2.1 Plant Cells, Tissues, and Organs

Cell Specialization cell specialization = when young cells develop from similar cells into ones that have specific functions Occurs in multicellular organisms Cell differentiation is the development stage in a young organism when specialized cells form Genes produce the proteins that cause cells to become specialized into a bone cell, nerve cell, muscle cell, ... all cells contain the same DNA certain genes “turn on” in one cell type, but not in others New cells come from pre-existing cells by mitosis (& cytokinesis), but they are not necessarily going to perform the same function

In all multicellular plants & animals: groups of specialized cells form tissues groups of tissues form organs groups of organs form systems In plants, unspecialized cells are called meristematic cells In animals, unspecialized cells are called stem cells Meristematic cells are sometimes called ‘permanent embryos’, because unlike many animal stem cells, they can produce cells that will become new tissues & organs at any point in a plant’s life in humans, true undifferentiated stem cells exist only as a fetus Plant: meristematic cells  tissues  organs  systems Animal: stem cells  tissues  organs  systems

Plant Animal

Meristematic Cells form Tissues Meristematic cells are constantly producing new cells these new cells become specialized to form tissues Types of plant tissues: 1. Dermal tissue 2. Ground tissue 3. Vascular tissue Figure 2.3 on p.58

Types of Plant Tissues 1. Dermal tissue  outermost part Made of epidermal cells These cells act as a barrier by: protecting the more delicate inner tissues controlling the exchange of materials 2. Ground tissue  middle layer Made of photosynthesizing cells & support cells 3. Vascular tissue  innermost tissue Made of phloem cells & xylem cells phloem cells transport nutrients like water & sugar throughout the plant xylem cells provide physical support

Replacing Specialized Cells Cells, tissues, and organs of multicellular animals like humans form as embryos inside the womb while some of the cells can be replaced, organs have to last for life! mammals cannot regenerate organs Unlike many animals, plants can replace cells, tissues and organs throughout their lives they can produce new cells which will become organs like leaves, roots, & stems Bud = cluster of meristematic cells that often specialize to form leaf or flower organs Most of a plant’s growth takes place at the terminal bud see Figure 2.4 and 2.5 on p. 59 terminal bud produces the plant hormone auxin which ‘holds back’ growth in other places

Plant Organs Tissues working together to form a leaf, one plant organ

How many plant organs can you identify in the diagram to the right? Leaf Stem Roots (Flower) reproductive organ

The Leaf Its most important job is to provide a large surface area for photosynthesis to take place Upper surface Made of a layer of dermal tissue called the epidermis Epidermal cells secrete a waxy cuticle (top layer) that: helps reduce water evaporation off of leaf protects the leaf so it can perform its primary function, photosynthesis sunlight passes through epidermal cells to photosynthesizing cells

Middle surface Made of mesophyll tissue (a special type of ground tissue), which is composed of 2 types of cells: palisade cells, which are specialized to perform most of the photosynthesis in the leaf spongy parenchyma cells, which form a loose network of open spaces (like a sponge), where gases for photosynthesis can be contained (CO2, H2O, O2) xylem and phloem cells are arranged in bundles that form veins to help the palisade cells conduct photo. & cell resp. xylem delivers water for photosynthesis phloem picks up sugars that have been produced in the palisade cells to transport to the rest of the plant (in the form of sap)

Lower surface (underside) Also made of a layer of dermal tissue, but the cells are different Guard cells allow gases to move in & out of the leaf these cells have special pores called stomata stomata are connected to the open spaces in the mesophyll transpiration happens through these pores CO2 enters, H2O and O2 leave See Figure 2.8 on p. 61 transpiration = evaporation of water from the leaves

Chloroplasts: the Leaf’s Key Organelle Located in palisade cells in the mesophyll Where photosynthesis takes place Palisade cells contain thylakoids thylakoids = sacs that contain chlorophyll to trap light when thylakoids are stacked, they are called granum (plural are grana) See Figure 2.9 on p. 63

A 3D look at a leaf organ

The Stem Has 2 main jobs: Xylem & phloem tissues are found here support transportation of water, nutrients and sugars Xylem & phloem tissues are found here Both are made of vertically stacked tubes Most of a plant’s xylem tissue is found in the stem when xylem cells die, they provide long, fibrous ‘pipes’ that allow water to be transported easily Phloem’s cell walls are porous to allow materials to move easily between phloem & neighbouring cells Xylem & phloem are grouped together in vascular bundles See Figure 2.10 on p. 64

The Roots Also has 2 main jobs: anchor the plant to the ground allow for water and minerals to be taken out of the soil In some plants, the roots act like a storage area for starch & minerals Special cells in the roots called cortex cells serve to store starch (long chains of glucose) Endodermis tissue controls water and mineral flow to vascular tissues Pericycle tissue surrounds the xylem and phloem it allows roots to branch out See Figure 2.11 on p. 65

Cross section of an Actual Root Also see Figure 2.12 on p. 65

Types of Roots 1. Taproot One main root that grows larger and thicker than the rest Allows plant to reach far underground for water ex: dandelion, thistle 2. Fibrous root Thin, hair-like roots Allows plant to absorb water from near the surface of the soil Help prevent erosion ex: grass, yarrow

Tissues in the 3 Main Organs