Cell structure

Fundamental Concepts in Human Biology

Instructor: Masoud Akhtar

Contact: m.akhtar.5@bham.ac.ukInstitution: University of Birmingham

Learning Objectives

  • Recognize diverse cell sizes, shapes, and functions.

  • Identify key organelles and their roles in eukaryotic cells.

  • Describe cell membrane structure.

  • Understand cell specialization for specific functions.

Size-Scales of Things

Measurement Scale: 1 nanometre to meters.Atoms, small molecules, macromolecules, organelles, cells, organisms.Examples: DNA, proteins, carbohydrates.

Cells: The Basic Unit of Life

  • Cells are alive, while subcellular components are not.

  • Major organelles include:

    • Nucleus

    • Golgi apparatus

    • Mitochondrion

    • Ribosome

    • Centrosome

    • Lysosome

    • Endoplasmic reticulum.

Composition of Living Organisms

  • All living organisms are composed of cells:

    • Bacteria, fungi, animals, and plants possess cellular structures.

  • Viruses: Not alive and not composed of cells.

Origin of Cells

  • Cells arise only from pre-existing cells.

  • Prokaryotic cells reproduce by binary fission.

  • Eukaryotic cells undergo mitosis or meiosis.

  • Spontaneous generation of cells is not feasible.

Prokaryotic Cells

Characteristics:

  • Lack a nucleus.

  • Minimal internal structures (e.g., absent endoplasmic reticulum, Golgi apparatus).

  • Generally smaller than eukaryotic cells.

Eukaryotic Animal Cells

Organelles include:

  • Lysosome

  • Ribosomes

  • Cilium

  • Centriole

  • Cell membrane

  • Centrosome

  • Peroxisome

  • Golgi apparatus

  • Secretory vesicles

  • Endoplasmic reticulum (smooth and rough)

  • Nuclear pore, nucleolus, nucleoplasm, nuclear envelope, mitochondrion.

Nucleus

  • Houses DNA as chromosomal structures.

  • Site for DNA replication, repair, and transcription.

  • Enclosed by a nuclear membrane with pores that control entry and exit.

  • Contains the nucleolus, which generates ribosomes and regulates protein functions.

Endoplasmic Reticulum (ER)

Structure: a network of membranes (cisternae).

  • Continuous with the nuclear membrane.

  • Types:

    • Smooth ER (SER): Synthesizes lipids and aids in carbohydrate metabolism.

    • Rough ER (RER): Studded with ribosomes; synthesizes proteins.

Ribosomes

  • Composed of proteins and RNA, forming two large subunits.

  • Function: Translation of mRNA into proteins; catalyzes peptide bond formation.

Golgi Apparatus

  • Membranous structure, phospholipid bilayer, folded.

  • Functions as a sorting center for proteins and lipids, packaging them into vesicles for transport.

Lysosomes

  • Types of vesicles, hollow spheres made of membranes.

  • Serve multiple functions, including molecular recycling (e.g., breaking down contents).

  • Example: Synaptic vesicles release neurotransmitters in nerve cells.

Mitochondria

  • Powerhouses of the cell; generate ATP from sugars and oxygen.

  • Contain mitochondrial DNA for ancestry tracing.

  • Structure: outer membrane, inter-membrane space, inner membrane with increased surface area.

Cytoplasm and Cytoskeleton

  • Cytosol: gel-like fluid (80% water) containing ions and proteins.

  • Cytoskeleton: supports cell structure and facilitates movement, composed of:

    • Actin cytoskeleton (cell structure and shape change).

    • Tubulin cytoskeleton (cellular transport).

Specialized Cells

  • Differentiate to perform various functions in the body:

    • Erythrocytes: Transport oxygen with hemoglobin; lifespan: 100 days.

    • Leukocytes: Immune functions (neutrophils and lymphocytes).

    • Muscle Cells (Myocytes): Three types (smooth, skeletal, cardiac).

    • Neurons: Process and transmit signals.

    • Adipocytes: Store energy as fat; size changes but number remains constant.

Plasma Membrane

  • Outermost boundary of the cell, forms a selective barrier.

  • Composed of a phospholipid bilayer embedded with proteins.

  • Functions: Regulates substance entry/exit and facilitates intercellular communication.

Phospholipids

  • Major components of cell membranes.

  • Synthesized by smooth ER and transported to the membrane.

  • Amphiphilic Nature of Phospholipids:

    • Characteristics: Hydrophobic tails and hydrophilic heads.

    • Allows for bilayer formation essential for membrane structure.

Membrane Fluidity

  • Essential for vesicle formation and cellular processes.

  • Influenced by temperature:

    • High temperatures increase fluidity, allowing for better movement of lipids and proteins within the membrane.

    • Lower temperatures increase viscosity, which can lead to a more rigid membrane, affecting protein mobility and interaction capabilities.

  • Adjustments in fluidity are crucial for maintaining membrane integrity and function under varying physiological conditions.

Fluid Mosaic Model

  • Membranes composed of phospholipids (fluid) and proteins (mosaic).

  • Membranes are dynamic structures allowing for flexibility and functionality.

Membrane Proteins

  • Integral Proteins: Span entire membrane, permanently attached.

  • Peripheral Proteins: Temporarily attached; participate in cell signaling and structural support.

Cholesterol in Membranes

  • Modulates membrane fluidity and permeability.

  • Present in cell membranes, adjusting structural integrity.

Summary

  • Cells vary in size, shape, and function.

  • Prokaryotic cells lack organelles (e.g., nucleus).

  • Eukaryotic cells possess various organelles, including nuclei, mitochondria, and Golgi apparatus.

Further Reading and Viewing

Focus areas: Cell Structure and Cell Membrane.