Plant Cells and Growth

Organ systems

  • Plant tissues form organs, which are arranged into two distinct organ systems.
  • Shoot system: usually aboveground; includes vegetative organs (leaves and stems) and reproductive organs (flowers and fruits).
  • Root system: usually underground; root is the primary organ; slide notes say “Only one organ: roots,” but standard biology recognizes two organ systems (shoot and root).

Determinate vs indeterminate growth

  • Determinate growth:
    • Maximum size genetically determined
    • Growth stops after a time
    • Usually cannot heal/regrow
    • Maximum size rarely achieved in nature
    • Associated with leaves, flowers, and fruits
  • Indeterminate growth:
    • No fixed maximum size; growth can continue through life
    • Usually can heal/regrow
    • Growth limited by resources and environmental factors
    • Occurs in roots and stems

Cell theory (timeline and basics)

  • 1665: Robert Hooke observes cork tissue and coins the term "cells".
  • 1838: Schleiden and Schwann state that all plants and animals are made of cells; cell is the basic unit of life.
  • 1858: Virchow adds that cells arise by reproduction from previous cells — "Omnis cellula e cellula".

Relative sizes of cells and organelles (logarithmic scale)

  • On a log scale from 0.1 nm to 1 m, typical categories include:
    • Atom, proteins, lipids, viruses, mitochondria, bacteria, plant cells, animal cells, frog eggs, chicken eggs, human eggs, ostrich eggs, adult eggs/males, etc.
  • Scale references: 0.1 nm, 1 nm, 10 nm, 100 nm, 1 μm, 10 μm, 100 μm, 1 mm, 10 mm, 100 mm, 1 m.
  • Visualization may include different microscope types: Naked eye, light microscope, electron microscope.

Cells: four universal components (prokaryotic and eukaryotic)

  • All cells contain four basic components:
    1) Plasma (cell) membrane
    2) Cytoplasm
    3) DNA
    4) Ribosomes

Plasma membrane

  • A phospholipid bilayer with embedded proteins.
  • Serves as a barrier between the cell and the environment.
  • Controls the passage of organic molecules, ions, water, oxygen, and waste ( CO₂ and ammonia ).

Cytoplasm

  • Region between the plasma membrane and the nuclear envelope.
  • Composed of the cytosol (gel-like) and the cytoskeleton (protein threads).
  • Houses organelles suspended within the cytoplasm.
  • Provides structural support for cell components.

DNA – the nucleus (eukaryotic cells)

  • DNA is typically housed in the nucleus.
  • Directs the synthesis of ribosomes and proteins.
  • Stores chromatin (DNA + proteins) and the nucleolus (ribosome synthesis).
  • Nuclear envelope (double membrane) separates DNA from the cytoplasm.

DNA – chromatin and chromosomes

  • During cellular replication, DNA becomes visible as chromosomes (linear in eukaryotes).
  • During growth/maintenance, proteins bind to chromosomes, creating a thread-like form called chromatin.

Ribosomes

  • Structures responsible for protein synthesis.
  • NOT considered true organelles.
  • Can be free-floating in cytoplasm or bound to the endoplasmic reticulum (rough ER).
  • Receive instructions from the nucleus in the form of mRNA to assemble amino acids into proteins.

Eukaryotic cells: endomembrane system and related components

  • Endomembrane system modifies, packages, and transports lipids and proteins.
  • Vesicles and peroxisomes bud from endomembrane system and act as transport units.
  • Major components:
    • Endoplasmic reticulum (ER)
    • Golgi apparatus
    • Mitochondria

Endoplasmic reticulum (ER) and Golgi apparatus

  • ER:
    • Interconnected sacs and tubules.
    • Modifies proteins and synthesizes lipids.
  • Golgi apparatus:
    • Series of flattened membranes.
    • Receives proteins and lipids from the ER.
    • Sorts, tags, packages, and distributes them to target destinations.

Mitochondria

  • The "powerhouse" of the cell; site of cellular respiration.
  • Produces ATP (main energy-carrying molecule) from glucose and other nutrients.
  • Uses oxygen and releases CO₂ as a waste product.
  • Contains its own DNA and ribosomes.

Plant cells: components unique to plants

  • Cell wall
  • Central vacuole
  • Plastids (e.g., chloroplasts)

Central vacuole

  • Large, membrane-bound structure that stores water and can occupy much of the cell.
  • Regulates water concentration via osmosis: releases water under dry conditions, absorbs water under wet conditions.
  • Stores nutrients, ions, and waste products.
  • Surrounded by the tonoplast (vacuolar membrane).

Chloroplasts and other plastids

  • Chloroplast: plastid specialized for photosynthesis;
    • Stores chlorophyll and other pigments.
    • Pigments stored in interconnected sacs called thylakoids, arranged in stacks called grana.
    • Contains its own DNA and ribosomes.
  • Other plastids:
    • Chromoplasts (store orange/yellow pigments)
    • Amyloplasts (store starch)

Cell wall structure and properties

  • Middle lamella: thin layer of pectin that binds adjacent cells.
  • Primary cell wall: forms on the surface of all plant cells, deposited on the middle lamella.
  • Composed of cellulose microfibrils bundled together.
  • Thin and elastic, allowing cell enlargement.
  • Some cells have no secondary wall; others do.

Cell wall: secondary wall and cellular death

  • Secondary cell wall: deposited inside the primary wall (layers).
  • Lignin is deposited with cellulose to add rigidity.
  • Thicker than the primary wall.
  • As cells mature, the secondary wall grows inward, contributing to cell death in many contexts.
  • Dead cells can make over 90% of wood tissue.

Cell wall: orientation and intercellular communication

  • The orientation of cellulose microfibrils determines the direction of cell enlargement (like a spring: longitudinal vs. lateral expansion).
  • Plasmodesmata: pores that allow communication and transport between adjacent plant cells.

Plant growth and meristems

  • Plant tissues originate from various types of cells and differentiate/specialize.
  • Plant tissue is totipotent: can regenerate an entire plant.
  • Meristems are regions of continuous cell division and growth found at different locations in the plant.
  • Three meristem types:
    • Apical meristems
    • Lateral meristems (secondary meristems)
    • Intercalary meristems (found only in grasses)

Apical meristem

  • Located at the tips of shoots (shoot apical meristems) and roots (root apical meristems).
  • Produce three primary meristematic tissues:
    • Protoderm → epidermis
    • Ground meristem → ground tissue (photosynthetic, storage, support)
    • Procambium → vascular tissue (transport)
  • Locations:
    • Shoot apical meristem
    • Root apical meristem

Lateral meristem (secondary meristem)

  • Responsible for secondary growth (increase in girth).
  • Located in two regions:
    • Vascular cambium (arises from procambium) → produces vascular tissue.
    • Cork cambium (arises from pericycle and cortex) → produces periderm (secondary dermal tissue).

Meristems in a seed

  • The three primary meristematic tissues can be observed in the embryo of a seed.
  • Indicates the earliest formation of primary tissues during seed development.