Cell Structure

Cell Structure Overview

• The basic unit of life.

  Three Basic Features of Cells

1. Compartmentalization

   • Allows for specialization.

2. Self Assembly and Homeostasis

3. Replication

   • Every cell is formed from a pre-existing cell.

   Specialization in Multicellular Organisms

• In multicellular organisms, specialization occurs between cells.

Cell Groups:

• Two basic groups of cells: eukaryotic (organelles surrounded by a membrane), and prokaryotic (no organelles surrounded by a membrane)

• Viruses contain genetic information and replicate inside host cells. Are viruses alive?

Cell Size and Measurement

Comparison of Cell Types

• Human Egg: 100 µm

• Paramecium: 1 mm (1,000 µm)

• Frog Egg: 1 mm (1,000 µm)

• Nucleus of Most Bacteria: 1 µm (1,000 nm)

• Mitochondrion: 1 µm (1,000 nm)

• Virus: 100 nm

• Ribosome: 10 nm

• Length of Some Nerve and Muscle Cells: 10 m

• Chicken Egg: 1 m (1,000 mm)

• Human Height: 10 m

• Most Plant and Animal Cells: 10 µm

• Proteins, Small Molecules, Lipids: 1 nm

• Atoms: 0.01 m

• Location-Based Size Perception:

  • Electron Microscope: Can resolve structures as small as 1 nm.

  • Light Microscope: * Can visualize cells larger than 1 µm.*

  • Unaided Eye: Can see objects as small as 0.1 mm; cannot see cells.

Understanding Cell Size Limitations

Minimum Size

• Limited by the ability to pack structural components into the cell.

Maximum Size

• Limited by the ratio of surface area to volume:

  • Volume formula: $ext{Volume} = L^3$

  • Surface area formula: $ext{Surface Area} = L^2 imes 6$

  • For a cube shape, relationship illustrated: Surface area increases at a slower rate than volume with increasing L.

Cell Connections in Multicellular Organisms

• Most cell-to-cell connections allow for the passage of materials between or through cells including:

  • Animal cells: Connections allow material flow.

  • Plant cells: Plasmodesmata and gap junctions allow material flow from one cell to another. In plants, many living cells are not deeply buried by other cells

  • Multi celled organisms need structures that maximize surface area relative to volume.

Structural Components of Cells

Membranes (sheet like structure)

• 4 Membrane Functions:

  1. Compartmentalization

  2. Selective Passage of Material: Semi-permeable

  3. Sites of Chemical Reactions

  4. Receptors for Cell Recognition

Cytoplasm

• Fluid medium where organelles are suspended.

Cytoskeleton

• Provides structure to the cell, anchors organelles, and controls movement of the plasma membrane and cell components.

  • ATP Motor Proteins: Use energy from ATP to move vesicles along the cytoskeleton.

  • Microtubules: Essential for vesicle movement.

Cell Wall

• Provides structure and protection.

• Not found in animal cells.

• Composed of cellulose in plant cells and chitin in fungal cells.

Organelles- structures within cells

Non-Membrane Bound Organelles

• Ribosomes:

  • Synthesize proteins.

Membrane Bound Organelles

• Nucleus:

  • Contains a double membrane and holds most of the genetic information (DNA) of the cell.

  • The nucleus is the control center of the cell.

The Endomembrane System

Components of the Endomembrane System

• Nuclear Membrane

• Rough Endoplasmic Reticulum (ER):

  • Contains ribosomes, involved in the production of proteins to be excreted from the cell.

• Smooth Endoplasmic Reticulum (ER):

  • Functions in lipid synthesis, carbohydrate metabolism, and detoxification.

• Golgi Apparatus:

  • Processes and packages secretory products from the endoplasmic reticulum to be secreted out of the cell.

Other Organelles

• Vacuoles: Storage areas within the cell.

• Chloroplasts:

  • Site of photosynthesis.

  • Double membrane with a highly folded internal membrane.

• Mitochondria:

  • Description: Site of cellular respiration in eukaryotic cells.

  • Contains ribosomes and DNA

  • Features: Composed of an inner membrane, outer membrane, matrix, and cristae.