Cell Structure and Function

Cell Structure and Function

Overview of Cells

  • Definition: Cells are the basic structural and functional units of every organism.

  • Common Features of All Cells:

    • Cells are bound by a plasma membrane.

    • Contain cytosol, the aqueous component of the cytoplasm.

    • Contain chromosomes, the structures that carry genetic information.

    • Contain ribosomes, the sites of protein synthesis.

Classification of Cells

  • Two Types of Cells: Prokaryotes and Eukaryotes.

    • Prokaryotes:

    • Domains: Bacteria and Archaea.

    • DNA is located in the nucleoid region.

    • Generally smaller in size than eukaryotes.

    • Eukaryotes:

    • Includes Protists, Fungi, Animals, and Plants.

    • DNA is contained within a nucleus.

    • Contain membrane-bound organelles.

Organelles

Definition of Organelles

  • Organelles: Membrane-bound structures found in eukaryotic cells.

Classification of Organelles

Endomembrane Organelles

  • Structures that are involved in the synthesis, modification, and transport of proteins.

  • Include:

    • Nuclear Envelope: Double membrane enclosing the nucleus.

    • Endoplasmic Reticulum (ER).

    • Golgi Complex (Golgi Apparatus).

    • Lysosomes: Contain hydrolytic enzymes for digestion.

    • Vesicles/Vacuoles: Transport materials within the cell and store substances.

    • Plasma Membrane: Protects the cell and controls what enters and leaves.

Energy Organelles

  • Specialized structures responsible for energy production.

  • Include:

    • Mitochondria: Site of cellular respiration, producing ATP.

    • Chloroplasts: Site of photosynthesis in plant cells, contain chlorophyll.

Compartmentalization

  • Definition: Compartmentalization refers to how organelles in eukaryotic cells are organized into distinct areas.

  • Importance:

    • Allows different metabolic reactions to occur in different locations within the cell.

    • Increases surface area for reactions.

    • Prevents interfering reactions from occurring in the same locale.

Unique Cell Components

Comparison of Plant and Animal Cell Components

  • Unique to Plants:

    • Chloroplasts: Organelles for photosynthesis.

    • Central Vacuole: Large vacuole providing turgor pressure and storage.

    • Cell Wall: Structural layer outside the plasma membrane.

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

  • Unique to Animals:

    • Lysosomes: Organelles for waste processing and digestion.

    • Centrosomes: Organelles that organize microtubules and assist in cell division.

    • Flagella: Structures enabling locomotion in certain cells.

The Nucleus

Structure and Function

  • Nucleus:

    • Contains chromosomes (genetic information), enclosed by a double membrane called the nuclear envelope.

    • Features nuclear pores that regulate material entry and exit.

  • Nucleolus:

    • Dense region within the nucleus responsible for synthesizing ribosomal RNA (rRNA).

    • rRNA combines with proteins to form ribosomal subunits, which exit through nuclear pores and assemble into functional ribosomes.

Ribosomes

Composition and Function

  • Definition: Comprised of ribosomal RNA and proteins.

  • Function: Synthesize proteins, translating messages from mRNA into polypeptides.

  • Locations:

    • Cytosol (Free Ribosomes): Produce proteins that function within the cytosol (e.g., enzymes).

    • Bound Ribosomes: Attached to the ER or nuclear envelope, synthesize proteins intended for secretion or membrane insertion.

Endoplasmic Reticulum (ER)

Structure and Types

  • Definition: A network of membranous sacs and tubes involved in various cellular processes.

  • Function:

    • Synthesizes membranes and proteins, compartmentalizing cellular components.

  • Types of ER:

    • Rough ER: Contains ribosomes on its surface; synthesizes proteins to be secreted or for membrane integration.

    • Smooth ER: Lacks ribosomes; functions in lipid synthesis, carbohydrate metabolism, and detoxification activities.

Golgi Complex

Structure and Function

  • Structure: Composed of flattened membranous sacs called cisternae, each sac separated and unconnected.

  • Directional Flow:

    • Cis Face: Receives transport vesicles from the ER.

    • Trans Face: Sends vesicles into the cytosol or towards the plasma membrane for secretion.

  • Functions:

    • Modifies received materials, sorts and packages them with molecular tags, ensuring newly formed proteins are properly folded and ready for transport.

Lysosomes and Peroxisomes

Lysosomes

  • Definition: Membranous sacs containing hydrolytic enzymes.

  • Function:

    • Hydrolyzes macromolecules in animal cells.

    • Involved in autophagy, recycling the cell's own materials to renew itself.

Peroxisomes

  • Definition: Membrane-bound metabolic compartments.

  • Function:

    • Catalyze reactions that produce hydrogen peroxide (H2O2), which is subsequently broken down to water by enzymes.

Vacuoles

Definition and Types

  • Vacuoles: Large vesicles that arise from the ER and Golgi, involved in storing and transporting substances.

  • Types of Vacuoles:

    • Food Vacuole: Formed through phagocytosis, containing digested substances by lysosomes.

    • Contractile Vacuole: Functions to maintain water levels in some cells.

    • Central Vacuole: Found in plant cells, stores inorganic ions and water, crucial for maintaining turgor pressure.

Endosymbiont Theory

Concept and Evidence

  • Definition: The endosymbiont theory posits that mitochondria and chloroplasts share similarities with prokaryotic cells. This theory suggests that an early eukaryotic cell engulfed a prokaryotic cell, creating a symbiotic relationship.

  • Evidence for the Theory:

    • Presence of a double membrane around mitochondria and chloroplasts.

    • Both organelles contain their own circular DNA and ribosomes.

    • They are capable of independent functioning.

Mitochondria

Structure and Function

  • Function: Site of cellular respiration, generating ATP.

  • Double Membrane Structure:

    • Outer Membrane: Smooth, separating the mitochondria from the cytosol.

    • Inner Membrane: Contains folds called cristae, increasing surface area for biochemical reactions.

  • Internal Compartments:

    • Intermembrane Space: Located between the inner and outer membranes.

    • Mitochondrial Matrix: Enclosed by the inner membrane, housing enzymes for the Krebs cycle, as well as mitochondrial DNA and ribosomes.

  • Correlation Between Mitochondria Count and Metabolic Activity:

    • Cells with high metabolic activity, such as muscle cells, contain a higher number of mitochondria.

Chloroplast

Structure and Function

  • Definition: Specialized organelles found in photosynthetic organisms, vital for photosynthesis.

  • Function: Contains chlorophyll, necessary for capturing light energy.

  • Double Membrane Structure:

    • Thylakoids: Membranous sacs organized into stacks called grana where light-dependent reactions occur.

  • Stroma: The fluid surrounding thylakoids, where the Calvin cycle occurs; contains chloroplast DNA, ribosomes, and enzymes necessary for photosynthetic reactions.

Cytoskeleton

Structure and Function

  • Definition: A network of fibers extending throughout the cytoplasm.

  • Purpose:

    • Provides structural support and mechanical support for animal cells, anchors organelles, and facilitates vesicular and cellular movement through interaction with motor proteins.

  • Types of Fibers:

    • Microtubules: Hollow structures made of tubulin; involved in organelle movement and chromosome separation during division.

    • Microfilaments: Solid rods made of actin; maintain cell shape, assist in muscle contraction, and facilitate cell division.

    • Intermediate Filaments: Fibrous proteins providing permanent structural support; maintain cell shape and anchor the nucleus and organelles.

Practice FRQs

Example 1 - Hepatitis C and Organelle Function

  • Context: Hepatitis C is known to attack liver cells and reduce albumin levels.

    • Question:

    • Identify the organelle affected and justify reasoning based on the function of the organelle.

Example 2 - Insulin Production in Pancreatic Cells

  • Context: Normal pancreatic cells synthesize insulin via the endoplasmic reticulum.

    • Question:

    • Describe the insulin production process in non-type 1 diabetic patients, addressing the role of the ER.

Example 3 - Alcohol Exposure in Liver Cells

  • Context: Experiment analyzing the effects of varying alcohol concentrations on liver cell tissues.

    • Question:

    • Identify the organelle for detoxification and an organelle responsible for removing damaged materials, detailing the steps involved in this process.