Exam 1

Plants are arranged in 2 different systems

Shoot and root

Shoot system

1. usually above ground

2. vegetative organs such as leaves and stems

3. Reproductive organs such as flowers and fruits

Root system

1. usually underground

2. only 1 organ : roots

Determinate growth

1. maximum size genetically determined

   1. growth stops after time

   2. usually cannot heal/regrow

   3. maximum size rarely achieved in nature

   4. leaves, flowers, and fruits

Indeterminate growth

1. no maximum size genetically

2. able to keep growing through life

3. usually can heal/regrow

4. limited by resources and environmental factors

5. roots and stems

cell theory

• 1665 – Robert Hooke observed cork under a microscope and saw small box-like structures. He called them “cells.”

• 1838 – Matthias Schleiden and Theodor Schwann proposed that:

  1. All plants and animals are made of cells

  2. The cell is the basic unit of life

• 1858 – Rudolf Virchow added that:
  3. All cells arise from pre-existing cells (“Omnis cellula e cellula”)

Robert Hooke

Observed cork under a microscope and saw small box like structures, He called then “cells”. 1665

Matthias Schleiden and Theodor Schwann

proposed that:

1. All plants and animals are made of cells

2. The cell is the basic unit of life

1838

Rudolf Virchow

added that:
3. All cells arise from pre-existing cells (“Omnis cellula e cellula”)

1858

All cells (prokaryotic and eukaryotic) contain

the same 4 components:

1. Plasma (cell) membrane

2. Cytoplasm

3. DNA

4. Ribosomes

Plasma membrane

1. phospholipid bilayer with embedded proteins

2. serves as a barrier between the cell and the environment

3. controls the passage of organic molecules, ions, water, oxygen, and cellular waste (CO2 and ammonia)

Cytoplasm

the gel-like substance enclosed by the cell membrane. In eukaryotic cells, it fills the space between the membrane and the nucleus. It acts as a staging ground for vital life processes, housing organelles, proteins, and the cytoskeleton.

DNA – the nucleus

known as nuclear DNA, is the biological blueprint for the cell. It contains the complete genetic instructions required to build, maintain, and reproduce organisms

• contains chromatin (DNA + Proteins) and the nucleolus (where ribosomes are made)

• separated from the rest of the cell by a double membrane called nuclear envelope

DNA – chromatin and chromosomes

• during cell division (replication), DNA becomes tightly coiled and is visible as chromosomes (linear in eukaryotes).

• During normal cell growth and maintenance (interphase), DNA is less condensed and is associated with proteins called histones, forming chromatin, which appears as loose, tangled threads.

Ribosomes

• Structures responsible for protein synthesis

• Considered non-membrane-bound organelles (or cellular machinery)

• Can float freely in the cytoplasm or be attached to the rough endoplasmic reticulum

• Receive instructions from the nucleus in the form of mRNA, which determines the amino acid sequence for building a protein

In addition, eukaryotic cells share several important internal structures:

• Endomembrane system – modifies, packages, and transports lipids and proteins

  • Includes the endoplasmic reticulum, Golgi apparatus, vesicles, and parts of the cell membrane system

• Endoplasmic reticulum (ER) – site of protein (rough ER) and lipid (smooth ER) synthesis

• Golgi apparatus – modifies, sorts, and ships proteins and lipids

• Vesicles – transport materials within the cell and to the cell membrane

• Peroxisomes – break down fatty acids and detoxify harmful substances (not part of the endomembrane system)

• Mitochondria – produce ATP (energy) through cellular respiration (independent of the endomembrane system)

Endoplasmic reticulum

a network of interconnected sacs and tubules that:

• modifies proteins (rough ER)

• synthesizes lipids (smooth ER)

Golgi apparatus

a stack of flattened membrane sacs that:

• receives proteins and lipids from the ER

• modifies, sorts, and “tags” them

• packages them into vesicles for transport to their final destinations

Mitochondria

Known as the “powerhouse” of the cell; they are the site of cellular respiration

• Produce ATP (adenosine triphosphate), the cell’s main energy-carrying molecule, from glucose and other nutrients

• Use oxygen and release carbon dioxide and water as waste products

• Contain their own DNA and ribosomes, allowing them to produce some of their own proteins

There are components that are unique to plant cells

1. cell wall

2. central vacuole

3. plastids such as chloroplast

Central Vacuole

A large, membrane-bound structure that stores water and can occupy most of the plant cell volume

• Helps regulate water balance (turgor pressure) by:

  • absorbing water in wet conditions

  • releasing water in dry conditions (osmosis-driven changes in turgor)

• Also stores nutrients, ions, pigments, and waste products

• Surrounded by a membrane called the tonoplast

Chloroplast (and other plastids)

A type of plastid that contains chlorophyll and other pigments for photosynthesis

• Internal membrane system forms flattened sacs called thylakoids

• Thylakoids are often stacked into structures called grana

• Contains its own DNA and ribosomes

Other plastids include:

• Chromoplasts – store red, orange, and yellow pigments

• Amyloplasts – store starch

Cell Wall

• The middle lamella is a pectin-rich layer that cements adjacent plant cells together

• The primary cell wall:

  • is present in all young plant cells

  • is laid down outside the middle lamella

  • contains cellulose microfibrils

  • is thin and flexible, allowing cell growth

• The secondary cell wall:

  • is deposited inside the primary wall after the cell stops growing

  • contains cellulose and often lignin, increasing strength and rigidity

  • is thicker and more rigid

  • can lead to cell death in some cells as it matures (common in xylem)

• Cellulose microfibril orientation determines the direction of cell expansion

  • analogy idea is good (spring), just remember it mainly restricts expansion perpendicular to microfibrils

• Plasmodesmata are microscopic channels that allow communication and transport between plant cells

Meristems

are regions of continuous cell division in plants. They are responsible for growth and are found in different parts of the plant. There are three main types:

• Apical meristems
  Located at the tips of roots and shoots. They are responsible for primary growth (lengthening the plant).

• Lateral meristems
  Found along the sides of stems and roots. They are responsible for secondary growth (increasing thickness/girth).

• Intercalary meristems
  Found between mature tissues, especially at the bases of leaves or nodes. They are common in grasses and allow rapid regrowth after cutting or grazing.

Apical Meristem

• Located at the very tips (apices) of:

  • Stems → shoot apical meristems

  • Roots → root apical meristems

  • Responsible for primary growth (increase in length)

Protoderm

• Apical meristem

  • → gives rise to epidermis (outer protective layer)

Ground meristem

gives rise to ground tissue (photosynthesis, storage, support)

Procambium

gives rise to vascular tissue (xylem and phloem for transport)

Lateral meristem (secondary meristem):

-Responsible for secondary growth (increase in girth/thickness)
-Located in two regions

Vascular cambium

• lateral meristem

(arises from procambium)
→ Produces secondary vascular tissue (secondary xylem and phloem)

Cork cambium

• (arises from pericycle and cortex)
  → Produces periderm (secondary dermal tissue)

Types of tissues

Parenchyma, collenchyma, sclerenchyma

Parenchyma cells:

Usually spherical or elongated, but can vary in shape
-Thin primary cell walls; may develop lignified secondary walls
-Living cells with many metabolic functions:
• Photosynthesis and respiration
• Storage and secretion
-Only plant cell type that can be reprogrammed when mature (tissue culture, wound healing)

Collenchyma cells and tissue:

-Elongated cells, often living at maturity, may contain chloroplasts
-Cell walls have uneven thickening with layers of pectin and cellulose (thickest at corners)
-Provides strong but flexible support in young stems and leaves
-Found in outer cortex, often in strands or cylinders around stems

Parenchyma tissue:

-Parenchyma cells form simple tissues such as:
• Cortex and pith → storage and support
• Mesophyll → photosynthesis (palisade) and gas exchange/regulation (spongy)

Sclerenchyma cells and tissue:

-Thick, lignified secondary cell walls; cells are dead at maturity
-Two types:

• Fibers (Sclerenchyma)

→ long, narrow, tapered cells; form strands or cylinders in stems

• Sclereids

→ variable shapes; can be stone-like or branched; form seed coats or clusters

Complex tissue: Vascular system

-Interconnected network of cells that transverses the entire plant
-Composed of:
• Vessel elements and tracheids (water-conducting cells; xylem)
• Fibers (sclerenchyma cells for support)
• Living parenchyma cells (assist in loading/unloading minerals and solutes)
• Sieve-tube members (food-conducting cells; phloem)
• Companion cells (specialized parenchyma cells that support sieve tubes)

Complex tissue: Epidermis

-Usually a single outer cell layer (up to ~6 layers in succulents)
-Covered with cutin (waxy protective layer)
-Composed of:
• Epidermal cells (flattened/elongated, usually lack chloroplasts)
• Guard cells (control stomata)
• Subsidiary cells (support guard cells)
• Trichomes (cellular outgrowths; in roots form root hairs, in shoots provide protection)

Complex tissue: Periderm

-Protective layer in older stems and roots; replaces epidermis
-Secondary tissue composed of:
• Phellem (cork) cells → outer layer; dead at maturity; walls contain suberin (waxy)
• Phellogen (cork cambium) → meristematic layer that produces periderm
• Phelloderm → inner layer of living, parenchyma-like cells between phellogen and

Stem

functions:
• Support: holds up leaves, flowers, and buds; can help orient leaves toward the Sun (tropisms)
• Conduction: transports water and minerals from roots to leaves, and moves sugars (products of photosynthesis) from leaves to the rest of the plant
• Photosynthesis: green stems can perform photosynthesis; in some plants (e.g. cacti), stems are the main photosynthetic organ
• Food storage: modified stems can store food, mainly in the form of starches

Stems - morphology:

-Plant stems are characterized by nodes (points of leaf attachment) spaced at regular intervals
-Internodes are the regions between nodes; they elongate early in development to separate leaves

-Axillary bud is found in the axil (the angle between a leaf and the stem)
• Can develop into a new shoot (axillary shoot) with its own apical meristem

-Note: Axillary buds help identify where a leaf begins; useful for pruning