Cell Biology: Cell Theory and Types

• The cell is the basic structural and functional unit of all known living organisms.
• It is the smallest unit of an organism that is considered alive.
• Cells are capable of independent existence.
• All cells come from pre-existing cells by division.
• The study of cells is called cell biology.

Cell Theory

• All known living things are made up of one or more cells.
• All living cells arise from pre-existing cells by division.
• The cell is the fundamental unit of structure and function in all living organisms.

Cell Types

• There are two main types of cells: prokaryotic and eukaryotic.

Prokaryotic Cells

• Prokaryotic cells lack a nucleus and other membrane-bound organelles.
• Genetic material is located in the cytoplasm in a region called the nucleoid.
• Prokaryotes are typically smaller and simpler than eukaryotes.
• Bacteria and Archaea are prokaryotic.
• Ribosomes are present to synthesize proteins.
• A cell wall provides structure and protection.
• Some prokaryotes have flagella for movement.
• Prokaryotic cells perform functions such as metabolism, growth, and reproduction.
• Prokaryotes have existed for billions of years and are highly diverse.
• They can thrive in a wide range of environments.

Eukaryotic Cells

• Eukaryotic cells have a nucleus and other membrane-bound organelles.
• Eukaryotes can be unicellular or multicellular
• Eukaryotes include plants, animals, fungi, and protists.
• The nucleus contains the cell's DNA, organized into chromosomes.
• Mitochondria produce energy through cellular respiration.
• The endoplasmic reticulum (ER) synthesizes and transports proteins and lipids.
• The Golgi apparatus modifies, sorts, and packages proteins and lipids.
• Lysosomes contain enzymes for breaking down cellular waste.
• Eukaryotic cells are more complex and larger than prokaryotic cells.
• They have a cytoskeleton for structural support and movement.

Cell Structures and Functions

Plasma Membrane

• The plasma membrane is the outer boundary of the cell.
• It is a selectively permeable barrier, regulating the passage of substances in and out of the cell.
• Composed of a phospholipid bilayer with embedded proteins.
• Proteins in the membrane can act as receptors, channels, or carriers.
• The membrane plays a role in cell signaling and communication.

Nucleus

• The nucleus contains the cell's genetic material (DNA).
• It controls the cell's growth, metabolism, and reproduction.
• Surrounded by a nuclear envelope with pores for transport.
• Contains the nucleolus, where ribosomes are assembled.
• DNA is organized into chromosomes.

Cytoplasm

• The cytoplasm is the region between the plasma membrane and the nucleus.
• It contains the cytosol, a gel-like substance, and various organelles.
• Many metabolic reactions occur in the cytoplasm.
• Provides a medium for the transport of substances within the cell.

Ribosomes

• Ribosomes are responsible for protein synthesis.
• They are found in the cytoplasm or attached to the endoplasmic reticulum.
• Ribosomes read mRNA and assemble amino acids into polypeptide chains.
• Composed of ribosomal RNA (rRNA) and proteins.

Endoplasmic Reticulum (ER)

• The ER is a network of membranes involved in protein and lipid synthesis.
• There are two types: rough ER (RER) and smooth ER (SER).
• RER has ribosomes attached and is involved in protein synthesis and modification.
• SER lacks ribosomes and is involved in lipid synthesis, detoxification, and calcium storage.

Golgi Apparatus

• The Golgi apparatus modifies, sorts, and packages proteins and lipids.
• It receives vesicles from the ER and processes their contents.
• Proteins and lipids are modified by glycosylation or other processes.
• The Golgi packages the modified molecules into vesicles for transport to other destinations.

Lysosomes

• Lysosomes contain enzymes for intracellular digestion.
• They break down cellular waste, debris, and foreign materials.
• Involved in autophagy (self-eating) to remove damaged organelles.
• Lysosomal enzymes are synthesized in the ER and processed in the Golgi.

Mitochondria

• Mitochondria are the powerhouses of the cell.
• They generate ATP (adenosine triphosphate) through cellular respiration.
• Have a double membrane structure: an outer membrane and an inner membrane with folds called cristae.
• Contain their own DNA and ribosomes.
• Involved in programmed cell death (apoptosis).

Cytoskeleton

• The cytoskeleton is a network of protein fibers that provides structural support and facilitates movement.
• There are three main types of fibers: microfilaments, intermediate filaments, and microtubules.
• Microfilaments are made of actin and are involved in cell shape and movement.
• Intermediate filaments provide structural support and stability.
• Microtubules are made of tubulin and are involved in cell division, intracellular transport, and cell motility.

Cell Wall

• The cell wall is a rigid outer layer that provides support and protection.
• Found in plant cells, bacteria, fungi, and algae.
• Plant cell walls are made of cellulose.
• Bacterial cell walls are made of peptidoglycan.
• Fungal cell walls are made of chitin.

Cell Size

• Cell size is limited by surface area to volume ratio.
• As a cell grows, its volume increases faster than its surface area.
• The plasma membrane must provide sufficient surface area for nutrient uptake and waste removal.
• Cells can increase surface area through folding or specialized structures.

Cell Communication

• Cells communicate with each other through chemical signals.
• Signals can be transmitted through direct contact, local signaling, or long-distance signaling.
• Receptor proteins on the cell surface bind to signaling molecules.
• Signal transduction pathways convert the signal into a cellular response.
• Cell communication is essential for coordinating cell activities and maintaining homeostasis.

Cell Division

• Cell division is the process by which cells reproduce.
• There are two main types of cell division: mitosis and meiosis.
• Mitosis results in two identical daughter cells.
• Meiosis results in four genetically different daughter cells with half the number of chromosomes.
• Cell division is essential for growth, repair, and reproduction.

Cell Metabolism

• Cell metabolism is the sum of all chemical reactions that occur within a cell.
• Catabolic pathways break down molecules to release energy.
• Anabolic pathways use energy to build complex molecules.
• Enzymes are essential for catalyzing metabolic reactions.
• ATP is the main energy currency of the cell.

Cell Differentiation

• Cell differentiation is the process by which cells become specialized in structure and function.
• During development, cells receive signals that determine their fate.
• Differential gene expression leads to different cell types.
• Cell differentiation is essential for the formation of tissues and organs.

Cell Specialization

• Specialized cells perform specific functions in multicellular organisms.
• Examples include nerve cells, muscle cells, and blood cells.
• Cell specialization results from differential gene expression and cell differentiation.

Stem Cells

• Stem cells are undifferentiated cells that can divide and differentiate into specialized cell types.
• Embryonic stem cells are pluripotent and can differentiate into any cell type in the body.
• Adult stem cells are multipotent and can differentiate into a limited range of cell types.
• Stem cells have potential for regenerative medicine and tissue engineering.

Cell Death

• Cell death is a normal and essential process in multicellular organisms.
• There are two main types of cell death: apoptosis and necrosis.
• Apoptosis is programmed cell death and is involved in development and tissue homeostasis.
• Necrosis is cell death caused by injury or infection.
• Dysregulation of cell death can lead to diseases such as cancer and neurodegenerative disorders.