Cell Structure, Membranes, and Transport: Study Notes for General Biology

The Cytoskeleton

Overview of the Cytoskeleton

• The cytoskeleton is a network of structural proteins extending throughout the cytoplasm of all cells.

• It consists of interconnected protein filaments, some permanent and some temporary, that reinforce, organize, and move cell structures.

• Major components include microfilaments, intermediate filaments, and microtubules.

Cytoskeletal Extensions and Functions

• Cilia and flagella are extensions of the cytoskeleton that allow for cell motility.

• Motor proteins use the cytoskeleton as highways to transport cellular materials, especially during processes like mitosis.

• Animal cells have centrosomes (containing centrioles) to organize microtubules during cell division.

External Cellular Structures

Cell Walls

• Cell walls are cross-linked networks of structural polysaccharides that provide support and protection.

• Plant cell walls are made of cellulose.

• Fungal cell walls are made of chitin.

• Bacterial cell walls are made of peptidoglycan.

Extracellular Matrix (ECM)

• Animal cells have an extracellular matrix (ECM), a network of connective proteins (like collagen) outside the cell membrane.

• The ECM acts as an external support scaffold and helps cells adhere together.

• Defects in ECM proteins (e.g., collagen) can cause tissues to tear easily.

Cell Junctions

• Proteins connect cells through intercellular junctions, which can create open channels for communication and transport between cells.

Cell Size

Why Cell Size Matters

• Cells must exchange substances with their environment at a rate that matches their metabolism.

• The cell membrane can only handle a limited number of exchanges at a time.

• The larger the cell's surface area, the more substances can cross the membrane at any given time.

Surface Area to Volume Ratio

• Efficient transport requires a surface area to volume ratio that is as high as possible.

• As cells increase in volume, their surface area to volume ratio decreases, making transport less efficient.

• Surface area is needed for transport of materials in and out of the cell.

Cell Size Equations

Shape	Surface Area (SA)	Volume (V)
Sphere	$SA = 4\pi r^2$	$V = \frac{4}{3}\pi r^3$
Rectangular Solid	$SA = 2lh + 2lw + 2wh$	$V = lwh$
Right Cylinder	$SA = 2\pi r^2 + 2\pi rh$	$V = \pi r^2 h$
Cube	$SA = 6a^2$	$V = a^3$

Limits to Cell Size

• Cells that are too large cannot exchange matter efficiently; nutrients and wastes cannot move fast enough.

• Cells that are too small cannot fit enough materials inside or retain heat/nutrients efficiently.

Solutions to Cell Size Limitations

• Multicellularity: Organisms maintain large volumes by increasing surface area with many individual cells (divide and conquer strategy).

• Cell Shape: Large cells increase surface area by stretching out or forming convolutions (e.g., microvilli in the intestine, long neurons).

Compartmentalization

• Organelles within eukaryotic cells allow for specialization and increased surface area for metabolic reactions.

The Cell Membrane

Cell Membrane Structure

• The cell membrane separates the internal environment from the external environment and compartmentalizes metabolism.

• It is composed of a double layer of phospholipids, which are amphipathic (having both hydrophilic and hydrophobic regions).

• Hydrophilic heads face outward toward water; hydrophobic tails face inward, away from water.

Fluid Mosaic Model

• Describes the organization of cell membranes as a fluid structure with a mosaic of proteins, glycoproteins, steroids, and cholesterol molecules embedded in the phospholipids.

• Phospholipids and proteins can move laterally within the layer, contributing to membrane fluidity.

Cell Membrane Function

• The membrane is semipermeable (selectively permeable), allowing only certain molecules to pass through.

Membrane Proteins

• Peripheral proteins temporarily attach to the membrane surface for signaling or communication.

• Integral proteins are permanently attached and often span the membrane (transmembrane proteins), functioning in transport or communication.

Functions of Membrane Proteins

• Intercellular joining

• Enzymatic activity

• Transport (active and passive)

• Cell-cell recognition

• Anchorage/attachment

• Signal transduction

Cholesterol

• Cholesterol molecules maintain membrane fluidity across temperature changes in animal cells.

• At high temperatures, cholesterol stabilizes the membrane; at low temperatures, it prevents phospholipids from packing too tightly.

• Plants adjust the proportion of saturated/unsaturated fatty acids instead of using cholesterol.

Carbohydrates

• Carbohydrate chains attached to proteins (glycoproteins) and lipids (glycolipids) are used for cell recognition and communication.

• These structures help the immune system recognize self vs. non-self cells (important in organ transplants).

Cellular Transport

Passive Transport

• Substances move down their concentration gradient (from high to low concentration) without using metabolic energy (ATP).

• Types:

  • Simple diffusion: Small, uncharged molecules pass through phospholipids.

  • Facilitated diffusion: Large or charged molecules pass through membrane proteins.

  • Osmosis: Diffusion of water across a membrane.

Diffusion

• Net movement of molecules from high to low concentration (down a concentration gradient).

• Factors affecting rate: molecule size, temperature, concentration gradient steepness, charge, and pressure.

Selective Permeability

• Cell membranes control which substances enter or leave, maintaining homeostasis.

• Small molecules (water, CO2, O2) cross easily; ions and large polar molecules require facilitated diffusion.

Active Transport

• Moves molecules from low to high concentration (against the gradient), requiring energy from ATP.

• Creates concentration gradients necessary for cellular processes.

Protein Pumps

• Activated by phosphorylation (addition of phosphate from ATP).

• Examples:

  • Na+/K+ pump: Maintains high sodium outside and high potassium inside neurons.

  • Proton (H+) pumps: Create electrochemical gradients across membranes.

Bulk Transport

• Cells ingest (endocytosis) or secrete (exocytosis) large particles or large amounts of material.

• Types:

  • Phagocytosis: Ingesting food particles.

  • Pinocytosis: Ingesting liquids.

• Requires energy to move vesicles and create new membrane patches.

Osmosis

Osmosis Mechanism

• Water diffuses through the phospholipid membrane (slowly) or through channel proteins called aquaporins (facilitated diffusion).

• Water moves to the side of the membrane with the highest solute concentration until dynamic equilibrium is reached.

Tonicity

• Isotonic: Equal solute concentration inside and outside the cell; water moves in and out at equal rates.

• Hypotonic: Lower solute concentration outside; water moves into the cell, which may swell and burst (animal cells) or become turgid (plant cells).

• Hypertonic: Higher solute concentration outside; water moves out, causing cells to shrivel (animal cells) or plasmolyze (plant cells).

Water Potential

• Water potential ($\Psi$) is the potential energy of water in a system.

• Equation: $\Psi = \Psi_s + \Psi_p$

  • $\Psi_s$ = solute potential

  • $\Psi_p$ = pressure potential

• Water moves from areas of higher water potential to lower water potential.