Study Unit 1

Lecture Unit 1: Cells and Cell Structure

Cell Theory

• Fundamental Principles:

  • Cells are the fundamental units of life

  • All living organisms are composed of cells.

  • All cells come from pre-existing cells.

  • Modern cells evolved from a common ancestor.

Core Functions of Cells

• Living Functions:

  • Reproduce

  • Respond to the environment

  • Harness energy

  • Evolve

Types of Cells

• Unicellular Organisms:

  • Composed of a single cell (e.g., bacteria, archaea, amoeba).

• Multicellular Organisms:

  • Composed of many cells (e.g., mammals, plants).

Origin of Cells

• Cell Division:

  • Parent cells divide to produce daughter cells.

  • All cells represent a continuous lineage from the first living cells.

Surface Area to Volume Ratio

• Why Cells are Small:

  • As an object's volume (x³) increases, its surface area (x²) increases at a different rate (volume increases at a faster rate).

  • Increased volume leads to increased chemical activities, needs for resources, and waste production, commonly outpacing surface area.

  • This discrepancy can hinder the cell’s ability to regulate internal conditions effectively, making it harder to maintain homeostasis.

  • A high SA:V ratio allows for more effective exchange of materials within the environment, ensuring that the cell can quickly respond to changes and sustain essential life functions.

  • To overcome these challenges, large organisms consist of many small cells.

Strategies for Small Cell Sizes

• Increase Membrane Infoldings:

  • Allows greater surface area in relation to volume.

  • Nerve cells may adopt elongated forms.

• Improve Substance Transport Rates:

  • To facilitate better absorption and release of materials.

Cell Membrane Functions

1. Selectively Permeable Barrier:

   • Prevents some substances from crossing while allowing others to enter/exit.

2. Regulation of Transport:

   • Enables homeostasis within the cell.

3. Boundary Protection:

   • Establishes communication with other cells and responds to environmental signals.

4. Protein Attachment:

   • Proteins embedded in the membrane help bind and adhere to adjacent cells, contributing to structure and shape.

Cytoplasm vs Cytosol

• Cytoplasm:

  • Contains everything inside the cell other than the nucleus.

• Cytosol:

  • The fluid part of the cytoplasm not enclosed in organelles.

Organelles

• Eukaryotic Organelles:

  • Membrane-enclosed compartments that separate biochemical reactions for regulation and efficiency (e.g., nucleus, endoplasmic reticulum).

Prokaryotic Cells

• Characteristics:

  • Ranges from 1–10 μm in diameter.

  • Basic structure includes a cell membrane, nucleoid (location of DNA), and ribosomes for protein synthesis.

  • Cytoplasm is dynamic, with substances in constant motion, aiding in processes like diffusion.

Common Features in Prokaryotes

• Cell Wall:

  • Most have a wall made of peptidoglycan for shape support.

  • Some have an outer membrane; others have a capsule for protection and adherence.

• Internal Structures:

  • Can include specialized membranes for energy reactions and photosynthesis.

• Flagella and Pili:

  • Used for locomotion and transferring genetic material, respectively.

• Cytoskeleton:

  • Composed of protein filaments supporting cell shape and division.

Eukaryotic Compartmentalization

• Organelles:

  • Perform specialized functions and separate biochemical reactions for efficiency (e.g., lysosomes for hydrolysis).

Plant vs Animal Cells

• Plant Cells:

  • Have cell walls, plasmodesmata, and plastids (e.g., chloroplasts).

• Animal Cells:

  • Have cell membranes and various junctions instead of cell walls.

Nucleus

• Functions:

  • Houses the majority of cellular DNA, where DNA replication and transcription occur.

  • Contains a nucleolus, responsible for ribosome assembly.

Endomembrane System Overview

• Components:

  • Include the cell membrane, nuclear envelope, endoplasmic reticulum, Golgi apparatus, and lysosomes, with vesicles enabling transport and communication.

Endoplasmic Reticulum

• Rough ER:

  • Studded with ribosomes; involved in protein synthesis and modification.

• Smooth ER:

  • Lacks ribosomes, responsible for lipid synthesis and detoxification.

Golgi Apparatus

• Structure:

  • Comprised of stacked cisternae and vesicles.

• Function:

  • Processes, modifies, and packages proteins for cellular use or export.

Lysosomes

• Functions:

  • Serve as cellular disposals for digestion of macromolecules.

  • Maintain acidic internal environments for enzymatic activity.

Mitochondria and Chloroplasts

• Mitochondria:

  • Generate ATP through cellular respiration; possess double membranes and independent DNA.

• Chloroplasts:

  • Sites of photosynthesis; contain chlorophyll and internal membranes (thylakoids) for energy conversion.

Peroxisomes and Glyoxysomes

• Peroxisomes:

  • Contain enzymes for breaking down toxic byproducts like hydrogen peroxide.

• Glyoxysomes:

  • Present in plants for converting stored lipids into carbohydrates for transport to growing cells.

Vacuoles

• Functions:

  • Storage site for water, nutrients, waste, and toxins; important for turgor pressure in plant cells.

Cytoskeleton

• Support Functions:

  • Maintains cell shape, organizes organelles, enables movement, and interacts with extracellular structures.

Cytoskeleton Components

1. Microfilaments:

   • Actin-based, involved in cell shape and movement, dynamic polymerization.

2. Intermediate Filaments:

   • Provide structural stability and resistance to tension.

3. Microtubules:

   • Form rigid structures aiding in organelle movement and chromosome segregation.

Motor Proteins

• Types:

  • Kinesins move materials toward the plus end of microtubules; dyneins toward the minus end; involved in organelle transport and flagellar movement.

Plant Cell Wall

1. Provides support and rigidity to the plant cell.

2. Acts as a barrier against infection.

3. Enables growth and shape during cell expansion.

Plasmodesmata

• Function:

  • Channels between adjacent plant cells facilitating communication and transport of materials.

Extracellular Matrix in Animal Cells

• Components:

  • Collagen (most abundant protein), proteoglycans, and linking proteins.

• Roles:

  • Structural support, tissue orientation, filtration, and signaling.

Endosymbiosis Theory

• Concept:

  • Explains the origin of mitochondria and plastids as past symbiotic relationships between cells, retaining some functions independent of the host cell.

Experiments on Microfilament Role in Amoeba Movement

• HYPOTHESIS:

  • Amoeboid movements are driven by the cytoskeleton (microfilaments).

• RESULTS:

  • Microfilaments are essential for cell movement; controls included various inhibitors affecting cellular processes.