Cell Structure and Function

Introduction to Cells

Cells are the fundamental units of life, forming the basis of all living organisms. Whether single-celled like bacteria and protists, or multicellular like plants, animals, and fungi, every living thing is composed of cells. The cell theory states that all living things are made of cells and that all cells arise from pre-existing cells through division.

• Cell Theory: All organisms are composed of cells; all cells come from other cells.

• Single-celled organisms: Examples include prokaryotes (bacteria, archaea) and protists.

• Multicellular organisms: Examples include plants, animals, and most fungi.

• Microscopy: Cells are studied using light and electron microscopes, which reveal their structure and organization.

Antibiotics and Bacterial Cells

Antibiotics are drugs designed to target and kill infectious bacteria without harming human cells. They exploit unique features of bacterial cells, such as cell walls and ribosomes, which differ from those in human cells.

• Antibiotics: Chemicals that disable or kill bacteria by targeting structures unique to bacterial cells.

• Selective toxicity: Most antibiotics do not harm human cells because they bind to bacterial-specific structures.

• Research: Ongoing efforts focus on discovering new antibiotics by exploiting bacterial cell features.

Types of Cells

Prokaryotic vs. Eukaryotic Cells

All life is classified into three domains: Bacteria, Archaea, and Eukarya. The first two domains consist of prokaryotic cells, while Eukarya includes eukaryotic cells found in protists, plants, fungi, and animals.

• Prokaryotic cells: Lack a nucleus and membrane-bound organelles; DNA is located in the nucleoid region.

• Eukaryotic cells: Have a true nucleus and various membrane-bound organelles.

Cellular Structures

Animal and Plant Cells

Both animal and plant cells share many organelles, but also have unique features. Plant cells have a cell wall and chloroplasts, while animal cells have lysosomes and an extracellular matrix.

• Common organelles: Nucleus, mitochondria, endoplasmic reticulum, Golgi apparatus, cytoskeleton.

• Plant-specific: Cell wall (cellulose), central vacuole, chloroplasts.

• Animal-specific: Lysosomes, centrosomes, extracellular matrix.

Plasma Membrane Structure

The plasma membrane separates the cell from its environment and regulates the movement of substances in and out. It is composed of a phospholipid bilayer with embedded proteins, forming a fluid mosaic.

• Phospholipid bilayer: Hydrophilic heads face outward; hydrophobic tails face inward.

• Fluid mosaic model: Proteins and lipids move freely within the membrane.

Cell Surfaces

Plant cells have a rigid cell wall made of cellulose, providing protection and structural support. Animal cells lack cell walls but have an extracellular matrix and cell junctions for tissue formation.

• Cell wall: Found in plants; made of cellulose.

• Extracellular matrix: Sticky coat secreted by animal cells.

• Cell junctions: Structures that connect animal cells into tissues.

Genetic Control: Nucleus and Ribosomes

Nucleus

The nucleus is the control center of the cell, containing DNA organized as chromatin and chromosomes. It is surrounded by a nuclear envelope with pores for material exchange.

• Chromatin: DNA and associated proteins; condenses into chromosomes during cell division.

• Nucleolus: Site of ribosome component synthesis.

• Nuclear envelope: Double membrane with pores.

Ribosomes

Ribosomes are responsible for protein synthesis. They can be free in the cytosol or attached to the endoplasmic reticulum.

• Free ribosomes: Make proteins for use within the cytosol.

• Bound ribosomes: Make proteins for membranes or secretion.

DNA to Protein: Information Flow

Genetic information flows from DNA to RNA to protein. Messenger RNA (mRNA) carries the genetic code from the nucleus to ribosomes, where it is translated into a specific amino acid sequence.

• Central Dogma:

• Transcription: DNA is transcribed to mRNA.

• Translation: Ribosomes translate mRNA into protein.

Endomembrane System

Components and Functions

The endomembrane system includes the nuclear envelope, endoplasmic reticulum (ER), Golgi apparatus, lysosomes, and vacuoles. These organelles are connected directly or via vesicles and coordinate the production, modification, and transport of cellular materials.

• Endoplasmic Reticulum (ER): Rough ER (with ribosomes) synthesizes proteins; smooth ER (without ribosomes) synthesizes lipids and detoxifies drugs.

• Golgi Apparatus: Receives, refines, stores, and distributes products from the ER.

• Lysosomes: Contain digestive enzymes; break down macromolecules and recycle organelles.

• Vacuoles: Store nutrients, water, pigments, and waste; central vacuole in plants provides structural support.

Energy-Transforming Organelles

Chloroplasts

Chloroplasts are unique to photosynthetic cells in plants and algae. They convert solar energy into chemical energy through photosynthesis.

• Photosynthesis:

• Structure: Contains thylakoids, stroma, and its own DNA.

Mitochondria

Mitochondria are found in almost all eukaryotic cells and are the site of cellular respiration, producing ATP from food molecules.

• Cellular respiration:

• Structure: Double membrane, cristae, matrix, and its own circular DNA.

Endosymbiotic Theory

Chloroplasts and mitochondria contain their own DNA and ribosomes, supporting the theory that they evolved from ancient free-living prokaryotes that established symbiotic relationships with host cells.

• Evidence: Circular DNA, reproduction by division, similarity to prokaryotic cells.

• Symbiosis: One species living inside another, leading to mutual benefit and co-evolution.

Cytoskeleton and Cell Movement

Cytoskeleton

The cytoskeleton is a network of protein fibers that provides structural support, maintains cell shape, and enables movement.

• Microtubules: Hollow tubes; involved in cell shape, organelle movement, and cell division.

• Intermediate filaments: Provide mechanical strength.

• Microfilaments: Thin, solid fibers; involved in cell movement and shape changes.

Cilia and Flagella

Cilia and flagella are extensions of the cytoskeleton that aid in cell movement. Flagella are longer and usually singular, while cilia are shorter and more numerous.

• Flagella: Propel cells with a whiplike motion (e.g., sperm cells).

• Cilia: Move in coordinated waves; can move cells or substances across cell surfaces (e.g., tracheal cells).

• Defects: Problems with cilia and flagella can cause infertility and respiratory issues.

Evolutionary Connections

Bacterial Resistance and Human Evolution

Persistent diseases like cholera can drive natural selection in human populations, favoring individuals with genetic resistance. Recent studies have identified mutations in Bangladeshi populations that confer increased resistance to cholera, offering insights into human evolution and potential new antibiotic targets.

• Natural selection: Disease presence can select for resistant individuals.

• Genetic mutations: Identified genes may help develop new antibiotics.

Major Themes in Biology Illustrated

• Structure and function: Cell structures are specialized for their functions.

• Information flow: DNA directs cellular activities.

• Energy transformation: Cells convert energy for biological processes.

• Interactions: Cells interact within systems and with their environment.

• Evolution: Biological traits change over time due to selection pressures.