The Cell

Overview of Cell Structure

The cell is the basic structural and functional unit of life. All organismal activities depend on the individual and collective activities of cells, and their biochemical activities are dictated by their subcellular structures. The continuity of life has a cellular basis, as described by the cell theory.

• Cell Diversity: Cells vary in structure and function, including fibroblasts, erythrocytes, epithelial cells, muscle cells, nerve cells, fat cells, and sperm cells.

• Generalized Cell Structure: Most cells share common features such as the plasma membrane, cytoplasm, and nucleus.

Structure of the Plasma Membrane: Fluid Mosaic Model

The plasma membrane is a dynamic structure composed of a phospholipid bilayer with embedded proteins, cholesterol, glycolipids, and glycoproteins. It separates the intracellular environment from the extracellular fluid and regulates the movement of substances into and out of the cell.

• Phospholipid Bilayer: Provides the basic structure, with hydrophilic heads facing outward and hydrophobic tails inward.

• Membrane Proteins: Integral and peripheral proteins serve various functions such as transport, signaling, and cell recognition.

• Carbohydrates: Attached to proteins and lipids on the extracellular surface, forming the glycocalyx for cell recognition.

Functions of Membrane Proteins

Membrane proteins are essential for various cellular functions, including transport, enzymatic activity, signal transduction, cell-cell recognition, intercellular joining, and attachment to the cytoskeleton and extracellular matrix (ECM).

• Transport: Channels and carriers move substances across the membrane.

• Enzymatic Activity: Some proteins act as enzymes to catalyze reactions.

• Signal Transduction: Receptors transmit signals from the extracellular environment to the cell's interior.

• Cell-Cell Recognition: Glycoproteins serve as identification tags.

• Intercellular Joining: Proteins help bind cells together.

• Attachment: Proteins anchor the membrane to the cytoskeleton and ECM.

Membrane Transport

Passive Processes

Passive transport does not require energy and relies on the movement of molecules down their concentration gradients. Types include diffusion (simple and facilitated), osmosis, and filtration.

• Diffusion: Movement of molecules from higher to lower concentration.

• Facilitated Diffusion: Uses carrier or channel proteins for molecules that cannot diffuse directly through the lipid bilayer.

• Osmosis: Diffusion of water across a semipermeable membrane.

• Filtration: Movement of solute-containing fluid driven by hydrostatic pressure.

Tonicity and Effects on Cells

Tonicity describes how a solution affects cell volume. Isotonic solutions cause no net movement of water, hypertonic solutions cause cells to shrink, and hypotonic solutions cause cells to swell and possibly burst.

• Isotonic: No net water movement; cell shape remains unchanged.

• Hypertonic: Water leaves the cell; cell shrinks (crenation).

• Hypotonic: Water enters the cell; cell swells and may lyse.

Active Processes

Active transport requires ATP to move substances against their concentration gradients. It includes primary and secondary active transport, as well as vesicular transport (endocytosis and exocytosis).

• Primary Active Transport: Direct use of ATP, e.g., Na+/K+ pump.

• Secondary Active Transport: Uses energy stored in gradients created by primary active transport.

• Vesicular Transport: Movement of large particles via vesicles (endocytosis and exocytosis).

Types of Endocytosis

• Phagocytosis: Cell engulfs large particles.

• Pinocytosis: Cell "gulps" extracellular fluid.

• Receptor-Mediated Endocytosis: Specific molecules are ingested after binding to receptors.

Generation and Maintenance of Membrane Potential

The resting membrane potential is established by the differential permeability of the plasma membrane to Na+ and K+, and by the activity of the Na+/K+ pump. This potential is essential for nerve and muscle function.

• Typical resting membrane potential: –45 to –90 mV.

• K+ diffusion: Out of the cell creates a negative charge inside.

• Na+/K+ pump: Maintains the gradient by pumping 3 Na+ out and 2 K+ in.

Cytoplasm and Organelles

Cytoplasm

The cytoplasm consists of cytosol (fluid), organelles (membranous and nonmembranous), and inclusions (stored nutrients, pigments, etc.).

• Membranous organelles: Mitochondria, peroxisomes, lysosomes, endoplasmic reticulum, Golgi apparatus.

• Nonmembranous organelles: Cytoskeleton, centrioles, ribosomes.

Mitochondria

Mitochondria are the powerhouses of the cell, generating ATP through aerobic respiration. They contain their own DNA and RNA.

Endoplasmic Reticulum (ER)

The ER is a network of membranes with two forms: rough ER (with ribosomes, site of protein synthesis) and smooth ER (involved in lipid synthesis, detoxification, and calcium storage).

Golgi Apparatus

The Golgi apparatus modifies, concentrates, and packages proteins and lipids for secretion or delivery to other organelles.

Peroxisomes and Lysosomes

• Peroxisomes: Contain enzymes for detoxification and neutralization of free radicals.

• Lysosomes: Digest bacteria, viruses, toxins, and worn-out organelles.

Cytoskeleton

The cytoskeleton provides structural support, maintains cell shape, and facilitates movement. It consists of microfilaments, intermediate filaments, and microtubules.

• Microfilaments: Actin filaments involved in cell movement and shape.

• Intermediate filaments: Provide tensile strength.

• Microtubules: Hollow tubes that determine cell shape and are involved in intracellular transport.

Centrioles and Cilia

Centrioles organize the mitotic spindle during cell division and form the bases of cilia and flagella, which are involved in cell movement and fluid propulsion.

Nucleus

The nucleus is the control center of the cell, containing genetic material (DNA) and the nucleolus (site of ribosome production). It is surrounded by a double membrane with nuclear pores for transport.

Cell Growth, Reproduction, and Protein Synthesis

Cell Cycle

The cell cycle consists of interphase (G1, S, G2 phases) and mitotic phase (mitosis and cytokinesis). DNA replication occurs during the S phase, and mitosis ensures equal distribution of genetic material.

DNA Structure and Replication

DNA is a double helix composed of nucleotides (adenine, thymine, cytosine, guanine). Replication is semiconservative, producing two identical DNA molecules.

• Key enzymes: Helicase (unwinds DNA), DNA polymerase (synthesizes new strands), DNA ligase (joins fragments).

Mitosis

Mitosis is the process of nuclear division, consisting of prophase, metaphase, anaphase, and telophase, followed by cytokinesis (division of cytoplasm).

From DNA to Protein: Protein Synthesis

Genes in DNA serve as templates for the synthesis of proteins through transcription (DNA to mRNA) and translation (mRNA to protein).

• Transcription: Occurs in the nucleus; RNA polymerase synthesizes mRNA from DNA.

• Translation: Occurs in the cytoplasm; ribosomes read mRNA and tRNAs bring amino acids to form a polypeptide chain.

Cell Differentiation and Aging

Cell Differentiation

Cell differentiation is the process by which cells develop specific and distinctive features. Programmed cell death (apoptosis) is a controlled process that removes unnecessary or damaged cells.

• Cell aging theories: Wear and tear, mitochondrial, immune, and genetic theories (e.g., telomere shortening).

Summary Table: Types of Membrane Transport