College Physics Chapter 4: Cell Structure

Studying Cells

Role of Cells in Organisms

  • Cells are the fundamental building blocks of all organisms.

  • In single-celled organisms, the cell performs all life functions.

Hierarchy of Multicellular Organisms

  • Cells: The basic unit.

  • Tissues: Composed of interconnected cells with a common function.

  • Organs: Formed by the combination of several tissues.

  • Organ Systems: Organs working together.

  • Organism: Multiple systems functioning together.

Biological Levels of Organization

  • Organelles: e.g., nucleus.

  • Cells: e.g., Human blood cells.

  • Tissues: e.g., Human skin tissue.

  • Organs and Organ Systems: e.g., stomach and intestine forming the human digestive system.

  • Organisms, Populations, and Communities: e.g., pine trees in a forest forming a population; all plant and animal species forming a community.

  • Ecosystems: Includes living organisms and their environment.

  • Biosphere: Encompasses all ecosystems on Earth.

Cell Size

  • Most cells are too small to be seen without microscopes.

Microscopy

  • Microscopes are essential for visualizing small cells.

Magnification and Resolving Power

  • Two key parameters in microscopy.

Magnification

  • The process of enlarging an object's appearance.

Resolution

  • Resolving power is the ability to distinguish two adjacent structures as separate.

  • Higher resolution provides better clarity and detail.

Types of Microscopes

  • Compound Light Microscopes: Use visible light and chemical stains to distinguish different parts of transparent objects like cells.

  • Electron Microscopes: Achieve higher magnification and resolution using beams of electrons.

  • Transmission Electron Microscopes (TEM): Show fine detail within cells.

  • Scanning Electron Microscopes (SEM): Provide 3-D exterior views.

Cell Theory

  • An underlying principle of biology:

    • Cells are the basic units of life.

    • All living organisms are made of cells.

    • All cells come from preexisting cells.

Common Cell Components

  • Plasma Membrane: Encloses the cell, separating the interior from the environment.

  • Cytoplasm: Made of cytosol, containing other cell components.

  • DNA: The genetic material of the cell.

  • Ribosomes: Synthesize proteins.

Prokaryotic Cells

Characteristics of Prokaryotes

  • Found in the domains Archaea and Bacteria.

  • Lack membrane-enclosed internal compartments (e.g., nucleus).

  • Most have a cell wall containing peptidoglycan.

  • Believed to be similar to the first cells.

Structure of a Prokaryotic Cell

  • Nucleoid: Where chromosomal DNA is localized.

  • Ribosomes: Located in the cytoplasm.

  • Cell Membrane: Surrounded by a cell wall.

  • Other structures may be present in some bacteria.

Size of Prokaryotic Cells

  • Smaller than eukaryotic cells.

Reasons for Small Size

  • More favorable surface area to volume ratio for material transport.

  • Lack modifications found in eukaryotes that aid internal transport.

Factors Limiting Cell Size

  • Ratio of Surface Area to Volume: Volume increases faster than surface area as cells grow.

  • Efficient material movement (e.g., oxygen and carbon dioxide) requires sufficient cell membrane surface area.

Eukaryotic Cells

Examples of Eukaryotic Organisms

  • Plants, animals, fungi, and many microorganisms.

Characteristics of Eukaryotic Cells

  • Contain internal membranes.

Eukaryotic Plasma Membrane

  • Phospholipid bilayer with embedded proteins.

    • Glycoprotein: Protein with carbohydrate attached.

    • Glycolipid: Lipid with carbohydrate attached.

    • Peripheral Membrane Protein

    • Integral Membrane Protein

    • Cholesterol

    • Protein Channel

    • Filaments of the Cytoskeleton

Cytoplasm

  • Region between the plasma membrane and the nuclear envelope.

  • Consists of organelles suspended in gel-like cytosol plus the cytoskeleton.

  • 70-80% water; semi-solid consistency due to proteins.

Nucleus

  • Usually one per cell.

  • Largest organelle.

  • Contains:

    • Nucleolus

    • Chromatin

    • Nucleoplasm

    • Nuclear Pore

    • Nuclear Envelope

Nuclear Envelope

  • Double membrane (two phospholipid bilayers).

  • Separates DNA from cytoplasm.

  • Nuclear Pores: Perforate the membrane, connecting nucleoplasm to cytoplasm and regulating molecular flow.

Nucleolus

  • Region inside the nucleus where ribosomal RNA (rRNA) is synthesized.

  • Ribosomes are assembled from rRNA and proteins.

Ribosomes

  • Made of two different-sized subunits.

  • Composed of rRNA and proteins.

  • Assemble amino acids into proteins during protein synthesis.

Mitochondrion

  • Converts chemical energy in glucose to ATP (cellular respiration).

  • Inner membrane is folded into cristae; encloses the mitochondrial matrix.

  • Contains its own DNA and ribosomes.

Chloroplasts

  • Double-membrane organelles; have their own ribosomes and DNA.

  • Inner membrane encloses stroma, containing interconnected and stacked fluid-filled membrane sacs called thylakoids.

  • Granum: A stack of thylakoids (plural = grana).

  • Site of photosynthesis in plants.

Endosymbiosis

  • Mitochondria and chloroplasts originated as independent prokaryotic organisms.

  • Became endosymbionts of the prokaryotic ancestors of eukaryotes.

  • Explains their distinct DNA and ribosomes, similar to those in prokaryotic cells.

  • Their size is similar to independent prokaryotes.

Centrosome

  • Consists of two centrioles at right angles to each other.

  • Each centriole is a cylinder made of nine triplets of microtubules.

  • Non-tubulin proteins hold the microtubule triplets together.

Contrasting Animal and Plant Cells

  • Plant Cells:

    • Cell wall

    • Chloroplasts

    • Large central vacuole

  • Animal Cells: Lack these structures.

Plant Cell Walls

  • Rigid protective structure external to the plasma membrane.

  • Made of cellulose, not peptidoglycan (as in prokaryotes).

Central Vacuole

  • Large vacuole occupying most of the cell area.

  • Regulates water concentration and contributes to cell expansion.

The Endomembrane System and Proteins

Components of the Endomembrane System

  • Internal membranes and organelles in eukaryotic cells working together.

  • Modifies, packages, and transports lipids and proteins.

  • Includes:

    • Nuclear envelope

    • Lysosomes

    • Vesicles

    • Endoplasmic reticulum

    • Golgi apparatus

    • Plasma membrane

Lysosomes

  • Contain digestive enzymes in animal cells.

  • Break down large biomolecules and worn-out organelles.

Endoplasmic Reticulum (ER)

  • Interconnected membranous sacs and tubules.

  • Lumen or cisternal space is the hollow portion of the ER tubules.

  • Membrane is continuous with the nuclear envelope.

    • Rough ER (RER): Modifies proteins.

    • Smooth ER (SER): Synthesizes lipids.

Rough Endoplasmic Reticulum

  • Ribosomes attached to the cytoplasmic surface manufacture proteins.

  • New proteins are modified (folding, acquisition of side chains) in the lumen.

  • Modified proteins are incorporated into cellular membranes or secreted.

Smooth Endoplasmic Reticulum

  • Continuous with the RER, but has few or no ribosomes.

Functions of the Smooth ER

  • Synthesis of carbohydrates, lipids, and steroid hormones.

  • Detoxification of medications and poisons.

  • Storage of Ca^{++}.

Sarcoplasmic Reticulum

  • Specialized SER in muscle cells.

  • Stores Ca^{++} needed for muscle cell contractions.

Golgi Apparatus

  • Sorts, packages, and tags lipids or proteins within transport vesicles.

  • Consists of flattened membranes.

  • Cis Face: Receiving side of the Golgi apparatus.

  • Trans Face: Opposite side.

  • Transport vesicles from the ER fuse with the cis face and empty their contents.

  • Proteins and lipids are further modified for sorting (e.g., adding sugar molecules).

The Cytoskeleton

Functions of the Cytoskeleton

  • Network of protein fibers.

  • Maintains cell shape.

  • Holds some organelles in specific positions.

  • Allows movement of cytoplasm and vesicles.

  • Enables cell movement in multicellular organisms.

Three Components of Cytoskeleton

  • Microfilaments

  • Intermediate Filaments

  • Microtubules

Microfilaments

  • Involved in movement (whole cell or internal parts).

  • Determine and stabilize shape.

  • Made from actin monomers.

Intermediate Filaments

  • Tough, flexible fibers assembled from protein subunits.

  • Provide mechanical strength and help stabilize cell shape.

Microtubules

  • Form rigid internal skeleton for some cells.

  • Provide framework for motor proteins to move structures within the cell.

  • Made of tubulin dimers (13 chains of dimers surround central cavity).

Cilia & Flagella

  • Ultrastructure: 9+2 array of microtubules.

    • 9 doublets on outside.

    • 2 unfused in center.

    • Spokes connect doublets to middle.

  • Cilia are shorter and more numerous than flagella.

Connections Between Cells and Cellular Activities

Extracellular Structures

  • Plant Cell Wall:

    • Support

    • Barrier to infection

    • Plasmodesmata connect cells

  • Extracellular Matrix (Animals):

    • 3 components

    • Collagens & other fibrous proteins

    • Glycoproteins called proteoglycans

    • Linking proteins

Tight Junctions

  • Watertight seals between animal cells.

  • Prevent materials from leaking between cells.

  • Found in epithelial cells lining internal organs and cavities.

Desmosomes

  • Short proteins (cadherins) that act as spot welds.

  • Join adjacent cells in tissues that stretch (e.g., heart, lungs, muscles).

  • Only present in animals.

Intercellular Junctions

  • Provide direct channels of communication between cells.

  • Differ between plants and animals.

Plasmodesmata

  • Channels passing between cell walls in plants.

  • Connect cytoplasm and allow materials to move from cell to cell.

  • Singular: plasmodesma; plural: plasmodesmata.

Gap Junctions

  • Connect animal cells; resemble plasmodesmata.

  • Form channels allowing ions, nutrients, and other materials to move between cells.

  • Formed when 6 proteins (connexins) form an elongated doughnut-like structure (connexon) in the plasma membrane.