controls cell activities and holds the instructions for heredity.

  • Cytoplasm: A gel-like fluid where cellular processes occur, containing organelles that perform specific functions.

  • Cell Membrane: The semi-permeable outer boundary of the cell that controls the entry and exit of substances.

Plasma (Cell) Membrane

Definition:

Outer covering of a cell regulating what enters and exits.

Components:

  • Glycoproteins: Proteins with carbohydrate groups attached; they play a crucial role in cell recognition and communication.

  • Proteins: Integral and peripheral proteins involved in various functions, such as transport and signaling.

  • Cholesterol: A lipid molecule that maintains membrane fluidity and stability.

  • Bilayer of phospholipid molecules: Composed of hydrophobic (water-fearing) tails and hydrophilic (water-loving) heads, forming the structural foundation of the membrane.

Functions of Plasma Membrane Proteins

  • Transport: Facilitating the movement of ions and molecules across the membrane, critical for maintaining homeostasis.

  • Enzymatic activity: Catalyzing biochemical reactions on the membrane surface.

  • Signal transduction: Relay signals from external stimuli to the cell's interior, triggering cellular responses.

  • Cell recognition: Identifying and interacting with other cells for immune response and tissue formation.

Cytoplasm

Consists of:

  • Cytosol: The fluid portion of the cytoplasm where organelles are suspended and various metabolic reactions occur.

  • Inclusions: Non-living substances such as lipid droplets, pigment granules, and glycogen that serve as storage or energy reserves.

  • Organelles: Specialized structures within cells performing specific functions essential for cell survival and activity.

Organelles

  • Endoplasmic Reticulum (ER):

    • Rough ER: Studded with ribosomes (causing the rough appearance); primarily functions in synthesizing and transporting proteins destined for secretion or incorporation into membranes.

    • Smooth ER: Lacks ribosomes (causing the smooth appearance); involved in lipid synthesis, detoxification of harmful substances, and calcium ion storage.

  • Ribosomes: Small structures made of RNA and proteins located in the cytoplasm or attached to the rough ER; essential for protein synthesis.

  • Golgi Apparatus: Often referred to as the "post office" of the cell; it modifies, sorts, and packages proteins and lipids for secretion or delivery to other organelles.

  • Mitochondria: Double-membraned organelles known as the powerhouse of the cell; responsible for ATP production through cellular respiration (glucose + O₂ → 36 ATP + CO2 + H₂O).

  • Lysosomes: Membrane-bound organelles containing hydrolytic enzymes that act like “little stomachs” by digest worn-out cell parts, bacteria, and other waste materials.

  • Centrioles: Cylindrical structures that play a key role in cell division by producing microtubules, forming the spindle fibers that separate chromosomes during mitosis. These help the cell function by creating an attachment site for organelles, and by organizing cellular components to ensure proper distribution of materials during cell division.

  • Cilia & Flagella:

    • Cilia: Short, hair-like structures that move fluids or mucus across cell surfaces, aiding in protection and secretion.

    • Flagella: Longer whip-like structures, such as sperm, that facilitate cell mobility.

Nucleus

Structure:

Surrounded by a double-layered nuclear envelope that contains nuclear pores for the transport of materials in and out.

Contents:

  • DNA: Comprising a total of 46 chromosomes; genetic information crucial for regulating cellular functions and inheritance.

  • Nucleolus: A dense structure within the nucleus, lacking a membrane, where ribosomal RNA is synthesized and assembled with proteins to form ribosome subunits.

  • Chromatin & Chromosomes:

    • Chromatin: Loose, extended form of DNA that allows for gene expression.

    • Chromosomes: Tightly packed DNA visible during cell division, ensuring genetic material is accurately distributed to daughter cells.

DNA

Definition:

Deoxyribonucleic acid; the hereditary material that carries genetic instructions essential for the development, functioning, and reproduction of living organisms.

Movement Through Cell Membranes

Membrane Functions:

The plasma membrane effectively regulates the entry and exit of substances through various mechanisms to maintain cellular homeostasis.

Types of Transport:

  • Passive Transport: No energy expenditure required, driven by concentration gradients.

    • Diffusion: Movement from areas of higher concentration to lower concentration until equilibrium is achieved.

      • Examples: oxygen, carbon dioxide, and fat

    • Osmosis: The specific movement of water across a selectively permeable membrane, critical in maintaining osmotic balance and pressure within cells.

      • Osmotic Pressure is caused by this movement

  • Tonicity:

    • Isotonic: Solutions with equal osmotic pressure; cells retain their normal shape.

    • Hypertonic: Solutions with higher osmotic pressure; cells lose water and shrink (crenate).

    • Hypotonic: Solutions with lower osmotic pressure; cells swell and may burst (lyse).

  • Facilitated Diffusion: Utilizes specific membrane proteins to transport polar or charged substances across the membrane; such as glucose.

  • Filtration: Driven by hydrostatic pressure; this process can force molecules through a membrane. A process notably occurring in kidneys to filter blood and form urine.

  • Active Transport: Requires ATP to move substances against their concentration gradient using transport proteins.

    • Solute Pump: An example is the sodium-potassium pump, crucial for maintaining the cell’s electrochemical gradient by pumping sodium out and potassium into the cell.

  • Vesicular Transport:

    • Endocytosis: The process of engulfing large molecules to transport them into the cell within a vesicle.

    • Exocytosis: A mechanism by which substances are secreted from the cell.

    • Phagocytosis: A form of endocytosis specifically for engulfing large particles, commonly utilized by white blood cells to remove pathogens.

Cell Cycle

Overview:

Describes the series of events from cell formation to cell division, crucial for growth, development, and repair.

Phases:

  • Interphase: Period of cell growth and DNA replication; most of a cell's life is spent here, divided into G1 (growth), S (synthesis of DNA), and G2 (preparation for mitosis).

  • Mitosis: The process of cell division producing two identical daughter cells, consisting of four stages.

Some cells are amitotic (do not divide):

Examples include neurons and cardiac muscle cells, which exit the cell cycle post-development.

Cell Division

Types:

  • Mitosis: The cell division process producing two identical daughter cells for growth and tissue repair.

  • Meiosis: Specialized cell division to produce gametes (sperm and oocytes) that occurs during reproductive cell formation.

Stages of Mitosis:

  1. Interphase: Cellular metabolism and preparation including DNA replication.

    • cells grow and synthesis new molecules and organelles

    • During the S phase the DNA is replicated

  2. Prophase: Chromatin condenses into visible chromosomes, and the nuclear membrane disappears.

    • Chromatin: a complex of DNA and proteins that forms chromosomes during cell division.

    • Chromosomes: structures that carry genetic information, consisting of two sister chromatids joined at the centromere.

  3. Metaphase: Chromosomes align along the cell equator.

  4. Anaphase: Centromeres split, and chromatids move toward opposite poles.

  5. Telophase: Nuclear membranes reform around the two sets of chromosomes; cytokinesis occurs, splitting the cytoplasm into two distinct cells.