The Cell

Overview

The cell is the basic structural and functional unit of the body. All living organisms are composed of cells, which perform essential life functions.

• Basic unit of life: Cells carry out all fundamental processes necessary for survival.

• Functions: Growth, metabolism, response to stimuli, and reproduction.

Principal Parts of the Cell

Main Components

Cells are composed of three principal parts, each with distinct roles in maintaining cellular function.

• Cell Membrane/Plasma Membrane/Plasmalemma: Forms the boundary of the cell.

• Cytoplasm: Gel-like substance inside the cell, containing organelles.

• Nucleus: Control center of the cell, contains genetic material.

Cell Membrane / Plasma Membrane / Plasmalemma

Structure and Composition

The cell membrane separates the inside and outside of the cell, regulating the movement of substances.

• Phospholipid bilayer:

  • Composed of phosphate head group (hydrophilic) and fatty acid tails (hydrophobic).

  • Divides Intracellular Fluid (ICF) (inside cell) and Extracellular Fluid (ECF) (outside cell).

• Cholesterol:

  • Scattered throughout the membrane.

  • Adds stability to the cell membrane.

• Membrane Proteins:

  • Integral proteins: Embedded within the membrane, often spanning its width (transmembrane proteins).

  • Peripheral proteins: Attached to either surface of integral proteins, located on the periphery of the membrane.

Functions of Membrane Proteins

• Enzymes

• Transporters

• Channels

• Receptors

• Anchors

• Identity markers (for immune system)

Specialized Structures

• Microvilli: Small projections of the cell membrane that increase surface area, found in cells of the small intestine and kidney.

• Fluid Mosaic Model: Describes the dynamic nature of the cell membrane, where proteins and lipids can move laterally within the layer.

Cytoplasm

Overview

The cytoplasm is the area between the cell membrane and the nucleus, containing cytosol and organelles.

• Cytosol: Gel-like intracellular fluid containing water, ions (e.g., K+, Na+), carbohydrates, proteins, and lipids. May contain inclusions such as melanin pigment and glycogen.

• Organelles: Structures that perform specific functions and are essential for life. Can be membranous (surrounded by a membrane) or non-membranous.

Non-membranous Organelles

• Ribosomes: Sites of protein synthesis, contain ribosomal RNA and proteins. Can be free in cytosol or attached to endoplasmic reticulum.

• Centrosomes: Composed of a pair of centrioles and pericentriolar material, organize microtubules and are important for cell division.

• Cytoskeleton: Provides structural support, cell shape, and movement. Formed from proteins and includes microfilaments, intermediate filaments, and microtubules.

Types of Cytoskeletal Elements

• Microfilaments: Composed of actin, involved in cell movement and muscle contraction.

• Intermediate filaments: Tissue-specific, e.g., keratin, provide mechanical strength.

• Microtubules: Hollow tubes made of tubulin, form centrioles, spindle apparatus, cilia, flagella, and move organelles within the cell.

Membranous Organelles

• Mitochondria: Site of ATP synthesis, contains own DNA, RNA, and proteins, has a double membrane.

• Endoplasmic Reticulum (ER): Network throughout cytoplasm.

  • Rough ER: Studded with ribosomes, synthesizes secretory, lysosomal, and membrane proteins.

  • Smooth ER: Lacks attached ribosomes, synthesizes lipids and steroid hormones.

• Golgi Apparatus/Complex: Stacks of membrane discs, modifies, sorts, packages, and delivers proteins and lipids.

• Lysosomes: Filled with digestive enzymes, digest bacteria, viruses, and worn-out organelles (cell clean-up function).

Nucleus

Structure and Function

The nucleus is the largest membranous organelle, controlling cell activities and containing genetic material.

• Nuclear envelope: Double membrane surrounding the nucleus.

• Nucleolus: Dense region containing DNA, RNA, and proteins, site of ribosome assembly.

• Chromosomes/Chromatin: Contain DNA and histone proteins.

  • Dispersed chromatin: DNA uncoiled and not individually visible, present when cell is not dividing.

  • Condensed chromatin: DNA tightly coiled and individually visible, present during cell division (mitosis/meiosis).

Summary Table: Cell Membrane Components

Key Equations

• Fluid Mosaic Model: No specific equation, but describes the dynamic and flexible nature of the cell membrane.

• ATP Synthesis (in mitochondria):

Example Application

• Microvilli in the small intestine: Increase surface area for nutrient absorption.

• Rough ER in pancreatic cells: Synthesizes digestive enzymes for secretion.

Additional info: Academic context and expanded explanations have been added to ensure completeness and clarity for college-level Anatomy & Physiology students.