Unit 3- The Cell: Structure, Components, and Functions

The Cell

Overview of the Cell

The cell is the basic structural and functional unit of the body. All living organisms are composed of cells, which are capable of performing all basic life functions necessary for survival.

• Basic structural and functional unit of the body

• Can perform all basic life functions

Principal Parts of the Cell

• Cell Membrane / Plasma Membrane / Plasmalemma

• Cytoplasm

• Nucleus

Cell Membrane / Plasma Membrane / Plasmalemma

Structure and Composition

The cell membrane forms the boundary between the inside and outside of a cell, regulating the movement of substances in and out.

• Phospholipid bilayer:

  • Composed of:

    • Phosphate head group (hydrophilic)

    • Fatty acid tails (hydrophobic)

  • Divides cell into:

    • Intracellular Fluid (ICF): inside the cell

    • Extracellular Fluid (ECF): outside the cell

• Cholesterol:

  • Scattered throughout the membrane

  • Adds stability to the cell membrane

• Membrane Proteins:

  • Two types:

    1. Integral proteins: integrated within the membrane, often spanning across (transmembrane proteins)

    2. Peripheral proteins: attached to the surface of integral proteins, on the periphery of the membrane

  • Functions of membrane proteins:

    • Enzymes

    • Transporters

    • Channels

    • Receptors

    • Anchors

    • Identity markers (important for immune system recognition)

Specializations of the Cell Membrane

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

• Fluid Mosaic Model:

  • Describes the dynamic nature of the membrane

  • Fluid: membrane constituents can move around (phospholipids and some proteins)

  • Mosaic: proteins dot the surface like tiles in a mosaic

Cytoplasm

Definition and Components

The cytoplasm is the area between the inside of the cell membrane and the outside of the nucleus. It contains various organelles and inclusions suspended in cytosol.

• Cytosol: gel-like intracellular fluid containing water, ions (e.g., K+, Na+), carbohydrates, proteins, and lipids

• Inclusions: non-living materials such as melanin (pigment), glycogen (stored glucose), and triglycerides

• Organelles: specialized structures that perform specific functions essential for life

  • Non-membranous organelles:

    1. Ribosomes: sites of protein synthesis, contain ribosomal RNA and proteins, may be free in cytosol or attached to endoplasmic reticulum (ER) or mitochondria

    2. Centrosomes: consist of centrioles and pericentriolar matrix, organize microtubules and spindle apparatus for cell division

    3. Cytoskeleton: network of protein filaments (microfilaments, intermediate filaments, microtubules) providing structural support, cell movement, and transport of organelles

  • Membranous organelles:

    1. Mitochondria: site of ATP synthesis, contain their own DNA, RNA, and proteins, have a double membrane

    2. Endoplasmic Reticulum (ER):

       • Rough ER (RER): ribosomes attached, synthesizes secretory, lysosomal, and membrane proteins

       • Smooth ER (SER): lacks attached ribosomes, synthesizes lipids and steroid hormones

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

    4. Lysosomes: filled with digestive enzymes, digest bacteria, viruses, and worn-out organelles (cellular clean-up)

Nucleus

Structure and Function

The nucleus is the largest membranous organelle and serves as the control center of the cell, containing genetic material and directing cellular activities.

• Cells may have one or more nuclei

• Parts of the nucleus:

  • Nuclear envelope: double membrane surrounding the nucleus

  • Nucleolus: dense region where ribosomal RNA is synthesized and ribosome assembly begins

  • Chromosomes/chromatin: contain DNA and histone proteins

    • Dispersed (chromatin): DNA uncoiled, not individually visible, present most of the time when the cell is not dividing

    • Condensed: DNA coiled and individually visible, found in dividing cells (mitosis/meiosis)