Cell_structure- Human Bio

The Human Body and Cells

  • The human body contains more than 100 trillion cells.

  • Cells vary considerably in shape and size but share common characteristics.

  • Variations in cell shape enable different functionalities.

Cell Theory

  • Historical Background:

    • Robert Hooke (late 1600s) coined the term "cell" while observing cork.

    • Van Leeuwenhoek (mid-late 1600s) observed unicellular organisms.

    • Schleiden (1838) and Schwann (1839) contributed to the understanding of plant and animal cells, respectively.

    • Virchow (1850s) stated that "every animal is composed of vital units" reflecting life characteristics.

  • Modern Formulation:

      1. A cell is the fundamental unit of life.

      1. All living things are made up of cells containing everything necessary for life.

      1. New cells arise from pre-existing cells.

    • Discussion on viruses and prions raises the question of what constitutes life.

Cell Size and Efficiency

  • Cells are generally small for functional efficiency.

  • The surface-area-to-volume ratio favors smaller cells:

    • Smaller cells provide greater surface area for nutrient absorption and waste expulsion.

    • Efficiency is limited as cell size increases.

Microscopy and Visualization of Cells

  • Common microscopes for cell observation:

    • Compound Light Microscope:

      • Magnification up to 1200x.

      • Utilizes visible light for imaging.

      • Effective in viewing micron scale structures (200 nm and above).

  • Electron Microscopy:

    • Scanning Electron Microscopy (SEM):

      • Resolution from 1 nm to 20 nm; 3D images of surface structures.

      • Requires metal coating; cannot view live specimens.

    • Transmission Electron Microscopy (TEM):

      • Provides 2D images of internal features through thin sections.

Types of Living Cells

  • Prokaryotic Cells:

    • Simplest form of life without a nucleus, represented by bacteria and archaea.

  • Eukaryotic Cells:

    • Possess a nucleus containing DNA and numerous membrane-bound organelles.

Common Features of Prokaryotic and Eukaryotic Cells

  • All cells share:

    • A plasma membrane surrounding them.

    • Cytoplasm for organelles to reside.

    • DNA as genetic material.

Origins of Eukaryotic Cells

  • Evolution from prokaryotic ancestors included:

    1. Formation of a nucleus via plasma membrane invagination.

    2. Development of an endomembrane system and mitochondria.

    3. Acquisition of chloroplasts in plant cells.

Structure of Eukaryotic Cells

  • Plasma Membrane: Regulates molecular passage.

  • Cytoskeleton: Maintains cell shape and facilitates movement, composed of:

    • Microtubules.

    • Intermediate filaments.

    • Actin filaments.

  • Organelles: Specialized structures with distinct functions.

  • Nucleus:

    • Enclosed by a nuclear envelope with pores.

    • Contains DNA and nucleolus for ribosome production.

Cell Membrane Structure and Function

  • Composed of a phospholipid bilayer:

    • Phospholipids (75%): form the bilayer with hydrophilic heads and hydrophobic tails.

    • Cholesterol (20%): contributes to membrane fluidity.

    • Glycolipids (5%): play a role in cellular recognition.

    • Membrane Proteins: Include transmembrane proteins, glycoproteins, and peripheral proteins.

Selective Permeability of Membranes

  • Membranes allow selective entry and exit of substances while serving as barriers.

Transport Mechanisms across Plasma Membranes

  1. Diffusion: Movement from high to low concentration.

  2. Osmosis: Diffusion of water across a selectively permeable membrane.

  3. Facilitated Transport: Movement via protein carriers without energy.

  4. Active Transport: Movement against concentration gradient requiring ATP.

  5. Endocytosis and Exocytosis: Processes for transporting large molecules into and out of the cell using vesicles.

Tonicity and its Effects on Cells

  • Hypertonic: High solute concentration outside causes cell shriveling (crenation).

  • Hypotonic: Low solute concentration outside leads to cell bursting (lysis).

  • Isotonic: Equal solute concentration maintains cell stability.

Facilitated Diffusion vs. Active Transport

  • Facilitated Diffusion: Passive transport using a specific carrier from higher to lower concentration.

  • Active Transport: Energy-requiring process moving substances against their concentration gradient.

Endocytosis and Exocytosis Defined

  • Endocytosis: Transport of materials into the cell via membrane invagination.

  • Exocytosis: Release of substances outside the cell through vesicle fusion with the plasma membrane.

Cytoplasm Structure

  • Cytosol: Fluid matrix, primarily water.

  • Organelles: Specialized structures that perform functions.

  • Cytoskeleton: Structural framework of protein fibers.

  • Inclusions: Non-essential materials stored within cells.

Organelles Functions

  • Mitochondria: Powerhouse of the cell, producing ATP via respiration. Contains its own DNA.

  • Ribosomes: Sites of protein synthesis, either free-floating or bound to the endoplasmic reticulum.

  • Endoplasmic Reticulum (ER):

    • Rough ER: Synthesizes proteins.

    • Smooth ER: Synthesizes lipids, does not have ribosomes.

  • Golgi Complex: Modifies, packages, and distributes proteins and lipids.

  • Lysosomes: Contain enzymes for digestion of waste and macromolecules.

Summary of the Endomembrane System

  • The endomembrane system includes various organelles that collectively manage the transport and processing of biomolecules in the cell.

Cytoskeleton Functions

  • The cytoskeleton maintains cell shape and anchors organelles.

  • Composed of three types of filaments indicating varied diameters and functions.

Mitochondrial Function

  • Mitochondria are involved in cellular respiration, converting glucose into ATP:

    1. Glycolysis (cytoplasm).

    2. Citric acid cycle (in mitochondria).

    3. Electron transport chain (in mitochondria).

Cellular Respiration Overview

  • Glycolysis: Occurs with or without oxygen, producing ATP and pyruvate.

  • Citric Acid Cycle: Requires oxygen, further processes pyruvate into ATP and carriers.

  • Electron Transport Chain: Final step for ATP production; requires oxygen as an electron acceptor.

Anaerobic ATP Production

  • When oxygen levels are low, ATP is generated through fermentation processes; produces limited ATP and lactate.

Enzymes in Cellular Processes

  • Enzymes are proteins that accelerate biochemical reactions:

    • Highly specific, with active sites binding substrates.

    • Enzymes are not consumed in reactions and can be reused.

    • Coenzymes may assist some enzymes in their activity.