Human Cell Structure and Function

The Cell

Introduction to Cells

  • Cells are the fundamental building blocks of all plants and animals.

  • Every cell originates from the division of pre-existing cells, making them the smallest units capable of performing all vital physiological functions.

Learning Outcomes

  • Describe a generalized cell.

  • Identify the three major regions of a cell: nucleus, cytoplasm, plasma membrane.

  • Explain the chemical composition of the plasma membrane and relate it to its functions.

  • Identify and describe major organelles and their roles within the cell.

Cell Diversity

  • Examples of specialized cells include:   - Red blood cells   - Nerve cells   - Skeletal muscle cells   - Epithelial cells

Structure of Human Cells

  • Cell Membrane: Encases cell contents.

  • Nucleus: Contains genetic information (DNA).

  • Cytoplasm: Comprises organelles and cytosol.

The Eukaryotic Cell Structure

  • Key Organelles and Structures:   - Rough Endoplasmic Reticulum (RER)   - Nucleus   - Nucleolus   - Nuclear Envelope   - Mitochondrion   - Lysosome   - Smooth Endoplasmic Reticulum (SER)   - Microtubule   - Plasma Membrane   - Golgi Apparatus   - Vesicles

Cell Membrane Functions

  • Forms a barrier between the intracellular and extracellular environments.

  • Regulates the passage of substances in and out of the cell.

  • Facilitates communication between cells.

Components of the Cell Membrane

  • Phospholipid Bilayer: Comprises two layers of phospholipids.   - Hydrophilic Heads: Form the exterior and interior surfaces of the membrane.   - Hydrophobic Tails: Located in the center of the membrane, repelling water.

Membrane Proteins

  • Channel and Carrier Proteins: Allow specific molecules to enter and exit the cell.

  • Receptor Proteins: For example, insulin receptors facilitate cellular responses to signals.

  • Enzymes: Catalyze biochemical reactions.

  • Linkers: Anchor cells and connect them to one another.

  • Recognition Proteins: Help identify cells to the immune system.

Nucleus

  • Enclosed by a nuclear envelope with nuclear pores.

  • Contains deoxyribonucleic acid (DNA) comprising approximately 20,000 genes, which serve as blueprints for proteins.

Organelles

  • Non-Membranous Organelles:   - Cytoskeleton: Structural support and movement.   - Cilia: Hair-like extensions that help in movement and transport.   - Microvilli: Increase surface area for absorption.   - Centrioles: Involved in cell division.   - Ribosomes: Produce proteins (free ribosomes in cytoplasm and fixed ribosomes on the RER).

  • Membranous Organelles:   - Mitochondria: Produce ATP energy, contain their own DNA with 37 genes.   - Endoplasmic Reticulum (ER):     - Rough ER: Covered with ribosomes, synthesizes and modifies proteins.     - Smooth ER: Synthesizes lipids, metabolizes carbohydrates, detoxifies drugs and alcohol.   - Golgi Apparatus: Modifies, sorts, and packages proteins and lipids for secretion or use within the cell.   - Lysosomes: Contain enzymes for digestion of cellular waste and recycling materials.

Summary of Cell Structure

  • Cells are the fundamental structural and functional units of life, characterized by diverse shapes and functions. They are encapsulated by a lipid bilayer membrane which houses various organelles, each serving distinct roles within the cell.

Membrane Transport

Learning Outcomes

  • Define important terms including:   - Concentration gradient   - Selective permeability   - Types of diffusion: simple, facilitated, osmosis   - Active and passive transport   - Specific processes: solute pumping, exocytosis, endocytosis, phagocytosis, pinocytosis, and the characteristics of hypertonic, hypotonic, and isotonic solutions.

  • Describe the structure of the plasma membrane and the transport processes governing the movement of substances.

Cell Membrane Characteristics

  • Phospholipid Bilayer:   - Integral and peripheral proteins move fluidly within the bilayer.   - The membrane is selectively permeable, allowing specific materials to pass while restricting others based on size, charge, and solubility.   - Highly permeable to small nonpolar molecules (e.g., O2, CO2), but impermeable to charged ions and large polar molecules (e.g., glucose).

Types of Membrane Transport

  • Membrane transport can occur via passive or active mechanisms:   - Passive Transport: No energy required (e.g., diffusion, osmosis).   - Active Transport: Requires energy (ATP).

Diffusion

  • Molecules are in constant random motion, colliding with one another. Over time, diffusion leads to an even distribution of substances, moving from areas of higher concentration to areas of lower concentration (concentration gradient).

Factors Affecting the Rate of Diffusion

  • Diffusion distance: Shorter distances lead to faster diffusion.

  • Molecule size: Smaller molecules diffuse faster.

  • Temperature: Increased temperature speeds up molecular motion.

  • Concentration gradient steepness: Greater differences in concentration improve diffusion rates.

Osmosis

  • A special case of diffusion, specifically regarding the movement of water across a selectively permeable membrane, typically in response to solute concentration differences. Water moves toward the area with higher solute concentration as it has the lower water concentration.

Tonicity

  • Refers to the solute concentration of the extracellular fluid (ECF):   - Isotonic Solution: Equal concentrations of solutes inside and outside the cell, with no net movement of water.   - Hypertonic Solution: ECF concentration is higher than the inside of cells, causing cells to shrink (crenate).   - Hypotonic Solution: ECF concentration is lower than inside the cell, causing cells to swell and potentially burst (hemolysis).

Active Transport

  • Unlike passive transport, active transport is not dependent on concentration gradients, allowing solutes to be transported against their gradient. It requires energy in the form of ATP.

  • Sodium-Potassium Pump: Essential for maintaining the concentration gradients of Na+ (sodium) and K+ (potassium) ions across the cell membrane:   - Pumps 3 Na+ out of the cell and 2 K+ into the cell per consumed ATP molecule.

Vesicular Transport

  • Involves transport of materials in membrane-bound vesicles. It includes:   - Endocytosis: Import of materials into the cell.   - Exocytosis: Export of materials out of the cell.

Summary of Membrane Transport

  • The cell membrane's selective permeability allows for the regulated passage of various substances, employing mechanisms of passive and active transport to maintain cellular homeostasis.

Proteins: Key to Cell Function

Learning Outcomes

  • Discuss the roles of genes in protein synthesis.

  • Describe the structure and packaging of DNA.

  • Explain the roles of DNA and the major types of RNA in protein synthesis.

  • Detail the processes of transcription and translation.

Different Cells - Different Proteins

  • Various types of proteins enable cells to perform different functions:   - Transport Proteins: Needed for moving substances across membranes.   - Receptor Proteins: Contain binding sites for signaling molecules.   - Enzymatic Proteins: Catalyze biochemical reactions.   - Structural Proteins: Provide support and shape to cells.

Genes Direct Formation of Proteins

  • DNA Structure: Consists of a sugar-phosphate backbone and nitrogenous bases (adenine, thymine, cytosine, and guanine).   - Base pairs: A-T and C-G form the rungs of the DNA ladder, which coil into a double helix.   - Genes are segments of DNA that encode proteins.

Different Cells - Different Gene Expression

  • Variability in protein production among cells pertains to differing gene expression patterns, causing cells to adopt distinct functions (e.g. fat cells vs. nerve cells).

Structure of a Gene

  • A gene contains:   - Promoter: Acts as the control switch for the gene.   - Coding Sequence: Encodes the amino acid sequence for protein.   - Terminator: Marks the end of the gene.

Transcription & Translation

  • Transcription Process: The synthesis of RNA from DNA.   - RNA polymerase binds to the promoter and unwinds the DNA.   - Builds an mRNA copy of the coding strand of DNA.

  • Translation Process: The production of proteins based on mRNA sequences by ribosomes in the cytoplasm.   - Codons (triplet sequences of bases) from mRNA specify corresponding amino acids.

Genetic Code

  • The genetic code is universal and consists of codons that instruct translation into specific amino acids, emphasizing that there are no spaces, commas, or overlaps in the codon reading frames.

  • Codons also signify start and stop signals in the translation process.

Summary of Protein Synthesis

  • Proteins are synthesized through coordinated processes of transcription and translation, reliant on RNA's role in transcribing and carrying genetic information from DNA to ribosomes for polypeptide assembly.

Cell Division

Learning Outcomes

  • Understand the principles of cell division.

  • Describe why and how cells divide.

  • Recall and outline the stages of the cell cycle.

  • Explain the process of DNA replication.

Introduction to Cell Division

  • Life begins as a single cell, growing to approximately 75 trillion cells in the human body.

  • Daily cellular turnover averages around 80 g, primarily of blood cells and gut epithelium.

Types of Cell Division

  • Mitosis: Produces 2 identical daughter cells; involves division of somatic cells.

  • Meiosis: Produces 4 genetically different daughter cells; involves gamete formation.

Regulation of Cell Division

  • Cell division is controlled by an ordered sequence of events known as the cell cycle, consisting of:   - Interphase: The phase of growth and DNA replication.   - Mitotic Phase: The actual division process.

Human Cell Life Spans

  • Different cell types in the body have varying lifespans:   - Neutrophils: Hours to a few days   - Stomach and intestinal cells: 3-5 days   - Skin cells: 2-3 weeks   - Red blood cells: 3-4 months   - Liver cells: 15-20 months

Interphase Stages

  • Comprises the G0, G1, S, and G2 phases:   - G0 Phase: A resting phase where cells can exit the cycle.   - G1 Phase: Cell grows and synthesizes proteins.   - S Phase: DNA replication occurs.   - G2 Phase: Prepares for mitosis through synthesis of crucial proteins and organelles.

DNA Replication

  • DNA must be replicated with high fidelity before division:   - Errors can lead to mutations; however, most errors (about 1 in 10 billion) are corrected.

  • The replication process is initiated by helicase unwinding the double helix and primase introducing RNA primers for DNA polymerase to act upon.

Stages of Mitosis

  • Mitosis is subdivided into four key stages:   - Prophase: Chromosomes condense, and the spindle apparatus forms.   - Metaphase: Chromosomes align at the metaphase plate.   - Anaphase: Centrosomes divide; chromatids are pulled to opposite poles.   - Telophase: Nuclear envelopes reform, and chromosomes decondense.

Cytokinesis

  • Occurs concurrently with the later stages of mitosis and involves the division of cytoplasm, leading to the final formation of two daughter cells.

Cell Cycle Check Points

  • Ensure that cells are healthy and ready to divide, thus preventing the distribution of damaged or incomplete cells during replication.

Summary of Cell Division

  • Cell division is essential for growth and tissue repair, necessitating accurate DNA replication and orchestrated events throughout the cell cycle to guarantee proper function of the resulting daughter cells.