Download Lecture 13- Plant Structure

BIO 120
Field Natural History
Spring
1
LECTURE 13
Structure of Plants
I. Introduction 1. Members of the phylum Chlorophyta were the ancestors to today’s green plants.
a. Green algae are aquatic and cannot survive on land.
b. They lack some basic adaptations that have made today’s land plants so successful.
2. Before the plants could live on land, they had to overcome three major challenges.
a. They had to absorb minerals from the rocky surface or hard soil.
b. They had to find a means of conserving water in a desiccating environment.
c. They had to develop a way to reproduce on land.
A. What are Plants?
1. Land plants are multicellular eukaryotes that are photosynthetic autotrophs.
2. They share the following characteristics with their green algal
ancestors.
a. Chloroplasts with photosynthetic pigments; chlorophyll a and chlorophyll b.
b. Cell walls containing cellulose - a key feature for remaining upright on land.
c. Food reserve is starch that is stored in cell.
d. Mitosis proceeds in same manner and a cell plate is formed during cytokinesis.
B. The Move to Land.
What were the major limitations faced by plants on land as compared to algae in the water?
1. The resources that land plants need are spatially separated.
a. Water and minerals are in the soil, but there is no light underground for photosynthesis.
Solution -Body of a plant is differentiated into a below ground part (the roots) that absorb
water and minerals and an aerial part (leaves and shoots) that conducts photosynthesis for
making sugars.
2. Regional specialization of plants solved one problem, but created another.
a. The roots anchor the plant in the soil (and are supported by it) but the shoot must stand
straight up in the air.
Solution – Plants have vascular tissue that with cell walls comprised of cellulose and lignin
and made like plywood. Creates a very rigid cell.
3. Because photosynthesis is confined to the aerial parts of the plants – plants must have a
physical mechanism to get water and minerals to the leaves from the soil.
Solution –Vascular tissue that moves water and minerals from roots to shoots.
4. They also must have a mechanism prevent desiccation. (Next lecture)
II. Adaptation to Terrestrial Living.
A. Roots – One of the first solutions to living on land.
1. Roots anchor plants to the soil.
2. Roots take up water and minerals from the soil.
3. As plants grow, the demand for water and minerals increases.
a. Roots grow to explore and exploit more water and minerals.
b. To increase the surface area of the roots, there are root hairs.
c. Early land plants did not have much of a root system. It is likely that they benefited from a
mutualistic association with mycorrhizae fungi.
2
4. Roots also serve as a storage organ for energy reserves.
a. May be used in the future for reproduction (flowering). E.g. Queen Anne’s lace – a
biennial. Live for two years. Year 1 grow and store food. Year 2. Produce flowers and
seeds.
b. Or for producing a new set of leaves. Deciduous trees.
c. Annuals – entire life cycle takes place in one year (season). Corn, radishes, beans.
d. Perennials – live for many years (100’s). May grow several years before they reproduce.
B. Stems – The above ground structures that support the light catching leaves.
1. The stem has cells that transport water and minerals from the roots to the leaves –xylem.
2. There are also cells that transport photosynthate from the leaves to the stem and the rootsphloem.
3. Together, phloem and xylem are called the vascular system because they function much like
our circulator system.
4. The water conducting xylem of many plants are just cylindrical cells stacked end to end to
create a large soda straw.
5. Plant cell walls are made of cellulose.
a. There are actually 4 layers to the cell wall.
b. In each layer, the fine, thread-like strands of cellulose are oriented at a different angle.
c. The resulting material is much like plywood, and very strong.
6. Many plants have additional material added to the xylem to make them more rigid- lignin.
a. Grasses have a compound called silicon dioxide, the presence of which greatly influence
horse evolution (via their teeth).
III. Leaves – are the primary organs of photosynthesis.
A. The structure of a leaf is a compromise between three conflicting evolutionary pressures.
1. To expose a maximum photosynthetic surface area to sunlight.
2. To conserve water.
3. While simultaneously providing for the exchange of gases necessary for photosynthesis.
B. Photosynthesis – a review. Sunlight + CO2 + H20 → Sugars (C6H12O6) + O2
C. External Leaf Structure
1. The basic structure of the leaf is the leaf blade.
2. Attached to the stem of the plant by a short stalk called a petiole.
3. If a leaf is compound, then the units that make up the leaf are called leaflets.
4. To solve problem of water loss, the outer layer of cells (epidermis) have a waxy, transparent
coating called the cuticle.
D. How do the gases CO2 and O2, get into and out of the leaf?
1. Tiny openings called stomates (stoma singl.)
2. The presence of openings in the leaf also permits water to escape. This is a bad thing and
a good thing.
a. The bad part - too much water escapes, the leaf dries out. The cells in a leaf need to be
wet so that CO2 can enter them. If CO2 cannot enter, then photosynthesis cannot proceed.
b. The good part is that water does need to leave the leaf. Water carries minerals to leaf.
c. How do plants get the minerals from the soil up to the top of a 10 story tree?
3
IV.
How does water get to the top of a tall tree?
A. Some basics.
1. Water enters at the bottom of the water column (xylem cells) via the roots.
a. Water enters roots via osmosis.
b. Osmosis is the name for the diffusion of water molecules in response to a water
concentration gradient between two regions that are separated by a selectively
permeable membrane.
c. This occurs when the concentrations of salts or minerals are greater on the inside of a
cell than the outside. This is how water is passively (no energy required) drawn into
the roots
d. The reverse is also possible - water can leave a cell. You observed this (plasmolysis)
with Elodea in your 4th lab.
e. Water entering the roots is pushed upward. This is called root pressure.
2. The cohesion-adhesion-tension theory.
a. The molecules of water, H20 have an attraction to the walls of the xylem cells. This is
called adhesion.
b. Capillary action - water adheres to the sides of a small hollow cylinder - i.e. xylem.
3. The molecules of water are attracted to each other (cohesion). They form weak chemical
bonds called hydrogen bonds that keep the water column together.
4. One other force plays a role – tension. Because the water molecules are held together by
cohesion, when the upper end of the water column is pulled by evaporation, the water
column is pulled up by tension.
5. Tensile strength is the resistance to breaking. The narrower the column of water, the
greater the tensile strength.
6. In plants, the passage of air across a leaf surface causes the evaporation of water,
transpiration, creating a “pull” at the upper end of the water column.
B. How do leaves keep from losing too much water?
1. Each stoma is bordered by a pair of guard cells.
2. When the guard cells are plump with water – condition called turgid – they become
bowed in shape, thus opening the stoma as wide as possible.
3. A number of factors influence the opening and closing of the stomates.
Sunlight causes them to open, and darkness to close.
Water loss causes the guard cells to become flaccid, hence they close the stoma.
Very high temperatures, above 86-95 °F.
C. Internal Structure.
1. The internal cells of the leaf are called mesophyll cells.
a. These cells contain chloroplasts and are the site of photosynthesis.
b. The mesophyll at the top are organized into columnar cells called palisade mesophyll.
c. Below the palisade is the spongy mesophyll. Called spongy because less organized.
d. Open space among the spongy mesophyll contributes to exchange of H2O, CO2 and
O2.
2. Bundles of xylem and phloem (vascular bundles) provide the leaf with water and a means
of transporting photosynthate out of the leaf.