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.